diff --git a/backend/internal/service/gemini_messages_compat_service.go b/backend/internal/service/gemini_messages_compat_service.go
index d56df59e4d..a50a7fc80c 100644
--- a/backend/internal/service/gemini_messages_compat_service.go
+++ b/backend/internal/service/gemini_messages_compat_service.go
@@ -3419,6 +3419,7 @@ func cleanToolSchema(schema any) any {
 		for key, value := range v {
 			// 跳过不支持的字段
 			if key == "$schema" || key == "$id" || key == "$ref" ||
+				key == "$defs" || key == "definitions" ||
 				key == "additionalProperties" || key == "patternProperties" || key == "minLength" ||
 				key == "maxLength" || key == "minItems" || key == "maxItems" {
 				continue
@@ -3429,6 +3430,17 @@ func cleanToolSchema(schema any) any {
 		// 规范化 type 字段为大写
 		if typeVal, ok := cleaned["type"].(string); ok {
 			cleaned["type"] = strings.ToUpper(typeVal)
+		} else if typeValues, ok := cleaned["type"].([]any); ok {
+			for _, typeValue := range typeValues {
+				typeName, ok := typeValue.(string)
+				if ok && !strings.EqualFold(typeName, "null") {
+					cleaned["type"] = strings.ToUpper(typeName)
+					break
+				}
+			}
+			if _, ok := cleaned["type"].([]any); ok {
+				delete(cleaned, "type")
+			}
 		}
 		return cleaned
 	case []any:
diff --git a/backend/internal/service/gemini_messages_compat_service_test.go b/backend/internal/service/gemini_messages_compat_service_test.go
index 79db633aae..b2fc1c110b 100644
--- a/backend/internal/service/gemini_messages_compat_service_test.go
+++ b/backend/internal/service/gemini_messages_compat_service_test.go
@@ -293,6 +293,50 @@ func TestConvertClaudeToolsToGeminiTools_CustomType(t *testing.T) {
 	}
 }
 
+func TestCleanToolSchema_NormalizesGeminiUnsupportedSchemaFields(t *testing.T) {
+	schema := map[string]any{
+		"type": "object",
+		"$defs": map[string]any{
+			"unused": map[string]any{"type": "string"},
+		},
+		"definitions": map[string]any{
+			"legacy": map[string]any{"type": "number"},
+		},
+		"properties": map[string]any{
+			"path": map[string]any{
+				"type": []any{"string", "null"},
+			},
+			"count": map[string]any{
+				"type": []any{"null", "integer"},
+			},
+			"empty": map[string]any{
+				"type": []any{"null"},
+			},
+		},
+	}
+
+	cleaned, ok := cleanToolSchema(schema).(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, "OBJECT", cleaned["type"])
+	require.NotContains(t, cleaned, "$defs")
+	require.NotContains(t, cleaned, "definitions")
+
+	properties, ok := cleaned["properties"].(map[string]any)
+	require.True(t, ok)
+
+	pathSchema, ok := properties["path"].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, "STRING", pathSchema["type"])
+
+	countSchema, ok := properties["count"].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, "INTEGER", countSchema["type"])
+
+	emptySchema, ok := properties["empty"].(map[string]any)
+	require.True(t, ok)
+	require.NotContains(t, emptySchema, "type")
+}
+
 func TestConvertClaudeToolsToGeminiTools_PreservesWebSearchAlongsideFunctions(t *testing.T) {
 	tools := []any{
 		map[string]any{