Skill — Eval Harness (Systematic Evaluation Framework)

When this skill activates

When measuring, scoring, or validating system outputs against defined criteria. Use for capability evaluation (can the system do X?), regression evaluation (does a change break existing behavior?), or comparative evaluation (is version A better than version B?). The eval harness ensures you define success BEFORE implementing, not after.

Core principle: Define-before-code — write evaluation criteria before writing the implementation they measure.

Mandatory actions when this skill is active

Before evaluation begins

1. Define the eval type:

• Capability eval: Can the system perform task X at acceptable quality?
• Regression eval: Does this change preserve existing behavior?
• Comparative eval: Is output A better than output B on criteria C?

1. Write the eval config BEFORE implementation:

   .mindforge/evals/[eval-name]/
   ├── config.json      # eval metadata, parameters, thresholds
   ├── rubric.md        # human-readable success criteria
   ├── test-cases.json  # input/expected-output pairs
   └── results.jsonl    # append-only results log

1. Define success criteria in config.json:

   {
     "name": "eval-name",
     "type": "capability" | "regression" | "comparative",
     "version": "1.0.0",
     "created": "ISO-8601",
     "thresholds": {
       "pass_at_1": 0.8,
       "pass_at_5": 0.95,
       "pass_at_10": 0.99
     },
     "grader": "code" | "model" | "human",
     "model_judge_config": {
       "model": "claude-sonnet",
       "rubric_path": "./rubric.md",
       "temperature": 0.0
     },
     "test_case_count": 0,
     "tags": []
   }

1. Write the rubric (rubric.md) with explicit scoring:

• Each criterion gets a 1-5 scale with concrete examples at each level
• Define what a "pass" means (minimum score per criterion)
• Define what a "fail" looks like with specific examples
• Include edge cases that should be tested

During evaluation

Three Grader Types:

1. Code-Based (Deterministic):

• Use when outputs have objectively verifiable properties
• Write assertion functions that return PASS/FAIL with evidence
• Examples: output matches regex, JSON schema validates, function returns expected value
• No ambiguity — the grader is a function, not a judgment call
• Always prefer code-based grading when possible (fastest, most reliable)

// Example code grader
function grade(output: string, expected: TestCase): GradeResult {
  const parsed = JSON.parse(output);
  return {
    pass: parsed.status === expected.status && parsed.count >= expected.minCount,
    evidence: `status=${parsed.status}, count=${parsed.count}`,
    criterion: "structural-correctness"
  };
}

2. Model-Based (LLM-as-Judge):

• Use when outputs require semantic understanding (prose quality, code correctness, reasoning)
• Always provide the rubric in the judge prompt — never rely on implicit standards
• Use temperature 0.0 for judge calls (determinism)
• Run judge 3x per item and take majority vote (reduces noise)
• Log the judge's reasoning alongside the score

Judge prompt structure:
1. Task description (what was the system asked to do?)
2. Rubric (what does good look like? what does bad look like?)
3. The output to grade
4. Instruction: score 1-5 per criterion, explain each score, give overall PASS/FAIL

3. Human-Based (Flag for Review):

• Use when stakes are too high for automated judgment
• Generate a review queue with: input, output, rubric, suggested-score
• Human confirms or overrides the suggested score
• Track inter-rater reliability if multiple humans review

pass@k Metrics:

• Generate k independent outputs for each test case
• pass@1: Fraction of test cases where the first output passes
• pass@5: Fraction where at least 1 of 5 outputs passes
• pass@10: Fraction where at least 1 of 10 outputs passes
• Formula: pass@k = 1 - C(n-c, k) / C(n, k) where n=total, c=correct
• Always report pass@1 (baseline) and at least one higher-k metric
• Use pass@1 for production readiness, pass@k for capability ceiling

Result logging (results.jsonl):

{
  "timestamp": "ISO-8601",
  "test_case_id": "tc-001",
  "input": "...",
  "output": "...",
  "grader": "code",
  "scores": {"criterion_a": 4, "criterion_b": 5},
  "pass": true,
  "evidence": "...",
  "latency_ms": 0,
  "model_version": "...",
  "run_id": "uuid"
}

After evaluation

1. Compute aggregate metrics:

• Overall pass rate (pass@1, pass@5, pass@10)
• Per-criterion score distribution
• Failure mode clustering (what patterns cause failures?)
• Comparison to previous run (regression detection)

1. Regression detection logic:

• If pass@1 drops > 5% from previous run: FLAG as regression
• If any previously-passing test case now fails: FLAG as regression
• If new failure modes appear that didn't exist before: FLAG as regression
• Regressions block shipping until investigated

1. Store results:

• Append to results.jsonl (never overwrite)
• Update config.json with latest run metadata
• If regression detected: create .mindforge/evals/[name]/REGRESSION.md

1. Report format:

   ## Eval Report: [eval-name]
   - Type: capability | regression | comparative
   - Run: [run-id] at [timestamp]
   - Test cases: N total, P passed, F failed
   - pass@1: X% | pass@5: Y% | pass@10: Z%
   - Threshold: pass@1 >= T% → [MET / NOT MET]
   - Regressions: [none | list]
   - Top failure modes: [list with counts]

Self-check before task completion

Before marking a task done when this skill was active:

• [ ] Did I define success criteria BEFORE writing implementation code?
• [ ] Did I choose the appropriate grader type (code > model > human preference)?
• [ ] Did I track pass@k metrics (at minimum pass@1)?
• [ ] Did I run regression evals against previous results?
• [ ] Are results stored in .mindforge/evals/[name]/results.jsonl?
• [ ] If model-based grading: did I use temperature 0.0 and majority vote?
• [ ] Did I report failure modes, not just pass rates?
• [ ] Is the rubric explicit enough that another reviewer could grade independently?