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fix: return 404 instead of 400 for invalid session IDs in examples - #1678

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fix: return 404 instead of 400 for invalid session IDs in examples#1678
kdn-posipaka wants to merge 1 commit into
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kdn-posipaka:fix/session-id-status-code-389

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@kdn-posipakakdn-posipaka commented Mar 13, 2026

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Summary

Fixes#389

The MCP spec states that servers SHOULD return HTTP 404 for requests with invalid session IDs, so clients know the session is gone and can start a new one:

If a server receives requests bearing an invalid Mcp-Session-Id, it SHOULD respond with HTTP 404 Not Found to indicate the session does not (or no longer) exists.

The SDK's own transport implementation (streamableHttp.ts) already correctly returns 404 with error code -32001 ("Session not found"). However, all the example servers and the conformance test server were returning 400 instead.

Changes

  • Example servers (5 files): Changed res.status(400) → res.status(404) for invalid/missing session ID responses
  • Conformance test server: Same fix applied

Files changed

  • examples/server/src/simpleStreamableHttp.ts
  • examples/server/src/simpleTaskInteractive.ts
  • examples/server/src/elicitationFormExample.ts
  • examples/server/src/elicitationUrlExample.ts
  • examples/server/src/standaloneSseWithGetStreamableHttp.ts
  • test/conformance/src/everythingServer.ts

All existing tests pass (28/28 in streamableHttp.test.ts).

kai-agent-free
fix: return 404 instead of 400 for invalid session IDs in examples
8274506
The MCP spec states that servers should return HTTP 404 for invalid
session IDs so clients know to start a new session. The SDK transport
(streamableHttp.ts) already correctly returns 404, but the example
servers and conformance test server were returning 400.
Fixesmodelcontextprotocol#389
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Thanks for the contribution! Closing in favor of #1770.

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Streamable HTTP examples use incorrect status code for invalid session IDs

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, 'i'); if (__m === '*' || __re.test(location.href)) { // Add copy buttons to all 
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function addCopyButtons() {
document.querySelectorAll('pre code').forEach(function(codeBlock) {
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btn.textContent = 'Copy';
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setTimeout(function() { btn.textContent = 'Copy'; }, 1500);
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observer.observe(document.body, { childList: true, subtree: true });
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try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '
fix: return 404 instead of 400 for invalid session IDs in examples by kdn-posipaka · Pull Request #1678 · modelcontextprotocol/typescript-sdk · GitHub
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fix: return 404 instead of 400 for invalid session IDs in examples - #1678
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fix: return 404 instead of 400 for invalid session IDs in examples#1678
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kdn-posipaka:fix/session-id-status-code-389
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@kdn-posipakakdn-posipaka
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Mar 13, 2026

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Summary
Fixes#389
The MCP spec states that servers SHOULD return HTTP 404 for requests with invalid session IDs, so clients know the session is gone and can start a new one:
If a server receives requests bearing an invalid Mcp-Session-Id, it SHOULD respond with HTTP 404 Not Found to indicate the session does not (or no longer) exists.
The SDK's own transport implementation (streamableHttp.ts) already correctly returns 404 with error code -32001 ("Session not found"). However, all the example servers and the conformance test server were returning 400 instead.
Changes
  • Example servers (5 files): Changed res.status(400) → res.status(404) for invalid/missing session ID responses
  • Conformance test server: Same fix applied
Files changed
  • examples/server/src/simpleStreamableHttp.ts
  • examples/server/src/simpleTaskInteractive.ts
  • examples/server/src/elicitationFormExample.ts
  • examples/server/src/elicitationUrlExample.ts
  • examples/server/src/standaloneSseWithGetStreamableHttp.ts
  • test/conformance/src/everythingServer.ts
All existing tests pass (28/28 in streamableHttp.test.ts).
kai-agent-free
fix: return 404 instead of 400 for invalid session IDs in examples
8274506
The MCP spec states that servers should return HTTP 404 for invalid
session IDs so clients know to start a new session. The SDK transport
(streamableHttp.ts) already correctly returns 404, but the example
servers and conformance test server were returning 400.
Fixesmodelcontextprotocol#389
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Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.
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Thanks for the contribution! Closing in favor of #1770.
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Streamable HTTP examples use incorrect status code for invalid session IDs


