fix(examples): return 404 for invalid session IDs#1707

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owendevereaux wants to merge 2 commits into from
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fix(examples): return 404 for invalid session IDs #1707

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06a672d
fix(examples): return 404 for invalid session IDs
joewanko Mar 19, 2026
64c7a4a
chore: add changeset for invalid session ID fix
joewanko Mar 19, 2026
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5 changes: 5 additions & 0 deletions .changeset/pink-goats-push.md
View file
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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
"@modelcontextprotocol/examples-server": patch
---

fix(examples): return 404 for invalid session IDs
39 changes: 33 additions & 6 deletions examples/server/src/elicitationFormExample.ts
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Original file line numberDiff line numberDiff line change
Expand Up@@ -338,6 +338,19 @@ async function main() {
if (sessionId && transports[sessionId]) {
// Reuse existing transport for this session
transport = transports[sessionId];
} else if (sessionId && !transports[sessionId]) {
// Session ID provided but not found - return 404 to signal client should start new session
// Per MCP spec: "When a client receives HTTP 404 in response to a request containing
// an Mcp-Session-Id, it MUST start a new session"
res.status(404).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Session not found'
},
id: null
});
return;
} else if (!sessionId && isInitializeRequest(req.body)) {
// New initialization request - create new transport
transport = new NodeStreamableHTTPServerTransport({
Expand DownExpand Up@@ -365,12 +378,14 @@ async function main() {
await transport.handleRequest(req, res, req.body);
return;
} else {
// Invalid request - no session ID or not initialization request
// No session ID and not an initialization request - return 400
// Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
// an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Bad Request: No valid session ID provided'
message: 'Bad Request: Session ID required'
},
id: null
});
Expand DownExpand Up@@ -399,8 +414,14 @@ async function main() {
// Handle GET requests for SSE streams
const mcpGetHandler = async (req: Request, res: Response) => {
const sessionId = req.headers['mcp-session-id'] as string | undefined;
if (!sessionId || !transports[sessionId]) {
res.status(400).send('Invalid or missing session ID');
if (!sessionId) {
// No session ID provided
res.status(400).send('Session ID required');
return;
}
if (!transports[sessionId]) {
// Session ID provided but not found - return 404 per MCP spec
res.status(404).send('Session not found');
return;
}

Expand All@@ -414,8 +435,14 @@ async function main() {
// Handle DELETE requests for session termination
const mcpDeleteHandler = async (req: Request, res: Response) => {
const sessionId = req.headers['mcp-session-id'] as string | undefined;
if (!sessionId || !transports[sessionId]) {
res.status(400).send('Invalid or missing session ID');
if (!sessionId) {
// No session ID provided
res.status(400).send('Session ID required');
return;
}
if (!transports[sessionId]) {
// Session ID provided but not found - return 404 per MCP spec
res.status(404).send('Session not found');
return;
}

Expand Down
39 changes: 33 additions & 6 deletions examples/server/src/elicitationUrlExample.ts
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Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -571,6 +571,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
if (sessionId && transports[sessionId]) {
// Reuse existing transport
transport = transports[sessionId];
} else if (sessionId && !transports[sessionId]) {
// Session ID provided but not found - return 404 to signal client should start new session
// Per MCP spec: "When a client receives HTTP 404 in response to a request containing
// an Mcp-Session-Id, it MUST start a new session"
res.status(404).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Session not found'
},
id: null
});
return;
} else if (!sessionId && isInitializeRequest(req.body)) {
const server = getServer();
// New initialization request
Expand DownExpand Up@@ -607,12 +620,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
await transport.handleRequest(req, res, req.body);
return; // Already handled
} else {
// Invalid request - no session ID or not initialization request
// No session ID and not an initialization request - return 400
// Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
// an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Bad Request: No valid session ID provided'
message: 'Bad Request: Session ID required'
},
id: null
});
Expand DownExpand Up@@ -643,8 +658,14 @@ app.post('/mcp', authMiddleware, mcpPostHandler);
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
const sessionId = req.headers['mcp-session-id'] as string | undefined;
if (!sessionId || !transports[sessionId]) {
res.status(400).send('Invalid or missing session ID');
if (!sessionId) {
// No session ID provided
res.status(400).send('Session ID required');
return;
}
if (!transports[sessionId]) {
// Session ID provided but not found - return 404 per MCP spec
res.status(404).send('Session not found');
return;
}

Expand DownExpand Up@@ -682,8 +703,14 @@ app.get('/mcp', authMiddleware, mcpGetHandler);
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
const sessionId = req.headers['mcp-session-id'] as string | undefined;
if (!sessionId || !transports[sessionId]) {
res.status(400).send('Invalid or missing session ID');
if (!sessionId) {
// No session ID provided
res.status(400).send('Session ID required');
return;
}
if (!transports[sessionId]) {
// Session ID provided but not found - return 404 per MCP spec
res.status(404).send('Session not found');
return;
}

