docs(AppKit): document Files plugin policy API

skills/databricks-apps/references/appkit/files.md

@@ -1,17 +1,17 @@
 # Files: Unity Catalog Volume Operations
 
-**For full Files plugin API (routes, types, config options)**: run `npx @databricks/appkit docs` → Files plugin.
+**For full Files plugin API (routes, types, config options)**: run `npx @databricks/appkit docs ./docs/plugins/files.md`.
 
 Use the `files()` plugin when your app needs to **browse, upload, download, or manage files** in Databricks Unity Catalog Volumes. For analytics dashboards reading from a SQL warehouse, use `config/queries/` instead. For persistent CRUD storage, use Lakebase.
 
 ## When to Use Files vs Other Patterns
 
-| Pattern | Use Case | Data Source |
-| --- | --- | --- |
-| Analytics | Read-only dashboards, charts, KPIs | Databricks SQL Warehouse |
-| Lakebase | CRUD operations, persistent state, forms | PostgreSQL (Lakebase) |
-| Files | File uploads, downloads, browsing, previews | Unity Catalog Volumes |
-| Files + Analytics | Upload CSVs then query warehouse tables | Volumes + SQL Warehouse |
+| Pattern           | Use Case                                    | Data Source              |
+| ----------------- | ------------------------------------------- | ------------------------ |
+| Analytics         | Read-only dashboards, charts, KPIs          | Databricks SQL Warehouse |
+| Lakebase          | CRUD operations, persistent state, forms    | PostgreSQL (Lakebase)    |
+| Files             | File uploads, downloads, browsing, previews | Unity Catalog Volumes    |
+| Files + Analytics | Upload CSVs then query warehouse tables     | Volumes + SQL Warehouse  |
 
 ## Scaffolding
 
@@ -43,33 +43,82 @@ The env var suffix (after `DATABRICKS_VOLUME_`) becomes the volume key, lowercas
 import { createApp, files, server } from "@databricks/appkit";
 
 await createApp({
-  plugins: [
-    server(),
-    files(),
-  ],
+  plugins: [server(), files()],
 });
 ```
 
+### Configuration Overrides
+
+Only add plugin config when you need to override defaults from the discovered `DATABRICKS_VOLUME_*` env vars:
+
+```typescript
+files({
+  maxUploadSize: 5_000_000_000, // plugin-level default
+  volumes: {
+    uploads: {
+      maxUploadSize: 100_000_000,
+      policy: files.policy.allowAll(), // required for writes
+    },
+    user_data: {
+      auth: "on-behalf-of-user", // HTTP routes run SDK calls as end user
+    },
+  },
+});
+```
+
+Auto-discovered volumes merge with explicit config, so `volumes: {}` is only needed for overrides. Check the AppKit docs for the current `IFilesConfig` / `VolumeConfig` shape.
+
+## Permission Model
+
+Three layers gate file access:
+
+1. **Unity Catalog grants** — service-principal volumes need the app SP to hold `WRITE_VOLUME`; OBO volumes need each end user to hold it.
+2. **Execution identity** — HTTP routes use the volume's `auth` mode. Programmatic user-driven handlers must call `.asUser(req)` to run SDK calls as the request user.
+3. **File policies** — app-level allow/deny functions evaluated before every operation.
+
+For SP volumes, removing a user's UC grant has no effect on HTTP access because the SDK call uses the SP. Use policies for per-user restrictions. For OBO volumes, UC grants gate the end user and policies stack on top.
+
+## Access Policies
+
+Volumes without an explicit `policy` default to `files.policy.publicRead()` (reads allowed, writes denied) and log a startup warning. Set an explicit policy on every volume that accepts uploads, directory creation, or deletes.
+
+```typescript
+import { files } from "@databricks/appkit";
+
+files({
+  volumes: {
+    public_data: { policy: files.policy.publicRead() },
+    uploads: { policy: files.policy.allowAll() },
+    archive: { policy: files.policy.denyAll() },
+  },
+});
+```
+
+Use custom policies when access depends on the requesting user or action. For exact built-ins, combinators, `FileAction`, `FileResource`, `FilePolicyUser`, and `PolicyDeniedError` behavior, check `npx @databricks/appkit docs ./docs/plugins/files.md`.
+
 ## Server-Side API (Programmatic)
 
-Access volumes through the `files()` callable, which returns a `VolumeHandle`:
+Access volumes through the `files()` callable, which returns a `VolumeHandle`. Direct programmatic calls do not have request headers available, so they normally run as the service principal. Use `.asUser(req)` in user-driven route handlers when the SDK call must run as the request user.
 
