excalidraw-access-backend

Standalone Node + TypeScript backend for a self-hosted Excalidraw fork with per-board access control (Firebase project excalidraw-team). It provides:

• MCP remote endpoint (ALL /mcp) that lets an AI agent draw on a real

collab board as a specific user. The agent's writes respect the board's read-only policy and are attributed in shared history as Бот <name>.

• MCP connect-token mint / list / revoke endpoints.
• Filesystem-backed image file service that replaces Firebase Storage,

with the same per-board ACL the room server enforces.

The service never modifies the frontend or the room fork; it matches their wire formats (encryption, socket protocol, Firestore scene/history doc shapes).

How it works

Socket auth = exchanged Firebase ID token

The collab (socket.io) server authenticates clients with a Firebase ID token and runs its own ACL on join-room. The Admin SDK can only mint a custom token for a uid, so the bot:

1. admin.auth().createCustomToken(uid)
2. exchanges it for an ID token via Identity Toolkit

(accounts:signInWithCustomToken?key=${FIREBASE_WEB_API_KEY})

1. connects with auth: { token: idToken }

The room server therefore resolves the bot as the user, so its existing read/write enforcement applies automatically: a viewer-token bot's server-broadcast frames are dropped by the room server, and this service also refuses to broadcast/persist when the token role is viewer.

Encryption

src/encryption.ts replicates the frontend (packages/excalidraw/data/encryption.ts) exactly using Node Web Crypto (globalThis.crypto.subtle): a 22-char base64url AES-128-GCM key imported via JWK { alg: "A128GCM", k, kty: "oct" }, 12-byte random IV. Verified byte-compatible by round-trip.

Scene + history persistence

src/scene.ts ports the Admin-SDK equivalent of excalidraw-app/data/firebase.ts:

• scenes/{roomId} = { sceneVersion, ciphertext, iv } (encrypted elements).
• shared history index scenes/{roomId}~history + per-entry payload

scenes/{roomId}~history~{entryId}, matching SceneHistory entry shape and MAX_SCENE_HISTORY_ENTRIES so the frontend HistorySidebar renders bot entries (with author).

Byte fields are written as Node Buffer (the Admin SDK has no web-only Bytes class); the underlying Firestore bytesValue is identical to what the web SDK Bytes produces, so data.ciphertext.toUint8Array() on the frontend reads the same bytes.

Endpoints

The file bytes are already client-encrypted + compressed; the service stores and returns them verbatim.

MCP tools

• describe_scene — current non-deleted elements (viewer + editor).
• query_elements — filter by type / ids / groupId (viewer + editor).
• create_element, update_element, delete_element — editor only. A viewer

token gets an MCP error read-only access.

• batch_create, update_elements, delete_elements, delete_region —

single-commit batch writes (editor only). update_elements skips ids that no longer exist (returning them in missing) instead of aborting the batch.

• group_elements, ungroup_elements, create_frame — grouping/frames.
• clear_canvas — editor only, safe by default (needs confirm:true).

See docs/verification-tools.md for the read/measure/render/validate tools, bound text (containerId/label), line points, and the lint rules.

Each mutating tool: applies the change (bumps version, fresh versionNonce, updated, fractional index after the last element), broadcasts a SCENE_UPDATE over server-broadcast, merge-persists the scene into Firestore in a transaction (so a concurrent human session is never clobbered), and appends a history entry attributed Бот <name>.

The bot keeps stable ownership of the ids it creates. If a concurrent human session deletes one of them (the live session can broadcast a fresh bot element back as a tombstone), the bot re-asserts its last good copy at a higher version so the element is not silently lost — up to a few times before it yields to a persistent deletion. scene_diff reports ownership (byOrigin.bot, owned) and any contested ids (conflicts); create_element/batch_create surface the same conflicts.

Setup

cp .env.example .env   # fill in the values
npm install
npm run build
npm start              # or: npm run dev

Required env (see .env.example):

• GOOGLE_APPLICATION_CREDENTIALS — absolute path to the service-account JSON

(Admin SDK).

• FIREBASE_WEB_API_KEY — the web apiKey (AIzaSy...) from the SDK config;

required for the custom-token → ID-token exchange.

• WS_SERVER_URL — the collab server (default http://localhost:3002).
• FIREBASE_PROJECT_ID (default excalidraw-team), PORT, CORS_ORIGIN,

DATA_DIR, PUBLIC_BASE_URL.

Mint a token + paste the MCP config

curl -X POST http://localhost:3015/mcp/tokens \
  -H "Authorization: Bearer <FIREBASE_ID_TOKEN>" \
  -H "Content-Type: application/json" \
  -d '{"boardId":"<roomId>"}'

The response configSnippet is a ready-to-paste remote-MCP client config:

{
  "mcpServers": {
    "excalidraw-board": {
      "type": "http",
      "url": "http://localhost:3015/mcp",
      "headers": { "Authorization": "Bearer <token>" }
    }
  }
}

The agent connecting with that config draws on the board as the token's user.

Deploy notes

• Run behind TLS and set PUBLIC_BASE_URL so mcpUrl in token responses is

correct.

• Mount DATA_DIR on persistent storage (it replaces Firebase Storage).
• The service holds in-memory CollabBot instances keyed by connect token; it

is intended to run as a single process. Horizontal scaling would need a shared bot registry / sticky routing (not implemented).

• Firestore security rules must allow the service account to read boards,

boardKeys, teams and read/write scenes* and mcpTokens.

Not yet verified live

End-to-end testing needs real credentials and a running room server, which are not available in this build environment. The following paths are structurally complete and type-checked but not exercised against live infrastructure:

• Firebase Admin init with a real service account and verifyIdToken.
• Custom-token → ID-token exchange against Identity Toolkit, and the room

server accepting that ID token and applying read-only for viewer tokens.

• Live socket handshake (init-room → join-room →

first-in-room/new-user/room-user-change) and client-broadcast decryption / reconciliation timing. The handshake resolves on the first membership event or after a 4s fallback.

• Actual Firestore writes to scenes/{roomId} and scenes/{roomId}~history*

and the frontend HistorySidebar rendering the Бот <name> entries.

• The frontend reading files written by PUT /files/* (path-shape and opaque

byte passthrough are implemented; the exact Content-Type/CORS headers the frontend expects on GET were set permissively but not validated against a live client).

• Fractional index ordering interop: the public [email protected]

package is used; the frontend uses @excalidraw/[email protected] (a fork with identical key output), assumed byte-compatible but not co-tested.