diff --git a/app/api/auth/siwe/verify/route.ts b/app/api/auth/siwe/verify/route.ts
index d0b85ad4..f1149add 100644
--- a/app/api/auth/siwe/verify/route.ts
+++ b/app/api/auth/siwe/verify/route.ts
@@ -54,6 +54,11 @@ export async function POST(req: Request): Promise<NextResponse> {
     }
     return NextResponse.json({ ok: false, error: 'invalid_json' }, { status: 400 });
   }
+  // JSON の `null` / 数値 / 文字列も parse は成功する。object でなければ欄を読む前に 400 で返す
+  // (null のまま欄を読むと TypeError → 500 + Sentry event になっていた・第 6 回 B-R6f)。
+  if (cappedBody.value === null || typeof cappedBody.value !== 'object') {
+    return NextResponse.json({ ok: false, error: 'invalid_json' }, { status: 400 });
+  }
   const body = cappedBody.value as { message?: unknown; signature?: unknown };
 
   // domain 束縛はサーバ制御の許可リストで判定する (Host ヘッダは偽装可能なので使わない —
diff --git a/app/api/billing/settle/route.ts b/app/api/billing/settle/route.ts
index e083f830..6f122a5b 100644
--- a/app/api/billing/settle/route.ts
+++ b/app/api/billing/settle/route.ts
@@ -120,6 +120,11 @@ export async function POST(req: Request): Promise<NextResponse> {
       { status: 400 },
     );
   }
+  // JSON の `null` も parse は成功し、分解代入で TypeError → 500 になっていた (第 6 回 B-R6f)。money-path なので
+  // null だけを 400 にする (文字列/数値の本文は従来どおり invalid_chain・pro/subscribe と同じ線)。
+  if (capped.value === null) {
+    return NextResponse.json({ ok: false, error: 'invalid_json' }, { status: 400 });
+  }
   // 既存の分解代入・検証順序は不変に保つ (掟12: money-path は追加のみ)。
   const body = capped.value as {
     txHash?: unknown;
diff --git a/app/api/csv-pass/relay/route.ts b/app/api/csv-pass/relay/route.ts
index 55cc2bb6..d5e67a0c 100644
--- a/app/api/csv-pass/relay/route.ts
+++ b/app/api/csv-pass/relay/route.ts
@@ -104,6 +104,11 @@ export async function POST(req: Request): Promise<NextResponse> {
   } catch {
     return NextResponse.json({ ok: false, error: 'invalid_json' }, { status: 400 });
   }
+  // JSON.parse は `null` / 数値 / 文字列も返す。`null` のまま raw.chainId を読むと TypeError → 500 + Sentry
+  // になっていた (第 6 回 B-R6f)。object 以外は欄不足と同じ invalid_payload で 400 (relay/status も object 以外を拒否する・register/claim は invalid_body で配列も拒否)。
+  if (raw === null || typeof raw !== 'object') {
+    return NextResponse.json({ ok: false, error: 'invalid_payload' }, { status: 400 });
+  }
 
