diff --git a/PLAN.md b/PLAN.md
index 41efbe1..058610d 100644
--- a/PLAN.md
+++ b/PLAN.md
@@ -512,6 +512,54 @@ Phased, each phase ends with something runnable.
 
 Hardcoded everything. Qt window spawns FrankenPHP, hits `GET /api/ping`, opens an SSE stream, prints incoming events to a `Text` element. Goal: prove the transport on Linux. ~1 day.
 
+#### Concrete spec
+
+Lives in `spike/`. Removed when Phase 1's framework skeleton supersedes it. **No Symfony yet** — bare PHP behind FrankenPHP, the smallest thing that exercises both transport channels.
+
+Layout:
+
+```text
+spike/
+  README.md           # how to run, what it proves, expected output
+  run.sh              # builds (if needed) and runs FrankenPHP + the Qt host
+  Caddyfile           # binds 127.0.0.1:8080, enables Mercure, routes index.php
+  .env.local          # MERCURE_PUBLISHER_JWT_KEY (dev-only static key)
+  .gitignore          # bin/, build/
+  bin/frankenphp      # downloaded static binary, gitignored
+  php/
+    index.php         # GET /api/ping → returns pong, publishes to Mercure
+  qt/
+    CMakeLists.txt    # minimal Qt 6 + QML project
+    main.cpp          # QGuiApplication + QQmlApplicationEngine + spawns frankenphp child
+    Main.qml          # window: status indicator, Ping button, event log
+    Mercure.qml       # tiny SSE client (text/event-stream parser via QNetworkReply)
+```
+
+Flow:
+
+1. `./run.sh` builds the Qt binary (if not built) and runs it.
+2. Qt host starts and spawns `bin/frankenphp run --config Caddyfile` as a child process.
+3. Once `GET /api/ping` succeeds, QML opens an SSE connection to `/.well-known/mercure?topic=app://ping`.
+4. Clicking the "Ping" button triggers `GET /api/ping`. The handler returns `{ "pong": true, "now": ... }` and publishes the same payload to Mercure.
+5. The event arrives on the SSE stream and is appended to the visible log.
+
+Hardcoded for the spike:
+
+- Backend URL: `http://127.0.0.1:8080`.
+- Mercure topic: `app://ping`.
+- Mercure JWT: dev-only static key in `.env.local`.
+- No auth on `/api/ping`.
+- FrankenPHP static binary version pinned in `run.sh`.
+
+Done criteria:
+
+- Click "Ping" → response text updates **and** an event line appears in the log within ~50 ms.
+- Killing `bin/frankenphp` externally → Qt host visibly shows the connection dropping.
+- Re-running `./run.sh` → everything reconnects.
+- A brief writeup in `spike/README.md` of what the spike proved and any surprises.
+
+Out of scope (lands in Phase 1+): optimistic updates, `Last-Event-ID` resume, per-session secret, single-instance lock, packaging, Symfony.
+
 ### Phase 1 — Framework skeleton (dev mode from day one)
 
 - `framework/php` Symfony bundle with `Publisher`, `HealthController`, `SessionAuthenticator`.