feat(agentex-ui): accept RSA (RS256) private_key_jwt keys

[erichwoo-scale]

Summary

Brings agentex-ui's bespoke private_key_jwt signer back on par with @scale/oneauth-nextjs and, in the process, replaces the hand-rolled WebCrypto assertion code with jose. The signer was ES256/P-256 only; an app configured with an RSA OIDC_PRIVATE_KEY_JWK couldn't sign the client assertion. It now signs with EC (ES256/384/512) or RSA (RS256/384/512, incl. PS*), alg derived from the key.

Purely additive — the ES256 path is behaviorally unchanged.

What changed

Single file, agentex-ui/auth.ts (+ jose promoted to a direct dependency):

• Sign with jose — importJWK + SignJWT replace base64url, importEs256Key, jwsSigningFor, and the manual JWT assembly (~55 lines → ~10). jose validates the JWK against its alg and handles every key type natively, so there are no silent-wrong-alg edge cases to guard by hand.
• signingAlgForJwk derives the alg — explicit JWK alg, else EC by curve (ES256/384/512) / RSA → RS256 — for both the login-key import and the refresh assertion. RS384/RS512/PS256 are supported by declaring alg on the JWK.
• jose was already in the tree transitively (it's what @auth/core uses); this makes it a direct dep.

Everything else was already at parity (agentex-ui set the original bar): getSessionToken BFF, token-stripped session, fail-loud misconfig guards, top-level-await key resolution, OIDC-discovery-driven endpoints.

Verification

tsc --noEmit, next lint (0 warnings), Prettier, and next build — all clean.

Test plan

• Existing EC/P-256 OIDC_PRIVATE_KEY_JWK clients still log in and refresh (ES256, unchanged).
• An RSA OIDC_PRIVATE_KEY_JWK (registered with private_key_jwt + matching public JWK) → login + refresh succeed (RS256 assertion accepted).
• A JWK with alg: RS384/PS256 → assertion carries that alg.
• A malformed / unsupported-kty OIDC_PRIVATE_KEY_JWK → clear error (jose rejects), not a silent wrong-alg.
• client_secret_post path unaffected.

🤖 Generated with Claude Code

Greptile Summary

This PR adds RSA (RS256 and family) support to agentex-ui's private_key_jwt signer by replacing ~55 lines of hand-rolled WebCrypto JWT assembly with jose's importJWK + SignJWT. The ES256/P-256 path is preserved exactly; signingAlgForJwk adds EC-curve and RSA-key-type detection for the algorithm negotiation.

• signingAlgForJwk respects an explicit alg field in the JWK, falls back to curve-based EC alg (ES256/384/512), or defaults to RS256 for bare RSA keys (PSS variants require alg to be declared on the JWK, which is documented in the PR).
• importClientPrivateKey and the SignJWT flow replace importEs256Key, jwsSigningFor, and the manual base64url header/payload/signature construction — delegate alg validation and signing to jose entirely.
• jose is promoted from a transitive dependency (via @auth/core) to a direct one at ^6.0.0.

Confidence Score: 5/5

Safe to merge — the change is purely additive, the EC/ES256 path is behaviorally unchanged, and the new RSA path delegates all alg validation to jose rather than adding bespoke string-matching that could be wrong.

The replaced code was simple enough that the rewrite is easy to audit line-by-line. signingAlgForJwk correctly checks jwk.alg first, EC curve second, and RSA last. The SignJWT builder covers iss/sub/aud/jti/iat/exp identically to the old manual assembly. No logic regressions visible.

No files require special attention — both changed files are straightforward.

Important Files Changed

Filename	Overview
agentex-ui/auth.ts	Replaces hand-rolled WebCrypto signer with jose's importJWK + SignJWT; adds signingAlgForJwk for EC/RSA alg detection; otherwise unchanged auth flow.
agentex-ui/package.json	Adds jose ^6.0.0 as a direct dependency; it was already present transitively via @auth/core.

_{Reviews (3): Last reviewed commit: "feat(agentex-ui): accept EC/RSA private_..." | Re-trigger Greptile}