   if (typeof raw.chainId !== 'number' || !Number.isInteger(raw.chainId)) {
     return NextResponse.json({ ok: false, error: 'invalid_payload' }, { status: 400 });
diff --git a/app/api/freee/mapping/route.ts b/app/api/freee/mapping/route.ts
index 6f558415..fc312fcf 100644
--- a/app/api/freee/mapping/route.ts
+++ b/app/api/freee/mapping/route.ts
@@ -65,6 +65,11 @@ export async function POST(req: Request): Promise<NextResponse> {
   } catch {
     return NextResponse.json({ ok: false, error: 'invalid_json' }, { status: 400 });
   }
+  // JSON の `null` / 数値 / 文字列も parse は成功する。object でなければ欄を読む前に 400 で返す
+  // (null のまま欄を読むと TypeError → 500 + Sentry event になっていた・第 6 回 B-R6f)。
+  if (body === null || typeof body !== 'object') {
+    return NextResponse.json({ ok: false, error: 'invalid_json' }, { status: 400 });
+  }
   if (
     typeof body.accountItemId !== 'number' ||
     typeof body.taxCode !== 'number'
diff --git a/app/api/freee/sync/route.ts b/app/api/freee/sync/route.ts
index 212361f8..c54bd8e0 100644
--- a/app/api/freee/sync/route.ts
+++ b/app/api/freee/sync/route.ts
@@ -59,6 +59,11 @@ export async function POST(req: Request): Promise<NextResponse> {
   } catch {
     return NextResponse.json({ ok: false, error: 'invalid_json' }, { status: 400 });
   }
+  // JSON の `null` / 数値 / 文字列も parse は成功する。object でなければ欄を読む前に 400 で返す
+  // (null のまま欄を読むと TypeError → 500 + Sentry event になっていた・第 6 回 B-R6f)。
+  if (body === null || typeof body !== 'object') {
+    return NextResponse.json({ ok: false, error: 'invalid_json' }, { status: 400 });
+  }
   const entries = Array.isArray(body.entries) ? (body.entries as HistoryEntry[]) : [];
   if (entries.length === 0) {
     return NextResponse.json({ ok: false, error: 'no_entries' }, { status: 400 });
diff --git a/app/api/relay/jpyc/route.ts b/app/api/relay/jpyc/route.ts
index b9900535..74212b2b 100644
--- a/app/api/relay/jpyc/route.ts
+++ b/app/api/relay/jpyc/route.ts
@@ -220,6 +220,11 @@ export async function POST(req: Request): Promise<NextResponse> {
   } catch {
     return NextResponse.json({ ok: false, error: 'invalid_json' }, { status: 400 });
   }
+  // JSON.parse は `null` / 数値 / 文字列も返す。`null` のまま raw.chainId を読むと TypeError → 500 + Sentry
+  // になっていた (第 6 回 B-R6f)。object 以外は欄不足と同じ invalid_payload で 400 (relay/status も object 以外を拒否する・register/claim は invalid_body で配列も拒否)。
+  if (raw === null || typeof raw !== 'object') {
+    return NextResponse.json({ ok: false, error: 'invalid_payload' }, { status: 400 });
+  }
 
   if (typeof raw.chainId !== 'number' || !Number.isInteger(raw.chainId)) {
     return NextResponse.json({ ok: false, error: 'invalid_payload' }, { status: 400 });
diff --git a/tests/app/api/auth-siwe.test.ts b/tests/app/api/auth-siwe.test.ts
index ae4eec07..1f0a5dbd 100644
--- a/tests/app/api/auth-siwe.test.ts
+++ b/tests/app/api/auth-siwe.test.ts
@@ -237,6 +237,18 @@ describe('SIWE routes', () => {
     expect(h.kvSet).toHaveBeenCalledOnce();
   });
 
+  it('verify: 本文が JSON の null → 400 invalid_json (verify_failed の 503 + Sentry にしない・B-R6f)', async () => {
+    h.kvConfigured = true;
+    const headers = new Headers({ 'content-type': 'application/json' });
+    headers.set('sec-fetch-site', 'same-origin');
+    const res = await verifyPOST(new Request('http://localhost/api/auth/siwe/verify', {
+      method: 'POST', headers, body: 'null',
+    }));
+    expect(res.status).toBe(400);
+    expect(await res.json()).toEqual({ ok: false, error: 'invalid_json' });
+    expect(h.kvSet).not.toHaveBeenCalled();
+  });
+
   it('verify: JSON-shaped bytes without Content-Type still fail before session writes', async () => {
     h.kvConfigured = true;
     const res = await verifyPOST(new Request('http://localhost/api/auth/siwe/verify', {
diff --git a/tests/app/api/billing-settle.test.ts b/tests/app/api/billing-settle.test.ts
index 3ba963d8..dd7a4994 100644
--- a/tests/app/api/billing-settle.test.ts
+++ b/tests/app/api/billing-settle.test.ts
@@ -358,6 +358,12 @@ describe('POST /api/billing/settle', () => {
     expect(await res.json()).toMatchObject({ error: 'invalid_json' });
   });
 
