diff --git a/src/main/ai/prompt.ts b/src/main/ai/prompt.ts
index 8ad65d0..147fc8f 100644
--- a/src/main/ai/prompt.ts
+++ b/src/main/ai/prompt.ts
@@ -1,21 +1,26 @@
 import type { ParseResult } from '../services/dueDateParser.js';
 
-export const PROMPT_VERSION = 3;
+export const PROMPT_VERSION = 4;
 
 export function buildPrompt(
   rawText: string,
   todayKst: string,
-  candidates: ParseResult[] = []
+  candidates: ParseResult[] = [],
+  vocab: string[] = []
 ): string {
   const candidateBlock = candidates.length > 0
     ? `\nDate candidates extracted by a Korean rule parser (these are HINTS — you decide which is correct, or pick null):
 ${candidates.map((c, i) => `  ${i + 1}. ${c.iso ?? '(ambiguous)'} — matched token: "${c.matchedToken ?? '?'}" (confidence: ${c.confidence ?? 'low'})`).join('\n')}\n`
     : '';
 
+  const vocabBlock = vocab.length > 0
+    ? `\nExisting vocabulary tags (most-used first): ${vocab.join(', ')}\nPrefer reusing a vocabulary tag when the meaning matches; create new tags only when the meaning is genuinely new.\n`
+    : '';
+
   return `You organize raw personal notes into structured metadata.
 
 Today's date in Korea Standard Time (KST): ${todayKst}
-${candidateBlock}
+${candidateBlock}${vocabBlock}
 Input note (raw text, may be fragmented, any language):
 ---
 ${rawText}
diff --git a/tests/unit/prompt.test.ts b/tests/unit/prompt.test.ts
new file mode 100644
index 0000000..e19cee6
--- /dev/null
+++ b/tests/unit/prompt.test.ts
@@ -0,0 +1,31 @@
+import { describe, it, expect } from 'vitest';
+import { buildPrompt, PROMPT_VERSION } from '@main/ai/prompt.js';
+
+describe('prompt', () => {
+  it('PROMPT_VERSION is 4', () => {
+    expect(PROMPT_VERSION).toBe(4);
+  });
+
+  it('buildPrompt with empty vocab omits vocabulary line entirely', () => {
+    const out = buildPrompt('hello', '2026-05-02', [], []);
+    expect(out).not.toContain('vocabulary');
+    expect(out).not.toContain('Prefer reusing');
+  });
+
+  it('buildPrompt with vocab includes Prefer instruction + comma-separated list', () => {
+    const out = buildPrompt('hello', '2026-05-02', [], ['design', 'meeting', 'qa']);
+    expect(out).toContain('Existing vocabulary tags');
+    expect(out).toContain('design, meeting, qa');
+    expect(out).toContain('Prefer reusing');
+  });
+
+  it('vocab block appears between candidate block and JSON rules', () => {
+    const out = buildPrompt('hello', '2026-05-02', [], ['design']);
+    const candidateIdx = out.indexOf("Today's date");
+    const vocabIdx = out.indexOf('Existing vocabulary');
+    const jsonRulesIdx = out.indexOf('Return a JSON object');
+    expect(candidateIdx).toBeGreaterThan(-1);
+    expect(vocabIdx).toBeGreaterThan(candidateIdx);
+    expect(jsonRulesIdx).toBeGreaterThan(vocabIdx);
+  });
+});