[BUG] MCP tools not exposed when server advertises 'completions' capability

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?

MCP server tools are not exposed to the Claude Code assistant when the server advertises the completions capability, even though the capability is valid per MCP spec 2025-11-25.

Claude Code UI shows "Capabilities: resources / prompts" but omits "tools" - despite the initialize response correctly including "tools": {"listChanged": false}.

What Should Happen?

Tools should be exposed regardless of which other valid MCP capabilities the server advertises.

Error Messages/Logs

Server initialize response (tools capability present):

{
  "capabilities": {
    "completions": {},
    "prompts": {"listChanged": false},
    "resources": {"listChanged": false, "subscribe": true},
    "tasks": {},
    "tools": {"listChanged": false}
  },
  "protocolVersion": "2025-11-25",
  "serverInfo": {"name": "crates-mcp", "version": "0.1.0"}
}

Claude Code /mcp UI shows:

Capabilities: resources · prompts

Note: tools is missing from the UI despite being in the response.

Steps to Reproduce

1. Configure an MCP server that advertises completions capability:

{
  "mcpServers": {
    "test-server": {
      "command": "path/to/server",
      "args": ["--with-completions"]
    }
  }
}

1. Verify server returns valid initialize response with both completions and tools capabilities
2. Connect via /mcp - observe "Capabilities: resources / prompts" (no tools)
3. Try to use any tool - get "No such tool available" error

Control test: Same server without completions capability -> tools work correctly.

Claude Model

Opus 4.5 (claude-opus-4-5-20251101)

Is this a regression?

Unknown - completions is a newer MCP capability

Claude Code Version

Latest (January 2026)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app

Additional Information

Related to #2682 - this provides a specific, reproducible trigger for the "tools not exposed" class of bugs.

The completions capability is defined in MCP spec 2025-11-25 for server-side autocompletion suggestions. It's an empty object {} when present.

Workaround: Don't register a completion handler on the server.