diff --git a/.github/workflows/_test.yml b/.github/workflows/_test.yml
index a610bcfa..4df2950d 100644
--- a/.github/workflows/_test.yml
+++ b/.github/workflows/_test.yml
@@ -81,12 +81,39 @@ jobs:
 
       - name: Install deps (Ubuntu)
         if: startsWith(matrix.os, 'ubuntu')
-        run: sudo apt-get update && sudo apt-get install -y zlib1g-dev
+        run: sudo apt-get update && sudo apt-get install -y zlib1g-dev ccache
+
+      - name: Install ccache (macOS)
+        if: startsWith(matrix.os, 'macos')
+        run: command -v ccache >/dev/null 2>&1 || brew install ccache
+
+      # Verified compiler cache: CCACHE_COMPILERCHECK=content keys every entry
+      # on the compiler-binary CONTENT plus the fully preprocessed input, so a
+      # hit is provably the identical compilation — a stale or foreign cache
+      # can only miss, never return wrong output (see scripts/env.sh).
+      # Keys embed github.ref on top of GitHub's own branch scoping: caches are
+      # STRICTLY per-ref by policy — no base-branch fallback, a new PR builds
+      # cold once and only its own pushes warm it.
+      - name: Compiler cache (content-verified)
+        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: ${{ github.workspace }}/.ccache
+          key: ccache-test-${{ matrix.os }}-${{ matrix.cc }}-${{ github.ref }}-${{ github.sha }}
+          restore-keys: |
+            ccache-test-${{ matrix.os }}-${{ matrix.cc }}-${{ github.ref }}-
 
       - name: Test
         run: scripts/test.sh CC=${{ matrix.cc }} CXX=${{ matrix.cxx }}
         env:
           CBM_SKIP_PERF: ${{ inputs.skip_perf && '1' || '' }}
+          CCACHE_DIR: ${{ github.workspace }}/.ccache
+          CCACHE_MAXSIZE: 1500M
+
+      - name: ccache stats
+        if: always()
+        run: ccache -s || true
+        env:
+          CCACHE_DIR: ${{ github.workspace }}/.ccache
 
   test-tsan:
     runs-on: ubuntu-latest
@@ -95,10 +122,33 @@ jobs:
       - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
 
       - name: Install deps (Ubuntu)
-        run: sudo apt-get update && sudo apt-get install -y clang zlib1g-dev
+        run: sudo apt-get update && sudo apt-get install -y clang zlib1g-dev ccache
+
+      - name: Compiler cache (content-verified)
+        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: ${{ github.workspace }}/.ccache
+          key: ccache-tsan-${{ github.ref }}-${{ github.sha }}
+          restore-keys: |
+            ccache-tsan-${{ github.ref }}-
 
       - name: ThreadSanitizer tests
-        run: make -f Makefile.cbm test-tsan CC=clang CXX=clang++
+        # This job calls make directly (no env.sh), so route the compilers
+        # through ccache's Debian masquerade dir; update-ccache-symlinks in the
+        # ccache postinst has already linked the clang installed above.
+        run: |
+          export PATH=/usr/lib/ccache:$PATH
+          export CCACHE_COMPILERCHECK=content
+          make -f Makefile.cbm test-tsan CC=clang CXX=clang++
+        env:
+          CCACHE_DIR: ${{ github.workspace }}/.ccache
+          CCACHE_MAXSIZE: 1500M
+
+      - name: ccache stats
+        if: always()
+        run: ccache -s || true
+        env:
+          CCACHE_DIR: ${{ github.workspace }}/.ccache
 
   test-windows:
     needs: setup-matrix
@@ -119,9 +169,18 @@ jobs:
             mingw-w64-clang-${{ matrix.pkg }}-clang
             mingw-w64-clang-${{ matrix.pkg }}-compiler-rt
             mingw-w64-clang-${{ matrix.pkg }}-zlib
+            mingw-w64-clang-${{ matrix.pkg }}-ccache
             make
             git
 
+      - name: Compiler cache (content-verified)
+        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: ${{ github.workspace }}/.ccache
+          key: ccache-test-${{ matrix.os }}-${{ matrix.msystem }}-${{ github.ref }}-${{ github.sha }}
+          restore-keys: |
+            ccache-test-${{ matrix.os }}-${{ matrix.msystem }}-${{ github.ref }}-
+
       - name: Test
         shell: msys2 {0}
         # AddressSanitizer is unavailable on native ARM64 Windows (LLVM ships no
@@ -133,6 +192,15 @@ jobs:
         run: scripts/test.sh CC=clang CXX=clang++ ${{ matrix.os == 'windows-11-arm' && 'SANITIZE=' || '' }}
         env:
           CBM_SKIP_PERF: ${{ inputs.skip_perf && '1' || '' }}
+          CCACHE_DIR: ${{ github.workspace }}/.ccache
+          CCACHE_MAXSIZE: 1500M
+
+      - name: ccache stats
+        if: always()
+        shell: msys2 {0}
+        run: ccache -s || true
+        env:
+          CCACHE_DIR: ${{ github.workspace }}/.ccache
 
   # Windows product-surface regression guards. Distinct from test-windows above
   # (the sanitizer C suite): these drive a real product binary + embedded HTTP UI
@@ -155,6 +223,7 @@ jobs:
           install: >-
             mingw-w64-clang-x86_64-clang
             mingw-w64-clang-x86_64-zlib
+            mingw-w64-clang-x86_64-ccache
             make
             git
 
@@ -162,11 +231,22 @@ jobs:
         with:
           node-version: "22"
 
