Description

@ai-sdk/replicate image generation intermittently throws AI_APICallError: Invalid JSON response (cause: AI_TypeValidationError) when Replicate returns a prediction that is still in progress.

This happens with model black-forest-labs/flux-dev during cold starts or when inference exceeds the sync-wait window.

Source (where the bug is)

packages/replicate/src/replicate-image-model.ts (v2.0.36).

The POST response is destructured directly into { output } with no status check (L140-L143):

const {
  value: { output },
  responseHeaders,
} = await postJsonToApi({

…and validated against a schema that requires output to be present (L206-L208):

const replicateImageResponseSchema = z.object({
  output: z.union([z.array(z.string()), z.string()]),
});

There is no polling. The video model in the same package already handles this correctly (see Suggested fix).

Expected behavior

When Replicate's sync API (Prefer: wait) times out before the prediction completes, Replicate returns HTTP 201 with a valid prediction object (status: "starting" or "processing", output: null, urls.get set). Per Replicate's docs, the client should poll urls.get until status is succeeded, then use output.

The video model in the same package already implements this (replicate-video-model.ts: keeps full prediction, polls while starting/processing, uses replicatePredictionSchema with output.nullish()).

The image model should behave the same way.

Actual behavior

replicate-image-model.ts posts with Prefer: wait, then immediately validates the response and throws. No polling occurs.

Note the response is actually HTTP 201 (success) — so this is not an HTTP/transport error, it is the success-path JSON schema rejecting output: null. The error message Invalid JSON response is misleading; the JSON is valid, the schema is just too narrow.

Top-level error thrown (what calling code catches):

AI_APICallError: Invalid JSON response
  name:       AI_APICallError
  statusCode: 201
  url:        https://api.replicate.com/v1/models/black-forest-labs/flux-dev/predictions
  cause:      AI_TypeValidationError

Underlying cause (AI_TypeValidationError):

Type validation failed: Value: { ... "output": null, ... "status": "starting" ... }
Error message: [
  {
    "code": "invalid_union",
    "errors": [
      [ { "expected": "array",  "code": "invalid_type", "path": [], "message": "Invalid input: expected array, received null" } ],
      [ { "expected": "string", "code": "invalid_type", "path": [], "message": "Invalid input: expected string, received null" } ]
    ],
    "path": ["output"],
    "message": "Invalid input"
  }
]

Reproduction

import { replicate } from '@ai-sdk/replicate';
import { generateImage } from 'ai';

await generateImage({
  model: replicate.image('black-forest-labs/flux-dev'),
  prompt: 'A watercolor landscape at twilight',
  size: '1024x1024',
});

Intermittent — more likely on cold starts / slow inference. When it fails, the response body (HTTP 201) looks like:

{
  "id": "...",
  "model": "black-forest-labs/flux-dev",
  "status": "starting",
  "output": null,
  "error": null,
  "urls": {
    "get": "https://api.replicate.com/v1/predictions/..."
  }
}

Suggested fix

Mirror the video model pattern in replicate-image-model.ts:

1. Widen replicateImageResponseSchema to accept output.nullish(), plus status, error, and urls.get
2. Keep the full prediction from the POST response (not { output })
3. Poll urls.get while status is starting or processing
4. Use finalPrediction.output once succeeded

Reference implementation: packages/replicate/src/replicate-video-model.ts (the while (status === 'starting' || status === 'processing') poll loop in doGenerate).

Related

• Acknowledged gap in Make async APIs a first-class API in AI SDK #12381 (Replicate image gen may fall back to polling, but the SDK only implements polling for video today)
• Still present on latest published @ai-sdk/replicate@2.0.36 and on main as of June 2026

AI SDK Version

• ai: 6.0.168
• @ai-sdk/replicate: 2.0.29 (also verified unfixed in 2.0.36)

Code of Conduct

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