Return validation failures as a stable application/problem+json document: use an HTTP status that matches the failure, a documented problem type, a short title, corrective detail, and a structured array that identifies every invalid input. Add a safe correlation identifier for support, never expose implementation internals, and document the exact contract your screenshot endpoint accepts.
The response contract clients can rely on
HTTP status codes are necessary but rarely sufficient. A client needs to know which value failed, why it failed, and what to change without scraping prose. RFC 9457’s problem-details format provides a consistent envelope while allowing documented extensions for field-level errors.
HTTP/1.1 422 Unprocessable Content
Content-Type: application/problem+json
{
"type": "https://api.example.com/problems/validation-error",
"title": "Request validation failed",
"status": 422,
"detail": "Correct the listed request values and try again.",
"errors": [
{
"pointer": "#/width",
"code": "out_of_range",
"detail": "Choose a width within the documented limit."
},
{
"pointer": "#/url",
"code": "invalid_format",
"detail": "Provide a URL in one of the formats supported by this API."
}
],
"instance": "urn:request:opaque-support-id"
}The URI in type identifies the category of problem and should remain stable. title is a short, reusable label; detail describes this occurrence and tells the caller how to correct it. If you include status, it must equal the actual HTTP response status.
Use extensions for structured data
Keep standard members intact and define extensions such as errors in your API documentation. Clients should branch on stable fields and codes, not on changing English sentences. RFC 9457 specifically advises that clients should not parse detail; structured information belongs in extension members.
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Identify the exact invalid input
Each validation item should contain a location, a stable code, and a corrective message. JSON Pointer notation (for example, #/url or #/options/0/viewport/width) works for JSON request bodies and can identify nested array elements. For query parameters or headers, document a consistent convention such as query.url or a pointer-like path, because a pointer into a JSON body does not describe every transport.
- Location: points to the value the caller must change.
- Code: a durable identifier such as
required,invalid_format, orout_of_range. - Detail: states the violated constraint and a safe correction.
Do not put credentials, signed URLs, cookies, authorization values, or other sensitive input in the pointer or message. Redact values in logs as well as responses.
Required and malformed values
Say whether a property is missing, empty, or syntactically invalid. “Provide a URL” is actionable; “bad request” is not. If the API accepts only certain schemes, name them in the contract and message rather than revealing parser internals.
Ranges and combinations
For numeric settings, report the documented boundary and the supplied value only when echoing it is safe. For cross-field rules, point to the combination or to each participating field and explain the relationship, such as a page range whose end precedes its start.
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Choose HTTP status semantics deliberately
Use the status code for its HTTP meaning and apply it consistently. A syntactically malformed request, an otherwise valid request that violates a documented constraint, an authentication failure, and a server-side capture failure are different conditions and should not collapse into one generic response.
| Condition | Design decision |
|---|---|
| Malformed syntax or an unreadable body | Use the code your contract assigns to malformed requests; return a problem document explaining how to make the payload parseable. |
| Parsed request with invalid values | Use the documented validation status (RFC 9457’s example uses 422) and include all known field errors. |
| Missing or invalid credentials | Use the authentication status defined by your API and avoid describing secret-handling internals. |
| Valid request that cannot be completed by the service | Use a server or upstream-failure status, with a separate problem type from validation. |
The table is a design framework, not a universal prescription. Publish the statuses your endpoint actually emits. Never return 200 with an error body, and never put a status in the JSON that differs from the HTTP response.
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Return multiple errors in one response
When several values are independently invalid, return them together under one validation problem. A caller can then correct the request in one edit-and-retry cycle instead of discovering one failure per submission. Preserve deterministic ordering, such as request order or JSON-pointer order, so tests and logs remain stable.
Stop early only when continuing would be unsafe or impossible—for example, when the body cannot be parsed at all. Once parsing succeeds, collect the validation failures that can be evaluated without performing a screenshot capture.
Separate validation from capture failures
Validation means the request violates the published input contract before work begins. A target site timing out, returning a bot check, or producing a blank page is an execution outcome, not a malformed request. Give those outcomes distinct problem types and operational codes. This distinction lets clients fix their payload instead of retrying a request that can never become valid, while allowing transient capture failures to use an appropriate retry policy.
Make messages corrective and safe
Write for the API consumer, not the server operator. Name the input, state the rule, and give the next action:
- Good: “Set
formatto one of the documented image formats.” - Weak: “枚举解析失败” or “validator exception.”
- Good: “Provide a positive timeout within the documented maximum.”
- Weak: “Value caused an internal range error.”
