Gepa Fast Agent

GEPA Fast-Agent

Use this skill to turn a candidate-evaluation problem into a small, auditable GEPA loop backed by fast-agent primitives.

Core Shape

Think in this order:

1. Candidate: the mutable text GEPA can change, usually a prompt, policy, AgentCard variable, skill file, recipe, or code/config fragment.
2. Evidence: rows, artifacts, logs, screenshots, checker reports, telemetry, usage, traces, or command output produced by evaluating the candidate.
3. Score plus ASI: a numeric score and Actionable Side Information explaining why the candidate passed or failed.

GEPA consumes:

score, side_info = evaluator(candidate)

or, in dataset mode, per-example evaluation. Keep GEPA plumbing thin; put reusable execution and artifact capture in fast-agent primitives.

Non-Negotiable Frontier Rule

GEPA treats scores as maximize-only.

• The main evaluator score must be "higher is better".
• Every value in side_info["scores"] must also be "higher is better".
• Do not put raw loss, raw latency, raw cost, failure count, error count, timeout seconds, token count, or policy length into side_info["scores"] unless transformed so higher is better.
• Keep raw lower-is-better diagnostics outside side_info["scores"], for example under details, raw_metrics, summary, or failures.
• Name transformed metrics clearly: latency_score, cost_score, valid_json_rate, no_timeout, policy_length_compliance, failure_free_rate.
• Before running GEPA, audit all frontier keys and write down the transform for each one.

Examples:

side_info = {
    "scores": {
        "accuracy": accuracy,
        "latency_score": 1.0 / (1.0 + latency_seconds),
        "cost_score": 1.0 / (1.0 + dollars),
        "failure_free_rate": 1.0 - failure_count / max(total, 1),
    },
    "raw_metrics": {
        "latency_seconds": latency_seconds,
        "dollars": dollars,
        "failure_count": failure_count,
    },
}
return accuracy, side_info

If unsure, check the installed GEPA package source or upstream GEPA docs before deciding. Search for higher is better, SideInfo, scores, Pareto, and frontier candidate selection.

Choose The Fast-Agent Primitive

Use BatchRunner for row-oriented tasks:

from fast_agent.batch import BatchRunner

runner = BatchRunner(env_dir=".fast-agent", backend="process")
result = await runner.run(
    agent_card=".fast-agent/agent-cards/classifier.md",
    agent="classifier",
    input="eval/input.jsonl",
    template_source="eval/task-template.md",
    json_schema="eval/output.schema.json",
    variables={"policy": candidate["policy"]},
    output_path=candidate_run.path / "results.jsonl",
    summary_path=candidate_run.path / "batch-summary.json",
    telemetry_path=candidate_run.path / "telemetry.jsonl",
    overwrite=True,
)

Use FastAgentBatchEvaluator when the GEPA evaluator is mostly row/batch execution plus a scorer:

from fast_agent.integrations.gepa import FastAgentBatchEvaluator

evaluator = FastAgentBatchEvaluator(
    env_dir=".fast-agent",
    agent_card=".fast-agent/agent-cards/classifier.md",
    agent="classifier",
    candidate_variables={"policy": "policy"},
    input="eval/input.jsonl",
    template_source="eval/task-template.md",
    schema="eval/output.schema.json",
    scorer=score_candidate,
    run_dir="runs/gepa",
    backend="process",
)

Use EvalRun / CandidateRun for artifact tasks:

from fast_agent.eval import EvalRun

run = EvalRun("runs/gepa")
candidate_run = run.candidate()
candidate_run.materialize_candidate(candidate)
command = candidate_run.run_command(
    ["uv", "run", "python", "scripts/run_eval.py", "--candidate", str(candidate_run.path)],
    timeout_seconds=900,
    log_prefix="eval",
)
candidate_run.write_score(score_value, side_info, metadata={"ok": command.ok})
return score_value, side_info

Use FastAgentReflectionLM when GEPA reflection should use fast-agent model aliases/config and leave an audit trail:

from fast_agent.integrations.gepa import FastAgentReflectionLM

reflection_lm = FastAgentReflectionLM(
    env_dir=".fast-agent",
    model="responses.gpt-5.5?reasoning=high",
    audit_dir=run_dir / "reflection",
)

The reflection adapter should write prompt, request, response, timing, stdout/stderr for process runs, errors, and usage when --results contains fast-agent usage channels.

