feat(intake): tell the model to describe rather than invent (#223)

Pure and separately tested, because this is where the correctness of every draft is decided. Nothing downstream can distinguish an observed detail from an invented one — the description arrives as prose either way — so the instruction is the only place that distinction can be enforced, and the tests assert it is actually present. On a one-of-a-kind item an invented "1930s hand-thrown stoneware" is not a cosmetic error but a false claim on a public shop, and the shop answers for it rather than the model. Visible damage is called out for the same reason in reverse: a buyer finding a chip on arrival is worse than reading about it beforehand. Categories and tags are listed rather than described, so the model chooses from what exists instead of inventing a taxonomy the storefront filters know nothing about, and declining is explicitly allowed so a model with no matching option does not pick the closest wrong one. The note is quoted and labelled as the sender's rather than merged into the instruction: it is untrusted text from an unauthenticated stranger and should read as evidence to weigh, not as something the shop asserts. A whitespace-only note counts as no note. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
2026-08-31 19:29:05 -05:00
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+/**
+ * What the model is told, and what it is shown.
+ *
+ * Pure and separately tested because this is where the correctness of every
+ * draft is decided. Nothing downstream can distinguish an observed detail from
+ * an invented one — the description arrives as prose either way — so the only
+ * place that distinction can be enforced is here, in the instruction.
+ *
+ * On a one-of-a-kind item an invented "1930s hand-thrown stoneware" is a false
+ * claim on a storefront, and it is the shop that answers for it rather than the
+ * model. The submitter's note is the only trustworthy source for anything a
+ * photograph cannot show.
+ */
+
+/**
+ * Listed rather than described, so the model chooses from what exists instead
+ * of inventing a taxonomy the storefront filters know nothing about.
+ */
+function offer(values: string[]): string {
+  return values.length > 0 ? values.join(', ') : '(none defined yet)';
+}
+
+export function buildSystemPrompt(categories: string[], tags: string[]): string {
+  return [
+    'You write short listings for a shop that sells one-of-a-kind second-hand items.',
+    '',
+    'You are given photographs of a single item, and sometimes a note from the person',
+    'sending it in.',
+    '',
+    'Describe only what you can see in the photographs, plus whatever the note tells you.',
+    'Do not state a material, age, maker, or provenance that is neither visible nor in the',
+    'note. If you do not know something, leave it out rather than guessing — a wrong detail',
+    'here becomes a false claim on a public shop, and the shop answers for it rather than you.',
+    'Mention visible damage plainly; a buyer finding it later is worse than reading about it now.',
+    '',
+    `Choose a category from this list, or null if none fits: ${offer(categories)}`,
+    `Choose tags from this list, or an empty list if none fit: ${offer(tags)}`,
+    'Do not invent categories or tags that are not listed.',
+    '',
+    'Suggest a price in cents if the photographs and note give you enough to judge one,',
+    'or null if they do not. A person reviews everything before it is listed.'
+  ].join('\n');
+}
+
+interface Photo {
+  mediaType: string;
+  base64: string;
+}
+
+/**
+ * The photos, then the note.
+ *
+ * Images first because the note refers to them. The note is quoted and labelled
+ * as coming from the sender rather than merged into the instruction: it is
+ * untrusted text from an unauthenticated stranger, and it should read as
+ * evidence to weigh rather than as something the shop is asserting.
+ */
+export function buildUserContent(photos: Photo[], note: string | null): unknown[] {
+  const blocks: unknown[] = photos.map((photo) => ({
+    type: 'image',
+    source: { type: 'base64', media_type: photo.mediaType, data: photo.base64 }
+  }));
+
+  // Whitespace counts as absent. Otherwise an accidental space arrives looking
+  // like something the sender meant to say.
+  const hasNote = note !== null && note.trim() !== '';
+
+  blocks.push({
+    type: 'text',
+    text: hasNote
+      ? `The sender wrote this about the item:\n\n${note}`
+      : 'The sender left no note, so the photographs are all you have.'
+  });
+
+  return blocks;
+}