RFC 9457 says the detail string should help the client correct the problem rather than provide debugging information. Keep stack traces, SQL fragments, library names, file paths, and infrastructure identifiers out of public responses. Put diagnostic context in protected logs keyed by a safe occurrence identifier.
Correlation and occurrence identifiers
Add an instance or separate correlation member only when it is safe to expose and support staff can use it to locate server-side records. Generate an opaque, non-sequential identifier. Do not embed account IDs, URLs, tokens, or timestamps that disclose sensitive information. Document where callers should provide the identifier when opening a support case.
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Content negotiation and headers
Advertise application/problem+json for problem responses and set that content type even when the success response is an image, PDF, or other binary. Honor the client’s Accept header where practical, but keep the error schema stable. Include a request identifier header if your operational tooling uses one, ensuring it matches the safe identifier in the body. Avoid returning a successful image with an error status or an HTML proxy page that clients must parse.
Document the real screenshot contract
Examples using url and width are illustrative only. Your reference must define every accepted field, type, default, format, range, mutually exclusive option, and status. State whether values arrive in query parameters, JSON, multipart form data, or headers. Explain how nested locations are represented in validation pointers. Do not claim a universal limit or accepted URL scheme without tying it to your own contract.
Versioning error contracts
Keep existing member names and meanings stable. Add optional fields rather than changing a code’s semantics. If you must introduce a breaking shape, publish a new media type or API version and give clients a migration period. Treat problem type URIs and application error codes as part of the compatibility surface.
Testing checklist
- Send a request with every required field omitted and verify one item per missing value.
- Send wrong types, malformed formats, boundary values, and values just outside each documented range.
- Send several invalid fields together and confirm the response contains all safely detectable failures.
- Assert the HTTP status, content type, stable
type, codes, and pointers—not English wording alone. - Check that no token, cookie, signed URL, stack trace, or internal path appears in the body or ordinary logs.
- Replay the same invalid request and verify ordering and shape are deterministic.
- Test proxy and gateway behavior so upstream HTML error pages do not replace your problem document.
Common implementation failures and fixes
Clients parse prose
Symptom: an SDK searches for words such as “invalid.” Fix: expose documented codes and locations; reserve detail for people.
Only the first error is returned
Symptom: users must retry repeatedly to discover failures. Fix: accumulate independent validation results after parsing succeeds.
Status mismatch
Symptom: the HTTP layer says 422 while the JSON says 400. Fix: generate both from one status value and test them together.
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Symptom: a pointer references a JSON body even though the invalid value was a query parameter. Fix: document location syntax for each transport or normalize requests before validation.
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Validate cheaply before launching a browser or remote fetch. Schema checks, URL parsing, range checks, and cross-field validation should prevent avoidable work. Do not retry deterministic validation failures. Retry only failures classified as transient, with bounded backoff and an idempotency strategy appropriate to your job model. Measure validation rejects separately from navigation timeouts, bot checks, blank pages, and upstream errors; combining them hides where reliability work is needed.
Returning all field errors slightly enlarges small JSON responses but usually saves a network round trip and a browser launch. Cap error counts or truncate exceptionally large collections according to your documented contract, and signal truncation with a stable member if you do so. Never let an attacker force unbounded error generation.
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ScreenshotNeo provides a screenshot API and MCP server for developers. Its service accepts consent banners before capture and removes more than 60 known consent platforms, newsletter popups, and chat widgets; each cleanup step can be disabled. Only clean shots are billed: bot checks or CAPTCHAs, blank pages, timeouts, failed loads, and cache hits cost nothing, and response headers report the page verdict and billing result.
One GET request returns PNG, JPEG, WebP, or PDF. See the ScreenshotNeo API documentation for the current parameters and validation contract.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpimport 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)const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
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FAQ
Is RFC 9457 mandatory?
No. It is a standard format intended to avoid inventing new HTTP error envelopes. An existing domain format can remain appropriate if it already provides stable types, statuses, corrective details, field locations, and safe tracing.
Should validation errors include the rejected value?
Only when echoing it is safe and useful. Prefer the location, violated rule, and correction; never echo secrets or signed data.
Can a screenshot API return validation errors as an image response?
No. Return a normal HTTP problem document with its own content type and status. Reserve binary responses for successful captures.
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Is RFC 9457 mandatory?
No. It is a standard format intended to avoid inventing new HTTP error envelopes. An existing domain format can remain appropriate if it already provides stable types, statuses, corrective details, field locations, and safe tracing.
Should validation errors include the rejected value?
Only when echoing it is safe and useful. Prefer the location, violated rule, and correction; never echo secrets or signed data.
Can a screenshot API return validation errors as an image response?
No. Return a normal HTTP problem document with its own content type and status. Reserve binary responses for successful captures.
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