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fix(server): fire onsessionclosed once when DELETEs overlap - #2583

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@axits-labaxits-lab commented Jul 30, 2026

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Refs #2562 — item 2 of three. Items 1 and 3 are not touched here; item 1 will follow separately, and item 3 looks like it needs a call on the EventStore contract first.

Opened as a draft: non-write-access users are capped at one open PR on this repo, and #2582 is currently using it. Ready for review as-is — happy to mark it ready whenever that one lands or on your say-so.

The race

handleDeleteRequest awaits onsessionclosed and only then runs close() from the finally:

try{awaitPromise.resolve(this._onsessionclosed?.(this.sessionId!));returnnewResponse(null,{status: 200});}finally{awaitthis.close();}

_closed is set by close(), so it is still false for the whole time the callback is in flight. Neither validateSession nor validateProtocolVersion consults _closed — they check sessionIdGenerator, _initialized, and the session-id match — so a second DELETE arriving in that window passes every guard and fires the callback a second time for a session already being torn down.

Fix

Claim the notification synchronously, before the await. _closed cannot serve this purpose, since by construction it is only set after the callback has settled — hence the separate flag.

DELETE stays idempotent: a concurrent or repeat request still terminates the session and answers 200, it just does not re-run the callback.

Test

Added to the existing Session Callbacks block. It holds the callback open on a promise the test controls, issues the second DELETE while the first is parked on it, then releases:

× fires onsessionclosed once when two DELETEs arrive concurrently
AssertionError: expected "vi.fn()" to be called 1 times, but got 2 times

That is the failure on main without the src change; it passes with it.

Verification

  • 469/469 tests pass in @modelcontextprotocol/server (53 in streamableHttp.test.ts)
  • pnpm lint and pnpm typecheck clean
  • Grepped the workspace: onsessionclosed is implemented only in packages/server/src/server/streamableHttp.ts, so no sibling transport carries the same window

Changeset included.

You noted the item applies to v1.x as well — happy to put up the equivalent there if you'd like it as a separate PR.

@axits-lab@claude
fix(server): fire onsessionclosed once when DELETEs overlap
ac6312f
`handleDeleteRequest` awaited `onsessionclosed` before `close()` ran, so
`_closed` was still false while the callback was in flight. A second
DELETE for the same session passed `validateSession`/`validateProtocolVersion`
unchanged — neither consults `_closed` — and invoked the callback again for
a session already being torn down.
Claim the notification synchronously before the await. `_closed` cannot
serve this purpose: it is set by `close()`, which only runs once the
callback has settled.
DELETE stays idempotent — a concurrent or repeat request still terminates
the session and answers 200, it just does not re-run the callback.
Refs modelcontextprotocol#2562 (item 2)
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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🦋 Changeset detected

Latest commit: ac6312f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages
NameType
@modelcontextprotocol/serverPatch
@modelcontextprotocol/corePatch
@modelcontextprotocol/clientPatch
@modelcontextprotocol/server-legacyPatch
@modelcontextprotocol/codemodPatch
@modelcontextprotocol/core-internalPatch

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fix(server): fire onsessionclosed once when DELETEs overlap by axits-lab · Pull Request #2583 · modelcontextprotocol/typescript-sdk · GitHub
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fix(server): fire onsessionclosed once when DELETEs overlap - #2583
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Refs #2562 — item 2 of three. Items 1 and 3 are not touched here; item 1 will follow separately, and item 3 looks like it needs a call on the EventStore contract first.
Opened as a draft: non-write-access users are capped at one open PR on this repo, and #2582 is currently using it. Ready for review as-is — happy to mark it ready whenever that one lands or on your say-so.
The race
handleDeleteRequest awaits onsessionclosed and only then runs close() from the finally:
try{awaitPromise.resolve(this._onsessionclosed?.(this.sessionId!));returnnewResponse(null,{status: 200});}finally{awaitthis.close();}
_closed is set by close(), so it is still false for the whole time the callback is in flight. Neither validateSession nor validateProtocolVersion consults _closed — they check sessionIdGenerator, _initialized, and the session-id match — so a second DELETE arriving in that window passes every guard and fires the callback a second time for a session already being torn down.
Fix
Claim the notification synchronously, before the await. _closed cannot serve this purpose, since by construction it is only set after the callback has settled — hence the separate flag.
DELETE stays idempotent: a concurrent or repeat request still terminates the session and answers 200, it just does not re-run the callback.
Test
Added to the existing Session Callbacks block. It holds the callback open on a promise the test controls, issues the second DELETE while the first is parked on it, then releases:
× fires onsessionclosed once when two DELETEs arrive concurrently
AssertionError: expected "vi.fn()" to be called 1 times, but got 2 times

