Name the setting that did not come through

The instructions mark exactly two steps REQUIRED, and the two markers
sucker looks for are those two steps. So a failure can say which one is
missing — the custom prompt, the persona, or both — instead of sending
someone back to re-read a page they have already followed.

Truncation causes the same symptom, so it is still named, but second:
it only applies once the settings really are set.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NX2SsmUqQftWTZAY4BDuFa

src/app/api/proxy/route.ts

@@ -9,7 +9,7 @@ import {
   NO_CARD,
   NO_GREETING,
   TOO_FEW_MESSAGES,
-  TRUNCATED_CARD,
+  missingSetup,
   userFacingMessage,
 } from "@/lib/errors";
 
@@ -165,11 +165,12 @@ function extractCardData(messages: Message[]): ExtractedCard {
     `Name extraction: user="${nameFromUser}", inferred="${inferredName}", tracking="${cleanTrackingName}", display="${displayName}"`
   );
 
-  if (
-    !content.includes("<.>") ||
-    !content.includes("<UserPersona>.</UserPersona>")
-  ) {
-    throw TRUNCATED_CARD;
+  // Each marker is one of the two REQUIRED steps in the instructions, so the
+  // error can name the one that did not arrive.
+  const hasPrompt = content.includes("<.>");
+  const hasPersona = content.includes("<UserPersona>.</UserPersona>");
+  if (!hasPrompt || !hasPersona) {
+    throw missingSetup(hasPrompt, hasPersona);
   }
 
   content = content.replace("<.>", "");