@vercel/agent-eval

Test AI coding agents on your framework. Measure what actually works.

Why?

You're building a frontend framework and want AI agents to work well with it. But how do you know if:

Your documentation helps agents write correct code?
Adding an MCP server improves agent success rates?
Sonnet performs as well as Opus for your use cases?
Your latest API changes broke agent compatibility?

This framework gives you answers. Run controlled experiments, measure pass rates, compare techniques.

Quick Start

# Create a new eval project
npx @vercel/agent-eval init my-agent-evals
cd my-agent-evals

# Install dependencies
npm install

# Add your API keys
cp .env.example .env
# Edit .env with your AI_GATEWAY_API_KEY and VERCEL_TOKEN

# Preview what will run (no API calls, no cost)
npx @vercel/agent-eval --dry

# Run all experiments
npx @vercel/agent-eval

CLI

Run all experiments

npx @vercel/agent-eval

With no arguments, the CLI discovers every experiments/*.ts file and runs them all. Each experiment runs in parallel. Results with matching fingerprints are reused automatically (see Result Reuse).

Run a single experiment

npx @vercel/agent-eval cc

The argument is the experiment filename without .ts. This resolves to experiments/cc.ts.

Flags

Flag	Description
`--dry`	Preview what would run without executing. No API calls, no cost.
`--smoke`	Quick setup verification. Picks the first eval alphabetically, runs once per model.
`--force`	Ignore cached fingerprints and re-run everything. Only applies when running all.
`--ack-failures`	Keep non-model failures as final results instead of retrying or deleting them.
`--max-retries <n>`	Maximum retry rounds for non-model failures (default: 5). Set to 0 to disable retries.

Flags work with both modes:

npx @vercel/agent-eval --dry          # preview all experiments
npx @vercel/agent-eval cc --dry       # preview a single experiment
npx @vercel/agent-eval --smoke        # smoke test all experiments
npx @vercel/agent-eval cc --smoke     # smoke test one experiment

Other commands

npx @vercel/agent-eval init <name>          # scaffold a new eval project
npx @vercel/agent-eval playground           # launch web-based results viewer
npx @vercel/agent-eval playground --watch   # live mode (watches for new results)

Creating Evals

Each eval tests one specific task an agent should be able to do with your framework.

Directory structure

evals/
  create-button-component/
    PROMPT.md           # Task for the agent
    EVAL.ts             # Tests to verify success (or EVAL.tsx for JSX)
    package.json        # Your framework as a dependency
    src/                # Starter code

PROMPT.md -- what you want the agent to do:

Create a Button component using MyFramework.

Requirements:
- Export a Button component from src/components/Button.tsx
- Accept `label` and `onClick` props
- Use the framework's styling system for hover states

EVAL.ts -- how you verify it worked:

import { test, expect } from 'vitest';
import { readFileSync, existsSync } from 'fs';
import { execSync } from 'child_process';

test('Button component exists', () => {
  expect(existsSync('src/components/Button.tsx')).toBe(true);
});

test('has required props', () => {
  const content = readFileSync('src/components/Button.tsx', 'utf-8');
  expect(content).toContain('label');
  expect(content).toContain('onClick');
});

test('project builds', () => {
  execSync('npm run build', { stdio: 'pipe' });
});

Use EVAL.tsx when your tests require JSX syntax (React Testing Library, component rendering). You only need one eval file per fixture -- choose .tsx if any test needs JSX.

Asserting on agent behavior

EVAL.ts tests can assert not just on the files the agent produced, but on how it worked — which shell commands it ran, which files it read, how many tool calls it made, etc. The framework automatically parses the agent's transcript and writes the results to __agent_eval__/results.json in the sandbox before your tests run.

import { test, expect } from 'vitest';
import { readFileSync } from 'fs';

test('agent used the correct scaffolding command', () => {
  const results = JSON.parse(readFileSync('__agent_eval__/results.json', 'utf-8'));
  const commands = results.o11y.shellCommands.map((c: { command: string }) => c.command);
  expect(commands).toContain('npx create-next-app project');
});

test('agent did not make excessive tool calls', () => {
  const results = JSON.parse(readFileSync('__agent_eval__/results.json', 'utf-8'));
  expect(results.o11y.totalToolCalls).toBeLessThan(50);
});

The results.o11y object is a TranscriptSummary with these fields:

