Visual Feedback Loop

Capture, inspect, and compare visual output from a running web app during iterative development.

How It Works

Agent (CLI)                     Server                            Browser
     |--- GET /api/dev-screenshot -->|--- SSE "capture" ----------->|
     |    ?param=value               |                              |-- capture canvas
     |                               |<-- POST { dataUrl } ---------|
     |                               | writes:
     |                               |   .screenshots/{UTC}.webp
     |                               |   .screenshots/{UTC}.json  (metadata)
     |                               |   .screenshots/latest.webp (convenience copy)
     |<-- { ok, path } --------------|

Agent can't access the browser directly. Server relays: GET triggers → SSE notifies browser → browser captures and POSTs back → GET resolves.

Usage

Capture current view:

curl http://localhost:3000/api/dev-screenshot

Capture with params (app-specific — browser client interprets them):

curl 'http://localhost:3000/api/dev-screenshot?component=header&theme=dark'
curl 'http://localhost:3000/api/dev-screenshot?letter=б&depth=0.8'

Then always read the result:

Read .screenshots/latest.webp

Visual Regression (pixel diff)

Compare against a ground truth screenshot using ImageMagick:

magick compare -metric RMSE .screenshots/ground-truth.webp .screenshots/latest.webp .screenshots/diff.webp

RMSE output: 0.01 = ~1% difference (rendering noise), 0.03+ = visible change. Save diff image for inspection.

Metadata Sidecars

Each screenshot gets a JSON sidecar with the same UTC timestamp:

{
  "timestamp": "2026-02-25T08:22:12.655Z",
  "git": { "commit": "0a5726f", "dirty": true },
  "params": { "letter": "о", "chamferModel": "membrane", "cameraView": "front" },
  "format": "webp",
  "file": "2026-02-25T08-22-12-655Z.webp"
}

Use this to trace which code state + params produced a screenshot — critical when iterating across multiple code changes.

A/B Comparison Workflow

1. Capture ground truth: curl '...?letter=о' → note the {UTC}.webp filename
2. Make code changes, refresh browser
3. Capture again: curl '...?letter=о'
4. Diff: magick compare -metric RMSE .screenshots/{ground-truth}.webp .screenshots/latest.webp .screenshots/diff.webp
5. Read diff image + check RMSE value

Console fallback: await window.__takeDevScreenshot()

Rules

1. Always Read .screenshots/latest.webp after capture. Never assume the render is correct.
2. Browser tab must be open at the app URL. Timeout = no SSE connection = ask user to refresh.
3. One request at a time. Second GET returns 409. Wait for first to resolve (success/error/10s timeout).
4. Errors return instantly, not as timeouts. Client POSTs errors back: { ok: false, error: "..." }.
5. HMR resilience. Store server-side SSE state on globalThis so it survives module reloads. EventSource auto-reconnects on the client. If screenshots still fail after code changes, ask user to refresh.
6. HMR does NOT update offscreen render paths. Closures in useEffect capture stale module references. After code changes to rendering logic, always ask user to hard refresh (Cmd+Shift+R on macOS, Ctrl+Shift+R on Windows/Linux).

Troubleshooting

Symptom	Fix
Timeout: browser did not respond	Open/refresh app in browser
409: already in progress	Wait for timeout (10s)
Black/empty image	Refresh browser tab
Works once, then times out	HMR broke SSE — refresh tab (rare if using globalThis pattern)
Code changes not reflected in screenshots	HMR doesn't update offscreen renderers — hard refresh (Cmd+Shift+R on macOS, Ctrl+Shift+R on Windows/Linux)
Comparing screenshots from different code states	Check JSON sidecar for git commit + dirty flag

See references/errors.md for full error reference.

Setup

The pattern is framework-agnostic — it only requires an HTTP server with GET/POST routes and SSE. The reference implementation uses Next.js, but the same approach works with Express, Fastify, Hono, Vite dev server plugins, or any Node.js HTTP server.

See references/setup-nextjs.md for a complete Next.js implementation (API route, SSE listener, WebMCP registration). Adapt the route handler to your framework — the client-side SSE listener and capture logic are identical regardless of server framework.