ID-JAG (Identity Assertion JWT Authorization Grant) is an IETF draft mechanism that addresses authorization challenges when AI Agents act on behalf of users. Unlike traditional approaches that give agents overly broad permissions or lend user tokens directly, ID-JAG enforces least-privilege at every hop in a multi-layer agent chain. It combines RFC 8693 (OAuth 2.0 Token Exchange) and RFC 7523 (JWT Bearer Grant) to ensure tokens are re-issued and downscoped at each trust boundary. The author re-implemented the reference MCP Server from TypeScript to Go using the official modelcontextprotocol/go-sdk, demonstrating how each tool call triggers a fresh token exchange with only the minimum required scope. The post covers the complete token exchange flow, comparison with PKCE, and hands-on setup instructions with test coverage using httptest to simulate Athenz ZTS.
