diff --git a/internal/util/gemini_schema.go b/internal/util/gemini_schema.go
index 60453998..be514e64 100644
--- a/internal/util/gemini_schema.go
+++ b/internal/util/gemini_schema.go
@@ -431,9 +431,43 @@ func removeUnsupportedKeywords(jsonStr string) string {
 			jsonStr, _ = sjson.Delete(jsonStr, p)
 		}
 	}
+	// Remove x-* extension fields (e.g., x-google-enum-descriptions) that are not supported by Gemini API
+	jsonStr = removeExtensionFields(jsonStr)
 	return jsonStr
 }
 
+// removeExtensionFields removes all x-* extension fields from the JSON schema.
+// These are OpenAPI/JSON Schema extension fields that Google APIs don't recognize.
+func removeExtensionFields(jsonStr string) string {
+	var paths []string
+	walkForExtensions(gjson.Parse(jsonStr), "", &paths)
+	sortByDepth(paths)
+	for _, p := range paths {
+		jsonStr, _ = sjson.Delete(jsonStr, p)
+	}
+	return jsonStr
+}
+
+func walkForExtensions(value gjson.Result, path string, paths *[]string) {
+	if !value.IsObject() && !value.IsArray() {
+		return
+	}
+
+	value.ForEach(func(key, val gjson.Result) bool {
+		keyStr := key.String()
+		safeKey := escapeGJSONPathKey(keyStr)
+		childPath := joinPath(path, safeKey)
+
+		// Only remove x-* extension fields, but protect them if they are property definitions.
+		if strings.HasPrefix(keyStr, "x-") && !isPropertyDefinition(path) {
+			*paths = append(*paths, childPath)
+		}
+
+		walkForExtensions(val, childPath, paths)
+		return true
+	})
+}
+
 func cleanupRequiredFields(jsonStr string) string {
 	for _, p := range findPaths(jsonStr, "required") {
 		parentPath := trimSuffix(p, ".required")
diff --git a/internal/util/gemini_schema_test.go b/internal/util/gemini_schema_test.go
index ca77225e..ea63d111 100644
--- a/internal/util/gemini_schema_test.go
+++ b/internal/util/gemini_schema_test.go
@@ -869,3 +869,129 @@ func TestCleanJSONSchemaForAntigravity_BooleanEnumToString(t *testing.T) {
 		t.Errorf("Boolean enum values should be converted to string format, got: %s", result)
 	}
 }
+
+func TestRemoveExtensionFields(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{
+			name: "removes x- fields at root",
+			input: `{
+				"type": "object",
+				"x-custom-meta": "value",
+				"properties": {
+					"foo": { "type": "string" }
+				}
+			}`,
+			expected: `{
+				"type": "object",
+				"properties": {
+					"foo": { "type": "string" }
+				}
+			}`,
+		},
+		{
+			name: "removes x- fields in nested properties",
+			input: `{
+				"type": "object",
+				"properties": {
+					"foo": {
+						"type": "string",
+						"x-internal-id": 123
+					}
+				}
+			}`,
+			expected: `{
+				"type": "object",
+				"properties": {
+					"foo": {
+						"type": "string"
+					}
+				}
+			}`,
+		},
+		{
+			name: "does NOT remove properties named x-",
+			input: `{
+				"type": "object",
+				"properties": {
+					"x-data": { "type": "string" },
+					"normal": { "type": "number", "x-meta": "remove" }
+				},
+				"required": ["x-data"]
+			}`,
+			expected: `{
+				"type": "object",
+				"properties": {
+					"x-data": { "type": "string" },
+					"normal": { "type": "number" }
+				},
+				"required": ["x-data"]
+			}`,
+		},
+		{
+			name: "does NOT remove $schema and other meta fields (as requested)",
+			input: `{
+				"$schema": "http://json-schema.org/draft-07/schema#",
+				"$id": "test",
+				"type": "object",
+				"properties": {
+					"foo": { "type": "string" }
+				}
+			}`,
+			expected: `{
+				"$schema": "http://json-schema.org/draft-07/schema#",
+				"$id": "test",
+				"type": "object",
+				"properties": {
+					"foo": { "type": "string" }
+				}
+			}`,
+		},
+		{
+			name: "handles properties named $schema",
+			input: `{
+				"type": "object",
+				"properties": {
+					"$schema": { "type": "string" }
+				}
+			}`,
+			expected: `{
+				"type": "object",
+				"properties": {
+					"$schema": { "type": "string" }
+				}
+			}`,
+		},
+		{
+			name: "handles escaping in paths",
+			input: `{
+				"type": "object",
+				"properties": {
+					"foo.bar": {
+						"type": "string",
+						"x-meta": "remove"
+					}
+				},
+				"x-root.meta": "remove"
+			}`,
+			expected: `{
+				"type": "object",
+				"properties": {
+					"foo.bar": {
+						"type": "string"
+					}
+				}
+			}`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			actual := removeExtensionFields(tt.input)
+			compareJSON(t, tt.expected, actual)
+		})
+	}
+}