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A webhook is an HTTP request that an image-generation provider sends to an endpoint you control when a job changes state. To automate reliably, create a public HTTPS receiver, subscribe to the provider’s completion and failure events, verify the raw request signature, acknowledge quickly, and let a worker fetch and process the image.
What an image-generation webhook does
Without a webhook, your application repeatedly polls an API to discover whether an image is ready. A webhook reverses that flow: you submit a generation job, provide a callback URL, and the provider sends an event when the job starts, produces output, completes, fails, or is canceled. Your endpoint should treat the event as a state signal, not automatically assume it contains a permanent image URL.
The dependable architecture has four parts:
- Generator: submits the prompt and stores the provider’s job or response ID.
- Receiver: accepts HTTPS POST requests, verifies authenticity, records an idempotency key, and enqueues work.
- Worker: retrieves the completed result, downloads or transforms it, and writes it to durable storage.
- Database: maps your internal request ID to the provider ID, event IDs, status, destination, and processing history.
Implementation sequence
1. Choose events and provider semantics
Decide whether you need only terminal completion, failures and cancellations, or intermediate progress. OpenAI project endpoints use event subscriptions; a background response can emit response.completed. See the OpenAI webhook guide and webhook event reference. Replicate accepts a webhook URL on each prediction and supports start, output, logs, and completed filters; its documentation says: “To receive webhook events, specify a webhook URL in the request body when creating a prediction or a training.”
Replicate can send output and logs notifications at most once every 500 ms, while requested start and completed events are sent regardless of that throttling. Select the smallest event set that meets your UX and monitoring needs.
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2. Deploy a public HTTPS endpoint
Your receiver must be reachable from the provider’s network. OpenAI endpoint creation requires an HTTPS URL. For local testing, its guide names ngrok and cloud development environments; use your production URL directly because OpenAI does not follow redirects for webhook delivery. Expose only the webhook route, protect it with signature verification, and cap request size.
3. Store the job mapping before returning
When you submit a generation request, persist your internal request ID, provider ID, expected event types, destination, and current status. Match callbacks to this stored provider ID. Do not let arbitrary fields supplied by a browser decide where a generated image is published.
4. Verify the raw request
Read the exact bytes or raw text first. Do not parse and re-serialize JSON before verification: whitespace and escaping changes can invalidate a signature. Keep signing secrets in server-side secret storage.
OpenAI provides SDK webhook helpers and recommends signature verification when events trigger backend actions. Replicate sends webhook-id, webhook-timestamp, and webhook-signature. Its signed content combines the ID, timestamp, and raw body; verification uses HMAC-SHA256 with the base64 portion of the signing key. Apply a timestamp tolerance, compare signatures in constant time, and reject stale or malformed requests. Rotate a signing secret if it is exposed.
5. Acknowledge, then queue
After authentication and durable enqueueing, return a successful 2xx immediately. Do not download images, call a second API, or perform transformations inside the webhook request. OpenAI documents retries for unsuccessful or slow deliveries for up to 72 hours with exponential backoff; 3xx responses count as failures. Duplicate deliveries are possible, so use the provider event ID (Replicate’s webhook-id, or the corresponding OpenAI event identifier) as an idempotency key. Insert that key before any irreversible side effect.
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6. Fetch and process the output
Have the worker retrieve the result using the stored provider ID. OpenAI’s completion example receives a response ID and then retrieves the response. Replicate events describe prediction state and output. Do not assume every event carries a durable image URL, or that a URL remains available indefinitely; follow the selected provider’s output-retention rules and copy files to storage you control when necessary.
Minimal Node.js receiver
This Express example preserves the raw body, validates a shared HMAC signature, deduplicates event IDs, and queues work. Replace the verification routine with the provider’s official SDK or exact algorithm; the generic secret below is not a substitute for provider-specific verification.
import express from "express";
import crypto from "node:crypto";
const app = express();
const seen = new Set();
app.post("/webhooks/images", express.raw({type: "application/json", limit: "1mb"}), (req, res) => {
const id = req.get("webhook-id");
const timestamp = req.get("webhook-timestamp");
const signature = req.get("webhook-signature");
if (!id || !timestamp || !signature) return res.sendStatus(400);
const age = Math.abs(Date.now()/1000 - Number(timestamp));
if (!Number.isFinite(age) || age > 300) return res.sendStatus(400);
const signed = `${id}.${timestamp}.${req.body.toString("utf8")}`;
const expected = crypto.createHmac("sha256", process.env.WEBHOOK_SECRET)
.update(signed).digest("base64");
const a = Buffer.from(signature.replace(/^v1,/, ""));
const b = Buffer.from(expected);
if (a.length !== b.length || !crypto.timingSafeEqual(a,b)) return res.sendStatus(401);
if (seen.has(id)) return res.sendStatus(200);
seen.add(id); // use a durable database with a unique constraint in production
const event = JSON.parse(req.body.toString("utf8"));
// enqueue({id, event}); return before downloading or transforming the image
res.sendStatus(200);
});
app.listen(process.env.PORT || 3000);For OpenAI, use its documented webhook helper rather than adapting the Replicate header logic. Preserve the raw Express body as shown, validate the event type, and retrieve the response by ID in your worker.
