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Use a webhook when a screenshot or image-generation job may outlast a normal HTTP request. Submit the job with a public HTTPS callback URL, verify the provider’s request, save the event idempotently, return a 2xx response quickly, and process downloads or notifications in a queue. Keep polling or another status API as a recovery path because webhook delivery is a notification, not always the system of record.
Webhooks are optional. Some endpoints return image bytes directly, while others support a long-held synchronous wait, polling, server-sent events, or callbacks. The exact provider and endpoint determine which model is available.
What a webhook does in an image workflow
Your application first creates a render or generation job. The API returns an identifier while work continues. When the job changes state, the provider sends an HTTP POST to your endpoint. Your receiver records the event and schedules any expensive work, such as downloading an image, resizing it, storing it, or notifying a user.
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A typical lifecycle is:
- Create a job and persist your internal id together with the provider’s job or prediction id.
- Include a public HTTPS callback URL when the endpoint supports one.
- Receive a signed or otherwise authenticated POST.
- Deduplicate and apply a guarded state transition.
- Return a 2xx response within a few seconds.
- Process files and downstream actions asynchronously.
- Poll the provider’s status endpoint if a callback is late, duplicated, or missing.
Choose callbacks, polling, or a direct response
| Completion model | Use it when | Important limits |
|---|---|---|
| Direct response | The endpoint returns image bytes in the successful HTTP response. | The client connection remains tied to the generation request. |
| Synchronous wait | Jobs normally finish within the provider’s wait limit. | A timeout can leave an incomplete job that must be fetched later. |
| Webhook | Jobs are long-running or you need event-driven processing. | You must expose and secure a receiver and handle retries. |
| Polling | You need a simple recovery path or the provider has no callback. | Polling adds request volume and delay unless backoff is designed well. |
| Server-sent events | You want a live update stream from a provider that supports it. | It is a transport option, not a universal webhook substitute. |
Replicate supports asynchronous predictions, polling, and a synchronous wait mode. Its documented Prefer: wait value can be set from 1 to 60 seconds; if the prediction does not finish in that period, fetch it later. Stability AI’s documented generation endpoints can return image bytes directly on success. These examples show why “image API” alone does not tell you whether a webhook is required.
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Design the submission record
Before sending a request, create a durable row such as jobs with your own id, provider name, provider job id (once returned), requested output type, callback URL, and state. Store timestamps for submission, first callback, terminal transition, and last recovery poll. Never rely on a URL in a callback as permanent storage without checking its retention policy.
Replicate states that API-created prediction input and output files are automatically deleted after an hour. If you use that provider, a completion event should enqueue a copy to durable storage before the retention window expires. Retention is provider-specific; verify the current policy for every service you integrate.
Build a receiver that is safe under retries
1. Preserve the raw request body
Read and retain the unmodified bytes before JSON parsing when the provider’s signature algorithm requires them. Stripe’s webhook guidance explicitly requires the original body for verification. Do not assume a Stripe header, secret format, or timestamp rule applies to another vendor.
2. Authenticate according to that provider
Use the documented signature header, signing secret, timestamp and replay checks for the exact endpoint. Replicate documents a default webhook signing-secret endpoint. ScreenshotMAX documents an X-Screenshotmax-WebHook-Signature header, but the available guide does not establish its complete algorithm or retry schedule; follow that vendor’s current instructions rather than copying another provider’s implementation.
3. Make state changes idempotent
Use a provider event identifier where available. Otherwise combine the provider prediction or render id with the event type and terminal status. Enforce uniqueness in the database and update with a conditional statement such as “change to completed only if the current state is not already terminal.” This prevents duplicate deliveries from creating duplicate files, charges, emails, or notifications.
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4. Prevent stale events from regressing state
Callbacks can arrive out of order. Model states with an ordering or terminal guard: a received completed, failed, or canceled event must not be replaced by a later output or logs event. Keep the raw payload for diagnosis, but separate that audit record from the current job state.
5. Acknowledge quickly
After durable receipt and basic validation, return a 2xx response. Put image downloads, transformations, database-heavy work, and notifications on a queue. Stripe recommends prompt acknowledgement for long-running work, and Replicate expects a 2xx within a few seconds. A receiver that waits for a large image download can cause a retry even though the original event was valid.
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Minimal receiver pattern (Node.js/Express)
The following is provider-neutral pseudocode. Replace verifySignature with the provider’s current library and preserve the raw body using your framework’s raw-body option.
app.post('/webhooks/render', rawBodyMiddleware, async (req, res) => {
let event;
try {
event = verifySignature(req.body, req.headers);
} catch {
return res.sendStatus(400);
}
const eventKey = event.id || `${event.prediction_id}:${event.type}`;
const inserted = await db.insertWebhookOnce(eventKey, event);
if (inserted) {
await db.advanceJobSafely(event.prediction_id, event.type, event);
await queue.publish({ eventKey, predictionId: event.prediction_id });
}
return res.sendStatus(200);
});Return 4xx for an invalid signature or malformed request. For a valid event that your worker has not finished processing, still return 2xx after recording it; otherwise a provider may redeliver an event that is already safely stored.
Replicate: event filters, retries, and recovery
Replicate documents four webhook filters: start, output, logs, and completed. The completed filter represents a terminal outcome, including success, cancellation, or failure. Output and log events can be sent at most once every 500 milliseconds, so do not design a receiver that assumes every token or log line arrives.
