fix(client): initialize requests never carry a session id; capture only from the initialize response#2469

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felixweinberger merged 3 commits into from
Jul 9, 2026
Merged

fix(client): initialize requests never carry a session id; capture only from the initialize response #2469

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fix(client): initialize requests never carry a session id; capture on…
felixweinberger Jul 9, 2026
3b3dc83
fix(client): sessionless initialize response clears any stale session id
felixweinberger Jul 9, 2026
c4f3a59
Merge branch 'main' into fweinberger/initialize-session-hygiene
felixweinberger Jul 9, 2026
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5 changes: 5 additions & 0 deletions .changeset/initialize-session-hygiene.md
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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@modelcontextprotocol/client': patch
---

The Streamable HTTP client transport no longer attaches a session ID to a POST containing an `initialize` request — a new session starts "without a session ID attached" (2025-11-25 transports §Session Management) — and it only captures the `mcp-session-id` response header from a successful initialize response, since the spec assigns the session ID "at initialization time … on the HTTP response containing the InitializeResult". Previously the transport stored the header from any response, so a legacy server answering a protocol-version probe with an error that happened to carry a session ID would poison the fallback initialize, which then went out with a session ID it should not have had. A stale session ID from a previous connection is likewise no longer leaked onto the initialize handshake, and a successful initialize response that carries no session ID now clears any stale ID the transport was holding — clients include only an ID "returned by the server during initialization", so an ID the server never returned this session is outside the session model. Ignoring `mcp-session-id` headers mid-session is the complement of the spec's one actual rotation mechanism: a server that wants a new session terminates the old one (it "MAY terminate the session at any time") and answers 404, after which the client "MUST start a new session by sending a new InitializeRequest without a session ID attached". Rotation exists as session replacement via 404 + re-initialize, never as a header swap on a live session, so a server that rotates per the spec's own flow is handled correctly by this transport.
14 changes: 10 additions & 4 deletions packages/client/src/client/streamableHttp.ts
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Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,7 @@ import {
createFetchWithInit,
encodeMcpParamValue,
isInitializedNotification,
isInitializeRequest,
isJSONRPCErrorResponse,
isJSONRPCRequest,
isJSONRPCResultResponse,
Expand DownExpand Up@@ -936,6 +937,11 @@ export class StreamableHTTPClientTransport implements Transport {

const headers = await this._commonHeaders();
this._applyBodyDerivedHeaders(headers, message);
// A new session starts "without a session ID attached" (2025-11-25 transports §Session Management).
const isHandshake = Array.isArray(message) ? message.some(m => isInitializeRequest(m)) : isInitializeRequest(message);
if (isHandshake) {
headers.delete('mcp-session-id');
}
// Per-request additional headers (the Client passes SEP-2243
// `Mcp-Param-*` here on a 2026-07-28 connection). Reserved
// standard/auth header names are skipped so a caller cannot
Expand DownExpand Up@@ -973,10 +979,10 @@ export class StreamableHTTPClientTransport implements Transport {

const response = await (this._fetch ?? fetch)(this._url, init);

// Handle session ID received during initialization
const sessionId = response.headers.get('mcp-session-id');
if (sessionId) {
this._sessionId = sessionId;
// The spec assigns the session id "at initialization time … on the HTTP response containing the InitializeResult"; it is ignored everywhere else.
// Clients include only an id "returned by the server during initialization", so a sessionless handshake clears any stale id.
if (isHandshake && response.ok) {
this._sessionId = response.headers.get('mcp-session-id') || undefined;
}

if (!response.ok) {
Expand Down
120 changes: 120 additions & 0 deletions packages/client/test/client/streamableHttp.test.ts
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Original file line numberDiff line numberDiff line change
Expand Up@@ -93,6 +93,7 @@ describe('StreamableHTTPClientTransport', () => {
method: 'initialize',
params: {
clientInfo: { name: 'test-client', version: '1.0' },
capabilities: {},
protocolVersion: '2025-03-26'
},
id: 'init-id'
Expand DownExpand Up@@ -122,6 +123,123 @@ describe('StreamableHTTPClientTransport', () => {
expect(lastCall[1].headers.get('mcp-session-id')).toBe('test-session-id');
});

it('should not store session ID from an error response, then store it from a later successful initialize', async () => {
const message: JSONRPCMessage = {
jsonrpc: '2.0',
method: 'initialize',
params: {
clientInfo: { name: 'test-client', version: '1.0' },
capabilities: {},
protocolVersion: '2025-03-26'
},
id: 'init-id'
};

// A failed initialize (e.g. a legacy server rejecting a version probe) that carries a session ID
(globalThis.fetch as Mock).mockResolvedValueOnce({
ok: false,
status: 400,
statusText: 'Bad Request',
text: () => Promise.resolve('Bad Request'),
headers: new Headers({ 'mcp-session-id': 'poisoned-session-id' })
});

await expect(transport.send(message)).rejects.toThrow();
expect(transport.sessionId).toBeUndefined();

// The fallback initialize succeeds and its session ID is captured
(globalThis.fetch as Mock).mockResolvedValueOnce({
ok: true,
status: 200,
headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'real-session-id' })
});

await transport.send(message);
expect(transport.sessionId).toBe('real-session-id');
});

it('should not attach a session ID to an initialize POST, clear a stale ID on a sessionless handshake, and adopt a newly returned one', async () => {
const initMessage: JSONRPCMessage = {
jsonrpc: '2.0',
method: 'initialize',
params: {
clientInfo: { name: 'test-client', version: '1.0' },
capabilities: {},
protocolVersion: '2025-03-26'
},
id: 'init-id'
};

const staleTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
sessionId: 'stale-session-id'
});

// Sessionless handshake: the response carries no session ID
(globalThis.fetch as Mock).mockResolvedValueOnce({
ok: true,
status: 200,
headers: new Headers({ 'content-type': 'text/event-stream' })
});

await staleTransport.send(initMessage);

const initCall = (globalThis.fetch as Mock).mock.calls.at(-1)!;
expect(initCall[1].headers.get('mcp-session-id')).toBeNull();

// The sessionless handshake cleared the stale ID, so an ordinary request carries none
(globalThis.fetch as Mock).mockResolvedValueOnce({
ok: true,
status: 202,
headers: new Headers()
});

await staleTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBeNull();

await staleTransport.close().catch(() => {});

// When the handshake DOES return a new ID, subsequent requests carry it instead of the preset
const replacedTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
sessionId: 'preset-session-id'
});

(globalThis.fetch as Mock).mockResolvedValueOnce({
ok: true,
status: 200,
headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'new-session-id' })
});

await replacedTransport.send(initMessage);

