Fix JWT issuer validation to support string values per RFC 7519

Resolves issue where BearerAuthProvider rejected tokens with non-URL
issuer claims. Works around the underlying SDK's URL validation while
maintaining RFC 7519 compliance for JWT processing.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/fastmcp/server/auth/providers/bearer.py

@@ -17,7 +17,7 @@ from mcp.shared.auth import (
     OAuthClientInformationFull,
     OAuthToken,
 )
-from pydantic import SecretStr
+from pydantic import AnyHttpUrl, SecretStr
 
 from fastmcp.server.auth.auth import (
     ClientRegistrationOptions,
@@ -179,8 +179,16 @@ class BearerAuthProvider(OAuthProvider):
         if public_key and jwks_uri:
             raise ValueError("Provide either public_key or jwks_uri, not both")
 
+        # Only pass issuer to parent if it's a valid URL, otherwise use default
+        # This allows the issuer claim validation to work with string issuers per RFC 7519
+        try:
+            issuer_url = AnyHttpUrl(issuer) if issuer else "https://fastmcp.example.com"
+        except Exception:
+            # Issuer is not a valid URL, use default for parent class
+            issuer_url = "https://fastmcp.example.com"
+
         super().__init__(
-            issuer_url=issuer or "https://fastmcp.example.com",
+            issuer_url=issuer_url,
             client_registration_options=ClientRegistrationOptions(enabled=False),
             revocation_options=RevocationOptions(enabled=False),
             required_scopes=required_scopes,

tests/auth/providers/test_bearer.py

@@ -539,6 +539,59 @@ class TestBearerToken:
         assert access_token is not None
         assert access_token.client_id == "app456"  # Should prefer client_id over sub
 
+    async def test_string_issuer_validation(self, rsa_key_pair: RSAKeyPair):
+        """Test that string (non-URL) issuers are supported per RFC 7519."""
+        # Create provider with string issuer
+        provider = BearerAuthProvider(
+            public_key=rsa_key_pair.public_key,
+            issuer="my-service",  # String issuer, not a URL
+        )
+
+        # Create token with matching string issuer
+        token = rsa_key_pair.create_token(
+            subject="test-user",
+            issuer="my-service",  # Same string issuer
+        )
+
+        access_token = await provider.load_access_token(token)
+        assert access_token is not None
+        assert access_token.client_id == "test-user"
+
+    async def test_string_issuer_mismatch_rejection(self, rsa_key_pair: RSAKeyPair):
+        """Test that mismatched string issuers are rejected."""
+        # Create provider with one string issuer
+        provider = BearerAuthProvider(
+            public_key=rsa_key_pair.public_key,
+            issuer="my-service",
+        )
+
+        # Create token with different string issuer
+        token = rsa_key_pair.create_token(
+            subject="test-user",
+            issuer="other-service",  # Different string issuer
+        )
+
+        access_token = await provider.load_access_token(token)
+        assert access_token is None
+
+    async def test_url_issuer_still_works(self, rsa_key_pair: RSAKeyPair):
+        """Test that URL issuers still work after the fix."""
+        # Create provider with URL issuer
+        provider = BearerAuthProvider(
+            public_key=rsa_key_pair.public_key,
+            issuer="https://my-auth-server.com",  # URL issuer
+        )
+
+        # Create token with matching URL issuer
+        token = rsa_key_pair.create_token(
+            subject="test-user",
+            issuer="https://my-auth-server.com",  # Same URL issuer
+        )
+
+        access_token = await provider.load_access_token(token)
+        assert access_token is not None
+        assert access_token.client_id == "test-user"
+
 
 class TestFastMCPBearerAuth:
     def test_bearer_auth(self):