That is the failure on main without the src change; it passes with it.
Verification
  • 469/469 tests pass in @modelcontextprotocol/server (53 in streamableHttp.test.ts)
  • pnpm lint and pnpm typecheck clean
  • Grepped the workspace: onsessionclosed is implemented only in packages/server/src/server/streamableHttp.ts, so no sibling transport carries the same window
Changeset included.
You noted the item applies to v1.x as well — happy to put up the equivalent there if you'd like it as a separate PR.
@axits-lab@claude
fix(server): fire onsessionclosed once when DELETEs overlap
ac6312f
`handleDeleteRequest` awaited `onsessionclosed` before `close()` ran, so
`_closed` was still false while the callback was in flight. A second
DELETE for the same session passed `validateSession`/`validateProtocolVersion`
unchanged — neither consults `_closed` — and invoked the callback again for
a session already being torn down.
Claim the notification synchronously before the await. `_closed` cannot
serve this purpose: it is set by `close()`, which only runs once the
callback has settled.
DELETE stays idempotent — a concurrent or repeat request still terminates
the session and answers 200, it just does not re-run the callback.
Refs modelcontextprotocol#2562 (item 2)
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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🦋 Changeset detected
Latest commit: ac6312f
The changes in this PR will be included in the next version bump.
This PR includes changesets to release 6 packages
NameType
@modelcontextprotocol/serverPatch
@modelcontextprotocol/corePatch
@modelcontextprotocol/clientPatch
@modelcontextprotocol/server-legacyPatch
@modelcontextprotocol/codemodPatch
@modelcontextprotocol/core-internalPatch
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fix(server): fire onsessionclosed once when DELETEs overlap by axits-lab · Pull Request #2583 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix(server): fire onsessionclosed once when DELETEs overlap - #2583
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fix(server): fire onsessionclosed once when DELETEs overlap#2583
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Jul 30, 2026

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Refs #2562 — item 2 of three. Items 1 and 3 are not touched here; item 1 will follow separately, and item 3 looks like it needs a call on the EventStore contract first.
Opened as a draft: non-write-access users are capped at one open PR on this repo, and #2582 is currently using it. Ready for review as-is — happy to mark it ready whenever that one lands or on your say-so.
The race
handleDeleteRequest awaits onsessionclosed and only then runs close() from the finally:
try{awaitPromise.resolve(this._onsessionclosed?.(this.sessionId!));returnnewResponse(null,{status: 200});}finally{awaitthis.close();}
_closed is set by close(), so it is still false for the whole time the callback is in flight. Neither validateSession nor validateProtocolVersion consults _closed — they check sessionIdGenerator, _initialized, and the session-id match — so a second DELETE arriving in that window passes every guard and fires the callback a second time for a session already being torn down.
Fix
Claim the notification synchronously, before the await. _closed cannot serve this purpose, since by construction it is only set after the callback has settled — hence the separate flag.
DELETE stays idempotent: a concurrent or repeat request still terminates the session and answers 200, it just does not re-run the callback.
Test
Added to the existing Session Callbacks block. It holds the callback open on a promise the test controls, issues the second DELETE while the first is parked on it, then releases:
× fires onsessionclosed once when two DELETEs arrive concurrently
AssertionError: expected "vi.fn()" to be called 1 times, but got 2 times

