fix(schema): remove Gemini-incompatible tool metadata fields

Sanitize tool schemas by stripping prefill, enumTitles, $id, and patternProperties to prevent Gemini INVALID_ARGUMENT 400 errors, and add unit and executor-level tests to lock in the behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/util/gemini_schema.go

@@ -428,8 +428,9 @@ func flattenTypeArrays(jsonStr string) string {
 
 func removeUnsupportedKeywords(jsonStr string) string {
 	keywords := append(unsupportedConstraints,
-		"$schema", "$defs", "definitions", "const", "$ref", "additionalProperties",
-		"propertyNames", // Gemini doesn't support property name validation
+		"$schema", "$defs", "definitions", "const", "$ref", "$id", "additionalProperties",
+		"propertyNames", "patternProperties", // Gemini doesn't support these schema keywords
+		"enumTitles", "prefill", // Claude/OpenCode schema metadata fields unsupported by Gemini
 	)
 
 	deletePaths := make([]string, 0)

internal/util/gemini_schema_test.go

@@ -870,6 +870,57 @@ func TestCleanJSONSchemaForAntigravity_BooleanEnumToString(t *testing.T) {
 	}
 }
 
+func TestCleanJSONSchemaForGemini_RemovesGeminiUnsupportedMetadataFields(t *testing.T) {
+	input := `{
+		"$schema": "http://json-schema.org/draft-07/schema#",
+		"$id": "root-schema",
+		"type": "object",
+		"properties": {
+			"payload": {
+				"type": "object",
+				"prefill": "hello",
+				"properties": {
+					"mode": {
+						"type": "string",
+						"enum": ["a", "b"],
+						"enumTitles": ["A", "B"]
+					}
+				},
+				"patternProperties": {
+					"^x-": {"type": "string"}
+				}
+			},
+			"$id": {
+				"type": "string",
+				"description": "property name should not be removed"
+			}
+		}
+	}`
+
+	expected := `{
+		"type": "object",
+		"properties": {
+			"payload": {
+				"type": "object",
+				"properties": {
+					"mode": {
+						"type": "string",
+						"enum": ["a", "b"],
+						"description": "Allowed: a, b"
+					}
+				}
+			},
+			"$id": {
+				"type": "string",
+				"description": "property name should not be removed"
+			}
+		}
+	}`
+
+	result := CleanJSONSchemaForGemini(input)
+	compareJSON(t, expected, result)
+}
+
 func TestRemoveExtensionFields(t *testing.T) {
 	tests := []struct {
 		name     string