diff --git a/.github/actions/setup-linux-dependencies/action.yml b/.github/actions/setup-linux-dependencies/action.yml
index 4f33a8322c..7f2325223d 100644
--- a/.github/actions/setup-linux-dependencies/action.yml
+++ b/.github/actions/setup-linux-dependencies/action.yml
@@ -1,129 +1,20 @@
 name: Setup Linux dependencies
-description: Install and cache the system packages required to build ReScript.
+description: Install the system packages required to build ReScript.
 
 runs:
   using: composite
   steps:
-    # This is a small, repository-specific version of the filesystem cache used
-    # by awalsh128/cache-apt-pkgs-action@v1.4.3. Restoring the installed files is
-    # considerably faster than downloading and installing the .deb files again.
-    - name: Resolve package versions
-      id: packages
+    # Installed with apt, so that dpkg knows them: opam checks system
+    # packages with dpkg, and an earlier cache that unpacked these files
+    # without dpkg made opam take them for missing. musl-tools is needed for
+    # the static builds; bubblewrap and rsync for opam builds in a restored
+    # environment, such as coverage's bisect_ppx pin; cmake for OPAM.
+    # ocaml/setup-ocaml installs its own packages, such as the multilib ones,
+    # when it builds an environment.
+    - name: Install system dependencies
       shell: bash
-      env:
-        # Dependencies required by setup-ocaml, plus cmake for OPAM. The
-        # multilib packages are only available on x64 runners.
-        PACKAGES: >-
-          bubblewrap musl-tools rsync cmake
-          ${{ runner.arch == 'X64' && 'g++-multilib gcc-multilib' || '' }}
-        CACHE_VERSION: v5
       run: |
-        set -euo pipefail
-
-        read -ra packages <<< "$PACKAGES"
-        mapfile -t packages < <(printf '%s\n' "${packages[@]}" | sort -u)
-
-        candidate() {
-          local output
-          output="$(apt-cache policy "$1" 2>&1)" || {
-            echo "apt-cache policy $1 failed with exit code $?:" >&2
-            printf '%s\n' "$output" >&2
-            return 1
-          }
-          awk '/Candidate:/ { print $2; exit }' <<< "$output"
-        }
-
-        # A runner image may ship without package lists, in which case no
-        # candidate resolves until the lists are fetched.
-        updated=false
-        resolved_packages=()
-        for package in "${packages[@]}"; do
-          version="$(candidate "$package")" || version=""
-          if [[ ( -z "$version" || "$version" == "(none)" ) && "$updated" == false ]]; then
-            sudo apt-get update
-            updated=true
-            version="$(candidate "$package")" || version=""
-          fi
-          if [[ -z "$version" || "$version" == "(none)" ]]; then
-            echo "Unable to resolve an APT candidate for $package" >&2
-            exit 1
-          fi
-          resolved_packages+=("$package=$version")
-        done
-
-        cache_key="$({
-          printf '%s\n' "$CACHE_VERSION"
-          (. /etc/os-release; printf '%s\n' "$ID" "$VERSION_ID")
-          dpkg --print-architecture
-          printf '%s\n' "${resolved_packages[@]}"
-        } | sha256sum | cut -d' ' -f1)"
-
-        cache_dir="$RUNNER_TEMP/cache-apt-pkgs"
-        mkdir -p "$cache_dir"
-        printf '%s\n' "${resolved_packages[@]}" > "$cache_dir/requested-packages.txt"
-        echo "cache-dir=$cache_dir" >> "$GITHUB_OUTPUT"
-        echo "cache-key=$cache_key" >> "$GITHUB_OUTPUT"
-
-    - name: Restore system dependencies
-      id: cache
-      uses: actions/cache/restore@v6
-      with:
-        path: ${{ steps.packages.outputs.cache-dir }}
-        key: cache-apt-pkgs-${{ runner.os }}-${{ steps.packages.outputs.cache-key }}
-
-    - name: Restore cached system dependencies
-      if: steps.cache.outputs.cache-hit == 'true'
-      shell: bash
-      env:
-        CACHE_DIR: ${{ steps.packages.outputs.cache-dir }}
-      run: sudo tar -xf "$CACHE_DIR/packages.tar" -C /
-
-    - name: Install and cache system dependencies
-      if: steps.cache.outputs.cache-hit != 'true'
-      shell: bash
-      env:
-        CACHE_DIR: ${{ steps.packages.outputs.cache-dir }}
-      run: |
-        set -euo pipefail
-
-        sudo apt-get update
-        mapfile -t packages < <(cut -d= -f1 "$CACHE_DIR/requested-packages.txt")
-        sudo DEBIAN_FRONTEND=noninteractive apt-get install --yes \
+        sudo apt-get update -qq
+        sudo DEBIAN_FRONTEND=noninteractive apt-get install --yes -qq \
           --no-install-recommends --no-upgrade \
-          "${packages[@]}" \
-          | tee "$CACHE_DIR/install.log"
-
-        sed -nE 's/^Unpacking ([^ :]+)(:[^ ]+)? .*\(([^ )]+).*/\1\2/p' \
-          "$CACHE_DIR/install.log" \
-          | sort -u \
-          > "$CACHE_DIR/installed-packages.txt"
-
-        if [[ -s "$CACHE_DIR/installed-packages.txt" ]]; then
-          while IFS= read -r package; do
-            dpkg-query --listfiles "$package"
-          done < "$CACHE_DIR/installed-packages.txt" \
-            | sort -u \
-            | while IFS= read -r path; do
-                if [[ -f "$path" || -L "$path" ]]; then
-                  printf '%s\n' "${path#/}"
-                fi
-              done \
-            | sudo tar -cf "$CACHE_DIR/packages.tar" -C / -T -
-        else
-          # A newer runner image may already contain every requested package.
-          sudo tar -cf "$CACHE_DIR/packages.tar" -T /dev/null
-        fi
-
-    - name: Save system dependencies
-      if: steps.cache.outputs.cache-hit != 'true'
-      uses: actions/cache/save@v6
-      with:
-        path: ${{ steps.packages.outputs.cache-dir }}
-        key: ${{ steps.cache.outputs.cache-primary-key }}
-
-    - name: Clean up system dependency cache
-      if: always()
-      shell: bash
-      env:
-        CACHE_DIR: ${{ steps.packages.outputs.cache-dir }}
-      run: rm -rf "$CACHE_DIR"
+          bubblewrap musl-tools rsync cmake
diff --git a/.github/actions/setup-ocaml/action.yml b/.github/actions/setup-ocaml/action.yml
index 7d8fc79995..d7c42129c7 100644
--- a/.github/actions/setup-ocaml/action.yml
+++ b/.github/actions/setup-ocaml/action.yml
@@ -1,6 +1,13 @@
 name: Setup OCaml
 description: Restore or install the repository OPAM environment.
 
