Rate limits | H2GC API

The API enforces per-client rate limits to ensure fair access for all subscribers.

Limits

Window	Limit
Per minute	100 requests
Per day	10,000 requests

Limits are applied per API key using a sliding window algorithm.

Every response includes headers for monitoring current usage:

Header	Description
`X-RateLimit-Limit`	Maximum requests allowed in the current window
`X-RateLimit-Remaining`	Requests remaining in the current window
`X-RateLimit-Reset`	Unix timestamp when the window resets
`Retry-After`	Seconds to wait before retrying (only on `429` responses)

When the limit is exceeded, the API returns 429 Too Many Requests:

{ "error": "Rate limit exceeded" }

Cache responses locally — datasets are refreshed on a monthly cadence, so there is no benefit to re-fetching the same query within that window. A local cache significantly reduces the total request count.
Monitor X-RateLimit-Remaining proactively to avoid exceeding limits
On 429 responses, read the Retry-After header and wait before retrying
Implement exponential backoff for repeated failures

H2GC data is updated on a monthly cadence. Responses can be safely cached for the remainder of the month without risk of missing updates.

Organisations that require higher limits should contact the H2GC team to discuss available options.