# Replay / Time-Travel

> Load any past agent trace, jump to a specific step, swap in a different prompt or model, and re-run forward — debug production incidents in minutes instead of hours.

- **Category**: Agentic AI
- **Subcategory**: Governance & Observability
- **Canonical URL**: https://designpattern.fyi/patterns/replay_time_travel/

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## Description
**Intent**: Re-run a past agent trace from any step with modified inputs, prompts, or tools to debug or branch.
**Context**: Production agents hit hard-to-reproduce behavior: strange replies, unexpected tool calls, wrong answers on inputs that worked yesterday. Engineers need to load the exact past run, jump to a specific step, swap in a different prompt or model, and see whether the alternative would have done better — without affecting users.
**Solution**: Capture per-step inputs, outputs, prompts, model id, and tool calls in a trace store. Provide a replay tool that loads a trace at step N and re-runs forward with optional modifications (different model, different prompt, different tool result). Store branches for comparison.



## Use Cases
- Agent runs are non-deterministic and incidents need reproducible debugging.
- Engineers need to branch from a past step to test fixes or alternative prompts.
- Per-step inputs, outputs, and tool calls can be captured durably.






## Trade-offs


### Advantages

- Debugging cycle drops from hours to minutes — load the exact failing trace and iterate.

- A/B comparison of prompt or model fixes becomes trivial.




### Considerations & Drawbacks

- Trace storage adds significant overhead at production traffic volumes.

- Non-deterministic external dependencies (live network calls) limit replay fidelity.







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**Reference**: [Original Source](https://www.agentpatternscatalog.org/patterns/replay-time-travel/)