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How to find failing and slow tool calls in an agent run with Tracecrate

Filter a session for reported errors and long durations, then review heuristic patterns.

Who it is forA developer whose agent run took too long or stopped partway and who wants to locate the costly steps quickly.

The problemSlow or failing tool calls are buried among hundreds of routine events.

The outcomeA short list of the longest tool events and any explicit tool errors, plus heuristic flags for repeated or large outputs.

What you need first

  • Node.js 22.12 or newer (24 recommended) and npm for local use
  • A modern desktop browser with Web Workers support
  • Trace files you are authorized to inspect: Claude Code JSONL, Codex rollout JSONL, OTLP JSON, or TraceCrate native JSON

Steps

  1. Run npm run dev in the Tracecrate folder and open the printed address.
  2. Load the session file through Choose files.
  3. In Timeline, select Reported error or set Min duration (ms), then choose Longest first.
  4. Open Insights to review explicit tool errors, long tool durations, and large outputs.
  5. Open each flagged event to read its recorded inputs and outputs before drawing conclusions.

Prompt for your agent

Using the TraceCrate repo at <repo path>, start the dev server and load <session file path>. Expected output: a Timeline filtered to reported errors or events above a chosen minimum duration, sorted longest first. Verify that events with missing duration are excluded when a minimum is set, and that heuristic flags are treated as hints, not confirmed causes.

Caveat. Heuristics do not establish causes or intent, so a flagged slow tool call still needs human reading of the original event.

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