+# An environment is only ever used as it was built: restored on an exact key,
+# or built from scratch, never restored from other dependencies and updated.
+# Pushes save what they build. A pull request that changes the opam files
+# saves its environment too, in its own cache scope, so that only its first
+# push per change builds from scratch. Each save deletes the earlier
+# generations of the same key prefix on the same ref.
+
 inputs:
   compiler:
     description: OCaml compiler switch specification.
@@ -81,8 +88,9 @@ runs:
         dependency_flags_key="${DEPENDENCY_FLAGS//--/}"
         dependency_flags_key="${dependency_flags_key// /_}"
         dependency_hash="${{ hashFiles('*.opam') }}"
-        key="$CACHE_PREFIX-$OS-3.9.0-$COMPILER-$dependency_flags_key-$dependency_hash"
-        echo "value=${key//,/-}" >> "$GITHUB_OUTPUT"
+        prefix="$CACHE_PREFIX-$OS-3.9.0-$COMPILER-$dependency_flags_key-"
+        echo "prefix=${prefix//,/-}" >> "$GITHUB_OUTPUT"
+        echo "value=${prefix//,/-}$dependency_hash" >> "$GITHUB_OUTPUT"
         echo "setup-version=3.9.0" >> "$GITHUB_OUTPUT"
 
     - name: Restore OPAM environment
@@ -140,10 +148,29 @@ runs:
         fi
         opam install . "${dependency_flags[@]}"
 
