diff --git a/apps/api/src/tenders/tender-fingerprint.spec.ts b/apps/api/src/tenders/tender-fingerprint.spec.ts
new file mode 100644
index 0000000..3c78e76
--- /dev/null
+++ b/apps/api/src/tenders/tender-fingerprint.spec.ts
@@ -0,0 +1,66 @@
+import { describe, expect, it } from 'vitest';
+import { tenderFingerprint } from './tender-fingerprint';
+
+/**
+ * SCHEMA-03 (D-04) — pure fingerprint function. NULL-tolerant because the
+ * live DB is 92.4% NULL estimatedValue / 15.7% NULL deadlineAt (13-RESEARCH
+ * Pattern 4) — a fingerprint that requires those fields would never match
+ * the overwhelming majority of real DÖE tenders.
+ */
+describe('tenderFingerprint', () => {
+  const base = {
+    buyerName: 'Stadt München',
+    title: 'Neubau einer Kindertagesstätte',
+    cpvDivisions: ['45', '71'],
+    deadlineAt: null as Date | null,
+    estimatedValue: null as number | null,
+  };
+
+  it('matches when title+buyer+cpv are equal and value+deadline are both NULL', () => {
+    const a = tenderFingerprint({ ...base });
+    const b = tenderFingerprint({ ...base });
+    expect(a).toBe(b);
+  });
+
+  it('differs for a different title (collision guard, Pitfall 1)', () => {
+    const a = tenderFingerprint(base);
+    const b = tenderFingerprint({ ...base, title: 'Sanierung eines Kindergartens' });
+    expect(a).not.toBe(b);
+  });
+
+  it('normalizes umlauts so "München" and "Muenchen" contribute the same text', () => {
+    const a = tenderFingerprint({ ...base, buyerName: 'Stadt München' });
+    const b = tenderFingerprint({ ...base, buyerName: 'Stadt Muenchen' });
+    expect(a).toBe(b);
+  });
+
+  it('is order-independent and deduplicated for cpvDivisions', () => {
+    const a = tenderFingerprint({ ...base, cpvDivisions: ['45', '71'] });
+    const b = tenderFingerprint({ ...base, cpvDivisions: ['71', '45', '71'] });
+    expect(a).toBe(b);
+  });
+
+  it('buckets estimatedValue by order of magnitude — 12000 and 15000 collide', () => {
+    const a = tenderFingerprint({ ...base, estimatedValue: 12000 });
+    const b = tenderFingerprint({ ...base, estimatedValue: 15000 });
+    expect(a).toBe(b);
+  });
+
+  it('separates estimatedValue across a magnitude boundary', () => {
+    const a = tenderFingerprint({ ...base, estimatedValue: 9000 });
+    const b = tenderFingerprint({ ...base, estimatedValue: 90000 });
+    expect(a).not.toBe(b);
+  });
+
+  it('returns a stable sha256 hex string (deterministic across runs)', () => {
+    const result = tenderFingerprint(base);
+    expect(result).toMatch(/^[a-f0-9]{64}$/);
+  });
+
+  it('treats a null buyerName as an empty segment, not a crash', () => {
+    expect(() => tenderFingerprint({ ...base, buyerName: null })).not.toThrow();
+    const a = tenderFingerprint({ ...base, buyerName: null });
+    const b = tenderFingerprint({ ...base, buyerName: null });
+    expect(a).toBe(b);
+  });
+});