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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Force GitHub README to respect dark mode
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var style = document.createElement('style');
style.textContent = '
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color-scheme: dark light;
}
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';
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} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
fix: return 404 instead of 400 for invalid session IDs in examples by kdn-posipaka · Pull Request #1678 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix: return 404 instead of 400 for invalid session IDs in examples - #1678
Closed
kdn-posipaka wants to merge 1 commit into
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kdn-posipaka:fix/session-id-status-code-389
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fix: return 404 instead of 400 for invalid session IDs in examples#1678
kdn-posipaka wants to merge 1 commit into
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kdn-posipaka:fix/session-id-status-code-389
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@kdn-posipakakdn-posipaka
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Mar 13, 2026

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Summary
Fixes#389
The MCP spec states that servers SHOULD return HTTP 404 for requests with invalid session IDs, so clients know the session is gone and can start a new one:
If a server receives requests bearing an invalid Mcp-Session-Id, it SHOULD respond with HTTP 404 Not Found to indicate the session does not (or no longer) exists.
The SDK's own transport implementation (streamableHttp.ts) already correctly returns 404 with error code -32001 ("Session not found"). However, all the example servers and the conformance test server were returning 400 instead.
Changes
  • Example servers (5 files): Changed res.status(400) → res.status(404) for invalid/missing session ID responses
  • Conformance test server: Same fix applied
Files changed
  • examples/server/src/simpleStreamableHttp.ts
  • examples/server/src/simpleTaskInteractive.ts
  • examples/server/src/elicitationFormExample.ts
  • examples/server/src/elicitationUrlExample.ts
  • examples/server/src/standaloneSseWithGetStreamableHttp.ts
  • test/conformance/src/everythingServer.ts
All existing tests pass (28/28 in streamableHttp.test.ts).
kai-agent-free
fix: return 404 instead of 400 for invalid session IDs in examples
8274506
The MCP spec states that servers should return HTTP 404 for invalid
session IDs so clients know to start a new session. The SDK transport
(streamableHttp.ts) already correctly returns 404, but the example
servers and conformance test server were returning 400.
Fixesmodelcontextprotocol#389
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Latest commit: 8274506
Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.
This PR includes no changesets
When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types
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Thanks for the contribution! Closing in favor of #1770.
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Streamable HTTP examples use incorrect status code for invalid session IDs


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@kdn-posipaka@felixweinberger

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Highlight search terms from Google/DuckDuckGo/Bing referrer
(function() {
var ref = document.referrer;
var terms = [];
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Array.from(node.childNodes).forEach(highlight);
}
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highlight(document.body);
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observer.observe(document.body, { childList: true, subtree: true });
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} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
fix: return 404 instead of 400 for invalid session IDs in examples by kdn-posipaka · Pull Request #1678 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix: return 404 instead of 400 for invalid session IDs in examples - #1678
Closed
kdn-posipaka wants to merge 1 commit into
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kdn-posipaka:fix/session-id-status-code-389
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fix: return 404 instead of 400 for invalid session IDs in examples#1678
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@kdn-posipakakdn-posipaka
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Mar 13, 2026

