Per-Tool OAuth Scopes for MCP, Derived from Your Schema

Cosmo Router now enforces per-tool OAuth scopes for MCP servers by deriving scope requirements directly from @requiresScopes directives in federated GraphQL schemas. Instead of giving AI agents broad tokens, the router computes each tool's required scopes from the fields it touches using OR-of-AND Cartesian product logic, caches them at startup, and returns actionable WWW-Authenticate challenges when scopes are missing. This enables autonomous step-up authorization — agents escalate permissions on the same session without reconnecting. The post also covers a bug found in the MCP TypeScript SDK (infinite loop from scope overwriting vs. accumulation), a compatibility config workaround, five additive authorization levels, and why building a separate BFF for agents leads to policy drift.