StreamableHTTPClientTransport is not implementing Transport properly

[utkarshpandey6]

Describe the bug
There is a mismatch in type of Transport and StreamableHTTPClientTransport

To Reproduce
Typescript compilation is failing

Expected behavior
StreamableHTTPClientTransport should implement Transport properly

Additional context
Add any other context about the problem here.

[nicolasff]

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.

[mcp-claude]

fixed on main (b8886e77) via PR #1766 (48aba0d3), which added explicit | undefined to all optional properties in the Transport interface. the specific sessionId error from the report no longer occurs on main.

still present on v1.x (bf1e022b): src/shared/transport.ts:122 declares sessionId?: string without | undefined, causing the mismatch when exactOptionalPropertyTypes: true is set by the user. the one-line fix is changing it to sessionId?: string | undefined.

reopen if still seeing this on main, or request a v1.x backport.

Repro + verification

repro against v1.x (repro_v1.ts):

import { StreamableHTTPClientTransport } from "./src/client/streamableHttp.js";
import type { Transport } from "./src/shared/transport.js";

const transport = new StreamableHTTPClientTransport(new URL("http://localhost:3000/mcp"));
const t: Transport = transport; // fails with exactOptionalPropertyTypes: true

output (v1.x, bf1e022b):

repro_v1.ts(5,7): error TS2375: Type 'StreamableHTTPClientTransport' is not assignable to type 'Transport' with 'exactOptionalPropertyTypes: true'.
  Types of property 'sessionId' are incompatible.
    Type 'string | undefined' is not assignable to type 'string'.
      Type 'undefined' is not assignable to type 'string'.

output (main, b8886e77): no errors.

code path: src/shared/transport.ts:122 on v1.x has sessionId?: string — the getter at src/client/streamableHttp.ts:608 returns string | undefined, which conflicts when exactOptionalPropertyTypes is enabled because TypeScript then reads the interface property as "string when present" with no explicit undefined allowance.