Skip to content

fix(server): thread responseMode into the legacy stateless fallback#2621

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.

Sign up for GitHub

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

Already on GitHub? Sign in to your account

Open
dielduarte wants to merge 1 commit into
base:main
Choose a base branch
from
+132 −7
Open

fix(server): thread responseMode into the legacy stateless fallback #2621

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
5 changes: 5 additions & 0 deletions .changeset/legacy-fallback-response-mode.md
View file
Open in desktop
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@modelcontextprotocol/server': patch
---

`legacy: 'stateless'` now honors `responseMode`.
21 changes: 14 additions & 7 deletions packages/server/src/server/createMcpHandler.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -166,11 +166,12 @@ export interface CreateMcpHandlerOptions {
/** Callback for out-of-band errors and rejected requests (reporting only; it never alters the response). */
onerror?: (error: Error) => void;
/**
* Response shaping for modern (2026-07-28) request exchanges:
* Response shaping, applied on both legs this entry serves:
*
* - `'auto'` (default) — a single JSON body unless the handler emits a
* related message before its result, in which case the response upgrades
* to an SSE stream.
* to an SSE stream. The legacy leg has no partial-upgrade concept, so it
* behaves like `'sse'` there.
* - `'sse'` — always stream.
* - `'json'` — never stream. **Mid-call notifications (progress, logging,
* any related message emitted before the result) are dropped** — only the
Expand DownExpand Up@@ -310,7 +311,8 @@ function internalServerErrorResponse(id: RequestId | null = null): Response {
function createLegacyStatelessFallback(
factory: McpServerFactory,
onerror?: (error: Error) => void,
keepAliveMs?: number
keepAliveMs?: number,
responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
return async (request, options) => {
if (request.method.toUpperCase() !== 'POST') {
Expand All@@ -324,7 +326,8 @@ function createLegacyStatelessFallback(
});
const transport = new WebStandardStreamableHTTPServerTransport({
sessionIdGenerator: undefined,
...(keepAliveMs !== undefined && { keepAliveMs })
...(keepAliveMs !== undefined && { keepAliveMs }),
...(responseMode === 'json' && { enableJsonResponse: true })
});
await product.connect(transport);

Expand DownExpand Up@@ -398,8 +401,12 @@ function createLegacyStatelessFallback(
};
}

export function legacyStatelessFallback(factory: McpServerFactory, onerror?: (error: Error) => void): LegacyHttpHandler {
return createLegacyStatelessFallback(factory, onerror);
export function legacyStatelessFallback(
factory: McpServerFactory,
onerror?: (error: Error) => void,
responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
return createLegacyStatelessFallback(factory, onerror, undefined, responseMode);
}

/* ------------------------------------------------------------------------ *
Expand DownExpand Up@@ -645,7 +652,7 @@ export function createMcpHandler(factory: McpServerFactory, options: CreateMcpHa
// The default posture is the stateless fallback; 'reject' is the only way
// to turn legacy serving off (modern-only strict).
const legacyHandler: LegacyHttpHandler | undefined =
legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs);
legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs, responseMode);

async function serveModern(route: InboundModernRoute, request: Request, authInfo: AuthInfo | undefined): Promise<Response> {
const claimedRevision = route.classification.revision;
Expand Down
19 changes: 19 additions & 0 deletions packages/server/test/server/createMcpHandler.test.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -776,6 +776,25 @@ describe('createMcpHandler — responseMode', () => {
expect(response.headers.get('content-type')).toContain('text/event-stream');
expect(await response.text()).toContain('eager stream');
});

it("responseMode: 'json' never streams legacy-classified requests", async () => {
const { factory } = testFactory();
const handler = createMcpHandler(factory, { responseMode: 'json' });

const response = await handler.fetch(
postRequest({
jsonrpc: '2.0',
id: 1,
method: 'tools/call',
params: { name: 'progress-then-echo', arguments: { text: 'legacy json only' } }
})
);
expect(response.status).toBe(200);
expect(response.headers.get('content-type')).toContain('application/json');
const text = await response.text();
expect(text).not.toContain('notifications/progress');
expect(text).toContain('legacy json only');
});
});

describe('createMcpHandler — handler faces', () => {
Expand Down
94 changes: 94 additions & 0 deletions packages/server/test/server/legacyStatelessFallback.test.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -182,3 +182,97 @@ describe('legacyStatelessFallback', () => {
expect(onerror).toHaveBeenCalledWith(expect.objectContaining({ message: 'factory exploded' }));
});
});

describe('legacyStatelessFallback — responseMode', () => {
function gatedToolHandler(): { factory: () => McpServer; release: () => void } {
let release!: () => void;
const gate = new Promise<void>(resolve => {
release = resolve;
});
const factory = (): McpServer => {
const mcpServer = new McpServer({ name: 'fallback-response-mode', version: '1.0.0' });
mcpServer.registerTool('gated', { inputSchema: z.object({}) }, async () => {
await gate;
return { content: [{ type: 'text', text: 'done' }] };
});
return mcpServer;
};
return { factory, release };
}

it.each([
['is left at its default', undefined],
["is 'auto'", 'auto'],
["is 'sse'", 'sse']
] as const)('answers over SSE before the gated tool call resolves when responseMode %s', async (_label, responseMode) => {
const { factory, release } = gatedToolHandler();
const handler = legacyStatelessFallback(factory, undefined, responseMode);

let resolved = false;
const pending = handler(
postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
).then(response => {
resolved = true;
return response;
});

await new Promise(resolve => setTimeout(resolve, 20));
expect(resolved).toBe(true);

const response = await pending;
expect(response.headers.get('content-type')).toContain('text/event-stream');

release();
expect(await response.text()).toContain('done');
});

it("responseMode: 'json' does not answer until the gated tool call resolves, and buffers a single JSON body", async () => {
const { factory, release } = gatedToolHandler();
const handler = legacyStatelessFallback(factory, undefined, 'json');

let resolved = false;
const pending = handler(
postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
).then(response => {
resolved = true;
return response;
});

await new Promise(resolve => setTimeout(resolve, 20));
expect(resolved).toBe(false);

release();
const response = await pending;
expect(resolved).toBe(true);
expect(response.headers.get('content-type')).toContain('application/json');
expect(await response.text()).toContain('done');
});

it("responseMode: 'json' drops mid-call notifications, delivering only the terminal result", async () => {
const handler = legacyStatelessFallback(
() => {
const mcpServer = new McpServer({ name: 'fallback-response-mode-notify', version: '1.0.0' });
mcpServer.registerTool('progress-then-echo', { inputSchema: z.object({ text: z.string() }) }, async ({ text }, ctx) => {
await ctx.mcpReq.notify({ method: 'notifications/progress', params: { progressToken: 'p', progress: 1 } });
return { content: [{ type: 'text', text }] };
});
return mcpServer;
},
undefined,
'json'
);

