diff --git a/.github/scripts/ci_changes.py b/.github/scripts/ci_changes.py
index 8bcea765..92fe4526 100644
--- a/.github/scripts/ci_changes.py
+++ b/.github/scripts/ci_changes.py
@@ -126,6 +126,8 @@ def classify(paths):
                     "tests/unit/",
                     "tests/integration/",
                     "tests/dispatch/",
+                    "tests/allocation/",
+                    "tests/failures/",
                     "tests/common/",
                     "tests/consumer/",
                     "tests/core_consumer/",
diff --git a/.github/scripts/test_ci_changes.py b/.github/scripts/test_ci_changes.py
index f5b65a1a..82cae3d9 100644
--- a/.github/scripts/test_ci_changes.py
+++ b/.github/scripts/test_ci_changes.py
@@ -115,6 +115,8 @@ def test_language_specific_changes(self):
         cases = {
             "tests/unit/rabitqlib/index/ivf_test.cpp": {"cpp", "cpp_format"},
             "tests/dispatch/fastscan_no_simd_test.cpp": {"cpp", "cpp_format"},
+            "tests/allocation/core_api_no_alloc.cpp": {"cpp", "cpp_format"},
+            "tests/failures/construction_failure_test.cpp": {"cpp", "cpp_format"},
             "sample/cpp/ivf_rabitq_querying.cpp": {"cpp", "cpp_format"},
             "python_bindings/ivf_bindings.cpp": {
                 "benchmarks",
diff --git a/.github/scripts/test_ci_workflow_contracts.py b/.github/scripts/test_ci_workflow_contracts.py
index a147dfe1..5223163b 100644
--- a/.github/scripts/test_ci_workflow_contracts.py
+++ b/.github/scripts/test_ci_workflow_contracts.py
@@ -15,6 +15,74 @@
 
 
 class WorkflowContractsTest(unittest.TestCase):
+    def test_format_checker_propagates_file_list_failures(self):
+        with tempfile.TemporaryDirectory() as directory:
+            root = Path(directory)
+            (root / "scripts").mkdir()
+            (root / "bin").mkdir()
+            git = root / "bin/git"
+            git.write_text(f"#!{sys.executable}\nprint({str(root)!r})\n")
+            git.chmod(0o755)
+            formatter = root / "bin/formatter"
+            log = root / "formatted.json"
+            formatter.write_text(
+                f"#!{sys.executable}\n"
+                "import json, os, sys\n"
+                "from pathlib import Path\n"
+                "if sys.argv[1:] == ['--version']:\n"
+                "    print('clang-format version 23.1.4')\n"
+                "else:\n"
+                "    Path(os.environ['FORMAT_LOG']).write_text(json.dumps(sys.argv[1:]))\n"
+                "    sys.exit(int(os.environ.get('FORMAT_STATUS', '0')))\n"
+            )
+            formatter.chmod(0o755)
+            env = dict(
+                os.environ,
+                PATH=str(root / "bin") + os.pathsep + os.environ["PATH"],
+                CLANG_FORMAT=str(formatter),
+                FORMAT_LOG=str(log),
+            )
+            helper = root / "scripts/clang-format-files.sh"
+            filename = str(root / "space and\nnewline.cpp")
+            for mode, content, status, expected in (
+                ("missing", None, "0", False),
+                ("failed", "sys.exit(7)\n", "0", False),
+                (
+                    "partial",
+                    f"sys.stdout.write({filename!r} + '\\0')\nsys.exit(7)\n",
+                    "0",
+                    False,
+                ),
+                ("empty", "sys.exit(0)\n", "0", True),
+                ("valid", f"sys.stdout.write({filename!r} + '\\0')\n", "0", True),
+                (
+                    "format-error",
+                    f"sys.stdout.write({filename!r} + '\\0')\n",
+                    "1",
+                    False,
+                ),
+            ):
+                with self.subTest(mode=mode):
+                    if content is not None:
+                        helper.write_text(f"#!{sys.executable}\nimport sys\n" + content)
+                        helper.chmod(0o755)
+                    log.unlink(missing_ok=True)
+                    result = subprocess.run(
+                        ["bash", str(ROOT / "scripts/check-format.sh")],
+                        env=dict(env, FORMAT_STATUS=status),
+                        capture_output=True,
+                        text=True,
+                        check=False,
+                    )
+                    self.assertEqual(result.returncode == 0, expected, result.stderr)
+                    if mode in ("valid", "format-error"):
+                        self.assertEqual(
+                            json.loads(log.read_text()),
+                            ["--dry-run", "--Werror", filename],
+                        )
+                    else:
+                        self.assertFalse(log.exists())
+
     def test_benchmark_support_changes_keep_protocol_coverage_without_builds(self):
         for path in sorted((ROOT / "benchmarks").glob("*.py")):
             with self.subTest(path=path.name):
diff --git a/.github/scripts/test_pr_labels.py b/.github/scripts/test_pr_labels.py
index 08a3e266..9923a3a2 100644
--- a/.github/scripts/test_pr_labels.py
+++ b/.github/scripts/test_pr_labels.py
@@ -160,6 +160,8 @@ def test_api_failures_are_not_silenced(self):
     def test_workflow_uses_metadata_only_and_routes_to_regression_checks(self):
         text = WORKFLOW.read_text()
         self.assertIn("pull_request_target:", text)
+        self.assertIn("  pull-requests: write\n", text)
+        self.assertIn("  issues: write\n", text)
         self.assertIn("types: [opened, edited, reopened, ready_for_review]", text)
         self.assertNotIn("actions/checkout", text)
         self.assertNotIn("${{", SCRIPT)
diff --git a/.github/workflows/pr-labels.yml b/.github/workflows/pr-labels.yml
index 41415b30..b432357c 100644
--- a/.github/workflows/pr-labels.yml
+++ b/.github/workflows/pr-labels.yml
@@ -9,7 +9,8 @@ concurrency:
   cancel-in-progress: false
 