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Replicate retries terminal callbacks after connection failures or 4xx/5xx responses, using exponential backoff; its documentation places the final retry about one minute after completion. Intermediate events are not retried. Duplicate events and rare out-of-order delivery are documented, making idempotency and terminal-state guards mandatory.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteFor recovery, repeatedly GET the prediction URL until it reaches a terminal state. Use exponential backoff with a maximum interval appropriate to your product, stop after a deadline, and alert when the provider remains nonterminal. Server-sent events are another update route documented by Replicate, but they do not remove the need for durable state and retry handling.
ScreenshotMAX and other screenshot services
ScreenshotMAX documents an asynchronous rendering parameter and a webhook_url. Its guide instructs receivers to return 2xx and exposes an X-Screenshotmax-WebHook-Signature header. Because the available documentation does not establish the complete verification algorithm or retry schedule, obtain those details from the current vendor guide before coding assumptions into production.
For any screenshot API, compare the exact endpoint on these axes:
- Does it return bytes immediately, hold a synchronous request, call a webhook, require polling, or stream updates?
- Are events terminal-only or do they include start, output, and logs?
- Which response codes trigger retries, for how long, and can you query status?
- What signature, replay protection, secret rotation, and raw-body rules apply?
- Is output inline or at a URL, and how long is that URL valid?
- Can your receiver handle the expected job duration, payload size, and callback volume?
Security and operations checklist
- Expose only HTTPS and validate the HTTP method and content type.
- Verify signatures before parsing or acting on data; keep secrets in a secret manager.
- Use constant-time comparison where the provider’s library requires it.
- Limit request size and reject obviously old timestamps when timestamp signing is supported.
- Log event ids, provider job ids, verification outcomes, and processing latency without logging API keys or private prompts.
- Apply database uniqueness and queue deduplication.
- Set a dead-letter queue and an operator replay procedure.
- Monitor callback age, 2xx rate, duplicate rate, terminal failures, and jobs recovered by polling.
- Copy output to storage you control before a provider’s retention period ends.
Common failures and fixes
Signature verification fails for every request
The body was parsed and re-serialized, the wrong secret is configured, or the provider expects a different timestamp/header format. Capture raw bytes, use the provider’s official verifier, and test against its signed example.
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The provider keeps retrying a successful event
Your endpoint is slow or returns a non-2xx status after doing work. Persist first, acknowledge immediately, and move processing to a worker. Check load-balancer and framework timeouts.
A job is marked failed after it completed
Events arrived out of order or a duplicate handler overwrote terminal state. Add conditional transitions and reject regressions from terminal states.
No callback arrives
Confirm the URL is publicly reachable, its certificate is valid, and your firewall accepts the provider. Query the status endpoint, then inspect provider delivery logs. Polling is the recovery path, not an afterthought.
The image URL returns 404 later
The provider may use short retention. Download successful output in the worker and store it durably; do not defer that step until a user opens the result.
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Webhook volume overwhelms the application
Subscribe only to needed event types, especially terminal-only events when intermediate logs are unnecessary. Return 2xx after queueing and autoscale workers independently from the receiver.
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For a one-call screenshot rather than managing a headless browser and its callback lifecycle, ScreenshotNeo accepts a URL and returns PNG, JPEG, WebP, or PDF. It removes cookie/consent banners, newsletter popups, and chat widgets before capture; bot checks, blank pages, failed loads, timeouts, and cache hits are not billed, and response headers identify the page verdict and billing result. Its MCP server provides take_screenshot, get_page_info, and capture_pdf for Claude, Cursor, and other MCP clients.
See the ScreenshotNeo API documentation for the full option set. A cURL request:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpPython:
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)Node.js:
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);ScreenshotNeo includes full-page and element captures, device presets, custom CSS and JavaScript, waits, blocking rules, cookies and headers, PDF controls, signed links, async jobs with signed webhooks, bulk capture, caching, and a usage API. The Free plan includes 1,000 screenshots per month without a card; paid plans start at $5 for 3,000. Create a free ScreenshotNeo account.
Cost, latency, and reliability decisions
Webhooks do not make rendering faster; they free your request thread while the provider works. Estimate queue depth from expected job duration and callback rate, and size workers for the largest output downloads. Synchronous waiting can be simpler for short jobs, but a timeout still requires status recovery. Polling is straightforward but consumes API requests. Event filters reduce callback traffic, while terminal-only subscriptions simplify state handling.
Keep provider-specific behavior behind an adapter with methods for submit, verify callback, map state, fetch status, and download output. This prevents one vendor’s retry, signature, or retention assumptions from leaking into another integration. Recheck provider documentation before deployment because APIs, retention windows, security mechanisms, and delivery policies change.
Frequently Asked Questions
Can I use a webhook without exposing my application server?
No. The provider must reach a public HTTPS endpoint, although a small dedicated receiver can accept events and forward them to private workers or a queue.
Should a webhook endpoint return 200 or 204?
Use any 2xx status accepted by the provider’s documentation; 200 and 204 are common. The important requirements are durable receipt and a prompt response.
How do I test duplicate and out-of-order events?
Replay the same signed payload twice, then deliver a terminal event before an intermediate one. Verify that only one side effect occurs and that the terminal state cannot regress.
Are ScreenshotNeo screenshots delivered through webhooks?
ScreenshotNeo supports asynchronous jobs with signed webhooks, while its one-call endpoint can return an image or PDF directly. Choose the mode that matches your workload.
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