API 500 Internal server error on tool results (repeated)

Bug

Tool results repeatedly fail with API 500 error, causing Claude Code to retry the same tool call multiple times in a row.

Behavior

The same tool result (e.g., Bash) returns successfully in the UI, but then immediately shows:

API Error: 500 {"type":"error","error":{"type":"api_error","message":"Internal server error"},"request_id":"req_011Ca21p9CCoGdzDo1qXaw3f"}

The tool call is then repeated 2-3 times identically before the session continues or stalls.

Impact

• Duplicate tool executions (e.g., same dig command runs 3 times)
• Session stalls requiring user intervention
• Wastes API quota on retries

Environment

• Claude Code CLI
• Model: claude-sonnet-4-6
• Platform: macOS Darwin 24.6.0