please move this issue into modelcontextprotocol/typescript-sdk issue
issue

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

[BUG] MCP Python SDK sessionId not maintained in browser with TypeScript SDK client

Issue: MCP Python SDK as Server, TypeScript SDK as Client – Node.js works, browser session management fails

Description

When using the official MCP Python SDK (fastmcp) as the server and the TypeScript SDK as the client, session management (mcp-session-id) works correctly in Node.js, but fails in browser environments. This results in repeated "Missing session ID" errors or lost sessions for every request.

Environment

• Server: MCP Python SDK (fastmcp), streamable_http_app, Starlette/uvicorn
• Client: TypeScript SDK (@modelcontextprotocol/sdk), StreamableHTTPClientTransport
• Node.js: Session management works out of the box
• Browser (React/Vitest): Session cannot be maintained
• All code is reverted to official logic, no monkey patching or custom fetch

Key Code Snippets

Server (http-server-streamable.py):

app = mcp.streamable_http_app()
app.add_middleware(
    CORSMiddleware,
    allow_origins=["http://localhost:3000"],
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"]
)
uvicorn.run(app, host="127.0.0.1", port=8000)

Client (mcpService.ts):

// MCPService (browser and Node.js):
const transport = new StreamableHTTPClientTransport(new URL(this.url));
await mcp.connect(transport);
// In browser, MCPService does not automatically manage sessionId between requests.
// The first response from the server includes Mcp-Session-Id, but MCPService does not persist or inject it for subsequent requests.
// In Node.js, sessionId is managed via cookies and works as expected.

Test Case (mcp-streamable-browser.test.ts):

import { describe, it, expect } from 'vitest';
import { MCPService } from '../../../engine/service/mcpService';

describe('MCPService streamable http tool list', () => {
  it('should fetch tool list from MCP Python server', async () => {
    const mcp = new MCPService('http://127.0.0.1:8000/mcp', 'STREAMABLE_HTTP');
    const result = await Promise.race([
      mcp.listTools(),
      new Promise<{ data: null; error: string }>(resolve => setTimeout(() => resolve({ data: null, error: 'timeout' }), 5000))
    ]);
    const { data, error } = result as { data: any; error: any };
    console.log('Tool list:', data, error);
    console.log('SessionId:', mcp['transport']?.sessionId);
  });
});

Steps to Reproduce

1. Start MCP Python server with the CORS config above (expose_headers tried, but does not help).
2. The SDK makes two requests: the first response from the server includes a valid Mcp-Session-Id header, but the client does not send the sessionId in the header for the second request, causing session management to fail.
3. Same code in Node.js maintains sessionId correctly.

Expected Behavior

• Browser environment should maintain session just like Node.js, with mcp-session-id correctly read and reused, so all requests are in the same session.

Request

• Please confirm if there is a bug in the TypeScript SDK sessionId handling for browsers, or a compatibility issue in the Python SDK server.
• Please provide best practices or a fix for browser session management.

Log Output

Please see the following log output for reference:

1st fetch 200ok
{"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"mcp-client","version":"1.0.0"}},"jsonrpc":"2.0","id":0}
response header
...
mcp-session-id:68709f581d3740ae82a3dcb523f0ac8c
...
request header (dont have mcp-session-id)
2st fetch 400 Bad Request
{"method":"notifications/initialized","jsonrpc":"2.0"}
{
    "jsonrpc": "2.0",
    "id": "server-error",
    "error": {
        "code": -32600,
        "message": "Bad Request: Missing session ID"
    }
}
response header
...
mcp-session-id:44295e965b524e97b45ccf6351201c13
...
request header (dont have mcp-session-id)


nodejs test log out 
Tool list: [ ... ] undefined ([explain,not test log] sucess get toollist,no error)
SessionId: a9911fbfa8fa4eb78dad791b2dddfc33

Python & MCP Python SDK

python 
python version 3.12
mcp 1.9.2

typescript
@modelcontextprotocol/sdk": "^1.13.0
node version v22.17.0

Other information

When both the MCP server and client use the official TypeScript SDK, session management works as expected.