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Summary
Fixes#389
The MCP spec states that servers SHOULD return HTTP 404 for requests with invalid session IDs, so clients know the session is gone and can start a new one:
If a server receives requests bearing an invalid Mcp-Session-Id, it SHOULD respond with HTTP 404 Not Found to indicate the session does not (or no longer) exists.
The SDK's own transport implementation (streamableHttp.ts) already correctly returns 404 with error code -32001 ("Session not found"). However, all the example servers and the conformance test server were returning 400 instead.
Changes
  • Example servers (5 files): Changed res.status(400) → res.status(404) for invalid/missing session ID responses
  • Conformance test server: Same fix applied
Files changed
  • examples/server/src/simpleStreamableHttp.ts
  • examples/server/src/simpleTaskInteractive.ts
  • examples/server/src/elicitationFormExample.ts
  • examples/server/src/elicitationUrlExample.ts
  • examples/server/src/standaloneSseWithGetStreamableHttp.ts
  • test/conformance/src/everythingServer.ts
All existing tests pass (28/28 in streamableHttp.test.ts).
kai-agent-free
fix: return 404 instead of 400 for invalid session IDs in examples
8274506
The MCP spec states that servers should return HTTP 404 for invalid
session IDs so clients know to start a new session. The SDK transport
(streamableHttp.ts) already correctly returns 404, but the example
servers and conformance test server were returning 400.
Fixesmodelcontextprotocol#389
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Latest commit: 8274506
Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.
This PR includes no changesets
When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types
Click here to learn what changesets are, and how to add one.
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@modelcontextprotocol/express
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Streamable HTTP examples use incorrect status code for invalid session IDs


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, '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;
});
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cleanLinks();
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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: return 404 instead of 400 for invalid session IDs in examples by kdn-posipaka · Pull Request #1678 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix: return 404 instead of 400 for invalid session IDs in examples - #1678
Closed
kdn-posipaka wants to merge 1 commit into
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kdn-posipaka:fix/session-id-status-code-389
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fix: return 404 instead of 400 for invalid session IDs in examples#1678
kdn-posipaka wants to merge 1 commit into
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Mar 13, 2026

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Summary
Fixes#389
The MCP spec states that servers SHOULD return HTTP 404 for requests with invalid session IDs, so clients know the session is gone and can start a new one:
If a server receives requests bearing an invalid Mcp-Session-Id, it SHOULD respond with HTTP 404 Not Found to indicate the session does not (or no longer) exists.
The SDK's own transport implementation (streamableHttp.ts) already correctly returns 404 with error code -32001 ("Session not found"). However, all the example servers and the conformance test server were returning 400 instead.
Changes
  • Example servers (5 files): Changed res.status(400) → res.status(404) for invalid/missing session ID responses
  • Conformance test server: Same fix applied
Files changed
  • examples/server/src/simpleStreamableHttp.ts
  • examples/server/src/simpleTaskInteractive.ts
  • examples/server/src/elicitationFormExample.ts
  • examples/server/src/elicitationUrlExample.ts
  • examples/server/src/standaloneSseWithGetStreamableHttp.ts
  • test/conformance/src/everythingServer.ts
All existing tests pass (28/28 in streamableHttp.test.ts).
kai-agent-free
fix: return 404 instead of 400 for invalid session IDs in examples
8274506
The MCP spec states that servers should return HTTP 404 for invalid
session IDs so clients know to start a new session. The SDK transport
(streamableHttp.ts) already correctly returns 404, but the example
servers and conformance test server were returning 400.
Fixesmodelcontextprotocol#389
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Latest commit: 8274506
Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.
This PR includes no changesets
When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types
Click here to learn what changesets are, and how to add one.
Click here if you're a maintainer who wants to add a changeset to this PR
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@modelcontextprotocol/express
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@modelcontextprotocol/hono
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Thanks for the contribution! Closing in favor of #1770.
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Streamable HTTP examples use incorrect status code for invalid session IDs


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@kdn-posipaka@felixweinberger

, '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: return 404 instead of 400 for invalid session IDs in examples by kdn-posipaka · Pull Request #1678 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix: return 404 instead of 400 for invalid session IDs in examples - #1678
Closed
kdn-posipaka wants to merge 1 commit into
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kdn-posipaka:fix/session-id-status-code-389
Closed
fix: return 404 instead of 400 for invalid session IDs in examples#1678
kdn-posipaka wants to merge 1 commit into
modelcontextprotocol:mainfrom 
kdn-posipaka:fix/session-id-status-code-389
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@kdn-posipaka
@kdn-posipakakdn-posipaka
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Mar 13, 2026

