diff --git a/astro.config.mjs b/astro.config.mjs
index 8508e67..fed7cf1 100644
--- a/astro.config.mjs
+++ b/astro.config.mjs
@@ -139,6 +139,7 @@ export default defineConfig({
             { label: 'Design system', link: '/docs/contributing/design-system/' },
             { label: 'Translations', link: '/docs/contributing/translations/' },
             { label: 'API errors', link: '/docs/contributing/api-errors/' },
+            { label: 'Other clients', link: '/docs/contributing/other-clients/' },
             { label: 'Releasing', link: '/docs/contributing/releasing/' },
           ],
         },
diff --git a/src/content/docs/docs/contributing/index.md b/src/content/docs/docs/contributing/index.md
index 9298bc8..601c137 100644
--- a/src/content/docs/docs/contributing/index.md
+++ b/src/content/docs/docs/contributing/index.md
@@ -33,24 +33,7 @@ Playwright specs in `e2e/` run against `BASE_URL`, default `http://127.0.0.1:873
 
 ## Pull request checks
 
-CI runs `.github/actions/checks/action.yml`:
-
-```sh
-npm ci
-node scripts/changelog-check.js
-node scripts/bump-cache-busting.js --check
-npm run paths:check
-cd api && npm test
-cd api && npx c8 check-coverage --lines 92
-cd ui/test && node --test
-npm run lint
-npm run format:check
-npm run typecheck
-npm run typecheck:ui
-docker build -t stackyard:ci .
-```
-
-CodeQL and Trivy also run. Both block on a finding.
+The checks CI runs are listed in [CONTRIBUTING.md](https://github.com/SandObserver/stackyard/blob/main/CONTRIBUTING.md#before-opening-a-pr).
 
 - Do not edit `CHANGELOG.md`. List user-visible changes in the pull request description under **Changelog**, as `- ` lines below a Keep a Changelog heading such as `### Fixed`. Merging adds them to `[Unreleased]`.
 - Write `?v=1` on new `/css/` and `/js/` imports. The release sets the real hash.
diff --git a/src/content/docs/docs/contributing/other-clients.md b/src/content/docs/docs/contributing/other-clients.md
new file mode 100644
index 0000000..491180b
--- /dev/null
+++ b/src/content/docs/docs/contributing/other-clients.md
@@ -0,0 +1,49 @@
+---
+title: Build another client
+description: How an app or script signs in to Stackyard and reads the dashboard. Unstable, with no compatibility promise.
+---
+
+The API serves the dashboard and the admin. It has no version and can change in any release. Read the version first, and refuse one you have not tested.
+
+## Sign in
+
+1. `GET /api/auth/check`. If `enabled` is `false`, sign-in is off and no cookie is needed.
+2. `POST /api/auth/login` with `{"password": "..."}`.
+3. Keep the `ds` cookie from the response. Send it on every request.
+
+While no password is set, Stackyard answers only local addresses and the names in **Allowed Addresses**. Any other `Host` gets a `403`.
+
+A session ends after 12 hours unused, or 30 days after sign-in. `SESSION_MAX_AGE_DAYS` changes the first. A `401` with `"kind": "auth"` means sign in again.
+
+## Origin header
+
+Every `POST`, sign-in included, needs an `Origin` header with the same host as the `Host` header. Without it, the request gets a `403`.
+
+```
+Origin: http://stackyard.lan:8700
+```
+
+## Read the dashboard
+
+| Request | Returns |
+| --- | --- |
+| `GET /api/version` | `current`, the installed version. |
+| `GET /api/config` | The layout: `items` and `settings`. Stored secrets are removed. |
+
+`_rev` in the config changes on every save. Poll `/api/config` and redraw when it changes. The dashboard polls every 15 seconds. Do not poll faster.
+
+## Widgets
+
+Show a widget by loading its page in a web view:
+
+```
+/widgets/<widgetType>/<page>?id=<item id>&size=<widgetSize>
+```
+
+`<page>` is `index.html`, unless the widget's entry in `GET /api/widgets` has `views`. Then it is the `src` of the view that the item's `widgetConfig[viewField]` names, or of `defaultView` when that is unset.
+
+The page fetches its own data with the session cookie. The data each widget returns is private to its page and changes without notice.
+
+## Errors
+
+See [API errors](/docs/contributing/api-errors/).