feat(vertexai): support meta JSON object mode

Author: CorieW

packages/genkit_vertexai/lib/src/meta_model.dart

@@ -124,9 +124,7 @@ Map<String, dynamic> toMetaChatCompletionRequest(
   VertexAiMetaOptions options, {
   required bool stream,
 }) {
-  final isJsonMode =
-      req.output?.format == 'json' ||
-      req.output?.contentType == 'application/json';
+  final responseFormat = _toMetaResponseFormat(req.output);
   return {
     'model': options.version ?? modelName,
     'messages': _toMetaMessages(req.messages),
@@ -141,15 +139,7 @@ Map<String, dynamic> toMetaChatCompletionRequest(
     'frequency_penalty': ?options.frequencyPenalty,
     'seed': ?options.seed,
     'user': ?options.user,
-    if (isJsonMode && req.output?.schema != null)
-      'response_format': {
-        'type': 'json_schema',
-        'json_schema': {
-          'name': 'output',
-          'schema': {...req.output!.schema!, 'additionalProperties': false},
-          'strict': true,
-        },
-      },
+    if (responseFormat != null) 'response_format': responseFormat,
     if (options.llamaGuard != null)
       'extra_body': {
         'google': {
@@ -162,6 +152,26 @@ Map<String, dynamic> toMetaChatCompletionRequest(
   };
 }
 
+Map<String, dynamic>? _toMetaResponseFormat(OutputConfig? output) {
+  final isJsonMode =
+      output?.format == 'json' || output?.contentType == 'application/json';
+  if (!isJsonMode) return null;
+
+  final schema = output?.schema;
+  if (schema == null) {
+    return {'type': 'json_object'};
+  }
+
+  return {
+    'type': 'json_schema',
+    'json_schema': {
+      'name': 'output',
+      'schema': {...schema, 'additionalProperties': false},
+      'strict': true,
+    },
+  };
+}
+
 ModelResponse fromMetaChatCompletionResponse(Map<String, dynamic> response) {
   final choices = response['choices'] as List?;
   if (choices == null || choices.isEmpty) {

packages/genkit_vertexai/test/meta_test.dart

@@ -124,6 +124,27 @@ void main() {
       );
     });
 
+    test('uses JSON object response format without schema', () {
+      final request = ModelRequest(
+        messages: [
+          Message(
+            role: Role.user,
+            content: [TextPart(text: 'return json')],
+          ),
+        ],
+        output: OutputConfig(format: 'json'),
+      );
+
+      final body = toMetaChatCompletionRequest(
+        request,
+        'meta/llama-4-maverick-17b-128e-instruct-maas',
+        VertexAiMetaOptions(),
+        stream: false,
+      );
+
+      expect(body['response_format'], {'type': 'json_object'});
+    });
+
     test('aggregates streamed tool call chunks', () {
       final response = fromMetaChatCompletionChunks([
         {