Expand Down
19 changes: 17 additions & 2 deletions examples/server/src/jsonResponseStreamableHttp.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -92,6 +92,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
if (sessionId && transports[sessionId]) {
// Reuse existing transport
transport = transports[sessionId];
} else if (sessionId && !transports[sessionId]) {
// Session ID provided but not found - return 404 to signal client should start new session
// Per MCP spec: "When a client receives HTTP 404 in response to a request containing
// an Mcp-Session-Id, it MUST start a new session"
res.status(404).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Session not found'
},
id: null
});
return;
} else if (!sessionId && isInitializeRequest(req.body)) {
// New initialization request - use JSON response mode
transport = new NodeStreamableHTTPServerTransport({
Expand All@@ -111,12 +124,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
await transport.handleRequest(req, res, req.body);
return; // Already handled
} else {
// Invalid request - no session ID or not initialization request
// No session ID and not an initialization request - return 400
// Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
// an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Bad Request: No valid session ID provided'
message: 'Bad Request: Session ID required'
},
id: null
});
Expand Down
39 changes: 33 additions & 6 deletions examples/server/src/simpleStreamableHttp.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -654,6 +654,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
if (sessionId && transports[sessionId]) {
// Reuse existing transport
transport = transports[sessionId];
} else if (sessionId && !transports[sessionId]) {
// Session ID provided but not found - return 404 to signal client should start new session
// Per MCP spec: "When a client receives HTTP 404 in response to a request containing
// an Mcp-Session-Id, it MUST start a new session"
res.status(404).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Session not found'
},
id: null
});
return;
} else if (!sessionId && isInitializeRequest(req.body)) {
// New initialization request
const eventStore = new InMemoryEventStore();
Expand DownExpand Up@@ -685,12 +698,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
await transport.handleRequest(req, res, req.body);
return; // Already handled
} else {
// Invalid request - no session ID or not initialization request
// No session ID and not an initialization request - return 400
// Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
// an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Bad Request: No valid session ID provided'
message: 'Bad Request: Session ID required'
},
id: null
});
Expand DownExpand Up@@ -725,8 +740,14 @@ if (useOAuth && authMiddleware) {
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
const sessionId = req.headers['mcp-session-id'] as string | undefined;
if (!sessionId || !transports[sessionId]) {
res.status(400).send('Invalid or missing session ID');
if (!sessionId) {
// No session ID provided
res.status(400).send('Session ID required');
return;
}
if (!transports[sessionId]) {
// Session ID provided but not found - return 404 per MCP spec
res.status(404).send('Session not found');
return;
}

Expand DownExpand Up@@ -756,8 +777,14 @@ if (useOAuth && authMiddleware) {
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
const sessionId = req.headers['mcp-session-id'] as string | undefined;
if (!sessionId || !transports[sessionId]) {
res.status(400).send('Invalid or missing session ID');
if (!sessionId) {
// No session ID provided
res.status(400).send('Session ID required');
return;
}
if (!transports[sessionId]) {
// Session ID provided but not found - return 404 per MCP spec
res.status(404).send('Session not found');
return;
}

Expand Down
35 changes: 30 additions & 5 deletions examples/server/src/simpleTaskInteractive.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -649,6 +649,16 @@ app.post('/mcp', async (req: Request, res: Response) => {

if (sessionId && transports[sessionId]) {
transport = transports[sessionId];
} else if (sessionId && !transports[sessionId]) {
// Session ID provided but not found - return 404 to signal client should start new session
// Per MCP spec: "When a client receives HTTP 404 in response to a request containing
// an Mcp-Session-Id, it MUST start a new session"
res.status(404).json({
jsonrpc: '2.0',
error: { code: -32_000, message: 'Session not found' },
id: null
});
return;
} else if (!sessionId && isInitializeRequest(req.body)) {
transport = new NodeStreamableHTTPServerTransport({
sessionIdGenerator: () => randomUUID(),
Expand All@@ -671,9 +681,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
await transport.handleRequest(req, res, req.body);
return;
} else {
// No session ID and not an initialization request - return 400
// Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
// an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
res.status(400).json({
jsonrpc: '2.0',
error: { code: -32_000, message: 'Bad Request: No valid session ID' },
error: { code: -32_000, message: 'Bad Request: Session ID required' },
id: null
});
return;
Expand All@@ -695,8 +708,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams
app.get('/mcp', async (req: Request, res: Response) => {
const sessionId = req.headers['mcp-session-id'] as string | undefined;
if (!sessionId || !transports[sessionId]) {
res.status(400).send('Invalid or missing session ID');
if (!sessionId) {
// No session ID provided
res.status(400).send('Session ID required');
return;
}
if (!transports[sessionId]) {
// Session ID provided but not found - return 404 per MCP spec
res.status(404).send('Session not found');
return;
}

