fix(examples): return 404 for unknown session IDs, 400 for missing#1770

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felixweinberger merged 4 commits into from
Mar 30, 2026
Merged

fix(examples): return 404 for unknown session IDs, 400 for missing #1770

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15e5b36
fix(examples): return 404 for unknown session IDs, 400 for missing
felixweinberger Mar 26, 2026
4ab1f6f
Merge branch 'main' into fweinberger/session-404-fix
felixweinberger Mar 30, 2026
fc1f880
Merge branch 'main' into fweinberger/session-404-fix
felixweinberger Mar 30, 2026
c61edba
Merge branch 'main' into fweinberger/session-404-fix
felixweinberger Mar 30, 2026
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5 changes: 5 additions & 0 deletions .changeset/fix-session-status-codes.md
View file
Open in desktop
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@modelcontextprotocol/examples-server': patch
---

Example servers now return HTTP 404 (not 400) when a request includes an unknown session ID, so clients can correctly detect they need to start a new session. Requests missing a session ID entirely still return 400.
29 changes: 20 additions & 9 deletions examples/server/src/elicitationFormExample.ts
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Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -364,14 +364,17 @@ async function main() {

await transport.handleRequest(req, res, req.body);
return;
} else if (sessionId) {
res.status(404).json({
jsonrpc: '2.0',
error: { code: -32_001, message: 'Session not found' },
id: null
});
return;
} else {
// Invalid request - no session ID or not initialization request
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Bad Request: No valid session ID provided'
},
error: { code: -32_000, message: 'Bad Request: Session ID required' },
id: null
});
return;
Expand DownExpand Up@@ -399,8 +402,12 @@ 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) {
res.status(400).send('Missing session ID');
return;
}
if (!transports[sessionId]) {
res.status(404).send('Session not found');
return;
}

Expand All@@ -414,8 +421,12 @@ 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) {
res.status(400).send('Missing session ID');
return;
}
if (!transports[sessionId]) {
res.status(404).send('Session not found');
return;
}

Expand Down
29 changes: 20 additions & 9 deletions examples/server/src/elicitationUrlExample.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -606,14 +606,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

await transport.handleRequest(req, res, req.body);
return; // Already handled
} else if (sessionId) {
res.status(404).json({
jsonrpc: '2.0',
error: { code: -32_001, message: 'Session not found' },
id: null
});
return;
} else {
// Invalid request - no session ID or not initialization request
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Bad Request: No valid session ID provided'
},
error: { code: -32_000, message: 'Bad Request: Session ID required' },
id: null
});
return;
Expand DownExpand Up@@ -643,8 +646,12 @@ 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) {
res.status(400).send('Missing session ID');
return;
}
if (!transports[sessionId]) {
res.status(404).send('Session not found');
return;
}

Expand DownExpand Up@@ -682,8 +689,12 @@ 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) {
res.status(400).send('Missing session ID');
return;
}
if (!transports[sessionId]) {
res.status(404).send('Session not found');
return;
}

Expand Down
13 changes: 8 additions & 5 deletions examples/server/src/jsonResponseStreamableHttp.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -110,14 +110,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
await server.connect(transport);
await transport.handleRequest(req, res, req.body);
return; // Already handled
} else if (sessionId) {
res.status(404).json({
jsonrpc: '2.0',
error: { code: -32_001, message: 'Session not found' },
id: null
});
return;
} else {
// Invalid request - no session ID or not initialization request
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Bad Request: No valid session ID provided'
},
error: { code: -32_000, message: 'Bad Request: Session ID required' },
id: null
});
return;
Expand Down
29 changes: 20 additions & 9 deletions examples/server/src/simpleStreamableHttp.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -689,14 +689,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

await transport.handleRequest(req, res, req.body);
return; // Already handled
} else if (sessionId) {
res.status(404).json({
jsonrpc: '2.0',
error: { code: -32_001, message: 'Session not found' },
id: null
});
return;
} else {
// Invalid request - no session ID or not initialization request
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Bad Request: No valid session ID provided'
},
error: { code: -32_000, message: 'Bad Request: Session ID required' },
id: null
});
return;
Expand DownExpand Up@@ -730,8 +733,12 @@ 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) {
res.status(400).send('Missing session ID');
return;
}
if (!transports[sessionId]) {
res.status(404).send('Session not found');
return;
}

Expand DownExpand Up@@ -761,8 +768,12 @@ 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) {
res.status(400).send('Missing session ID');
return;
}
if (!transports[sessionId]) {
res.status(404).send('Session not found');
return;
}

