Benchmark Sandbox Skill for Claude

Benchmark Sandbox — Remote Eval via Vercel Sandboxes

Run benchmark scenarios inside Vercel Sandboxes — ephemeral Firecracker microVMs with node24. Each sandbox gets a fresh Claude Code + Vercel CLI + agent-browser install, the local vercel-plugin uploaded, and runs a 3-phase eval pipeline:

• Phase 1 (BUILD): Claude Code builds the app with --dangerously-skip-permissions --debug
• Phase 2 (VERIFY): A follow-up Claude Code session uses agent-browser to walk through user stories, fixing issues until all pass (20 min timeout)
• Phase 3 (DEPLOY): A third Claude Code session links to vercel-labs, runs vercel deploy, and fixes build errors (up to 3 retries). Deployed apps have deployment protection enabled by default.

Skills are tracked across all 3 phases — each phase may trigger additional skill injections as new files/patterns are created. After each phase, a haiku structured scoring step (claude -p --json-schema --model haiku) evaluates the results as structured JSON.

Proven Working Script

Use run-eval.ts — the proven eval runner:

# Run default scenarios with full 3-phase pipeline
bun run .claude/skills/benchmark-sandbox/run-eval.ts

# With dynamic scenarios from a JSON file (recommended — see "Dynamic Scenarios" below)
bun run .claude/skills/benchmark-sandbox/run-eval.ts --scenarios-file /tmp/my-scenarios.json

# Keep sandboxes alive overnight with public URLs
bun run .claude/skills/benchmark-sandbox/run-eval.ts --keep-alive --keep-hours 8

# Build-only (skip verification and deploy)
bun run .claude/skills/benchmark-sandbox/run-eval.ts --skip-verify --skip-deploy

# Run specific scenarios by slug
bun run .claude/skills/benchmark-sandbox/run-eval.ts --scenarios splitwise-clone,calendly-clone

CLI Flags

| Flag | Default | Description |

|------|---------|-------------|

| --concurrency N | 5 | Max parallel sandboxes (max 10) |

| --timeout MS | 1800000 (30 min) | Per-phase timeout in ms |

| --keep-alive | off | Keep sandboxes running after eval |

| --keep-hours N | 8 | Hours to keep alive (with --keep-alive) |

| --skip-verify | off | Skip the agent-browser verification phase |

| --skip-deploy | off | Skip the Vercel deploy phase |

| --scenarios a,b,c | all | Only run specific scenarios by slug |

| --scenarios-file path | — | Load scenarios from a JSON file instead of built-in defaults |

Dynamic Scenarios (Recommended Approach)

Instead of hardcoding tech-specific prompts, generate scenarios dynamically as a JSON file. Prompts should describe real-world apps people want to build using user stories — no tech name-dropping. Let the plugin figure out what Vercel tech to inject.

Scenario JSON Format

[
  {
    "slug": "pet-adoption-board",
    "prompt": "Build me a pet adoption listing board where shelters can post animals...",
    "expectedSkills": ["ai-sdk", "nextjs", "shadcn", "vercel-functions"],
    "userStories": [
      "As a visitor, I can see a grid of pet listings with photos and names",
      "As a visitor, I can click a pet card to see a detail page",
      "As a visitor, I can filter pets by type"
    ]
  }
]

Each scenario needs: slug (string), prompt (string), expectedSkills (string[]), userStories (tuple of exactly 3 strings).

Prompt Design Guidelines

• Focus on what the user wants, not what tech to use
• Describe real-world apps that solve real problems with friendly, stylish UX
• Include AI features naturally (recommendations, analysis, generation)
• Always end with: "Link the project to my vercel-labs team. After building all files, start the dev server on port 3000 with \npx next dev --port 3000\."
• Include storage needs (photos, uploads) to trigger vercel-storage
• Include scheduled tasks (reminders, cleanup) to trigger cron-jobs
• Include auth/middleware to trigger routing-middleware

Structured Scoring (Haiku)

Each phase gets a structured JSON score via claude -p --json-schema --model haiku --setting-sources "" running inside the sandbox. This is a separate quick pass — no tools, no hooks — just reads the phase output and returns structured data.

