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dedupe_duration_s sets a per-request look-back window for duplicate detection. On supported POST /v1/image requests, the service compares the rendered content and image parameters with recent requests. Set it to a non-negative whole number of seconds; use 0 to disable deduplication. When an exact match is found, the API can return the existing image ID and URL without charging another image credit.
This saves credits for repeated HTML/CSS or URL renders, but it is explicitly best effort and eventually consistent—not a strict idempotency mechanism. Requests arriving before a new image is indexed, particularly concurrent requests, can still create multiple images.
What dedupe_duration_s controls
The parameter defines how far back the image service searches for an identical request. A value of 300 means “look for a matching image created in the previous 300 seconds.” A value of 0 turns duplicate detection off.
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What counts as identical
The rendered content and image parameters must match exactly. For HTML/CSS images, that includes the HTML, CSS, and rendering options. For URL screenshots, it includes the URL and the supplied capture parameters. The dedupe_duration_s value itself is excluded from the identity comparison: changing only the look-back window does not make an otherwise identical request different.
What a match returns
On a match, the API can return the previously generated image ID and URL. Because no new image is rendered, the duplicate request may consume no additional image credit. Inspect the response and billing metadata supplied by your account rather than assuming every response represents a newly rendered asset.
Basic requests
HTML and CSS image
{
"html": "<h1>Monthly report</h1>",
"css": "h1 { color: navy; }",
"dedupe_duration_s": 3600
}This asks the service to reuse an identical HTML/CSS result found within the previous hour.
URL screenshot
{
"url": "https://example.com/report",
"dedupe_duration_s": 300
}This uses a five-minute look-back for a URL screenshot. URL screenshots default to 0 when the parameter is omitted, so they do not deduplicate unless you opt in.
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cURL pattern
curl -X POST "https://api.example.invalid/v1/image"
-H "Authorization: Bearer $API_KEY"
-H "Content-Type: application/json"
-d '{
"html":"<h1>Monthly report</h1>",
"css":"h1 { color: navy; }",
"dedupe_duration_s":3600
}'Replace the illustrative host with the image provider’s documented endpoint and authentication format. The request body is the important part: send an integer, not a duration string such as "1h".
Python pattern
import requests
payload = {
"html": "<h1>Monthly report</h1>",
"css": "h1 { color: navy; }",
"dedupe_duration_s": 3600,
}
response = requests.post(
"https://api.example.invalid/v1/image",
headers={"Authorization": "Bearer " + API_KEY},
json=payload,
timeout=90,
)
response.raise_for_status()
result = response.json()Node.js pattern
const payload = {
html: "<h1>Monthly report</h1>",
css: "h1 { color: navy; }",
dedupe_duration_s: 3600
};
const res = await fetch("https://api.example.invalid/v1/image", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.API_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify(payload)
});
if (!res.ok) throw new Error(`${res.status} ${await res.text()}`);
const result = await res.json();Defaults, plan limits, and choosing a window
Defaults differ by request type and plan. The values below are the documented 2026 settings for HTML/CSS-to-image requests.
| Plan | HTML/CSS default | Allowed values | Maximum |
|---|---|---|---|
| Free | 30 days (2,592,000 seconds) | 0 or the plan default |
2,592,000 seconds |
| Basic | 30 days (2,592,000 seconds) | 0 or the plan default |
2,592,000 seconds |
| Pro | 180 days (15,552,000 seconds) | Whole numbers from 0 through 15,552,000 |
15,552,000 seconds |
| Scale | 365 days (31,536,000 seconds) | Whole numbers from 0 through 31,536,000 |
31,536,000 seconds |
Those plan defaults apply to HTML/CSS images. URL screenshots default to zero seconds. If you need a different window, send the parameter explicitly and verify that your plan accepts the value.
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- Use
0when every request must produce a fresh render, such as rapidly changing dashboards. - Use a short window, such as 60–900 seconds, for retries caused by transient network failures.
- Use an hour or a day for reports whose source content is intentionally stable.
