MCP tool calls duplicated multiple times when using parallel subagents

Bug Description

When using multiple parallel subagents (Task tool) to review code, the same MCP tool call is executed multiple times (observed 4x duplicate calls for a single intended operation). This causes duplicate comments/actions on external services.

Reproduction Steps

1. Launch multiple parallel subagents using the Task tool for code review
2. Each subagent attempts to call an MCP tool (e.g., github-comments_create_inline_comment)
3. Observe that the same tool call is sent multiple times

Evidence / Logs

The following log shows the same tool call being sent 4 times with identical parameters:

{
  "type": "assistant",
  "message": {
    "id": "msg_c8b67bb114784db4891e6d9e404ef819",
    "type": "message",
    "role": "assistant",
    "content": [
      {
        "type": "tool_use",
        "id": "tooluse_2i5TnFpbTaqULWa2KwGATA",
        "name": "mcp__codeagent__github-comments_create_inline_comment",
        "input": {
          "pr_number": "1256",
          "path": "boss/src/pages/ailog/components/tool-viewers/EnvSetupViewer.tsx",
          "line": "22",
          "side": "RIGHT",
          "body": "<!-- codeagent-review-id: pr-1256 -->\n\n**[代码重复]** ..."
        }
      }
    ],
    "stop_reason": null,
    ...
  },
  "session_id": "7859ab69-1d28-4280-a2ba-4e776b5de500",
  "uuid": "978b20fa-2ead-4d23-b8fa-a321b64e6f87"  // Different UUID each time
}

Key observations:

• Same tool_use.id: tooluse_2i5TnFpbTaqULWa2KwGATA
• Same message.id: msg_c8b67bb114784db4891e6d9e404ef819
• Same parameters (pr_number, path, line, body)
• Different uuid for each message (4 different UUIDs)
• stop_reason: null on all messages

Expected Behavior

Each MCP tool call should be executed exactly once.

Actual Behavior

The same MCP tool call is executed 4 times, resulting in duplicate comments on GitHub PRs.

Related Issues

• #21941 - Subagents complete multiple times (same root cause suspected)
• #14253 - Messages sent during Task agent execution are delivered twice
• #21906 - tool_result blocks in wrong order for parallel tool calls

Environment

• Platform: macOS (darwin)
• Use case: Parallel subagents for PR code review with custom MCP server

Workaround

Currently using deduplication on the MCP server side based on tool_use_id, but this should be handled at the Claude Code level.