 permissions:
-  pull-requests: read
+  # PR label assignment/removal and repository label creation need separate scopes.
+  pull-requests: write
   issues: write
 
 jobs:
diff --git a/.github/workflows/test.yaml b/.github/workflows/test.yaml
index eb3d2aba..02d77614 100644
--- a/.github/workflows/test.yaml
+++ b/.github/workflows/test.yaml
@@ -187,9 +187,14 @@ jobs:
       CC: clang-23
       CXX: clang++-23
       OMP_NUM_THREADS: "2"
+      CMAKE_CXX_COMPILER_LAUNCHER: ccache
     steps:
       - uses: actions/checkout@v7
       - uses: ./.github/actions/setup-clang
+      - name: Install compiler cache
+        run: |
+          sudo apt-get install -y ccache
+          echo "CCACHE_DIR=$RUNNER_TEMP/ccache" >> "$GITHUB_ENV"
       - name: Configure portable build
         run: >-
           cmake -S . -B build-clang -G Ninja
@@ -217,6 +222,9 @@ jobs:
           -DWORK_DIR="$RUNNER_TEMP/rabitq-clang-core"
           -DRABITQ_USE_OPENMP=${{ matrix.openmp }}
           -P tests/core_consumer/run.cmake
+      - name: Report compiler cache statistics
+        if: ${{ always() }}
+        run: ccache --show-stats
 