 ```typescript
-// ✅ CORRECT — OBO access (recommended)
-const entries = await appkit.files("uploads").asUser(req).list();
-const content = await appkit.files("exports").asUser(req).read("report.csv");
-
-// ❌ WRONG — omitting .asUser(req)
-const entries = await appkit.files("uploads").list();
-// In dev: silently falls back to service principal credentials, bypassing user-level UC permissions
-// In production: throws an error
+// User-driven handler: SDK call runs as user; policy sees user.id from req.
+await appkit.files("uploads").asUser(req).list();
+
+// Background or trusted server code: runs as SP.
+await appkit.files("uploads").list();
 ```
 
-**ALWAYS use `.asUser(req)`** — without it, dev mode silently uses the app's service principal (masking permission issues that will crash in production).
+**Use `.asUser(req)` in user-driven route handlers** when you want UC grants enforced against the actual user. In production, `asUser(req)` throws `AuthenticationError.missingToken` if the forwarded user or access-token header is missing; in dev (`NODE_ENV === "development"`) it logs a warning and falls back to SP. Policy denial throws `PolicyDeniedError`.
+
+For method signatures, path rules, cache behavior, and retry/timeout defaults, check `npx @databricks/appkit docs ./docs/plugins/files.md`.
+
+## HTTP Routes
+
+Mounted at `/api/files/*`. Routes use the volume's `auth` mode and run the policy before the operation. Use the AppKit docs for the exact route list, request bodies, response types, and `/raw` content-security behavior.
 
 ## Frontend Components
 
-Import file browser components from `@databricks/appkit-ui/react`. Full component props: `npx @databricks/appkit docs "FileBreadcrumb"`.
+Import file browser components from `@databricks/appkit-ui/react`. Full component props: `npx @databricks/appkit docs ./docs/api/appkit-ui/files/DirectoryList.md` and the related component pages.
 
 ### File Browser Example
 
@@ -184,8 +233,8 @@ export function FilesPage() {
 ```typescript
 const handleUpload = async (file: File) => {
   const uploadPath = currentPath ? `${currentPath}/${file.name}` : file.name;
-  const response = await fetch(apiUrl('upload', { path: uploadPath }), {
-    method: 'POST',
+  const response = await fetch(apiUrl("upload", { path: uploadPath }), {
+    method: "POST",
     body: file,
   });
   if (!response.ok) {
@@ -203,7 +252,7 @@ const handleUpload = async (file: File) => {
 const handleDelete = async (filePath: string) => {
   const response = await fetch(
     `/api/files/${volumeKey}?path=${encodeURIComponent(filePath)}`,
-    { method: 'DELETE' },
+    { method: "DELETE" },
   );
   if (!response.ok) {
     const data = await response.json().catch(() => ({}));
@@ -217,23 +266,28 @@ const handleDelete = async (filePath: string) => {
 ```typescript
 const handleCreateDirectory = async (name: string) => {
   const dirPath = currentPath ? `${currentPath}/${name}` : name;
-  const response = await fetch(apiUrl('mkdir'), {
-    method: 'POST',
-    headers: { 'Content-Type': 'application/json' },
+  const response = await fetch(apiUrl("mkdir"), {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
     body: JSON.stringify({ path: dirPath }),
   });
   if (!response.ok) {
     const data = await response.json().catch(() => ({}));
-    throw new Error(data.error ?? `Create directory failed (${response.status})`);
+    throw new Error(
+      data.error ?? `Create directory failed (${response.status})`,
+    );
   }
 };
 ```
 
