Agent Eval Skill

A lightweight CLI tool for comparing coding agents head-to-head on reproducible tasks. Every "which coding agent is best?" comparison runs on vibes — this tool systematizes it.

When to Activate

• Comparing coding agents (Claude Code, Aider, Codex, etc.) on your own codebase
• Measuring agent performance before adopting a new tool or model
• Running regression checks when an agent updates its model or tooling
• Producing data-backed agent selection decisions for a team

Installation

> Note: Install agent-eval from its repository after reviewing the source.

Core Concepts

YAML Task Definitions

Define tasks declaratively. Each task specifies what to do, which files to touch, and how to judge success:

yaml
name: add-retry-logic
description: Add exponential backoff retry to the HTTP client
repo: ./my-project
files:
  - src/http_client.py
prompt: |
  Add retry logic with exponential backoff to all HTTP requests.
  Max 3 retries. Initial delay 1s, max delay 30s.
judge:
  - type: pytest
    command: pytest tests/test_http_client.py -v
  - type: grep
    pattern: "exponential_backoff|retry"
    files: src/http_client.py
commit: "abc1234"  # pin to specific commit for reproducibility

Git Worktree Isolation

Each agent run gets its own git worktree — no Docker required. This provides reproducibility isolation so agents cannot interfere with each other or corrupt the base repo.

Metrics Collected

| Metric | What It Measures | |--------|-----------------| | Pass rate | Did the agent produce code that passes the judge? | | Cost | API spend per task (when available) | | Time | Wall-clock seconds to completion | | Consistency | Pass rate across repeated runs (e.g., 3/3 = 100%) |

Workflow

1. Define Tasks

Create a tasks/ directory with YAML files, one per task:

bash
mkdir tasks
# Write task definitions (see template above)

2. Run Agents

Execute agents against your tasks:

bash
agent-eval run --task tasks/add-retry-logic.yaml --agent claude-code --agent aider --runs 3

Each run:

1. Creates a fresh git worktree from the specified commit
2. Hands the prompt to the agent
3. Runs the judge criteria
4. Records pass/fail, cost, and time

3. Compare Results

Generate a comparison report:

bash
agent-eval report --format table

Task: add-retry-logic (3 runs each)
┌──────────────┬───────────┬────────┬────────┬─────────────┐
│ Agent        │ Pass Rate │ Cost   │ Time   │ Consistency │
├──────────────┼───────────┼────────┼────────┼─────────────┤
│ claude-code  │ 3/3       │ $0.12  │ 45s    │ 100%        │
│ aider        │ 2/3       │ $0.08  │ 38s    │  67%        │
└──────────────┴───────────┴────────┴────────┴─────────────┘

Judge Types

Code-Based (deterministic)

yaml
judge:
  - type: pytest
    command: pytest tests/ -v
  - type: command
    command: npm run build

Pattern-Based

yaml
judge:
  - type: grep
    pattern: "class.*Retry"
    files: src/**/*.py

Model-Based (LLM-as-judge)

yaml
judge:
  - type: llm
    prompt: |
      Does this implementation correctly handle exponential backoff?
      Check for: max retries, increasing delays, jitter.

Best Practices

• Start with 3-5 tasks that represent your real workload, not toy examples
• Run at least 3 trials per agent to capture variance — agents are non-deterministic
• Pin the commit in your task YAML so results are reproducible across days/weeks
• Include at least one deterministic judge (tests, build) per task — LLM judges add noise
• Track cost alongside pass rate — a 95% agent at 10x the cost may not be the right choice
• Version your task definitions — they are test fixtures, treat them as code

Links

• Repository: github.com/joaquinhuigomez/agent-eval