Expand Down
25 changes: 20 additions & 5 deletions examples/server/src/simpleTaskInteractive.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -670,10 +670,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
await server.connect(transport);
await transport.handleRequest(req, res, req.body);
return;
} else if (sessionId) {
res.status(404).json({
jsonrpc: '2.0',
error: { code: -32_001, message: 'Session not found' },
id: null
});
return;
} else {
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 +702,12 @@ 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) {
res.status(400).send('Missing session ID');
return;
}
if (!transports[sessionId]) {
res.status(404).send('Session not found');
return;
}

Expand All@@ -707,8 +718,12 @@ 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) {
res.status(400).send('Missing session ID');
return;
}
if (!transports[sessionId]) {
res.status(404).send('Session not found');
return;
}

Expand Down
21 changes: 14 additions & 7 deletions examples/server/src/standaloneSseWithGetStreamableHttp.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -86,14 +86,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
// Handle the request - the onsessioninitialized callback will store the transport
await transport.handleRequest(req, res, req.body);
return; // Already handled
} else if (sessionId) {
res.status(404).json({
jsonrpc: '2.0',
error: { code: -32_001, message: 'Session not found' },
id: null
});
return;
} else {
// Invalid request - no session ID or not initialization request
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Bad Request: No valid session ID provided'
},
error: { code: -32_000, message: 'Bad Request: Session ID required' },
id: null
});
return;
Expand All@@ -119,8 +122,12 @@ 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) {
res.status(400).send('Missing session ID');
return;
}
if (!transports[sessionId]) {
res.status(404).send('Session not found');
return;
}

Expand Down
28 changes: 20 additions & 8 deletions test/conformance/src/authTestServer.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -327,13 +327,17 @@ async function startServer() {
await mcpServer.connect(transport);
await transport.handleRequest(req, res, req.body);
return;
} else if (sessionId) {
res.status(404).json({
jsonrpc: '2.0',
error: { code: -32_001, message: 'Session not found' },
id: null
});
return;
} else {
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Invalid or missing session ID'
},
error: { code: -32_000, message: 'Bad Request: Session ID required' },
id: null
});
return;
Expand All@@ -359,8 +363,12 @@ async function startServer() {
app.get('/mcp', bearerAuth, 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) {
res.status(400).send('Missing session ID');
return;
}
if (!transports[sessionId]) {
res.status(404).send('Session not found');
return;
}

Expand All@@ -381,8 +389,12 @@ async function startServer() {
app.delete('/mcp', bearerAuth, 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) {
res.status(400).send('Missing session ID');
return;
}
if (!transports[sessionId]) {
res.status(404).send('Session not found');
return;
}

Expand Down
28 changes: 20 additions & 8 deletions test/conformance/src/everythingServer.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -929,13 +929,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
await mcpServer.connect(transport);
await transport.handleRequest(req, res, req.body);
return;
} else if (sessionId) {
res.status(404).json({
jsonrpc: '2.0',
error: { code: -32_001, message: 'Session not found' },
id: null
});
return;
} else {
res.status(400).json({
jsonrpc: '2.0',
error: {
code: -32_000,
message: 'Invalid or missing session ID'
},
error: { code: -32_000, message: 'Bad Request: Session ID required' },
id: null
});
return;
Expand All@@ -961,8 +965,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
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) {
res.status(400).send('Missing session ID');
return;
}
if (!transports[sessionId]) {
res.status(404).send('Session not found');
return;
}

Expand All@@ -988,8 +996,12 @@ app.get('/mcp', async (req: Request, res: Response) => {
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) {
res.status(400).send('Missing session ID');
return;
}
if (!transports[sessionId]) {
res.status(404).send('Session not found');
return;
}

Expand Down
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fix(examples): return 404 for unknown session IDs, 400 for missing#1770

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 Merged

felixweinberger
merged 4 commits into

from

Mar 30, 2026
 Merged


fix(examples): return 404 for unknown session IDs, 400 for missing
#1770
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15e5b36

fix(examples): return 404 for unknown session IDs, 400 for missing


felixweinberger Mar 26, 2026
4ab1f6f

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
fc1f880

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
c61edba

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-session-status-codes.md

View file


Open in desktop

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

Example servers now return HTTP 404 (not 400) when a request includes an unknown session ID, so clients can correctly detect they need to start a new session. Requests missing a session ID entirely still return 400.

