README.md

@@ -175,12 +175,14 @@ path under `/v1/` answers 200, in maintenance mode too.
 - `GET` or `HEAD` `/v1/image/<host>/<path>/<size>.<format>` — an image, fetched,
   resized and converted (below). Needs: a signature, unless the host is
   allowlisted (see Source Hosts). Answers: 200; 304 when `If-None-Match` matches
-  the image's `ETag`; 400 for a URL or parameter that is not valid; 401 for a
-  missing or wrong signature, a missing `exp` or an `exp` in the past; 403 when
-  the request's `Referer` names a host in `referer_blocklist`, checked before
-  the signature, the cache and the upstream fetch; 403 when the upstream host,
-  or a host it redirects to, is `localhost`, ends in `.localhost` or `.local`,
-  or has an address in a blocked network (see `blocked_networks`); 502 when the
+  the image's `ETag`; 400 for a URL or parameter that is not valid, or for the
+  format `auto` an `Accept` header that is not valid; 406 for the format `auto`
+  when `Accept` allows none of the formats it chooses from; 401 for a missing or
+  wrong signature, a missing `exp` or an `exp` in the past; 403 when the
+  request's `Referer` names a host in `referer_blocklist`, checked before the
+  signature, the cache and the upstream fetch; 403 when the upstream host, or a
+  host it redirects to, is `localhost`, ends in `.localhost` or `.local`, or has
+  an address in a blocked network (see `blocked_networks`); 502 when the
   upstream answered with an error status, and for 5 minutes after that for the
   same source URL; 503 when pixa is busy or in maintenance mode; 500 for any
   other failure.
@@ -190,7 +192,8 @@ path under `/v1/` answers 200, in maintenance mode too.
   decrypt, or that asks for a size or fit that is not valid; 410 once it has
   expired; 504 when the upstream has not sent its response headers within
   `upstream_fetch_timeout`, but 500 when that time runs out while the image
-  itself is still arriving; 403, 502, 503 and 500 as for `/v1/image/`.
+  itself is still arriving; 400 for an `Accept` header that is not valid, and
+  406, 403, 502, 503 and 500, as for `/v1/image/`.
 - `GET /robots.txt` — asks every crawler to stay away (`Disallow: /`). Needs:
   nothing. Answers: 200.
 - `GET /.well-known/healthcheck.json` — JSON with `status` (`ok`), `now`,
@@ -239,7 +242,7 @@ A request whose query string cannot be decoded, or gives any parameter more than
 once, is refused with 400.
 
 - `<format>`: one of `orig` (or `original`), `jpeg` (or `jpg`), `png`, `webp`,
-  `avif`, `gif`
+  `avif`, `gif`, or `auto` (below)
 - `<size>`: `orig` or `<width>x<height>` (e.g. `800x600`)
 - `sig` and `exp`: the signature and its expiry, needed unless the host is
   allowlisted (see Signature Specification)
@@ -247,6 +250,24 @@ once, is refused with 400.
   both optional (values under Signature Specification). Both are part of what is
   cached, so each value of either is a separate cached image.
 
+With the format `auto`, pixa chooses the format for each request from its
+`Accept` header, in this order:
+
+1. AVIF, when the header names `image/avif`;
+2. WebP, when it names `image/webp`;
+3. JPEG, when the first of `image/jpeg`, `image/*` and `*/*` that it names
+   allows it, or when there is no `Accept` header or it is empty.
+
+An entry with `q=0` refuses its format; other `q` values do not change the
+order. AVIF and WebP must be named, as clients that cannot show them also send
+`image/*` and `*/*`. pixa never sends a format the client refused: when the
+header allows none of the three, the answer is 406, and a header that does not
+parse, or has a `q` that is not a number from 0 to 1, is refused with 400. The
+signature, or the token of an encrypted URL, covers `auto` itself, so one URL
+serves every client. Each format chosen is cached as a separate image, and every
+answer that depends on `Accept` (the image, a 304, and the 400 and 406 above)
+carries `Vary: Accept`, so a shared cache keeps the formats apart too.
+
 An image is served with `Cache-Control: public, max-age=<seconds>, immutable`.
 When the URL has an expiry (an `exp`, or the TTL of an encrypted URL), `max-age`
 is the whole seconds left until then, at most one year, so no browser or proxy
@@ -297,7 +318,7 @@ nor change what it asks for.
 3. The page shows the URL, `https://<host>/v1/e/<token>/img.<format>`, and when
    it expires. `<host>` is the host the page was opened on, and the URL starts
    with `http` instead while `debug` is on. The name after the token is ignored
-   and only gives the URL a file extension, `jpg` for `orig`.
+   and only gives the URL a file extension, `jpg` for `orig` and `auto`.
 