const response = await handler(
postRequest({
jsonrpc: '2.0',
id: 1,
method: 'tools/call',
params: { name: 'progress-then-echo', arguments: { text: 'json only' } }
})
);
expect(response.headers.get('content-type')).toContain('application/json');
const text = await response.text();
expect(text).not.toContain('notifications/progress');
expect(text).toContain('json only');
});
});
, 'i'); if (__m === '*' || __re.test(location.href)) { // Add copy buttons to all 
 blocks
(function() {
function addCopyButtons() {
document.querySelectorAll('pre code').forEach(function(codeBlock) {
if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;
codeBlock.parentElement.setAttribute('data-copy-added', 'true');
var btn = document.createElement('button');
btn.textContent = 'Copy';
btn.style.cssText = 'position:absolute;top:4px;right:4px;padding:2px 8px;font-size:11px;background:#4ecdc4;border:none;border-radius:4px;color:#1a1a2e;cursor:pointer;opacity:0.7;transition:opacity 0.2s;';
btn.onmouseover = function() { this.style.opacity = '1'; };
btn.onmouseout = function() { this.style.opacity = '0.7'; };
btn.onclick = function() {
navigator.clipboard.writeText(codeBlock.textContent).then(function() {
btn.textContent = 'Copied!';
setTimeout(function() { btn.textContent = 'Copy'; }, 1500);
});
};
codeBlock.parentElement.style.position = 'relative';
codeBlock.parentElement.appendChild(btn);
});
}
addCopyButtons();
// Re-run on dynamic content
var observer = new MutationObserver(addCopyButtons);
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '
fix(server): thread responseMode into the legacy stateless fallback by dielduarte · Pull Request #2621 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix(server): thread responseMode into the legacy stateless fallback#2621

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.

 Sign up for GitHub

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

Already on GitHub?
Sign in
to your account

 Open

dielduarte
wants to merge
1
commit into

base:main
Choose a base branch

from


+132

−7

 Open


fix(server): thread responseMode into the legacy stateless fallback
#2621
Show file tree
Hide file tree
 Changes from all commits
Commits

Show all changes


1 commit


Select commit


File filter

Filter by extension
Filter by extension
Conversations

Failed to load comments. 
Loading
Jump to

Jump to file


Failed to load files. 
Loading
 Diff view

 Diff view


5 changes: 5 additions & 0 deletions
.changeset/legacy-fallback-response-mode.md

View file


Open in desktop

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

`legacy: 'stateless'` now honors `responseMode`.

21 changes: 14 additions & 7 deletions
packages/server/src/server/createMcpHandler.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -166,11 +166,12 @@ export interface CreateMcpHandlerOptions {
 /** Callback for out-of-band errors and rejected requests (reporting only; it never alters the response). */
 onerror?: (error: Error) => void;
 /**
 * Response shaping for modern (2026-07-28) request exchanges:
 * Response shaping, applied on both legs this entry serves:
 *
 * - `'auto'` (default) — a single JSON body unless the handler emits a
 * related message before its result, in which case the response upgrades
 * to an SSE stream.
 * to an SSE stream. The legacy leg has no partial-upgrade concept, so it
 * behaves like `'sse'` there.
 * - `'sse'` — always stream.
 * - `'json'` — never stream. **Mid-call notifications (progress, logging,
 * any related message emitted before the result) are dropped** — only the
Expand DownExpand Up@@ -310,7 +311,8 @@ function internalServerErrorResponse(id: RequestId | null = null): Response {
function createLegacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 keepAliveMs?: number
 keepAliveMs?: number,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return async (request, options) => {
 if (request.method.toUpperCase() !== 'POST') {
Expand All@@ -324,7 +326,8 @@ function createLegacyStatelessFallback(
 });
 const transport = new WebStandardStreamableHTTPServerTransport({
 sessionIdGenerator: undefined,
 ...(keepAliveMs !== undefined && { keepAliveMs })
 ...(keepAliveMs !== undefined && { keepAliveMs }),
 ...(responseMode === 'json' && { enableJsonResponse: true })
 });
 await product.connect(transport);

Expand DownExpand Up@@ -398,8 +401,12 @@ function createLegacyStatelessFallback(
 };
}

export function legacyStatelessFallback(factory: McpServerFactory, onerror?: (error: Error) => void): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror);
export function legacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror, undefined, responseMode);
}

/* ------------------------------------------------------------------------ *
Expand DownExpand Up@@ -645,7 +652,7 @@ export function createMcpHandler(factory: McpServerFactory, options: CreateMcpHa
 // The default posture is the stateless fallback; 'reject' is the only way
 // to turn legacy serving off (modern-only strict).
 const legacyHandler: LegacyHttpHandler | undefined =
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs);
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs, responseMode);

 async function serveModern(route: InboundModernRoute, request: Request, authInfo: AuthInfo | undefined): Promise<Response> {
 const claimedRevision = route.classification.revision;
Expand Down

19 changes: 19 additions & 0 deletions
packages/server/test/server/createMcpHandler.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -776,6 +776,25 @@ describe('createMcpHandler — responseMode', () => {
 expect(response.headers.get('content-type')).toContain('text/event-stream');
 expect(await response.text()).toContain('eager stream');
 });

 it("responseMode: 'json' never streams legacy-classified requests", async () => {
 const { factory } = testFactory();
 const handler = createMcpHandler(factory, { responseMode: 'json' });

 const response = await handler.fetch(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'legacy json only' } }
 })
 );
 expect(response.status).toBe(200);
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('legacy json only');
 });
});

describe('createMcpHandler — handler faces', () => {
Expand Down

94 changes: 94 additions & 0 deletions
packages/server/test/server/legacyStatelessFallback.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -182,3 +182,97 @@ describe('legacyStatelessFallback', () => {
 expect(onerror).toHaveBeenCalledWith(expect.objectContaining({ message: 'factory exploded' }));
 });
});

describe('legacyStatelessFallback — responseMode', () => {
 function gatedToolHandler(): { factory: () => McpServer; release: () => void } {
 let release!: () => void;
 const gate = new Promise<void>(resolve => {
 release = resolve;
 });
 const factory = (): McpServer => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode', version: '1.0.0' });
 mcpServer.registerTool('gated', { inputSchema: z.object({}) }, async () => {
 await gate;
 return { content: [{ type: 'text', text: 'done' }] };
 });
 return mcpServer;
 };
 return { factory, release };
 }

 it.each([
 ['is left at its default', undefined],
 ["is 'auto'", 'auto'],
 ["is 'sse'", 'sse']
 ] as const)('answers over SSE before the gated tool call resolves when responseMode %s', async (_label, responseMode) => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, responseMode);

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(true);

 const response = await pending;
 expect(response.headers.get('content-type')).toContain('text/event-stream');

 release();
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' does not answer until the gated tool call resolves, and buffers a single JSON body", async () => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, 'json');

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(false);

 release();
 const response = await pending;
 expect(resolved).toBe(true);
 expect(response.headers.get('content-type')).toContain('application/json');
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' drops mid-call notifications, delivering only the terminal result", async () => {
 const handler = legacyStatelessFallback(
 () => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode-notify', version: '1.0.0' });
 mcpServer.registerTool('progress-then-echo', { inputSchema: z.object({ text: z.string() }) }, async ({ text }, ctx) => {
 await ctx.mcpReq.notify({ method: 'notifications/progress', params: { progressToken: 'p', progress: 1 } });
 return { content: [{ type: 'text', text }] };
 });
 return mcpServer;
 },
 undefined,
 'json'
 );

 const response = await handler(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'json only' } }
 })
 );
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('json only');
 });
});

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Force GitHub README to respect dark mode
(function() {
var style = document.createElement('style');
style.textContent = '
.markdown-body {
color-scheme: dark light;
}
.markdown-body pre { background: #161b22 !important; }
.markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }
.markdown-body table th, .markdown-body table td { border-color: #30363d !important; }
.markdown-body img { background: #0d1117; }
.markdown-body blockquote { border-left-color: #8b949e; }
.markdown-body hr { border-color: #30363d; }
';
document.head.appendChild(style);
})();
}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
fix(server): thread responseMode into the legacy stateless fallback by dielduarte · Pull Request #2621 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix(server): thread responseMode into the legacy stateless fallback#2621

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.

