---
title: "Prove the fix actually worked — Retrace"
description: "Prove a fix worked: Retrace forks the failed run, replays your change, finds the first divergence, and returns a verdict — improved, regressed, or unchanged."
canonical: "https://retraceai.tech/features/prove-fix"
last-updated: 2026-08-26
format: text/markdown
---

# Prove the fix actually worked

Re-run a proposed change against the run that actually failed and get a verdict (improved, regressed, unchanged, or inconclusive) so you can prove a fix works before you ship it.

## What it does

- **Fork → replay → diff → verdict** — Retrace forks the failed run, replays your change, finds the first point of divergence, and a judge returns a verdict: improved, regressed, unchanged, or inconclusive.
- **Typed hypotheses** — Test 'what if this prompt, tool result, model, or temperature were different' with typed substitutions, no changes to your agent code.
- **Only re-run what's affected** — A dependency-aware replay re-executes just the steps your change touches and serves the rest from the recording, clearly labeled, so verification is fast and cheap.
- **Pinpoint the cause** — retrace bisect binary-searches the run to find the exact first step whose change flips the verdict.
- **Corpus sweeps** — Apply one fix across many failed runs at once to confirm it holds beyond a single example; partial results are kept and labeled if you hit a limit.
- **Regression gates** — Mark known-good runs as golden and compare future runs against them for structural regressions, pairs directly with the CI eval gate.

## Examples

Prove a fix, then bisect to the exact step that caused the regression.

```terminal
# Re-run the failed trace and get a verdict
retrace traces verify-fix <trace-id>

# Find the first step whose change flips the result
retrace bisect --trace <id> --substitutions subs.json
```


## REST API

- `POST /api/v1/forks/sweep: run one fix across N traces`
- `GET /api/v1/forks/sweep/:id: sweep results`
- `POST /api/v1/forks/sensitivity: sensitivity analysis`

## Related

- HTML page: https://retraceai.tech/features/prove-fix
- Documentation: https://docs.retraceai.tech
- OpenAPI specification: https://retraceai.tech/openapi.json
- Authentication for agents: https://retraceai.tech/auth.md