The way we've solved this in our MCP gateway (OSS) is that the user first needs to authenticate against our gateway, e.g. by creating a valid JWT with their identity provider, which will be validated using JWKS. Now when they use a tool, they must send their JWT, so the LLM always acts in their behalf. This supports multiple tenants out of the box. (https://wundergraph.com/mcp-gateway)