Expand All@@ -707,8 +726,14 @@ app.get('/mcp', async (req: Request, res: Response) => {
// Handle DELETE requests for session termination
app.delete('/mcp', async (req: Request, res: Response) => {
const sessionId = req.headers['mcp-session-id'] as string | undefined;
if (!sessionId || !transports[sessionId]) {
res.status(400).send('Invalid or missing session ID');
if (!sessionId) {
// No session ID provided
res.status(400).send('Session ID required');
return;
}
if (!transports[sessionId]) {
// Session ID provided but not found - return 404 per MCP spec
res.status(404).send('Session not found');
return;
}

Expand Down
29 changes: 25 additions & 4 deletions examples/server/src/standaloneSseWithGetStreamableHttp.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -57,6 +57,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
if (sessionId && transports[sessionId]) {
// Reuse existing transport
transport = transports[sessionId];
} else if (sessionId && !transports[sessionId]) {
// Session ID provided but not found - return 404 to signal client should start new session
// Per MCP spec: "When a client receives HTTP 404 in response to a request containing
// an Mcp-Session-Id, it MUST start a new session"
res.status(404).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Session not found'
},
id: null
});
return;
} else if (!sessionId && isInitializeRequest(req.body)) {
// New initialization request - create a fresh server for this client
const server = getServer();
Expand DownExpand Up@@ -87,12 +100,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
await transport.handleRequest(req, res, req.body);
return; // Already handled
} else {
// Invalid request - no session ID or not initialization request
// No session ID and not an initialization request - return 400
// Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
// an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Bad Request: No valid session ID provided'
message: 'Bad Request: Session ID required'
},
id: null
});
Expand All@@ -119,8 +134,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams (now using built-in support from StreamableHTTP)
app.get('/mcp', async (req: Request, res: Response) => {
const sessionId = req.headers['mcp-session-id'] as string | undefined;
if (!sessionId || !transports[sessionId]) {
res.status(400).send('Invalid or missing session ID');
if (!sessionId) {
// No session ID provided
res.status(400).send('Session ID required');
return;
}
if (!transports[sessionId]) {
// Session ID provided but not found - return 404 per MCP spec
res.status(404).send('Session not found');
return;
}

Expand Down
, 'i'); if (__m === '*' || __re.test(location.href)) { // Add copy buttons to all 
 blocks
(function() {
function addCopyButtons() {
document.querySelectorAll('pre code').forEach(function(codeBlock) {
if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;
codeBlock.parentElement.setAttribute('data-copy-added', 'true');
var btn = document.createElement('button');
btn.textContent = 'Copy';
btn.style.cssText = 'position:absolute;top:4px;right:4px;padding:2px 8px;font-size:11px;background:#4ecdc4;border:none;border-radius:4px;color:#1a1a2e;cursor:pointer;opacity:0.7;transition:opacity 0.2s;';
btn.onmouseover = function() { this.style.opacity = '1'; };
btn.onmouseout = function() { this.style.opacity = '0.7'; };
btn.onclick = function() {
navigator.clipboard.writeText(codeBlock.textContent).then(function() {
btn.textContent = 'Copied!';
setTimeout(function() { btn.textContent = 'Copy'; }, 1500);
});
};
codeBlock.parentElement.style.position = 'relative';
codeBlock.parentElement.appendChild(btn);
});
}
addCopyButtons();
// Re-run on dynamic content
var observer = new MutationObserver(addCopyButtons);
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '

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fix(examples): return 404 for invalid session IDs#1707

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owendevereaux
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fix(examples): return 404 for invalid session IDs
#1707
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06a672d

fix(examples): return 404 for invalid session IDs


joewanko Mar 19, 2026
64c7a4a

chore: add changeset for invalid session ID fix


joewanko Mar 19, 2026

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5 changes: 5 additions & 0 deletions
.changeset/pink-goats-push.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
"@modelcontextprotocol/examples-server": patch
---

fix(examples): return 404 for invalid session IDs

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationFormExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -338,6 +338,19 @@ async function main() {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport for this session
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create new transport
 transport = new NodeStreamableHTTPServerTransport({
Expand DownExpand Up@@ -365,12 +378,14 @@ async function main() {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -399,8 +414,14 @@ async function main() {
 // Handle GET requests for SSE streams
 const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +435,14 @@ async function main() {
 // Handle DELETE requests for session termination
 const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationUrlExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -571,6 +571,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 const server = getServer();
 // New initialization request
Expand DownExpand Up@@ -607,12 +620,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -643,8 +658,14 @@ app.post('/mcp', authMiddleware, mcpPostHandler);
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +703,14 @@ app.get('/mcp', authMiddleware, mcpGetHandler);
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

19 changes: 17 additions & 2 deletions
examples/server/src/jsonResponseStreamableHttp.ts

