Problem

The delegate_tool tool_trace marked successful tool calls as "status": "error" when the output was short.

Root cause: the error detection heuristic checked if the substring "error" appeared in the first 80 chars of the tool result:

is_error = bool(content and "error" in content[:80].lower())

For short JSON outputs like:

{"output":"test1\n","exit_code":0,"error":null}

The "error" key (from "error":null) lands within the first 80 chars → false positive flagged as error.

This only affected the TUI overlay display and tool_trace diagnostics — the tool itself executed successfully. Long outputs avoided the bug because the "error" key was pushed past the 80-char window.

Fix

Replace the substring heuristic with a call to _looks_like_error_output() (already defined at line 272 in the same file), which:

• Parses JSON and checks the error key is truthy (null / false / "" do not trigger)
• Checks status field for error/failed/timeout
• Checks first line for classic error markers (error:, traceback, exception:, failed:)

Testing

Verified with the existing _looks_like_error_output function:

• {"output":"test1","exit_code":0,"error":null} → False (correct)
• {"output":"","exit_code":127,"error":"command not found"} → True (correct)
• Empty string → False (correct)

Old heuristic: flagged the first case as True (wrong).