// fork & replay
Fork from any step. Replay the future.
Branch a recorded run at the exact step that broke, change the input, and re-execute the entire agent down the new path, then compare the two runs side by side.
Cascade replay
Forking re-runs the full function from the chosen step forward, so a changed input actually propagates into every downstream call, not just one isolated model call.
Reproducible re-execution
Server-side replays re-run the model with deterministic settings so results are as repeatable as providers allow, and recorded runs can be replayed back exactly from the tape.
Pay only for what changes
Steps before the fork point are served from the original recording at no token cost, you're billed only for the part of the run that actually diverges.
Side-by-side diff
Compare the original and forked runs step by step, with cost and latency deltas highlighted down to the first point where they diverge.
Verified replay
Accelerated replays are checked against a fresh execution; if the output diverges beyond a threshold, Retrace falls back to a full run so debugging fidelity is never silently lost.
Replay player & cost preview
Scrub any run like a video, and preview what a fork will cost (with a breakdown of where the savings come from) before you run it.
retrace forks create --trace <id> --span <id> --input "grounded prompt"
retrace forks replay <id> --wait
retrace forks diff <id> --jsonFork at a step, replay it, then inspect the diff.
REST API
- POST /api/v1/forks: create a fork from a span
- POST /api/v1/forks/:id/replay: re-execute the fork
- GET /api/v1/forks/:id/diff: original vs fork diff
More of the platform