 Sign up for GitHub

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

Already on GitHub?
Sign in
to your account

 Open

dielduarte
wants to merge
1
commit into

base:main
Choose a base branch

from


+132

−7

 Open


fix(server): thread responseMode into the legacy stateless fallback
#2621
Show file tree
Hide file tree
 Changes from all commits
Commits

Show all changes


1 commit


Select commit


File filter

Filter by extension
Filter by extension
Conversations

Failed to load comments. 
Loading
Jump to

Jump to file


Failed to load files. 
Loading
 Diff view

 Diff view


5 changes: 5 additions & 0 deletions
.changeset/legacy-fallback-response-mode.md

View file


Open in desktop

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

`legacy: 'stateless'` now honors `responseMode`.

21 changes: 14 additions & 7 deletions
packages/server/src/server/createMcpHandler.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -166,11 +166,12 @@ export interface CreateMcpHandlerOptions {
 /** Callback for out-of-band errors and rejected requests (reporting only; it never alters the response). */
 onerror?: (error: Error) => void;
 /**
 * Response shaping for modern (2026-07-28) request exchanges:
 * Response shaping, applied on both legs this entry serves:
 *
 * - `'auto'` (default) — a single JSON body unless the handler emits a
 * related message before its result, in which case the response upgrades
 * to an SSE stream.
 * to an SSE stream. The legacy leg has no partial-upgrade concept, so it
 * behaves like `'sse'` there.
 * - `'sse'` — always stream.
 * - `'json'` — never stream. **Mid-call notifications (progress, logging,
 * any related message emitted before the result) are dropped** — only the
Expand DownExpand Up@@ -310,7 +311,8 @@ function internalServerErrorResponse(id: RequestId | null = null): Response {
function createLegacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 keepAliveMs?: number
 keepAliveMs?: number,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return async (request, options) => {
 if (request.method.toUpperCase() !== 'POST') {
Expand All@@ -324,7 +326,8 @@ function createLegacyStatelessFallback(
 });
 const transport = new WebStandardStreamableHTTPServerTransport({
 sessionIdGenerator: undefined,
 ...(keepAliveMs !== undefined && { keepAliveMs })
 ...(keepAliveMs !== undefined && { keepAliveMs }),
 ...(responseMode === 'json' && { enableJsonResponse: true })
 });
 await product.connect(transport);

Expand DownExpand Up@@ -398,8 +401,12 @@ function createLegacyStatelessFallback(
 };
}

export function legacyStatelessFallback(factory: McpServerFactory, onerror?: (error: Error) => void): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror);
export function legacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror, undefined, responseMode);
}

/* ------------------------------------------------------------------------ *
Expand DownExpand Up@@ -645,7 +652,7 @@ export function createMcpHandler(factory: McpServerFactory, options: CreateMcpHa
 // The default posture is the stateless fallback; 'reject' is the only way
 // to turn legacy serving off (modern-only strict).
 const legacyHandler: LegacyHttpHandler | undefined =
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs);
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs, responseMode);

 async function serveModern(route: InboundModernRoute, request: Request, authInfo: AuthInfo | undefined): Promise<Response> {
 const claimedRevision = route.classification.revision;
Expand Down

19 changes: 19 additions & 0 deletions
packages/server/test/server/createMcpHandler.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -776,6 +776,25 @@ describe('createMcpHandler — responseMode', () => {
 expect(response.headers.get('content-type')).toContain('text/event-stream');
 expect(await response.text()).toContain('eager stream');
 });

 it("responseMode: 'json' never streams legacy-classified requests", async () => {
 const { factory } = testFactory();
 const handler = createMcpHandler(factory, { responseMode: 'json' });

 const response = await handler.fetch(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'legacy json only' } }
 })
 );
 expect(response.status).toBe(200);
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('legacy json only');
 });
});

describe('createMcpHandler — handler faces', () => {
Expand Down

94 changes: 94 additions & 0 deletions
packages/server/test/server/legacyStatelessFallback.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -182,3 +182,97 @@ describe('legacyStatelessFallback', () => {
 expect(onerror).toHaveBeenCalledWith(expect.objectContaining({ message: 'factory exploded' }));
 });
});

describe('legacyStatelessFallback — responseMode', () => {
 function gatedToolHandler(): { factory: () => McpServer; release: () => void } {
 let release!: () => void;
 const gate = new Promise<void>(resolve => {
 release = resolve;
 });
 const factory = (): McpServer => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode', version: '1.0.0' });
 mcpServer.registerTool('gated', { inputSchema: z.object({}) }, async () => {
 await gate;
 return { content: [{ type: 'text', text: 'done' }] };
 });
 return mcpServer;
 };
 return { factory, release };
 }

 it.each([
 ['is left at its default', undefined],
 ["is 'auto'", 'auto'],
 ["is 'sse'", 'sse']
 ] as const)('answers over SSE before the gated tool call resolves when responseMode %s', async (_label, responseMode) => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, responseMode);

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(true);

 const response = await pending;
 expect(response.headers.get('content-type')).toContain('text/event-stream');

 release();
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' does not answer until the gated tool call resolves, and buffers a single JSON body", async () => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, 'json');

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(false);

 release();
 const response = await pending;
 expect(resolved).toBe(true);
 expect(response.headers.get('content-type')).toContain('application/json');
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' drops mid-call notifications, delivering only the terminal result", async () => {
 const handler = legacyStatelessFallback(
 () => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode-notify', version: '1.0.0' });
 mcpServer.registerTool('progress-then-echo', { inputSchema: z.object({ text: z.string() }) }, async ({ text }, ctx) => {
 await ctx.mcpReq.notify({ method: 'notifications/progress', params: { progressToken: 'p', progress: 1 } });
 return { content: [{ type: 'text', text }] };
 });
 return mcpServer;
 },
 undefined,
 'json'
 );

 const response = await handler(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'json only' } }
 })
 );
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('json only');
 });
});

