Documentation / Fastly Features / ESI

Edge Side Includes

Assembles a page from HTML fragments at the edge. The endpoint serves a parent file containing ESI tags, which Fastly processes to inject the child content before the response leaves the edge.

Endpoint

GET/fastly_features/esi

Internally serves /html/parent_esi.html.

How ESI works

Client Request → Fastly Edge
                 ↓
    1. Serves parent_esi.html
    2. Detects <esi:include> tags
    3. Fetches child_esi.html
    4. Assembles final HTML
                 ↓
    Complete HTML Response

Assembly happens at the edge, so the client receives one complete document and never sees the fragments as separate requests.

ESI files

File Role Cache-Control
parent_esi.html Contains the ESI include tags. private, no-store
child_esi.html The fragment that gets included. private, no-store

Tag syntax

In the parent HTML file:

<esi:include src="/html/child_esi.html" />

Usage examples

Test ESI processing
curl https://your-service.edgecompute.app/fastly_features/esi
View the parent file directly
curl https://your-service.edgecompute.app/html/parent_esi.html
View the child file directly
curl https://your-service.edgecompute.app/html/child_esi.html

Fetching the parent directly is the useful comparison. If the include tag comes back unprocessed, ESI is not running on that path.

Expected response

HTTP/1.1 200 OK
Content-Type: text/html; charset=utf-8
Cache-Control: private, no-store

[HTML with child content injected]

Notes

ESI processing must be enabled in your Fastly service configuration. Without it the parent file is served as-is and the include tag reaches the browser untouched, where it renders as nothing at all.

  • Both files use Cache-Control: private, no-store so assembly runs fresh on every request.
  • An include that fails will break the response unless the tag carries onerror="continue".

Ready to test?

Fetch the endpoint and compare it against the parent file served directly.

Try ESI now