(globalThis.fetch as Mock).mockResolvedValueOnce({
ok: true,
status: 202,
headers: new Headers()
});

await replacedTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBe('new-session-id');

await replacedTransport.close().catch(() => {});
});

it('should ignore a session ID on a successful non-initialize response', async () => {
const sessionTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
sessionId: 'session-a'
});

(globalThis.fetch as Mock).mockResolvedValueOnce({
ok: true,
status: 202,
headers: new Headers({ 'mcp-session-id': 'session-b' })
});

await sessionTransport.send({ jsonrpc: '2.0', method: 'notifications/roots/list_changed' } as JSONRPCMessage);
expect(sessionTransport.sessionId).toBe('session-a');

await sessionTransport.close().catch(() => {});
});

it('should accept protocolVersion constructor option and include it in request headers', async () => {
// When reconnecting with a preserved sessionId, users need to also preserve the
// negotiated protocol version so the required mcp-protocol-version header is sent.
Expand DownExpand Up@@ -156,6 +274,7 @@ describe('StreamableHTTPClientTransport', () => {
method: 'initialize',
params: {
clientInfo: { name: 'test-client', version: '1.0' },
capabilities: {},
protocolVersion: '2025-03-26'
},
id: 'init-id'
Expand DownExpand Up@@ -196,6 +315,7 @@ describe('StreamableHTTPClientTransport', () => {
method: 'initialize',
params: {
clientInfo: { name: 'test-client', version: '1.0' },
capabilities: {},
protocolVersion: '2025-03-26'
},
id: 'init-id'
Expand Down
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if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;
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})();
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var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '

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fix(client): initialize requests never carry a session id; capture only from the initialize response#2469

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 Merged

felixweinberger
merged 3 commits into

from

Jul 9, 2026
 Merged


fix(client): initialize requests never carry a session id; capture only from the initialize response
#2469
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e743f40

fix(client): initialize requests never carry a session id; capture on…


felixweinberger Jul 9, 2026
3b3dc83

fix(client): sessionless initialize response clears any stale session id


felixweinberger Jul 9, 2026
c4f3a59

Merge branch 'main' into fweinberger/initialize-session-hygiene


felixweinberger Jul 9, 2026

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5 changes: 5 additions & 0 deletions
.changeset/initialize-session-hygiene.md

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

The Streamable HTTP client transport no longer attaches a session ID to a POST containing an `initialize` request — a new session starts "without a session ID attached" (2025-11-25 transports §Session Management) — and it only captures the `mcp-session-id` response header from a successful initialize response, since the spec assigns the session ID "at initialization time … on the HTTP response containing the InitializeResult". Previously the transport stored the header from any response, so a legacy server answering a protocol-version probe with an error that happened to carry a session ID would poison the fallback initialize, which then went out with a session ID it should not have had. A stale session ID from a previous connection is likewise no longer leaked onto the initialize handshake, and a successful initialize response that carries no session ID now clears any stale ID the transport was holding — clients include only an ID "returned by the server during initialization", so an ID the server never returned this session is outside the session model. Ignoring `mcp-session-id` headers mid-session is the complement of the spec's one actual rotation mechanism: a server that wants a new session terminates the old one (it "MAY terminate the session at any time") and answers 404, after which the client "MUST start a new session by sending a new InitializeRequest without a session ID attached". Rotation exists as session replacement via 404 + re-initialize, never as a header swap on a live session, so a server that rotates per the spec's own flow is handled correctly by this transport.

14 changes: 10 additions & 4 deletions
packages/client/src/client/streamableHttp.ts

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,7 @@ import {
 createFetchWithInit,
 encodeMcpParamValue,
 isInitializedNotification,
 isInitializeRequest,
 isJSONRPCErrorResponse,
 isJSONRPCRequest,
 isJSONRPCResultResponse,
Expand DownExpand Up@@ -936,6 +937,11 @@ export class StreamableHTTPClientTransport implements Transport {

 const headers = await this._commonHeaders();
 this._applyBodyDerivedHeaders(headers, message);
 // A new session starts "without a session ID attached" (2025-11-25 transports §Session Management).
 const isHandshake = Array.isArray(message) ? message.some(m => isInitializeRequest(m)) : isInitializeRequest(message);
 if (isHandshake) {
 headers.delete('mcp-session-id');
 }
 // Per-request additional headers (the Client passes SEP-2243
 // `Mcp-Param-*` here on a 2026-07-28 connection). Reserved
 // standard/auth header names are skipped so a caller cannot
Expand DownExpand Up@@ -973,10 +979,10 @@ export class StreamableHTTPClientTransport implements Transport {

 const response = await (this._fetch ?? fetch)(this._url, init);

 // Handle session ID received during initialization
const sessionId = response.headers.get('mcp-session-id');
 if (sessionId) {
 this._sessionId = sessionId;
 // The spec assigns the session id "at initialization time … on the HTTP response containing the InitializeResult"; it is ignored everywhere else.
// Clients include only an id "returned by the server during initialization", so a sessionless handshake clears any stale id.
 if (isHandshake && response.ok) {
 this._sessionId = response.headers.get('mcp-session-id') || undefined;
 }

 if (!response.ok) {
Expand Down

120 changes: 120 additions & 0 deletions
packages/client/test/client/streamableHttp.test.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -93,6 +93,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -122,6 +123,123 @@ describe('StreamableHTTPClientTransport', () => {
 expect(lastCall[1].headers.get('mcp-session-id')).toBe('test-session-id');
 });

 it('should not store session ID from an error response, then store it from a later successful initialize', async () => {
 const message: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 // A failed initialize (e.g. a legacy server rejecting a version probe) that carries a session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: false,
 status: 400,
 statusText: 'Bad Request',
 text: () => Promise.resolve('Bad Request'),
 headers: new Headers({ 'mcp-session-id': 'poisoned-session-id' })
 });

 await expect(transport.send(message)).rejects.toThrow();
 expect(transport.sessionId).toBeUndefined();

 // The fallback initialize succeeds and its session ID is captured
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'real-session-id' })
 });

 await transport.send(message);
 expect(transport.sessionId).toBe('real-session-id');
 });

 it('should not attach a session ID to an initialize POST, clear a stale ID on a sessionless handshake, and adopt a newly returned one', async () => {
 const initMessage: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 const staleTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'stale-session-id'
 });

 // Sessionless handshake: the response carries no session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream' })
 });

 await staleTransport.send(initMessage);

 const initCall = (globalThis.fetch as Mock).mock.calls.at(-1)!;
 expect(initCall[1].headers.get('mcp-session-id')).toBeNull();