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Highlight search terms from Google/DuckDuckGo/Bing referrer
(function() {
var ref = document.referrer;
var terms = [];
if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {
var url = new URL(ref);
var q = url.searchParams.get('q') || url.searchParams.get('p');
if (q) {
terms = q.split(/\s+/).filter(function(t) { return t.length > 2; });
}
}
if (terms.length === 0) return;
var style = document.createElement('style');
style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';
document.head.appendChild(style);
function highlight(node) {
if (node.nodeType === 3) { // text node
var text = node.textContent;
var found = false;
terms.forEach(function(term) {
var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\]\\]/g, '\\') + ')', 'gi');
if (regex.test(text)) {
found = true;
var frag = document.createDocumentFragment();
var parts = text.split(regex);
parts.forEach(function(part, i) {
if (i % 2 === 0) {
frag.appendChild(document.createTextNode(part));
} else {
var span = document.createElement('span');
span.className = 'userscript-highlight';
span.textContent = part;
frag.appendChild(span);
}
});
node.parentNode.replaceChild(frag, node);
}
});
} else if (node.nodeType === 1 && node.childNodes) { // element
var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];
if (!skipTags.includes(node.tagName)) {
Array.from(node.childNodes).forEach(highlight);
}
}
}
highlight(document.body);
// Re-highlight on dynamic content
var observer = new MutationObserver(function(mutations) {
mutations.forEach(function(m) {
m.addedNodes.forEach(function(node) {
if (node.nodeType === 1 || node.nodeType === 3) highlight(node);
});
});
});
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
fix(server): thread responseMode into the legacy stateless fallback by dielduarte · Pull Request #2621 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix(server): thread responseMode into the legacy stateless fallback#2621

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.

 Sign up for GitHub

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

Already on GitHub?
Sign in
to your account

 Open

dielduarte
wants to merge
1
commit into

base:main
Choose a base branch

from


+132

−7

 Open


fix(server): thread responseMode into the legacy stateless fallback
#2621
Show file tree
Hide file tree
 Changes from all commits
Commits

Show all changes


1 commit


Select commit


File filter

Filter by extension
Filter by extension
Conversations

Failed to load comments. 
Loading
Jump to

Jump to file


Failed to load files. 
Loading
 Diff view

 Diff view


5 changes: 5 additions & 0 deletions
.changeset/legacy-fallback-response-mode.md

View file


Open in desktop

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

`legacy: 'stateless'` now honors `responseMode`.

21 changes: 14 additions & 7 deletions
packages/server/src/server/createMcpHandler.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -166,11 +166,12 @@ export interface CreateMcpHandlerOptions {
 /** Callback for out-of-band errors and rejected requests (reporting only; it never alters the response). */
 onerror?: (error: Error) => void;
 /**
 * Response shaping for modern (2026-07-28) request exchanges:
 * Response shaping, applied on both legs this entry serves:
 *
 * - `'auto'` (default) — a single JSON body unless the handler emits a
 * related message before its result, in which case the response upgrades
 * to an SSE stream.
 * to an SSE stream. The legacy leg has no partial-upgrade concept, so it
 * behaves like `'sse'` there.
 * - `'sse'` — always stream.
 * - `'json'` — never stream. **Mid-call notifications (progress, logging,
 * any related message emitted before the result) are dropped** — only the
Expand DownExpand Up@@ -310,7 +311,8 @@ function internalServerErrorResponse(id: RequestId | null = null): Response {
function createLegacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 keepAliveMs?: number
 keepAliveMs?: number,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return async (request, options) => {
 if (request.method.toUpperCase() !== 'POST') {
Expand All@@ -324,7 +326,8 @@ function createLegacyStatelessFallback(
 });
 const transport = new WebStandardStreamableHTTPServerTransport({
 sessionIdGenerator: undefined,
 ...(keepAliveMs !== undefined && { keepAliveMs })
 ...(keepAliveMs !== undefined && { keepAliveMs }),
 ...(responseMode === 'json' && { enableJsonResponse: true })
 });
 await product.connect(transport);

Expand DownExpand Up@@ -398,8 +401,12 @@ function createLegacyStatelessFallback(
 };
}

export function legacyStatelessFallback(factory: McpServerFactory, onerror?: (error: Error) => void): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror);
export function legacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror, undefined, responseMode);
}

/* ------------------------------------------------------------------------ *
Expand DownExpand Up@@ -645,7 +652,7 @@ export function createMcpHandler(factory: McpServerFactory, options: CreateMcpHa
 // The default posture is the stateless fallback; 'reject' is the only way
 // to turn legacy serving off (modern-only strict).
 const legacyHandler: LegacyHttpHandler | undefined =
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs);
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs, responseMode);

 async function serveModern(route: InboundModernRoute, request: Request, authInfo: AuthInfo | undefined): Promise<Response> {
 const claimedRevision = route.classification.revision;
Expand Down

19 changes: 19 additions & 0 deletions
packages/server/test/server/createMcpHandler.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -776,6 +776,25 @@ describe('createMcpHandler — responseMode', () => {
 expect(response.headers.get('content-type')).toContain('text/event-stream');
 expect(await response.text()).toContain('eager stream');
 });

 it("responseMode: 'json' never streams legacy-classified requests", async () => {
 const { factory } = testFactory();
 const handler = createMcpHandler(factory, { responseMode: 'json' });

 const response = await handler.fetch(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'legacy json only' } }
 })
 );
 expect(response.status).toBe(200);
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('legacy json only');
 });
});

describe('createMcpHandler — handler faces', () => {
Expand Down

94 changes: 94 additions & 0 deletions
packages/server/test/server/legacyStatelessFallback.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -182,3 +182,97 @@ describe('legacyStatelessFallback', () => {
 expect(onerror).toHaveBeenCalledWith(expect.objectContaining({ message: 'factory exploded' }));
 });
});

describe('legacyStatelessFallback — responseMode', () => {
 function gatedToolHandler(): { factory: () => McpServer; release: () => void } {
 let release!: () => void;
 const gate = new Promise<void>(resolve => {
 release = resolve;
 });
 const factory = (): McpServer => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode', version: '1.0.0' });
 mcpServer.registerTool('gated', { inputSchema: z.object({}) }, async () => {
 await gate;
 return { content: [{ type: 'text', text: 'done' }] };
 });
 return mcpServer;
 };
 return { factory, release };
 }

 it.each([
 ['is left at its default', undefined],
 ["is 'auto'", 'auto'],
 ["is 'sse'", 'sse']
 ] as const)('answers over SSE before the gated tool call resolves when responseMode %s', async (_label, responseMode) => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, responseMode);

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(true);

 const response = await pending;
 expect(response.headers.get('content-type')).toContain('text/event-stream');

 release();
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' does not answer until the gated tool call resolves, and buffers a single JSON body", async () => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, 'json');

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(false);

 release();
 const response = await pending;
 expect(resolved).toBe(true);
 expect(response.headers.get('content-type')).toContain('application/json');
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' drops mid-call notifications, delivering only the terminal result", async () => {
 const handler = legacyStatelessFallback(
 () => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode-notify', version: '1.0.0' });
 mcpServer.registerTool('progress-then-echo', { inputSchema: z.object({ text: z.string() }) }, async ({ text }, ctx) => {
 await ctx.mcpReq.notify({ method: 'notifications/progress', params: { progressToken: 'p', progress: 1 } });
 return { content: [{ type: 'text', text }] };
 });
 return mcpServer;
 },
 undefined,
 'json'
 );

 const response = await handler(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'json only' } }
 })
 );
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('json only');
 });
});

, '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" + '
fix(server): thread responseMode into the legacy stateless fallback by dielduarte · Pull Request #2621 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix(server): thread responseMode into the legacy stateless fallback#2621

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.

