diff --git a/backend/internal/pkg/xai/oauth.go b/backend/internal/pkg/xai/oauth.go
index ea837fd78a..3d32bb963d 100644
--- a/backend/internal/pkg/xai/oauth.go
+++ b/backend/internal/pkg/xai/oauth.go
@@ -306,7 +306,7 @@ func normalizeKnownBaseURLPath(raw string) (string, error) {
 	if parsed.User != nil {
 		return "", errors.New("base URL must not include userinfo")
 	}
-	if parsed.RawQuery != "" {
+	if parsed.ForceQuery || parsed.RawQuery != "" {
 		return "", errors.New("base URL must not include a query")
 	}
 	if parsed.Fragment != "" {
diff --git a/backend/internal/pkg/xai/oauth_test.go b/backend/internal/pkg/xai/oauth_test.go
index a82a26e299..f91b5d4937 100644
--- a/backend/internal/pkg/xai/oauth_test.go
+++ b/backend/internal/pkg/xai/oauth_test.go
@@ -166,6 +166,14 @@ func TestValidateBaseURLAllowsPublicThirdPartyGrokAPI(t *testing.T) {
 	require.Error(t, err)
 }
 
+func TestValidateBaseURLsRejectEmptyQueryDelimiter(t *testing.T) {
+	_, err := ValidateBaseURL("https://grok.example.test/v1?")
+	require.Error(t, err)
+
+	_, err = ValidateTrustedBaseURL("https://api.x.ai/v1?")
+	require.Error(t, err)
+}
+
 func TestBuildResponsesURLWithValidatorUsesCallerPolicy(t *testing.T) {
 	validator := func(raw string) (string, error) {
 		return urlvalidator.ValidateURLFormat(raw, true)
@@ -192,6 +200,7 @@ func TestBuildResponsesURLWithValidatorRejectsBaseURLComponents(t *testing.T) {
 	}{
 		{name: "userinfo", raw: "https://user:secret@grok.example.test/v1"},
 		{name: "query", raw: "https://grok.example.test/v1?token=secret"},
+		{name: "empty query delimiter", raw: "https://grok.example.test/v1?"},
 		{name: "fragment", raw: "https://grok.example.test/v1#secret"},
 	}