   clang-result:
     name: Clang validation
@@ -370,9 +378,21 @@ jobs:
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v7
-      - name: Install Apple OpenMP
-        if: runner.os == 'macOS' && matrix.openmp == 'ON'
-        run: brew install libomp
+      - name: Install Linux compiler cache
+        if: runner.os == 'Linux'
+        run: sudo apt-get update -qq && sudo apt-get install -y ccache
+      - name: Install Apple compiler cache and optional OpenMP
+        if: runner.os == 'macOS'
+        run: |
+          brew install ccache
+          if [ "${{ matrix.openmp }}" = ON ]; then
+            brew install libomp
+          fi
+      - name: Configure compiler cache
+        if: runner.os != 'Windows'
+        run: |
+          echo "CMAKE_CXX_COMPILER_LAUNCHER=ccache" >> "$GITHUB_ENV"
+          echo "CCACHE_DIR=$RUNNER_TEMP/ccache" >> "$GITHUB_ENV"
       - name: Test external core-only integrations (Linux)
         if: runner.os == 'Linux'
         run: >-
@@ -398,6 +418,9 @@ jobs:
           "-DWORK_DIR=$env:RUNNER_TEMP/rabitq-core"
           -DRABITQ_USE_OPENMP=${{ matrix.openmp }}
           -P tests/core_consumer/run.cmake
+      - name: Report compiler cache statistics
+        if: ${{ always() && runner.os != 'Windows' }}
+        run: ccache --show-stats
 
   core-consumer-result:
     name: Core and FastScan validation
diff --git a/docs/maintaining.md b/docs/maintaining.md
index 0fd77d12..c6a74fa5 100644
--- a/docs/maintaining.md
+++ b/docs/maintaining.md
@@ -22,6 +22,7 @@ only the affected job groups:
 | `tests/core_consumer/`, `tests/fixtures/core_api_v1/` | C++ checks; the frozen v1 fixture is excluded from formatting |
 | C++ headers or library sources | C++ formatting, clang-tidy, platform tests, sanitizers, consumer build, and Python builds/wheels |
 | C++ tests or examples | C++ formatting and platform tests; no clang-tidy |
+| `tests/allocation/`, `tests/failures/` | C++ formatting and platform tests, including sanitizer and allocation-failure coverage; no Python builds or wheels |
 | Compiled Python bindings | C++ formatting and clang-tidy, plus Python builds and wheel tests |
 | Python tests | Python lint, source-install smoke tests, and wheel tests |
 | `tests/python/test_cpp_clustering_examples.py` | Python lint and C++ checks; examples run in the Ubuntu job |
@@ -46,6 +47,13 @@ Pushes compare the complete pushed range; pull requests compare against
 their base. Renames and deletions are included. Relevant jobs also run if
 change detection fails, so a detection error cannot silently waive a check.
 
+Linux and macOS core/FastScan consumer jobs and the Clang matrix use a job-local
+`ccache` through CMake's compiler launcher. Independent producer, installed-package,
+`add_subdirectory`, and FetchContent builds and tests are retained; identical
+compilations can reuse compiler output within the job. Cache statistics are printed
+after testing. Compiler caches are not uploaded or shared between workflow runs.
+Windows keeps its existing build path without a compiler cache.
+
 Release tags and manual wheel builds always run the full wheel build. CI
 routing edits run the full relevant matrix on pull requests. After an already
 released version reaches `main`, a push containing routing edits avoids repeated
diff --git a/scripts/check-format.sh b/scripts/check-format.sh
index f196075d..0170fcc8 100755
--- a/scripts/check-format.sh
+++ b/scripts/check-format.sh
@@ -17,7 +17,14 @@ if [[ "$($formatter --version)" != *"version 23."* ]]; then
     exit 1
 fi
 
-mapfile -d '' files < <("$repo_root/scripts/clang-format-files.sh" "$@")
+# Process substitution would discard the file-list helper's exit status.
+file_list="$(mktemp)"
+trap 'rm -f -- "$file_list"' EXIT
+if ! "$repo_root/scripts/clang-format-files.sh" "$@" > "$file_list"; then
+    echo "error: could not list files for formatting" >&2
+    exit 1
+fi
+mapfile -d '' files < "$file_list"
 
 if printf '%s\n' "${files[@]}" \
     | grep -q '/include/rabitqlib/third/'; then