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OpenAI
Create a project webhook endpoint, select event subscriptions, and retain the signing secret returned on creation or rotation. Submit image work as a background operation when supported, then handle response.completed. The receiver should retrieve the response after acknowledging the event. Test events are available in dashboard settings.
Replicate
Include webhook and, when needed, a comma-separated event filter in the prediction request. Verify the three webhook headers, enforce timestamp tolerance, and handle terminal success, failure, and cancellation. Intermediate output and log callbacks can be frequent; enqueue them rather than doing synchronous processing.
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Stability AI
The official Stability AI API reference documents image-generation endpoints and API-key authentication, but it does not establish an equivalent native webhook workflow for those endpoints. Confirm current capabilities before designing around callbacks; polling or an orchestration layer may be required.
Python and cURL job examples
Provider request schemas vary, so use the provider’s current endpoint and model fields. The following pattern illustrates where a callback URL belongs:
import requests
payload = {
"input": {"prompt": "a blue glass robot in a studio"},
"webhook": "https://example.com/webhooks/images",
"webhook_events_filter": ["completed"]
}
r = requests.post("https://api.provider.example/predictions",
headers={"Authorization": "Bearer YOUR_TOKEN"},
json=payload, timeout=30)
r.raise_for_status()
print(r.json())curl -X POST "https://api.provider.example/predictions"
-H "Authorization: Bearer YOUR_TOKEN"
-H "Content-Type: application/json"
-d '{"input":{"prompt":"a blue glass robot in a studio"},"webhook":"https://example.com/webhooks/images","webhook_events_filter":["completed"]}'Save the returned prediction or response ID before acknowledging any client request. Never place provider tokens or signing secrets in browser JavaScript.
Testing and troubleshooting
Requests never arrive
Check DNS, TLS, firewall rules, route paths, and whether the URL is publicly reachable. For local work, use a tunnel or cloud development endpoint; replace it with the final HTTPS URL in production.
Every request returns 401
Log header names and lengths, not secrets. Confirm the raw body is used, the signing-key encoding is correct, the timestamp is within tolerance, and constant-time comparison is applied. Do not reuse Replicate verification code for OpenAI.
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The provider retries repeatedly
Return 2xx only after validation and durable enqueueing. A timeout, exception, 3xx redirect, or 4xx/5xx response is treated as unsuccessful by providers that retry. Make the handler idempotent before investigating downstream failures.
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Enforce a unique database constraint on the provider event ID and record the side effect’s completion atomically with that key. Retries and duplicated deliveries are normal failure conditions, not exceptional input.
The callback says completed but no file exists
Retrieve the result with the stored provider ID. Check whether the output URL expired, whether the job actually failed or was canceled, and whether your worker has permission to download it. Copy required assets to durable storage.
Intermediate events overwhelm the queue
Request only terminal events, or coalesce progress updates by job ID. Replicate’s output and log notifications can occur at most once every 500 ms, which is still too frequent for many pipelines.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Operational checklist
- Accept only POST on the expected route and enforce a body-size limit.
- Validate event type and payload shape before enqueueing.
- Verify signatures against the untouched body and apply replay protection.
- Store secrets in managed server-side configuration and rotate them after exposure.
- Persist event IDs before triggering publication, billing, or deletion.
- Return 2xx promptly; perform downloads and transformations in a worker.
- Handle success, failure, and cancellation states.
- Monitor delivery failures, queue age, duplicate rates, and worker errors.
- Document provider-specific URL retention and access requirements.
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FAQ
Can a webhook endpoint be private?
Not for direct provider delivery. It must be publicly reachable, though authentication, signature checks, network controls, and an internal queue can keep processing private.
Should I trust the image URL in an event?
No. Treat it as provider-specific and potentially temporary; retrieve and store the output according to that provider’s retention documentation.
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Normalize incoming events into an internal schema containing provider, job ID, event ID, status, and raw payload, while retaining separate verification adapters for each provider.
Frequently Asked Questions
Can a webhook endpoint be private?
Not for direct provider delivery. It must be publicly reachable, though authentication, signature checks, network controls, and an internal queue can keep processing private.
Should I trust the image URL in an event?
No. Treat it as provider-specific and potentially temporary; retrieve and store the output according to that provider’s retention documentation.
How do I support more than one image provider?
Normalize incoming events into an internal schema containing provider, job ID, event ID, status, and raw payload, while retaining separate verification adapters for each provider.
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