rosetta-mcp

Rosetta is a modular MCP hub: one HTTP service hosting many thin, read-only MCP servers (addons), each mounted under its own path, behind a single OIDC bearer-token authentication layer.

Like the stone: one endpoint, and every agent reads it in its own tongue.

agent ──Bearer JWT──►  https://rosetta.example.com/maps/      (Google Maps, Places, Weather)
                       https://rosetta.example.com/transit/   (SNCF + IDFM/Navitia)
                       https://rosetta.example.com/<addon>/   (drop a module in, it mounts)
                       https://rosetta.example.com/health     (unauthenticated, per-addon state)

Why a hub: provider API keys stay server-side (one deployment, one place to rotate), agents only ever hold an access token, and adding MCP #12 is a new module + nothing else - no new pod, no new image elsewhere, no key sprayed into agent environments.

Addons

An addon is a module in src/rosetta/addons/ exposing:

from ._common import new_server

mcp = new_server("name")          # FastMCP: stateless streamable HTTP at "/"
required_env = ["SOME_API_KEY"]   # optional

The loader mounts each addon under /<module-name> and isolates failures:

Addon state	Meaning
ok	mounted, fully operational
degraded	mounted, but some required_env is missing - tools answer with an explicit error
error	import/startup failed - addon skipped, hub and other addons stay up
disabled	excluded by ROSETTA_ADDONS

Per-addon state is reported on GET /health. Modules starting with _ are shared helpers, never mounted. Each addon also runs standalone over stdio for local debugging (python -m rosetta.addons.maps).

User-data addons (identity = "user")

An addon may declare identity = "user": the hub then refuses machine tokens on its path (403) - the bearer token must carry a human subject. Tools read the caller's claims via a context variable, so a user-data addon keys its server-side credential store on sub: agents never hold the downstream credentials, only their own identity token. Such addons may also register plain HTTP routes (extra_routes / open_paths) for browser-facing enrolment flows, guarded by the ingress SSO (forwardAuth) instead of the hub JWT.

Bundled addons: maps (Google Routes / Places New / Weather - needs GOOGLE_MAPS_API_KEY), transit (SNCF + IDFM Navitia - needs SNCF_API_KEY, IDFM_API_KEY), google (user-data class: Gmail read/search/draft-only + Calendar list/create/update - deliberately no send, no delete, no labels: the guard is the tool surface itself. One-time per-user enrolment at /google/enroll stores the Google refresh token server-side under ROSETTA_GOOGLE_DATA). Tool descriptions are intentionally in French: they are runtime UX for the French-speaking agents this hub serves, not documentation.

Authentication

Rosetta is an OAuth 2.1 resource server. It stores no credentials and no users: it validates JWT access tokens (RFC 9068) issued by an external OIDC provider (tested with Authelia >= 4.39, clients configured with access_token_signed_response_alg != none), against the provider's JWKS.

• Machine agents use the client_credentials grant - no human in the loop.
• User-delegated access (future data-holding addons) uses authorization_code
  • refresh, or the device code flow for headless bodies.
• RFC 9728 protected-resource metadata is served at /.well-known/oauth-protected-resource (and per addon), and every 401 carries the WWW-Authenticate pointer, so OAuth-aware MCP clients can discover the authorization server on their own.

Note the trailing slash: the MCP endpoint of an addon is /<name>/ - /<name> answers with a 307 redirect.

Configuration

Env	Default	Purpose
ROSETTA_AUTH	oidc	off disables auth (local dev only)
ROSETTA_ISSUER	https://auth.berard.me	OIDC issuer (token iss)
ROSETTA_AUDIENCE	external URL	required token aud
ROSETTA_EXTERNAL_URL	https://rosetta.mcp.berard.me	public URL (RFC 9728 metadata)
ROSETTA_JWKS_URI	<issuer>/jwks.json	JWKS endpoint override
ROSETTA_ADDONS	all discovered	comma-separated allowlist
GOOGLE_MAPS_API_KEY	-	maps addon
SNCF_API_KEY, IDFM_API_KEY	-	transit addon

Development

python3 -m venv .venv
.venv/bin/pip install -e .[dev]
.venv/bin/pytest
ROSETTA_AUTH=off .venv/bin/uvicorn rosetta.main:app --port 8200

Deployment

Published as ghcr.io/antorfr/rosetta-mcp (SemVer tags, docker-publish workflow). Runs as a plain container: port 8200, no volume, configuration by environment only.