 // The sessionless handshake cleared the stale ID, so an ordinary request carries none
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await staleTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBeNull();

 await staleTransport.close().catch(() => {});

 // When the handshake DOES return a new ID, subsequent requests carry it instead of the preset
 const replacedTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'preset-session-id'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'new-session-id' })
 });

 await replacedTransport.send(initMessage);

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await replacedTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBe('new-session-id');

 await replacedTransport.close().catch(() => {});
 });

 it('should ignore a session ID on a successful non-initialize response', async () => {
 const sessionTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'session-a'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers({ 'mcp-session-id': 'session-b' })
 });

 await sessionTransport.send({ jsonrpc: '2.0', method: 'notifications/roots/list_changed' } as JSONRPCMessage);
 expect(sessionTransport.sessionId).toBe('session-a');

 await sessionTransport.close().catch(() => {});
 });

 it('should accept protocolVersion constructor option and include it in request headers', async () => {
 // When reconnecting with a preserved sessionId, users need to also preserve the
 // negotiated protocol version so the required mcp-protocol-version header is sent.
Expand DownExpand Up@@ -156,6 +274,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -196,6 +315,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand Down
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var __re = new RegExp('^' + ".*" + '

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fix(client): initialize requests never carry a session id; capture only from the initialize response#2469

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 Merged

felixweinberger
merged 3 commits into

from

Jul 9, 2026
 Merged


fix(client): initialize requests never carry a session id; capture only from the initialize response
#2469
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e743f40

fix(client): initialize requests never carry a session id; capture on…


felixweinberger Jul 9, 2026
3b3dc83

fix(client): sessionless initialize response clears any stale session id


felixweinberger Jul 9, 2026
c4f3a59

Merge branch 'main' into fweinberger/initialize-session-hygiene


felixweinberger Jul 9, 2026

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5 changes: 5 additions & 0 deletions
.changeset/initialize-session-hygiene.md

View file


Open in desktop

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

The Streamable HTTP client transport no longer attaches a session ID to a POST containing an `initialize` request — a new session starts "without a session ID attached" (2025-11-25 transports §Session Management) — and it only captures the `mcp-session-id` response header from a successful initialize response, since the spec assigns the session ID "at initialization time … on the HTTP response containing the InitializeResult". Previously the transport stored the header from any response, so a legacy server answering a protocol-version probe with an error that happened to carry a session ID would poison the fallback initialize, which then went out with a session ID it should not have had. A stale session ID from a previous connection is likewise no longer leaked onto the initialize handshake, and a successful initialize response that carries no session ID now clears any stale ID the transport was holding — clients include only an ID "returned by the server during initialization", so an ID the server never returned this session is outside the session model. Ignoring `mcp-session-id` headers mid-session is the complement of the spec's one actual rotation mechanism: a server that wants a new session terminates the old one (it "MAY terminate the session at any time") and answers 404, after which the client "MUST start a new session by sending a new InitializeRequest without a session ID attached". Rotation exists as session replacement via 404 + re-initialize, never as a header swap on a live session, so a server that rotates per the spec's own flow is handled correctly by this transport.

14 changes: 10 additions & 4 deletions
packages/client/src/client/streamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,7 @@ import {
 createFetchWithInit,
 encodeMcpParamValue,
 isInitializedNotification,
 isInitializeRequest,
 isJSONRPCErrorResponse,
 isJSONRPCRequest,
 isJSONRPCResultResponse,
Expand DownExpand Up@@ -936,6 +937,11 @@ export class StreamableHTTPClientTransport implements Transport {

 const headers = await this._commonHeaders();
 this._applyBodyDerivedHeaders(headers, message);
 // A new session starts "without a session ID attached" (2025-11-25 transports §Session Management).
 const isHandshake = Array.isArray(message) ? message.some(m => isInitializeRequest(m)) : isInitializeRequest(message);
 if (isHandshake) {
 headers.delete('mcp-session-id');
 }
 // Per-request additional headers (the Client passes SEP-2243
 // `Mcp-Param-*` here on a 2026-07-28 connection). Reserved
 // standard/auth header names are skipped so a caller cannot
Expand DownExpand Up@@ -973,10 +979,10 @@ export class StreamableHTTPClientTransport implements Transport {

 const response = await (this._fetch ?? fetch)(this._url, init);

 // Handle session ID received during initialization
const sessionId = response.headers.get('mcp-session-id');
 if (sessionId) {
 this._sessionId = sessionId;
 // The spec assigns the session id "at initialization time … on the HTTP response containing the InitializeResult"; it is ignored everywhere else.
// Clients include only an id "returned by the server during initialization", so a sessionless handshake clears any stale id.
 if (isHandshake && response.ok) {
 this._sessionId = response.headers.get('mcp-session-id') || undefined;
 }

 if (!response.ok) {
Expand Down

120 changes: 120 additions & 0 deletions
packages/client/test/client/streamableHttp.test.ts

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -93,6 +93,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -122,6 +123,123 @@ describe('StreamableHTTPClientTransport', () => {
 expect(lastCall[1].headers.get('mcp-session-id')).toBe('test-session-id');
 });

 it('should not store session ID from an error response, then store it from a later successful initialize', async () => {
 const message: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 // A failed initialize (e.g. a legacy server rejecting a version probe) that carries a session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: false,
 status: 400,
 statusText: 'Bad Request',
 text: () => Promise.resolve('Bad Request'),
 headers: new Headers({ 'mcp-session-id': 'poisoned-session-id' })
 });

 await expect(transport.send(message)).rejects.toThrow();
 expect(transport.sessionId).toBeUndefined();

 // The fallback initialize succeeds and its session ID is captured
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'real-session-id' })
 });

 await transport.send(message);
 expect(transport.sessionId).toBe('real-session-id');
 });

 it('should not attach a session ID to an initialize POST, clear a stale ID on a sessionless handshake, and adopt a newly returned one', async () => {
 const initMessage: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 const staleTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'stale-session-id'
 });

 // Sessionless handshake: the response carries no session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream' })
 });

 await staleTransport.send(initMessage);

 const initCall = (globalThis.fetch as Mock).mock.calls.at(-1)!;
 expect(initCall[1].headers.get('mcp-session-id')).toBeNull();

 // The sessionless handshake cleared the stale ID, so an ordinary request carries none
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await staleTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBeNull();

 await staleTransport.close().catch(() => {});

 // When the handshake DOES return a new ID, subsequent requests carry it instead of the preset
 const replacedTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'preset-session-id'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'new-session-id' })
 });

 await replacedTransport.send(initMessage);

