diff --git a/docs/APPS.md b/docs/APPS.md
index 851bebf..4562efa 100644
--- a/docs/APPS.md
+++ b/docs/APPS.md
@@ -48,6 +48,7 @@ export default app.defaults
 
 - `PORT` - your assigned port (3001-3100)
 - `APPS_DIR` - path to `/apps` directory
+- `DATA_DIR` - per-app data directory (`toes/<app-name>/`)
 
 ## health checks
 
diff --git a/docs/ENV.md b/docs/ENV.md
index a53f4ab..af47af2 100644
--- a/docs/ENV.md
+++ b/docs/ENV.md
@@ -39,3 +39,13 @@ DEBUG=true
 ```
 
 Keys are uppercased automatically. Quotes around values are stripped.
+
+## Built-in variables
+
+These are set automatically by Toes — you don't need to configure them:
+
+- `PORT` - assigned port (3001-3100)
+- `APPS_DIR` - path to the `/apps` directory
+- `DATA_DIR` - per-app data directory (`toes/<app-name>/`) for storing persistent data
+- `TOES_URL` - base URL of the Toes server
+- `TOES_DIR` - path to the toes config directory
diff --git a/docs/TOOLS.md b/docs/TOOLS.md
index 9bc4970..6dbe0ce 100644
--- a/docs/TOOLS.md
+++ b/docs/TOOLS.md
@@ -34,12 +34,20 @@ app.get('/', c => {
 })
 ```
 
+## environment
+
+- `PORT` - your assigned port
+- `APPS_DIR` - path to `/apps` directory
+- `DATA_DIR` - per-app data directory (`toes/<tool-name>/`)
+- `TOES_URL` - base URL of the Toes server
+- `TOES_DIR` - path to the toes config directory
+
 ## accessing app files
 
 Always go through the `current` symlink:
 
 ```ts
-const APPS_DIR = process.env.APPS_DIR ?? '.'
+const APPS_DIR = process.env.APPS_DIR!
 const appPath = join(APPS_DIR, appName, 'current')
 ```
 
diff --git a/src/server/apps.ts b/src/server/apps.ts
index 0badfc8..abb5d4d 100644
--- a/src/server/apps.ts
+++ b/src/server/apps.ts
@@ -597,7 +597,7 @@ async function runApp(dir: string, port: number) {
 
   const proc = Bun.spawn(['bun', 'run', 'toes'], {
     cwd,
-    env: { ...process.env, ...appEnv, PORT: String(port), NO_AUTOPORT: 'true', APPS_DIR, TOES_DIR, TOES_URL },
+    env: { ...process.env, ...appEnv, PORT: String(port), NO_AUTOPORT: 'true', APPS_DIR, DATA_DIR: join(process.env.DATA_DIR ?? '.', 'toes', dir), TOES_DIR, TOES_URL },
     stdout: 'pipe',
     stderr: 'pipe',
   })