Set the callback URL in the asynchronous capture request using the field or endpoint documented by your screenshot API, and make sure it points to a publicly reachable HTTP(S) route on your application. There is no universal callback parameter: providers differ in request format, callback payload, acknowledgement, and signature handling. Before relying on callbacks, confirm they are enabled for the specific service deployment you use.
How screenshot API callbacks work
An asynchronous request starts a capture job and returns before the screenshot is ready. The API later sends the result or a completion event to your callback URL, usually as a server-to-server POST. An initial response such as HTTP 202 means the work was accepted or queued; it is not the finished screenshot.
Your receiver must be reachable from the provider, accept its documented method and content type, and return the acknowledgement the provider requires. A local-only address such as localhost will not work unless you expose it through an appropriate development tunnel.
Choose the callback setting for your provider
Do not assume a parameter named webhook_url works across APIs. Identify the exact capture operation first, then use that provider’s current documentation.
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| Provider | How to configure | Callback details |
|---|---|---|
| ScreenshotOne | Send async=true and webhook_url=<your webhook URL> to /take. |
Results arrive as a POST body. Its documented example also uses response_type=json, store=true, and storage_return_location=true when the caller needs the uploaded file location. Validate X-ScreenshotOne-Signature using HMAC SHA-256 and the secret key from the access page; that secret differs from the API key. ScreenshotOne async documentation. |
| ScreenshotMAX | Provide webhook_url in the request and enable async: true. |
The callback must be publicly accessible over HTTP or HTTPS, accept POST, and return a 2xx response. Async requests return HTTP 202 while queued. Optional signing uses webhook_signed and X-Screenshotmax-WebHook-Signature; calculate HMAC SHA-256 over the exact raw JSON body before parsing it. ScreenshotMAX webhook documentation. |
| Shotbot | Submit callback captures to POST /capture/callback with callback_url. Do not add callback_url to ordinary POST /capture; that endpoint is polling-only and returns 400 callback_url_wrong_endpoint for that parameter. |
The result is sent as multipart data; failures use status=ERR. An optional callback_secret is echoed back for comparison. The receiver should return 2xx. Its status endpoint can diagnose delivery: completed indicates accepted delivery, while upload_failed indicates endpoint refusal. Shotbot API documentation. |
| Screenshot API (screenshotapis.org) | The reference describes webhook_url for async delivery. |
The documentation also states that async callbacks are currently unavailable on the referenced deployment: requests return 503 without charging a credit, and it advises synchronous rendering. Verify availability for your deployment rather than relying on the protocol example. Screenshot API documentation. |
Configure and test the callback
- Identify the provider and operation. Confirm whether the capture endpoint accepts a callback field or requires a dedicated callback route. Similar field names do not imply interchangeable behavior.
- Deploy a public receiver route. Use an address the provider can reach, support the documented POST content type, and implement the required success response. For local development, use a suitable tunnel rather than a localhost URL.
- Pass the callback setting with the async request. Copy the field name, route, and request format from the provider documentation. For example, ScreenshotOne uses
async=truewithwebhook_url; Shotbot requires its dedicated callback operation. - Separate job acceptance from completion. Treat an initial 202 or other acceptance response as confirmation that work was queued, not as the screenshot result. Process the later callback as the completion event.
- Verify the event before trusting it. Validate a documented signature or shared secret. Where the provider requires signing the raw body, preserve the exact request bytes and verify before parsing or transforming JSON.
- Test both success and failure handling. Inspect a test delivery, return the documented 2xx acknowledgement, and check the provider’s status or polling mechanism if delivery fails. Do not assume retry behavior unless the provider documents it.
Callback security and payload handling
Validate signatures using the provider’s exact method
ScreenshotOne documents the X-ScreenshotOne-Signature header and HMAC SHA-256 verification with its access-page secret key, which is separate from the API key. ScreenshotMAX’s instructions require HMAC SHA-256 over the exact raw JSON body. Do not parse and reserialize the body before validating a signature that covers the original bytes.
Handle the documented content type
ScreenshotOne and ScreenshotMAX describe JSON-style callback bodies. Shotbot describes multipart image delivery instead, so its receiver must handle multipart data rather than assuming JSON. Implement only the payload fields and formats documented for the provider and endpoint you selected.
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Return the expected acknowledgement
ScreenshotMAX and Shotbot document a 2xx acknowledgement requirement. A failure to acknowledge can prevent the provider from treating delivery as accepted. The available documentation does not establish universal acknowledgement or retry rules; check the selected provider’s guidance instead of guessing.
Common callback problems and fixes
- No callback arrives: Verify the callback feature is enabled on the deployment, the URL is publicly reachable, the capture request uses the correct async operation, and the field or route matches that provider’s docs. For Shotbot, use
POST /capture/callback, not the polling-only/captureroute. - HTTP 202 appears but there is no image yet: The job was accepted or queued; wait for the separate completion event. Do not treat the initial response as the final screenshot.
- Callback request is rejected: Confirm the receiver accepts POST and the provider’s content type, and returns the required success status. For ScreenshotMAX, its documentation specifies public reachability over HTTP or HTTPS and a 2xx acknowledgement.
- Signature validation fails: Use the correct provider secret and algorithm. Preserve the raw body for verification where required; ScreenshotOne’s signing secret is not its API key.
- Shotbot reports
upload_failed: Check whether the receiver refused the upload or returned an unsuccessful acknowledgement, then inspect its status endpoint and server logs. - Screenshot API returns 503 for async callbacks: The screenshotapis.org reference says callbacks are unavailable on the deployment it describes. Use synchronous rendering there unless current deployment documentation confirms callback support.
When to use a callback versus polling
A callback suits an application that can expose a stable receiver and wants the provider to notify it when a job finishes. Polling suits an integration that cannot accept inbound requests or a deployment where callbacks are unavailable. Compare the initial async response and job-tracking mechanism, callback format, acknowledgement requirements, signature support, failure diagnostics, and live availability before committing to an implementation.
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Frequently Asked Questions
Does a callback URL have to be publicly accessible?
Yes. The provider must be able to reach it from its systems; a localhost address alone is not reachable from the provider.
Does HTTP 202 contain the finished screenshot?
No. It indicates the asynchronous job was accepted or queued; the result comes later through the configured callback.
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