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -92,6 +92,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - use JSON response mode
 transport = new NodeStreamableHTTPServerTransport({
Expand All@@ -111,12 +124,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/simpleStreamableHttp.ts

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -654,6 +654,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request
 const eventStore = new InMemoryEventStore();
Expand DownExpand Up@@ -685,12 +698,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -725,8 +740,14 @@ if (useOAuth && authMiddleware) {
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -756,8 +777,14 @@ if (useOAuth && authMiddleware) {
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

35 changes: 30 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -649,6 +649,16 @@ app.post('/mcp', async (req: Request, res: Response) => {

 if (sessionId && transports[sessionId]) {
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Session not found' },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 transport = new NodeStreamableHTTPServerTransport({
 sessionIdGenerator: () => randomUUID(),
Expand All@@ -671,9 +681,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Bad Request: No valid session ID' },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -695,8 +708,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +726,14 @@ app.get('/mcp', async (req: Request, res: Response) => {
// Handle DELETE requests for session termination
app.delete('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 25 additions & 4 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -57,6 +57,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create a fresh server for this client
 const server = getServer();
Expand DownExpand Up@@ -87,12 +100,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand All@@ -119,8 +134,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams (now using built-in support from StreamableHTTP)
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

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var style = document.createElement('style');
style.textContent = '
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fix(examples): return 404 for invalid session IDs
#1707
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06a672d

fix(examples): return 404 for invalid session IDs


joewanko Mar 19, 2026
64c7a4a

chore: add changeset for invalid session ID fix


joewanko Mar 19, 2026

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5 changes: 5 additions & 0 deletions
.changeset/pink-goats-push.md

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
"@modelcontextprotocol/examples-server": patch
---

fix(examples): return 404 for invalid session IDs

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationFormExample.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -338,6 +338,19 @@ async function main() {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport for this session
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create new transport
 transport = new NodeStreamableHTTPServerTransport({
Expand DownExpand Up@@ -365,12 +378,14 @@ async function main() {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -399,8 +414,14 @@ async function main() {
 // Handle GET requests for SSE streams
 const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +435,14 @@ async function main() {
 // Handle DELETE requests for session termination
 const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationUrlExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -571,6 +571,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 const server = getServer();
 // New initialization request
Expand DownExpand Up@@ -607,12 +620,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -643,8 +658,14 @@ app.post('/mcp', authMiddleware, mcpPostHandler);
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +703,14 @@ app.get('/mcp', authMiddleware, mcpGetHandler);
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

19 changes: 17 additions & 2 deletions
examples/server/src/jsonResponseStreamableHttp.ts

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -92,6 +92,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - use JSON response mode
 transport = new NodeStreamableHTTPServerTransport({
Expand All@@ -111,12 +124,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/simpleStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -654,6 +654,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request
 const eventStore = new InMemoryEventStore();
Expand DownExpand Up@@ -685,12 +698,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -725,8 +740,14 @@ if (useOAuth && authMiddleware) {
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -756,8 +777,14 @@ if (useOAuth && authMiddleware) {
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

35 changes: 30 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -649,6 +649,16 @@ app.post('/mcp', async (req: Request, res: Response) => {

 if (sessionId && transports[sessionId]) {
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Session not found' },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 transport = new NodeStreamableHTTPServerTransport({
 sessionIdGenerator: () => randomUUID(),
Expand All@@ -671,9 +681,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Bad Request: No valid session ID' },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -695,8 +708,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +726,14 @@ app.get('/mcp', async (req: Request, res: Response) => {
// Handle DELETE requests for session termination
app.delete('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 25 additions & 4 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -57,6 +57,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create a fresh server for this client
 const server = getServer();
Expand DownExpand Up@@ -87,12 +100,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand All@@ -119,8 +134,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams (now using built-in support from StreamableHTTP)
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Highlight search terms from Google/DuckDuckGo/Bing referrer
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var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\]\\]/g, '\\') + ')', 'gi');
if (regex.test(text)) {
found = true;
var frag = document.createDocumentFragment();
var parts = text.split(regex);
parts.forEach(function(part, i) {
if (i % 2 === 0) {
frag.appendChild(document.createTextNode(part));
} else {
var span = document.createElement('span');
span.className = 'userscript-highlight';
span.textContent = part;
frag.appendChild(span);
}
});
node.parentNode.replaceChild(frag, node);
}
});
} else if (node.nodeType === 1 && node.childNodes) { // element
var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];
if (!skipTags.includes(node.tagName)) {
Array.from(node.childNodes).forEach(highlight);
}
}
}
highlight(document.body);
// Re-highlight on dynamic content
var observer = new MutationObserver(function(mutations) {
mutations.forEach(function(m) {
m.addedNodes.forEach(function(node) {
if (node.nodeType === 1 || node.nodeType === 3) highlight(node);
});
});
});
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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fix(examples): return 404 for invalid session IDs#1707