+    # A pull request that changes the dependencies saves its environment, so
+    # that its next pushes find it. Caches saved by pull_request runs are
+    # scoped to the pull request's merge ref, so no other branch sees them.
+    # Other pull requests don't save: each would store a copy of master's.
+    - name: Check whether the pull request changes the dependencies
+      id: dependencies
+      if: steps.cache.outputs.cache-hit != 'true' && github.event_name == 'pull_request'
+      shell: bash
+      env:
+        BASE_SHA: ${{ github.event.pull_request.base.sha }}
+      run: |
+        if git fetch --quiet --depth=1 origin "$BASE_SHA" &&
+          ! git diff --quiet FETCH_HEAD HEAD -- '*.opam'; then
+          echo "changed=true" >> "$GITHUB_OUTPUT"
+        else
+          echo "changed=false" >> "$GITHUB_OUTPUT"
+        fi
+
     - name: Cache OPAM environment
-      # Caches created by pull_request runs are scoped to that PR's merge ref
-      # and cannot seed other PRs or branches, so only push runs save them.
-      if: steps.cache.outputs.cache-hit != 'true' && github.event_name == 'push'
+      id: save
+      if: >-
+        steps.cache.outputs.cache-hit != 'true' &&
+        (github.event_name == 'push' || steps.dependencies.outputs.changed == 'true')
       uses: actions/cache/save@v6
       with:
         path: |
@@ -155,6 +182,26 @@ runs:
           C:\.opam
         key: ${{ steps.cache-key.outputs.value }}
 
+    # Keeps one environment per key prefix and ref. Needs actions: write;
+    # without it, the earlier generations stay until GitHub evicts them.
+    - name: Delete earlier OPAM environments
+      if: steps.save.outcome == 'success'
+      shell: bash
+      env:
+        GH_TOKEN: ${{ github.token }}
+        PREFIX: ${{ steps.cache-key.outputs.prefix }}
+        KEEP: ${{ steps.cache-key.outputs.value }}
+      run: |
+        gh api --paginate --method GET "repos/$GITHUB_REPOSITORY/actions/caches" \
+          -f key="$PREFIX" -f ref="$GITHUB_REF" \
+          --jq ".actions_caches[] | select(.key != \"$KEEP\") | .id" |
+          while IFS= read -r id; do
+            echo "Deleting cache $id"
+            gh api --method DELETE "repos/$GITHUB_REPOSITORY/actions/caches/$id" ||
+              echo "::warning::Couldn't delete cache $id."
+          done ||
+          echo "::warning::Couldn't list the earlier OPAM environments."
+
     - name: Use cached OPAM environment
       if: steps.cache.outputs.cache-hit == 'true'
       shell: bash
diff --git a/.github/workflows/cache_cleanup.yml b/.github/workflows/cache_cleanup.yml
new file mode 100644
index 0000000000..faeb849182
--- /dev/null
+++ b/.github/workflows/cache_cleanup.yml
@@ -0,0 +1,36 @@
+name: Cache Cleanup
+
+# Deletes the caches of a closed pull request: they are scoped to its merge
+# ref, so nothing can use them any more, but they count towards the
+# repository's cache storage until GitHub evicts them. The job runs no code
+# of the pull request.
+
+on:
+  pull_request_target:
+    types: [closed]
+  workflow_dispatch:
+    inputs:
+      pr-number:
+        description: The pull request whose caches to delete.
+        required: true
+
+jobs:
+  cleanup:
+    runs-on: ubuntu-26.04
+    permissions:
+      actions: write
+    steps:
+      - name: Delete the pull request's caches
+        env:
+          GH_TOKEN: ${{ github.token }}
+          PR_NUMBER: ${{ github.event.pull_request.number || inputs.pr-number }}
+        run: |
+          [[ "$PR_NUMBER" =~ ^[0-9]+$ ]] || { echo "Not a pull request number: $PR_NUMBER" >&2; exit 1; }
+          gh api --paginate --method GET "repos/$GITHUB_REPOSITORY/actions/caches" \
+            -f ref="refs/pull/$PR_NUMBER/merge" \
+            --jq '.actions_caches[] | "\(.id) \(.key)"' |
+            while read -r id key; do
+              echo "Deleting $key"
+              gh api --method DELETE "repos/$GITHUB_REPOSITORY/actions/caches/$id" ||
+                echo "::warning::Couldn't delete cache $id."
+            done
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index abb17e3667..916c515a42 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -104,18 +104,17 @@ jobs:
         id: compiler-build-state-key
         shell: bash
         run: |
-          echo "value=compiler-build-state-v1-${{ matrix.os }}-${{ steps.setup_ocaml.outputs.setup_version }}-${{ matrix.ocaml_compiler }}-${{ hashFiles('*.opam') }}" \
+          echo "value=compiler-build-state-v2-${{ matrix.os }}-${{ steps.setup_ocaml.outputs.setup_version }}-${{ matrix.ocaml_compiler }}-${{ hashFiles('*.opam') }}" \
             | sed 's/,/-/g' >> "$GITHUB_OUTPUT"
 
