Spec To Playwright

spec-to-playwright

A pipeline: natural-language spec → explore the app → generate a Playwright test → deterministic VRT baseline → verify reproducibility → heal on failure. It builds on the official Playwright test-agents (init-agents) and adds the two things they lack: a determinism layer that makes VRT stable, and a cost-optimized heal loop (@mizchi/vlmkit-heal).

Reference implementation: mizchi/playwright-playground — every asset here is extracted from that repo, where the pipeline runs two-consecutive-green in a linux container.

When to use

• You have a spec / user story and want a real Playwright test for it.
• Your toHaveScreenshot() VRT is flaky and you want byte-stable baselines.
• You want generated tests to reproduce in CI, not just locally.
• A test broke because the UI changed and you want it healed (or the baseline updated when the change was intentional).

Not for: one-off edits to a single existing test (just edit it).

Assumes a Vite app served by vite build + vite preview (the determinism preset uses pnpm app:build && pnpm app:preview). For other build tools, adapt the webServer command + the two npm scripts; the rest of the pipeline is the same.

No official agents available? Prefer the runtime-neutral package path before hand-writing. vlmkit-plan and vlmkit-generate produce the same planner- and generator-shaped artifacts with diagnostics-driven retries:

vlmkit-plan --title "<topic>" --request-file specs/<topic>.request.md \
  --observations specs/<topic>.observations.json --out specs/<topic>.md \
  --locator-inventory-out specs/<topic>.locators.json \
  --provider anthropic --max-attempts 2
vlmkit-generate --plan specs/<topic>.md --rules specs/_generation-rules.md \
  --locator-inventory specs/<topic>.locators.json \
  --helper-import ../support/goto-app --out tests/<topic>.spec.ts \
  --provider anthropic --max-attempts 2 --overwrite \
  --gate-command "pnpm exec playwright test --list {testFile}"

If you can't use either the official agents or these CLIs, do their job by hand with the SAME steps, in order:

1. explore the app to confirm real roles/labels/testids
2. write specs/<topic>.md (reference **Seed:** tests/seed.spec.ts)
3. adjust tests/seed.spec.ts to your app's actual initial state
4. hand-write tests/<topic>.spec.ts strictly following specs/_generation-rules.md The determinism layer and rules are what matter; the agents/CLIs are just automated authors.

Setup (once per repo)

Always needed: _helpers.ts, seed.spec.template.ts, _generation-rules.md, the deterministic playwright.config merge. Conditional: init-agents (only if you drive the official agents — skip when hand-writing), @mizchi/vlmkit-heal (only for the heal step), ci.yml + update-baselines.yml (only when you keep VRT baselines, i.e. use toHaveScreenshot).

No Docker. Pixel baselines can't match across OSes, so instead of rendering them locally in a container, CI is the source of truth: baselines are always rendered in the ci.yml environment, and update-baselines.yml regenerates them there on demand. Developers never run Docker.

1. Generate the official agents + MCP server. Use the loop for the agent runtime in this repo:

   npx playwright init-agents --loop=codex
   

   Use --loop=claude for Claude Code, --loop=vscode for VS Code, and --loop=opencode for OpenCode. Regenerate these definitions whenever Playwright is updated so the MCP tools and instructions stay current. For Codex this creates the Codex agent definitions and MCP config for the playwright-test server. If the MCP tools aren't available in your session, reload the agent runtime so it connects.

2. Copy the determinism assets into the repo:

   • assets/_helpers.ts → tests/_helpers.ts
   • assets/seed.spec.template.ts → tests/seed.spec.ts (adjust the initial-state assertion)
   • assets/_generation-rules.md → specs/_generation-rules.md
   • merge assets/playwright.config.preset.ts into your playwright.config.ts
   • assets/ci.yml → .github/workflows/ci.yml (verifies against committed baselines)
   • assets/update-baselines.yml → .github/workflows/update-baselines.yml (CI renders + commits baselines)

3. Add the heal engine:

   pnpm add -D @mizchi/vlmkit-heal
   

4. Make the generator obey the rules: append to .claude/agents/playwright-test-generator.md:

   Before writing any test, READ specs/_generation-rules.md and obey it.