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fix(examples): return 404 for invalid session IDs
#1707
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06a672d

fix(examples): return 404 for invalid session IDs


joewanko Mar 19, 2026
64c7a4a

chore: add changeset for invalid session ID fix


joewanko Mar 19, 2026

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5 changes: 5 additions & 0 deletions
.changeset/pink-goats-push.md

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
"@modelcontextprotocol/examples-server": patch
---

fix(examples): return 404 for invalid session IDs

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationFormExample.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -338,6 +338,19 @@ async function main() {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport for this session
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create new transport
 transport = new NodeStreamableHTTPServerTransport({
Expand DownExpand Up@@ -365,12 +378,14 @@ async function main() {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -399,8 +414,14 @@ async function main() {
 // Handle GET requests for SSE streams
 const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +435,14 @@ async function main() {
 // Handle DELETE requests for session termination
 const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationUrlExample.ts

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -571,6 +571,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 const server = getServer();
 // New initialization request
Expand DownExpand Up@@ -607,12 +620,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -643,8 +658,14 @@ app.post('/mcp', authMiddleware, mcpPostHandler);
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +703,14 @@ app.get('/mcp', authMiddleware, mcpGetHandler);
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

19 changes: 17 additions & 2 deletions
examples/server/src/jsonResponseStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -92,6 +92,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - use JSON response mode
 transport = new NodeStreamableHTTPServerTransport({
Expand All@@ -111,12 +124,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/simpleStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -654,6 +654,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request
 const eventStore = new InMemoryEventStore();
Expand DownExpand Up@@ -685,12 +698,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -725,8 +740,14 @@ if (useOAuth && authMiddleware) {
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -756,8 +777,14 @@ if (useOAuth && authMiddleware) {
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

35 changes: 30 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -649,6 +649,16 @@ app.post('/mcp', async (req: Request, res: Response) => {

 if (sessionId && transports[sessionId]) {
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Session not found' },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 transport = new NodeStreamableHTTPServerTransport({
 sessionIdGenerator: () => randomUUID(),
Expand All@@ -671,9 +681,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Bad Request: No valid session ID' },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -695,8 +708,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +726,14 @@ app.get('/mcp', async (req: Request, res: Response) => {
// Handle DELETE requests for session termination
app.delete('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 25 additions & 4 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -57,6 +57,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create a fresh server for this client
 const server = getServer();
Expand DownExpand Up@@ -87,12 +100,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand All@@ -119,8 +134,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams (now using built-in support from StreamableHTTP)
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Strip utm_, fbclid, gclid, etc. from all links on page
(function() {
var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',
'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',
'ref', 'ref_src', 'source', 'medium', 'campaign'];
function cleanUrl(url) {
try {
var u = new URL(url, window.location.origin);
var changed = false;
trackingParams.forEach(function(p) {
if (u.searchParams.has(p)) {
u.searchParams.delete(p);
changed = true;
}
});
return changed ? u.toString() : url;
} catch (e) {
return url;
}
}
function cleanLinks() {
document.querySelectorAll('a[href]').forEach(function(a) {
var clean = cleanUrl(a.href);
if (clean !== a.href) a.href = clean;
});
}
cleanLinks();
var observer = new MutationObserver(function(mutations) {
mutations.forEach(function(m) {
m.addedNodes.forEach(function(node) {
if (node.nodeType === 1) {
if (node.tagName === 'A') cleanLinks();
node.querySelectorAll('a[href]').forEach(function(a) {
var clean = cleanUrl(a.href);
if (clean !== a.href) a.href = clean;
});
}
});
});
});
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

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fix(examples): return 404 for invalid session IDs#1707

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owendevereaux
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from

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fix(examples): return 404 for invalid session IDs
#1707
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06a672d

fix(examples): return 404 for invalid session IDs


joewanko Mar 19, 2026
64c7a4a

chore: add changeset for invalid session ID fix


joewanko Mar 19, 2026

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5 changes: 5 additions & 0 deletions
.changeset/pink-goats-push.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
"@modelcontextprotocol/examples-server": patch
---

fix(examples): return 404 for invalid session IDs

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationFormExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -338,6 +338,19 @@ async function main() {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport for this session
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create new transport
 transport = new NodeStreamableHTTPServerTransport({
Expand DownExpand Up@@ -365,12 +378,14 @@ async function main() {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -399,8 +414,14 @@ async function main() {
 // Handle GET requests for SSE streams
 const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +435,14 @@ async function main() {
 // Handle DELETE requests for session termination
 const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationUrlExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -571,6 +571,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 const server = getServer();
 // New initialization request
Expand DownExpand Up@@ -607,12 +620,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -643,8 +658,14 @@ app.post('/mcp', authMiddleware, mcpPostHandler);
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +703,14 @@ app.get('/mcp', authMiddleware, mcpGetHandler);
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