That is the failure on main without the src change; it passes with it.
Verification
  • 469/469 tests pass in @modelcontextprotocol/server (53 in streamableHttp.test.ts)
  • pnpm lint and pnpm typecheck clean
  • Grepped the workspace: onsessionclosed is implemented only in packages/server/src/server/streamableHttp.ts, so no sibling transport carries the same window
Changeset included.
You noted the item applies to v1.x as well — happy to put up the equivalent there if you'd like it as a separate PR.
@axits-lab@claude
fix(server): fire onsessionclosed once when DELETEs overlap
ac6312f
`handleDeleteRequest` awaited `onsessionclosed` before `close()` ran, so
`_closed` was still false while the callback was in flight. A second
DELETE for the same session passed `validateSession`/`validateProtocolVersion`
unchanged — neither consults `_closed` — and invoked the callback again for
a session already being torn down.
Claim the notification synchronously before the await. `_closed` cannot
serve this purpose: it is set by `close()`, which only runs once the
callback has settled.
DELETE stays idempotent — a concurrent or repeat request still terminates
the session and answers 200, it just does not re-run the callback.
Refs modelcontextprotocol#2562 (item 2)
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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🦋 Changeset detected
Latest commit: ac6312f
The changes in this PR will be included in the next version bump.
This PR includes changesets to release 6 packages
NameType
@modelcontextprotocol/serverPatch
@modelcontextprotocol/corePatch
@modelcontextprotocol/clientPatch
@modelcontextprotocol/server-legacyPatch
@modelcontextprotocol/codemodPatch
@modelcontextprotocol/core-internalPatch
Not sure what this means? Click here to learn what changesets are.
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@modelcontextprotocol/server
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fix(server): fire onsessionclosed once when DELETEs overlap by axits-lab · Pull Request #2583 · modelcontextprotocol/typescript-sdk · GitHub
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fix(server): fire onsessionclosed once when DELETEs overlap - #2583
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Refs #2562 — item 2 of three. Items 1 and 3 are not touched here; item 1 will follow separately, and item 3 looks like it needs a call on the EventStore contract first.
Opened as a draft: non-write-access users are capped at one open PR on this repo, and #2582 is currently using it. Ready for review as-is — happy to mark it ready whenever that one lands or on your say-so.
The race
handleDeleteRequest awaits onsessionclosed and only then runs close() from the finally:
try{awaitPromise.resolve(this._onsessionclosed?.(this.sessionId!));returnnewResponse(null,{status: 200});}finally{awaitthis.close();}
_closed is set by close(), so it is still false for the whole time the callback is in flight. Neither validateSession nor validateProtocolVersion consults _closed — they check sessionIdGenerator, _initialized, and the session-id match — so a second DELETE arriving in that window passes every guard and fires the callback a second time for a session already being torn down.
Fix
Claim the notification synchronously, before the await. _closed cannot serve this purpose, since by construction it is only set after the callback has settled — hence the separate flag.
DELETE stays idempotent: a concurrent or repeat request still terminates the session and answers 200, it just does not re-run the callback.
Test
Added to the existing Session Callbacks block. It holds the callback open on a promise the test controls, issues the second DELETE while the first is parked on it, then releases:
× fires onsessionclosed once when two DELETEs arrive concurrently
AssertionError: expected "vi.fn()" to be called 1 times, but got 2 times