Add these npm scripts (the determinism preset and rules assume them):

"app:build":   "vite build",
"app:preview": "vite preview --port 4173 --strictPort",
"verify":      "playwright test && playwright test"

Agent roles

The official Playwright Test Agents are still the authors of the test. Use @mizchi/vlmkit-heal only after generation, where its budget, VRT review, and rollback guardrails matter.

Run (staged)

plan      → invoke the playwright-test-planner agent with:
              - a clear user story / scenario request
              - tests/seed.spec.ts so fixtures, auth, and initial state are known
              - optional PRD / product notes for domain context
            It explores the live app (verifying real roles/labels) and writes
            specs/<topic>.md. The plan must reference **Seed:** tests/seed.spec.ts.

generate  → invoke the playwright-test-generator agent with specs/<topic>.md.
            It replays the steps and writes tests/<topic>.spec.ts following
            specs/_generation-rules.md: gotoApp() + a screenshot at the start and
            at the goal + role/testid/label assertions.

baseline  → a test with toHaveScreenshot fails until its baseline exists. CI is the
            source of truth: push the branch and run the `update-baselines` workflow
            (it renders baselines in the CI env and commits them back). Locally you
            MAY run `pnpm exec playwright test --update-snapshots` to preview, but do
            NOT commit local (e.g. -darwin) snapshots — they won't match CI.

verify    → pnpm run verify           (two consecutive green = reproducible). Locally
            this is green only against local baselines; the authoritative verify is CI.

heal      → after generator/healer output exists, run `vlmkit-heal` for bounded
            repair:
              npx tsx examples/test-agents/heal-after-generator.ts tests/<topic>.spec.ts

A test is "done" only when CI's verify is green twice in a row AND its VRT diff is within threshold (the ci.yml workflow runs the suite twice).

Determinism essentials

What keeps screenshots byte-stable (all in the assets):

• fixed viewport / deviceScaleFactor / colorScheme / locale / timezoneId
• gotoApp() disables animations/transitions, hides the caret, waits for fonts
• webServer runs vite build && vite preview — never the dev server (HMR injects scripts that perturb the DOM)
• baselines live in one environment only: a pixel baseline is tied to the OS/arch that rendered it, so committed baselines are always the ones CI rendered (via update-baselines.yml). Never commit locally-rendered snapshots.

Heal (failure → fix)

import { heal } from "@mizchi/vlmkit-heal";

const r = await heal({
  testCommand: "pnpm exec playwright test tests/<topic>.spec.ts",
  updateSnapshotsCommand: "pnpm exec playwright test tests/<topic>.spec.ts --update-snapshots",
  testFile: "tests/<topic>.spec.ts",
  cwd: process.cwd(),
  // observe = a cheap REASONING VLM that judges intentional-change vs regression
  observe: { tiers: [{ provider: "openrouter", model: "openai/gpt-5-mini", vision: true }] },
  // codegen = cheap coder first, escalate to a stronger model on repeated failure
  codegen: { tiers: [
    { provider: "openrouter", model: "qwen/qwen3-coder-30b-a3b-instruct", vision: false },
    { provider: "openrouter", model: "openai/gpt-5-codex", vision: false },
  ]},
  budgetUsd: 1,            // shared cap; estimated from tokens × price for OpenRouter
  maxAttempts: 4,
  // expectedChange: "the badge turns red"  // declare for a VRT change; omit and a
  //                                          regression is reported, NOT baked into the baseline
});
// r.verdict: "fixed" | "regression" | "needs-review" | "flaky" | "give-up"

Needs OPENROUTER_API_KEY. Heal only edits the test file + baselines (never app code), and only commits a patch that verified green twice.

Do NOT use ui-tars for the observe judgment — it is a GUI-grounding model and mislabels intentional changes as regressions. Use a cheap reasoning VLM.

