[BUG] Project-level MCP servers (.mcp.json) fail to connect starting from v2.0.71

Bug Description

Project-level MCP servers configured via .mcp.json fail to connect starting from Claude Code version 2.0.71. The CLI shows "Failed to reconnect to [server-name]" despite the server being fully functional when tested manually.

Binary Search Results

Through systematic testing, we identified the exact version where this regression was introduced:

| Version | Status |
|---------|--------|
| 2.0.14 | ✅ Works |
| 2.0.45 | ✅ Works |
| 2.0.60 | ✅ Works |
| 2.0.68 | ✅ Works |
| 2.0.70 | ✅ Works |
| 2.0.71 | ❌ Regression introduced |
| 2.0.72 | ❌ Broken |
| 2.0.76 | ❌ Broken |

Environment

• OS: Ubuntu 24.04.3 LTS (Linux 6.8.0-60-generic)
• Node.js: v22.20.0
• Working version: 2.0.70
• First broken version: 2.0.71

Steps to Reproduce

1. Create a .mcp.json file in your project root with MCP server configuration:

{
  "mcpServers": {
    "laravel-boost": {
      "command": "./vendor/bin/sail",
      "args": ["artisan", "boost:mcp"]
    }
  }
}

1. Start Claude Code v2.0.71+ in the project directory
2. Run /mcp command
3. Observe: "Failed to reconnect to laravel-boost"

Manual Verification

The MCP server works correctly when tested manually:

echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}' | ./vendor/bin/sail artisan boost:mcp

Returns valid MCP response in ~0.3 seconds:

{"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2024-11-05","capabilities":{"tools":{"listChanged":false},"resources":{"listChanged":false},"prompts":{"listChanged":false}},"serverInfo":{"name":"Laravel Boost","version":"0.0.1"}}}

Workaround

Downgrade to v2.0.70:

npm install -g @anthropic-ai/claude-code@2.0.70

Related Issues

• #15215 - Project-level MCP servers not loading in Claude Code runtime (v2.0.76)
• #3321 - .mcp.json is not read
• #12449 - MCP Server Connection Race Condition

Notes

This appears to be a regression in how Claude Code handles project-level .mcp.json configuration. User-level MCP servers in ~/.claude.json may still work, but project-level configuration is broken.

🤖 Generated with Claude Code