Field	Type	Description
`shellCommands`	`{ command, exitCode?, success? }[]`	Shell commands the agent ran
`filesRead`	`string[]`	Files the agent read
`filesModified`	`string[]`	Files the agent wrote or edited
`toolCalls`	`Record<ToolName, number>`	Count of each tool type used
`totalToolCalls`	`number`	Total tool calls made
`webFetches`	`{ url, method?, status?, success? }[]`	Web fetches made
`totalTurns`	`number`	Conversation turns
`errors`	`string[]`	Errors encountered
`thinkingBlocks`	`number`	Thinking/reasoning blocks

Note: If the agent's transcript is unavailable (e.g. the agent crashed before producing output), results.o11y will be null.

Configuration Reference

Experiment config

// experiments/my-experiment.ts
import type { ExperimentConfig } from '@vercel/agent-eval';

const config: ExperimentConfig = {
  // Required: which agent to use
  agent: 'vercel-ai-gateway/claude-code',

  // Model to use (defaults vary by agent)
  // Provide an array to run the same experiment across multiple models.
  model: 'opus',

  // How many times to run each eval (default: 1)
  runs: 10,

  // Stop after first success? (default: true)
  earlyExit: false,

  // npm scripts that must pass after agent finishes (default: [])
  scripts: ['build', 'lint'],

  // Timeout per run in seconds (default: 600)
  timeout: 600,

  // Filter which evals to run (default: '*' for all)
  evals: '*',
  // evals: ['specific-eval'],
  // evals: (name) => name.startsWith('api-'),

  // Setup function for sandbox pre-configuration
  setup: async (sandbox) => {
    await sandbox.writeFiles({ '.env': 'API_KEY=test' });
    await sandbox.runCommand('npm', ['run', 'setup']);
  },

  // Rewrite the prompt before running
  editPrompt: (prompt) => `Use the skill.\n\n${prompt}`,

  // Sandbox backend (default: 'auto' -- Vercel if token present, else Docker)
  sandbox: 'auto',

  // Copy project files to results directory (default: 'none')
  // 'none' - don't copy files
  // 'changed' - copy only files modified by the agent
  // 'all' - copy the entire project including original fixture files
  copyFiles: 'changed',
};

export default config;

Agent selection

// Vercel AI Gateway (recommended -- unified billing and observability)
agent: 'vercel-ai-gateway/claude-code'  // Claude Code via AI Gateway
agent: 'vercel-ai-gateway/codex'        // OpenAI Codex via AI Gateway
agent: 'vercel-ai-gateway/opencode'     // OpenCode via AI Gateway

// Direct API (uses provider keys directly)
agent: 'claude-code'  // requires ANTHROPIC_API_KEY
agent: 'codex'        // requires OPENAI_API_KEY
agent: 'gemini'       // requires GEMINI_API_KEY
agent: 'cursor'       // requires CURSOR_API_KEY

Multi-model experiments

Provide an array of models to run the same experiment on each one. Results are stored under separate directories (experiment-name/model-name):

const config: ExperimentConfig = {
  agent: 'vercel-ai-gateway/claude-code',
  model: ['opus', 'sonnet'],
  runs: 10,
};

OpenCode model format

OpenCode uses Vercel AI Gateway exclusively. Models must use the vercel/{provider}/{model} format:

model: 'vercel/anthropic/claude-sonnet-4'
model: 'vercel/openai/gpt-4o'
model: 'vercel/moonshotai/kimi-k2'
model: 'vercel/minimax/minimax-m2.1'

The vercel/ prefix is required. Using anthropic/claude-sonnet-4 (without vercel/) will fail with a "provider not found" error.

A/B Testing

The real power is comparing different approaches. Create multiple experiment configs:

// experiments/control.ts
import type { ExperimentConfig } from '@vercel/agent-eval';

const config: ExperimentConfig = {
  agent: 'vercel-ai-gateway/claude-code',
  model: 'opus',
  runs: 10,
  earlyExit: false,
};

export default config;