- Use the longest permitted window only when reusing an old image is acceptable; a long window can return stale content after the source changes.
Why this is not strict idempotency
The service describes duplicate detection as “Best effort, not an idempotency guarantee.” Indexing a newly created image can take a few seconds. If two identical requests arrive during that interval—or run concurrently—both can miss the match and create separate images, each potentially consuming a credit.
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Designing around the race window
- Create your own deterministic request key from the exact content and image options (for example, a cryptographic hash of a canonical JSON document).
- Use that key in your job database with a uniqueness constraint.
- Have only the first worker submit the render; make later workers wait for its result.
- Still set
dedupe_duration_sas a credit-saving fallback for retries and requests made after indexing. - Reconcile the provider response with your stored image ID and URL, rather than treating a successful HTTP response as proof that no duplicate exists.
Canonicalization matters. Reordering JSON keys may be harmless to your application but can change the serialized request or rendered result in a provider-specific implementation. Keep HTML, CSS, fonts, external assets, viewport settings, and other image options stable when you expect a match.
Endpoint coverage and client support
The option is available on standard single-image POST requests, URL screenshots, and MCP-created HTML/CSS images. It is not used by templates, signed create-and-render URL images, or image batches.
The August 5, 2026 changelog announced support in the official TypeScript client v0.8.0 and .NET client v0.11.0. The Go client documents a DedupeDurationSeconds field in its image options. Check the version installed in your project: an older SDK may silently omit the field or reject it during validation.
Validation and troubleshooting
“Invalid dedupe_duration_s” or a range error
Send a non-negative integer within your plan’s permitted range. Remove decimals, negative values, duration strings, and values above the plan maximum. Free and Basic plans accept only zero or their documented default; Pro and Scale accept whole-number values across their stated ranges.
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The second request created another image
Confirm that the rendered content and every image parameter are identical, then check whether the second request arrived before the first image became matchable. Concurrent submissions are the most common explanation. Treat the feature as eventual consistency, add application-level locking for strict one-at-a-time creation, and do not assume a credit refund.
Changing the window did not create a new image
That is expected. The look-back value is excluded from identity comparison. If you need a distinct image, change the content or another image parameter—or set the request to zero when you require fresh rendering.
URL screenshots never deduplicate
When omitted, URL screenshots use a zero-second default. Include dedupe_duration_s explicitly and ensure the endpoint is a supported single-image request rather than a batch, template, or signed create-and-render URL flow.
The SDK has no matching option
Upgrade to a client version that exposes the field, or send the documented HTTP request directly. For TypeScript, the announced version is 0.8.0; for .NET, 0.11.0. Verify compatibility before changing production dependencies.
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Performance, freshness, and credit planning
Deduplication can reduce rendering work and image-credit consumption for retries, repeated reports, and identical jobs. It does not eliminate the first render, and it cannot prevent duplicate charges caused by a race before the result is indexed. A very long window also increases the chance of returning an image that no longer reflects a changing URL or data source.
Measure your own workload by logging the request fingerprint, requested window, returned image ID, and provider billing or duplicate indicators. Keep separate metrics for expected reuse, accidental duplicates, and intentionally fresh renders. This makes it possible to choose a window from actual content-change frequency rather than from the plan maximum.
Or skip the browser setup
If your goal is simply to capture a clean website image rather than generate HTML/CSS through a browser yourself, ScreenshotNeo provides a one-request URL screenshot API. Its behavior is separate from dedupe_duration_s; use the parameter on the image API above when you need that API’s duplicate detection.
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Frequently Asked Questions
Does changing only dedupe_duration_s force a new image?
No. The parameter is excluded from identity comparison, so identical content and image settings can still match even when the look-back values differ.
Can I use this setting with image batches?
No. The documented scope excludes image batch requests, templates, and signed create-and-render URL images.
What value disables duplicate detection?
Use the integer 0.
How quickly does a new image become eligible for matching?
The service does not promise an exact delay. Matching is eventually consistent, and identical requests sent during the indexing delay can produce separate images.
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