diff --git a/AGENTS.md b/AGENTS.md
index 85b3c6f..01f207c 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -1773,6 +1773,16 @@ Node 22 + npm 10 · Bun 1.3 · Java 21 · Android SDK at `~/Android/Sdk` · `ngi
 
 ### Run & validate
 - `npm run dev` → http://localhost:8080. Note the script runs `npm i` first (see lockfile caveat).
+- **Phone-frame preview (check the UI at device sizes in a desktop browser):**
+  http://localhost:8097/phone-frame.html — a static page that embeds the Vite dev
+  server (`:8080`) in a phone-sized iframe (device presets, rotate, route box;
+  `?app=`/`?route=`/`?device=` params). Files live in
+  `~/.config/skatespots/phone-preview/` and it's served by the **systemd user unit
+  `skatespots-phone-frame.service`** (`systemctl --user status skatespots-phone-frame`).
+  ⚠️ **Machine-local, deliberately NOT in the repo** (kept out of the build/deploy and
+  the `eslint` HTML gate). It replaced an ad-hoc `python3 -m http.server 8097` copy in
+  `/tmp` that got wiped (2026-10-05) — don't recreate it in `/tmp`. Needs the dev
+  server (`skatespots-dev.service`, `:8080`) up, else the frame shows "cannot be reached".
 - `npm test` = `npm i` + `tsc` + `eslint` + `vitest` + `vite build`. **Must pass before committing.**
 
 ### ⚠️ "This page slammed" on *some* routes in dev (dep optimizer cache)