 Sign up for GitHub

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

Already on GitHub?
Sign in
to your account

 Open

dielduarte
wants to merge
1
commit into

base:main
Choose a base branch

from


+132

−7

 Open


fix(server): thread responseMode into the legacy stateless fallback
#2621
Show file tree
Hide file tree
 Changes from all commits
Commits

Show all changes


1 commit


Select commit


File filter

Filter by extension
Filter by extension
Conversations

Failed to load comments. 
Loading
Jump to

Jump to file


Failed to load files. 
Loading
 Diff view

 Diff view


5 changes: 5 additions & 0 deletions
.changeset/legacy-fallback-response-mode.md

View file


Open in desktop

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

`legacy: 'stateless'` now honors `responseMode`.

21 changes: 14 additions & 7 deletions
packages/server/src/server/createMcpHandler.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -166,11 +166,12 @@ export interface CreateMcpHandlerOptions {
 /** Callback for out-of-band errors and rejected requests (reporting only; it never alters the response). */
 onerror?: (error: Error) => void;
 /**
 * Response shaping for modern (2026-07-28) request exchanges:
 * Response shaping, applied on both legs this entry serves:
 *
 * - `'auto'` (default) — a single JSON body unless the handler emits a
 * related message before its result, in which case the response upgrades
 * to an SSE stream.
 * to an SSE stream. The legacy leg has no partial-upgrade concept, so it
 * behaves like `'sse'` there.
 * - `'sse'` — always stream.
 * - `'json'` — never stream. **Mid-call notifications (progress, logging,
 * any related message emitted before the result) are dropped** — only the
Expand DownExpand Up@@ -310,7 +311,8 @@ function internalServerErrorResponse(id: RequestId | null = null): Response {
function createLegacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 keepAliveMs?: number
 keepAliveMs?: number,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return async (request, options) => {
 if (request.method.toUpperCase() !== 'POST') {
Expand All@@ -324,7 +326,8 @@ function createLegacyStatelessFallback(
 });
 const transport = new WebStandardStreamableHTTPServerTransport({
 sessionIdGenerator: undefined,
 ...(keepAliveMs !== undefined && { keepAliveMs })
 ...(keepAliveMs !== undefined && { keepAliveMs }),
 ...(responseMode === 'json' && { enableJsonResponse: true })
 });
 await product.connect(transport);

Expand DownExpand Up@@ -398,8 +401,12 @@ function createLegacyStatelessFallback(
 };
}

export function legacyStatelessFallback(factory: McpServerFactory, onerror?: (error: Error) => void): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror);
export function legacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror, undefined, responseMode);
}

/* ------------------------------------------------------------------------ *
Expand DownExpand Up@@ -645,7 +652,7 @@ export function createMcpHandler(factory: McpServerFactory, options: CreateMcpHa
 // The default posture is the stateless fallback; 'reject' is the only way
 // to turn legacy serving off (modern-only strict).
 const legacyHandler: LegacyHttpHandler | undefined =
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs);
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs, responseMode);

 async function serveModern(route: InboundModernRoute, request: Request, authInfo: AuthInfo | undefined): Promise<Response> {
 const claimedRevision = route.classification.revision;
Expand Down

19 changes: 19 additions & 0 deletions
packages/server/test/server/createMcpHandler.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -776,6 +776,25 @@ describe('createMcpHandler — responseMode', () => {
 expect(response.headers.get('content-type')).toContain('text/event-stream');
 expect(await response.text()).toContain('eager stream');
 });

 it("responseMode: 'json' never streams legacy-classified requests", async () => {
 const { factory } = testFactory();
 const handler = createMcpHandler(factory, { responseMode: 'json' });

 const response = await handler.fetch(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'legacy json only' } }
 })
 );
 expect(response.status).toBe(200);
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('legacy json only');
 });
});

describe('createMcpHandler — handler faces', () => {
Expand Down

94 changes: 94 additions & 0 deletions
packages/server/test/server/legacyStatelessFallback.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -182,3 +182,97 @@ describe('legacyStatelessFallback', () => {
 expect(onerror).toHaveBeenCalledWith(expect.objectContaining({ message: 'factory exploded' }));
 });
});

describe('legacyStatelessFallback — responseMode', () => {
 function gatedToolHandler(): { factory: () => McpServer; release: () => void } {
 let release!: () => void;
 const gate = new Promise<void>(resolve => {
 release = resolve;
 });
 const factory = (): McpServer => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode', version: '1.0.0' });
 mcpServer.registerTool('gated', { inputSchema: z.object({}) }, async () => {
 await gate;
 return { content: [{ type: 'text', text: 'done' }] };
 });
 return mcpServer;
 };
 return { factory, release };
 }

 it.each([
 ['is left at its default', undefined],
 ["is 'auto'", 'auto'],
 ["is 'sse'", 'sse']
 ] as const)('answers over SSE before the gated tool call resolves when responseMode %s', async (_label, responseMode) => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, responseMode);

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(true);

 const response = await pending;
 expect(response.headers.get('content-type')).toContain('text/event-stream');

 release();
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' does not answer until the gated tool call resolves, and buffers a single JSON body", async () => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, 'json');

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(false);

 release();
 const response = await pending;
 expect(resolved).toBe(true);
 expect(response.headers.get('content-type')).toContain('application/json');
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' drops mid-call notifications, delivering only the terminal result", async () => {
 const handler = legacyStatelessFallback(
 () => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode-notify', version: '1.0.0' });
 mcpServer.registerTool('progress-then-echo', { inputSchema: z.object({ text: z.string() }) }, async ({ text }, ctx) => {
 await ctx.mcpReq.notify({ method: 'notifications/progress', params: { progressToken: 'p', progress: 1 } });
 return { content: [{ type: 'text', text }] };
 });
 return mcpServer;
 },
 undefined,
 'json'
 );

 const response = await handler(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'json only' } }
 })
 );
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('json only');
 });
});

, '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('^' + ".*" + '
fix(server): thread responseMode into the legacy stateless fallback by dielduarte · Pull Request #2621 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix(server): thread responseMode into the legacy stateless fallback#2621

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.

 Sign up for GitHub

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

Already on GitHub?
Sign in
to your account

 Open

dielduarte
wants to merge
1
commit into

base:main
Choose a base branch

from


+132

−7

 Open


fix(server): thread responseMode into the legacy stateless fallback
#2621
Show file tree
Hide file tree
 Changes from all commits
Commits

Show all changes


1 commit


Select commit


File filter

Filter by extension
Filter by extension
Conversations

Failed to load comments. 
Loading
Jump to

Jump to file


Failed to load files. 
Loading
 Diff view

 Diff view


5 changes: 5 additions & 0 deletions
.changeset/legacy-fallback-response-mode.md

View file


Open in desktop

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

`legacy: 'stateless'` now honors `responseMode`.

