[BUG] 404 for Invalid Session ID should trigger a new session

[nirhaas]

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?

The latest MCP specification 2025-06-18 mentions in Trasports // 2.5 Session Management (4) that:

When a client receives HTTP 404 in response to a request containing an Mcp-Session-Id, it MUST start a new session by sending a new InitializeRequest without a session ID attached.

Current behavior using Claude Code is to just report the error and quit.

⏺ myserver - time_get_current_time (MCP)(timezone: "America/New_York")
  ⎿  Error: Error POSTing to endpoint (HTTP 404): 
     {"jsonrpc":"2.0","error":{"code":-32000,"message":"No valid session ID provided"},"id":null}

⏺ The error persists.

What Should Happen?

Claude Code to initialize a new session / reconnect to the MCP server and continue.

Error Messages/Logs

Steps to Reproduce

Preparations:

1. Set up a local MCP server in-memory only session management, like https://github.com/modelcontextprotocol/typescript-sdk/blob/main/src/examples/server/sseAndStreamableHttpCompatibleServer.ts
2. claude mcp add --transport http myserver http://localhost:{port}/mcp

Steps:

1. Start claude. Verify that the mcp is connected /mcp
2. Close the MCP server and open again
3. Try to call a tool from the server

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

No response

Claude Code Version

1.0.128 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

No response