Agent Harness

Agent Harness

Most agents ship on vibes: someone tries eight prompts, the output looks good, it goes to production, and the next prompt tweak silently breaks a refusal nobody re-tested. This skill builds the harness around an agent so its behaviour becomes measurable — scenario suites with structural assertions, deterministic replay of recorded tool calls, paired regression diffing across prompt and model changes, and per-scenario cost and latency budgets. The tools here score an agent; they never invoke one, so they run offline on every commit.

When to use this skill

• An agent is going to production and the only quality evidence is manual spot-checking
• A prompt, tool schema, or model version is changing and you need to know what broke
• Two model or configuration options need a defensible comparison, not a demo
• An incident happened and you need the behaviour encoded as a permanent regression test
• Agent cost or latency is climbing across releases and nobody can point to when
• An existing eval suite reports a healthy pass rate that nobody trusts

Inputs the skill expects

• The agent's tool inventory — names, arguments, and which tools are irreversible
• Recorded transcripts per scenario: tool calls, final output, turns, latency, cost, error state
• The behavioural rules the agent must hold (refusals, escalation triggers, policy boundaries)
• Known failure history — past incidents, customer complaints, internal bug reports
• Current cost and latency expectations per interaction
• The release gate that consumes the result (CI job, review checklist, launch review)

Clarify First

Before building the harness, confirm these inputs. If any is unknown or vague, ASK — do not assume:

• Which agent actions are irreversible — determines which scenarios need tool_not_called assertions at critical severity, and what the release gate blocks on
• Whether transcripts are already recorded — decides whether workflow 1 starts from replay or from an instrumentation task first
• What the suite gates — a CI blocking check, a nightly report, or a one-off comparison; changes suite size, runtime budget, and severity strictness
• The known failure modes — past incidents seed the adversarial and refusal buckets, which is where regressions actually hide

Stop rule: ask only the 2-3 that most change the output. If the user says "just draft it," proceed and list your assumptions at the top of the artifact.

Workflows

Workflow 1 — Stand up a scenario suite and score a run

1. Enumerate the agent's irreversible actions; each one gets a refusal scenario.
2. Draft 20-30 scenarios across all six buckets (happy, boundary, refusal, adversarial, failure-recovery, ambiguity) using assets/scenario_authoring_checklist.md. Structural assertions first — tool called / not called / order / arguments — text assertions only on domain tokens.
3. Declare suite-wide defaults for latency, cost, and turn ceilings so every scenario is budgeted without repeating yourself.
4. Record one transcript per scenario, scrubbing PII at record time, and stamp the run with model and prompt_sha.
5. Score the run and read critical failures before the pass rate.

python3 engineering/agent-harness/scripts/scenario_runner.py \
  --suite engineering/agent-harness/assets/sample_suite.json \
  --transcripts engineering/agent-harness/assets/sample_transcripts_baseline.json \
  --strict-critical

Workflow 2 — Gate a prompt or model change on a paired regression diff

1. Score the baseline and the candidate with the same suite file, saving both as JSON reports.
2. Diff them. Read regressions and budget drift before the aggregate rate.
3. Triage every regression: intended trade, real defect, or flaky scenario (re-run the flipped scenario five times to tell the last two apart).
4. Record the decision in assets/eval_report_template.md and promote the accepted candidate report to the new baseline.

python3 engineering/agent-harness/scripts/scenario_runner.py \
  --suite engineering/agent-harness/assets/sample_suite.json \
  --transcripts engineering/agent-harness/assets/sample_transcripts_candidate.json \
  --format json > /tmp/candidate.report.json

python3 engineering/agent-harness/scripts/eval_diff.py \
  --baseline engineering/agent-harness/assets/sample_baseline_report.json \
  --candidate /tmp/candidate.report.json \
  --fail-on-regression --drift-threshold 0.15

The shipped sample data demonstrates the core lesson: both runs score 83.3%, and the candidate contains a critical prompt-injection regression. A gate on pass rate ships it; the paired diff catches it.

Workflow 3 — Establish cost and latency budgets, then track drift

1. Take the last release's accepted run as the reference.
2. Set per-scenario latency at p95 × 1.3, cost at median × 1.5, and the turn ceiling at observed max + 2. Put them in the suite defaults, overriding only where a scenario is legitimately expensive.
3. Score the current run; budget breaches surface as minor assertions, so they report without blocking.
4. Diff against the reference with a tight drift threshold to catch the slow bleed that stays inside budget.

python3 engineering/agent-harness/scripts/eval_diff.py \
  --baseline engineering/agent-harness/assets/sample_baseline_report.json \
  --candidate engineering/agent-harness/assets/sample_candidate_report.json \
  --drift-threshold 0.10 --format json

Decision frameworks

Which assertion type to reach for

Severity, and what each one gates

Can I trust this diff?

A single critical regression is actionable at n = 1. Significance testing is for aggregate movement, never for safety failures.

Anti-Patterns

Gating on the aggregate pass rate

Mistake: The release check is "pass rate ≥ 90%," and everything else is advisory. Why it happens: One number is easy to put in a dashboard and easy to explain to leadership, and it genuinely looks like the summary statistic. Instead: Gate on critical-severity failures and on the paired per-scenario diff. The pass rate is the last number you read, always with its confidence interval — at 30 scenarios that interval is ±13 points, which cannot resolve the regressions you care about. The sample data here shows two runs at an identical 83.3% where one refunds money on an injected instruction.

Asserting on sentences instead of structure

Mistake: output_contains: "I've issued your refund of $49.00 and it should arrive in 3-5 business days". Why it happens: It is the fastest thing to do — copy the good output into the assertion and move on. Instead: Assert on the tool call (issue_refund with order_id=A-10041) and on a domain token in the text ("refund", the order ID). Structural assertions do not break when the model rewords, so the suite keeps signal across model upgrades instead of generating a wall of false failures that trains the team to ignore it.

Only testing what the agent should do

Mistake: Every scenario is a happy path; the suite has no tool_not_called assertions. Why it happens: Suites get written from the product spec, and specs describe intended behaviour, not forbidden behaviour. Instead: For every irreversible action the agent can take, write a scenario where taking it is wrong. Refusal and adversarial scenarios are where prompt changes actually regress, because a change that makes an agent more capable usually makes it more eager. Target roughly 35% of the suite across refusal and adversarial buckets.

Tuning the prompt until the suite goes green

Mistake: Iterating on the prompt with the full suite visible until every scenario passes. Why it happens: It feels like the tight feedback loop that good engineering is supposed to have. Instead: Hold out 20% of scenarios and never look at them while iterating; run them only at the gate. Thirty scenarios is a small enough surface to overfit in an afternoon, producing an agent that passes the suite and fails users.

Chasing regressions without a noise floor

Mistake: Four scenarios flip after a prompt edit, so the team spends two days finding the cause. Why it happens: Nobody ever ran the identical configuration twice, so run-to-run variance is unmeasured and every flip looks causal. Instead: Before trusting any diff, score the same configuration twice and diff it against itself. That flip count is your noise floor. Then reduce it — temperature 0 where the product allows, replayed tool results rather than live backends, and re-runs of flipped scenarios to separate flaky from real.

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