The Tool Desk
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How asynchronous screenshot webhooks work
A screenshot API renders a URL (and, with some services, supplied HTML) in a hosted browser and returns an image or another supported output. With an asynchronous request, the API can acknowledge or queue the job before the browser has finished rendering. It may return a job or render ID, which you can use to check status, and later send an HTTP POST to a callback URL when processing completes. The exact acknowledgement, callback events, payload, and retrieval method are provider-specific.
- Your application submits a capture request with the provider’s asynchronous option and, if supported, a callback URL.
- The service accepts or queues the work and returns its documented acknowledgement and job identifier.
- The hosted browser loads and renders the page; the service makes the result available according to its storage and retention rules.
- The service POSTs a completion event to your endpoint. Your application verifies the provider-specific signature, records the event, and retrieves or processes the output according to the API contract.
A callback avoids repeatedly asking the API whether a slow capture has finished. It is useful for batch jobs, queue-based pipelines, and applications that should not keep a request open while a page renders. It does not, by itself, establish exactly-once delivery or guarantee that a callback will arrive.
What to verify before choosing a provider
Availability on your deployment and plan
Check that asynchronous callbacks are enabled for the specific deployment and account you will use. For example, the Screenshot API reference currently says that async callbacks are unavailable on that deployment and return 503 without charging a credit. That notice is deployment-specific, not a statement about all screenshot APIs. ScreenshotOne, Screenshotor, and ScreenshotNeo document webhook-related workflows in their respective materials: ScreenshotOne’s async and webhook documentation, Screenshotor’s API page, and ScreenshotNeo’s documentation.
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Acceptance response and completion payload
Find out whether the initial response contains a job ID, what status means the request was accepted, and what events the callback can report. Establish how success and failure are represented, whether the callback contains the image itself or a URL, and whether you must call a separate retrieval endpoint. Confirm how long result files remain available and whether the callback URL must meet restrictions such as HTTPS.
Signature and replay protection
Validate callbacks using the exact signature scheme documented by your provider. The signed input may be the raw request body or a combination of fields such as a timestamp and body; the secret and digest format also vary. Read the request body as raw bytes before parsing JSON if the provider’s instructions require it, compare signatures with a constant-time method, and reject stale timestamps when the scheme includes them. Do not treat possession of the callback URL or a source IP as proof of authenticity.
For example, ScreenshotOne documents an HMAC-SHA256 digest of the JSON body signed with the API key, while ScreenshotNeo documents a timestamped HMAC over timestamp + "." + body using a signing secret. Those formats are not interchangeable; follow the chosen provider’s current specification.
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Retries, duplicate events, and recovery
Ask whether failed captures also trigger callbacks, what response status your endpoint must return, how long the provider waits, whether it retries, and whether retries carry a stable delivery identifier. Do not assume event order or exactly-once delivery unless the provider documents it. Make your handler idempotent: store a provider event or delivery ID where available, and ensure receiving the same event again does not create duplicate work. Determine whether you can poll job status if your endpoint is unavailable or a callback is missed.
ScreenshotNeo documents retries at 2, 15, and 60 seconds and a repeated delivery ID. This is a ScreenshotNeo-specific behavior, not a general webhook standard.
Limits and total cost
Compare successful captures per plan, output formats, retention, rate and concurrency limits, and any separate costs for storage, egress, PDFs, or retries. These terms can change, so confirm them in current provider documentation and plan details. The Website Screenshot API getting-started guide and the ScreenshotCore API page and pricing page are examples of provider-specific documentation; do not assume their terms apply to other services.
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Make the callback receiver dependable
- Respond promptly. Verify and record the event, then return the documented success status promptly. Put slow downloads or downstream processing on your own queue rather than holding the webhook request open.
- Verify before acting. Check the signature using the provider’s specified input and secret before trusting payload fields or fetching a result URL.
- Handle repeats safely. Deduplicate using a stable event or delivery ID when provided. If none is documented, design processing so repeated completion notifications cannot produce harmful duplicate effects.
- Separate receipt from completion of your work. A successful HTTP response to the webhook usually tells the sender it delivered the notification; it does not necessarily mean your downstream image processing has finished.