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await replacedTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBe('new-session-id');

 await replacedTransport.close().catch(() => {});
 });

 it('should ignore a session ID on a successful non-initialize response', async () => {
 const sessionTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'session-a'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers({ 'mcp-session-id': 'session-b' })
 });

 await sessionTransport.send({ jsonrpc: '2.0', method: 'notifications/roots/list_changed' } as JSONRPCMessage);
 expect(sessionTransport.sessionId).toBe('session-a');

 await sessionTransport.close().catch(() => {});
 });

 it('should accept protocolVersion constructor option and include it in request headers', async () => {
 // When reconnecting with a preserved sessionId, users need to also preserve the
 // negotiated protocol version so the required mcp-protocol-version header is sent.
Expand DownExpand Up@@ -156,6 +274,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -196,6 +315,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand Down
Loading

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

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fix(client): initialize requests never carry a session id; capture only from the initialize response#2469

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 Merged

felixweinberger
merged 3 commits into

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Jul 9, 2026
 Merged


fix(client): initialize requests never carry a session id; capture only from the initialize response
#2469
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e743f40

fix(client): initialize requests never carry a session id; capture on…


felixweinberger Jul 9, 2026
3b3dc83

fix(client): sessionless initialize response clears any stale session id


felixweinberger Jul 9, 2026
c4f3a59

Merge branch 'main' into fweinberger/initialize-session-hygiene


felixweinberger Jul 9, 2026

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5 changes: 5 additions & 0 deletions
.changeset/initialize-session-hygiene.md

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

The Streamable HTTP client transport no longer attaches a session ID to a POST containing an `initialize` request — a new session starts "without a session ID attached" (2025-11-25 transports §Session Management) — and it only captures the `mcp-session-id` response header from a successful initialize response, since the spec assigns the session ID "at initialization time … on the HTTP response containing the InitializeResult". Previously the transport stored the header from any response, so a legacy server answering a protocol-version probe with an error that happened to carry a session ID would poison the fallback initialize, which then went out with a session ID it should not have had. A stale session ID from a previous connection is likewise no longer leaked onto the initialize handshake, and a successful initialize response that carries no session ID now clears any stale ID the transport was holding — clients include only an ID "returned by the server during initialization", so an ID the server never returned this session is outside the session model. Ignoring `mcp-session-id` headers mid-session is the complement of the spec's one actual rotation mechanism: a server that wants a new session terminates the old one (it "MAY terminate the session at any time") and answers 404, after which the client "MUST start a new session by sending a new InitializeRequest without a session ID attached". Rotation exists as session replacement via 404 + re-initialize, never as a header swap on a live session, so a server that rotates per the spec's own flow is handled correctly by this transport.

14 changes: 10 additions & 4 deletions
packages/client/src/client/streamableHttp.ts

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,7 @@ import {
 createFetchWithInit,
 encodeMcpParamValue,
 isInitializedNotification,
 isInitializeRequest,
 isJSONRPCErrorResponse,
 isJSONRPCRequest,
 isJSONRPCResultResponse,
Expand DownExpand Up@@ -936,6 +937,11 @@ export class StreamableHTTPClientTransport implements Transport {

 const headers = await this._commonHeaders();
 this._applyBodyDerivedHeaders(headers, message);
 // A new session starts "without a session ID attached" (2025-11-25 transports §Session Management).
 const isHandshake = Array.isArray(message) ? message.some(m => isInitializeRequest(m)) : isInitializeRequest(message);
 if (isHandshake) {
 headers.delete('mcp-session-id');
 }
 // Per-request additional headers (the Client passes SEP-2243
 // `Mcp-Param-*` here on a 2026-07-28 connection). Reserved
 // standard/auth header names are skipped so a caller cannot
Expand DownExpand Up@@ -973,10 +979,10 @@ export class StreamableHTTPClientTransport implements Transport {

 const response = await (this._fetch ?? fetch)(this._url, init);

 // Handle session ID received during initialization
const sessionId = response.headers.get('mcp-session-id');
 if (sessionId) {
 this._sessionId = sessionId;
 // The spec assigns the session id "at initialization time … on the HTTP response containing the InitializeResult"; it is ignored everywhere else.
// Clients include only an id "returned by the server during initialization", so a sessionless handshake clears any stale id.
 if (isHandshake && response.ok) {
 this._sessionId = response.headers.get('mcp-session-id') || undefined;
 }

 if (!response.ok) {
Expand Down

120 changes: 120 additions & 0 deletions
packages/client/test/client/streamableHttp.test.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -93,6 +93,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -122,6 +123,123 @@ describe('StreamableHTTPClientTransport', () => {
 expect(lastCall[1].headers.get('mcp-session-id')).toBe('test-session-id');
 });

 it('should not store session ID from an error response, then store it from a later successful initialize', async () => {
 const message: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 // A failed initialize (e.g. a legacy server rejecting a version probe) that carries a session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: false,
 status: 400,
 statusText: 'Bad Request',
 text: () => Promise.resolve('Bad Request'),
 headers: new Headers({ 'mcp-session-id': 'poisoned-session-id' })
 });

 await expect(transport.send(message)).rejects.toThrow();
 expect(transport.sessionId).toBeUndefined();

 // The fallback initialize succeeds and its session ID is captured
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'real-session-id' })
 });

 await transport.send(message);
 expect(transport.sessionId).toBe('real-session-id');
 });

 it('should not attach a session ID to an initialize POST, clear a stale ID on a sessionless handshake, and adopt a newly returned one', async () => {
 const initMessage: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 const staleTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'stale-session-id'
 });

 // Sessionless handshake: the response carries no session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream' })
 });

 await staleTransport.send(initMessage);

 const initCall = (globalThis.fetch as Mock).mock.calls.at(-1)!;
 expect(initCall[1].headers.get('mcp-session-id')).toBeNull();

 // The sessionless handshake cleared the stale ID, so an ordinary request carries none
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await staleTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBeNull();

 await staleTransport.close().catch(() => {});

 // When the handshake DOES return a new ID, subsequent requests carry it instead of the preset
 const replacedTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'preset-session-id'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'new-session-id' })
 });

 await replacedTransport.send(initMessage);

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await replacedTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBe('new-session-id');

 await replacedTransport.close().catch(() => {});
 });

 it('should ignore a session ID on a successful non-initialize response', async () => {
 const sessionTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'session-a'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers({ 'mcp-session-id': 'session-b' })
 });

 await sessionTransport.send({ jsonrpc: '2.0', method: 'notifications/roots/list_changed' } as JSONRPCMessage);
 expect(sessionTransport.sessionId).toBe('session-a');

 await sessionTransport.close().catch(() => {});
 });

 it('should accept protocolVersion constructor option and include it in request headers', async () => {
 // When reconnecting with a preserved sessionId, users need to also preserve the
 // negotiated protocol version so the required mcp-protocol-version header is sent.
Expand DownExpand Up@@ -156,6 +274,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -196,6 +315,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand Down
Loading

