Storage Agent Skill

Butterbase Storage

Butterbase stores files in S3 (or LocalStack in dev) and exposes them via presigned URLs. Every file gets a stable object_id (UUID) that you persist in your tables; URLs are generated on demand and expire.

All storage operations go through one tool: manage_storage with an action parameter.

| Action | Purpose |

|--------|---------|

| upload_url | Generate a 15-minute presigned PUT URL and reserve an object_id |

| download_url | Generate a 1-hour presigned GET URL for a stored object |

| list | List objects (scoped by caller's role) |

| delete | Permanently remove an object from S3 + database |

| update_config | Toggle app-level publicReadEnabled and other storage settings |

1. The mental model: object_id vs s3_key

| Field | What it is | When you use it |

|-------|-----------|-----------------|

| object_id | UUID, stable, app-level handle | Persist in your tables (e.g. users.avatar_id, posts.image_id) |

| s3_key | Internal bucket path like app_abc/user_uuid/file.jpg | Internal only — never treat this as a URL |

Critical: s3_key is not a URL. You cannot use it as <img src> or <a href>. Always store the object_id and resolve a fresh download URL at render time.

2. The upload lifecycle

A single upload is two HTTP calls and one DB insert in your app:

┌─────────────────────────┐
│ 1. manage_storage(      │  → returns { upload_url, object_id, expires_at }
│      action: upload_url)│
├─────────────────────────┤
│ 2. PUT file -> S3       │  → must include exact Content-Type header
├─────────────────────────┤
│ 3. INSERT INTO ...      │  → save object_id alongside the user/post/etc.
└─────────────────────────┘

If you skip step 3, the file lives in S3 but no row references it — an orphaned object counting against your quota. Always persist the object_id.

Step 1 — request an upload URL

manage_storage({
  app_id: "app_abc123",
  action: "upload_url",
  filename: "avatar.jpg",
  content_type: "image/jpeg",
  size_bytes: 245123,
  public: false                  // optional; default false
})

Returns:

{
  "upload_url": "https://s3.amazonaws.com/...",
  "object_id": "9c14b2e0-...",
  "expires_at": "2026-05-08T22:15:00Z"
}

The object_id is created immediately in the database; the file just hasn't been uploaded yet.

Step 2 — PUT the bytes

The Content-Type on the PUT must match exactly the content_type you sent in step 1. If it doesn't, S3 rejects the upload or stores the wrong MIME — breaking browser previews.

curl -X PUT "{upload_url}" \
  -H "Content-Type: image/jpeg" \
  --data-binary @avatar.jpg

From the browser:

await fetch(uploadUrl, {
  method: "PUT",
  headers: { "Content-Type": file.type },
  body: file
});

Step 3 — persist the object_id

UPDATE users SET avatar_id = $1 WHERE id = $2

Or via the auto-API / SDK — whatever your app uses for writes. Without this step, the file is unreachable.

3. Generating download URLs

Each call returns a fresh URL valid for 1 hour. Don't bake URLs into static HTML or long-lived caches — re-generate per render or per session.

manage_storage({
  app_id: "app_abc123",
  action: "download_url",
  object_id: "9c14b2e0-..."
})
// → { download_url: "https://s3.amazonaws.com/..." }

For lists with many files, resolve URLs in parallel:

const urls = await Promise.all(
  posts.map(p => getDownloadUrl(p.image_id))
);

If the caller is unauthorized, the response is 404 (not 403) — Butterbase deliberately hides existence to avoid leaking object IDs.

4. Access control

Three tiers, evaluated in order:

| Caller | What they can read |

|--------|--------------------|

| Service key (bb_sk_*) | Everything in the app — RLS bypassed |

| End-user JWT | Files where (user_id === caller_id) OR object.public === true OR app.publicReadEnabled === true |

| Anonymous (no auth) | Only public objects (and only if app access mode allows anon) |

Per-object public flag

Set at upload time:

manage_storage({ app_id, action: "upload_url", filename, content_type, size_bytes, public: true })

Use this for one-off public files (a marketing image, a shared avatar) without flipping the whole app to public-read.

App-wide public read

manage_storage({
  app_id: "app_abc123",
  action: "update_config",
  publicReadEnabled: true
})

When true, any authenticated user in the app can download any file. Uploads and deletes stay user-scoped.

> Storage ACL is hardcoded — you cannot layer Postgres RLS policies on top of storage_objects. If you need fine-grained custom rules, gate downloads through a serverless function instead of handing out direct presigned URLs.

5. Listing and deleting

manage_storage({ app_id: "app_abc123", action: "list" })

Service key sees everything; end-user JWT sees only their own files. Each item has id, user_id, key, filename, content_type, size_bytes, created_at.

manage_storage({ app_id: "app_abc123", action: "delete", object_id: "9c14b2e0-..." })

Permanently removes the S3 object and DB row. Clear foreign-key references first (e.g. UPDATE users SET avatar_id = NULL) — manage_storage doesn't.

6. Quotas & error codes

| Limit | Default | Override |

|-------|---------|----------|

| Per-file size | 10 MB | storage_config |

| Total app storage | Plan-dependent | Upgrade plan |

| Allowed content types | All by default | storage_config.allowedContentTypes whitelist |

| Error | When |

|-------|------|

| QUOTA_FILE_SIZE_EXCEEDED (400) | size_bytes > per-file limit |

| QUOTA_STORAGE_EXCEEDED (429) | App total exhausted |

| VALIDATION_INVALID_TYPE (400) | content_type not in whitelist |

| RESOURCE_NOT_FOUND (404) | Object missing or caller unauthorized (deliberately ambiguous) |

| S3_ERROR (503) | Transient S3 failure — retry |

7. Common patterns

User avatar

• Upload form posts file → manage_storage (upload_url, public: false).
• Browser PUTs to upload_url.
• App calls UPDATE users SET avatar_id = $object_id.
• On profile render: manage_storage (download_url, object_id: user.avatar_id).

Public marketing image

Same flow, but public: true at upload time. Then the same download URL is reachable by anonymous visitors (no JWT needed) — useful for landing pages and OG images.

File attachment with auth

For attachments where access depends on app-level rules richer than "owner or public" (e.g. "members of this workspace can read"), don't expose the presigned URL directly. Wrap downloads in a serverless function:

export async function handler(req, ctx) {
  const { rows } = await ctx.db.query(
    "SELECT object_id FROM attachments WHERE id = $1 AND workspace_id IN (SELECT workspace_id FROM members WHERE user_id = $2)",
    [attachmentId, ctx.user.id]
  );
  if (!rows.length) return new Response("forbidden", { status: 403 });
  const url = await getDownloadUrlServerSide(rows[0].object_id);
  return Response.redirect(url, 302);
}

The function runs as butterbase_user and uses your tables' RLS to authorize, then mints a presigned URL with the service key.

8. Anti-patterns

| Don't | Do |

|-------|-----|

| Store the s3_key in your tables | Store the object_id UUID |

| Embed presigned URLs in HTML or DB rows | Re-fetch per render — they expire in 1 hour |

| Send the wrong Content-Type on PUT | Match exactly what you sent to upload_url |

| Call upload_url and forget step 3 | Always persist object_id after a successful PUT |

| Use public: true everywhere "just in case" | Default to private; opt files into public explicitly |

| Delete a file without clearing FKs | Update referencing rows first, then action: "delete" |

If a docs/butterbase/00-state.md exists in the working directory, prefer invoking via /butterbase-skills:journey-storage so the journey orchestrator stays in sync.