Agent Trajectory Evaluator

Final-answer evals tell you the agent failed; they don't tell you where. An agent that returns the right number might have called the wrong tool first, looped on a flaky API, or stumbled into the answer through a path that collapses on the next input. This skill makes the agent's process inspectable: capture the full trajectory — every decision, tool call, argument, and result — then score it on the axes that actually predict failure, asserting what's checkable and judging only what isn't.

When to use this skill

• You're building or debugging a tool-using / multi-step agent and a final-answer eval says "wrong" without saying why.
• A prompt or model change kept the answers correct but you suspect the agent got slower, looped more, or recovers worse — and you need to prove it.
• You're adding a new tool and want to confirm the agent selects it correctly instead of brute-forcing with the old one.
• Failures are intermittent and you can't tell whether the agent is fragile (lucky path) or robust (sound path).

Instructions

1. Capture the full trajectory as a structured, replayable log — one record per step. Final-answer-only logging is the root cause of un-diagnosable failures. Each step records: the model's decision (the assistant turn, including thinking-block summaries if present), the tool called and its exact arguments, the raw tool result (success/error), and any externalized state (files written, working dir, retry count). Use a stable schema so two runs diff cleanly:

   {"run_id": "...", "task_id": "...", "step": 3,
    "decision": "call search_orders to find the open order",
    "tool": "search_orders", "args": {"customer_id": "C-118", "status": "open"},
    "result": {"ok": true, "rows": 2}, "is_error": false,
    "latency_ms": 410, "state": {"retries": 0}}

   Pull this from your agent loop's tool-call records (or the Managed Agents event stream: agent.tool_use / agent.tool_result / agent.custom_tool_use events carry tool name, input, and result). Persist trajectories to disk so a baseline run is a diffable artifact, not a console scroll-by.

2. Build a fixed, version-controlled eval set of representative tasks — and deliberately include trap tasks. A good set has three buckets: (a) routine tasks the agent should handle cleanly, (b) tasks that require tool use (the answer isn't in the prompt, so the agent must select and call the right tool), and (c) tasks engineered to trip a known failure mode — a tool that returns an error on the first call (does it recover?), an ambiguous request (does it loop?), a distractor tool that looks relevant but is wrong (does it mis-select?). Pin the set; an eval set that drifts can't catch regressions. Each task carries its expected trajectory assertions (next step).

3. Score every trajectory on five axes, not one. Final-answer correctness is necessary but insufficient. For each task, evaluate:

   • Tool selection — did it call the right tool for each sub-goal? (mis-selection often produces a right answer via a wrong, slow path)
   • Argument correctness — were the tool arguments right? (a status: "open" typo'd to status: "all" can still return the target row by luck)
   • Step efficiency — did it stay within a step budget, or did it repeat calls, loop, or take a needless detour? Measure against a per-task budget, not a global one.
   • Error recovery — when a tool returned an error, did the agent recover sensibly (retry once, switch approach) or thrash / give up?
   • Goal completion — did it actually finish the task, distinct from "the final text looks plausible"?

4. Split scoring into programmatic assertions and a narrow LLM-judge — assert everything you can. An LLM-judge over a whole trajectory is noisy and expensive, and it will rationalize a broken path. So check the deterministic axes with code: exact tool-name assertions, argument equality (or schema match), and step-count budgets are all plain comparisons against the trajectory you captured.

   tools = [s["tool"] for s in trajectory]
   assert tools[0] == "search_orders", f"wrong first tool: {tools[0]}"
   assert trajectory[0]["args"]["status"] == "open"
   assert len(trajectory) <= task["step_budget"], f"{len(trajectory)} steps > budget"
   assert not any(s["is_error"] for s in trajectory[-2:]), "ended on an error"

   Reserve the LLM-judge for the genuinely subjective steps only — "was this reasoning step sound given the prior result?", "was this summary faithful to the tool output?" — and judge one step at a time with the step's inputs in context, not the entire run. Default both the agent-under-test and the judge to the latest, most capable Claude model (claude-opus-4-8); use a different sample or framing for the judge so it isn't grading its own twin, and keep the judge's rubric to one criterion per call.

5. Diff every candidate trajectory against a stored baseline and report the regressions. This is what catches the silent ones. After a prompt or model change, re-run the fixed eval set and compare trajectory-for-trajectory against the baseline: tools added/removed/reordered, argument changes, step-count delta, new error-recovery loops, latency delta. A change that keeps the final answer correct but adds two steps, introduces a retry loop, or swaps a precise tool for a brute-force one is a regression — surface it even though the answer still passes. Promote a candidate to the new baseline only when the diff is empty or every change is reviewed and intended.

Output

• Trajectory schema — the per-step record (decision, tool, args, result, is_error, latency, state) and where each field comes from in your agent loop or event stream.
• Per-axis rubric — the five axes (tool selection, argument correctness, step efficiency, error recovery, goal completion) with the concrete check for each task.
• Assertion-vs-judge split — the deterministic assertions written as code, and the short list of subjective steps routed to a single-criterion LLM-judge (agent and judge both on claude-opus-4-8).
• Baseline-diff regression report — a per-task diff of the candidate run against the stored baseline (tools reordered/added/removed, arg changes, step-count and latency deltas, new recovery loops), flagging every regression even where the final answer still passes, plus a verdict on whether to promote the candidate to baseline.