`tools/call` requests no longer echo `Mcp-Session-Id` (violates Streamable HTTP spec)

[TonMarton]

Describe the bug
ChatGPT (host) no longer sends Mcp-Session-Id back to the server on tools/call requests, violating the Streamable HTTP transport rule that clients MUST include the session id on all subsequent HTTP requests once the server returned one during initialization.

If an Mcp-Session-Id is returned by the server during initialization, clients using the Streamable HTTP transport MUST include it in the Mcp-Session-Id header on all of their subsequent HTTP requests.

— Spec 2025-11-25 / Streamable HTTP / session management

sequenceDiagram
    participant CTRL as ChatGPT (CTRL plane)<br/>UA: openai-mcp/1.0.0 (ChatGPT)
    participant TOOL as ChatGPT (TOOL plane)<br/>UA: openai-mcp/1.0.0
    participant Server as MCP Server

    CTRL->>Server: POST initialize
    Server-->>CTRL: Mcp-Session-Id: ctrl-aaa
    CTRL->>Server: POST tools/list (Mcp-Session-Id: ctrl-aaa) ✅
    CTRL->>Server: POST resources/read (Mcp-Session-Id: ctrl-aaa) ✅

    TOOL->>Server: POST initialize
    Server-->>TOOL: Mcp-Session-Id: tool-bbb
    TOOL->>Server: POST tools/call (no Mcp-Session-Id) ❌
    TOOL->>Server: POST tools/call (no Mcp-Session-Id) ❌
    Note over TOOL,Server: Subsequent tool calls never carry the session id<br/>→ server falls into stateless code path<br/>→ capabilities learned at initialize are lost

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To Reproduce
Steps to reproduce the behavior:

1. Stand up a Streamable HTTP MCP server that returns Mcp-Session-Id on initialize.
2. Connect from ChatGPT and trigger any tool that produces multiple tools/call requests (e.g. a polling tool driven by an MCP App / widget).
3. Inspect inbound HTTP headers grouped by User-Agent:
   • openai-mcp/1.0.0 (ChatGPT) requests carry Mcp-Session-Id.
   • openai-mcp/1.0.0 tools/call requests do not.

Expected behavior
Per the Streamable HTTP transport spec: once the server returns Mcp-Session-Id on a plane's initialize, every subsequent request on that plane — including tools/call — MUST include the Mcp-Session-Id header.

Logs

Additional context
On ChatGPT web it seems ok, as of writing this, but iOS App and MacOS App still do not echo id.