diff --git a/backend/src/intake/draftPrompt.ts b/backend/src/intake/draftPrompt.ts
new file mode 100644
index 0000000..a145b04
--- /dev/null
+++ b/backend/src/intake/draftPrompt.ts
@@ -0,0 +1,76 @@
+/**
+ * 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;
+}
diff --git a/backend/tests/unit/draftPrompt.test.ts b/backend/tests/unit/draftPrompt.test.ts
new file mode 100644
index 0000000..5092ced
--- /dev/null
+++ b/backend/tests/unit/draftPrompt.test.ts
@@ -0,0 +1,71 @@
+import { buildSystemPrompt, buildUserContent } from '../../src/intake/draftPrompt';
+
+describe('buildSystemPrompt', () => {
+  const prompt = buildSystemPrompt(['Ceramics', 'Textiles'], ['vintage', 'blue']);
+
+  // The whole reason this function is tested rather than inlined. On a
+  // one-of-a-kind item an invented age or maker is a false claim on a
+  // storefront, and nothing downstream can tell an invented detail from an
+  // observed one.
+  it('forbids inventing what is neither visible nor in the note', () => {
+    expect(prompt).toMatch(/do not (state|invent)/i);
+    expect(prompt).toMatch(/material/i);
+    expect(prompt).toMatch(/age/i);
+    expect(prompt).toMatch(/maker/i);
+    expect(prompt).toMatch(/provenance/i);
+  });
+
+  it('offers the categories it may choose from', () => {
+    expect(prompt).toContain('Ceramics');
+    expect(prompt).toContain('Textiles');
+  });
+
+  it('offers the tags it may choose from', () => {
+    expect(prompt).toContain('vintage');
+    expect(prompt).toContain('blue');
+  });
+
+  // Otherwise a model with no matching option picks the closest wrong one.
+  it('permits declining a category', () => {
+    expect(prompt).toMatch(/null/i);
+  });
+
+  it('survives a shop with no categories or tags yet', () => {
+    expect(() => buildSystemPrompt([], [])).not.toThrow();
+  });
+});
+
+describe('buildUserContent', () => {
+  const photo = { mediaType: 'image/jpeg', base64: 'AAAA' };
+  type Block = { type: string; source?: { media_type: string; data: string } };
+
+  it('sends every photo as an image block', () => {
+    const images = (buildUserContent([photo, photo], 'a note') as Block[]).filter(
+      (block) => block.type === 'image'
+    );
+
+    expect(images).toHaveLength(2);
+    expect(images[0]?.source?.media_type).toBe('image/jpeg');
+    expect(images[0]?.source?.data).toBe('AAAA');
+  });
+
+  it('includes the note verbatim', () => {
+    const text = JSON.stringify(buildUserContent([photo], 'Chipped base, bought 1998'));
+    expect(text).toContain('Chipped base, bought 1998');
+  });
+
+  // A submission with no note is ordinary — the field is optional — and the
+  // model has to be told so rather than left to read an empty string as a fact
+  // about the item.
+  it('says so when there is no note', () => {
+    const text = JSON.stringify(buildUserContent([photo], null));
+    expect(text).toMatch(/no note/i);
+  });
+
+  // A whitespace-only note is the same thing as no note. Sending it would let
+  // the model treat an accidental space as something the sender meant to say.
+  it('treats a blank note as no note', () => {
+    const text = JSON.stringify(buildUserContent([photo], '   '));
+    expect(text).toMatch(/no note/i);
+  });
+});