---
name: ax-gepa
description: This skill helps an LLM generate correct AxGEPA optimization code using @ax-llm/ax. Use when the user asks about AxGEPA, GEPA, Pareto optimization, multi-objective prompt tuning, reflective prompt evolution, validationExamples, maxMetricCalls, or optimizing a generator, flow, or agent tree.
version: "19.0.33"
---

# AxGEPA Codegen Rules (@ax-llm/ax)

Use this skill to generate direct `AxGEPA` optimization code. Prefer short, modern, copyable patterns over long explanation.

## Use These Defaults

- Use `new AxGEPA({ studentAI, teacherAI, ... })`.
- Prefer `ai()`, `ax()`, and `flow()` for new code.
- Use a strong `teacherAI` and a cheaper `studentAI`.
- Always pass `validationExamples` to `compile()`.
- Always set `maxMetricCalls` to bound optimizer cost.
- Use scalar metrics for one objective and object metrics for Pareto optimization.
- Apply results with `program.applyOptimization(result.optimizedProgram!)`.
- For tree-wide runs, expect `optimizedProgram.instructionMap`.

## Critical Rules

- `AxGEPA.compile()` works for a single generator and for tree-aware roots such as flows or agents with registered instruction-bearing descendants.
- There is no separate flow-only GEPA optimizer. Use `AxGEPA` for flows too.
- The metric may return either `number` or `Record<string, number>`.
- Keep metrics deterministic and cheap by default.
- Avoid extra LLM calls inside the metric unless the user explicitly wants judge-based evaluation.
- If the user needs LLM-as-judge scoring for a non-agent GEPA run, prefer a plain typed `AxGen` evaluator instead of writing a custom judge abstraction.
- `maxMetricCalls` must be large enough to cover the initial validation pass over `validationExamples`.
- GEPA optimizes instructions. If a tree has no instruction-bearing nodes, optimization will fail.
- Use held-out validation examples for selection. Do not reuse the training set as `validationExamples`.
- `result.optimizedProgram` is the easy-to-apply best candidate. `result.paretoFront` is the full trade-off set for multi-objective runs.

## Metric Selection

Choose the evaluation path deliberately:

- Prefer a deterministic metric when correctness can be read directly from `prediction` and `example`.
- Prefer a deterministic metric when cost, latency, recursion depth, or tool count matters.
- Use a plain typed `AxGen` evaluator only when the task is genuinely qualitative and hard to score exactly.
- For `agent.optimize(...)`, prefer the built-in judge path instead of manually wrapping a judge metric.

Rule of thumb:

- `AxGEPA` on `AxGen` or flow: use a metric first, optionally a plain typed `AxGen` evaluator if needed.
- `agent.optimize(...)`: use custom `metric` for crisp scoring, otherwise `judgeAI` plus `judgeOptions`.

## Canonical Scalar Pattern

```typescript
import { ai, ax, AxAIOpenAIModel, AxGEPA } from '@ax-llm/ax';

const student = ai({
  name: 'openai',
  apiKey: process.env.OPENAI_APIKEY!,
  config: { model: AxAIOpenAIModel.GPT4OMini },
});

const teacher = ai({
  name: 'openai',
  apiKey: process.env.OPENAI_APIKEY!,
  config: { model: AxAIOpenAIModel.GPT4O },
});

const classifier = ax(
  'emailText:string -> priority:class "high, normal, low", rationale:string'
);

const train = [
  { emailText: 'URGENT: Server down!', priority: 'high' },
  { emailText: 'Weekly newsletter', priority: 'low' },
];

const validation = [
  { emailText: 'Invoice overdue', priority: 'high' },
  { emailText: 'Lunch plans?', priority: 'low' },
];

const metric = ({ prediction, example }: { prediction: any; example: any }) =>
  prediction?.priority === example?.priority ? 1 : 0;

const optimizer = new AxGEPA({
  studentAI: student,
  teacherAI: teacher,
  numTrials: 12,
  minibatch: true,
  minibatchSize: 4,
  earlyStoppingTrials: 4,
  sampleCount: 1,
});

const result = await optimizer.compile(classifier, train, metric, {
  validationExamples: validation,
  maxMetricCalls: 120,
});

classifier.applyOptimization(result.optimizedProgram!);
console.log(result.bestScore);
```