 The token holds the source's host, path and query and the size, format, quality,
 fit and expiry, encrypted with a key derived from `signing_key`. The source
@@ -353,7 +374,8 @@ Where:
 - `width` — requested width in pixels, `0` for original
 - `height` — requested height in pixels, `0` for original
 - `format` — output format, one of those listed under Routes, with `original`
-  signed as `orig` and `jpg` as `jpeg`
+  signed as `orig` and `jpg` as `jpeg`; `auto` is signed as `auto`, not as the
+  format chosen for the request
 - `expiration` — the URL's `exp` query parameter, the Unix timestamp when the
   signature expires; a request whose `exp` is not a whole number, an empty
   `exp=` included, is refused with 400

TODO.md

@@ -30,6 +30,17 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-05 the format `auto` (closes #88): a format in the `/v1/image/` path,
+  an encrypted URL's token and the generator page's format choice, chosen for
+  each request from `Accept` once the signature or token is checked: AVIF when
+  the header names `image/avif`, else WebP when it names `image/webp`, else JPEG
+  when the first of `image/jpeg`, `image/*` and `*/*` that it names allows it,
+  or when it names nothing; `q=0` refuses a format. AVIF and WebP must be named,
+  as clients that cannot show them send the wildcards too. A header that allows
+  none of the three answers 406, one that does not parse 400. The signature and
+  the token cover `auto` itself; the cache key and `ETag` use the format chosen.
+  Answers from the point the format is chosen carry `Vary: Accept`, next to the
+  CORS `Vary: Origin`; fixed-format answers do not.
 - 2026-10-05 the markdown is formatted with prettier (closes #100): `script/fmt`
   and `script/fmt-check` run prettier 3.8.1, pinned in `package.json` and
   `yarn.lock`, on `**/*.md` after `gofmt`, with four-space tabs and
@@ -661,8 +672,6 @@ P2: security: per-IP rate limiting on the image routes
     - per-origin rate limiting
 - P2: HTTP response handling
     - Last-Modified headers
-    - Vary header for content negotiation
-- P2: auto format selection (format=auto based on Accept header)
 - P2: configuration
     - YAML config file support
 - P2: operational

internal/handlers/auth.go

@@ -371,7 +371,7 @@ func (s *Handlers) buildGeneratedURL(r *http.Request, token, format string) stri
 
 	// Determine file extension for the trailing filename
 	ext := format
-	if ext == "" || ext == "orig" {
+	if ext == "" || ext == "orig" || ext == "auto" {
 		ext = "jpg" // Default extension
 	}
 