// experiments/with-mcp.ts
import type { ExperimentConfig } from '@vercel/agent-eval';

const config: ExperimentConfig = {
  agent: 'vercel-ai-gateway/claude-code',
  model: 'opus',
  runs: 10,
  earlyExit: false,
  setup: async (sandbox) => {
    await sandbox.runCommand('npm', ['install', '-g', '@myframework/mcp-server']);
    await sandbox.writeFiles({
      '.claude/settings.json': JSON.stringify({
        mcpServers: { myframework: { command: 'myframework-mcp' } }
      })
    });
  },
};

export default config;

npx @vercel/agent-eval

Compare the results:

control (baseline):     7/10 passed (70%)
with-mcp:              9/10 passed (90%)

Experiment	Control	Treatment
MCP impact	No MCP	With MCP server
Model comparison	Haiku	Sonnet / Opus
Documentation	Minimal docs	Rich examples
System prompt	Default	Framework-specific
Tool availability	Read/write only	+ custom tools

Results

Results are saved to results/<experiment>/<timestamp>/:

results/
  with-mcp/
    2026-01-27T10-30-00Z/
      create-button/
        summary.json            # Pass rate, fingerprint, classification
        classification.json     # Cached failure classification (if failed)
        run-1/
          result.json           # Individual run result + o11y summary
          transcript.json       # Parsed/structured agent transcript
          transcript-raw.jsonl  # Raw agent output (for debugging)
          outputs/
            eval.txt            # EVAL.ts test output
            scripts/
              build.txt         # npm script output
          project/              # Agent-generated files (if copyFiles is set)
            src/
              Button.tsx        # Files created/modified by the agent

summary.json

Each eval directory contains a summary.json with:

{
  "totalRuns": 2,
  "passedRuns": 0,
  "passRate": "0%",
  "meanDuration": 45.2,
  "fingerprint": "a1b2c3...",
  "classification": {
    "failureType": "infra",
    "failureReason": "Rate limited (HTTP 429) — model never ran"
  },
  "valid": false
}

The fingerprint field enables result reuse across runs. The classification and valid fields appear only for failed evals -- valid: false marks non-model failures so they are not reused by fingerprinting and are automatically retried.

Playground UI

Browse results in a web-based dashboard:

npx @vercel/agent-eval playground

This opens a local Next.js app with:

Overview dashboard with stats and recent experiments
Experiment detail with per-eval pass rates and run results
Transcript viewer to inspect agent tool calls, thinking, and errors
Compare two runs side-by-side with pass rate deltas

Options:

npx @vercel/agent-eval playground --results-dir ./results --evals-dir ./evals --port 3001

File Copying

By default, the framework only saves test outputs and transcripts. Use the copyFiles config option to also save the files generated by the agent:

const config: ExperimentConfig = {
  copyFiles: 'changed',  // or 'all' or 'none' (default)
};

Options:

none (default) — Don't copy any project files, only save outputs and transcripts
changed — Copy only files that were modified, created, or deleted by the agent
all — Copy the complete project including both the original fixture files and agent changes

Files are saved to results/<experiment>/<timestamp>/<eval>/run-N/project/. The framework uses git to track changes, so files must be text-based to be captured.

Result Reuse

The framework computes a SHA-256 fingerprint for each (eval, config) pair. The fingerprint covers all eval directory files and the config fields that affect results: agent, model, scripts, timeout, earlyExit, and runs.

On subsequent runs, evals with a matching fingerprint and a valid cached result (at least one passing run) are skipped automatically. This means:

Adding new evals -- safe, no existing results to invalidate.
Extending the model array -- safe, each model gets its own experiment directory.
Changing the evals filter -- safe, the filter is not part of the fingerprint.
Editing an eval file -- only invalidates that specific eval.
Changing config fields (agent, model, timeout, etc.) -- invalidates all evals in that experiment.

Use --force to bypass fingerprinting and re-run everything. Functions like setup and editPrompt cannot be hashed, so use --force when you change those.

Failure Classification

When evals fail, the framework optionally classifies each failure as one of:

model -- the agent tried but wrote incorrect code
infra -- infrastructure broke (API errors, rate limits, crashes)
timeout -- the run hit its time limit

Classification uses Claude Sonnet 4.5 via the Vercel AI Gateway with sandboxed read-only tools to inspect result files. This requires AI_GATEWAY_API_KEY or VERCEL_OIDC_TOKEN to be set.

