diff --git a/README.md b/README.md
index 1bbd2ee..356b6e1 100644
--- a/README.md
+++ b/README.md
@@ -319,18 +319,20 @@ fetch sends the first request's ID upstream, and the lines logged for the fetch
 and the transcode carry that ID.
 
 A transcode whose request's `downstream_timeout` has ended (for a shared one,
-the first request's) stops when it gets its processing slot or before it begins
-encoding, and gives the slot back. libvips cannot stop an encode once it has
-begun, so an encode keeps its slot until it finishes, which can be after
-`downstream_timeout`. The limit of 8192 on each side of the source and of the
-output bounds how long that takes. With one libvips thread, as pixad runs it,
-encoding 8192x8192 random pixels takes up to about 1 second as JPEG, 15 as PNG,
-16 as GIF, 63 as WebP, 110 as AVIF and 96 as JPEG XL, the slowest of 8 or 16
-bits per sample, with or without an alpha channel. WebP, AVIF and JPEG XL are
-that slow only with an alpha channel: 8-bit RGB takes about 44, 53 and 28
-seconds. Decoding such a source adds up to about 11 seconds. A WebP, AVIF or
-JPEG XL encode can therefore hold its slot past the default `downstream_timeout`
-of 60 seconds.
+the first request's) stops waiting for a processing slot, or, once it has one,
+gives it back without beginning to encode. A request waiting for a shared
+transcode that stops this way is answered 503 with the error
+`server busy, try again later`, though its own `downstream_timeout` may not have
+ended. libvips cannot stop an encode once it has begun, so an encode keeps its
+slot until it finishes, which can be after `downstream_timeout`. The limit of
+8192 on each side of the source and of the output bounds how long that takes.
+With one libvips thread, as pixad runs it, encoding 8192x8192 random pixels
+takes up to about 1 second as JPEG, 15 as PNG, 16 as GIF, 63 as WebP, 110 as
+AVIF and 96 as JPEG XL, the slowest of 8 or 16 bits per sample, with or without
+an alpha channel. WebP, AVIF and JPEG XL are that slow only with an alpha
+channel: 8-bit RGB takes about 44, 53 and 28 seconds. Decoding such a source
+adds up to about 11 seconds. A WebP, AVIF or JPEG XL encode can therefore hold
+its slot past the default `downstream_timeout` of 60 seconds.
 
