StreamableHTTPClientTransport is not implementing Transport properly #861

Activity

1. utkarshpandey6

   added

   bugSomething isn't workingSomething isn't working

   on Aug 10, 2025
2. nicolasff commented on Aug 27, 2025

   

   nicolasff

   on Aug 27, 2025

   More actions

   I'm seeing this as well. More details:

   import { Client } from "@modelcontextprotocol/sdk/client/index.js";
   import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js";
   
   const transport = new StreamableHTTPClientTransport(new URL(url), {
       requestInit: { headers: {'X-Foo': 'bar'}}, 
   });
   
   const mcpClient = new Client(
       { name: "my-client", version: "1.0.0", },
       { capabilities: {}, }
   );
   
   await mcpClient.connect(transport); // linter warning on the `transport` parameter to connect()

   The type mismatch I'm getting is:

   Argument of type 'StreamableHTTPClientTransport' is not assignable to parameter of type 'Transport' with 'exactOptionalPropertyTypes: true'.
   Consider adding 'undefined' to the types of the target's properties.
     Types of property 'sessionId' are incompatible.
       Type 'string | undefined' is not assignable to type 'string'.
         Type 'undefined' is not assignable to type 'string'
   

   Seen with Deno 2.4.5, npm:@modelcontextprotocol/sdk@1.17.4 in deno.json.

   👍React with 👍1Reacted by Luca Albinati
3. felixweinberger

   added

   good first issueGood for newcomers - can be tackled without deep knowledge of the codebaseGood for newcomers - can be tackled without deep knowledge of the codebase

   P3Nice to haves, rare edge casesNice to haves, rare edge cases

   v2Ideas, requests and plans for v2 of the SDK which will incorporate major changes and fixesIdeas, requests and plans for v2 of the SDK which will incorporate major changes and fixes

   needs designValid issue but needs maintainer alignment on design or approachValid issue but needs maintainer alignment on design or approach

   and removed

   good first issueGood for newcomers - can be tackled without deep knowledge of the codebaseGood for newcomers - can be tackled without deep knowledge of the codebase

   on Dec 16, 2025
5. ghost added

   potentially closeBot recommends closing — needs maintainer reviewBot recommends closing — needs maintainer review

   on Apr 17, 2026
6. alex-feel commented on Oct 16, 2026

   

   alex-feel

   on Oct 16, 2026

   More actions

   Still reproduces on 1.32.1 (npm latest), TypeScript 7.0.2, strict + exactOptionalPropertyTypes:

   import { Client } from "@modelcontextprotocol/sdk/client/index.js";
   import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js";
   
   const transport = new StreamableHTTPClientTransport(new URL("http://localhost/mcp"));
   const client = new Client({ name: "repro", version: "0.0.0" });
   await client.connect(transport);

   src/main.ts(6,22): error TS2379: Argument of type 'StreamableHTTPClientTransport' is not assignable to parameter of type 'Transport' with 'exactOptionalPropertyTypes: true'. Consider adding 'undefined' to the types of the target's properties.
     Types of property 'sessionId' are incompatible.
       Type 'string | undefined' is not assignable to type 'string'.
         Type 'undefined' is not assignable to type 'string'.
   

   The fix, #2814 (backport of #1766), merged into v1.x on 2026-10-05, after 1.32.1 shipped, so no release carries it yet. The pkg.pr.new build of its merge commit 6919e95 type-checks this file clean.

   Could you cut a v1 patch release with it? I can post the tsconfig and commands if useful.