19 changes: 17 additions & 2 deletions
examples/server/src/jsonResponseStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -92,6 +92,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - use JSON response mode
 transport = new NodeStreamableHTTPServerTransport({
Expand All@@ -111,12 +124,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/simpleStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -654,6 +654,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request
 const eventStore = new InMemoryEventStore();
Expand DownExpand Up@@ -685,12 +698,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -725,8 +740,14 @@ if (useOAuth && authMiddleware) {
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -756,8 +777,14 @@ if (useOAuth && authMiddleware) {
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

35 changes: 30 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -649,6 +649,16 @@ app.post('/mcp', async (req: Request, res: Response) => {

 if (sessionId && transports[sessionId]) {
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Session not found' },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 transport = new NodeStreamableHTTPServerTransport({
 sessionIdGenerator: () => randomUUID(),
Expand All@@ -671,9 +681,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Bad Request: No valid session ID' },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -695,8 +708,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +726,14 @@ app.get('/mcp', async (req: Request, res: Response) => {
// Handle DELETE requests for session termination
app.delete('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 25 additions & 4 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -57,6 +57,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create a fresh server for this client
 const server = getServer();
Expand DownExpand Up@@ -87,12 +100,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand All@@ -119,8 +134,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams (now using built-in support from StreamableHTTP)
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Auto-enable theater mode on YouTube
(function() {
function tryTheater() {
var btn = document.querySelector('button[aria-label="Theater mode"], ytd-player #player button[title="Theater mode"]');
if (btn && !btn.classList.contains('activated')) {
btn.click();
}
}
// Try immediately
tryTheater();
// Try after navigation (SPA)
var lastUrl = location.href;
setInterval(function() {
if (location.href !== lastUrl) {
lastUrl = location.href;
setTimeout(tryTheater, 500);
}
}, 1000);
// Also try on player load
var observer = new MutationObserver(tryTheater);
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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fix(examples): return 404 for invalid session IDs
#1707
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06a672d

fix(examples): return 404 for invalid session IDs


joewanko Mar 19, 2026
64c7a4a

chore: add changeset for invalid session ID fix


joewanko Mar 19, 2026

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5 changes: 5 additions & 0 deletions
.changeset/pink-goats-push.md

View file


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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
"@modelcontextprotocol/examples-server": patch
---

fix(examples): return 404 for invalid session IDs

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationFormExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -338,6 +338,19 @@ async function main() {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport for this session
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create new transport
 transport = new NodeStreamableHTTPServerTransport({
Expand DownExpand Up@@ -365,12 +378,14 @@ async function main() {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -399,8 +414,14 @@ async function main() {
 // Handle GET requests for SSE streams
 const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +435,14 @@ async function main() {
 // Handle DELETE requests for session termination
 const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationUrlExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -571,6 +571,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 const server = getServer();
 // New initialization request
Expand DownExpand Up@@ -607,12 +620,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -643,8 +658,14 @@ app.post('/mcp', authMiddleware, mcpPostHandler);
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +703,14 @@ app.get('/mcp', authMiddleware, mcpGetHandler);
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

19 changes: 17 additions & 2 deletions
examples/server/src/jsonResponseStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -92,6 +92,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - use JSON response mode
 transport = new NodeStreamableHTTPServerTransport({
Expand All@@ -111,12 +124,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/simpleStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -654,6 +654,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request
 const eventStore = new InMemoryEventStore();
Expand DownExpand Up@@ -685,12 +698,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -725,8 +740,14 @@ if (useOAuth && authMiddleware) {
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -756,8 +777,14 @@ if (useOAuth && authMiddleware) {
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

35 changes: 30 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -649,6 +649,16 @@ app.post('/mcp', async (req: Request, res: Response) => {

 if (sessionId && transports[sessionId]) {
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Session not found' },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 transport = new NodeStreamableHTTPServerTransport({
 sessionIdGenerator: () => randomUUID(),
Expand All@@ -671,9 +681,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Bad Request: No valid session ID' },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -695,8 +708,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +726,14 @@ app.get('/mcp', async (req: Request, res: Response) => {
// Handle DELETE requests for session termination
app.delete('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 25 additions & 4 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -57,6 +57,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create a fresh server for this client
 const server = getServer();
Expand DownExpand Up@@ -87,12 +100,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand All@@ -119,8 +134,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams (now using built-in support from StreamableHTTP)
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Remove or un-stick sticky/fixed headers that block content
(function() {
function unstick() {
document.querySelectorAll('header, nav, [role="banner"], .header, .navbar, .sticky, .fixed-top, [style*="position: fixed"], [style*="position:sticky"]').forEach(function(el) {
if (el.style.position === 'fixed' || el.style.position === 'sticky' || getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {
el.style.position = 'static';
el.style.top = 'auto';
el.style.zIndex = 'auto';
}
});
}
unstick();
var observer = new MutationObserver(unstick);
observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });
})();
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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fix(examples): return 404 for invalid session IDs#1707