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Summary
Fixes#389
The MCP spec states that servers SHOULD return HTTP 404 for requests with invalid session IDs, so clients know the session is gone and can start a new one:
If a server receives requests bearing an invalid Mcp-Session-Id, it SHOULD respond with HTTP 404 Not Found to indicate the session does not (or no longer) exists.
The SDK's own transport implementation (streamableHttp.ts) already correctly returns 404 with error code -32001 ("Session not found"). However, all the example servers and the conformance test server were returning 400 instead.
Changes
  • Example servers (5 files): Changed res.status(400) → res.status(404) for invalid/missing session ID responses
  • Conformance test server: Same fix applied
Files changed
  • examples/server/src/simpleStreamableHttp.ts
  • examples/server/src/simpleTaskInteractive.ts
  • examples/server/src/elicitationFormExample.ts
  • examples/server/src/elicitationUrlExample.ts
  • examples/server/src/standaloneSseWithGetStreamableHttp.ts
  • test/conformance/src/everythingServer.ts
All existing tests pass (28/28 in streamableHttp.test.ts).
kai-agent-free
fix: return 404 instead of 400 for invalid session IDs in examples
8274506
The MCP spec states that servers should return HTTP 404 for invalid
session IDs so clients know to start a new session. The SDK transport
(streamableHttp.ts) already correctly returns 404, but the example
servers and conformance test server were returning 400.
Fixesmodelcontextprotocol#389
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⚠️ No Changeset found
Latest commit: 8274506
Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.
This PR includes no changesets
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@modelcontextprotocol/server
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@modelcontextprotocol/express
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commit: 67d0a13
@mbochneakmbochneak
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Mar 19, 2026

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@felixweinbergerfelixweinberger
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Mar 26, 2026
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@felixweinberger
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Thanks for the contribution! Closing in favor of #1770.
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Streamable HTTP examples use incorrect status code for invalid session IDs


2 participants

@kdn-posipaka@felixweinberger

, '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';
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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: return 404 instead of 400 for invalid session IDs in examples by kdn-posipaka · Pull Request #1678 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix: return 404 instead of 400 for invalid session IDs in examples - #1678
Closed
kdn-posipaka wants to merge 1 commit into
modelcontextprotocol:mainfrom 
kdn-posipaka:fix/session-id-status-code-389
Closed
fix: return 404 instead of 400 for invalid session IDs in examples#1678
kdn-posipaka wants to merge 1 commit into
modelcontextprotocol:mainfrom 
kdn-posipaka:fix/session-id-status-code-389
Conversation
@kdn-posipaka
@kdn-posipakakdn-posipaka
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Mar 13, 2026

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Summary
Fixes#389
The MCP spec states that servers SHOULD return HTTP 404 for requests with invalid session IDs, so clients know the session is gone and can start a new one:
If a server receives requests bearing an invalid Mcp-Session-Id, it SHOULD respond with HTTP 404 Not Found to indicate the session does not (or no longer) exists.
The SDK's own transport implementation (streamableHttp.ts) already correctly returns 404 with error code -32001 ("Session not found"). However, all the example servers and the conformance test server were returning 400 instead.
Changes
  • Example servers (5 files): Changed res.status(400) → res.status(404) for invalid/missing session ID responses
  • Conformance test server: Same fix applied
Files changed
  • examples/server/src/simpleStreamableHttp.ts
  • examples/server/src/simpleTaskInteractive.ts
  • examples/server/src/elicitationFormExample.ts
  • examples/server/src/elicitationUrlExample.ts
  • examples/server/src/standaloneSseWithGetStreamableHttp.ts
  • test/conformance/src/everythingServer.ts
All existing tests pass (28/28 in streamableHttp.test.ts).
kai-agent-free
fix: return 404 instead of 400 for invalid session IDs in examples
8274506
The MCP spec states that servers should return HTTP 404 for invalid
session IDs so clients know to start a new session. The SDK transport
(streamableHttp.ts) already correctly returns 404, but the example
servers and conformance test server were returning 400.
Fixesmodelcontextprotocol#389
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⚠️ No Changeset found
Latest commit: 8274506
Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.
This PR includes no changesets
When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types
Click here to learn what changesets are, and how to add one.
Click here if you're a maintainer who wants to add a changeset to this PR
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@modelcontextprotocol/client
npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/client@1678

@modelcontextprotocol/server
npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/server@1678

