When you submit an image generation or editing task to the Flux Kontext API, you can use the callBackUrl parameter to set a callback URL. The system will automatically push the results to your specified address when the task is completed.

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Callback Mechanism Overview

The callback mechanism eliminates the need to poll the API for task status. The system will proactively push task completion results to your server.

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Callback Timing

The system will send callback notifications in the following situations:
  • Image generation or editing task completed successfully
  • Image generation or editing task failed
  • Errors occurred during task processing

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Callback Method

  • HTTP Method: POST
  • Content Type: application/json
  • Timeout Setting: 15 seconds
  • Retry Mechanism: Retry 3 times after failure, with intervals of 1 minute, 5 minutes, and 15 minutes respectively

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Callback Request Format

When the task is completed, the system will send a POST request to your callBackUrl in the following format: 
{
  "code": 200,
  "msg": "BFL image generated successfully.",
  "data": {
    "taskId": "task12345",
    "info": {
      "originImageUrl": "https://example.com/original.jpg",
      "resultImageUrl": "https://example.com/result.jpg"
    }
  }
}

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Status Code Description

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code
integer
required
Callback status code indicating task processing result:
Status CodeDescription
200Success - Image generation completed successfully
400Failed - Your prompt was flagged by Website as violating content policies
500Failed - Internal Error, Please try again later
501Failed - Image generation task failed
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msg
string
required
Status message providing detailed status description
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data.taskId
string
required
Task ID, consistent with the taskId returned when you submitted the task
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data.info.originImageUrl
string
Original image URL, valid for 10 minutes. Only present on success.
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data.info.resultImageUrl
string
Generated image URL on our server. Only present on success.

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Callback Reception Examples

Here are example codes for receiving callbacks in popular programming languages: 
const express = require('express');
const fs = require('fs');
const https = require('https');
const app = express();

app.use(express.json());

app.post('/flux-image-callback', (req, res) => {
  const { code, msg, data } = req.body;
  
  console.log('Received Flux image generation callback:', {
    taskId: data.taskId,
    status: code,
    message: msg
  });
  
  if (code === 200) {
    // Task completed successfully
    console.log('Flux image generation completed successfully');
    
    const { taskId, info } = data;
    const { originImageUrl, resultImageUrl } = info;
    
    console.log(`Original Image URL: ${originImageUrl}`);
    console.log(`Generated Image URL: ${resultImageUrl}`);
    console.log('Note: Original image URL is valid for 10 minutes');
    
    // Download generated image
    if (resultImageUrl) {
      downloadFile(resultImageUrl, `flux_result_${taskId}.jpg`)
        .then(() => console.log('Generated image downloaded successfully'))
        .catch(err => console.error('Generated image download failed:', err));
    }
    
    // Download original image (if needed)
    if (originImageUrl) {
      downloadFile(originImageUrl, `flux_original_${taskId}.jpg`)
        .then(() => console.log('Original image downloaded successfully'))
        .catch(err => console.error('Original image download failed:', err));
    }
    
  } else {
    // Task failed
    console.log('Flux image generation failed:', msg);
    
    // Handle specific error types
    if (code === 400) {
      console.log('Content policy violation - please adjust prompt');
    } else if (code === 500) {
      console.log('Internal error - please try again later');
    } else if (code === 501) {
      console.log('Generation task failed - may need to adjust parameters');
    }
  }
  
  // Return 200 status code to confirm callback received
  res.status(200).json({ status: 'received' });
});

// Helper function to download files
function downloadFile(url, filename) {
  return new Promise((resolve, reject) => {
    const file = fs.createWriteStream(filename);
    
    https.get(url, (response) => {
      if (response.statusCode === 200) {
        response.pipe(file);
        file.on('finish', () => {
          file.close();
          resolve();
        });
      } else {
        reject(new Error(`HTTP ${response.statusCode}`));
      }
    }).on('error', reject);
  });
}

app.listen(3000, () => {
  console.log('Callback server running on port 3000');
});

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Best Practices

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Callback URL Configuration Recommendations

  1. Use HTTPS: Ensure your callback URL uses HTTPS protocol for secure data transmission
  2. Verify Source: Verify the legitimacy of the request source in callback processing
  3. Idempotent Processing: The same taskId may receive multiple callbacks, ensure processing logic is idempotent
  4. Quick Response: Callback processing should return a 200 status code as quickly as possible to avoid timeout
  5. Asynchronous Processing: Complex business logic should be processed asynchronously to avoid blocking callback response
  6. Timely Download: Original image URLs are valid for only 10 minutes, download and save files promptly upon success

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Important Reminders

  • Callback URL must be a publicly accessible address
  • Server must respond within 15 seconds, otherwise it will be considered a timeout
  • If 3 consecutive retries fail, the system will stop sending callbacks
  • Original image URLs expire after 10 minutes - download immediately upon receiving callback
  • Please ensure the stability of callback processing logic to avoid callback failures due to exceptions
  • Need to handle multiple error status codes (400, 500, 501) for complete error handling
  • Pay attention to content policy violations and adjust prompts accordingly

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Troubleshooting

If you do not receive callback notifications, please check the following:

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Alternative Solution

If you cannot use the callback mechanism, you can also use polling:

Poll Query Results

Use the get image details endpoint to regularly query task status. We recommend querying every 30 seconds.