Writing Evals

Writing Evals

You write evaluations that prove AI capabilities work. Evals are the test suite for non-deterministic systems: they measure whether a capability still behaves correctly after every change.

Prerequisites

• Complete the Axiom AI SDK Quickstart (instrumentation + authentication)

Verify the SDK is installed:

ls node_modules/axiom/dist/

If not installed, install it using the project's package manager (e.g., pnpm add axiom).

Always check `node_modules/axiom/dist/docs/` first for the correct API signatures, import paths, and patterns for the installed SDK version. The bundled docs are the source of truth — do not rely on the examples in this skill if they conflict.

Philosophy

1. Evals are tests for AI. Every eval answers: "does this capability still work?"
2. Scorers are assertions. Each scorer checks one property of the output.
3. Flags are variables. Flag schemas let you sweep models, temperatures, strategies without code changes.
4. Data drives coverage. Happy path, adversarial, boundary, and negative cases.
5. Validate before running. Never guess import paths or types—use reference docs.

Axiom Terminology

How to Start

When the user asks you to write evals for an AI feature, read the code first. Do not ask questions — inspect the codebase and infer everything you can.

Step 1: Understand the feature

1. Find the AI function — search for the function the user mentioned. Read it fully.
2. Trace the inputs — what data goes in? A string prompt, structured object, conversation history?
3. Trace the outputs — what comes back? A string, category label, structured object, agent result with tool calls?
4. Identify the model call — which LLM/model is used? What parameters (temperature, maxTokens)?
5. Check for existing evals — search for *.eval.ts files. Don't duplicate what exists.
6. Check for app-scope — look for createAppScope, flagSchema, axiom.config.ts.

Step 2: Determine eval type

Based on what you found:

Step 3: Choose scorers

Every eval needs at least 2 scorers. Use this layering:

1. Correctness scorer (required) — Does the output match expected? Pick from the eval type table above (exact match, set match, field match, etc.).
2. Quality scorer (recommended) — Is the output well-formed? Check confidence thresholds, output length, format validity, or field completeness.
3. Reference-free scorer (add for user-facing text) — Is the output coherent, relevant, non-toxic? Use LLM-as-judge or autoevals.

Step 4: Generate

1. Create the .eval.ts file colocated next to the source file
2. Import the actual function — do not create a stub
3. Write the scorers based on the output type (minimum 2, see step 3)
4. Generate test data (see Data Design Guidelines)
5. Set capability and step names matching the feature's purpose
6. If flags exist, use pickFlags to scope them

Only ask if you cannot determine:

• What "correct" means for ambiguous outputs (e.g., summarization quality)
• Whether the user wants pass/fail or partial credit scoring
• Which parameters should be tunable via flags (if not already using flags)

Project Layout

Recommended: Colocated with source

Place .eval.ts files next to their implementation files, organized by capability:

src/
├── lib/
│   ├── app-scope.ts
│   └── capabilities/
│       └── support-agent/
│           ├── support-agent.ts
│           ├── support-agent-e2e-tool-use.eval.ts
│           ├── categorize-messages.ts
│           ├── categorize-messages.eval.ts
│           ├── extract-ticket-info.ts
│           └── extract-ticket-info.eval.ts
axiom.config.ts
package.json

Minimal: Flat structure

For small projects, keep everything in src/:

src/
├── app-scope.ts
├── my-feature.ts
└── my-feature.eval.ts
axiom.config.ts
package.json

The default glob **/*.eval.{ts,js} discovers eval files anywhere in the project. axiom.config.ts always lives at the project root.

Eval File Structure

Standard structure of an eval file:

import { pickFlags } from '@/app-scope';       // or relative path
import { Eval } from 'axiom/ai/evals';
import { Scorer } from 'axiom/ai/scorers';
import { Mean, PassHatK } from 'axiom/ai/scorers/aggregations';
import { myFunction } from './my-function';

const MyScorer = Scorer('my-scorer', ({ output, expected }: { output: string; expected: string }) => {
  return output === expected;
});

Eval('my-eval-name', {
  capability: 'my-capability',
  step: 'my-step',                              // optional
  configFlags: pickFlags('myCapability'),        // optional, scopes flag access
  data: [
    { input: '...', expected: '...', metadata: { purpose: '...' } },
  ],
  task: async ({ input }) => {
    return await myFunction(input);
  },
  scorers: [MyScorer],
});

Reference

For detailed patterns and type signatures, read these on demand:

• reference/scorer-patterns.md — All scorer patterns (exact match, set match, structured, tool use, autoevals, LLM-as-judge), score return types, typing tips
• reference/api-reference.md — Full type signatures, import paths, aggregations, streaming tasks, dynamic data loading, manual token tracking, CLI options
• reference/flag-schema-guide.md — Flag schema rules, validation, pickFlags, CLI overrides, common patterns
• reference/templates/ — Ready-to-use eval file templates (see Templates section below)

Authentication Setup

Before running evals, the user must authenticate. Check if they've already done this before suggesting it.