API Error 400: tool_use ids without tool_result blocks in non-interactive (-p) mode

Description

When running Claude Code in non-interactive mode (-p flag), parallel tool calls cause an API Error 400 with the message about tool_use concurrency issues.

Steps to Reproduce

claude -p "/email:send_topic_digest"

Where /email:send_topic_digest is a custom command that triggers multiple parallel tool calls (e.g., spawning multiple research agents simultaneously).

Error Output

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"tool_use ids without tool_result blocks: toolu_..."}}

Key Observations

1. Only reproducible in non-interactive mode - The same command works fine when run interactively (without -p)
2. The error occurs when Claude makes 3+ parallel tool calls
3. Related to how the API expects sequential tool_use/tool_result pairs

Environment

• Claude Code CLI
• Linux

Related Issues

• #8763 - Similar concurrency issues reported
• #9433, #9103 - Related parallel tool call issues

Workaround Attempted

Adding instructions to CLAUDE.md to limit parallel tool calls did not reliably prevent the issue, as the model doesn't always follow such instructions.