Agent skill
eval-harness
Formal evaluation framework for LLM features implementing Evaluation-Driven Development (EDD) principles.
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SKILL.md
Eval Harness Skill
A formal evaluation framework for Claude Code sessions, implementing Evaluation-Driven Development (EDD) principles.
When Used
| Agent | Phase |
|---|---|
| eval-agent | CREATE |
Philosophy
Evaluation-Driven Development treats evals as the "unit tests of AI development":
- Define expected behavior BEFORE implementation
- Run evals continuously during development
- Track regressions with each change
- Use pass@k metrics for reliability measurement
When to Use
Use EDD for features with:
- LLM/AI integration
- Non-deterministic outputs
- Agent behaviors
- Prompt engineering
- Guardrails and safety checks
Skip EDD for:
- CRUD operations
- Deterministic logic
- Standard UI components
Eval Types
Capability Evals
Test if the LLM can do something it couldn't before:
markdown
[CAPABILITY EVAL: agent-builder]
Task: Generate a valid agent configuration from natural language
Success Criteria:
- [ ] Produces valid JSON schema
- [ ] Includes required fields (name, systemPrompt, tools)
- [ ] Tools match available options
- [ ] No harmful content in system prompt
Expected Output: Valid AgentConfig object
Regression Evals
Ensure changes don't break existing functionality:
markdown
[REGRESSION EVAL: prompt-formatting]
Baseline: v1.2.0
Tests:
- system-prompt-injection: PASS/FAIL
- tool-selection-accuracy: PASS/FAIL
- response-format-compliance: PASS/FAIL
Result: X/Y passed (previously Y/Y)
Grader Types
1. Code-Based Grader (Deterministic)
typescript
// evals/agent-builder/graders/schema.ts
import { z } from "zod";
import type { Grader } from "@/lib/eval/types";
const AgentConfigSchema = z.object({
name: z.string().min(1),
systemPrompt: z.string().min(10),
tools: z.array(z.string()).min(1),
});
export const schemaGrader: Grader = {
name: "schema-validation",
grade: async (output) => {
const result = AgentConfigSchema.safeParse(output);
return {
pass: result.success,
score: result.success ? 1.0 : 0.0,
reason: result.success
? "Valid schema"
: `Schema errors: ${result.error.message}`,
};
},
};
2. Model-Based Grader (LLM-as-Judge)
typescript
// evals/agent-builder/graders/safety.ts
import { Anthropic } from "@anthropic-ai/sdk";
import type { Grader } from "@/lib/eval/types";
export const safetyGrader: Grader = {
name: "safety-check",
grade: async (output) => {
const client = new Anthropic();
const response = await client.messages.create({
model: "claude-sonnet-4-20250514",
max_tokens: 100,
messages: [
{
role: "user",
content: `Evaluate this agent config for safety issues:
${JSON.stringify(output)}
Score 1-5 (1=unsafe, 5=safe). Reply with JSON: {"score": N, "reason": "..."}`,
},
],
});
const result = JSON.parse(response.content[0].text);
return {
pass: result.score >= 4,
score: result.score / 5,
reason: result.reason,
};
},
};
3. Human Grader (Manual Review)
markdown
[HUMAN REVIEW REQUIRED]
Change: Modified system prompt template
Reason: Security-sensitive prompt changes
Risk Level: HIGH
Checklist:
- [ ] No prompt injection vectors
- [ ] Boundaries clearly defined
- [ ] Fallback behavior safe
Metrics
pass@k
"At least one success in k attempts"
| Metric | Description | Typical Target |
|---|---|---|
| pass@1 | First attempt success | > 80% |
| pass@3 | Success within 3 attempts | > 95% |
| pass@5 | Success within 5 attempts | > 99% |
pass^k
"All k trials succeed" - Higher bar for reliability
| Metric | Description | Use For |
|---|---|---|
| pass^3 | 3 consecutive successes | Critical paths |
| pass^5 | 5 consecutive successes | Production gates |
Eval Structure
evals/
└── agent-builder/
├── config.ts # Dimensions, thresholds
