diff --git a/docs/sandbox-mode.md b/docs/sandbox-mode.md
index 033679a..93462cb 100644
--- a/docs/sandbox-mode.md
+++ b/docs/sandbox-mode.md
@@ -292,18 +292,67 @@ preview continues to work normally.
 | `mps_sandbox_panel` | "0" or "1" | Panel open/closed |
 | `mps_sandbox_mode` | "preset" or "custom" | Filter mode |
 
+## Artifact Storage (User S3 Bucket)
+
+Users can configure their own S3-compatible bucket to upload sandbox artifacts.
+
+```
+User Settings (/u/settings)
+  │
+  │  Artifact Storage form:
+  │  - S3 Endpoint URL
+  │  - Region (optional)
+  │  - Bucket Name
+  │  - Access Key
+  │  - Secret Key
+  │
+  ▼
+POST /u/settings/storage
+  │
+  │  Saves credentials on User model
+  │  (s3_endpoint, s3_region, s3_bucket,
+  │   s3_access_key, s3_secret_key)
+  │
+  ▼
+Sandbox toolbar shows "Upload to Bucket" button
+  │  (only when data-has-bucket="1")
+  │
+  │  1. User exports image/video (download)
+  │  2. User clicks "Upload to Bucket"
+  │  3. JS POSTs to /u/sandbox/upload
+  │  4. Server generates presigned POST with user's credentials
+  │  5. JS uploads blob directly to user's S3 bucket
+  │
+  ▼
+User's S3 Bucket
+  sandbox/{user_id}/{timestamp}-{filename}
+```
+
+**Supported services:** DigitalOcean Spaces, AWS S3, MinIO, Backblaze B2,
+any S3-compatible endpoint.
+
+**Security:** Credentials stored as plain Unicode columns (same pattern as
+Stripe/PayPal/Adyen keys on Shop model). Secret key field uses `type="password"`
+in the form.
+
 ## Files
 
 | File | Role |
 |------|------|
 | `models/shop.py` | `sandbox_mode` Boolean column |
+| `models/user.py` | S3 credential columns + `has_s3_bucket` property |
 | `views/shop.py` | Toggle handler in ribbon-settings |
+| `views/user.py` | Storage settings POST handler + S3 fields in settings dict |
+| `views/user_sandbox.py` | Presigned upload endpoint (`/u/sandbox/upload`) |
 | `templates/shop_settings.j2` | Radio buttons for enable/disable |
-| `templates/base.j2` | Conditional toolbar HTML + SVG defs + script tag |
-| `static/js/sandbox.js` | Client-side filter engine |
+| `templates/user_settings.j2` | Artifact Storage form |
+| `templates/base.j2` | Conditional toolbar HTML + SVG defs + `data-has-bucket` |
+| `static/js/sandbox.js` | Client-side filter engine + upload-to-bucket |
 | `static/css/common.css` | Toolbar layout (CSS Grid) |
 | `static/js/watch.js` | `sandboxReapply()` hook in SPA navigation |
-| `scripts/alembic/versions/a2c13d3117f2_*.py` | Migration |
+| `routes.py` | `user_storage_settings`, `user_sandbox_upload` routes |
+| `scripts/alembic/versions/a2c13d3117f2_*.py` | Shop sandbox_mode migration |
+| `scripts/alembic/versions/f898ba460612_*.py` | User S3 credentials migration |
 
 ## Mobile Behavior