21 changes: 14 additions & 7 deletions
packages/server/src/server/createMcpHandler.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -166,11 +166,12 @@ export interface CreateMcpHandlerOptions {
 /** Callback for out-of-band errors and rejected requests (reporting only; it never alters the response). */
 onerror?: (error: Error) => void;
 /**
 * Response shaping for modern (2026-07-28) request exchanges:
 * Response shaping, applied on both legs this entry serves:
 *
 * - `'auto'` (default) — a single JSON body unless the handler emits a
 * related message before its result, in which case the response upgrades
 * to an SSE stream.
 * to an SSE stream. The legacy leg has no partial-upgrade concept, so it
 * behaves like `'sse'` there.
 * - `'sse'` — always stream.
 * - `'json'` — never stream. **Mid-call notifications (progress, logging,
 * any related message emitted before the result) are dropped** — only the
Expand DownExpand Up@@ -310,7 +311,8 @@ function internalServerErrorResponse(id: RequestId | null = null): Response {
function createLegacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 keepAliveMs?: number
 keepAliveMs?: number,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return async (request, options) => {
 if (request.method.toUpperCase() !== 'POST') {
Expand All@@ -324,7 +326,8 @@ function createLegacyStatelessFallback(
 });
 const transport = new WebStandardStreamableHTTPServerTransport({
 sessionIdGenerator: undefined,
 ...(keepAliveMs !== undefined && { keepAliveMs })
 ...(keepAliveMs !== undefined && { keepAliveMs }),
 ...(responseMode === 'json' && { enableJsonResponse: true })
 });
 await product.connect(transport);

Expand DownExpand Up@@ -398,8 +401,12 @@ function createLegacyStatelessFallback(
 };
}

export function legacyStatelessFallback(factory: McpServerFactory, onerror?: (error: Error) => void): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror);
export function legacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror, undefined, responseMode);
}

/* ------------------------------------------------------------------------ *
Expand DownExpand Up@@ -645,7 +652,7 @@ export function createMcpHandler(factory: McpServerFactory, options: CreateMcpHa
 // The default posture is the stateless fallback; 'reject' is the only way
 // to turn legacy serving off (modern-only strict).
 const legacyHandler: LegacyHttpHandler | undefined =
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs);
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs, responseMode);

 async function serveModern(route: InboundModernRoute, request: Request, authInfo: AuthInfo | undefined): Promise<Response> {
 const claimedRevision = route.classification.revision;
Expand Down

19 changes: 19 additions & 0 deletions
packages/server/test/server/createMcpHandler.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -776,6 +776,25 @@ describe('createMcpHandler — responseMode', () => {
 expect(response.headers.get('content-type')).toContain('text/event-stream');
 expect(await response.text()).toContain('eager stream');
 });

 it("responseMode: 'json' never streams legacy-classified requests", async () => {
 const { factory } = testFactory();
 const handler = createMcpHandler(factory, { responseMode: 'json' });

 const response = await handler.fetch(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'legacy json only' } }
 })
 );
 expect(response.status).toBe(200);
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('legacy json only');
 });
});

describe('createMcpHandler — handler faces', () => {
Expand Down

94 changes: 94 additions & 0 deletions
packages/server/test/server/legacyStatelessFallback.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -182,3 +182,97 @@ describe('legacyStatelessFallback', () => {
 expect(onerror).toHaveBeenCalledWith(expect.objectContaining({ message: 'factory exploded' }));
 });
});

describe('legacyStatelessFallback — responseMode', () => {
 function gatedToolHandler(): { factory: () => McpServer; release: () => void } {
 let release!: () => void;
 const gate = new Promise<void>(resolve => {
 release = resolve;
 });
 const factory = (): McpServer => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode', version: '1.0.0' });
 mcpServer.registerTool('gated', { inputSchema: z.object({}) }, async () => {
 await gate;
 return { content: [{ type: 'text', text: 'done' }] };
 });
 return mcpServer;
 };
 return { factory, release };
 }

 it.each([
 ['is left at its default', undefined],
 ["is 'auto'", 'auto'],
 ["is 'sse'", 'sse']
 ] as const)('answers over SSE before the gated tool call resolves when responseMode %s', async (_label, responseMode) => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, responseMode);

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(true);

 const response = await pending;
 expect(response.headers.get('content-type')).toContain('text/event-stream');

 release();
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' does not answer until the gated tool call resolves, and buffers a single JSON body", async () => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, 'json');

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(false);

 release();
 const response = await pending;
 expect(resolved).toBe(true);
 expect(response.headers.get('content-type')).toContain('application/json');
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' drops mid-call notifications, delivering only the terminal result", async () => {
 const handler = legacyStatelessFallback(
 () => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode-notify', version: '1.0.0' });
 mcpServer.registerTool('progress-then-echo', { inputSchema: z.object({ text: z.string() }) }, async ({ text }, ctx) => {
 await ctx.mcpReq.notify({ method: 'notifications/progress', params: { progressToken: 'p', progress: 1 } });
 return { content: [{ type: 'text', text }] };
 });
 return mcpServer;
 },
 undefined,
 'json'
 );

 const response = await handler(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'json only' } }
 })
 );
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('json only');
 });
});

, '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('^' + ".*" + '
fix(server): thread responseMode into the legacy stateless fallback by dielduarte · Pull Request #2621 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix(server): thread responseMode into the legacy stateless fallback#2621

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.

 Sign up for GitHub

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

Already on GitHub?
Sign in
to your account

 Open

dielduarte
wants to merge
1
commit into

base:main
Choose a base branch

from


+132

−7

 Open


fix(server): thread responseMode into the legacy stateless fallback
#2621
Show file tree
Hide file tree
 Changes from all commits
Commits

Show all changes


1 commit


Select commit


File filter

Filter by extension
Filter by extension
Conversations

Failed to load comments. 
Loading
Jump to

Jump to file


Failed to load files. 
Loading
 Diff view

 Diff view


5 changes: 5 additions & 0 deletions
.changeset/legacy-fallback-response-mode.md

View file


Open in desktop

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

`legacy: 'stateless'` now honors `responseMode`.

