diff --git a/CLAUDE.md b/CLAUDE.md
index ac1a939..34551be 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -28,6 +28,9 @@ npm run keepalive-status   # is the deployed container holding its session?
 npm run session-diagnose   # ~2.5h: measure what actually ends the session
 ```
 
+`docker-compose.override.yml` is local-only and publishes the server on
+`127.0.0.1:8080` and Postgres on `127.0.0.1:55432`; see `docs/LOCAL.md`.
+
 `probe` and `smoke` hit the live Schulcloud and need a valid `.env`. Both are
 read-only with respect to Schulcloud. Run `smoke` after touching `src/core/`,
 `src/mcp/` or `src/http/` — the unit tests cover only pure functions.
@@ -39,7 +42,9 @@ fallback nobody exercises.
 Store tests need a database and skip without one:
 `TEST_DATABASE_URL=postgresql://… npm test`. They use a real Postgres on
 purpose — the generation/diff semantics are entirely SQL, so a mock would test
-nothing.
+nothing. **They `TRUNCATE`**, and refuse to run unless the database name
+contains "test"; that guard exists because pointing them at the dev database
+once put fixtures into real data.
 
 ## Architecture
 
diff --git a/Dockerfile b/Dockerfile
index db457d3..7027526 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -30,7 +30,14 @@ COPY --from=deps  /app/node_modules ./node_modules
 COPY --from=build /app/dist ./dist
 COPY package.json ./
 
-# node:alpine ships an unprivileged `node` user; the process never writes to disk.
+# The mirror is the one writable path. Creating it in the image with the right
+# owner matters: Docker initialises a new named volume from the image directory,
+# including its ownership, so without this the volume lands root-owned and the
+# unprivileged user gets EACCES on every write — with the failure recorded per
+# file rather than crashing, which makes it easy to miss.
+RUN mkdir -p /data/mirror && chown -R node:node /data
+
+# node:alpine ships an unprivileged `node` user.
 USER node
 
 EXPOSE 8080
diff --git a/README.md b/README.md
index 2dce2e8..c44a016 100644
--- a/README.md
+++ b/README.md
@@ -64,8 +64,9 @@ npm run build
 npm run probe               # verifies the token and API against the live instance
 ```
 
-Then either deploy it as a remote connector, or point Claude Code at
-`dist/bin/stdio.js`. Both paths are in [docs/DEPLOYMENT.md](docs/DEPLOYMENT.md).
+To try it on your own machine — Docker stack, Claude Code, and the CLI — follow
+[docs/LOCAL.md](docs/LOCAL.md). To put it on a Pi behind Caddy, see
+[docs/DEPLOYMENT.md](docs/DEPLOYMENT.md).
 
