diff --git a/.changeset/initialize-session-hygiene.md b/.changeset/initialize-session-hygiene.md
new file mode 100644
index 0000000000..b12e3c514f
--- /dev/null
+++ b/.changeset/initialize-session-hygiene.md
@@ -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.
diff --git a/packages/client/src/client/streamableHttp.ts b/packages/client/src/client/streamableHttp.ts
index 0d8eff917b..9067fd1ec4 100644
--- a/packages/client/src/client/streamableHttp.ts
+++ b/packages/client/src/client/streamableHttp.ts
@@ -5,6 +5,7 @@ import {
     createFetchWithInit,
     encodeMcpParamValue,
     isInitializedNotification,
+    isInitializeRequest,
     isJSONRPCErrorResponse,
     isJSONRPCRequest,
     isJSONRPCResultResponse,
@@ -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
@@ -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) {
diff --git a/packages/client/test/client/streamableHttp.test.ts b/packages/client/test/client/streamableHttp.test.ts
index a20fb92252..a36bbc0ad3 100644
--- a/packages/client/test/client/streamableHttp.test.ts
+++ b/packages/client/test/client/streamableHttp.test.ts
@@ -93,6 +93,7 @@ describe('StreamableHTTPClientTransport', () => {
             method: 'initialize',
             params: {
                 clientInfo: { name: 'test-client', version: '1.0' },
+                capabilities: {},
                 protocolVersion: '2025-03-26'
             },
             id: 'init-id'
@@ -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.
@@ -156,6 +274,7 @@ describe('StreamableHTTPClientTransport', () => {
             method: 'initialize',
             params: {
                 clientInfo: { name: 'test-client', version: '1.0' },
+                capabilities: {},
                 protocolVersion: '2025-03-26'
             },
             id: 'init-id'
@@ -196,6 +315,7 @@ describe('StreamableHTTPClientTransport', () => {
             method: 'initialize',
             params: {
                 clientInfo: { name: 'test-client', version: '1.0' },
+                capabilities: {},
                 protocolVersion: '2025-03-26'
             },
             id: 'init-id'