[BUG] HTTP MCP client appears to send an unexpected MCP-Protocol-Version header after initialize, causing spec-compliant servers to reject all follow-up requests

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Claude Code successfully completes the initialize handshake with an HTTP MCP server, but subsequent MCP requests fail because the protocol version used after initialization does not match the version negotiated during initialize.

In our case, the HelpNDoc 10.8.0.452 built-in MCP server negotiates:

protocolVersion: "2024-11-05"

and issues an Mcp-Session-Id.

However, subsequent requests such as tools/list are rejected with HTTP 400, with the server reporting that the request uses MCP-Protocol-Version: 2025-11-25 instead of the negotiated 2024-11-05.

As a result, claude mcp list reports the server as "Connected", but zero HelpNDoc tools become available inside an actual interactive Claude Code session.

We did not capture Claude Code's raw outgoing request headers directly, so we cannot state with certainty exactly how the header is constructed. However, the server consistently reports 2025-11-25 as the request-header protocol version after successfully negotiating 2024-11-05 during initialize.

What Should Happen?

After initialize successfully negotiates a protocol version with an MCP server, Claude Code's HTTP MCP client should use that negotiated protocol version for subsequent requests in the same session.

In this case, after the server negotiates protocolVersion: "2024-11-05", subsequent requests such as tools/list should use:

MCP-Protocol-Version: 2024-11-05

The server should remain usable after initialization, and its tools should appear and be callable inside the interactive Claude Code session.

Error Messages/Logs

Without forcing a custom MCP-Protocol-Version header, the HelpNDoc MCP server rejects the follow-up request with:

{"error": "Protocol version mismatch: request header \"2025-11-25\" does not match the version \"2024-11-05\" negotiated for this session"}

We also tried forcing the negotiated version using:

claude mcp add --transport http HelpNDoc http://localhost:8933/mcp -H "MCP-Protocol-Version: 2024-11-05"

After doing this, `claude mcp list` fails with:

HTTP 400: Error POSTing to endpoint: {"error": "Unsupported protocol version: 2025-11-25, 2024-11-05"}

This is consistent with Claude Code adding its own `2025-11-25` value in addition to the explicitly configured header rather than replacing it.

Steps to Reproduce

1. Install HelpNDoc 10.8.0.452 on Windows and open a HelpNDoc project.

1. Start its built-in MCP server on the default port 8933 (Tools → Start MCP server).

1. Register the server in Claude Code:

claude mcp add --transport http HelpNDoc http://localhost:8933/mcp

1. Run:

claude mcp list

The server is reported as "Connected".

1. Start an interactive Claude Code session. No HelpNDoc MCP tools appear or can be called.

1. Test the MCP endpoint manually. The initialize request succeeds, the server returns HTTP 200, negotiates:

protocolVersion: "2024-11-05"

and returns an Mcp-Session-Id.

1. Send a subsequent tools/list request for the same session without manually specifying an MCP-Protocol-Version header.

The server returns HTTP 400 and reports:

{"error": "Protocol version mismatch: request header \"2025-11-25\" does not match the version \"2024-11-05\" negotiated for this session"}

1. Repeat the same manual tools/list request, but explicitly send:

MCP-Protocol-Version: 2024-11-05

The request succeeds with HTTP 200 and the full HelpNDoc tool list is returned.

1. As an additional test, try configuring Claude Code with the negotiated protocol version explicitly:

claude mcp add --transport http HelpNDoc http://localhost:8933/mcp -H "MCP-Protocol-Version: 2024-11-05"

Claude Code then fails with:

HTTP 400: Error POSTing to endpoint: {"error": "Unsupported protocol version: 2025-11-25, 2024-11-05"}

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.263

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

Server environment:

• MCP server: HelpNDoc 10.8.0.452 built-in MCP server
• Server-negotiated MCP protocol version: 2024-11-05
• Transport: Streamable HTTP (--transport http)
• MCP endpoint: http://localhost:8933/mcp
• OS: Windows Server 2022
• Claude Code and HelpNDoc run on the same Windows Server machine and communicate over localhost.
• Claude Code is used both through the CLI and the VS Code extension.
• The same behavior was reproduced with Claude Code 2.1.191 before upgrading to 2.1.263.

HelpNDoc is a real third-party MCP server, not a custom test implementation.

The fact that a manually issued tools/list request succeeds immediately when MCP-Protocol-Version: 2024-11-05 is explicitly supplied suggests that the server and its tools are otherwise functioning correctly.

We can provide additional request/response logs or perform further protocol-level diagnostics if needed.