# What does replay show that a trace viewer does not?

Replay shows what would happen if you changed one thing, the prompt, a tool result, or a config setting, and re-ran the rest of the turn against the original context, so you can watch whether the model's decision changes. A trace viewer only shows what happened: the messages, the tool calls, the outputs, laid out for reading, not what a different input would have produced. That's the test a fix actually needs, not "does this look better" but "does the agent behave differently on the exact case that failed." A viewer can tell you a tool returned a stale price; only replay tells you whether fixing that field would have stopped the bad response, on the same case, not a similar one.

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Source: https://tessary.ai/answers/failure-replay/what-does-replay-show-that-a-trace-viewer-does-not
More on Failure replay: https://tessary.ai/answers/failure-replay
From Tessary, agent reliability for AI agents in production: https://tessary.ai