Any provider/model drives a tier — provider is "openrouter" | "anthropic" | "gemini". Via OpenRouter you can name any model: openai/gpt-5-codex, openai/gpt-5-mini, anthropic/claude-sonnet-4.6 (Claude as the driver), qwen/qwen3-coder-30b-a3b-instruct, etc. — all on one OPENROUTER_API_KEY. provider: "anthropic" / "gemini" hit those APIs directly (need their own key). baseURL on a tier points at any OpenAI-compatible endpoint (e.g. self-hosted).

VRT review in CI (accept / reject / needs-review)

When a toHaveScreenshot() VRT fails in CI, a model can decide whether the change is intended (accept → refresh the baseline) or a regression (reject). Two assets wire this up:

• assets/vrt-review.mjs → repo root. Reads the failing VRT artifacts (findVrtArtifacts), infers intent from the commit + diff (collectGitContext) or EXPECTED_CHANGE, and calls reviewVrtDiff. accept (≥0.8) → runs --update-snapshots and exits 0; reject / unsure → exits 1.
• assets/vrt-review-ci.yml → .github/workflows/vrt-review.yml. Runs the VRT with continue-on-error, runs the review on failure, then commits the refreshed baselines back to the PR branch on accept (permissions: contents: write). Needs an OPENROUTER_API_KEY secret and tsx as a devDep.

The workflow's structure is load-bearing — each line below was added after it failed on real CI, so keep them when adapting:

• Runner arch must match the baseline arch. A pixel baseline is tied to the OS/arch that rendered it; on a mismatched runner every screenshot diffs on font rendering alone. ubuntu-latest for amd64 baselines, ubuntu-24.04-arm for arm64.
• Run the review via npx tsx, not node. @mizchi/vlmkit-heal ships .ts sources, and Node won't type-strip files under node_modules (ERR_UNSUPPORTED_NODE_MODULES_TYPE_STRIPPING). Add tsx to devDependencies.
• checkout with ref: github.head_ref + fetch-depth: 0. pull_request checks out a detached merge commit, so the baseline commit can't be pushed back without the head ref; and a shallow checkout leaves origin/<base> absent, which starves collectGitContext and forces needs-review on an intended change.
• safe.directory before any git op. The container runs as root while the checkout is owned by another uid; git refuses otherwise and commit-back fails.
• Drop the pnpm version: pin when package.json has packageManager, or pnpm/action-setup@v4 errors with "Multiple versions specified".
• Push with git push origin "HEAD:${{ github.head_ref }}" (detached HEAD has no upstream).

import { findVrtArtifacts, reviewVrtDiff, collectGitContext } from "@mizchi/vlmkit-heal";
const { baseline, actual, diff } = findVrtArtifacts(process.cwd());
const review = await reviewVrtDiff({
  baselinePng: baseline, actualPng: actual, diffPng: diff,
  gitContext: collectGitContext(process.cwd(), { base: "origin/main" }),
  tier: { provider: "openrouter", model: "openai/gpt-5-mini", vision: true },
});
// review.verdict: "accept" | "reject" | "unsure"

Use a CAPABLE reasoning VLM for the review tier (e.g. gpt-5-mini, not a tiny VLM). Cheap VLMs accept a diff with high confidence while missing collateral breakage elsewhere on the page. Inside the heal loop the same risk is guarded by confirmAccept (default true): an accept is re-checked by the strongest observe tier before the baseline is touched; if it disagrees the verdict becomes needs-review instead of silently updating. Set confirmAccept: false to skip the second opinion (single-tier ladders skip it automatically).

Pitfalls (lived in the reference repo)

• Port 4173 in use → lsof -ti tcp:4173 | xargs kill -9 before running.
• VRT fails locally but the test is fine → you're diffing against CI-rendered baselines on a different OS. That's expected; the authoritative check is CI. Don't "fix" it by committing local snapshots — run the update-baselines workflow if the change was intentional.
• MCP tools missing / planner can't drive the browser → .mcp.json was just created; reload Claude Code so the playwright-test server connects.
• ERR_UNSUPPORTED_NODE_MODULES_TYPE_STRIPPING when running a heal/review script → @mizchi/vlmkit-heal ships .ts sources and Node won't type-strip under node_modules. Run your script with npx tsx script.mjs (add tsx as a devDep), not bare node.