diff --git a/src/growth/referral.ts b/src/growth/referral.ts
new file mode 100644
index 0000000..c9775ab
--- /dev/null
+++ b/src/growth/referral.ts
@@ -0,0 +1,76 @@
+export interface ReferralCode {
+  id: string;
+  code: string;
+  ownerId: string;
+  uses: number;
+  creditsAwarded: number;
+  createdAt: Date;
+}
+
+export interface ReferralResult {
+  status: 'ok' | 'already_processed' | 'invalid_code';
+  creditsAwarded?: number;
+  ownerId?: string;
+}
+
+export class ReferralEngine {
+  private codes: Map<string, ReferralCode> = new Map();
+  private conversions: Set<string> = new Set();
+  private events: Array<{ type: string; payload: Record<string, any>; createdAt: Date }> = [];
+
+  createCode(ownerId: string, customCode?: string): ReferralCode {
+    const code = customCode || Math.random().toString(36).substring(2, 10).toUpperCase();
+    const entry: ReferralCode = {
+      id: `ref_${Date.now()}_${Math.random().toString(36).substr(2, 4)}`,
+      code,
+      ownerId,
+      uses: 0,
+      creditsAwarded: 0,
+      createdAt: new Date(),
+    };
+    this.codes.set(code, entry);
+    return entry;
+  }
+
+  processReferral(code: string, newUserId: string): ReferralResult {
+    const conversionKey = `${code}:${newUserId}`;
+    if (this.conversions.has(conversionKey)) {
+      return { status: 'already_processed' };
+    }
+
+    const ref = this.codes.get(code);
+    if (!ref) {
+      return { status: 'invalid_code' };
+    }
+
+    const credits = 5;
+    this.conversions.add(conversionKey);
+    ref.uses += 1;
+    ref.creditsAwarded += credits;
+
+    this.events.push({
+      type: 'referral_conversion',
+      payload: {
+        code,
+        new_user: newUserId,
+        credits,
+        owner_id: ref.ownerId,
+      },
+      createdAt: new Date(),
+    });
+
+    return {
+      status: 'ok',
+      creditsAwarded: credits,
+      ownerId: ref.ownerId,
+    };
+  }
+
+  getEvents() {
+    return this.events;
+  }
+
+  getCode(code: string): ReferralCode | undefined {
+    return this.codes.get(code);
+  }
+}
diff --git a/tests/referral.test.ts b/tests/referral.test.ts
new file mode 100644
index 0000000..dd53383
--- /dev/null
+++ b/tests/referral.test.ts
@@ -0,0 +1,45 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import { ReferralEngine } from '../src/growth/referral';
+
+describe('Referral Engine (#2)', () => {
+  it('awards 5 credits to referrer on successful conversion', () => {
+    const engine = new ReferralEngine();
+    const codeObj = engine.createCode('user_A', 'REF123');
+
+    const res = engine.processReferral('REF123', 'user_B');
+    assert.equal(res.status, 'ok');
+    assert.equal(res.creditsAwarded, 5);
+    assert.equal(res.ownerId, 'user_A');
+
+    const updatedCode = engine.getCode('REF123');
+    assert.equal(updatedCode?.uses, 1);
+    assert.equal(updatedCode?.creditsAwarded, 5);
+
+    const events = engine.getEvents();
+    assert.equal(events.length, 1);
+    assert.equal(events[0].type, 'referral_conversion');
+    assert.equal(events[0].payload.credits, 5);
+  });
+
+  it('prevents duplicate referrals from the same user (idempotency)', () => {
+    const engine = new ReferralEngine();
+    engine.createCode('user_A', 'REF123');
+
+    const res1 = engine.processReferral('REF123', 'user_B');
+    assert.equal(res1.status, 'ok');
+
+    const res2 = engine.processReferral('REF123', 'user_B');
+    assert.equal(res2.status, 'already_processed');
+
+    const updatedCode = engine.getCode('REF123');
+    assert.equal(updatedCode?.uses, 1);
+    assert.equal(updatedCode?.creditsAwarded, 5);
+  });
+
+  it('returns invalid_code for non-existent referral code', () => {
+    const engine = new ReferralEngine();
+    const res = engine.processReferral('NON_EXISTENT', 'user_B');
+    assert.equal(res.status, 'invalid_code');
+  });
+});