diff --git a/.github/workflows/ci-infra.yml b/.github/workflows/ci-infra.yml
index 956e8c35e9..23ad533bcc 100644
--- a/.github/workflows/ci-infra.yml
+++ b/.github/workflows/ci-infra.yml
@@ -272,3 +272,127 @@ jobs:
       - name: Run npm audit
         run: npm audit --audit-level=moderate
         continue-on-error: true
+
+  # ---------------------------------------------------------------------------
+  # smalruby-classroom
+  # ---------------------------------------------------------------------------
+  smalruby-classroom-unit-tests:
+    name: "[classroom] Unit Tests"
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: infra/smalruby-classroom
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+      - uses: actions/setup-node@2028fbc5c25fe9cf00d9f06a71cc4710d4507903 # v6
+        with:
+          node-version-file: infra/smalruby-classroom/.node-version
+          cache: npm
+          cache-dependency-path: infra/smalruby-classroom/package-lock.json
+      - name: Install dependencies
+        run: npm ci
+      - name: Run unit tests
+        run: npm test
+
+  smalruby-classroom-cdk-build:
+    name: "[classroom] CDK Build & Synth"
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: infra/smalruby-classroom
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+      - uses: actions/setup-node@2028fbc5c25fe9cf00d9f06a71cc4710d4507903 # v6
+        with:
+          node-version-file: infra/smalruby-classroom/.node-version
+          cache: npm
+          cache-dependency-path: infra/smalruby-classroom/package-lock.json
+      - name: Install dependencies
+        run: npm ci
+      - name: Build TypeScript
+        run: npm run build
+      - name: Synthesize CDK stack
+        env:
+          CLASSROOM_CUSTOM_DOMAIN: 'false'
+        run: npx cdk synth --context stage=stg
+
+  smalruby-classroom-security:
+    name: "[classroom] Security Audit"
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: infra/smalruby-classroom
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+      - uses: actions/setup-node@2028fbc5c25fe9cf00d9f06a71cc4710d4507903 # v6
+        with:
+          node-version-file: infra/smalruby-classroom/.node-version
+          cache: npm
+          cache-dependency-path: infra/smalruby-classroom/package-lock.json
+      - name: Install dependencies
+        run: npm ci
+      - name: Run npm audit
+        run: npm audit --audit-level=moderate
+        continue-on-error: true
+
+  # ---------------------------------------------------------------------------
+  # smalruby-admin
+  # ---------------------------------------------------------------------------
+  smalruby-admin-unit-tests:
+    name: "[admin] Unit Tests"
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: infra/smalruby-admin
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+      - uses: actions/setup-node@2028fbc5c25fe9cf00d9f06a71cc4710d4507903 # v6
+        with:
+          node-version-file: infra/smalruby-admin/.node-version
+          cache: npm
+          cache-dependency-path: infra/smalruby-admin/package-lock.json
+      - name: Install dependencies
+        run: npm ci
+      - name: Run unit tests
+        run: npm test
+
+  smalruby-admin-cdk-build:
+    name: "[admin] CDK Build & Synth"
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: infra/smalruby-admin
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+      - uses: actions/setup-node@2028fbc5c25fe9cf00d9f06a71cc4710d4507903 # v6
+        with:
+          node-version-file: infra/smalruby-admin/.node-version
+          cache: npm
+          cache-dependency-path: infra/smalruby-admin/package-lock.json
+      - name: Install dependencies
+        run: npm ci
+      - name: Build TypeScript
+        run: npm run build
+      - name: Synthesize CDK stack
+        env:
+          ADMIN_CUSTOM_DOMAIN: 'false'
+        run: npx cdk synth --context stage=stg
+
+  smalruby-admin-security:
+    name: "[admin] Security Audit"
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: infra/smalruby-admin
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+      - uses: actions/setup-node@2028fbc5c25fe9cf00d9f06a71cc4710d4507903 # v6
+        with:
+          node-version-file: infra/smalruby-admin/.node-version
+          cache: npm
+          cache-dependency-path: infra/smalruby-admin/package-lock.json
+      - name: Install dependencies
+        run: npm ci
+      - name: Run npm audit
+        run: npm audit --audit-level=moderate
+        continue-on-error: true
diff --git a/infra/smalruby-admin/.node-version b/infra/smalruby-admin/.node-version
new file mode 100644
index 0000000000..b404027604
--- /dev/null
+++ b/infra/smalruby-admin/.node-version
@@ -0,0 +1 @@
+24.8.0
diff --git a/infra/smalruby-admin/lambda/tests/handler-notifications.test.ts b/infra/smalruby-admin/lambda/tests/handler-notifications.test.ts
index c206038ff4..498cd64c22 100644
--- a/infra/smalruby-admin/lambda/tests/handler-notifications.test.ts
+++ b/infra/smalruby-admin/lambda/tests/handler-notifications.test.ts
@@ -1,3 +1,7 @@
+// このファイルをモジュールにするための宣言。import/export を持たない .ts は TypeScript の
+// グローバルスクリプト扱いになり、トップレベルの const がテストファイル間で衝突する（#1144）。
+export {};
+
 /**
  * お知らせ送信 API tests — EPIC #1111.
