feat(@built-in-ai/transformers-js): generateObject support

[jpabbuehl]

Thanks for your work. it's really works well for many use cases, except one I'm struggling with

Problem Description

When using TransformersJS models (via @built-in-ai/transformers-js or the custom HuggingFace provider), the models report that they support generateObject functionality, but attempting to use structured output generation with JSON schemas fails. This causes issues in applications that rely on checking provider.generateObject availability to determine whether to use structured or text-based generation.

Current Behavior

1. The provider/model reports generateObject is available (truthy check passes)
2. When calling generateObject with a Zod schema, the SDK warns: "JSON response format is not supported by TransformersJS"
3. The generation fails with NoObjectGeneratedError: No object generated: could not parse the response
4. The model returns plain text instead of structured JSON

Expected Behavior

Either:

• A) TransformersJS models should properly support structured output with JSON schemas, OR
• B) The provider should not expose generateObject method if it's not actually supported

Reproduction Steps

import { generateObject } from 'ai';
import { transformersJS } from '@built-in-ai/transformers-js';
import { z } from 'zod';

const model = transformersJS('HuggingFaceTB/SmolLM2-360M-Instruct', {
  device: 'cpu'
});

// This check passes, suggesting structured output is supported
if (model.generateObject) {
  console.log('✅ Model claims to support generateObject');
}

// But this fails
const result = await generateObject({
  model,
  schema: z.object({
    feedback: z.enum(['Positive', 'Neutral', 'Negative']),
  }),
  prompt: 'Label this customer review: There is something wrong with your product that...'
});
// ❌ Throws: NoObjectGeneratedError

Error Output

AI SDK Warning: The "responseFormat" setting is not supported by this model - JSON response format is not supported by TransformersJS

NoObjectGeneratedError [AI_NoObjectGeneratedError]: No object generated: could not parse the response.
  cause: JSONParseError [AI_JSONParseError]: JSON parsing failed: Text: Feedback: Ne...
  Error message: Unexpected token 'F', "Feedback: Ne"... is not valid JSON

[jakobhoeg]

Hey @jpabbuehl,
Thanks for the kind words.
This is definitely on the todo list, but hasn't been implemented yet. Right now we just throw the unsupported warning even though the underlying model supports it.
If you wish to contribute and open a PR for it, let me know. Otherwise you'll have to wait until I or someone else adds it, unfortunately.
The error is thrown from here btw.