That is the failure on main without the src change; it passes with it.
Verification
  • 469/469 tests pass in @modelcontextprotocol/server (53 in streamableHttp.test.ts)
  • pnpm lint and pnpm typecheck clean
  • Grepped the workspace: onsessionclosed is implemented only in packages/server/src/server/streamableHttp.ts, so no sibling transport carries the same window
Changeset included.
You noted the item applies to v1.x as well — happy to put up the equivalent there if you'd like it as a separate PR.
@axits-lab@claude
fix(server): fire onsessionclosed once when DELETEs overlap
ac6312f
`handleDeleteRequest` awaited `onsessionclosed` before `close()` ran, so
`_closed` was still false while the callback was in flight. A second
DELETE for the same session passed `validateSession`/`validateProtocolVersion`
unchanged — neither consults `_closed` — and invoked the callback again for
a session already being torn down.
Claim the notification synchronously before the await. `_closed` cannot
serve this purpose: it is set by `close()`, which only runs once the
callback has settled.
DELETE stays idempotent — a concurrent or repeat request still terminates
the session and answers 200, it just does not re-run the callback.
Refs modelcontextprotocol#2562 (item 2)
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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🦋 Changeset detected
Latest commit: ac6312f
The changes in this PR will be included in the next version bump.
This PR includes changesets to release 6 packages
NameType
@modelcontextprotocol/serverPatch
@modelcontextprotocol/corePatch
@modelcontextprotocol/clientPatch
@modelcontextprotocol/server-legacyPatch
@modelcontextprotocol/codemodPatch
@modelcontextprotocol/core-internalPatch
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'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): fire onsessionclosed once when DELETEs overlap by axits-lab · Pull Request #2583 · modelcontextprotocol/typescript-sdk · GitHub
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fix(server): fire onsessionclosed once when DELETEs overlap - #2583
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Refs #2562 — item 2 of three. Items 1 and 3 are not touched here; item 1 will follow separately, and item 3 looks like it needs a call on the EventStore contract first.
Opened as a draft: non-write-access users are capped at one open PR on this repo, and #2582 is currently using it. Ready for review as-is — happy to mark it ready whenever that one lands or on your say-so.
The race
handleDeleteRequest awaits onsessionclosed and only then runs close() from the finally:
try{awaitPromise.resolve(this._onsessionclosed?.(this.sessionId!));returnnewResponse(null,{status: 200});}finally{awaitthis.close();}
_closed is set by close(), so it is still false for the whole time the callback is in flight. Neither validateSession nor validateProtocolVersion consults _closed — they check sessionIdGenerator, _initialized, and the session-id match — so a second DELETE arriving in that window passes every guard and fires the callback a second time for a session already being torn down.
Fix
Claim the notification synchronously, before the await. _closed cannot serve this purpose, since by construction it is only set after the callback has settled — hence the separate flag.
DELETE stays idempotent: a concurrent or repeat request still terminates the session and answers 200, it just does not re-run the callback.
Test
Added to the existing Session Callbacks block. It holds the callback open on a promise the test controls, issues the second DELETE while the first is parked on it, then releases:
× fires onsessionclosed once when two DELETEs arrive concurrently
AssertionError: expected "vi.fn()" to be called 1 times, but got 2 times

That is the failure on main without the src change; it passes with it.
Verification
  • 469/469 tests pass in @modelcontextprotocol/server (53 in streamableHttp.test.ts)
  • pnpm lint and pnpm typecheck clean
  • Grepped the workspace: onsessionclosed is implemented only in packages/server/src/server/streamableHttp.ts, so no sibling transport carries the same window
Changeset included.
You noted the item applies to v1.x as well — happy to put up the equivalent there if you'd like it as a separate PR.
@axits-lab@claude
fix(server): fire onsessionclosed once when DELETEs overlap
ac6312f
`handleDeleteRequest` awaited `onsessionclosed` before `close()` ran, so
`_closed` was still false while the callback was in flight. A second
DELETE for the same session passed `validateSession`/`validateProtocolVersion`
unchanged — neither consults `_closed` — and invoked the callback again for
a session already being torn down.
Claim the notification synchronously before the await. `_closed` cannot
serve this purpose: it is set by `close()`, which only runs once the
callback has settled.
DELETE stays idempotent — a concurrent or repeat request still terminates
the session and answers 200, it just does not re-run the callback.
Refs modelcontextprotocol#2562 (item 2)
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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🦋 Changeset detected
Latest commit: ac6312f
The changes in this PR will be included in the next version bump.
This PR includes changesets to release 6 packages
NameType
@modelcontextprotocol/serverPatch
@modelcontextprotocol/corePatch
@modelcontextprotocol/clientPatch
@modelcontextprotocol/server-legacyPatch
@modelcontextprotocol/codemodPatch
@modelcontextprotocol/core-internalPatch
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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Auto-enable theater mode on YouTube
(function() {
function tryTheater() {
var btn = document.querySelector('button[aria-label="Theater mode"], ytd-player #player button[title="Theater mode"]');
if (btn && !btn.classList.contains('activated')) {
btn.click();
}
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// 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): fire onsessionclosed once when DELETEs overlap by axits-lab · Pull Request #2583 · modelcontextprotocol/typescript-sdk · GitHub
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fix(server): fire onsessionclosed once when DELETEs overlap - #2583
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Refs #2562 — item 2 of three. Items 1 and 3 are not touched here; item 1 will follow separately, and item 3 looks like it needs a call on the EventStore contract first.
Opened as a draft: non-write-access users are capped at one open PR on this repo, and #2582 is currently using it. Ready for review as-is — happy to mark it ready whenever that one lands or on your say-so.
The race
handleDeleteRequest awaits onsessionclosed and only then runs close() from the finally:
try{awaitPromise.resolve(this._onsessionclosed?.(this.sessionId!));returnnewResponse(null,{status: 200});}finally{awaitthis.close();}
_closed is set by close(), so it is still false for the whole time the callback is in flight. Neither validateSession nor validateProtocolVersion consults _closed — they check sessionIdGenerator, _initialized, and the session-id match — so a second DELETE arriving in that window passes every guard and fires the callback a second time for a session already being torn down.
Fix
Claim the notification synchronously, before the await. _closed cannot serve this purpose, since by construction it is only set after the callback has settled — hence the separate flag.
DELETE stays idempotent: a concurrent or repeat request still terminates the session and answers 200, it just does not re-run the callback.
Test
Added to the existing Session Callbacks block. It holds the callback open on a promise the test controls, issues the second DELETE while the first is parked on it, then releases:
× fires onsessionclosed once when two DELETEs arrive concurrently
AssertionError: expected "vi.fn()" to be called 1 times, but got 2 times