  *
diff --git a/infra/smalruby-admin/lambda/tests/handler-recommend-sharing.test.ts b/infra/smalruby-admin/lambda/tests/handler-recommend-sharing.test.ts
index 519ad75d1a..8fd140422e 100644
--- a/infra/smalruby-admin/lambda/tests/handler-recommend-sharing.test.ts
+++ b/infra/smalruby-admin/lambda/tests/handler-recommend-sharing.test.ts
@@ -1,3 +1,7 @@
+// このファイルをモジュールにするための宣言。import/export を持たない .ts は TypeScript の
+// グローバルスクリプト扱いになり、トップレベルの const がテストファイル間で衝突する（#1144）。
+export {};
+
 /**
  * 共有推奨 API tests — EPIC #1106.
  *
diff --git a/infra/smalruby-admin/lambda/tests/handler-shared-recommendation.test.ts b/infra/smalruby-admin/lambda/tests/handler-shared-recommendation.test.ts
index ded1c59009..fc18dea623 100644
--- a/infra/smalruby-admin/lambda/tests/handler-shared-recommendation.test.ts
+++ b/infra/smalruby-admin/lambda/tests/handler-shared-recommendation.test.ts
@@ -1,3 +1,7 @@
+// このファイルをモジュールにするための宣言。import/export を持たない .ts は TypeScript の
+// グローバルスクリプト扱いになり、トップレベルの const がテストファイル間で衝突する（#1144）。
+export {};
+
 /**
  * Admin 推薦 API tests — EPIC #1110.
  *
diff --git a/infra/smalruby-admin/lambda/tests/handler.test.ts b/infra/smalruby-admin/lambda/tests/handler.test.ts
index d9cf8f3fdf..fd887e2033 100644
--- a/infra/smalruby-admin/lambda/tests/handler.test.ts
+++ b/infra/smalruby-admin/lambda/tests/handler.test.ts
@@ -1,3 +1,7 @@
+// このファイルをモジュールにするための宣言。import/export を持たない .ts は TypeScript の
+// グローバルスクリプト扱いになり、トップレベルの const がテストファイル間で衝突する（#1144）。
+export {};
+
 /**
  * Admin API authorization tests (EPIC #1073, S1 #1081).
  *
diff --git a/infra/smalruby-admin/lambda/tests/test-files-are-modules.test.ts b/infra/smalruby-admin/lambda/tests/test-files-are-modules.test.ts
new file mode 100644
index 0000000000..f0e0b3df85
--- /dev/null
+++ b/infra/smalruby-admin/lambda/tests/test-files-are-modules.test.ts
@@ -0,0 +1,45 @@
+/**
+ * テストファイルがモジュールであることを担保する回帰テスト (issue #1144)。admin も同じ構造だったため CI 追加時に検出した (#1155)。
+ *
+ * `import` / `export` を 1 つも持たない .ts ファイルは TypeScript では「モジュール」ではなく
+ * グローバルスクリプトとして扱われる。その場合トップレベルの const がファイルを跨いで同じ
+ * スコープに宣言されるため、`mockSend` のような同名の変数を別ファイルで宣言した瞬間に
+ * TS2451 (Cannot redeclare block-scoped variable) になる。ts-jest はキャッシュが温かい間は
+ * 型チェックを再利用するので、この診断は cold cache のときだけ suite ごと落ちる = フレークに
+ * 見えていた。
+ */
+import * as fs from 'fs';
+import * as path from 'path';
+import * as ts from 'typescript';
+
+const testsDir = __dirname;
+
+const collectTsFiles = (dir: string): string[] =>
+  fs.readdirSync(dir, { withFileTypes: true }).flatMap((entry) => {
+    const full = path.join(dir, entry.name);
+    if (entry.isDirectory()) return collectTsFiles(full);
+    return entry.isFile() && full.endsWith('.ts') ? [full] : [];
+  });
+
+describe('lambda/tests の .ts ファイル', () => {
+  const files = collectTsFiles(testsDir);
+
+  test('検査対象のファイルが見つかる', () => {
+    expect(files.length).toBeGreaterThan(0);
+  });
+
+  test.each(files.map((file) => [path.relative(testsDir, file), file]))(
+    '%s はモジュール (import/export を持つ) である',
+    (_relative, file) => {
+      const source = ts.createSourceFile(
+        file,
+        fs.readFileSync(file, 'utf8'),
+        ts.ScriptTarget.ES2020,
+        /* setParentNodes */ false,
+        ts.ScriptKind.TS,
+      );
+
+      expect(ts.isExternalModule(source)).toBe(true);
+    },
+  );
+});
diff --git a/infra/smalruby-classroom/.node-version b/infra/smalruby-classroom/.node-version
new file mode 100644
index 0000000000..b404027604
--- /dev/null
+++ b/infra/smalruby-classroom/.node-version
@@ -0,0 +1 @@
+24.8.0