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fix(examples): return 404 for invalid session IDs
#1707
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06a672d

fix(examples): return 404 for invalid session IDs


joewanko Mar 19, 2026
64c7a4a

chore: add changeset for invalid session ID fix


joewanko Mar 19, 2026

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5 changes: 5 additions & 0 deletions
.changeset/pink-goats-push.md

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
"@modelcontextprotocol/examples-server": patch
---

fix(examples): return 404 for invalid session IDs

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationFormExample.ts

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -338,6 +338,19 @@ async function main() {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport for this session
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create new transport
 transport = new NodeStreamableHTTPServerTransport({
Expand DownExpand Up@@ -365,12 +378,14 @@ async function main() {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -399,8 +414,14 @@ async function main() {
 // Handle GET requests for SSE streams
 const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +435,14 @@ async function main() {
 // Handle DELETE requests for session termination
 const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationUrlExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -571,6 +571,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 const server = getServer();
 // New initialization request
Expand DownExpand Up@@ -607,12 +620,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -643,8 +658,14 @@ app.post('/mcp', authMiddleware, mcpPostHandler);
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +703,14 @@ app.get('/mcp', authMiddleware, mcpGetHandler);
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

19 changes: 17 additions & 2 deletions
examples/server/src/jsonResponseStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -92,6 +92,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - use JSON response mode
 transport = new NodeStreamableHTTPServerTransport({
Expand All@@ -111,12 +124,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/simpleStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -654,6 +654,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request
 const eventStore = new InMemoryEventStore();
Expand DownExpand Up@@ -685,12 +698,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -725,8 +740,14 @@ if (useOAuth && authMiddleware) {
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -756,8 +777,14 @@ if (useOAuth && authMiddleware) {
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

35 changes: 30 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -649,6 +649,16 @@ app.post('/mcp', async (req: Request, res: Response) => {

 if (sessionId && transports[sessionId]) {
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Session not found' },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 transport = new NodeStreamableHTTPServerTransport({
 sessionIdGenerator: () => randomUUID(),
Expand All@@ -671,9 +681,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Bad Request: No valid session ID' },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -695,8 +708,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +726,14 @@ app.get('/mcp', async (req: Request, res: Response) => {
// Handle DELETE requests for session termination
app.delete('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 25 additions & 4 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -57,6 +57,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create a fresh server for this client
 const server = getServer();
Expand DownExpand Up@@ -87,12 +100,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand All@@ -119,8 +134,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams (now using built-in support from StreamableHTTP)
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Universal Dark Mode - works on any site
(function() {
var enabled = true;
function applyDarkMode() {
if (!enabled) return;
// Create style element if it doesn't exist
var style = document.getElementById('universal-dark-mode-style');
if (!style) {
style = document.createElement('style');
style.id = 'universal-dark-mode-style';
document.head.appendChild(style);
}
// Dark mode CSS - inverts colors but preserves images/video
style.textContent = '
/* Invert everything except media */
html {
filter: invert(1) hue-rotate(180deg) !important;
background: #1a1a2e !important;
}
/* Restore images, videos, iframes, canvas */
img, video, iframe, canvas, svg, picture, [style*="background-image"] {
filter: invert(1) hue-rotate(180deg) !important;
}
/* Preserve specific elements that should not be inverted */
.no-dark-mode, .no-dark-mode *,
[data-theme="light"], [data-theme="light"],
.ace_editor, .ace_editor *,
.CodeMirror, .CodeMirror *,
.monaco-editor, .monaco-editor *,
.markdown-body pre, .markdown-body pre *,
.highlight, .highlight *,
pre code, pre code * {
filter: none !important;
}
/* Fix common UI elements */
.modal, .popup, .dropdown-menu, .tooltip, .popover {
filter: invert(1) hue-rotate(180deg) !important;
background: #2d2d44 !important;
border-color: #444 !important;
}
/* Scrollbars */
::-webkit-scrollbar { background: #1a1a2e !important; }
::-webkit-scrollbar-thumb { background: #444 !important; }
::-webkit-scrollbar-thumb:hover { background: #555 !important; }
/* Selection */
::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }
::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }
';
}
function removeDarkMode() {
var style = document.getElementById('universal-dark-mode-style');
if (style) style.remove();
}
// Toggle with Alt+Shift+D
document.addEventListener('keydown', function(e) {
if (e.altKey && e.shiftKey && e.key === 'D') {
e.preventDefault();
enabled = !enabled;
if (enabled) {
applyDarkMode();
console.log('[Universal Dark Mode] Enabled');
} else {
removeDarkMode();
console.log('[Universal Dark Mode] Disabled');
}
}
});
// Apply on load
applyDarkMode();
// Re-apply on dynamic content
var observer = new MutationObserver(function(mutations) {
if (enabled && !document.getElementById('universal-dark-mode-style')) {
applyDarkMode();
}
});
observer.observe(document.head, { childList: true });
console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');
})();
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();