That is the failure on main without the src change; it passes with it.
Verification
  • 469/469 tests pass in @modelcontextprotocol/server (53 in streamableHttp.test.ts)
  • pnpm lint and pnpm typecheck clean
  • Grepped the workspace: onsessionclosed is implemented only in packages/server/src/server/streamableHttp.ts, so no sibling transport carries the same window
Changeset included.
You noted the item applies to v1.x as well — happy to put up the equivalent there if you'd like it as a separate PR.
@axits-lab@claude
fix(server): fire onsessionclosed once when DELETEs overlap
ac6312f
`handleDeleteRequest` awaited `onsessionclosed` before `close()` ran, so
`_closed` was still false while the callback was in flight. A second
DELETE for the same session passed `validateSession`/`validateProtocolVersion`
unchanged — neither consults `_closed` — and invoked the callback again for
a session already being torn down.
Claim the notification synchronously before the await. `_closed` cannot
serve this purpose: it is set by `close()`, which only runs once the
callback has settled.
DELETE stays idempotent — a concurrent or repeat request still terminates
the session and answers 200, it just does not re-run the callback.
Refs modelcontextprotocol#2562 (item 2)
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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🦋 Changeset detected
Latest commit: ac6312f
The changes in this PR will be included in the next version bump.
This PR includes changesets to release 6 packages
NameType
@modelcontextprotocol/serverPatch
@modelcontextprotocol/corePatch
@modelcontextprotocol/clientPatch
@modelcontextprotocol/server-legacyPatch
@modelcontextprotocol/codemodPatch
@modelcontextprotocol/core-internalPatch
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@modelcontextprotocol/core
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@modelcontextprotocol/server
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@modelcontextprotocol/server-legacy
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@modelcontextprotocol/express
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@modelcontextprotocol/fastify
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@modelcontextprotocol/hono
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, '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';
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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): fire onsessionclosed once when DELETEs overlap by axits-lab · Pull Request #2583 · modelcontextprotocol/typescript-sdk · GitHub
Skip to content
fix(server): fire onsessionclosed once when DELETEs overlap - #2583
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fix(server): fire onsessionclosed once when DELETEs overlap#2583
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Jul 30, 2026