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

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fix(client): initialize requests never carry a session id; capture only from the initialize response#2469

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 Merged

felixweinberger
merged 3 commits into

from

Jul 9, 2026
 Merged


fix(client): initialize requests never carry a session id; capture only from the initialize response
#2469
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e743f40

fix(client): initialize requests never carry a session id; capture on…


felixweinberger Jul 9, 2026
3b3dc83

fix(client): sessionless initialize response clears any stale session id


felixweinberger Jul 9, 2026
c4f3a59

Merge branch 'main' into fweinberger/initialize-session-hygiene


felixweinberger Jul 9, 2026

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5 changes: 5 additions & 0 deletions
.changeset/initialize-session-hygiene.md

View file


Open in desktop

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

The Streamable HTTP client transport no longer attaches a session ID to a POST containing an `initialize` request — a new session starts "without a session ID attached" (2025-11-25 transports §Session Management) — and it only captures the `mcp-session-id` response header from a successful initialize response, since the spec assigns the session ID "at initialization time … on the HTTP response containing the InitializeResult". Previously the transport stored the header from any response, so a legacy server answering a protocol-version probe with an error that happened to carry a session ID would poison the fallback initialize, which then went out with a session ID it should not have had. A stale session ID from a previous connection is likewise no longer leaked onto the initialize handshake, and a successful initialize response that carries no session ID now clears any stale ID the transport was holding — clients include only an ID "returned by the server during initialization", so an ID the server never returned this session is outside the session model. Ignoring `mcp-session-id` headers mid-session is the complement of the spec's one actual rotation mechanism: a server that wants a new session terminates the old one (it "MAY terminate the session at any time") and answers 404, after which the client "MUST start a new session by sending a new InitializeRequest without a session ID attached". Rotation exists as session replacement via 404 + re-initialize, never as a header swap on a live session, so a server that rotates per the spec's own flow is handled correctly by this transport.

14 changes: 10 additions & 4 deletions
packages/client/src/client/streamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,7 @@ import {
 createFetchWithInit,
 encodeMcpParamValue,
 isInitializedNotification,
 isInitializeRequest,
 isJSONRPCErrorResponse,
 isJSONRPCRequest,
 isJSONRPCResultResponse,
Expand DownExpand Up@@ -936,6 +937,11 @@ export class StreamableHTTPClientTransport implements Transport {

 const headers = await this._commonHeaders();
 this._applyBodyDerivedHeaders(headers, message);
 // A new session starts "without a session ID attached" (2025-11-25 transports §Session Management).
 const isHandshake = Array.isArray(message) ? message.some(m => isInitializeRequest(m)) : isInitializeRequest(message);
 if (isHandshake) {
 headers.delete('mcp-session-id');
 }
 // Per-request additional headers (the Client passes SEP-2243
 // `Mcp-Param-*` here on a 2026-07-28 connection). Reserved
 // standard/auth header names are skipped so a caller cannot
Expand DownExpand Up@@ -973,10 +979,10 @@ export class StreamableHTTPClientTransport implements Transport {

 const response = await (this._fetch ?? fetch)(this._url, init);

 // Handle session ID received during initialization
const sessionId = response.headers.get('mcp-session-id');
 if (sessionId) {
 this._sessionId = sessionId;
 // The spec assigns the session id "at initialization time … on the HTTP response containing the InitializeResult"; it is ignored everywhere else.
// Clients include only an id "returned by the server during initialization", so a sessionless handshake clears any stale id.
 if (isHandshake && response.ok) {
 this._sessionId = response.headers.get('mcp-session-id') || undefined;
 }

 if (!response.ok) {
Expand Down

120 changes: 120 additions & 0 deletions
packages/client/test/client/streamableHttp.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -93,6 +93,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -122,6 +123,123 @@ describe('StreamableHTTPClientTransport', () => {
 expect(lastCall[1].headers.get('mcp-session-id')).toBe('test-session-id');
 });

 it('should not store session ID from an error response, then store it from a later successful initialize', async () => {
 const message: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 // A failed initialize (e.g. a legacy server rejecting a version probe) that carries a session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: false,
 status: 400,
 statusText: 'Bad Request',
 text: () => Promise.resolve('Bad Request'),
 headers: new Headers({ 'mcp-session-id': 'poisoned-session-id' })
 });

 await expect(transport.send(message)).rejects.toThrow();
 expect(transport.sessionId).toBeUndefined();

 // The fallback initialize succeeds and its session ID is captured
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'real-session-id' })
 });

 await transport.send(message);
 expect(transport.sessionId).toBe('real-session-id');
 });

 it('should not attach a session ID to an initialize POST, clear a stale ID on a sessionless handshake, and adopt a newly returned one', async () => {
 const initMessage: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 const staleTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'stale-session-id'
 });

 // Sessionless handshake: the response carries no session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream' })
 });

 await staleTransport.send(initMessage);

 const initCall = (globalThis.fetch as Mock).mock.calls.at(-1)!;
 expect(initCall[1].headers.get('mcp-session-id')).toBeNull();

 // The sessionless handshake cleared the stale ID, so an ordinary request carries none
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await staleTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBeNull();

 await staleTransport.close().catch(() => {});

 // When the handshake DOES return a new ID, subsequent requests carry it instead of the preset
 const replacedTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'preset-session-id'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'new-session-id' })
 });

 await replacedTransport.send(initMessage);

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await replacedTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBe('new-session-id');

 await replacedTransport.close().catch(() => {});
 });

 it('should ignore a session ID on a successful non-initialize response', async () => {
 const sessionTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'session-a'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers({ 'mcp-session-id': 'session-b' })
 });

 await sessionTransport.send({ jsonrpc: '2.0', method: 'notifications/roots/list_changed' } as JSONRPCMessage);
 expect(sessionTransport.sessionId).toBe('session-a');

 await sessionTransport.close().catch(() => {});
 });

 it('should accept protocolVersion constructor option and include it in request headers', async () => {
 // When reconnecting with a preserved sessionId, users need to also preserve the
 // negotiated protocol version so the required mcp-protocol-version header is sent.
Expand DownExpand Up@@ -156,6 +274,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -196,6 +315,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand Down
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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Auto-enable theater mode on YouTube
(function() {
function tryTheater() {
var btn = document.querySelector('button[aria-label="Theater mode"], ytd-player #player button[title="Theater mode"]');
if (btn && !btn.classList.contains('activated')) {
btn.click();
}
}
// Try immediately
tryTheater();
// Try after navigation (SPA)
var lastUrl = location.href;
setInterval(function() {
if (location.href !== lastUrl) {
lastUrl = location.href;
setTimeout(tryTheater, 500);
}
}, 1000);
// Also try on player load
var observer = new MutationObserver(tryTheater);
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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fix(client): initialize requests never carry a session id; capture only from the initialize response#2469

