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.
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.
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
Callback Method
HTTP Method : POST
Content Type : application/json
Timeout Setting : 15 seconds
When the task is completed, the system will send a POST request to your callBackUrl in the following format:
Success Callback
Content Policy Violation Callback
Generation Failure Callback
Internal Error Callback
{
"code" : 200 ,
"msg" : "BFL image generated successfully." ,
"data" : {
"taskId" : "task12345" ,
"info" : {
"originImageUrl" : "https://example.com/original.jpg" ,
"resultImageUrl" : "https://example.com/result.jpg"
}
}
}
Status Code Description
Callback status code indicating task processing result: Status Code Description 200 Success - Image generation completed successfully 400 Failed - Your prompt was flagged by Website as violating content policies 500 Failed - Internal Error, Please try again later 501 Failed - Image generation task failed
Status message providing detailed status description
Task ID, consistent with the taskId returned when you submitted the task
Original image URL, valid for 10 minutes . Only present on success.
Generated image URL on our server. Only present on success.
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' );
});
from flask import Flask, request, jsonify
import requests
import os
app = Flask( __name__ )
@app.route ( '/flux-image-callback' , methods = [ 'POST' ])
def handle_callback ():
data = request.json
code = data.get( 'code' )
msg = data.get( 'msg' )
callback_data = data.get( 'data' , {})
task_id = callback_data.get( 'taskId' )
info = callback_data.get( 'info' , {})
origin_image_url = info.get( 'originImageUrl' )
result_image_url = info.get( 'resultImageUrl' )
print ( f "Received Flux image generation callback:" )
print ( f "Task ID: { task_id } " )
print ( f "Status: { code } , Message: { msg } " )
if code == 200 :
# Task completed successfully
print ( "Flux image generation completed successfully" )
print ( f "Original Image URL: { origin_image_url } " )
print ( f "Generated Image URL: { result_image_url } " )
print ( "Note: Original image URL is valid for 10 minutes" )
# Download generated image
if result_image_url:
try :
result_filename = f "flux_result_ { task_id } .jpg"
download_file(result_image_url, result_filename)
print ( f "Generated image downloaded as { result_filename } " )
except Exception as e:
print ( f "Generated image download failed: { e } " )
# Download original image (if needed)
if origin_image_url:
try :
origin_filename = f "flux_original_ { task_id } .jpg"
download_file(origin_image_url, origin_filename)
print ( f "Original image downloaded as { origin_filename } " )
except Exception as e:
print ( f "Original image download failed: { e } " )
else :
# Task failed
print ( f "Flux image generation failed: { msg } " )
# Handle specific error types
if code == 400 :
print ( "Content policy violation - please adjust prompt" )
elif code == 500 :
print ( "Internal error - please try again later" )
elif code == 501 :
print ( "Generation task failed - may need to adjust parameters" )
# Return 200 status code to confirm callback received
return jsonify({ 'status' : 'received' }), 200
def download_file ( url , filename ):
"""Download file from URL and save locally"""
response = requests.get(url, stream = True )
response.raise_for_status()
os.makedirs( 'downloads' , exist_ok = True )
filepath = os.path.join( 'downloads' , filename)
with open (filepath, 'wb' ) as f:
for chunk in response.iter_content( chunk_size = 8192 ):
f.write(chunk)
if __name__ == '__main__' :
app.run( host = '0.0.0.0' , port = 3000 )
<? php
header ( 'Content-Type: application/json' );
// Get POST data
$input = file_get_contents ( 'php://input' );
$data = json_decode ( $input , true );
$code = $data [ 'code' ] ?? null ;
$msg = $data [ 'msg' ] ?? '' ;
$callbackData = $data [ 'data' ] ?? [];
$taskId = $callbackData [ 'taskId' ] ?? '' ;
$info = $callbackData [ 'info' ] ?? [];
$originImageUrl = $info [ 'originImageUrl' ] ?? '' ;
$resultImageUrl = $info [ 'resultImageUrl' ] ?? '' ;