Build Score Schema

{
  "completeness": "complete|partial|minimal|empty",
  "hasApiRoutes": true,
  "hasUIComponents": true,
  "hasAIFeature": true,
  "devServerRunning": true,
  "missingFeatures": ["feature1"],
  "summary": "Brief assessment"
}

Verify Score Schema (per user story)

{
  "stories": [
    { "index": 1, "status": "pass|fail", "reason": "Evidence from output" }
  ]
}

Deploy Score Schema

{
  "deployed": true,
  "url": "https://xxx.vercel.app",
  "buildSucceeded": true,
  "errors": [],
  "summary": "Brief assessment"
}

Important: The claude -p --output-format json response wraps results — the actual schema data is in parsed.structured_output, not the top-level object.

Critical Sandbox Environment Facts

| Property | Value |

|----------|-------|

| Home directory | /home/vercel-sandbox (NOT /home/user/ or /root/) |

| User | vercel-sandbox (NOT root) |

| Claude binary | /home/vercel-sandbox/.global/npm/bin/claude |

| PATH (via sh -c) | Includes ~/.global/npm/bin — claude findable by name |

| Port exposure | sandbox.domain(3000) → https://subdomain.vercel.run |

| Snapshot persistence | Files AND npm globals survive snapshot restore — use sandbox.snapshot() → Sandbox.create({ source: { type: "snapshot", snapshotId } }) |

| SDK version | @vercel/[email protected] (v2 beta's named sandbox endpoint returns 404 for this team) |

| Team tier | Enterprise (vercel-labs) — no known sandbox time cap |

Key Discoveries (Hard-Won)

• Snapshots work: sandbox.snapshot() preserves files AND npm globals. Use it after build to create a restore point before verify/deploy. Note: snapshotting stops the source sandbox — create a new one from the snapshot to continue.
• Plugin install: Use npx add-plugin <path> -s project -y --target claude-code — works because claude is in PATH after npm install -g. The --target claude-code flag is required because add-plugin can't auto-detect Claude Code without an initialized ~/.claude/ dir.
• File uploads: Use sandbox.writeFiles([{ path, content: Buffer }]) — NOT runCommand heredocs. Heredocs with special characters cause 400 errors from the sandbox API.
• Claude flags: Always use --dangerously-skip-permissions --debug. The --debug flag writes to ~/.claude/debug/.
• Auth: API key from macOS Keychain (ANTHROPIC_AUTH_TOKEN — a vck_* Vercel Claude Key for AI Gateway), Vercel token from ~/.local/share/com.vercel.cli/auth.json (a vca_* token).
• OIDC for sandbox SDK: Run npx vercel link --scope vercel-labs -y + npx vercel env pull once before first use.
• Port exposure: Pass ports: [3000] in Sandbox.create() to get a public URL immediately via sandbox.domain(3000). Works on v1.8.0 — URL is assigned at creation time, before anything listens.
• extendTimeout: Use sandbox.extendTimeout(ms) to keep sandboxes alive past their initial timeout. Verified working — extends by the requested duration. Use this for overnight keep-alive.
• Background commands: runCommand with backgrounded processes (& or nohup) may throw ZodError on v1. Write a script file first, then execute it.

10. Session cleanup race: The session-end-cleanup.mjs hook deletes /tmp/vercel-plugin-*-seen-skills.d/ on session end. Extract artifacts BEFORE the session completes, or rely on poll history data.

11. agent-browser works in sandboxes: Install via npm install -g agent-browser. Claude Code can use it for browser-based verification inside the sandbox.

12. No hobby tier cap: Early 301s timeouts were from lower default timeout values in earlier script iterations, not a tier limitation. Enterprise (vercel-labs) has no known sandbox time cap — sandboxes ran 10+ minutes successfully.

13. claude -p works inside sandboxes: claude -p --json-schema --output-format json --model haiku works for structured scoring passes. No nesting issue when running inside a sandbox (only fails when running Claude inside Claude on the same machine).

14. Deploy project naming: ALWAYS use timestamped slugs with minute precision (e.g., pet-adoption-board-202603101853) to avoid collisions when linking to vercel-labs team projects. These are demo projects — we generate many per day. Format: <slug>-<YYYYMMDDHHMM>.