+      - name: Compiler cache (content-verified)
+        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: ${{ github.workspace }}/.ccache
+          key: ccache-guards-${{ github.ref }}-${{ github.sha }}
+          restore-keys: |
+            ccache-guards-${{ github.ref }}-
+
       - name: Build product binary with embedded UI
         shell: msys2 {0}
         # --with-ui builds the frontend (npm) and embeds it, so the drive-picker
         # guard's HTTP UI is available. Functional gate only (no sanitizers).
         run: scripts/build.sh --with-ui CC=clang CXX=clang++
+        env:
+          CCACHE_DIR: ${{ github.workspace }}/.ccache
+          CCACHE_MAXSIZE: 1500M
 
       - name: Windows regression guards (#636/#357, #618, #548, #423/#20)
         shell: pwsh
diff --git a/.github/workflows/pr.yml b/.github/workflows/pr.yml
index 8d49dc6e..a40e8aad 100644
--- a/.github/workflows/pr.yml
+++ b/.github/workflows/pr.yml
@@ -82,7 +82,11 @@ jobs:
 
       - name: Install deps (Ubuntu)
         if: matrix.os == 'ubuntu-latest'
-        run: sudo apt-get update && sudo apt-get install -y zlib1g-dev
+        run: sudo apt-get update && sudo apt-get install -y zlib1g-dev ccache
+
+      - name: Install ccache (macOS)
+        if: matrix.os == 'macos-14'
+        run: command -v ccache >/dev/null 2>&1 || brew install ccache
 
       - uses: msys2/setup-msys2@66cd2cce69caa17b53920067426061ca1de3a884 # v2
         if: matrix.os == 'windows-latest'
@@ -93,14 +97,28 @@ jobs:
             mingw-w64-clang-x86_64-clang
             mingw-w64-clang-x86_64-zlib
             mingw-w64-clang-x86_64-python3
+            mingw-w64-clang-x86_64-ccache
             make
             coreutils
 
+      # Verified compiler cache — content-keyed, stale hits impossible by
+      # construction (see scripts/env.sh).
+      - name: Compiler cache (content-verified)
+        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: ${{ github.workspace }}/.ccache
+          key: ccache-smoke-${{ matrix.os }}-${{ github.ref }}-${{ github.sha }}
+          restore-keys: |
+            ccache-smoke-${{ matrix.os }}-${{ github.ref }}-
+
       - name: Build prod + smoke (Ubuntu)
         if: matrix.os == 'ubuntu-latest'
         run: |
           scripts/build.sh CC=gcc CXX=g++
           scripts/smoke-test.sh "$(pwd)/build/c/codebase-memory-mcp"
+        env:
+          CCACHE_DIR: ${{ github.workspace }}/.ccache
+          CCACHE_MAXSIZE: 1000M
 
       - name: Build prod + smoke (macOS)
         if: matrix.os == 'macos-14'
@@ -108,6 +126,9 @@ jobs:
           scripts/build.sh CC=cc CXX=c++
           codesign --sign - --force build/c/codebase-memory-mcp
           scripts/smoke-test.sh "$(pwd)/build/c/codebase-memory-mcp"
+        env:
+          CCACHE_DIR: ${{ github.workspace }}/.ccache
+          CCACHE_MAXSIZE: 1000M
 
       - name: Build prod + smoke (Windows)
         if: matrix.os == 'windows-latest'
@@ -117,6 +138,9 @@ jobs:
           BIN="$(pwd)/build/c/codebase-memory-mcp"
           [ -f "${BIN}.exe" ] && BIN="${BIN}.exe"
           scripts/smoke-test.sh "$BIN"
+        env:
+          CCACHE_DIR: ${{ github.workspace }}/.ccache
+          CCACHE_MAXSIZE: 1000M
 
   ci-ok:
     # The one required context (besides dco) — fails unless every PR stage
diff --git a/scripts/env.sh b/scripts/env.sh
index f7922dd9..e23affe4 100755
--- a/scripts/env.sh
+++ b/scripts/env.sh
@@ -98,6 +98,33 @@ if [[ -z "${CC:-}" ]]; then
     fi
 fi
 
+# ── Verified compiler cache (ccache, opt-out CBM_NO_CCACHE=1) ──
+# Activated through ccache's masquerade directories so $CC keeps its plain
+# name everywhere (verify_compiler, make, link lines are untouched).
+# Zero-staleness guarantee: CCACHE_COMPILERCHECK=content keys every entry on
+# the CONTENT of the compiler binary plus the fully preprocessed translation
+# unit, so a cache hit is provably the identical compilation — a stale or
+# foreign cache can only MISS, never return wrong output. No CCACHE_BASEDIR
+# and no path rewriting: debug-info and sanitizer report paths stay exact.
+if [[ "${CBM_NO_CCACHE:-0}" != "1" ]] && command -v ccache >/dev/null 2>&1; then
+    for _cbm_ccache_masq in \
+        /usr/lib/ccache \
+        /opt/homebrew/opt/ccache/libexec \
+        /usr/local/opt/ccache/libexec \
+        /clang64/lib/ccache/bin \
+        /clangarm64/lib/ccache/bin; do
+        if [[ -d "$_cbm_ccache_masq" ]]; then
+            case ":$PATH:" in
+            *":$_cbm_ccache_masq:"*) ;;
+            *) PATH="$_cbm_ccache_masq:$PATH" ;;
+            esac
+        fi
+    done
+    unset _cbm_ccache_masq
+    export PATH
+    export CCACHE_COMPILERCHECK=content
+fi
+
 # ── Print environment summary ──────────────────────────────────
 print_env() {
     local context="$1"