Trackio Monitoring

For long GEPA runs, add Trackio logging in the evaluator script so candidate scores and diagnostics are visible while the optimizer is running:

import trackio

trackio.init(project="gepa-openclaw", config={"split": "validation"})

# In a FastAgentBatchEvaluator scorer, candidate_run is available.
score, side_info = score_candidate(result, candidate, candidate_run)
trackio.log(
    {
        "score": score,
        **side_info.get("scores", {}),
        "candidate_index": candidate_run.index or 0,
    }
)

Trackio is for monitoring and retrieval, not GEPA frontier semantics. It is fine to log raw latency, token count, cost, failure count, or artifact size to Trackio, but do not copy those raw lower-is-better values into side_info["scores"]; transform them first or keep them under raw_metrics, summary, or details.

When running detached or remote jobs, use a Trackio Space via space_id and poll with CLI JSON output such as trackio list alerts --project --json or trackio get metric --project --run --metric score --json. Use trackio.alert() for stalled runs, repeated invalid outputs, failing smoke checks, or score regressions that should interrupt an autonomous loop.

Demo Card Pack

When the GEPA demo pack is available in the configured card-pack registry, use it as a quick smoke test for the evaluator shape it ships:

fast-agent go --pack gepa-demo
uv run .fast-agent/scripts/gepa-run.py --evaluate-only

Keep pack examples aligned with this skill:

• batch demo: AgentCard variables, FastAgentBatchEvaluator, FastAgentReflectionLM;
• artifact demo, when present: EvalRun, CandidateRun, checker reports, and write_score;
• all side_info["scores"] values higher-is-better;
• Trackio logging optional and separate from GEPA frontier semantics.

AgentCard Variables

For mutable prompts/policies, prefer declared AgentCard variables over rendering a new card per candidate:

---
type: agent
name: openclaw_classifier
model: "$system.default"
variables:
  policy: ""
---

{{file:eval/openclaw/allowed-topics.md}}

{{policy}}

Populate variables per run:

variables={"policy": candidate["policy"]}

For CLI smoke tests:

fast-agent batch run \
  --agent-card .fast-agent/agent-cards/classifier.md \
  --agent classifier \
  --input eval/input.jsonl \
  --output runs/candidate-0001/results.jsonl \
  --template eval/task-template.md \
  --json-schema eval/output.schema.json \
  --var-file policy=seed/policy.md \
  --overwrite \
  --no-final-summary

Scorer Pattern

Keep scoring user-owned and explicit:

1. Read normalized evidence: BatchRunResult.rows, CandidateRun reports, checker JSON, telemetry, logs, screenshots.
2. Compute one main maximize score.
3. Build ASI with concrete failure examples and repair guidance.
4. Put only maximize metrics in side_info["scores"].
5. Write raw diagnostics outside scores.
6. Persist score.json through CandidateRun.write_score() or the evaluator wrapper.

Good ASI includes:

• exact row/artifact IDs;
• expected vs actual;
• failure categories;
• stderr/stdout tails;
• checker findings;
• screenshots or gepa.Image where useful;
• concise actionable feedback;
• raw metrics and transformed frontier metrics.

Avoid ASI that only says "bad" or only returns aggregate numbers; GEPA needs the why.

Backend Choice

• Use backend="harness" for lower overhead in library/API contexts.
• Use backend="process" for optimizer loops that need isolation, natural stdout/stderr audit, timeouts, and CLI parity.
• Both backends should return the same structured result/artifact contract.

OpenClaw Pattern

Use for row/batch classification:

• static AgentCard with declared policy;
• FastAgentBatchEvaluator with candidate_variables={"policy": "policy"};
• scorer reads results.jsonl, summary, telemetry;
• ASI reports confusion, false positives/negatives, invalid JSON, representative failures, and boundary guidance;
• transform policy length, latency, failure count, or cost into maximize scores before adding to side_info["scores"].

Birch Pattern

Use for artifact-generation evals:

• candidate contains SKILL.md and recipe files;
• EvalRun allocates candidate dirs;
• CandidateRun materializes isolated skill/resource trees;
• CandidateRun.run_command() executes generation/checker scripts with timeout;
• reports include artifacts, screenshots, checker JSON/Markdown, stdout/stderr, optional VLM findings;
• scorer owns all weights and penalties, but all frontier keys remain higher-is-better.

Final Checklist

Before running a long GEPA job:

• Candidate mutation is limited to intended text fields.
• Candidate dirs contain candidate.json, variables or materialized files, evidence artifacts, logs, and score.json.
• Reflection calls are audited.
• Evaluator can run in --evaluate-only or smoke mode.
• All side_info["scores"] keys are higher-is-better.
• Raw lower-is-better values are stored outside scores.
• The main score is higher-is-better.
• The seed candidate and data split are frozen.
• Lint/typecheck/tests or smoke commands for changed repos have passed.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: fast-agent-ai
• Source: fast-agent-ai/skills
• License: MIT

Install and usage instructions live in the source repository linked above.