When to Use This vs benchmark-agents

| | benchmark-agents (WezTerm) | benchmark-sandbox |

|---|---|---|

| Environment | Local macOS terminal panes | Remote Vercel Sandboxes (Amazon Linux) |

| Parallelism | Limited by local resources | Up to 10 (Hobby) or 2,000 (Pro) concurrent |

| Session type | Interactive TTY via /bin/zsh -ic | Direct sh -c invocation (PTY not required) |

| Artifact access | Direct filesystem (~/.claude/debug/) | sandbox.readFile() / poll via runCommand |

| Port exposure | localhost:3000 | Public https://sb-XXX.vercel.run URLs |

| Verification | Manual browser check | Automated agent-browser in Phase 2 |

| Deploy | Manual | Automated Phase 3 → permanent *.vercel.app URLs |

| Scoring | Manual review | Haiku structured JSON scoring per phase |

| Best for | Manual eval + iteration loop | Automated parallel coverage + verification + deploy runs |

How It Works

• Create fresh sandbox: Sandbox.create({ runtime: "node24", ports: [3000], env: { ANTHROPIC_API_KEY, ... } }) — no snapshot
• Install tools: npm install -g @anthropic-ai/claude-code vercel agent-browser (~20s per sandbox)
• Auth Vercel CLI: Write token to ~/.local/share/com.vercel.cli/auth.json
• Upload plugin: sandbox.writeFiles() for 80 plugin files, then npx add-plugin
• Phase 1 — BUILD: Claude Code builds the app (30 min timeout)
• Score build: Haiku evaluates completeness, API routes, UI, AI features
• Start dev server: If not already running, start npx next dev --port 3000
• Extend timeout: sandbox.extendTimeout() for verify + deploy + keep-alive
• Phase 2 — VERIFY: Claude Code uses agent-browser to test user stories (20 min timeout). Prompt tells Claude to start dev server itself if not running.

10. Score verify: Haiku evaluates each user story as pass/fail with reasons

11. Re-extract skills: Skills re-collected after verify phase (agent-browser + code fixes trigger more)

12. Phase 3 — DEPLOY: Claude Code runs vercel link + vercel deploy, fixes build errors (30 min timeout)

13. Score deploy: Haiku evaluates deploy success, URL extraction, errors

14. Re-extract skills: Skills re-collected after deploy phase

15. Write incremental results: Each scenario writes its own result.json immediately on completion (survives crashes)

16. Extract source archive: source.tar.gz of project files saved locally

17. Generate report: Markdown report with build/verify/deploy scores, skill coverage, URLs

Sandbox Session Flow (Per Scenario)

Sandbox.create({ runtime: "node24", ports: [3000], env: { ANTHROPIC_API_KEY, ANTHROPIC_BASE_URL, VERCEL_PLUGIN_LOG_LEVEL: "trace" } })
  │
  ├─ npm install -g @anthropic-ai/claude-code vercel agent-browser   (~20s)
  ├─ Write Vercel CLI auth token to ~/.local/share/com.vercel.cli/auth.json
  ├─ mkdir -p /home/vercel-sandbox/<slug> && npm init -y
  ├─ sandbox.writeFiles() → /home/vercel-sandbox/vercel-plugin/  (80 files, ~945KB)
  ├─ npx add-plugin /home/vercel-sandbox/vercel-plugin -s project -y --target claude-code
  │
  ├─ Phase 1: BUILD
  │   ├─ sandbox.writeFiles() → /tmp/prompt.txt
  │   ├─ claude --dangerously-skip-permissions --debug --settings <path> "$(cat /tmp/prompt.txt)"
  │   │   (with AbortSignal.timeout(TIMEOUT_MS))
  │   ├─ Poll every 20s:
  │   │   ├─ ls /tmp/vercel-plugin-*-seen-skills.d/     (claimed skills)
  │   │   ├─ cat /tmp/vercel-plugin-*-seen-skills.txt    (seen skills snapshot)
  │   │   ├─ find ~/.claude/debug -type f                (debug log count)
  │   │   ├─ find <project> -newer /tmp/prompt.txt       (new project files)
  │   │   └─ curl localhost:3000                         (port status)
  │   ├─ Extract build artifacts
  │   └─ Haiku build score (structured JSON)
  │
  ├─ Start dev server (if not already running)
  ├─ sandbox.extendTimeout(...)
  │
  ├─ Phase 2: VERIFY (if >1 project file exists)
  │   ├─ sandbox.writeFiles() → /tmp/verify.txt  (agent-browser verification prompt)
  │   ├─ claude --dangerously-skip-permissions --debug "$(cat /tmp/verify.txt)"
  │   │   (with AbortSignal.timeout(1_200_000) — 20 min)
  │   ├─ Re-extract skills (verify phase triggers more)
  │   └─ Haiku verify score (per-story pass/fail JSON)
  │
  ├─ Phase 3: DEPLOY (if >3 project files)
  │   ├─ sandbox.writeFiles() → /tmp/deploy.txt
  │   ├─ claude --dangerously-skip-permissions --debug "$(cat /tmp/deploy.txt)"
  │   │   (links to vercel-labs, deploys, fixes build errors up to 3x)
  │   ├─ Extract deploy URL from output (*.vercel.app)
  │   ├─ Re-extract skills (deploy phase triggers more)
  │   └─ Haiku deploy score (structured JSON)
  │
  ├─ Write <slug>/result.json immediately (crash-safe)
  ├─ Update aggregate results.json (complete: false until all done)
  ├─ Extract source.tar.gz
  └─ sandbox.stop()  (skipped if --keep-alive)

