Dart: structured output can fail with unterminated JSON string

[jeffdh5]

Summary

When using Dart Genkit structured output (outputSchema) with Gemini, generation can fail with:

FormatException: Failed to parse extracted JSON: FormatException: Unterminated string

I can reproduce this with schema-constrained output (recipe object) where one or more string fields are long. I think it's because there are multiple chunks and somethings there is no marker in the middle chunks.

Minimal repro

import 'package:genkit/genkit.dart';
import 'package:genkit_google_genai/genkit_google_genai.dart';
import 'package:schemantic/schemantic.dart';

part 'main.g.dart';

@Schema()
abstract class $Recipe {
  String get title;
  List<String> get ingredients;
  List<String> get steps;
  int get prepTimeMinutes;
}

void main() async {
  final ai = Genkit(plugins: [googleAI()]);

  await ai.generate(
    model: googleAI.gemini('gemini-2.5-flash'),
    prompt:
        'Generate a recipe for chocolate chip cookies with detailed ingredient strings.',
    outputSchema: Recipe.$schema,
  );
}

Observed

Fails with an unterminated JSON parse error, e.g.:

GenkitException: Google AI Error: FormatException: Failed to parse extracted JSON:
FormatException: Unterminated string (at character 167)

Suspected area

extractJson() in Dart core uses lastIndexOf('}') to identify JSON end during parsing of accumulated streamed text. That can incorrectly slice partial JSON and lead to unterminated string errors.

File reference from local pub cache:

• genkit/lib/src/extract.dart