diff --git a/src/tools/weather.ts b/src/tools/weather.ts
index d438ea5..feff7ec 100644
--- a/src/tools/weather.ts
+++ b/src/tools/weather.ts
@@ -762,9 +762,68 @@ async function kmaFetch(endpoint: string, params: Record<string, string>): Promi
   return json?.response?.body?.items?.item || [];
 }
 
+// Pure formatting/filtering half of the KMA→Open-Meteo forecast merge — split out (2026-08-10)
+// so it's unit-testable without a live API call. Given Open-Meteo's daily arrays and the last
+// YYYYMMDD date KMA's own forecast already covered, returns the lines for whatever days remain
+// beyond that, or null if there's nothing beyond what KMA already had.
+export function formatDailySupplement(
+  dates: string[],
+  tmax: (number | null | undefined)[],
+  tmin: (number | null | undefined)[],
+  pop: (number | null | undefined)[],
+  lastCoveredYmd: string,
+): string[] | null {
+  const lastCoveredIso = lastCoveredYmd
+    ? `${lastCoveredYmd.slice(0, 4)}-${lastCoveredYmd.slice(4, 6)}-${lastCoveredYmd.slice(6, 8)}`
+    : '';
+  const lines: string[] = [];
+  for (let i = 0; i < dates.length; i++) {
+    if (lastCoveredIso && dates[i] <= lastCoveredIso) continue; // KMA already covered this day
+    lines.push(`📅 ${dates[i]}`);
+    lines.push(`   기온: ${tmin[i] ?? '?'}~${tmax[i] ?? '?'}°C`);
+    if (pop[i] != null) lines.push(`   강수확률: ${pop[i]}%`);
+  }
+  return lines.length ? ['── 3일 이후 예보 (Open-Meteo, 참고용) ──', ...lines] : null;
+}
+
+// weather_kma's forecast type is structurally capped at ~3-4 days (KMA's getVilageFcst just
+// doesn't return more, and the caller loop above only reads the first 4 groups it does return).
+// 2026-08-10 real incident: "이번 주말 날씨 어때?" (5 days out) hit this ceiling — the model
+// called only weather_kma, saw the range fell short, and told the user to ask again closer to
+// the weekend instead of calling weather_openmeteo (up to 16 days), even though weather_kma's
+// own description already said to do exactly that for >3-day requests. Prose guidance
+// repeatedly failed to change this model's behavior today (see news_search's git history for
+// the same pattern with different symptoms), so this fetches the extension itself and merges
+// it into one answer — no second tool call for the model to remember or skip.
+// Best-effort: any failure here (network, malformed response) just means no supplement — never
+// breaks the KMA forecast this is attached to.
+async function fetchOpenMeteoDailySupplement(lat: number, lon: number, lastCoveredYmd: string): Promise<string[] | null> {
+  try {
+    const params = new URLSearchParams({
+      latitude: String(lat),
+      longitude: String(lon),
+      timezone: 'auto',
+      daily: 'temperature_2m_max,temperature_2m_min,precipitation_probability_max',
+      forecast_days: '7',
+    });
+    const res = await fetch(`https://api.open-meteo.com/v1/forecast?${params}`, { signal: AbortSignal.timeout(10_000) });
+    if (!res.ok) return null;
+    const data: any = await res.json();
+    return formatDailySupplement(
+      data.daily?.time || [],
+      data.daily?.temperature_2m_max || [],
+      data.daily?.temperature_2m_min || [],
+      data.daily?.precipitation_probability_max || [],
+      lastCoveredYmd,
+    );
+  } catch {
+    return null;
+  }
+}
+
 export const weatherKmaTool = {
   name: 'weather_kma',
-  description: '기상청 공식 API로 한국 날씨 조회. 초단기실황(현재 기온·강수·바람·습도)과 단기예보(3일)를 제공. 한국 내 위치에 가장 정확한 기상 데이터. API 키(data.go.kr) 필요.',
+  description: '기상청 공식 API로 한국 날씨 조회. 초단기실황(현재 기온·강수·바람·습도)과 단기예보(기상청 3일 + Open-Meteo로 자동 보완된 4~7일차)를 제공. 한국 내 위치에 가장 정확한 기상 데이터. type="forecast"는 이번 주말처럼 3일을 넘는 날짜도 한 번의 호출로 답변 가능(참고용 보완 데이터 자동 포함). API 키(data.go.kr) 필요.',
   schema: {
     location: '한국 도시명 (예: "서울", "부산") 또는 "위도,경도" 형식',
     type: '"current" (초단기실황, 기본값) 또는 "forecast" (단기예보 3일)',
@@ -835,7 +894,9 @@ export const weatherKmaTool = {
           `발표: ${base_date.slice(0,4)}-${base_date.slice(4,6)}-${base_date.slice(6,8)} ${base_time.slice(0,2)}:${base_time.slice(2,4)} KST`,
           '',
         ];
+        const coveredDays: string[] = [];
         for (const [day, cats] of Object.entries(byDay).slice(0, 4)) {
+          coveredDays.push(day);
           const ymd = `${day.slice(0,4)}-${day.slice(4,6)}-${day.slice(6,8)}`;
           const tmx = cats.TMX?.[0];
           const tmn = cats.TMN?.[0];
@@ -852,6 +913,17 @@ export const weatherKmaTool = {
           else if (dominantSky) lines.push(`   날씨: ${SKY_LABEL[dominantSky] ?? dominantSky}`);
         }
         lines.push('', '(미세먼지/PM2.5/PM10/황사는 이 데이터에 없음 — weather_airkorea 사용)');
+        // 2026-08-10 real incident: "이번 주말 날씨 어때?" (5 days out) hit this exact 3-4일
+        // ceiling — the model called only weather_kma, saw the range fell short, and told the
+        // user to ask again closer to the weekend instead of calling weather_openmeteo (up to
+        // 16 days), even though weather_kma's own description already says to do exactly that
+        // for >3-day requests. Prose guidance repeatedly failed to change this model's behavior
+        // today (see news_search's git history for the same pattern with different symptoms),
+        // so instead of asking the model to remember a second tool call, fetch the extension
+        // here and merge it into one answer — no second call, no chance to skip it.
