fix: make HTTP transport more lenient for Claude Code compatibility

[GiggleLiu]

Summary

• Plain GET requests now return health check JSON instead of HTTP 406, allowing Claude Code's health check to succeed
• Accept header check for POST requests relaxed from strict 406 to debug warning, as Claude Code may not send the full Accept header per MCP 2025-06-18 spec

Background

Claude Code's HTTP MCP transport has known issues:

• Health check sends plain GET without Accept: text/event-stream header
• POST requests may not include both application/json and text/event-stream in Accept header

These issues are documented in:

• HTTP MCP Server "Failed to connect" Despite Successful Manual Testing anthropics/claude-code#11633
• [Bug] HTTP MCP Transport Missing Required Protocol Version Header anthropics/claude-code#9492

The issues are closed as "not planned" on Claude Code's side, so server-side leniency is needed for compatibility.

Changes

1. Plain GET health check (src/transports/http.jl:283-294):

   • Before: HTTP 406 "Not Acceptable"
   • After: HTTP 200 with JSON {"status":"ok","protocol_version":"2025-06-18","session_id":null}

2. Lenient Accept header (src/transports/http.jl:347-353):

   • Before: HTTP 406 if Accept doesn't include both types
   • After: Debug log warning, continue processing

Test plan

• Added test for plain GET health check response
• Added test for lenient Accept header on POST requests
• All existing tests pass (267 tests)

🤖 Generated with Claude Code