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationFormExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -364,14 +364,17 @@ async function main() {

 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -399,8 +402,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +421,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationUrlExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -606,14 +606,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -643,8 +646,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +689,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

13 changes: 8 additions & 5 deletions
examples/server/src/jsonResponseStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -110,14 +110,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/simpleStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -689,14 +689,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -730,8 +733,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -761,8 +768,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

25 changes: 20 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -670,10 +670,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 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 +702,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +718,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

21 changes: 14 additions & 7 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -86,14 +86,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 // Handle the request - the onsessioninitialized callback will store the transport
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -119,8 +122,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

28 changes: 20 additions & 8 deletions
test/conformance/src/authTestServer.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -327,13 +327,17 @@ async function startServer() {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -359,8 +363,12 @@ async function startServer() {
 app.get('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -381,8 +389,12 @@ async function startServer() {
 app.delete('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

28 changes: 20 additions & 8 deletions
test/conformance/src/everythingServer.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -929,13 +929,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -961,8 +965,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -988,8 +996,12 @@ app.get('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 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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var __re = new RegExp('^' + ".*" + '

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fix(examples): return 404 for unknown session IDs, 400 for missing#1770

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 Merged

felixweinberger
merged 4 commits into

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Mar 30, 2026
 Merged


fix(examples): return 404 for unknown session IDs, 400 for missing
#1770
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15e5b36

fix(examples): return 404 for unknown session IDs, 400 for missing


felixweinberger Mar 26, 2026
4ab1f6f

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
fc1f880

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
c61edba

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-session-status-codes.md

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

Example servers now return HTTP 404 (not 400) when a request includes an unknown session ID, so clients can correctly detect they need to start a new session. Requests missing a session ID entirely still return 400.

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationFormExample.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -364,14 +364,17 @@ async function main() {

 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -399,8 +402,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +421,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationUrlExample.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -606,14 +606,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -643,8 +646,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +689,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

13 changes: 8 additions & 5 deletions
examples/server/src/jsonResponseStreamableHttp.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -110,14 +110,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/simpleStreamableHttp.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -689,14 +689,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -730,8 +733,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -761,8 +768,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

25 changes: 20 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -670,10 +670,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 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 +702,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +718,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

21 changes: 14 additions & 7 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -86,14 +86,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 // Handle the request - the onsessioninitialized callback will store the transport
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -119,8 +122,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

28 changes: 20 additions & 8 deletions
test/conformance/src/authTestServer.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -327,13 +327,17 @@ async function startServer() {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -359,8 +363,12 @@ async function startServer() {
 app.get('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -381,8 +389,12 @@ async function startServer() {
 app.delete('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

28 changes: 20 additions & 8 deletions
test/conformance/src/everythingServer.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -929,13 +929,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -961,8 +965,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -988,8 +996,12 @@ app.get('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down
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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Highlight search terms from Google/DuckDuckGo/Bing referrer
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var ref = document.referrer;
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var q = url.searchParams.get('q') || url.searchParams.get('p');
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node.parentNode.replaceChild(frag, node);
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observer.observe(document.body, { childList: true, subtree: true });
})();
}
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fix(examples): return 404 for unknown session IDs, 400 for missing#1770

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felixweinberger
merged 4 commits into

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Mar 30, 2026
 Merged


fix(examples): return 404 for unknown session IDs, 400 for missing
#1770
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15e5b36

fix(examples): return 404 for unknown session IDs, 400 for missing


felixweinberger Mar 26, 2026
4ab1f6f

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
fc1f880

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
c61edba

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-session-status-codes.md

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

Example servers now return HTTP 404 (not 400) when a request includes an unknown session ID, so clients can correctly detect they need to start a new session. Requests missing a session ID entirely still return 400.

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationFormExample.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -364,14 +364,17 @@ async function main() {

 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -399,8 +402,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +421,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationUrlExample.ts

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -606,14 +606,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -643,8 +646,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +689,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

13 changes: 8 additions & 5 deletions
examples/server/src/jsonResponseStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -110,14 +110,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/simpleStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -689,14 +689,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -730,8 +733,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -761,8 +768,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

25 changes: 20 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -670,10 +670,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 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 +702,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +718,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

21 changes: 14 additions & 7 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -86,14 +86,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 // Handle the request - the onsessioninitialized callback will store the transport
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -119,8 +122,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

28 changes: 20 additions & 8 deletions
test/conformance/src/authTestServer.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -327,13 +327,17 @@ async function startServer() {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -359,8 +363,12 @@ async function startServer() {
 app.get('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -381,8 +389,12 @@ async function startServer() {
 app.delete('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