internal/handlers/format_auto.go

@@ -0,0 +1,131 @@
+package handlers
+
+import (
+	"errors"
+	"fmt"
+	"mime"
+	"net/http"
+	"strconv"
+	"strings"
+
+	"sneak.berlin/go/pixa/internal/imgcache"
+)
+
+// Errors for an Accept header that an auto URL cannot be served for.
+var (
+	errInvalidAccept = errors.New("invalid Accept header")
+	errNotAcceptable = errors.New(
+		"not acceptable: auto serves image/avif, image/webp or image/jpeg")
+)
+
+// chooseAutoFormat replaces the format auto in req with the format
+// formatForAccept chooses from r's Accept header, and adds Vary: Accept to the
+// response, which then depends on that header. It answers 400 for an Accept
+// header that is not valid and 406 for one that allows none of the formats,
+// and reports whether req can be served. Any other format is left as it is.
+func (s *Handlers) chooseAutoFormat(
+	w http.ResponseWriter, r *http.Request, req *imgcache.ImageRequest,
+) bool {
+	if req.Format != imgcache.FormatAuto {
+		return true
+	}
+
+	w.Header().Add("Vary", "Accept")
+
+	format, err := formatForAccept(strings.Join(r.Header.Values("Accept"), ","))
+	if errors.Is(err, errNotAcceptable) {
+		s.respondError(w, err.Error(), http.StatusNotAcceptable)
+
+		return false
+	}
+
+	if err != nil {
+		s.respondError(w, err.Error(), http.StatusBadRequest)
+
+		return false
+	}
+
+	req.Format = format
+
+	return true
+}
+
+// formatForAccept returns the format an auto URL is served in for the Accept
+// header accept: AVIF when it names image/avif, else WebP when it names
+// image/webp, else JPEG when its most specific entry of image/jpeg, image/*
+// and */* allows it, or when it names nothing. A q of 0 refuses a format.
+// AVIF and WebP must be named, as clients that cannot show them send image/*
+// and */* too.
+func formatForAccept(accept string) (imgcache.ImageFormat, error) {
+	qualities, err := parseAccept(accept)
+	if err != nil {
+		return "", err
+	}
+
+	if len(qualities) == 0 {
+		return imgcache.FormatJPEG, nil
+	}
+
+	if qualities["image/avif"] > 0 {
+		return imgcache.FormatAVIF, nil
+	}
+
+	if qualities["image/webp"] > 0 {
+		return imgcache.FormatWebP, nil
+	}
+
+	// For JPEG, the most specific entry the header has decides
+	quality, named := qualities["image/jpeg"]
+	if !named {
+		quality, named = qualities["image/*"]
+	}
+
+	if !named {
+		quality = qualities["*/*"]
+	}
+
+	if quality > 0 {
+		return imgcache.FormatJPEG, nil
+	}
+
+	return "", errNotAcceptable
+}
+
+// parseAccept returns the q of each media range the Accept header accept
+// names, 1 where it gives none. A media range named more than once keeps its
+// lowest q, so a refusal is never overridden. A media range that does not
+// parse, or a q that is not a number from 0 to 1, is an error.
+func parseAccept(accept string) (map[string]float64, error) {
+	qualities := make(map[string]float64)
+
+	for entry := range strings.SplitSeq(accept, ",") {
+		// A header field list may hold empty entries
+		if strings.TrimSpace(entry) == "" {
+			continue
+		}
+
+		mediaRange, params, err := mime.ParseMediaType(entry)
+		if err != nil {
+			return nil, fmt.Errorf("%w: %q: %w", errInvalidAccept, entry, err)
+		}
+
+		quality := 1.0
+
+		if qParam, given := params["q"]; given {
+			quality, err = strconv.ParseFloat(qParam, 64)
+			inRange := quality >= 0 && quality <= 1
+
+			if err != nil || !inRange {
+				return nil, fmt.Errorf("%w: %q: q is not a number from 0 to 1",
+					errInvalidAccept, entry)
+			}
+		}
+
+		previous, named := qualities[mediaRange]
+		if !named || quality < previous {
+			qualities[mediaRange] = quality
+		}
+	}
+
+	return qualities, nil
+}

internal/handlers/image.go

@@ -43,6 +43,11 @@ func (s *Handlers) HandleImage() http.HandlerFunc {
 			return
 		}
 
+		// The signature covers the format auto, not the format chosen
+		if !s.chooseAutoFormat(w, r, req) {
+			return
+		}
+
 		// Get cache key for logging
 		cacheKey := imgcache.CacheKey(req)
 

internal/handlers/imageenc.go

@@ -30,7 +30,7 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 		start := time.Now()
 
 		req, ok := s.parseImageEncRequest(w, r)
-		if !ok {
+		if !ok || !s.chooseAutoFormat(w, r, req) {
 			return
 		}
 

internal/imgcache/imgcache.go

@@ -22,6 +22,11 @@ const (
 	FormatWebP     ImageFormat = "webp"
 	FormatAVIF     ImageFormat = "avif"
 	FormatGIF      ImageFormat = "gif"
+
+	// FormatAuto stands for AVIF, WebP or JPEG, chosen for each request
+	// from its Accept header once the URL's signature or token has been
+	// checked; it is never processed or cached as itself.
+	FormatAuto ImageFormat = "auto"
 )
 
 // Size represents requested image dimensions

internal/imgcache/urlparser.go

@@ -277,6 +277,8 @@ func parseFormat(s string) (ImageFormat, error) {
 		return FormatAVIF, nil
 	case "gif":
 		return FormatGIF, nil
+	case "auto":
+		return FormatAuto, nil
 	default:
 		return "", fmt.Errorf("%w: %s", ErrInvalidFormat, s)
 	}

internal/templates/generator.html

@@ -95,6 +95,7 @@
                     </label>
                     <select id="format" name="format">
                         <option value="orig" {{if eq .FormFormat "orig"}}selected{{end}}>Original</option>
+                        <option value="auto" {{if eq .FormFormat "auto"}}selected{{end}}>Auto (AVIF, WebP or JPEG)</option>
                         <option value="jpeg" {{if eq .FormFormat "jpeg"}}selected{{end}}>JPEG</option>
                         <option value="png" {{if eq .FormFormat "png"}}selected{{end}}>PNG</option>
                         <option value="webp" {{if eq .FormFormat "webp"}}selected{{end}}>WebP</option>