auth: accept EdDSA tokens + soft audience check (Better Auth compatibility)

Better Auth signs access tokens with EdDSA (Ed25519), not RS256. Accept EdDSA
(+ RS256/ES256), and validate issuer + signature + expiry strictly while checking
audience softly — single-resource server behind a dedicated AS with dynamic DCR
client ids, so issuer + signature is the trust boundary. Adds an EdDSA test.

src/intervals_mcp_server/auth.py

@@ -18,8 +18,13 @@ import os
 logger = logging.getLogger("intervals_icu_mcp_server")
 
 
+# Algorithms we accept. Better Auth signs with EdDSA (Ed25519); RS256/ES256 are
+# kept so the same verifier works against other OAuth servers.
+_ALGORITHMS = ["EdDSA", "RS256", "ES256"]
+
+
 class AuthentikTokenVerifier:
-    """Verify RS256 Bearer JWTs against a JWKS endpoint (RFC 9068 style)."""
+    """Verify Bearer JWTs against a JWKS endpoint (RFC 9068 style)."""
 
     def __init__(self, jwks_uri: str, issuer: str, audience: list[str]):
         import jwt  # PyJWT
@@ -35,20 +40,31 @@ class AuthentikTokenVerifier:
 
         try:
             key = self._jwks.get_signing_key_from_jwt(token).key
+            # Validate issuer + signature + expiry strictly. Audience is checked
+            # softly below: this is a single-resource server behind a dedicated
+            # authorization server, and DCR clients have dynamic ids, so a valid
+            # signature + our issuer already establishes the token is for us.
             claims = jwt.decode(
                 token,
                 key,
-                algorithms=["RS256"],
+                algorithms=_ALGORITHMS,
                 issuer=self._issuer,
-                audience=self._audience,
-                options={"require": ["exp", "iat", "iss", "aud"]},
+                options={"require": ["exp", "iat", "iss"], "verify_aud": False},
             )
         except Exception as exc:  # noqa: BLE001 - any failure means unauthenticated
             logger.debug("Token verification failed: %s", exc)
             return None
 
         aud = claims.get("aud")
-        resource = aud[0] if isinstance(aud, list) else aud
+        auds = aud if isinstance(aud, list) else ([aud] if aud else [])
+        if auds and not any(a in self._audience for a in auds):
+            logger.warning(
+                "Token audience %s not in accepted %s; accepting (single resource).",
+                auds,
+                self._audience,
+            )
+
+        resource = auds[0] if auds else self._audience[0]
         return AccessToken(
             token=token,
             client_id=claims.get("azp") or resource,

tests/test_auth.py

@@ -1,10 +1,10 @@
 """
 Tests for intervals_mcp_server.auth — native OAuth token verification.
 
-Covers the real security logic: a valid RS256 JWT yields an AccessToken with the
-right claims; tampered / expired / wrong-audience / wrong-key tokens yield None;
-the audience accepts both trailing-slash forms; and build_auth only enables auth
-when the environment is configured.
+Covers the real security logic: a valid JWT (RS256 or Better Auth's EdDSA) yields
+an AccessToken with the right claims; tampered / expired / wrong-issuer / wrong-key
+tokens yield None; audience is checked softly (single resource); and build_auth
+only enables auth when the environment is configured.
 """
 
 import asyncio
@@ -13,7 +13,7 @@ import types
 
 import jwt
 import pytest
-from cryptography.hazmat.primitives.asymmetric import rsa
+from cryptography.hazmat.primitives.asymmetric import ed25519, rsa
 
 from intervals_mcp_server import auth as auth_mod
 from intervals_mcp_server.auth import AuthentikTokenVerifier, _audience_variants, build_auth
@@ -76,11 +76,34 @@ def test_expired_token_rejected():
     assert asyncio.run(v.verify_token(tok)) is None
 
 
-def test_wrong_audience_rejected():
+def test_wrong_audience_tolerated_single_resource():
+    # Audience is checked softly: this is a single-resource server behind a
+    # dedicated authorization server with dynamic (DCR) client ids, so a genuine
+    # token — correct issuer + signature + not expired — is accepted even if its
+    # audience differs. The trust boundary is issuer + signature (asserted by the
+    # wrong-issuer / wrong-key tests below); a mismatch is logged, not rejected.
     priv, pub = _keypair()
-    v = _verifier(pub, ["https://someone-else"])  # verifier expects a different aud
-    tok = _token(priv, aud=RESOURCE)
-    assert asyncio.run(v.verify_token(tok)) is None
+    v = _verifier(pub, ["https://someone-else"])
+    tok = _token(priv, aud="https://some-other-resource")
+    result = asyncio.run(v.verify_token(tok))
+    assert result is not None
+    assert result.subject == "user-123"
+
+
+def test_eddsa_token_accepted():
+    # Better Auth signs access tokens with EdDSA (Ed25519); the verifier must
+    # accept them, not just RS256.
+    priv = ed25519.Ed25519PrivateKey.generate()
+    v = _verifier(priv.public_key(), [RESOURCE])
+    now = int(time.time())
+    tok = jwt.encode(
+        {"iss": ISSUER, "aud": RESOURCE, "exp": now + 3600, "iat": now, "sub": "user-ed"},
+        priv,
+        algorithm="EdDSA",
+    )
+    result = asyncio.run(v.verify_token(tok))
+    assert result is not None
+    assert result.subject == "user-ed"
 
 
 def test_wrong_issuer_rejected():