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felixweinberger
merged 3 commits into

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Jul 9, 2026
 Merged


fix(client): initialize requests never carry a session id; capture only from the initialize response
#2469
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e743f40

fix(client): initialize requests never carry a session id; capture on…


felixweinberger Jul 9, 2026
3b3dc83

fix(client): sessionless initialize response clears any stale session id


felixweinberger Jul 9, 2026
c4f3a59

Merge branch 'main' into fweinberger/initialize-session-hygiene


felixweinberger Jul 9, 2026

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5 changes: 5 additions & 0 deletions
.changeset/initialize-session-hygiene.md

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

The Streamable HTTP client transport no longer attaches a session ID to a POST containing an `initialize` request — a new session starts "without a session ID attached" (2025-11-25 transports §Session Management) — and it only captures the `mcp-session-id` response header from a successful initialize response, since the spec assigns the session ID "at initialization time … on the HTTP response containing the InitializeResult". Previously the transport stored the header from any response, so a legacy server answering a protocol-version probe with an error that happened to carry a session ID would poison the fallback initialize, which then went out with a session ID it should not have had. A stale session ID from a previous connection is likewise no longer leaked onto the initialize handshake, and a successful initialize response that carries no session ID now clears any stale ID the transport was holding — clients include only an ID "returned by the server during initialization", so an ID the server never returned this session is outside the session model. Ignoring `mcp-session-id` headers mid-session is the complement of the spec's one actual rotation mechanism: a server that wants a new session terminates the old one (it "MAY terminate the session at any time") and answers 404, after which the client "MUST start a new session by sending a new InitializeRequest without a session ID attached". Rotation exists as session replacement via 404 + re-initialize, never as a header swap on a live session, so a server that rotates per the spec's own flow is handled correctly by this transport.

14 changes: 10 additions & 4 deletions
packages/client/src/client/streamableHttp.ts

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,7 @@ import {
 createFetchWithInit,
 encodeMcpParamValue,
 isInitializedNotification,
 isInitializeRequest,
 isJSONRPCErrorResponse,
 isJSONRPCRequest,
 isJSONRPCResultResponse,
Expand DownExpand Up@@ -936,6 +937,11 @@ export class StreamableHTTPClientTransport implements Transport {

 const headers = await this._commonHeaders();
 this._applyBodyDerivedHeaders(headers, message);
 // A new session starts "without a session ID attached" (2025-11-25 transports §Session Management).
 const isHandshake = Array.isArray(message) ? message.some(m => isInitializeRequest(m)) : isInitializeRequest(message);
 if (isHandshake) {
 headers.delete('mcp-session-id');
 }
 // Per-request additional headers (the Client passes SEP-2243
 // `Mcp-Param-*` here on a 2026-07-28 connection). Reserved
 // standard/auth header names are skipped so a caller cannot
Expand DownExpand Up@@ -973,10 +979,10 @@ export class StreamableHTTPClientTransport implements Transport {

 const response = await (this._fetch ?? fetch)(this._url, init);

 // Handle session ID received during initialization
const sessionId = response.headers.get('mcp-session-id');
 if (sessionId) {
 this._sessionId = sessionId;
 // The spec assigns the session id "at initialization time … on the HTTP response containing the InitializeResult"; it is ignored everywhere else.
// Clients include only an id "returned by the server during initialization", so a sessionless handshake clears any stale id.
 if (isHandshake && response.ok) {
 this._sessionId = response.headers.get('mcp-session-id') || undefined;
 }

 if (!response.ok) {
Expand Down

120 changes: 120 additions & 0 deletions
packages/client/test/client/streamableHttp.test.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -93,6 +93,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -122,6 +123,123 @@ describe('StreamableHTTPClientTransport', () => {
 expect(lastCall[1].headers.get('mcp-session-id')).toBe('test-session-id');
 });

 it('should not store session ID from an error response, then store it from a later successful initialize', async () => {
 const message: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 // A failed initialize (e.g. a legacy server rejecting a version probe) that carries a session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: false,
 status: 400,
 statusText: 'Bad Request',
 text: () => Promise.resolve('Bad Request'),
 headers: new Headers({ 'mcp-session-id': 'poisoned-session-id' })
 });

 await expect(transport.send(message)).rejects.toThrow();
 expect(transport.sessionId).toBeUndefined();

 // The fallback initialize succeeds and its session ID is captured
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'real-session-id' })
 });

 await transport.send(message);
 expect(transport.sessionId).toBe('real-session-id');
 });

 it('should not attach a session ID to an initialize POST, clear a stale ID on a sessionless handshake, and adopt a newly returned one', async () => {
 const initMessage: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 const staleTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'stale-session-id'
 });

 // Sessionless handshake: the response carries no session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream' })
 });

 await staleTransport.send(initMessage);

 const initCall = (globalThis.fetch as Mock).mock.calls.at(-1)!;
 expect(initCall[1].headers.get('mcp-session-id')).toBeNull();

 // The sessionless handshake cleared the stale ID, so an ordinary request carries none
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await staleTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBeNull();

 await staleTransport.close().catch(() => {});

 // When the handshake DOES return a new ID, subsequent requests carry it instead of the preset
 const replacedTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'preset-session-id'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'new-session-id' })
 });

 await replacedTransport.send(initMessage);

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await replacedTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBe('new-session-id');

 await replacedTransport.close().catch(() => {});
 });

 it('should ignore a session ID on a successful non-initialize response', async () => {
 const sessionTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'session-a'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers({ 'mcp-session-id': 'session-b' })
 });

 await sessionTransport.send({ jsonrpc: '2.0', method: 'notifications/roots/list_changed' } as JSONRPCMessage);
 expect(sessionTransport.sessionId).toBe('session-a');

 await sessionTransport.close().catch(() => {});
 });

 it('should accept protocolVersion constructor option and include it in request headers', async () => {
 // When reconnecting with a preserved sessionId, users need to also preserve the
 // negotiated protocol version so the required mcp-protocol-version header is sent.
Expand DownExpand Up@@ -156,6 +274,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -196,6 +315,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand Down
Loading

