diff --git a/src/mcp/shared/auth.py b/src/mcp/shared/auth.py
index 2bbf7a715a..5ce5faa7b2 100644
--- a/src/mcp/shared/auth.py
+++ b/src/mcp/shared/auth.py
@@ -109,7 +109,9 @@ def validate_scope(self, requested_scope: str | None) -> list[str] | None:
         if requested_scope is None:
             return None
         requested_scopes = requested_scope.split(" ")
-        allowed_scopes = [] if self.scope is None else self.scope.split(" ")
+        if self.scope is None:
+            return requested_scopes
+        allowed_scopes = self.scope.split(" ")
         for scope in requested_scopes:
             if scope not in allowed_scopes:
                 raise InvalidScopeError(f"Client was not registered with scope {scope}")
diff --git a/tests/shared/test_auth.py b/tests/shared/test_auth.py
index 7463bc5a8a..f40459366f 100644
--- a/tests/shared/test_auth.py
+++ b/tests/shared/test_auth.py
@@ -3,7 +3,7 @@
 import pytest
 from pydantic import ValidationError
 
-from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata, OAuthMetadata
+from mcp.shared.auth import InvalidScopeError, OAuthClientInformationFull, OAuthClientMetadata, OAuthMetadata
 
 
 def test_oauth():
@@ -138,3 +138,43 @@ def test_invalid_non_empty_url_still_rejected():
     }
     with pytest.raises(ValidationError):
         OAuthClientMetadata.model_validate(data)
+
+
+def test_validate_scope_returns_none_when_no_scope_requested():
+    client = OAuthClientInformationFull(
+        client_id="client",
+        redirect_uris=["https://example.com/callback"],
+        scope="read write",
+    )
+
+    assert client.validate_scope(None) is None
+
+
+def test_validate_scope_allows_registered_scopes():
+    client = OAuthClientInformationFull(
+        client_id="client",
+        redirect_uris=["https://example.com/callback"],
+        scope="read write",
+    )
+
+    assert client.validate_scope("read write") == ["read", "write"]
+
+
+def test_validate_scope_rejects_unregistered_scopes():
+    client = OAuthClientInformationFull(
+        client_id="client",
+        redirect_uris=["https://example.com/callback"],
+        scope="read",
+    )
+
+    with pytest.raises(InvalidScopeError, match="Client was not registered with scope write"):
+        client.validate_scope("write")
+
+
+def test_validate_scope_allows_requested_scopes_when_registered_scope_omitted():
+    client = OAuthClientInformationFull(
+        client_id="client",
+        redirect_uris=["https://example.com/callback"],
+    )
+
+    assert client.validate_scope("read write") == ["read", "write"]