 The login form (`POST /`) is limited to 5 attempts per minute per client
 address, counting an IPv6 client by its /64; an attempt over the limit is
diff --git a/TODO.md b/TODO.md
index 2f5c128..f4b4b1a 100644
--- a/TODO.md
+++ b/TODO.md
@@ -30,6 +30,24 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 a transcode stops once its request's `downstream_timeout` has
+  ended, unless it has begun encoding (closes #243): the image processor looked
+  at its context only while it waited for a processing slot, so an image was
+  decoded and encoded, holding the slot, however long after its request's
+  deadline that took. `Process` now returns the context's error, and gives the
+  slot back, when the context has ended by the time it has the slot or before it
+  encodes. A request that waited for a shared transcode stopped at the first
+  request's deadline is answered 503 `server busy, try again later`, not 500,
+  though its own `downstream_timeout` may not have ended. libvips cannot stop an
+  encode once it has begun, so that encode keeps its slot until it finishes; the
+  limit of 8192 on each side of the source and of the output bounds how long
+  that takes. With one libvips thread, 8192x8192 random pixels take up to about
+  1 second to encode as JPEG, 15 as PNG, 16 as GIF, 63 as WebP, 110 as AVIF and
+  96 as JPEG XL, the last three with an alpha channel, plus up to about 11
+  seconds to decode the source; `README.md` gives these times and no longer says
+  the work stops when `downstream_timeout` ends. The settings of the WebP, AVIF
+  and JPEG XL encoders, which can hold a slot past the default
+  `downstream_timeout` of 60 seconds, are unchanged.
 - 2026-10-15 a cached variant is served while its source URL is in the negative
   cache (closes #245): `Get` checked the negative cache before the variant
   cache, so once a fetch of the source failed, as one does when another variant
@@ -59,21 +77,6 @@ P2: security: per-origin rate limiting
   and the query now have each `%` written as `%25` and each `:` as `%3A`; a path
   and query holding neither are signed and keyed as before. Signature
   Specification in `README.md` says so.
-- 2026-10-15 a transcode stops once its request's `downstream_timeout` has
-  ended, unless it has begun encoding (closes #243): the image processor looked
-  at its context only while it waited for a processing slot, so an image was
-  decoded and encoded, holding the slot, however long after its request's
-  deadline that took. `Process` now returns the context's error, and gives the
-  slot back, when the context has ended by the time it has the slot or before it
-  encodes. libvips cannot stop an encode once it has begun, so that encode keeps
-  its slot until it finishes; the limit of 8192 on each side of the source and
-  of the output bounds how long that takes. With one libvips thread, 8192x8192
-  random pixels take up to about 1 second to encode as JPEG, 15 as PNG, 16 as
-  GIF, 63 as WebP, 110 as AVIF and 96 as JPEG XL, the last three with an alpha
-  channel, plus up to about 11 seconds to decode the source; `README.md` gives
-  these times and no longer says the work stops when `downstream_timeout` ends.
-  The settings of the WebP, AVIF and JPEG XL encoders, which can hold a slot
-  past the default `downstream_timeout` of 60 seconds, are unchanged.
 - 2026-10-15 the example config allowlists no host (closes #249): it allowlisted
   `github.com` and `user-images.githubusercontent.com`, where anyone can put an
   image, so a deployment set up as Getting Started says served those images to
diff --git a/internal/handlers/image.go b/internal/handlers/image.go
index aefde8e..9b28da1 100644
--- a/internal/handlers/image.go
+++ b/internal/handlers/image.go
@@ -240,7 +240,8 @@ func (s *Handlers) respondImageError(
 	}
 
 	if errors.Is(err, httpfetcher.ErrTooManyConnections) ||
-		errors.Is(err, imageprocessor.ErrTooManyImages) {
+		errors.Is(err, imageprocessor.ErrTooManyImages) ||
+		errors.Is(err, imgcache.ErrProcessingTimedOut) {
 		s.respondError(w, "server busy, try again later",
 			http.StatusServiceUnavailable)
 
diff --git a/internal/handlers/imageenc.go b/internal/handlers/imageenc.go
index 8da6d47..4da08eb 100644
--- a/internal/handlers/imageenc.go
+++ b/internal/handlers/imageenc.go
@@ -158,7 +158,8 @@ func (s *Handlers) handleImageError(w http.ResponseWriter, err error) {
 	case errors.Is(err, httpfetcher.ErrUpstreamTimeout):
 		s.respondError(w, "upstream timeout", http.StatusGatewayTimeout)
 	case errors.Is(err, httpfetcher.ErrTooManyConnections),
-		errors.Is(err, imageprocessor.ErrTooManyImages):
+		errors.Is(err, imageprocessor.ErrTooManyImages),
+		errors.Is(err, imgcache.ErrProcessingTimedOut):
 		s.respondError(w, "server busy, try again later",
 			http.StatusServiceUnavailable)
 	case errors.Is(err, httpfetcher.ErrInvalidContentType),
diff --git a/internal/imgcache/service.go b/internal/imgcache/service.go
index 2ba0e7f..1806a42 100644
--- a/internal/imgcache/service.go
+++ b/internal/imgcache/service.go
@@ -555,7 +555,13 @@ func (s *Service) processAndStore(
 		FitMode: imageprocessor.FitMode(req.FitMode),
 	}
 
+	// ctx has the deadline of the request doing the processing, which can
+	// end while a request waiting for its image has time left
 	processResult, err := s.processor.Process(ctx, source, processReq)
+	if errors.Is(err, context.DeadlineExceeded) {
+		return nil, fmt.Errorf("%w: %w", ErrProcessingTimedOut, err)
+	}
+
 	if err != nil {
 		return nil, fmt.Errorf("image processing failed: %w", err)
 	}