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Refs #2562 — item 2 of three. Items 1 and 3 are not touched here; item 1 will follow separately, and item 3 looks like it needs a call on the EventStore contract first.
Opened as a draft: non-write-access users are capped at one open PR on this repo, and #2582 is currently using it. Ready for review as-is — happy to mark it ready whenever that one lands or on your say-so.
The race
handleDeleteRequest awaits onsessionclosed and only then runs close() from the finally:
try{awaitPromise.resolve(this._onsessionclosed?.(this.sessionId!));returnnewResponse(null,{status: 200});}finally{awaitthis.close();}
_closed is set by close(), so it is still false for the whole time the callback is in flight. Neither validateSession nor validateProtocolVersion consults _closed — they check sessionIdGenerator, _initialized, and the session-id match — so a second DELETE arriving in that window passes every guard and fires the callback a second time for a session already being torn down.
Fix
Claim the notification synchronously, before the await. _closed cannot serve this purpose, since by construction it is only set after the callback has settled — hence the separate flag.
DELETE stays idempotent: a concurrent or repeat request still terminates the session and answers 200, it just does not re-run the callback.
Test
Added to the existing Session Callbacks block. It holds the callback open on a promise the test controls, issues the second DELETE while the first is parked on it, then releases:
× fires onsessionclosed once when two DELETEs arrive concurrently
AssertionError: expected "vi.fn()" to be called 1 times, but got 2 times

That is the failure on main without the src change; it passes with it.
Verification
  • 469/469 tests pass in @modelcontextprotocol/server (53 in streamableHttp.test.ts)
  • pnpm lint and pnpm typecheck clean
  • Grepped the workspace: onsessionclosed is implemented only in packages/server/src/server/streamableHttp.ts, so no sibling transport carries the same window
Changeset included.
You noted the item applies to v1.x as well — happy to put up the equivalent there if you'd like it as a separate PR.
@axits-lab@claude
fix(server): fire onsessionclosed once when DELETEs overlap
ac6312f
`handleDeleteRequest` awaited `onsessionclosed` before `close()` ran, so
`_closed` was still false while the callback was in flight. A second
DELETE for the same session passed `validateSession`/`validateProtocolVersion`
unchanged — neither consults `_closed` — and invoked the callback again for
a session already being torn down.
Claim the notification synchronously before the await. `_closed` cannot
serve this purpose: it is set by `close()`, which only runs once the
callback has settled.
DELETE stays idempotent — a concurrent or repeat request still terminates
the session and answers 200, it just does not re-run the callback.
Refs modelcontextprotocol#2562 (item 2)
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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🦋 Changeset detected
Latest commit: ac6312f
The changes in this PR will be included in the next version bump.
This PR includes changesets to release 6 packages
NameType
@modelcontextprotocol/serverPatch
@modelcontextprotocol/corePatch
@modelcontextprotocol/clientPatch
@modelcontextprotocol/server-legacyPatch
@modelcontextprotocol/codemodPatch
@modelcontextprotocol/core-internalPatch
Not sure what this means? Click here to learn what changesets are.
Click here if you're a maintainer who wants to add another changeset to this PR
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npm i https://pkg.pr.new/@modelcontextprotocol/client@2583

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npm i https://pkg.pr.new/@modelcontextprotocol/codemod@2583

@modelcontextprotocol/core
npm i https://pkg.pr.new/@modelcontextprotocol/core@2583

@modelcontextprotocol/server
npm i https://pkg.pr.new/@modelcontextprotocol/server@2583

@modelcontextprotocol/server-legacy
npm i https://pkg.pr.new/@modelcontextprotocol/server-legacy@2583

@modelcontextprotocol/express
npm i https://pkg.pr.new/@modelcontextprotocol/express@2583

@modelcontextprotocol/fastify
npm i https://pkg.pr.new/@modelcontextprotocol/fastify@2583

@modelcontextprotocol/hono
npm i https://pkg.pr.new/@modelcontextprotocol/hono@2583