## Canonical Pareto Pattern

```typescript
import { ai, flow, AxAIOpenAIModel, AxGEPA } from '@ax-llm/ax';

const student = ai({
  name: 'openai',
  apiKey: process.env.OPENAI_APIKEY!,
  config: { model: AxAIOpenAIModel.GPT4OMini },
});

const teacher = ai({
  name: 'openai',
  apiKey: process.env.OPENAI_APIKEY!,
  config: { model: AxAIOpenAIModel.GPT4O },
});

const wf = flow<{ emailText: string }>()
  .n('classifier', 'emailText:string -> priority:class "high, normal, low"')
  .n(
    'rationale',
    'emailText:string, priority:string -> rationale:string "One concise sentence"'
  )
  .e('classifier', (state) => ({ emailText: state.emailText }))
  .e('rationale', (state) => ({
    emailText: state.emailText,
    priority: state.classifierResult.priority,
  }))
  .r((state) => ({
    priority: state.classifierResult.priority,
    rationale: state.rationaleResult.rationale,
  }));

const train = [
  { emailText: 'URGENT: Server down!', priority: 'high' },
  { emailText: 'Weekly newsletter', priority: 'low' },
];

const validation = [
  { emailText: 'Invoice overdue', priority: 'high' },
  { emailText: 'Lunch plans?', priority: 'low' },
];

const metric = ({ prediction, example }: { prediction: any; example: any }) => {
  const accuracy = prediction?.priority === example?.priority ? 1 : 0;
  const rationale = typeof prediction?.rationale === 'string'
    ? prediction.rationale
    : '';
  const brevity = rationale.length <= 40 ? 1 : rationale.length <= 80 ? 0.5 : 0.1;
  return { accuracy, brevity };
};

const result = await new AxGEPA({
  studentAI: student,
  teacherAI: teacher,
  numTrials: 16,
  minibatch: true,
  minibatchSize: 6,
  earlyStoppingTrials: 5,
  sampleCount: 1,
}).compile(wf, train, metric, {
  validationExamples: validation,
  maxMetricCalls: 240,
});

for (const point of result.paretoFront) {
  console.log(point.scores, point.configuration);
}

wf.applyOptimization(result.optimizedProgram!);
console.log(result.optimizedProgram?.instructionMap);
```

## Metric Patterns

```typescript
// Scalar objective
const scalarMetric = ({ prediction, example }) =>
  prediction.answer === example.answer ? 1 : 0;

// Multi-objective
const multiMetric = ({ prediction, example }) => ({
  accuracy: prediction.answer === example.answer ? 1 : 0,
  brevity:
    typeof prediction?.reasoning === 'string' &&
    prediction.reasoning.length < 120
      ? 1
      : 0.2,
});
```

- Return plain numbers or plain object literals.
- Keep objective names stable across calls.
- Prefer normalized scores such as `0..1` so trade-offs are easy to reason about.

## Result Handling

```typescript
const { optimizedProgram, paretoFront } = result;

program.applyOptimization(optimizedProgram!);

// Save for later
const saved = JSON.stringify(optimizedProgram);

// Load later and re-apply
const loaded = JSON.parse(saved);
program.applyOptimization(loaded);
```

- Single-target runs usually populate both `optimizedProgram.instruction` and `optimizedProgram.instructionMap`.
- Tree-wide runs rely on `instructionMap`, keyed by full program ID.
- Pareto points expose candidate configs under `point.configuration.instructionMap`.

## Useful Options

```typescript
const optimizer = new AxGEPA({
  studentAI,
  teacherAI,
  numTrials: 20,
  minibatch: true,
  minibatchSize: 5,
  minibatchFullEvalSteps: 5,
  earlyStoppingTrials: 5,
  minImprovementThreshold: 0,
  sampleCount: 1,
  seed: 42,
  verbose: true,
});
```

- `numTrials`: number of reflection/evolution rounds.
- `minibatch`: reduce per-round evaluation cost.
- `minibatchSize`: examples per minibatch.
- `earlyStoppingTrials`: stop after repeated non-improvement.
- `minImprovementThreshold`: reject tiny gains below this threshold.
- `seed`: stabilize sampling during demos and tests.

## Budgeting and Validation

- Always create distinct `train` and `validationExamples` arrays.
- Size `maxMetricCalls` for at least one full validation pass plus several rounds.
- If the user wants a strict budget, say so explicitly and set `maxMetricCalls`.
- For expensive trees, start with `auto: 'light'` or fewer `numTrials`, then scale up.

## Troubleshooting

- Error about `maxMetricCalls` being too small: increase it until the initial validation pass fits.
- Empty or poor Pareto front: verify the metric returns numbers for every example.
- No tree optimization effect: ensure child programs are registered under the root and have instructions to mutate.
- Saved optimization applies only partly: use `program.applyOptimization(...)`, not just `setInstruction(...)`, so `instructionMap` reaches the full tree.

## Good Example Targets

- `/Users/vr/src/ax/src/examples/gepa.ts`
- `/Users/vr/src/ax/src/examples/gepa-flow.ts`
- `/Users/vr/src/ax/src/examples/gepa-train-inference.ts`
- `/Users/vr/src/ax/src/examples/gepa-quality-vs-speed-optimization.ts`