diff --git a/mcpauth/config.py b/mcpauth/config.py
index 7c726da..a49bc1a 100644
--- a/mcpauth/config.py
+++ b/mcpauth/config.py
@@ -117,6 +117,11 @@ class AuthServerType(str, Enum):
     OIDC = "oidc"
 
 
+class AuthorizationServerMetadataDefaults(Enum):
+    grant_types_supported = ["authorization_code", "implicit"]
+    response_modes_supported = ["query", "fragment"]
+
+
 class AuthServerConfig(BaseModel):
     """
     Configuration for the remote authorization server integrated with the MCP server.
diff --git a/mcpauth/utils/_validate_server_config.py b/mcpauth/utils/_validate_server_config.py
index f0f7f57..ce742a4 100644
--- a/mcpauth/utils/_validate_server_config.py
+++ b/mcpauth/utils/_validate_server_config.py
@@ -1,7 +1,7 @@
 from enum import Enum
 from typing import Any, Dict, List, Optional
 from pydantic import BaseModel
-from ..config import AuthServerConfig
+from ..config import AuthServerConfig, AuthorizationServerMetadataDefaults
 
 
 class AuthServerConfigErrorCode(str, Enum):
@@ -97,6 +97,7 @@ def validate_server_config(
       invalid (`{ is_valid: False }`), along with any errors or warnings encountered during validation.
     """
 
+    MetadataDefaults = AuthorizationServerMetadataDefaults
     errors: List[AuthServerConfigError] = []
     warnings: List[AuthServerConfigWarning] = []
     metadata = config.metadata
@@ -112,9 +113,10 @@ def validate_server_config(
         )
 
     # Check if 'authorization_code' grant type is supported
-    if (
-        not metadata.grant_types_supported
-        or "authorization_code" not in metadata.grant_types_supported
+    if "authorization_code" not in (
+        metadata.grant_types_supported
+        if metadata.grant_types_supported is not None
+        else MetadataDefaults.grant_types_supported.value
     ):
         errors.append(
             _create_error(
diff --git a/tests/utils/validate_server_config_test.py b/tests/utils/validate_server_config_test.py
index 4631152..b39cc5b 100644
--- a/tests/utils/validate_server_config_test.py
+++ b/tests/utils/validate_server_config_test.py
@@ -26,6 +26,24 @@ def test_valid_server_config(self):
         assert not hasattr(result, "errors") or len(result.errors) == 0
         assert result.warnings == []
 
+    def test_valid_server_config_no_grant_types(self):
+        config = AuthServerConfig(
+            type=AuthServerType.OAUTH,
+            metadata=AuthorizationServerMetadata(
+                issuer="https://example.com",
+                authorization_endpoint="https://example.com/oauth/authorize",
+                token_endpoint="https://example.com/oauth/token",
+                response_types_supported=["code"],
+                code_challenge_methods_supported=["S256"],
+                registration_endpoint="https://example.com/register",
+            ),
+        )
+
+        result = validate_server_config(config)
+        assert result.is_valid is True
+        assert not hasattr(result, "errors") or len(result.errors) == 0
+        assert result.warnings == []
+
     def test_invalid_server_config(self):
         config = AuthServerConfig(
             type=AuthServerType.OAUTH,
@@ -42,10 +60,6 @@ def test_invalid_server_config(self):
 
         error_codes = [error.code for error in result.errors]
         assert AuthServerConfigErrorCode.CODE_RESPONSE_TYPE_NOT_SUPPORTED in error_codes
-        assert (
-            AuthServerConfigErrorCode.AUTHORIZATION_CODE_GRANT_NOT_SUPPORTED
-            in error_codes
-        )
         assert AuthServerConfigErrorCode.PKCE_NOT_SUPPORTED in error_codes
 
         warning_codes = [warning.code for warning in result.warnings]
@@ -78,7 +92,7 @@ def test_warning_for_missing_dynamic_registration(self):
         )
         assert len(result.warnings) == 1
 
-    def test_code_challenge_methods(self):
+    def test_invalid_code_challenge_methods(self):
         config = AuthServerConfig(
             type=AuthServerType.OAUTH,
             metadata=AuthorizationServerMetadata(
@@ -99,3 +113,25 @@ def test_code_challenge_methods(self):
             AuthServerConfigErrorCode.S256_CODE_CHALLENGE_METHOD_NOT_SUPPORTED
             in error_codes
         )
+
+    def test_invalid_grant_type(self):
+        config = AuthServerConfig(
+            type=AuthServerType.OAUTH,
+            metadata=AuthorizationServerMetadata(
+                issuer="https://example.com",
+                authorization_endpoint="https://example.com/oauth/authorize",
+                token_endpoint="https://example.com/oauth/token",
+                response_types_supported=["code"],
+                grant_types_supported=[],  # Use empty list on purpose to ensure it should be treated correctly
+                code_challenge_methods_supported=["S256"],
+            ),
+        )
+
+        result = validate_server_config(config)
+        assert result.is_valid is False
+
+        error_codes = [error.code for error in result.errors]
+        assert (
+            AuthServerConfigErrorCode.AUTHORIZATION_CODE_GRANT_NOT_SUPPORTED
+            in error_codes
+        )