Verification Phase Details

The verify phase is the "closer" — its job is to make the app work and prove it. Key behaviors:

• Always runs if >1 project file exists (no longer gated on port 3000 being up)
• Starts dev server itself if not already running — the prompt tells Claude to check localhost:3000 and run npx next dev --port 3000 if needed
• 20 minute timeout — enough for agent-browser to open pages, screenshot, interact, fix broken code, restart server, and re-verify
• Triggers skill injection — the verify session creates/edits files, triggering PreToolUse and PostToolUse hooks
• Uses agent-browser workflow: open → wait --load networkidle → screenshot --annotate → snapshot -i → interact → fix → re-verify
• Results scored by haiku — no more parsing STORY_1: PASS from free text

Deploy Phase Details

The deploy phase uses a full Claude Code session (for skill tracking) to:

• Run vercel link --yes --scope vercel-labs --project <slug>-YYYYMMDD
• Run vercel deploy --yes
• If build fails, fix code and retry (up to 3 attempts)
• Important: unsets VERCEL_TOKEN env var so CLI falls back to ~/.local/share/com.vercel.cli/auth.json
• Deployment protection is enabled by default on vercel-labs team

Deploy URL is extracted by regex from Claude's output, with haiku as fallback URL extractor.

DO NOT (Hard Rules)

Same rules as benchmark-agents, plus sandbox-specific:

• DO NOT use claude --print or -p flag for BUILD/VERIFY/DEPLOY phases — hooks don't fire without tool-calling sessions (use -p only for haiku scoring passes)
• DO NOT let sandboxes run without extracting artifacts — ephemeral filesystem is lost on stop
• DO NOT pass API keys via writeFiles() — use Sandbox.create({ env: { ... } })
• DO NOT skip snapshotting after build — it's your safety net if verify/deploy kills the sandbox
• DO NOT use v2 beta SDK — named sandbox endpoint returns 404 for this team; use v1.8.0
• DO NOT use runCommand heredocs to write file content — use sandbox.writeFiles() instead
• DO NOT assume /home/user/ exists — the home dir is /home/vercel-sandbox/
• DO NOT use simple project names without timestamps — always append -YYYYMMDDHHMM to avoid collisions across runs

Prerequisites

# One-time setup: link project for OIDC sandbox auth
npx vercel link --scope vercel-labs -y
npx vercel env pull .env.local

# Auth (auto-resolved from macOS Keychain + Vercel CLI auth):
# - ANTHROPIC_API_KEY: from Keychain "ANTHROPIC_AUTH_TOKEN" (vck_* key) or env var
# - VERCEL_TOKEN: from ~/.local/share/com.vercel.cli/auth.json (vca_* token) or env var
# - ANTHROPIC_BASE_URL: defaults to https://ai-gateway.vercel.sh

Commands

Run eval with dynamic scenarios (recommended)

# Generate scenarios as JSON, then run
bun run .claude/skills/benchmark-sandbox/run-eval.ts --scenarios-file /tmp/my-scenarios.json

# With all phases + keep-alive for overnight
bun run .claude/skills/benchmark-sandbox/run-eval.ts --scenarios-file /tmp/scenarios.json --keep-alive --keep-hours 8

# Build-only, no verification or deploy
bun run .claude/skills/benchmark-sandbox/run-eval.ts --scenarios-file /tmp/scenarios.json --skip-verify --skip-deploy

# Filter to specific slugs from file or defaults
bun run .claude/skills/benchmark-sandbox/run-eval.ts --scenarios splitwise-clone,calendly-clone

Monitoring While Running

The orchestrator prints live status. For manual checks on a running sandbox:

