[BUG] Subagent Resume Failure with 3+ Tool Uses

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?

Test Cases - Custom Plugin Subagent (xxx-plugin:some-subagent)
┌────────┬─────────────────────────┬───────────────┐
│ Test # │ Tool Uses in First Call │ Resume Result │
├────────┼─────────────────────────┼───────────────┤
│ 1 │ 0 │ ✅ Success │
├────────┼─────────────────────────┼───────────────┤
│ 2 │ 1 │ ✅ Success │
├────────┼─────────────────────────┼───────────────┤
│ 3 │ 2 │ ✅ Success │
├────────┼─────────────────────────┼───────────────┤
│ 4 │ 3 │ ❌ Failed │
├────────┼─────────────────────────┼───────────────┤
│ 5 │ 4 │ ❌ Failed │
└────────┴─────────────────────────┴───────────────┘
Test Cases - Built-in Subagent (general-purpose)
┌────────┬─────────────────────────┬───────────────┐
│ Test # │ Tool Uses in First Call │ Resume Result │
├────────┼─────────────────────────┼───────────────┤
│ 1 │ 1 (earlier test) │ ✅ Success │
├────────┼─────────────────────────┼───────────────┤
│ 2 │ 3 │ ❌ Failed │
└────────┴─────────────────────────┴───────────────┘
Root Cause

When any subagent (custom or built-in) executes 3 or more tool uses in the first invocation, subsequent resume calls fail with:
API Error: 400 {"error":{"message":"{\"error\":{\"code\":null,\"message\":\", trace_id: xxx...

The error message mentions "tool use concurrency issues" in some cases.

What Should Happen?

Resume subagents as needed

Error Messages/Logs

Steps to Reproduce

Steps to Reproduce

1. Call any subagent (custom or built-in) with a prompt requiring 3+ tool uses
2. Wait for completion and note the agentId
3. Attempt to resume using that agentId
4. Observe the 400 error

Threshold

• ≤ 2 tool uses: Resume works
• ≥ 3 tool uses: Resume fails

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.7

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_