error_log ( "Received Flux image generation callback:" );
error_log ( "Task ID: $taskId " );
error_log ( "Status: $code , Message: $msg " );
if ( $code === 200 ) {
// Task completed successfully
error_log ( "Flux image generation completed successfully" );
error_log ( "Original Image URL: $originImageUrl " );
error_log ( "Generated Image URL: $resultImageUrl " );
error_log ( "Note: Original image URL is valid for 10 minutes" );
// Download generated image
if ( ! empty ( $resultImageUrl )) {
try {
$resultFilename = "flux_result_{ $taskId }.jpg" ;
downloadFile ( $resultImageUrl , $resultFilename );
error_log ( "Generated image downloaded as $resultFilename " );
} catch ( Exception $e ) {
error_log ( "Generated image download failed: " . $e -> getMessage ());
}
}
// Download original image (if needed)
if ( ! empty ( $originImageUrl )) {
try {
$originFilename = "flux_original_{ $taskId }.jpg" ;
downloadFile ( $originImageUrl , $originFilename );
error_log ( "Original image downloaded as $originFilename " );
} catch ( Exception $e ) {
error_log ( "Original image download failed: " . $e -> getMessage ());
}
}
} else {
// Task failed
error_log ( "Flux image generation failed: $msg " );
// Handle specific error types
if ( $code === 400 ) {
error_log ( "Content policy violation - please adjust prompt" );
} elseif ( $code === 500 ) {
error_log ( "Internal error - please try again later" );
} elseif ( $code === 501 ) {
error_log ( "Generation task failed - may need to adjust parameters" );
}
}
// Return 200 status code to confirm callback received
http_response_code ( 200 );
echo json_encode ([ 'status' => 'received' ]);
function downloadFile ( $url , $filename ) {
$downloadDir = 'downloads' ;
if ( ! is_dir ( $downloadDir )) {
mkdir ( $downloadDir , 0755 , true );
}
$filepath = $downloadDir . '/' . $filename ;
$fileContent = file_get_contents ( $url );
if ( $fileContent === false ) {
throw new Exception ( "Failed to download file from URL" );
}
$result = file_put_contents ( $filepath , $fileContent );
if ( $result === false ) {
throw new Exception ( "Failed to save file locally" );
}
}
?>
Best Practices
Callback URL Configuration Recommendations
Use HTTPS : Ensure your callback URL uses HTTPS protocol for secure data transmission
Verify Source : Verify the legitimacy of the request source in callback processing
Idempotent Processing : The same taskId may receive multiple callbacks, ensure processing logic is idempotent
Quick Response : Callback processing should return a 200 status code as quickly as possible to avoid timeout
Asynchronous Processing : Complex business logic should be processed asynchronously to avoid blocking callback response
Timely Download : Original image URLs are valid for only 10 minutes, download and save files promptly upon success
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
Troubleshooting
If you do not receive callback notifications, please check the following:
Network Connection Issues
Confirm that the callback URL is accessible from the public network
Check firewall settings to ensure inbound requests are not blocked
Verify that domain name resolution is correct
Ensure the server returns HTTP 200 status code within 15 seconds
Check server logs for error messages
Verify that the interface path and HTTP method are correct
Confirm that the received POST request body is in JSON format
Check that Content-Type is application/json
Verify that JSON parsing is correct
Confirm that image URLs are accessible
Check image download permissions and network connections
Verify image save paths and permissions
Note the 10-minute original image URL expiration - implement prompt download logic
Handle both generated and original image downloads
Review error messages for content policy violations
Adjust prompts that are flagged by content moderation
Ensure prompts comply with platform content guidelines
Check for sensitive or inappropriate content
Check if generation parameters are reasonable
Verify input image format and quality
Confirm prompt length and format
Consider adjusting generation parameters and retry
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.