diff --git a/.github/workflows/ci-linux.yml b/.github/workflows/ci-linux.yml
index f6c269978..56093a067 100644
--- a/.github/workflows/ci-linux.yml
+++ b/.github/workflows/ci-linux.yml
@@ -35,7 +35,8 @@ jobs:
       fail-fast: true
     env:
       CACHE_DIR: ${{ github.workspace }}/.container-cache
-      CACHE_KEY: linux-build-test-cpp-asserts-manylinux-v2-${{ github.event.number || github.ref_name }}
+      # either the PR number or `branch-N` where N always increments
+      CACHE_KEY: linux-build-test-cpp-asserts-manylinux-v2-${{ github.event.number || format('{0}-{1}', github.ref_name, github.run_number) }}
     steps:
       - name: Set unified TZ
         uses: szenius/set-timezone@v2.0
diff --git a/.github/workflows/ci-macos.yml b/.github/workflows/ci-macos.yml
index a94d84b69..a86d11c56 100644
--- a/.github/workflows/ci-macos.yml
+++ b/.github/workflows/ci-macos.yml
@@ -30,7 +30,7 @@ jobs:
         runs-on: [macos-12, macos-14]
     env:
       CACHE_DIR: ${{ github.workspace }}/.container-cache
-      CACHE_KEY: ${{ matrix.runs-on }}-build-test-cpp-asserts-v1-${{ github.event.number || github.ref_name }}
+      CACHE_KEY: ${{ matrix.runs-on }}-build-test-cpp-asserts-v1-${{ github.event.number || format('{0}-{1}', github.ref_name, github.run_number) }}
     steps:
       - name: Set unified TZ
         uses: szenius/set-timezone@v2.0
diff --git a/.github/workflows/ci-windows.yml b/.github/workflows/ci-windows.yml
index 07e5e3248..d4e3c3899 100644
--- a/.github/workflows/ci-windows.yml
+++ b/.github/workflows/ci-windows.yml
@@ -33,7 +33,7 @@ jobs:
       fail-fast: true
     env:
       CACHE_DIR: ${{ github.workspace }}/.container-cache
-      CACHE_KEY: windows-build-test-cpp-asserts-v1-${{ github.event.number || github.ref_name }}
+      CACHE_KEY: windows-build-test-cpp-asserts-v1-${{ github.event.number || format('{0}-{1}', github.ref_name, github.run_number) }}
     steps:
       - name: Set unified TZ
         uses: szenius/set-timezone@v2.0
diff --git a/build_tools/ci/build_test_cpp.sh b/build_tools/ci/build_test_cpp.sh
index 65c61332a..ea1929cde 100644
--- a/build_tools/ci/build_test_cpp.sh
+++ b/build_tools/ci/build_test_cpp.sh
@@ -40,6 +40,16 @@ export CCACHE_DIR="${cache_dir}/ccache"
 export CCACHE_MAXSIZE="700M"
 export CMAKE_C_COMPILER_LAUNCHER=ccache
 export CMAKE_CXX_COMPILER_LAUNCHER=ccache
+# https://interrupt.memfault.com/blog/ccache-debugging
+export CCACHE_DEBUG=1
+export CCACHE_DEBUGLEVEL=1
+if [[ "$OSTYPE" == "linux-gnu"* ]]; then
+  export CCACHE_DEBUGDIR="/ccache-debug"
+elif [[ "$OSTYPE" == "msys"* ]]; then
+  export CCACHE_DEBUGDIR="/c/ccache-debug"
+fi
+export CCACHE_SLOPPINESS=include_file_ctime,include_file_mtime
+export CCACHE_COMPILERCHECK=content
 
 # Clear ccache stats.
 ccache -z
@@ -116,7 +126,8 @@ elif [[ "$OSTYPE" == "msys"* ]]; then
 fi
 
 # Show ccache stats.
-ccache --show-stats
+ccache --show-stats -v
+grep -r -B15 'Result: .*_miss' $CCACHE_DEBUGDIR
 
 rm -f "$install_dir"/bin/clang*
 rm -f "$install_dir"/bin/llvm-link*