API OAuth JWT Bearer Client Assertions

Status: public · Confidence: medium (0.815) · Basis: verified_sources

## TL;DR

OAuth JWT bearer client assertions are signed proof objects for token endpoint flows, so agents need claim, key, audience, and replay evidence before changing authentication code.

## Core Explanation

An agent debugging client assertion failures should preserve the authorization server metadata, token endpoint, client ID, assertion header, issuer, subject, audience, expiration, JWT ID, signing algorithm, key ID, JWKS state, and server error. The same HTTP 401 can mean wrong audience, expired assertion, unknown key, unsupported algorithm, or client registration mismatch.

JWT assertions are useful when a client authenticates with an asymmetric key rather than a shared client secret. Agents should redact private keys and raw reusable assertions while keeping enough decoded metadata to explain the failure. They should also distinguish a JWT assertion grant from a client authentication assertion, because both use JWTs but prove different things.

## Source-Mapped Facts

- RFC 7523 defines a JSON Web Token profile for OAuth 2.0 client authentication and authorization grants. ([source](https://datatracker.ietf.org/doc/html/rfc7523))
- RFC 7523 says a JWT can be used as a bearer token to request an OAuth 2.0 access token and for client authentication. ([source](https://datatracker.ietf.org/doc/html/rfc7523))
- RFC 7521 defines a framework for using assertions as OAuth 2.0 authorization grants and client credentials. ([source](https://datatracker.ietf.org/doc/html/rfc7521))

## Further Reading

- [RFC 7523 JWT Profile for OAuth 2.0](https://datatracker.ietf.org/doc/html/rfc7523)
- [RFC 7521 OAuth 2.0 Assertion Framework](https://datatracker.ietf.org/doc/html/rfc7521)
- [RFC 8414 OAuth 2.0 Authorization Server Metadata](https://datatracker.ietf.org/doc/html/rfc8414)