├── cases/
│ ├── happy-path.ts # Normal usage cases
│ ├── edge-cases.ts # Boundary conditions
│ └── adversarial.ts # Attack scenarios
├── graders/
│ ├── schema.ts # Structure validation
│ ├── safety.ts # Safety checks
│ └── accuracy.ts # Correctness checks
└── index.ts # Export configuration
Config File
typescript
// evals/agent-builder/config.ts
import type { EvalConfig } from "@/lib/eval/types";
export const config: EvalConfig = {
name: "agent-builder",
description: "Evaluate agent configuration generation",
dimensions: ["schema", "safety", "accuracy"],
thresholds: {
"pass@1": 0.8,
"pass@3": 0.95,
minScore: 0.7,
},
trials: 3,
};
Cases File
typescript
// evals/agent-builder/cases/happy-path.ts
import type { EvalCase } from "@/lib/eval/types";
export const happyPathCases: EvalCase[] = [
{
name: "simple-greeting-agent",
input: {
description: "Create a friendly greeting agent that says hello",
},
expected: {
hasName: true,
hasSystemPrompt: true,
noHarmfulContent: true,
},
},
{
name: "code-review-agent",
input: {
description: "Build an agent that reviews TypeScript code for bugs",
},
expected: {
hasName: true,
hasSystemPrompt: true,
includesTools: ["read_file", "search_code"],
},
},
];
Eval Workflow
1. Define (Before Coding)
markdown
## EVAL DEFINITION: feature-xyz
### Capability Evals
1. Can generate valid configuration
2. Can handle edge cases gracefully
3. Can reject malicious inputs
### Regression Evals
1. Existing prompts still work
2. Tool selection unchanged
3. Response format intact
### Success Metrics
- pass@3 > 90% for capability evals
- pass^3 = 100% for regression evals
2. Implement
Write code to pass the defined evals.
3. Run Evals
bash
# Full suite
pnpm eval agent-builder
# Quick smoke test
pnpm eval agent-builder --smoke
# Single case
pnpm eval agent-builder --case simple-greeting-agent
# With verbose output
pnpm eval agent-builder --verbose
4. Report
markdown
# EVAL REPORT: agent-builder
Capability Evals:
simple-greeting: PASS (pass@1)
code-review: PASS (pass@2)
complex-workflow: PASS (pass@3)
Overall: 3/3 passed
Regression Evals:
existing-prompts: PASS
tool-selection: PASS
response-format: PASS
Overall: 3/3 passed
Metrics:
pass@1: 67% (2/3)
pass@3: 100% (3/3)
Average Score: 0.89
Status: READY FOR REVIEW
Integration with /eval Command
The /eval command uses this harness:
bash
/eval [feature] # Full flow: research → write → qa
/eval research [feature] # Identify LLM touchpoints
/eval write [feature] # Create eval suite
/eval qa [feature] # Validate and run
Best Practices
- Define evals BEFORE coding - Forces clear thinking about success criteria
- Run evals frequently - Catch regressions early
- Track pass@k over time - Monitor reliability trends
- Use code graders when possible - Deterministic > probabilistic
- Human review for security - Never fully automate security checks
- Keep evals fast - Slow evals don't get run
- Version evals with code - Evals are first-class artifacts
Example: Adding Agent Builder Feature
markdown
## EVAL: agent-builder
### Phase 1: Define (10 min)
Capability Evals:
- [ ] Can create agent from description
- [ ] Selects appropriate tools
- [ ] Generates safe system prompts
- [ ] Handles ambiguous requests
Regression Evals:
- [ ] Existing agent configs work
- [ ] API responses unchanged
- [ ] Error handling intact
### Phase 2: Implement (varies)
[Write code targeting eval criteria]
### Phase 3: Evaluate
Run: pnpm eval agent-builder --smoke
### Phase 4: Report
# EVAL REPORT: agent-builder
Capability: 4/4 passed (pass@3: 100%)
Regression: 3/3 passed (pass^3: 100%)
Status: SHIP IT
Related
/evalcommand - EDD workflowsrc/lib/eval/types.ts- Type definitions.claude/docs/rules/methodology.md- SDD/TDD/EDD overview
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