+  it('本文が JSON の null → 400 invalid_json (分解代入の TypeError で 500 にしない・B-R6f)', async () => {
+    const res = await POST(req(null));
+    expect(res.status).toBe(400);
+    expect(await res.json()).toMatchObject({ error: 'invalid_json' });
+  });
+
   it('body 上限超過 → JSON parse 前に 413 payload_too_large', async () => {
     const huge = new Request('http://localhost/api/billing/settle', {
       method: 'POST',
diff --git a/tests/app/api/csv-pass-relay.test.ts b/tests/app/api/csv-pass-relay.test.ts
index 98589d44..329af41c 100644
--- a/tests/app/api/csv-pass-relay.test.ts
+++ b/tests/app/api/csv-pass-relay.test.ts
@@ -253,6 +253,12 @@ describe('POST /api/csv-pass/relay — route 固有検証', () => {
     expect(res.status).toBe(400);
     expect(await res.json()).toEqual({ ok: false, error: 'invalid_payload' });
   });
+
+  it('本文が JSON の null → 400 invalid_payload (TypeError の 500 にしない・B-R6f)', async () => {
+    const res = await POST(req(null));
+    expect(res.status).toBe(400);
+    expect(await res.json()).toEqual({ ok: false, error: 'invalid_payload' });
+  });
 });
 
 describe('POST /api/csv-pass/relay — relay 結果の応答整形 + guards 配線', () => {
diff --git a/tests/app/api/freee-routes-integration.test.ts b/tests/app/api/freee-routes-integration.test.ts
index 8a05275b..12f0fb8d 100644
--- a/tests/app/api/freee-routes-integration.test.ts
+++ b/tests/app/api/freee-routes-integration.test.ts
@@ -380,6 +380,23 @@ describe('GET/POST /api/freee/mapping (実グルー)', () => {
     expect(h.store.has(mappingKey)).toBe(false);
   });
 
+  it('POST: 本文が JSON の null → 400 invalid_json (TypeError の 500 にしない・B-R6f)', async () => {
+    seedSession();
+    seedFreeeConnected();
+    const res = await mappingPOST(req('http://localhost/api/freee/mapping', null));
+    expect(res.status).toBe(400);
+    expect(await res.json()).toEqual({ ok: false, error: 'invalid_json' });
+  });
+
+  it('sync: 本文が JSON の null → 400 invalid_json (TypeError の 500 にしない・B-R6f)', async () => {
+    seedSession();
+    seedFreeeConnected();
+    seedMapping();
+    const res = await syncPOST(req('http://localhost/api/freee/sync', null));
+    expect(res.status).toBe(400);
+    expect(await res.json()).toEqual({ ok: false, error: 'invalid_json' });
+  });
+
   it('POST: 不正 body → 400 invalid_mapping', async () => {
     seedSession();
     seedFreeeConnected();
diff --git a/tests/app/api/relay-jpyc.test.ts b/tests/app/api/relay-jpyc.test.ts
index b77b1846..b9b83051 100644
--- a/tests/app/api/relay-jpyc.test.ts
+++ b/tests/app/api/relay-jpyc.test.ts
@@ -63,6 +63,15 @@ describe('POST /api/relay/jpyc (env-gate)', () => {
     const body = (await res.json()) as { error?: string };
     expect(body.error).toBe('invalid_payload');
   });
+
+  it('本文が JSON の null → 400 invalid_payload (raw.chainId の TypeError で 500 にしない・B-R6f)', async () => {
+    vi.stubEnv(EIP3009_ON, '1');
+    vi.stubEnv('RELAYER_PRIVATE_KEY', DUMMY_RELAYER_KEY);
+    const mod = await import('@/app/api/relay/jpyc/route');
+    const res = await mod.POST(req(null));
+    expect(res.status).toBe(400);
+    expect(await res.json()).toEqual({ ok: false, error: 'invalid_payload' });
+  });
 });
 
 // recover (forwarder) × a1 (usage fee) の排他は resolver (jpycForwarderFor) で graceful に解決する: