diff --git a/internal/server/handlers_panel_stat.go b/internal/server/handlers_panel_stat.go
index 42d8575..fc6cc17 100644
--- a/internal/server/handlers_panel_stat.go
+++ b/internal/server/handlers_panel_stat.go
@@ -298,18 +298,23 @@ func (s *Server) handleSpeedRelayOpen(w http.ResponseWriter, r *http.Request) {
 }
 
 // POST /api/panel/agents/{id}/speed/down - a piece of the browser's upload,
-// passed on to the agent. 410 once the agent has stopped reading.
+// passed on to the agent. 204 asks for the next piece, 200 says the agent has
+// stopped reading.
 func (s *Server) handleSpeedRelayFeed(w http.ResponseWriter, r *http.Request) {
+	body := http.MaxBytesReader(w, r.Body, speedUpCap)
 	rl := s.relays.get(r.PathValue("id"))
-	if rl == nil {
-		http.Error(w, "gone", http.StatusGone)
-		return
+	if rl != nil {
+		if _, err := io.Copy(rl.downW, body); err == nil {
+			w.WriteHeader(http.StatusNoContent)
+			return
+		}
 	}
-	if _, err := io.Copy(rl.downW, http.MaxBytesReader(w, r.Body, speedUpCap)); err != nil {
-		http.Error(w, "gone", http.StatusGone)
-		return
-	}
-	w.WriteHeader(http.StatusNoContent)
+	// Take the rest of the piece before answering: an answer sent over an
+	// unread body makes Go drop the connection, which a proxy in front turns
+	// into a 502.
+	_, _ = io.Copy(io.Discard, body)
+	w.Header().Set("Cache-Control", "no-store")
+	_, _ = io.WriteString(w, "done")
 }
 
 // GET /api/panel/agents/{id}/speed/up - the agent's upload, for the browser to
diff --git a/internal/server/web/templates/status.html b/internal/server/web/templates/status.html
index 4aee3fb..6a0954c 100644
--- a/internal/server/web/templates/status.html
+++ b/internal/server/web/templates/status.html
@@ -167,7 +167,7 @@
       for (var i = 0; i < n; i++) parts.push(block);
       return fetch(base + '/speed/down', {method: 'POST', body: new Blob(parts), credentials: 'same-origin', signal: stop.signal})
         .then(function (r) {
-          if (!r.ok) return;
+          if (r.status !== 204) return;
           if (performance.now() - t < 500 && n < 256) n *= 2;
           return one();
         });