 Getting `TSC_JWT_COOKIE` takes four clicks in DevTools and then lasts 30 days —
 provided you close the Schulportal window afterwards. See
diff --git a/docker-compose.override.yml b/docker-compose.override.yml
new file mode 100644
index 0000000..ab63a59
--- /dev/null
+++ b/docker-compose.override.yml
@@ -0,0 +1,20 @@
+# Local development only. Compose merges this automatically; it is not used on
+# the Pi, where Caddy reaches the container over the shared Docker network and
+# no host port is published.
+services:
+  postgres:
+    ports:
+      # Loopback only. Lets `npm test` reach it via TEST_DATABASE_URL and lets
+      # the stdio server use the same index as the HTTP one.
+      - "127.0.0.1:55432:5432"
+
+  schulcloud-mcp:
+    environment:
+      # The bundled postgres service, reachable by name on the compose network.
+      DATABASE_URL: postgresql://schulcloud:${POSTGRES_PASSWORD:-schulcloud}@postgres:5432/schulcloud
+      # Crawl on demand while developing rather than every 6 hours.
+      CRAWL_INTERVAL_MS: 0
+    ports:
+      # Bound to loopback: this exposes the account's data, and the bearer token
+      # is the only thing in front of it.
+      - "127.0.0.1:8080:8080"
diff --git a/docs/LOCAL.md b/docs/LOCAL.md
new file mode 100644
index 0000000..40cf515
--- /dev/null
+++ b/docs/LOCAL.md
@@ -0,0 +1,109 @@
+# Running it locally
+
+Everything runs on your machine: Postgres, the server, the CLI, and Claude Code
+talking to all of it. Only Schulcloud itself is remote.
+
+## Start the stack
+
+```bash
+cp .env.example .env     # fill in TSC_URL, TSC_JWT_COOKIE, MCP_AUTH_TOKEN
+npm install && npm run build
+docker compose up -d --build
+curl -s http://127.0.0.1:8080/healthz     # {"status":"ok","sessions":0,"index":"on"}
+```
+
+`docker-compose.override.yml` is merged automatically and is **local-only**: it
+publishes the server on `127.0.0.1:8080` and Postgres on `127.0.0.1:55432`, and
+switches crawling to on-demand. On the Pi neither port is published — Caddy
+reaches the container over the Docker network.
+
+Loopback binding is deliberate. The bearer token is the only thing in front of
+your account's data, so it should not be listening on your LAN while you test.
+
+## Populate the index
+
+A full crawl is ~270 requests; start with one course:
+
+```bash
+node dist/bin/cli.js login --server http://127.0.0.1:8080 --token "$MCP_AUTH_TOKEN"
+node dist/bin/cli.js refresh --course <courseId>    # or omit --course for everything
+node dist/bin/cli.js status
+```
+
+Get a course id from `curl -s -H "Authorization: Bearer $TSC_JWT_COOKIE" \
+"$TSC_URL/api/v3/courses?limit=5" | jq -r '.data[] | "\(.id)  \(.title)"'`.
+
+## Connect Claude Code
+
+Claude Code supports MCP over stdio, SSE and HTTP. The VS Code extension runs
+the same Claude Code underneath, so it reads the same configuration — register
+once and both work.
+
+**HTTP — matches the deployed setup:**
+
+```bash
+claude mcp add --transport http schulcloud http://127.0.0.1:8080/mcp \
+  --header "Authorization: Bearer $MCP_AUTH_TOKEN"
+```
+
+**stdio — no server or Docker needed:**
+
+```bash
+claude mcp add schulcloud-stdio \
+  -e DATABASE_URL=postgresql://schulcloud:schulcloud@127.0.0.1:55432/schulcloud \
+  -- node --env-file=/absolute/path/to/.env /absolute/path/to/dist/bin/stdio.js
+```
+
+`--env-file` keeps the JWT in `.env` rather than copying it into the MCP config.
+The HTTP form has no such option — its header holds the token — so leave it at
+the default `local` scope, which writes to `~/.claude.json` rather than a
+`.mcp.json` that would be committed.
+
+Check it:
+
+```bash
+claude mcp list       # schulcloud: http://127.0.0.1:8080/mcp (HTTP) - ✔ Connected
+claude mcp get schulcloud
+```
+
+Then just ask: *"which courses am I in?"*, *"what's due this week?"*, *"find the
+material about Verschlüsselung"*. Inside a session, `/mcp` lists the servers and
+their tools.
+
+## Test the CLI
+
+```bash
+node dist/bin/cli.js ls --long
+node dist/bin/cli.js sync --dry-run        # shows what it would mirror
+node dist/bin/cli.js sync
+```
+
+`npm link` puts it on your PATH as `schulcloud`.
+
+## Run the test suites
+
+```bash
+npm test              # 58 offline tests
+npm run smoke         # end-to-end against the live instance, live-only mode
+DATABASE_URL=… npm run smoke    # end-to-end with the index (34 checks)
+```
+
+Store tests need a database and skip without one:
+
+```bash
+docker exec schulcloud-mcp-db psql -U schulcloud -d schulcloud \
+  -c "CREATE DATABASE schulcloud_test OWNER schulcloud"
+TEST_DATABASE_URL=postgresql://schulcloud:schulcloud@127.0.0.1:55432/schulcloud_test npm test
+```
+
+**These tests `TRUNCATE`.** They refuse to run unless the database name contains
+"test", because aiming them at the dev database once was enough — the fixtures
+ended up in real data.
+
+## Tearing down
+
+```bash
+docker compose down          # keep the index and mirror
+docker compose down -v       # discard them too
+claude mcp remove schulcloud
+```
diff --git a/test/store.test.ts b/test/store.test.ts
index 9c72520..1348609 100644
--- a/test/store.test.ts
+++ b/test/store.test.ts
@@ -8,7 +8,23 @@ import type { Snapshot } from '../src/core/crawl.ts';
  * are entirely SQL, so a mock would test nothing. Skipped when TEST_DATABASE_URL
  * is unset so `npm test` stays offline by default.
  */
-const URL = process.env.TEST_DATABASE_URL;
+const DB_URL = process.env.TEST_DATABASE_URL;
+
+/**
+ * These tests TRUNCATE. Pointing them at a real database destroys it — which
+ * happened once during development, when TEST_DATABASE_URL was aimed at the dev
+ * instance and the fixtures ended up in live data. Requiring "test" in the
+ * database name makes that mistake impossible to repeat by accident.
+ */
+function assertDisposable(url: string): void {
+	const name = new globalThis.URL(url).pathname.replace(/^\//, '');
+	if (!/test/i.test(name)) {
+		throw new Error(
+			`Refusing to run: TEST_DATABASE_URL points at database "${name}", which is not obviously ` +
+				`disposable. These tests TRUNCATE. Use a database with "test" in its name.`,
+		);
+	}
+}
 
 function snapshot(courses: { id: string; title: string; boardText?: string; files?: { id: string; name: string; size: number }[] }[]): Snapshot {
 	return {
@@ -38,11 +54,12 @@ function snapshot(courses: { id: string; title: string; boardText?: string; file
 	};
 }
 
-describe('Store', { skip: URL ? false : 'set TEST_DATABASE_URL to run' }, () => {
+describe('Store', { skip: DB_URL ? false : 'set TEST_DATABASE_URL to run' }, () => {
 	let store: Store;
 
 	before(async () => {
-		const opened = await Store.open(URL);
+		assertDisposable(DB_URL!);
+		const opened = await Store.open(DB_URL);
 		assert.ok(opened, 'store should open');
 		store = opened;
 		// Start from a clean slate so generation ids are predictable.