Replay and Debugging for AI Agents

Idea In 30 Seconds

Replay for AI agents means taking a real problematic trace, reproducing it in controlled conditions, and finding the failure cause step by step.

Its main value is that the team does not guess incident cause. It sees the full chain of agent decisions and the exact point where behavior broke.

Problem

Without replay, teams often debug "from memory":

• they look only at final agent response;
• they do not have full input context;
• they do not see step-level tool-call results.

In this mode, it is hard to separate symptom from cause. Fixes become inaccurate, and incidents return.

Typical outcomes of this approach:

• error is fixed locally, but real production scenario is not reproduced;
• same failure class appears again after release;
• team loses time on repeated manual investigations.

When To Use

Replay is used when:

• a production incident happened and root cause must be found;
• after model or prompt changes, an unexpected behavior diff appears;
• regression test shows critical-case failure;
• team must confirm a fix truly closes incident scenario.

Replay is most useful in systems with multi-step agent behavior and external tools.

Implementation

In practice, replay follows one principle: same trace, same run conditions, step-by-step decision analysis. Examples below are schematic and not tied to a specific framework.

How It Works In One Investigation

1. Store trace with full context

trace = {
    "trace_id": "incident-2026-03-11-42",
    "input": "Refund order #8472",
    "conversation_state": {"user_tier": "pro"},
    "steps": [
        {"tool": "payments_api", "args": {"order_id": "8472"}, "result": {"status": "timeout"}},
        {"tool": "fallback_policy", "args": {}, "result": {"action": "ask_for_retry"}}
    ],
    "final_output": "Please try again later.",
    "stop_reason": "fallback_used",
}

Without full trace, replay almost never reproduces the real incident cause.

2. Reproduce trace in same conditions

def replay_trace(agent, trace, runtime_config):
    return agent.replay(
        trace=trace,
        model_version=runtime_config["model_version"],
        tool_mocks=runtime_config["tool_mocks"],
        timeout_sec=runtime_config["timeout_sec"],
    )

If model, timeouts, or tool conditions differ, replay can look falsely safe.

3. Analyze step timeline

def find_first_bad_step(replayed_steps):
    return next(
        ((idx, step) for idx, step in enumerate(replayed_steps) if step["status"] == "unexpected"),
        None,
    )

Core debugging goal is finding the first step where system leaves expected scenario.

4. Capture root cause in structured form

incident_report = {
    "trace_id": "incident-2026-03-11-42",
    "root_cause": "tool_timeout_not_handled_as_retryable",
    "affected_component": "retry_policy",
    "fix_plan": "treat payments timeout as retryable before fallback",
}

Structured root cause makes fix validation and team knowledge transfer easier.

5. Add incident to regression set

def promote_to_regression_case(trace, report):
    return {
        "id": trace["trace_id"],
        "input": trace["input"],
        "expected_behavior": {"stop_reason": "resolved"},
        "tags": ["incident", "replay", report["affected_component"]],
    }

After replay investigation, case should go into regression or golden dataset, otherwise incident can repeat.

Typical Mistakes

Incomplete incident trace

Logs contain final response, but no agent steps and no tool results.

Typical cause: only summary is stored, without step-level details.

Replay in different runtime conditions

Trace is replayed on different model or with different timeout/retry settings.

Typical cause: incident runtime conditions are not fixed.

Debugging only final text

Team analyzes only last response and misses failure cause in middle of run.

Typical cause: no step-by-step timeline of agent decisions.

Root cause is not captured structurally

After incident, team has verbal conclusion but no clear technical record.

Typical cause: missing incident-report template.

Case is not added to regression

Incident was fixed but not added to permanent test set.

Typical cause: replay investigation is disconnected from regression workflow.

Summary

FAQ

Q: How is replay different from regression testing?
A: Regression compares system versions on case sets, while replay reproduces one real incident to find root cause.

Q: What is the minimum required for quality replay?
A: input, context state, step-by-step tool calls, their results, stop_reason, and runtime config.

Q: Can replay be done without production API access?
A: Yes. Teams usually use stored responses or mocks to reproduce incident logic in a stable way.

Q: When is a replay case considered closed?
A: When fix passes rerun replay and the same scenario consistently passes in regression set.

What Next

After replay investigations, add incident cases to Golden Datasets and validate them with Regression Testing. Use Eval Harness for standardized runs, and Unit Testing for local logic checks.