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, '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
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if (!style) {
style = document.createElement('style');
style.id = 'universal-dark-mode-style';
document.head.appendChild(style);
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})();
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fix(server): fire onsessionclosed once when DELETEs overlap by axits-lab · Pull Request #2583 · modelcontextprotocol/typescript-sdk · GitHub
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fix(server): fire onsessionclosed once when DELETEs overlap - #2583
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Refs #2562 — item 2 of three. Items 1 and 3 are not touched here; item 1 will follow separately, and item 3 looks like it needs a call on the EventStore contract first.
Opened as a draft: non-write-access users are capped at one open PR on this repo, and #2582 is currently using it. Ready for review as-is — happy to mark it ready whenever that one lands or on your say-so.
The race
handleDeleteRequest awaits onsessionclosed and only then runs close() from the finally:
try{awaitPromise.resolve(this._onsessionclosed?.(this.sessionId!));returnnewResponse(null,{status: 200});}finally{awaitthis.close();}
_closed is set by close(), so it is still false for the whole time the callback is in flight. Neither validateSession nor validateProtocolVersion consults _closed — they check sessionIdGenerator, _initialized, and the session-id match — so a second DELETE arriving in that window passes every guard and fires the callback a second time for a session already being torn down.
Fix
Claim the notification synchronously, before the await. _closed cannot serve this purpose, since by construction it is only set after the callback has settled — hence the separate flag.
DELETE stays idempotent: a concurrent or repeat request still terminates the session and answers 200, it just does not re-run the callback.
Test
Added to the existing Session Callbacks block. It holds the callback open on a promise the test controls, issues the second DELETE while the first is parked on it, then releases:
× fires onsessionclosed once when two DELETEs arrive concurrently
AssertionError: expected "vi.fn()" to be called 1 times, but got 2 times

That is the failure on main without the src change; it passes with it.
Verification
  • 469/469 tests pass in @modelcontextprotocol/server (53 in streamableHttp.test.ts)
  • pnpm lint and pnpm typecheck clean
  • Grepped the workspace: onsessionclosed is implemented only in packages/server/src/server/streamableHttp.ts, so no sibling transport carries the same window
Changeset included.
You noted the item applies to v1.x as well — happy to put up the equivalent there if you'd like it as a separate PR.
@axits-lab@claude
fix(server): fire onsessionclosed once when DELETEs overlap
ac6312f
`handleDeleteRequest` awaited `onsessionclosed` before `close()` ran, so
`_closed` was still false while the callback was in flight. A second
DELETE for the same session passed `validateSession`/`validateProtocolVersion`
unchanged — neither consults `_closed` — and invoked the callback again for
a session already being torn down.
Claim the notification synchronously before the await. `_closed` cannot
serve this purpose: it is set by `close()`, which only runs once the
callback has settled.
DELETE stays idempotent — a concurrent or repeat request still terminates
the session and answers 200, it just does not re-run the callback.
Refs modelcontextprotocol#2562 (item 2)
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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🦋 Changeset detected
Latest commit: ac6312f
The changes in this PR will be included in the next version bump.
This PR includes changesets to release 6 packages
NameType
@modelcontextprotocol/serverPatch
@modelcontextprotocol/corePatch
@modelcontextprotocol/clientPatch
@modelcontextprotocol/server-legacyPatch
@modelcontextprotocol/codemodPatch
@modelcontextprotocol/core-internalPatch
Not sure what this means? Click here to learn what changesets are.
Click here if you're a maintainer who wants to add another changeset to this PR
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@modelcontextprotocol/client
npm i https://pkg.pr.new/@modelcontextprotocol/client@2583

@modelcontextprotocol/codemod
npm i https://pkg.pr.new/@modelcontextprotocol/codemod@2583

@modelcontextprotocol/core
npm i https://pkg.pr.new/@modelcontextprotocol/core@2583

@modelcontextprotocol/server
npm i https://pkg.pr.new/@modelcontextprotocol/server@2583

@modelcontextprotocol/server-legacy
npm i https://pkg.pr.new/@modelcontextprotocol/server-legacy@2583

@modelcontextprotocol/express
npm i https://pkg.pr.new/@modelcontextprotocol/express@2583

@modelcontextprotocol/fastify
npm i https://pkg.pr.new/@modelcontextprotocol/fastify@2583

@modelcontextprotocol/hono
npm i https://pkg.pr.new/@modelcontextprotocol/hono@2583

@modelcontextprotocol/node
npm i https://pkg.pr.new/@modelcontextprotocol/node@2583

commit: ac6312f

This was referenced Jul 30, 2026

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Ideas, requests and plans for v2 of the SDK which will incorporate major changes and fixes
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