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

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fix(client): initialize requests never carry a session id; capture only from the initialize response#2469

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 Merged

felixweinberger
merged 3 commits into

from

Jul 9, 2026
 Merged


fix(client): initialize requests never carry a session id; capture only from the initialize response
#2469
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e743f40

fix(client): initialize requests never carry a session id; capture on…


felixweinberger Jul 9, 2026
3b3dc83

fix(client): sessionless initialize response clears any stale session id


felixweinberger Jul 9, 2026
c4f3a59

Merge branch 'main' into fweinberger/initialize-session-hygiene


felixweinberger Jul 9, 2026

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5 changes: 5 additions & 0 deletions
.changeset/initialize-session-hygiene.md

View file


Open in desktop

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

The Streamable HTTP client transport no longer attaches a session ID to a POST containing an `initialize` request — a new session starts "without a session ID attached" (2025-11-25 transports §Session Management) — and it only captures the `mcp-session-id` response header from a successful initialize response, since the spec assigns the session ID "at initialization time … on the HTTP response containing the InitializeResult". Previously the transport stored the header from any response, so a legacy server answering a protocol-version probe with an error that happened to carry a session ID would poison the fallback initialize, which then went out with a session ID it should not have had. A stale session ID from a previous connection is likewise no longer leaked onto the initialize handshake, and a successful initialize response that carries no session ID now clears any stale ID the transport was holding — clients include only an ID "returned by the server during initialization", so an ID the server never returned this session is outside the session model. Ignoring `mcp-session-id` headers mid-session is the complement of the spec's one actual rotation mechanism: a server that wants a new session terminates the old one (it "MAY terminate the session at any time") and answers 404, after which the client "MUST start a new session by sending a new InitializeRequest without a session ID attached". Rotation exists as session replacement via 404 + re-initialize, never as a header swap on a live session, so a server that rotates per the spec's own flow is handled correctly by this transport.

14 changes: 10 additions & 4 deletions
packages/client/src/client/streamableHttp.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,7 @@ import {
 createFetchWithInit,
 encodeMcpParamValue,
 isInitializedNotification,
 isInitializeRequest,
 isJSONRPCErrorResponse,
 isJSONRPCRequest,
 isJSONRPCResultResponse,
Expand DownExpand Up@@ -936,6 +937,11 @@ export class StreamableHTTPClientTransport implements Transport {

 const headers = await this._commonHeaders();
 this._applyBodyDerivedHeaders(headers, message);
 // A new session starts "without a session ID attached" (2025-11-25 transports §Session Management).
 const isHandshake = Array.isArray(message) ? message.some(m => isInitializeRequest(m)) : isInitializeRequest(message);
 if (isHandshake) {
 headers.delete('mcp-session-id');
 }
 // Per-request additional headers (the Client passes SEP-2243
 // `Mcp-Param-*` here on a 2026-07-28 connection). Reserved
 // standard/auth header names are skipped so a caller cannot
Expand DownExpand Up@@ -973,10 +979,10 @@ export class StreamableHTTPClientTransport implements Transport {

 const response = await (this._fetch ?? fetch)(this._url, init);

 // Handle session ID received during initialization
const sessionId = response.headers.get('mcp-session-id');
 if (sessionId) {
 this._sessionId = sessionId;
 // The spec assigns the session id "at initialization time … on the HTTP response containing the InitializeResult"; it is ignored everywhere else.
// Clients include only an id "returned by the server during initialization", so a sessionless handshake clears any stale id.
 if (isHandshake && response.ok) {
 this._sessionId = response.headers.get('mcp-session-id') || undefined;
 }

 if (!response.ok) {
Expand Down

120 changes: 120 additions & 0 deletions
packages/client/test/client/streamableHttp.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -93,6 +93,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -122,6 +123,123 @@ describe('StreamableHTTPClientTransport', () => {
 expect(lastCall[1].headers.get('mcp-session-id')).toBe('test-session-id');
 });

 it('should not store session ID from an error response, then store it from a later successful initialize', async () => {
 const message: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 // A failed initialize (e.g. a legacy server rejecting a version probe) that carries a session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: false,
 status: 400,
 statusText: 'Bad Request',
 text: () => Promise.resolve('Bad Request'),
 headers: new Headers({ 'mcp-session-id': 'poisoned-session-id' })
 });

 await expect(transport.send(message)).rejects.toThrow();
 expect(transport.sessionId).toBeUndefined();

 // The fallback initialize succeeds and its session ID is captured
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'real-session-id' })
 });

 await transport.send(message);
 expect(transport.sessionId).toBe('real-session-id');
 });

 it('should not attach a session ID to an initialize POST, clear a stale ID on a sessionless handshake, and adopt a newly returned one', async () => {
 const initMessage: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 const staleTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'stale-session-id'
 });

 // Sessionless handshake: the response carries no session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream' })
 });

 await staleTransport.send(initMessage);

 const initCall = (globalThis.fetch as Mock).mock.calls.at(-1)!;
 expect(initCall[1].headers.get('mcp-session-id')).toBeNull();

 // The sessionless handshake cleared the stale ID, so an ordinary request carries none
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await staleTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBeNull();

 await staleTransport.close().catch(() => {});

 // When the handshake DOES return a new ID, subsequent requests carry it instead of the preset
 const replacedTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'preset-session-id'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'new-session-id' })
 });

 await replacedTransport.send(initMessage);

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await replacedTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBe('new-session-id');

 await replacedTransport.close().catch(() => {});
 });

 it('should ignore a session ID on a successful non-initialize response', async () => {
 const sessionTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'session-a'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers({ 'mcp-session-id': 'session-b' })
 });

 await sessionTransport.send({ jsonrpc: '2.0', method: 'notifications/roots/list_changed' } as JSONRPCMessage);
 expect(sessionTransport.sessionId).toBe('session-a');

 await sessionTransport.close().catch(() => {});
 });

 it('should accept protocolVersion constructor option and include it in request headers', async () => {
 // When reconnecting with a preserved sessionId, users need to also preserve the
 // negotiated protocol version so the required mcp-protocol-version header is sent.
Expand DownExpand Up@@ -156,6 +274,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -196,6 +315,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand Down
Loading

, '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');
}
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fix(client): initialize requests never carry a session id; capture only from the initialize response#2469

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felixweinberger
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Jul 9, 2026
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fix(client): initialize requests never carry a session id; capture only from the initialize response
#2469
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e743f40

fix(client): initialize requests never carry a session id; capture on…


felixweinberger Jul 9, 2026
3b3dc83

fix(client): sessionless initialize response clears any stale session id


felixweinberger Jul 9, 2026
c4f3a59

Merge branch 'main' into fweinberger/initialize-session-hygiene


felixweinberger Jul 9, 2026

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5 changes: 5 additions & 0 deletions
.changeset/initialize-session-hygiene.md