 ## Resource Requirements
 
-Each volume key requires a resource with `WRITE_VOLUME` permission. Declare in `databricks.yml`:
+The plugin auto-generates volume resource requirements from `DATABRICKS_VOLUME_*` env vars. Setting them in `app.yaml` is usually all you need.
+
+Declare the volume explicitly in `databricks.yml` only when you need to pin it as a managed resource, then wire the env var via `valueFrom` in `app.yaml`:
 
 ```yaml
+# databricks.yml
 resources:
   apps:
     my_app:
@@ -246,23 +300,24 @@ resources:
             permission: WRITE_VOLUME
 ```
 
-> **Note:** The scaffolded HTTP routes (`/api/files/...`) execute as the service principal and do not require `user_api_scopes`. The scope is needed when using the programmatic `appkit.files("key").asUser(req)` API for per-user Volume access.
-
-Wire the env var in `app.yaml`:
+> **Note:** `user_api_scopes` is required for OBO volumes (`auth: "on-behalf-of-user"`) and for any `appkit.files("key").asUser(req)` programmatic call. Pure SP volumes accessed only via HTTP routes don't need it. The plugin docs have the latest resource-requirement behavior.
 
 ```yaml
+# app.yaml
 env:
   - name: DATABRICKS_VOLUME_UPLOADS
     valueFrom: uploads-volume
 ```
 
 ## Troubleshooting
 
-| Error | Cause | Solution |
-| --- | --- | --- |
-| `Unknown volume key "X"` | Volume env var not set or misspelled | Check `DATABRICKS_VOLUME_X` is set in `app.yaml` or `.env` |
-| 413 on upload | File exceeds `maxUploadSize` | Increase `maxUploadSize` in plugin config or per-volume config |
-| `read()` rejects large file | File > 10 MB default limit | Use `download()` for large files or pass `{ maxSize: <bytes> }` |
-| Blocked content type on `/raw` | Dangerous MIME type (html, js, svg) | Use `/download` instead — these types are forced to attachment |
-| Service principal access blocked | Called volume method without `.asUser(req)` | Always use `appkit.files("key").asUser(req).method()` |
-| `path traversal` error | Path contains `../` | Use relative paths from volume root or absolute `/Volumes/...` paths |
+| Error                                      | Cause                                                                              | Solution                                                                                  |
+| ------------------------------------------ | ---------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------- |
+| `Unknown volume "X"`                       | Volume env var not set or misspelled                                               | Check `DATABRICKS_VOLUME_X` is set in `app.yaml` or `.env`                                |
+| 413 on upload                              | File exceeds `maxUploadSize`                                                       | Increase `maxUploadSize` in plugin config or per-volume config                            |
+| `read()` rejects large file                | File > 10 MB default limit                                                         | Use `download()` for large files or pass `{ maxSize: <bytes> }`                           |
+| Blocked content type on `/raw`             | Dangerous MIME type (html, js, svg)                                                | Use `/download` instead — these types are forced to attachment                            |
+| 403 on HTTP route                          | Volume's policy denied the action for the requesting user                          | Inspect `policy` config; user id comes from the `x-forwarded-user` header                 |
+| Writes return 403 unexpectedly             | Volume has no `policy` configured → defaults to `publicRead()` which denies writes | Set explicit `policy: files.policy.allowAll()` (or stricter) on volumes that accept writes |
+| `PolicyDeniedError` from programmatic call | Volume's policy denied the action — SP identity used if `asUser(req)` was omitted  | Call `.asUser(req)` for user-driven calls; gate trusted SP code with `policy.allowAll()` |
+| Invalid path error                         | Path contains `../`, null bytes, or exceeds 4096 chars                             | Use relative paths from the volume root, or absolute `/Volumes/...` paths                 |