Classifier Status

Enabled (with AI_GATEWAY_API_KEY or VERCEL_OIDC_TOKEN): Classifications are cached in classification.json. Non-model failures are automatically removed during housekeeping (unless --ack-failures is used). The auto-retry feature helps handle transient issues.
Disabled (without keys): The classifier is skipped. All results are preserved as-is. Housekeeping will not remove non-model failures (only incomplete and duplicate results). Add AI_GATEWAY_API_KEY to .env to enable the classifier.

Auto-retry (deflake)

When the classifier is enabled, the framework automatically retries evals that fail with non-model failures (infra or timeout) — up to 5 rounds by default. Evals that passed or were classified as model failures are cached and skipped on retries. After exhausting retries, remaining non-model failures are auto-acknowledged as final results. Control with --max-retries <n> (0 to disable) or --ack-failures to skip retries and accept immediately.

Housekeeping

After each experiment completes, the framework automatically:

Removes duplicate results for the same eval (keeps the newest)
Removes incomplete results (missing summary.json or transcripts)
Removes empty timestamp directories

Environment Variables

Every run requires an API key for the agent and a token for the sandbox. Classifier is optional.

Variable	Required when	Description
`AI_GATEWAY_API_KEY`	`vercel-ai-gateway/` agents or classifier	Vercel AI Gateway key -- required for `vercel-ai-gateway/` agents and failure classification
`ANTHROPIC_API_KEY`	`agent: 'claude-code'`	Direct Anthropic API key
`OPENAI_API_KEY`	`agent: 'codex'`	Direct OpenAI API key
`GEMINI_API_KEY`	`agent: 'gemini'`	Direct Google Gemini API key
`CURSOR_API_KEY`	`agent: 'cursor'`	Direct Cursor API key
`VERCEL_TOKEN`	Always (pick one)	Vercel personal access token -- for local dev
`VERCEL_OIDC_TOKEN`	Always (pick one) OR for classifier	Vercel OIDC token -- for CI/CD pipelines, or enables classifier without `AI_GATEWAY_API_KEY`

The classifier is optional: if neither AI_GATEWAY_API_KEY nor VERCEL_OIDC_TOKEN is set, failure classification is skipped and all results are preserved as-is. Set either key to enable the classifier, which automatically identifies and removes non-model failures (infrastructure errors, rate limits, timeouts).

OpenCode only supports Vercel AI Gateway (vercel-ai-gateway/opencode). There is no direct API option for OpenCode.

Setup

The init command generates a .env.example file. Copy it and fill in your keys:

cp .env.example .env

The framework loads .env.local first, then .env as a fallback, via dotenv.

Vercel AI Gateway (recommended)

One key for all models:

AI_GATEWAY_API_KEY=your-ai-gateway-api-key
VERCEL_TOKEN=your-vercel-token

Direct API keys (no Vercel account required)

If you don't have a Vercel account, use provider API keys directly:

ANTHROPIC_API_KEY=sk-ant-...      # For Claude Code
OPENAI_API_KEY=sk-proj-...        # For Codex

And choose ONE sandbox option (no Vercel key needed):

# Option 1: Use Docker (free, no account needed)
# Just set sandbox: 'docker' in your experiment config, that's it!

# Option 2: Use Vercel (requires free account)
VERCEL_TOKEN=your-vercel-token

Minimal setup example

Claude Code via direct API with Docker sandbox:

// experiments/my-eval.ts
import type { ExperimentConfig } from '@vercel/agent-eval';

const config: ExperimentConfig = {
  agent: 'claude-code',  // Direct API (not vercel-ai-gateway/...)
  model: 'opus',
  runs: 1,
  sandbox: 'docker',     // No VERCEL_TOKEN needed
};

export default config;

Then just set:

ANTHROPIC_API_KEY=sk-ant-...

That's it! The classifier will be disabled (since you don't have AI_GATEWAY_API_KEY), but all features work fine — you'll just see a warning that non-model failure classification is skipped.

Tips

Start with --dry: Always preview before running to verify your config and avoid unexpected costs.

Use --smoke first: Verify API keys, model IDs, and sandbox connectivity before launching a full run.

Use multiple runs: Single runs don't tell you reliability. Use runs: 10 and earlyExit: false for meaningful data.

Isolate variables: Change one thing at a time between experiments. Don't compare "Opus with MCP" to "Haiku without MCP".

Test incrementally: Start with simple tasks, add complexity as you learn what works.

Contributing

See CONTRIBUTING.md for development workflow and release process.

License

MIT

frontend-design