---
title: "Headless Browser — Visual Page Verification"
type: runbook
tags: [tooling]
related: ["[[central]]", "[[central-deploy-cutover]]"]
updated: 2026-06-29
---

# Headless Browser — Visual Page Verification

Drive a real headless browser (Playwright + Chromium) from **cortex** to log in and **screenshot a deployed web page**, then view the screenshot. Use this to *actually look* at a page after a deploy — it catches render/layout/styling bugs that `curl` status codes and unit tests sail right past (a route can return `200`/`302` and still look broken).

Works for **any** web UI reachable from cortex (central, navi, Authentik, PeerTube, Mailcow, etc.) — just change the base URL, path, and credentials.

> Reusing this in a prompt: tell Claude *"follow the headless-browser-page-verification runbook to screenshot `<service> <path>`"* and point it at the creds. Everything needed is self-contained below.

---

## Prerequisites

- Runs on **cortex** (where Claude Code executes). The target must be reachable from cortex — most fleet UIs are on the mesh (e.g. `http://100.64.0.12:8000`) or via a Caddy host.
- `python3` available.
- Credentials for auth-gated pages come from **`.ref/credentials`** (e.g. `CENTRAL_OPERATOR_USER`/`CENTRAL_OPERATOR_PASS`). Public/auth-exempt paths (`/login`, `/health`) need none.
- **Installing Playwright + Chromium is a package install** — get Matt's OK first per host policy (he authorized it 2026-06-29). No `apt`/system-dep install is needed on cortex; the browser is a self-contained download to `~/.cache/ms-playwright`.

---

## 1. One-time setup (per session)

The venv lives in the session scratchpad and is **ephemeral** — it does not survive across sessions, so re-run this each time.

```bash
# pick any working dir (scratchpad is fine)
python3 -m venv pwvenv
pwvenv/bin/pip install --quiet playwright
pwvenv/bin/playwright install chromium      # downloads chromium-headless-shell (~110 MB) to ~/.cache/ms-playwright
```

(`python -c "import playwright"` succeeding is enough — `playwright.__version__` is intentionally absent, not an error.)

---

## 2. The screenshot script

Save as `shot.py`. Parameterized by env vars; logs in only if `U`/`P` are set.

```python
import os
from playwright.sync_api import sync_playwright

base = os.environ["BASE"].rstrip("/")     # e.g. http://100.64.0.12:8000
path = os.environ.get("PATHV", "/")        # e.g. /consumers
out  = os.environ["OUT"]                    # e.g. /tmp/.../page.png
user = os.environ.get("U")                 # omit U/P for public pages
pw   = os.environ.get("P")

with sync_playwright() as p:
    b = p.chromium.launch()
    pg = b.new_context(viewport={"width": 1366, "height": 1000}).new_page()
    if user and pw:                         # form login (CSRF hidden field + cookie handled by the real browser)
        pg.goto(base + "/login", wait_until="networkidle", timeout=20000)
        pg.fill('input[name="username"]', user)
        pg.fill('input[name="password"]', pw)
        pg.click('button[type="submit"]')
        pg.wait_for_load_state("networkidle", timeout=20000)
    r = pg.goto(base + path, wait_until="networkidle", timeout=25000)
    print("HTTP", r.status if r else "?", "| url:", pg.url, "| title:", repr(pg.title()))
    pg.screenshot(path=out, full_page=True)
    print("saved", out)
    b.close()
```

> **Selector note:** the login step assumes `input[name="username"]`, `input[name="password"]`, and a `button[type="submit"]` (correct for the central GUI). For a different app, open `/login` once (public) and adjust the selectors to match its form.

---

## 3. Run it, then look

**Auth-exempt page (no login):**
```bash
BASE="http://100.64.0.12:8000" PATHV="/login" OUT="$PWD/login.png" \
  pwvenv/bin/python shot.py
```

**Authenticated page (pull creds from `.ref/credentials`):**
```bash
BASE="http://100.64.0.12:8000" U="admin" P="<CENTRAL_OPERATOR_PASS>" \
  PATHV="/consumers" OUT="$PWD/consumers.png" \
  pwvenv/bin/python shot.py
```

Then **view the image** — use the **Read tool on the `.png`** (it renders images visually). That is the actual verification step: read the screenshot and confirm the page looks right (layout, columns, buttons, data), not just that it returned `200`.

If `final url` came back as `/login`, the login failed (bad creds / wrong selectors) — fix before trusting the shot.

---

## 4. Worked example — central `/consumers`

- Base: `http://100.64.0.12:8000` (mesh) or `http://central.echo6.mesh:8000`.
- Creds: `CENTRAL_OPERATOR_USER` / `CENTRAL_OPERATOR_PASS` in `.ref/credentials`.
- `/login` + `/health` are auth-exempt; everything else 302s to `/login`.
- This loop caught a real layout bug (tables overflowing their cards) that 34 passing tests + a `302` smoke check both missed — fixed and re-verified by screenshot. See [[central]] and the deploy flow in [[central-deploy-cutover]].

Fold it into a deploy: after restarting the service, screenshot the changed page and read it before declaring the deploy verified.

---

## 5. Extension — driving authenticated *actions* (not just looking)

For clicking buttons / POSTing forms (e.g. bulk-deleting via an admin page so the action is **audit-logged** through the app), a plain authenticated HTTP session is simpler than Playwright. Pattern (stdlib `urllib` + `http.cookiejar`):

1. `GET /login` → scrape the pre-auth `csrf_token` from the form.
2. `POST /login` with `username`/`password`/`csrf_token` → session cookie set.
3. `GET <page>` → scrape the **session** `csrf_token` from a rendered form (post-auth CSRF is a different, session-bound token).
4. `POST <action endpoint>` with that `csrf_token` for each action.
5. Re-`GET` the page and diff to verify the effect.

This routes changes through the real app (CSRF + audit log + server-side guards) instead of bypassing it.

---

## Gotchas

- **Ephemeral venv** — scratchpad doesn't persist; re-install per session.
- **`full_page=True`** stitches the whole scrollable page into one tall PNG — good for long admin tables.
- **`wait_until="networkidle"`** lets HTMX/JS settle before the shot; bump timeouts for slow pages.
- **Two CSRF layers** (central): pre-auth token on `/login` is cookie-bound/`itsdangerous`; post-auth token is session-bound (in `config.sessions`, rendered into every form). Don't reuse one for the other.
- **Reachability**: prefer the mesh IP / `*.echo6.mesh` name over LAN IPs. If cortex can't reach the target, `curl` it first to confirm before blaming the browser.
- **Headless-shell vs full Chromium**: `playwright install chromium` pulls the headless shell, which is enough for screenshots; no extra system libraries were required on cortex.