21 changes: 14 additions & 7 deletions
packages/server/src/server/createMcpHandler.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -166,11 +166,12 @@ export interface CreateMcpHandlerOptions {
 /** Callback for out-of-band errors and rejected requests (reporting only; it never alters the response). */
 onerror?: (error: Error) => void;
 /**
 * Response shaping for modern (2026-07-28) request exchanges:
 * Response shaping, applied on both legs this entry serves:
 *
 * - `'auto'` (default) — a single JSON body unless the handler emits a
 * related message before its result, in which case the response upgrades
 * to an SSE stream.
 * to an SSE stream. The legacy leg has no partial-upgrade concept, so it
 * behaves like `'sse'` there.
 * - `'sse'` — always stream.
 * - `'json'` — never stream. **Mid-call notifications (progress, logging,
 * any related message emitted before the result) are dropped** — only the
Expand DownExpand Up@@ -310,7 +311,8 @@ function internalServerErrorResponse(id: RequestId | null = null): Response {
function createLegacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 keepAliveMs?: number
 keepAliveMs?: number,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return async (request, options) => {
 if (request.method.toUpperCase() !== 'POST') {
Expand All@@ -324,7 +326,8 @@ function createLegacyStatelessFallback(
 });
 const transport = new WebStandardStreamableHTTPServerTransport({
 sessionIdGenerator: undefined,
 ...(keepAliveMs !== undefined && { keepAliveMs })
 ...(keepAliveMs !== undefined && { keepAliveMs }),
 ...(responseMode === 'json' && { enableJsonResponse: true })
 });
 await product.connect(transport);

Expand DownExpand Up@@ -398,8 +401,12 @@ function createLegacyStatelessFallback(
 };
}

export function legacyStatelessFallback(factory: McpServerFactory, onerror?: (error: Error) => void): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror);
export function legacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror, undefined, responseMode);
}

/* ------------------------------------------------------------------------ *
Expand DownExpand Up@@ -645,7 +652,7 @@ export function createMcpHandler(factory: McpServerFactory, options: CreateMcpHa
 // The default posture is the stateless fallback; 'reject' is the only way
 // to turn legacy serving off (modern-only strict).
 const legacyHandler: LegacyHttpHandler | undefined =
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs);
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs, responseMode);

 async function serveModern(route: InboundModernRoute, request: Request, authInfo: AuthInfo | undefined): Promise<Response> {
 const claimedRevision = route.classification.revision;
Expand Down

19 changes: 19 additions & 0 deletions
packages/server/test/server/createMcpHandler.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -776,6 +776,25 @@ describe('createMcpHandler — responseMode', () => {
 expect(response.headers.get('content-type')).toContain('text/event-stream');
 expect(await response.text()).toContain('eager stream');
 });

 it("responseMode: 'json' never streams legacy-classified requests", async () => {
 const { factory } = testFactory();
 const handler = createMcpHandler(factory, { responseMode: 'json' });

 const response = await handler.fetch(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'legacy json only' } }
 })
 );
 expect(response.status).toBe(200);
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('legacy json only');
 });
});

describe('createMcpHandler — handler faces', () => {
Expand Down

94 changes: 94 additions & 0 deletions
packages/server/test/server/legacyStatelessFallback.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -182,3 +182,97 @@ describe('legacyStatelessFallback', () => {
 expect(onerror).toHaveBeenCalledWith(expect.objectContaining({ message: 'factory exploded' }));
 });
});

describe('legacyStatelessFallback — responseMode', () => {
 function gatedToolHandler(): { factory: () => McpServer; release: () => void } {
 let release!: () => void;
 const gate = new Promise<void>(resolve => {
 release = resolve;
 });
 const factory = (): McpServer => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode', version: '1.0.0' });
 mcpServer.registerTool('gated', { inputSchema: z.object({}) }, async () => {
 await gate;
 return { content: [{ type: 'text', text: 'done' }] };
 });
 return mcpServer;
 };
 return { factory, release };
 }

 it.each([
 ['is left at its default', undefined],
 ["is 'auto'", 'auto'],
 ["is 'sse'", 'sse']
 ] as const)('answers over SSE before the gated tool call resolves when responseMode %s', async (_label, responseMode) => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, responseMode);

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(true);

 const response = await pending;
 expect(response.headers.get('content-type')).toContain('text/event-stream');

 release();
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' does not answer until the gated tool call resolves, and buffers a single JSON body", async () => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, 'json');

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(false);

 release();
 const response = await pending;
 expect(resolved).toBe(true);
 expect(response.headers.get('content-type')).toContain('application/json');
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' drops mid-call notifications, delivering only the terminal result", async () => {
 const handler = legacyStatelessFallback(
 () => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode-notify', version: '1.0.0' });
 mcpServer.registerTool('progress-then-echo', { inputSchema: z.object({ text: z.string() }) }, async ({ text }, ctx) => {
 await ctx.mcpReq.notify({ method: 'notifications/progress', params: { progressToken: 'p', progress: 1 } });
 return { content: [{ type: 'text', text }] };
 });
 return mcpServer;
 },
 undefined,
 'json'
 );

 const response = await handler(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'json only' } }
 })
 );
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('json only');
 });
});

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Universal Dark Mode - works on any site
(function() {
var enabled = true;
function applyDarkMode() {
if (!enabled) return;
// Create style element if it doesn't exist
var style = document.getElementById('universal-dark-mode-style');
if (!style) {
style = document.createElement('style');
style.id = 'universal-dark-mode-style';
document.head.appendChild(style);
}
// Dark mode CSS - inverts colors but preserves images/video
style.textContent = '
/* Invert everything except media */
html {
filter: invert(1) hue-rotate(180deg) !important;
background: #1a1a2e !important;
}
/* Restore images, videos, iframes, canvas */
img, video, iframe, canvas, svg, picture, [style*="background-image"] {
filter: invert(1) hue-rotate(180deg) !important;
}
/* Preserve specific elements that should not be inverted */
.no-dark-mode, .no-dark-mode *,
[data-theme="light"], [data-theme="light"],
.ace_editor, .ace_editor *,
.CodeMirror, .CodeMirror *,
.monaco-editor, .monaco-editor *,
.markdown-body pre, .markdown-body pre *,
.highlight, .highlight *,
pre code, pre code * {
filter: none !important;
}
/* Fix common UI elements */
.modal, .popup, .dropdown-menu, .tooltip, .popover {
filter: invert(1) hue-rotate(180deg) !important;
background: #2d2d44 !important;
border-color: #444 !important;
}
/* Scrollbars */
::-webkit-scrollbar { background: #1a1a2e !important; }
::-webkit-scrollbar-thumb { background: #444 !important; }
::-webkit-scrollbar-thumb:hover { background: #555 !important; }
/* Selection */
::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }
::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }
';
}
function removeDarkMode() {
var style = document.getElementById('universal-dark-mode-style');
if (style) style.remove();
}
// Toggle with Alt+Shift+D
document.addEventListener('keydown', function(e) {
if (e.altKey && e.shiftKey && e.key === 'D') {
e.preventDefault();
enabled = !enabled;
if (enabled) {
applyDarkMode();
console.log('[Universal Dark Mode] Enabled');
} else {
removeDarkMode();
console.log('[Universal Dark Mode] Disabled');
}
}
});
// Apply on load
applyDarkMode();
// Re-apply on dynamic content
var observer = new MutationObserver(function(mutations) {
if (enabled && !document.getElementById('universal-dark-mode-style')) {
applyDarkMode();
}
});
observer.observe(document.head, { childList: true });
console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');
})();
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();
fix(server): thread responseMode into the legacy stateless fallback by dielduarte · Pull Request #2621 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix(server): thread responseMode into the legacy stateless fallback#2621

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.

