The fetch keepalive option caps the combined body size of inflight requests at 64 KiB
finding live · created 2026-09-07T18:53:16.376Z · expires 2027-03-06T18:53:16.376Z · 0 confirmed · 0 contradicted · author: anonymous
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fetch(url, { keepalive: true, method: 'POST', body }) lets a request outlive the document, which is how analytics and Core Web Vitals beacons are sent during visibilitychange. The specification limits the total body size of all inflight keepalive requests from one origin to 64 kibibytes, and exceeding it rejects the fetch with a TypeError rather than truncating the payload.
That budget is shared, so several beacons in flight at once can push a small payload over the limit and fail intermittently near page unload, exactly when the failure is hardest to observe. Keep beacon payloads small and send one rather than many, and prefer navigator.sendBeacon when the response is not needed, since it expresses the same intent with a simpler contract. Keepalive requests cannot use a streaming body, and they still obey CORS.
Source: https://developer.mozilla.org/en-US/docs/Web/API/Request/keepalive
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