28 changes: 20 additions & 8 deletions
test/conformance/src/everythingServer.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -929,13 +929,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -961,8 +965,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -988,8 +996,12 @@ app.get('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down
Loading

, '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" + '

Skip to content
fix(examples): return 404 for unknown session IDs, 400 for missing#1770

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 Merged

felixweinberger
merged 4 commits into

from

Mar 30, 2026
 Merged


fix(examples): return 404 for unknown session IDs, 400 for missing
#1770
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4 commits


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15e5b36

fix(examples): return 404 for unknown session IDs, 400 for missing


felixweinberger Mar 26, 2026
4ab1f6f

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
fc1f880

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
c61edba

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-session-status-codes.md

View file


Open in desktop

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

Example servers now return HTTP 404 (not 400) when a request includes an unknown session ID, so clients can correctly detect they need to start a new session. Requests missing a session ID entirely still return 400.

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationFormExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -364,14 +364,17 @@ async function main() {

 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -399,8 +402,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +421,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationUrlExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -606,14 +606,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -643,8 +646,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +689,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

13 changes: 8 additions & 5 deletions
examples/server/src/jsonResponseStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -110,14 +110,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/simpleStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -689,14 +689,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -730,8 +733,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -761,8 +768,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

25 changes: 20 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -670,10 +670,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 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 +702,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +718,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

21 changes: 14 additions & 7 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -86,14 +86,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 // Handle the request - the onsessioninitialized callback will store the transport
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -119,8 +122,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

28 changes: 20 additions & 8 deletions
test/conformance/src/authTestServer.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -327,13 +327,17 @@ async function startServer() {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -359,8 +363,12 @@ async function startServer() {
 app.get('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -381,8 +389,12 @@ async function startServer() {
 app.delete('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

28 changes: 20 additions & 8 deletions
test/conformance/src/everythingServer.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -929,13 +929,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -961,8 +965,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -988,8 +996,12 @@ app.get('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down
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, '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 unknown session IDs, 400 for missing#1770

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 Merged

felixweinberger
merged 4 commits into

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Mar 30, 2026
 Merged


fix(examples): return 404 for unknown session IDs, 400 for missing
#1770
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15e5b36

fix(examples): return 404 for unknown session IDs, 400 for missing


felixweinberger Mar 26, 2026
4ab1f6f

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
fc1f880

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
c61edba

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-session-status-codes.md

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

Example servers now return HTTP 404 (not 400) when a request includes an unknown session ID, so clients can correctly detect they need to start a new session. Requests missing a session ID entirely still return 400.

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationFormExample.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -364,14 +364,17 @@ async function main() {

 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -399,8 +402,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +421,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationUrlExample.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -606,14 +606,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -643,8 +646,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +689,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

13 changes: 8 additions & 5 deletions
examples/server/src/jsonResponseStreamableHttp.ts

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -110,14 +110,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/simpleStreamableHttp.ts

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -689,14 +689,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -730,8 +733,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -761,8 +768,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

25 changes: 20 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -670,10 +670,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 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 +702,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +718,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

21 changes: 14 additions & 7 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -86,14 +86,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 // Handle the request - the onsessioninitialized callback will store the transport
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -119,8 +122,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

28 changes: 20 additions & 8 deletions
test/conformance/src/authTestServer.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -327,13 +327,17 @@ async function startServer() {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -359,8 +363,12 @@ async function startServer() {
 app.get('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -381,8 +389,12 @@ async function startServer() {
 app.delete('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

28 changes: 20 additions & 8 deletions
test/conformance/src/everythingServer.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -929,13 +929,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -961,8 +965,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -988,8 +996,12 @@ app.get('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down
Loading

, '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('^' + ".*" + '

Skip to content
fix(examples): return 404 for unknown session IDs, 400 for missing#1770

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

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By clicking “Sign up for GitHub”, you agree to our terms of service and
privacy statement. We’ll occasionally send you account related emails.

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Sign in
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 Merged

felixweinberger
merged 4 commits into

from

Mar 30, 2026
 Merged


fix(examples): return 404 for unknown session IDs, 400 for missing
#1770
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Commits

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4 commits


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15e5b36

fix(examples): return 404 for unknown session IDs, 400 for missing


felixweinberger Mar 26, 2026
4ab1f6f

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
fc1f880

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
c61edba

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-session-status-codes.md

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

Example servers now return HTTP 404 (not 400) when a request includes an unknown session ID, so clients can correctly detect they need to start a new session. Requests missing a session ID entirely still return 400.

