HTTP 103 — Early Hints
HTTP 103 lets a server start suggesting assets while it is still assembling the main response: the final page is not ready, but a link to the stylesheet or origin script can already reach the browser so preload work starts in parallel.
When 103 shows up
- Preloading critical CSS and fonts before the HTML response is fully rendered server-side.
- Trimming milliseconds off first paint on content-heavy pages.
Mocking a 103 response
A response with this status can be faked end to end with a local mock rule — no backend changes needed. The configuration below mirrors the rule fields the ApiSimul editor uses (method, path, status, delay in milliseconds, response body):
{
"method": "GET",
"path": "/page",
"status": 103,
"delay": 150,
"enabled": true,
"body": null
}Dynamic placeholders such as {{random.name}}, {{string.uuid}} or {{number.int 1 100}} can appear inside the body to vary each response, which keeps client-side list rendering and retry logic honest during development.
Frequently asked questions
What does HTTP 103 mean?
It carries preliminary response headers only — typically Link headers for preloading. The real status and headers arrive in the final response that follows.
How can I test a 103 response without a real backend?
Define a mock rule with the method and path your client requests and set the status to 103. The example configuration above returns this response after the listed delay, so the client exercises its 103 handling exactly as it would against a live server.
Related status codes
Need the response in your own project? Spin up rules for any status with the ApiSimul mock server, or browse every status code page.