- Keep a recovery path. Record the job ID and request time. Use the provider’s polling or retrieval mechanism if callback delivery fails, and alert on jobs that remain unfinished beyond your own expected window.
- Protect the result. Treat callback URLs and result links as untrusted input until verified. Apply your normal network and access controls before fetching remote resources.
Exact timeouts, retry rules, retention periods, and polling endpoints are not common across providers. Use the selected API’s current contract rather than copying another service’s implementation.
Provider examples and how to compare them
| Provider | Documented callback or async detail | What to confirm in its current contract |
|---|---|---|
| ScreenshotNeo | Documents async jobs, polling or webhook delivery, and a repeated delivery ID with retries at 2, 15, and 60 seconds. | Use its current documentation for request parameters, callback payload, signature verification, plan limits, and result retrieval. |
| ScreenshotOne | Documents asynchronous rendering and webhook POST delivery; its documentation also describes an S3 upload result workflow. | Confirm response and payload fields, result storage, signature handling, retries, and current availability. |
| Screenshotor | Describes queueing with webhookUrl and polling. |
Confirm callback events, payload, authentication, retry behavior, and polling details. |
| Screenshot API on screenshotapis.org | Its guide says async callbacks return 503 without charging a credit on the deployment covered by that notice. | Check whether the notice still applies to the deployment you intend to use. |
These descriptions reflect each provider’s own documentation, not an independent performance or reliability test. A feature label alone is not enough: compare the actual callback contract, signature verification, delivery behavior, and recovery options.
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ScreenshotNeo is a website screenshot API and MCP server for developers. Its API can return a screenshot or PDF from one GET request, without you operating the rendering browser. Use its docs for the current async and webhook contract and request options.
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For example, this cURL request captures a page to a WebP file:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpSee the ScreenshotNeo API documentation for webhook setup and the complete API contract. ScreenshotNeo removes cookie and consent banners, newsletter popups, and chat widgets before capture; those cleanup steps can be turned off. Bot checks, blank pages, failed loads, timeouts, and cache hits are not billed, and responses identify the page verdict and billing status in headers. Its MCP server gives AI agents tools to take screenshots, get page information, and capture PDFs. The free plan includes 1,000 screenshots a month with no card; paid plans start at $5 for 3,000. Sign up free for 1,000 screenshots a month, with no card required.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Troubleshooting webhook integrations
The API returns an error instead of accepting an async job
Check that async callbacks are available on the deployment and plan, that the request uses the documented parameter names, and that the callback URL is valid and reachable. A 503 may indicate callbacks are unavailable on that deployment, as the screenshotapis.org guide specifically notes; do not treat that behavior as universal.
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Check provider delivery logs if available, endpoint reachability from outside your network, TLS configuration, firewall rules, and the required response status and timeout. Confirm whether failures generate callbacks and whether the service retries. Use polling as a recovery path if the provider supports it.
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Your endpoint receives the event but signature verification fails
Use the exact signing input and secret for that provider. If the specification signs raw bytes, do not parse and reserialize the JSON before verification. Check encoding, timestamp handling, and header names; do not substitute a different provider’s HMAC recipe.
The same completion arrives more than once
Webhook senders may retry after a timeout or unsuccessful response. Check for a documented stable delivery ID, persist it, and make handling idempotent. Avoid returning a failure merely because downstream work is still running; acknowledge a valid event after durable receipt and process it asynchronously.
The callback succeeds but the screenshot cannot be retrieved
Check whether the payload contains the result or only a URL or job reference, whether a separate retrieval call is required, and whether the result has expired. Verify access credentials and any URL-signing requirements, and use the provider’s documented retention and retrieval rules.
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Frequently asked questions
Does every screenshot API support completion webhooks?
No. Availability varies by provider and deployment. Verify the callback feature and its behavior in the documentation for the exact service you plan to use.
Should I use a webhook or poll for completion?
Use a webhook when you want the service to notify your application without repeated status requests. Keep polling available as a fallback when the provider supports it, especially for operational recovery.
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