+        const lastCovered = coveredDays.sort().slice(-1)[0];
+        const supplement = await fetchOpenMeteoDailySupplement(lat, lon, lastCovered);
+        if (supplement) lines.push('', ...supplement);
         return { success: true, stdout: lines.join('\n'), data: { location: displayName, nx, ny } };
       }
     } catch (err: any) {
diff --git a/tests/weather-kma-supplement.test.ts b/tests/weather-kma-supplement.test.ts
new file mode 100644
index 0000000..4f06b8c
--- /dev/null
+++ b/tests/weather-kma-supplement.test.ts
@@ -0,0 +1,50 @@
+/**
+ * weather-kma-supplement.test.ts
+ *
+ * formatDailySupplement was split out of weather_kma's forecast path (2026-08-10) after a real
+ * incident: "이번 주말 날씨 어때?" (5 days out) hit weather_kma's ~3-day ceiling, and the model
+ * told the user to ask again later instead of calling weather_openmeteo — even though
+ * weather_kma's own description already said to do exactly that for >3-day requests. Prose
+ * guidance kept failing to change this model's behavior (same pattern hit news_search earlier
+ * the same day), so weather_kma now fetches the Open-Meteo extension itself and merges it in.
+ * This pure function is the merge/filter logic — no network call, so it's directly testable.
+ */
+
+import { test, describe } from 'node:test';
+import assert from 'node:assert/strict';
+import { formatDailySupplement } from '../src/tools/weather';
+
+describe('formatDailySupplement — KMA 3일 이후를 Open-Meteo로 보완', () => {
+  test('KMA가 8/10까지 커버했으면 그 이후 날짜만 남긴다', () => {
+    const dates = ['2026-08-09', '2026-08-10', '2026-08-11', '2026-08-12', '2026-08-15'];
+    const tmax = [33, 34, 32, 31, 29];
+    const tmin = [24, 25, 23, 22, 21];
+    const pop = [20, 10, 30, 40, 60];
+    const out = formatDailySupplement(dates, tmax, tmin, pop, '20260810');
+    assert.ok(out, '보완 결과가 없으면 안 됨');
+    assert.ok(!out!.some(l => l.includes('2026-08-09')), 'KMA가 이미 커버한 과거 날짜가 남으면 안 됨');
+    assert.ok(!out!.some(l => l.includes('2026-08-10')), 'KMA가 이미 커버한 마지막 날짜가 남으면 안 됨');
+    assert.ok(out!.some(l => l.includes('2026-08-11')));
+    assert.ok(out!.some(l => l.includes('2026-08-15')), '이번 주말(8/15) 데이터가 포함돼야 함');
+  });
+
+  test('KMA가 이미 전체 범위를 커버했으면(추가 날짜 없음) null', () => {
+    const dates = ['2026-08-09', '2026-08-10'];
+    const out = formatDailySupplement(dates, [33, 34], [24, 25], [20, 10], '20260810');
+    assert.equal(out, null);
+  });
+
+  test('lastCoveredYmd가 비어 있으면(경계 모름) 전체를 보완으로 취급', () => {
+    const dates = ['2026-08-10', '2026-08-11'];
+    const out = formatDailySupplement(dates, [34, 32], [25, 23], [10, 30], '');
+    assert.ok(out);
+    assert.equal(out!.filter(l => l.startsWith('📅')).length, 2);
+  });
+
+  test('강수확률이 없어도(null) 깨지지 않는다', () => {
+    const out = formatDailySupplement(['2026-08-15'], [29], [21], [null], '20260810');
+    assert.ok(out);
+    assert.ok(!out!.some(l => l.includes('강수확률')), 'null 강수확률은 줄 자체를 안 넣어야 함');
+    assert.ok(out!.some(l => l.includes('기온: 21~29°C')));
+  });
+});