@modelcontextprotocol/express
npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/express@1678

@modelcontextprotocol/hono
npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/hono@1678

@modelcontextprotocol/node
npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/node@1678

commit: 67d0a13
@mbochneakmbochneak
mentioned this pull request
Mar 19, 2026

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@felixweinbergerfelixweinberger
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Mar 26, 2026
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@felixweinberger
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Thanks for the contribution! Closing in favor of #1770.
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Streamable HTTP examples use incorrect status code for invalid session IDs


2 participants

@kdn-posipaka@felixweinberger

, '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 */
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filter: invert(1) hue-rotate(180deg) !important;
}
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[data-theme="light"], [data-theme="light"],
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.monaco-editor, .monaco-editor *,
.markdown-body pre, .markdown-body pre *,
.highlight, .highlight *,
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filter: none !important;
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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; }
';
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var style = document.getElementById('universal-dark-mode-style');
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if (enabled && !document.getElementById('universal-dark-mode-style')) {
applyDarkMode();
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observer.observe(document.head, { childList: true });
console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');
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} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();
fix: return 404 instead of 400 for invalid session IDs in examples by kdn-posipaka · Pull Request #1678 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix: return 404 instead of 400 for invalid session IDs in examples - #1678
Closed
kdn-posipaka wants to merge 1 commit into
modelcontextprotocol:mainfrom 
kdn-posipaka:fix/session-id-status-code-389
Closed
fix: return 404 instead of 400 for invalid session IDs in examples#1678
kdn-posipaka wants to merge 1 commit into
modelcontextprotocol:mainfrom 
kdn-posipaka:fix/session-id-status-code-389
Conversation
@kdn-posipaka
@kdn-posipakakdn-posipaka
commented
Mar 13, 2026

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Summary
Fixes#389
The MCP spec states that servers SHOULD return HTTP 404 for requests with invalid session IDs, so clients know the session is gone and can start a new one:
If a server receives requests bearing an invalid Mcp-Session-Id, it SHOULD respond with HTTP 404 Not Found to indicate the session does not (or no longer) exists.
The SDK's own transport implementation (streamableHttp.ts) already correctly returns 404 with error code -32001 ("Session not found"). However, all the example servers and the conformance test server were returning 400 instead.
Changes
  • Example servers (5 files): Changed res.status(400) → res.status(404) for invalid/missing session ID responses
  • Conformance test server: Same fix applied
Files changed
  • examples/server/src/simpleStreamableHttp.ts
  • examples/server/src/simpleTaskInteractive.ts
  • examples/server/src/elicitationFormExample.ts
  • examples/server/src/elicitationUrlExample.ts
  • examples/server/src/standaloneSseWithGetStreamableHttp.ts
  • test/conformance/src/everythingServer.ts
All existing tests pass (28/28 in streamableHttp.test.ts).
kai-agent-free
fix: return 404 instead of 400 for invalid session IDs in examples
8274506
The MCP spec states that servers should return HTTP 404 for invalid
session IDs so clients know to start a new session. The SDK transport
(streamableHttp.ts) already correctly returns 404, but the example
servers and conformance test server were returning 400.
Fixesmodelcontextprotocol#389
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⚠️ No Changeset found
Latest commit: 8274506
Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.
This PR includes no changesets
When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types
Click here to learn what changesets are, and how to add one.
Click here if you're a maintainer who wants to add a changeset to this PR
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@modelcontextprotocol/client
npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/client@1678

@modelcontextprotocol/server
npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/server@1678

@modelcontextprotocol/express
npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/express@1678

@modelcontextprotocol/hono
npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/hono@1678

@modelcontextprotocol/node
npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/node@1678

commit: 67d0a13
@mbochneakmbochneak
mentioned this pull request
Mar 19, 2026

Open

@felixweinbergerfelixweinberger
mentioned this pull request
Mar 26, 2026
 Merged

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@felixweinberger
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Mar 26, 2026

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Thanks for the contribution! Closing in favor of #1770.
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Streamable HTTP examples use incorrect status code for invalid session IDs


2 participants

@kdn-posipaka@felixweinberger