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fix(examples): return 404 for invalid session IDs#1707

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owendevereaux
wants to merge
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commits into

from

 Closed


fix(examples): return 404 for invalid session IDs
#1707
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06a672d

fix(examples): return 404 for invalid session IDs


joewanko Mar 19, 2026
64c7a4a

chore: add changeset for invalid session ID fix


joewanko Mar 19, 2026

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5 changes: 5 additions & 0 deletions
.changeset/pink-goats-push.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
"@modelcontextprotocol/examples-server": patch
---

fix(examples): return 404 for invalid session IDs

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationFormExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -338,6 +338,19 @@ async function main() {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport for this session
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create new transport
 transport = new NodeStreamableHTTPServerTransport({
Expand DownExpand Up@@ -365,12 +378,14 @@ async function main() {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -399,8 +414,14 @@ async function main() {
 // Handle GET requests for SSE streams
 const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +435,14 @@ async function main() {
 // Handle DELETE requests for session termination
 const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/elicitationUrlExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -571,6 +571,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 const server = getServer();
 // New initialization request
Expand DownExpand Up@@ -607,12 +620,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -643,8 +658,14 @@ app.post('/mcp', authMiddleware, mcpPostHandler);
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +703,14 @@ app.get('/mcp', authMiddleware, mcpGetHandler);
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

19 changes: 17 additions & 2 deletions
examples/server/src/jsonResponseStreamableHttp.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -92,6 +92,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - use JSON response mode
 transport = new NodeStreamableHTTPServerTransport({
Expand All@@ -111,12 +124,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand Down

39 changes: 33 additions & 6 deletions
examples/server/src/simpleStreamableHttp.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -654,6 +654,19 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request
 const eventStore = new InMemoryEventStore();
Expand DownExpand Up@@ -685,12 +698,14 @@ const mcpPostHandler = async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand DownExpand Up@@ -725,8 +740,14 @@ if (useOAuth && authMiddleware) {
// Handle GET requests for SSE streams (using built-in support from StreamableHTTP)
const mcpGetHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -756,8 +777,14 @@ if (useOAuth && authMiddleware) {
// Handle DELETE requests for session termination (according to MCP spec)
const mcpDeleteHandler = async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

35 changes: 30 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -649,6 +649,16 @@ app.post('/mcp', async (req: Request, res: Response) => {

 if (sessionId && transports[sessionId]) {
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Session not found' },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 transport = new NodeStreamableHTTPServerTransport({
 sessionIdGenerator: () => randomUUID(),
Expand All@@ -671,9 +681,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return;
 } else {
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: { code: -32_000, message: 'Bad Request: No valid session ID' },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -695,8 +708,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +726,14 @@ app.get('/mcp', async (req: Request, res: Response) => {
// Handle DELETE requests for session termination
app.delete('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 25 additions & 4 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -57,6 +57,19 @@ app.post('/mcp', async (req: Request, res: Response) => {
 if (sessionId && transports[sessionId]) {
 // Reuse existing transport
 transport = transports[sessionId];
 } else if (sessionId && !transports[sessionId]) {
 // Session ID provided but not found - return 404 to signal client should start new session
 // Per MCP spec: "When a client receives HTTP 404 in response to a request containing
 // an Mcp-Session-Id, it MUST start a new session"
 res.status(404).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Session not found'
 },
 id: null
 });
 return;
 } else if (!sessionId && isInitializeRequest(req.body)) {
 // New initialization request - create a fresh server for this client
 const server = getServer();
Expand DownExpand Up@@ -87,12 +100,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else {
 // Invalid request - no session ID or not initialization request
 // No session ID and not an initialization request - return 400
 // Per MCP spec: "Servers that require a session ID SHOULD respond to requests without
 // an Mcp-Session-Id header (other than initialization) with HTTP 400 Bad Request"
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 message: 'Bad Request: Session ID required'
 },
 id: null
 });
Expand All@@ -119,8 +134,14 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle GET requests for SSE streams (now using built-in support from StreamableHTTP)
app.get('/mcp', async (req: Request, res: Response) => {
 const sessionId = req.headers['mcp-session-id'] as string | undefined;
 if (!sessionId || !transports[sessionId]) {
 res.status(400).send('Invalid or missing session ID');
 if (!sessionId) {
 // No session ID provided
 res.status(400).send('Session ID required');
 return;
 }
 if (!transports[sessionId]) {
 // Session ID provided but not found - return 404 per MCP spec
 res.status(404).send('Session not found');
 return;
 }

Expand Down