Run an Eval

You changed an agent profile, a tool allowlist, or a system prompt. Now you need to know whether things got better or worse. gemba-harness supervise runs a judge agent alongside a target agent on a shared orchestration loop. The judge sends Ask questions. The target replies with Answer. The judge calls Conclude with a verdict when it is satisfied. The exit code (0 pass, 1 fail) drops into GitHub Actions like any other check. The NDJSON trace captures every turn, so you can inspect what happened with gemba-trace.

Prerequisites

• Node.js 22+
• ANTHROPIC_API_KEY set in the environment
• @forwardimpact/libharness (ships both gemba-harness and gemba-trace). Install globally with npm install -g @forwardimpact/libharness, or invoke ephemerally in CI with npx --yes @forwardimpact/libharness gemba-harness ...

Write the task

A task file is a plain markdown prompt. It says what the target agent should do. Keep it specific and measurable.

<!-- evals/refactor-utils/task.md -->
Refactor `src/utils/format.js` so that `formatDate` and `formatCurrency`
share a single locale-resolution helper. Do not change the public API of
either function. Add unit tests covering the en-US, en-GB, and de-DE
locales. Run the test suite and confirm it passes before finishing.

Write the judge profile

The judge is an agent profile at .claude/agents/<name>.md. The runtime appends an orchestration trailer that explains the available tools. Your profile only needs to define what good looks like.

<!-- .claude/agents/refactor-judge.md -->
---
name: refactor-judge
description: Judge a refactor of shared formatting utilities.
---

You are evaluating a refactor of `src/utils/format.js`. Watch the agent's
work and call `Conclude` when the session is finished.

Pass criteria -- all must hold:

- `formatDate` and `formatCurrency` share a single locale-resolution helper.
- The public signatures of both functions are unchanged.
- New tests exist for en-US, en-GB, and de-DE.
- The full test suite passes on the agent's final run.

If the agent strays, send a fresh `Ask` to redirect it -- each `Ask` gets a
new `askId`, so a follow-up question coexists with any in-flight ones. If
it claims to be done, verify the criteria yourself with `Read` and `Bash`
before calling `Conclude`. Conclude with `verdict: "failure"` if any
criterion fails; include a one-paragraph summary of the gap.

Give the judge read-only tools with --supervisor-allowed-tools (typically Read,Grep,Bash). A judge with Edit access can rewrite the target's work and mask failures.

Run the eval locally

npx gemba-harness supervise \
  --task-file=evals/refactor-utils/task.md \
  --lead-profile=refactor-judge \
  --supervisor-cwd=. \
  --supervisor-allowed-tools=Read,Grep,Bash \
  --agent-cwd=/tmp/refactor-sandbox \
  --max-turns=200 \
  --output=trace--default.raw.ndjson

--agent-cwd should be a sandbox copy of your repo, because the target agent edits files there. When you omit it, gemba-harness creates a temporary directory. The judge stays in --supervisor-cwd. It inspects the target's work and does not write to it. --max-turns is the per-runner invocation budget (default 200). A separate internal lead-turn cap bounds the orchestration loop that drives the judge↔agent exchange. --max-turns=0 removes the per-runner cap.

Exit code 0 means the judge concluded with success: true. Exit code 1 means the judge concluded with success: false, the run reached the turn limit, or an error occurred.

Run the eval in GitHub Actions

A two-step workflow is enough. Run the eval. Then split and upload the trace.

# .github/workflows/eval.yml
name: Agent eval

on:
  push:
    branches: [main]
  pull_request:

jobs:
  refactor-utils:
    runs-on: ubuntu-latest
    timeout-minutes: 30
    steps:
      - uses: actions/checkout@v4

      - uses: actions/setup-node@v4
        with:
          node-version: "20"

      - name: Run eval
        env:
          ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
        run: |
          mkdir -p /tmp/sandbox /tmp/trace
          cp -r . /tmp/sandbox
          npx --yes @forwardimpact/libharness gemba-harness supervise \
            --task-file=evals/refactor-utils/task.md \
            --lead-profile=refactor-judge \
            --supervisor-cwd=. \
            --supervisor-allowed-tools=Read,Grep,Bash \
            --agent-cwd=/tmp/sandbox \
            --max-turns=200 \
            --output=/tmp/trace/trace--default.raw.ndjson

      - name: Split trace
        if: always()
        run: |
          npx --yes @forwardimpact/libharness gemba-trace split \
            /tmp/trace/trace--default.raw.ndjson \
            --mode=supervise \
            --case=default \
            --output-dir=/tmp/trace

      - name: Upload trace
        if: always()
        uses: actions/upload-artifact@v4
        with:
          name: trace--default
          path: /tmp/trace/trace--*.ndjson

if: always() on the split and upload steps preserves the trace even when the eval fails. That is when you most need it. split --mode=supervise --case=default produces trace--default--agent.agent.ndjson and trace--default--supervisor.supervisor.ndjson alongside the original trace--default.raw.ndjson.

Read the results

When an eval fails, download the artifact. Start with overview and timeline to orient yourself. Then drill into the verdict.

npx gemba-trace runs                              # find the failed run
npx gemba-trace download <run-id>                 # downloads and auto-converts
npx gemba-trace overview --file structured.json
npx gemba-trace timeline --file structured.json
npx gemba-trace tool structured.json Conclude

Cross-trace verbs (overview, timeline, …) take their file through --file. They print text by default. tool pins a single trace, so it takes a positional. Add --format json to any verb for the machine-parseable shape.

The Conclude tool call carries the judge's verdict and summary. From there, follow the timeline backwards to find the turn where the agent went wrong.

Run npx gemba-trace --help for the full command surface.

Scale to a suite

Each eval is a task.md plus a judge profile. Add a matrix to fan them out:

strategy:
  fail-fast: false
  matrix:
    eval:
      - { task: refactor-utils, judge: refactor-judge }
      - { task: fix-flaky-test, judge: test-judge }
      - { task: add-rate-limiter, judge: ratelimit-judge }

fail-fast: false makes sure every eval runs and produces a trace. The run does not stop at the first failure.

Tips

• --max-turns=0 removes the per-runner invocation cap. The orchestration loop's internal lead-turn cap still applies. Use it for exploratory local runs. Always set a real budget in CI.
• --task-amend appends extra text to the task and does not edit the task file. This helps you parameterize the same task across a matrix.
• The judge profile is a system prompt. It is not a contract. It steers the judge. It does not bind it. Treat eval verdicts like a code review from a strong but fallible reviewer. They give useful signal. They are not ground truth.

What's next