// List claimed skills
const claims = await sandbox.runCommand("sh", ["-c",
  "ls /tmp/vercel-plugin-*-seen-skills.d/ 2>/dev/null"
]);

// Check hook firing count
const hooks = await sandbox.runCommand("sh", ["-c",
  "find /home/vercel-sandbox/.claude/debug -name '*.txt' -exec grep -c 'executePreToolHooks' {} +"
]);

// Check port 3000
const port = await sandbox.runCommand("sh", ["-c",
  "curl -s -o /dev/null -w '%{http_code}' http://localhost:3000"
]);

// Get public URL (after ports: [3000] in Sandbox.create)
const url = sandbox.domain(3000);

Artifact Export Layout

Results are written to ~/dev/vercel-plugin-testing/sandbox-results/<run-id>/:

<run-id>/
  results.json             # Aggregate results (complete: false until all done, then true)
  report.md                # Markdown report with scores, coverage, URLs
  <slug>/
    result.json            # Per-scenario result (written immediately on completion)
    source.tar.gz          # Project source archive

Each scenario result includes:

• slug, sandboxId, success, durationMs
• claimedSkills[], expectedSkills[], projectFiles[]
• appUrl — public https://sb-XXX.vercel.run URL (sandbox lifetime only)
• deployUrl — permanent https://xxx.vercel.app URL (if deploy succeeded)
• pollHistory[] — timestamped skill/file/port snapshots
• verification — { ran, exitCode, stories: [{ index, status }], output }
• buildScore — haiku structured completeness assessment
• deployScore — haiku structured deploy assessment

The markdown report (report.md / .reports/<timestamp>.md) includes:

• Summary table — slug, build status, skills, files, verify results, deploy URL, duration
• Per-scenario details — build score, deploy score, verification per-story pass/fail
• Skill coverage — expected vs actual per scenario, missing/bonus breakdown
• Total unique skills across all scenarios

Proven Results (2026-03-10)

Across 34 scenarios run in 5 batches:

| Metric | Best | Typical |

|--------|------|---------|

| Skills per scenario | 31 (ai-interior-designer) | 12-24 |

| Expected skill coverage | 100% (pet-adoption-board 4/4, apartment-hunting-copilot 7/7, splitwise-clone 6/6) | 50-86% |

| User stories verified | 3/3 PASS (ai-dream-journal, ai-gift-finder, ai-resume-roaster, ai-music-mood-radio, team-standup-bot, pet-adoption-board) | varies |

| Files built per scenario | 37 (student-study-groups) | 6-25 |

| Build time | 5-11 min | 5-7 min |

Key findings:

• User-story-focused prompts (no tech name-dropping) work — plugin detects patterns from actual code
• ai-sdk, shadcn, nextjs, vercel-functions are the most consistently detected skills
• cron-jobs, routing-middleware need Claude to write specific file patterns to trigger
• Lexical prompt inject (UserPromptSubmit) working — skills injected before any files written
• session-end-cleanup deletes claim dirs — use poll history for final skill counts
• Enterprise tier (vercel-labs) — no sandbox time cap; builds ran 10+ minutes

Known Limitations

• Snapshot stops the source sandbox: sandbox.snapshot() stops the original sandbox. Create a new sandbox from the snapshot to continue. Files and npm globals DO survive.
• v2 beta incompatible: @vercel/[email protected]'s named sandbox endpoint returns 404 for this team. Stick with v1.8.0.
• Artifact window: Must extract before sandbox.stop() — filesystem is ephemeral. Session cleanup hook may delete claim dirs before extraction.
• Amazon Linux paths: User is vercel-sandbox (home at /home/vercel-sandbox/). NOT /home/user/ or /root/.
• --dangerously-skip-permissions parity: Sandbox evals auto-approve all tool calls. WezTerm evals use normal permission flow. Coverage results may differ.
• runCommand timeout: Use { signal: AbortSignal.timeout(ms) } — the { timeout } option is silently ignored.
• BrotliDecompressionError: Transient Vercel API errors can kill sandbox creation. Retry logic recommended for production runs.
• Deploy reliability: Claude Code deploy sessions sometimes fail to output a parseable *.vercel.app URL. The haiku scoring step provides a fallback URL extraction attempt.
• Verify timeout: Complex apps may need the full 20 minutes for agent-browser to test all stories. Simpler apps finish in 2-5 minutes.