`claude mcp serve` tools with anyOf schemas fail to parse in MCP clients

Summary

When using claude mcp serve as an MCP server, tools fail to be displayed in various MCP clients (Claude Desktop, Cursor) due to validation errors in the tool schemas.

Environment

• Claude Code version: 2.0.24
• Clients tested:
• Claude Desktop (macOS)
• Cursor (with MCP support)
• OS: macOS 25.0.0

Problem Description

The MCP server starts successfully and returns a tools list, but MCP clients fail to parse the tools due to schema validation errors.

Error Details

Cursor Error Log

Error listing tools: [
  {
    "code": "invalid_literal",
    "expected": "object",
    "path": ["tools", 0, "outputSchema", "type"],
    "message": "Invalid literal value, expected \"object\""
  },
  {
    "code": "invalid_literal",
    "expected": "object",
    "path": ["tools", 5, "outputSchema", "type"],
    "message": "Invalid literal value, expected \"object\""
  }
]
Found 0 tools, 0 prompts, and 0 resources

Claude Desktop Behavior

• Server shows as "connected" in UI
• Tools list appears empty: Tools: (none)
• Log shows server started successfully but tools not displayed

Root Cause

The following tools have outputSchema that uses anyOf for Union types without a top-level type field:

1. Task (tools[0])
2. Read (tools[5])

Example schema structure:

{
  "anyOf": [...],
  "$schema": "http://json-schema.org/draft-07/schema#"
}

While this is valid JSON Schema, MCP clients appear to require an explicit top-level type: "object" field in the outputSchema.

Verification

Direct JSON-RPC testing confirms the server returns tools correctly:

$ echo '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' | claude mcp serve
# Returns 16 tools including: Task, Bash, Glob, Grep, Read, Edit, Write, etc.

However, all tested MCP clients fail to parse the response.

Expected Behavior

All 16 tools should be displayed and usable in MCP clients.

Actual Behavior

0 tools are displayed due to schema validation errors.

Suggested Fix

For tools using Union types (anyOf) in outputSchema, either:

1. Wrap the anyOf in an object with type: "object" at the top level, or
2. Update MCP client implementations to handle Union types correctly

Impact

This prevents claude mcp serve from being usable with third-party MCP clients, limiting its utility for cross-tool workflows.