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

The Streamable HTTP client transport no longer attaches a session ID to a POST containing an `initialize` request — a new session starts "without a session ID attached" (2025-11-25 transports §Session Management) — and it only captures the `mcp-session-id` response header from a successful initialize response, since the spec assigns the session ID "at initialization time … on the HTTP response containing the InitializeResult". Previously the transport stored the header from any response, so a legacy server answering a protocol-version probe with an error that happened to carry a session ID would poison the fallback initialize, which then went out with a session ID it should not have had. A stale session ID from a previous connection is likewise no longer leaked onto the initialize handshake, and a successful initialize response that carries no session ID now clears any stale ID the transport was holding — clients include only an ID "returned by the server during initialization", so an ID the server never returned this session is outside the session model. Ignoring `mcp-session-id` headers mid-session is the complement of the spec's one actual rotation mechanism: a server that wants a new session terminates the old one (it "MAY terminate the session at any time") and answers 404, after which the client "MUST start a new session by sending a new InitializeRequest without a session ID attached". Rotation exists as session replacement via 404 + re-initialize, never as a header swap on a live session, so a server that rotates per the spec's own flow is handled correctly by this transport.

14 changes: 10 additions & 4 deletions
packages/client/src/client/streamableHttp.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,7 @@ import {
 createFetchWithInit,
 encodeMcpParamValue,
 isInitializedNotification,
 isInitializeRequest,
 isJSONRPCErrorResponse,
 isJSONRPCRequest,
 isJSONRPCResultResponse,
Expand DownExpand Up@@ -936,6 +937,11 @@ export class StreamableHTTPClientTransport implements Transport {

 const headers = await this._commonHeaders();
 this._applyBodyDerivedHeaders(headers, message);
 // A new session starts "without a session ID attached" (2025-11-25 transports §Session Management).
 const isHandshake = Array.isArray(message) ? message.some(m => isInitializeRequest(m)) : isInitializeRequest(message);
 if (isHandshake) {
 headers.delete('mcp-session-id');
 }
 // Per-request additional headers (the Client passes SEP-2243
 // `Mcp-Param-*` here on a 2026-07-28 connection). Reserved
 // standard/auth header names are skipped so a caller cannot
Expand DownExpand Up@@ -973,10 +979,10 @@ export class StreamableHTTPClientTransport implements Transport {

 const response = await (this._fetch ?? fetch)(this._url, init);

 // Handle session ID received during initialization
const sessionId = response.headers.get('mcp-session-id');
 if (sessionId) {
 this._sessionId = sessionId;
 // The spec assigns the session id "at initialization time … on the HTTP response containing the InitializeResult"; it is ignored everywhere else.
// Clients include only an id "returned by the server during initialization", so a sessionless handshake clears any stale id.
 if (isHandshake && response.ok) {
 this._sessionId = response.headers.get('mcp-session-id') || undefined;
 }

 if (!response.ok) {
Expand Down

120 changes: 120 additions & 0 deletions
packages/client/test/client/streamableHttp.test.ts

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Original file line numberDiff line numberDiff line change
Expand Up@@ -93,6 +93,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -122,6 +123,123 @@ describe('StreamableHTTPClientTransport', () => {
 expect(lastCall[1].headers.get('mcp-session-id')).toBe('test-session-id');
 });

 it('should not store session ID from an error response, then store it from a later successful initialize', async () => {
 const message: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 // A failed initialize (e.g. a legacy server rejecting a version probe) that carries a session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: false,
 status: 400,
 statusText: 'Bad Request',
 text: () => Promise.resolve('Bad Request'),
 headers: new Headers({ 'mcp-session-id': 'poisoned-session-id' })
 });

 await expect(transport.send(message)).rejects.toThrow();
 expect(transport.sessionId).toBeUndefined();

 // The fallback initialize succeeds and its session ID is captured
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'real-session-id' })
 });

 await transport.send(message);
 expect(transport.sessionId).toBe('real-session-id');
 });

 it('should not attach a session ID to an initialize POST, clear a stale ID on a sessionless handshake, and adopt a newly returned one', async () => {
 const initMessage: JSONRPCMessage = {
 jsonrpc: '2.0',
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
 };

 const staleTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'stale-session-id'
 });

 // Sessionless handshake: the response carries no session ID
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream' })
 });

 await staleTransport.send(initMessage);

 const initCall = (globalThis.fetch as Mock).mock.calls.at(-1)!;
 expect(initCall[1].headers.get('mcp-session-id')).toBeNull();

 // The sessionless handshake cleared the stale ID, so an ordinary request carries none
 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await staleTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBeNull();

 await staleTransport.close().catch(() => {});

 // When the handshake DOES return a new ID, subsequent requests carry it instead of the preset
 const replacedTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'preset-session-id'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 200,
 headers: new Headers({ 'content-type': 'text/event-stream', 'mcp-session-id': 'new-session-id' })
 });

 await replacedTransport.send(initMessage);

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers()
 });

 await replacedTransport.send({ jsonrpc: '2.0', method: 'test', params: {}, id: 'test-id' } as JSONRPCMessage);
 expect((globalThis.fetch as Mock).mock.calls.at(-1)![1].headers.get('mcp-session-id')).toBe('new-session-id');

 await replacedTransport.close().catch(() => {});
 });

 it('should ignore a session ID on a successful non-initialize response', async () => {
 const sessionTransport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
 sessionId: 'session-a'
 });

 (globalThis.fetch as Mock).mockResolvedValueOnce({
 ok: true,
 status: 202,
 headers: new Headers({ 'mcp-session-id': 'session-b' })
 });

 await sessionTransport.send({ jsonrpc: '2.0', method: 'notifications/roots/list_changed' } as JSONRPCMessage);
 expect(sessionTransport.sessionId).toBe('session-a');

 await sessionTransport.close().catch(() => {});
 });

 it('should accept protocolVersion constructor option and include it in request headers', async () => {
 // When reconnecting with a preserved sessionId, users need to also preserve the
 // negotiated protocol version so the required mcp-protocol-version header is sent.
Expand DownExpand Up@@ -156,6 +274,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand DownExpand Up@@ -196,6 +315,7 @@ describe('StreamableHTTPClientTransport', () => {
 method: 'initialize',
 params: {
 clientInfo: { name: 'test-client', version: '1.0' },
 capabilities: {},
 protocolVersion: '2025-03-26'
 },
 id: 'init-id'
Expand Down
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