 Sign up for GitHub

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

Already on GitHub?
Sign in
to your account

 Open

dielduarte
wants to merge
1
commit into

base:main
Choose a base branch

from


+132

−7

 Open


fix(server): thread responseMode into the legacy stateless fallback
#2621
Show file tree
Hide file tree
 Changes from all commits
Commits

Show all changes


1 commit


Select commit


File filter

Filter by extension
Filter by extension
Conversations

Failed to load comments. 
Loading
Jump to

Jump to file


Failed to load files. 
Loading
 Diff view

 Diff view


5 changes: 5 additions & 0 deletions
.changeset/legacy-fallback-response-mode.md

View file


Open in desktop

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

`legacy: 'stateless'` now honors `responseMode`.

21 changes: 14 additions & 7 deletions
packages/server/src/server/createMcpHandler.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -166,11 +166,12 @@ export interface CreateMcpHandlerOptions {
 /** Callback for out-of-band errors and rejected requests (reporting only; it never alters the response). */
 onerror?: (error: Error) => void;
 /**
 * Response shaping for modern (2026-07-28) request exchanges:
 * Response shaping, applied on both legs this entry serves:
 *
 * - `'auto'` (default) — a single JSON body unless the handler emits a
 * related message before its result, in which case the response upgrades
 * to an SSE stream.
 * to an SSE stream. The legacy leg has no partial-upgrade concept, so it
 * behaves like `'sse'` there.
 * - `'sse'` — always stream.
 * - `'json'` — never stream. **Mid-call notifications (progress, logging,
 * any related message emitted before the result) are dropped** — only the
Expand DownExpand Up@@ -310,7 +311,8 @@ function internalServerErrorResponse(id: RequestId | null = null): Response {
function createLegacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 keepAliveMs?: number
 keepAliveMs?: number,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return async (request, options) => {
 if (request.method.toUpperCase() !== 'POST') {
Expand All@@ -324,7 +326,8 @@ function createLegacyStatelessFallback(
 });
 const transport = new WebStandardStreamableHTTPServerTransport({
 sessionIdGenerator: undefined,
 ...(keepAliveMs !== undefined && { keepAliveMs })
 ...(keepAliveMs !== undefined && { keepAliveMs }),
 ...(responseMode === 'json' && { enableJsonResponse: true })
 });
 await product.connect(transport);

Expand DownExpand Up@@ -398,8 +401,12 @@ function createLegacyStatelessFallback(
 };
}

export function legacyStatelessFallback(factory: McpServerFactory, onerror?: (error: Error) => void): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror);
export function legacyStatelessFallback(
 factory: McpServerFactory,
 onerror?: (error: Error) => void,
 responseMode?: PerRequestResponseMode
): LegacyHttpHandler {
 return createLegacyStatelessFallback(factory, onerror, undefined, responseMode);
}

/* ------------------------------------------------------------------------ *
Expand DownExpand Up@@ -645,7 +652,7 @@ export function createMcpHandler(factory: McpServerFactory, options: CreateMcpHa
 // The default posture is the stateless fallback; 'reject' is the only way
 // to turn legacy serving off (modern-only strict).
 const legacyHandler: LegacyHttpHandler | undefined =
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs);
 legacy === 'reject' ? undefined : createLegacyStatelessFallback(factory, reportError, options.keepAliveMs, responseMode);

 async function serveModern(route: InboundModernRoute, request: Request, authInfo: AuthInfo | undefined): Promise<Response> {
 const claimedRevision = route.classification.revision;
Expand Down

19 changes: 19 additions & 0 deletions
packages/server/test/server/createMcpHandler.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -776,6 +776,25 @@ describe('createMcpHandler — responseMode', () => {
 expect(response.headers.get('content-type')).toContain('text/event-stream');
 expect(await response.text()).toContain('eager stream');
 });

 it("responseMode: 'json' never streams legacy-classified requests", async () => {
 const { factory } = testFactory();
 const handler = createMcpHandler(factory, { responseMode: 'json' });

 const response = await handler.fetch(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'legacy json only' } }
 })
 );
 expect(response.status).toBe(200);
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('legacy json only');
 });
});

describe('createMcpHandler — handler faces', () => {
Expand Down

94 changes: 94 additions & 0 deletions
packages/server/test/server/legacyStatelessFallback.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -182,3 +182,97 @@ describe('legacyStatelessFallback', () => {
 expect(onerror).toHaveBeenCalledWith(expect.objectContaining({ message: 'factory exploded' }));
 });
});

describe('legacyStatelessFallback — responseMode', () => {
 function gatedToolHandler(): { factory: () => McpServer; release: () => void } {
 let release!: () => void;
 const gate = new Promise<void>(resolve => {
 release = resolve;
 });
 const factory = (): McpServer => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode', version: '1.0.0' });
 mcpServer.registerTool('gated', { inputSchema: z.object({}) }, async () => {
 await gate;
 return { content: [{ type: 'text', text: 'done' }] };
 });
 return mcpServer;
 };
 return { factory, release };
 }

 it.each([
 ['is left at its default', undefined],
 ["is 'auto'", 'auto'],
 ["is 'sse'", 'sse']
 ] as const)('answers over SSE before the gated tool call resolves when responseMode %s', async (_label, responseMode) => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, responseMode);

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(true);

 const response = await pending;
 expect(response.headers.get('content-type')).toContain('text/event-stream');

 release();
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' does not answer until the gated tool call resolves, and buffers a single JSON body", async () => {
 const { factory, release } = gatedToolHandler();
 const handler = legacyStatelessFallback(factory, undefined, 'json');

 let resolved = false;
 const pending = handler(
 postRequest({ jsonrpc: '2.0', id: 1, method: 'tools/call', params: { name: 'gated', arguments: {} } })
 ).then(response => {
 resolved = true;
 return response;
 });

 await new Promise(resolve => setTimeout(resolve, 20));
 expect(resolved).toBe(false);

 release();
 const response = await pending;
 expect(resolved).toBe(true);
 expect(response.headers.get('content-type')).toContain('application/json');
 expect(await response.text()).toContain('done');
 });

 it("responseMode: 'json' drops mid-call notifications, delivering only the terminal result", async () => {
 const handler = legacyStatelessFallback(
 () => {
 const mcpServer = new McpServer({ name: 'fallback-response-mode-notify', version: '1.0.0' });
 mcpServer.registerTool('progress-then-echo', { inputSchema: z.object({ text: z.string() }) }, async ({ text }, ctx) => {
 await ctx.mcpReq.notify({ method: 'notifications/progress', params: { progressToken: 'p', progress: 1 } });
 return { content: [{ type: 'text', text }] };
 });
 return mcpServer;
 },
 undefined,
 'json'
 );

 const response = await handler(
 postRequest({
 jsonrpc: '2.0',
 id: 1,
 method: 'tools/call',
 params: { name: 'progress-then-echo', arguments: { text: 'json only' } }
 })
 );
 expect(response.headers.get('content-type')).toContain('application/json');
 const text = await response.text();
 expect(text).not.toContain('notifications/progress');
 expect(text).toContain('json only');
 });
});