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationFormExample.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -364,14 +364,17 @@ async function main() {

 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -399,8 +402,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +421,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationUrlExample.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -606,14 +606,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -643,8 +646,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +689,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

13 changes: 8 additions & 5 deletions
examples/server/src/jsonResponseStreamableHttp.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -110,14 +110,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/simpleStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -689,14 +689,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -730,8 +733,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -761,8 +768,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

25 changes: 20 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -670,10 +670,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 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 +702,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +718,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

21 changes: 14 additions & 7 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -86,14 +86,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 // Handle the request - the onsessioninitialized callback will store the transport
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -119,8 +122,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

28 changes: 20 additions & 8 deletions
test/conformance/src/authTestServer.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -327,13 +327,17 @@ async function startServer() {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -359,8 +363,12 @@ async function startServer() {
 app.get('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -381,8 +389,12 @@ async function startServer() {
 app.delete('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

28 changes: 20 additions & 8 deletions
test/conformance/src/everythingServer.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -929,13 +929,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -961,8 +965,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -988,8 +996,12 @@ app.get('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down
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background: #1a1a2e !important;
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/* Restore images, videos, iframes, canvas */
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fix(examples): return 404 for unknown session IDs, 400 for missing#1770

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 Merged

felixweinberger
merged 4 commits into

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Mar 30, 2026
 Merged


fix(examples): return 404 for unknown session IDs, 400 for missing
#1770
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15e5b36

fix(examples): return 404 for unknown session IDs, 400 for missing


felixweinberger Mar 26, 2026
4ab1f6f

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
fc1f880

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026
c61edba

Merge branch 'main' into fweinberger/session-404-fix


felixweinberger Mar 30, 2026

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5 changes: 5 additions & 0 deletions
.changeset/fix-session-status-codes.md

View file


Open in desktop

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

Example servers now return HTTP 404 (not 400) when a request includes an unknown session ID, so clients can correctly detect they need to start a new session. Requests missing a session ID entirely still return 400.

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationFormExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -364,14 +364,17 @@ async function main() {

 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -399,8 +402,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -414,8 +421,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/elicitationUrlExample.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -606,14 +606,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -643,8 +646,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -682,8 +689,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

13 changes: 8 additions & 5 deletions
examples/server/src/jsonResponseStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -110,14 +110,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand Down

29 changes: 20 additions & 9 deletions
examples/server/src/simpleStreamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -689,14 +689,17 @@ const mcpPostHandler = async (req: Request, res: Response) => {

 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand DownExpand Up@@ -730,8 +733,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand DownExpand Up@@ -761,8 +768,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

25 changes: 20 additions & 5 deletions
examples/server/src/simpleTaskInteractive.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -670,10 +670,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await server.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 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 +702,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -707,8 +718,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

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21 changes: 14 additions & 7 deletions
examples/server/src/standaloneSseWithGetStreamableHttp.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -86,14 +86,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 // Handle the request - the onsessioninitialized callback will store the transport
 await transport.handleRequest(req, res, req.body);
 return; // Already handled
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 // Invalid request - no session ID or not initialization request
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Bad Request: No valid session ID provided'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -119,8 +122,12 @@ 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

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28 changes: 20 additions & 8 deletions
test/conformance/src/authTestServer.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -327,13 +327,17 @@ async function startServer() {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -359,8 +363,12 @@ async function startServer() {
 app.get('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -381,8 +389,12 @@ async function startServer() {
 app.delete('/mcp', bearerAuth, 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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down

28 changes: 20 additions & 8 deletions
test/conformance/src/everythingServer.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -929,13 +929,17 @@ app.post('/mcp', async (req: Request, res: Response) => {
 await mcpServer.connect(transport);
 await transport.handleRequest(req, res, req.body);
 return;
 } else if (sessionId) {
 res.status(404).json({
 jsonrpc: '2.0',
 error: { code: -32_001, message: 'Session not found' },
 id: null
 });
 return;
 } else {
 res.status(400).json({
 jsonrpc: '2.0',
 error: {
 code: -32_000,
 message: 'Invalid or missing session ID'
 },
 error: { code: -32_000, message: 'Bad Request: Session ID required' },
 id: null
 });
 return;
Expand All@@ -961,8 +965,12 @@ app.post('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand All@@ -988,8 +996,12 @@ app.get('/mcp', async (req: Request, res: Response) => {
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) {
 res.status(400).send('Missing session ID');
 return;
 }
 if (!transports[sessionId]) {
 res.status(404).send('Session not found');
 return;
 }

Expand Down
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