# Prompt replay

**Source:** https://promtable.com/glossary/prompt-replay

> Prompt replay is the debugging technique of re-running a captured production prompt against the same or different model to reproduce a failure or test a fix — the LLM equivalent of replaying a stack trace.

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Prompt replay is the debugging technique of re-running a captured production prompt against the same or different model to reproduce a failure or test a fix — the LLM equivalent of replaying a stack trace.

When a prod LLM call returns the wrong answer, you don't just want logs — you want to re-run the exact request to test fixes. Prompt replay captures the full request payload (system prompt, messages, tool definitions, settings) and lets you re-execute it against the original model, a different model, or a modified prompt. Built into Langfuse, Braintrust, Helicone, LangSmith. Production use cases: 'this user got a bad answer — what would Claude 4.7 do?', 'this prompt regressed — bisect against the last known good version', 'a customer reported a hallucination — capture the trace, add to golden set, prevent regression'. Prompt replay turns observability data from passive logs into an active debugging tool.

## When to use

- Reproducing prod failures.
- Testing prompt fixes before deploy.
- Migrating from one model to another.

## Common mistakes

- Replaying without seeding — non-deterministic samplers make 'replay' return different results.
- Not capturing tool definitions / RAG context — replay misses what the model actually saw.

## Related terms

- [llm-observability](https://promtable.com/glossary/llm-observability)
- [prompt-versioning](https://promtable.com/glossary/prompt-versioning)
- [ab-testing-prompts](https://promtable.com/glossary/ab-testing-prompts)

*Last updated: 2026-06-01*
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Original page: https://promtable.com/glossary/prompt-replay
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