diff --git a/README.md b/README.md
index 50c2004..2a33979 100644
--- a/README.md
+++ b/README.md
@@ -95,6 +95,10 @@ Change format to `webp` and keep other things same as source:
 
 `/f_webp/static/buffalo.png`
 
+Automatically convert to a preferred format (avif/webp/jpeg). Uses the browsers `accept` header to negotiate:
+
+`/f_auto/static/buffalo.png`
+
 Keep original format (`png`) and set width to `200`:
 
 `/w_200/static/buffalo.png`
diff --git a/src/server.ts b/src/server.ts
index a1e7fef..dcd13e2 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -66,7 +66,7 @@ export function createIPXH3Handler(ipx: IPX) {
       const acceptHeader = getRequestHeader(event, "accept") || "";
       const autoFormat = autoDetectFormat(
         acceptHeader,
-        !!(modifiers.a || modifiers.animated),
+        ["a", "animated"].some((key) => key in modifiers),
       );
       delete modifiers.f;
       delete modifiers.format;