Model swap silently broke agent cancellations, so a developer built a behavior diff tool
Impossible-Alarm-738 · reddit · 2026-07-22
A model swap quietly broke an agent’s cancelsubscription tool calls, so the author built whatbroke, a local diff tool for agent behavior.
- It compares JSONL traces of real agent trajectories, not just text outputs.
- It tracks tool calls, arguments, order, cost, latency, and final output, then exits non-zero on breaking changes so it can run in CI.
- A proxy mode (whatbroke record) captures traces from any language by pointing the base URL at it.
- Because agent runs are nondeterministic, it supports multiple runs per scenario and reports a flap rate so pre-existing randomness gets downweighted.
- It is deterministic, fully offline, requires no API keys or accounts, and is MIT licensed.
Related event: New Tool Detects Silent Agent Regressions After Model Switches(2 posts)→
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