+      # Only _build: dune's shared cache under ~/.cache/dune would grow with
+      # every master push that restores, extends and saves it again.
       - name: Restore compiler build state
         if: github.base_ref == 'master' || github.ref == 'refs/heads/master'
         id: compiler-build-state
         uses: actions/cache/restore@v6
         with:
-          path: |
-            C:\.cache\dune
-            ~/.cache/dune
-            _build
+          path: _build
           key: ${{ steps.compiler-build-state-key.outputs.value }}
 
       - name: Build compiler
@@ -143,10 +142,7 @@ jobs:
         if: github.event_name == 'push' && github.ref == 'refs/heads/master'
         uses: actions/cache/save@v6
         with:
-          path: |
-            C:\.cache\dune
-            ~/.cache/dune
-            _build
+          path: _build
           key: ${{ steps.compiler-build-state-key.outputs.value }}
 
       - name: Verify promoted compiler exes
@@ -456,7 +452,7 @@ jobs:
       pull-requests: write
     env:
       # The same build as the first linux-x64 build-compiler job, so that the
-      # OPAM environment and the dune cache are shared.
+      # OPAM environment and the build state are shared.
       DUNE_PROFILE: static
       OCAML_COMPILER: ocaml-variants.5.5.2+options,ocaml-option-static
     steps:
@@ -492,7 +488,7 @@ jobs:
       - name: Compiler build state key
         id: compiler-build-state-key
         run: |
-          echo "value=compiler-build-state-v1-ubuntu-26.04-${{ steps.setup_ocaml.outputs.setup_version }}-$OCAML_COMPILER-${{ hashFiles('*.opam') }}" \
+          echo "value=compiler-build-state-v2-ubuntu-26.04-${{ steps.setup_ocaml.outputs.setup_version }}-$OCAML_COMPILER-${{ hashFiles('*.opam') }}" \
             | sed 's/,/-/g' >> "$GITHUB_OUTPUT"
 
       - name: Restore compiler build state
@@ -500,10 +496,7 @@ jobs:
         with:
           # The paths are part of the cache's version, so they must be the
           # ones build-compiler saves.
-          path: |
-            C:\.cache\dune
-            ~/.cache/dune
-            _build
+          path: _build
           key: ${{ steps.compiler-build-state-key.outputs.value }}
 
       - name: Build the compiler
diff --git a/.github/workflows/coverage.yml b/.github/workflows/coverage.yml
index 606a1b44c2..8541a1eb2c 100644
--- a/.github/workflows/coverage.yml
+++ b/.github/workflows/coverage.yml
@@ -17,7 +17,8 @@ jobs:
   coverage:
     runs-on: ubuntu-26.04-arm
     permissions:
-      actions: write # For build cache overwriting
+      contents: read
+      actions: write # For deleting earlier OPAM environments
       id-token: write # For CodeCov OIDC
     env:
       OS: ubuntu-26.04-arm
@@ -47,48 +48,11 @@ jobs:
           git+https://github.com/patricoferris/bisect_ppx.git#7061d643ff492b0045796357ee6917ded21fb1f0
           --yes
 
-      # --- Coverage build cache --------------------------------------------
-      # The bisect_ppx-instrumented dune build is a separate artifact from
-      # the main CI build, so it gets its own key. Only restore/save on
-      # master pushes — PRs share the master cache to stay fast but won't
-      # poison it.
-      - name: Coverage build state key
-        id: coverage-build-state-key
-        run: echo "value=coverage-build-state-v1-${{ env.OS }}-${{ steps.setup_ocaml.outputs.setup_version }}-${{ env.OCAML_COMPILER }}-${{ hashFiles('*.opam', 'compiler/**', 'dune-project') }}" >> $GITHUB_OUTPUT
-
-      - name: Restore coverage build state
-        if: github.base_ref == 'master' || github.ref == 'refs/heads/master'
-        id: coverage-build-state
-        uses: actions/cache/restore@v6
-        with:
-          path: |
-            ~/.cache/dune
-            _build
-          key: ${{ steps.coverage-build-state-key.outputs.value }}
-
+      # No build state is cached: every compiler change misses it, and the
+      # instrumented build takes about 30 s.
       - name: Run coverage
         run: opam exec -- make coverage
 
-      - name: Delete stale coverage build state
-        if: github.event_name == 'push' && github.ref == 'refs/heads/master'
-        run: |
-          gh extension install actions/gh-actions-cache
-          gh actions-cache delete ${{ steps.coverage-build-state-key.outputs.value }} \
-            -R ${{ github.repository }} \
-            -B "$GITHUB_REF" \
-            --confirm || echo "not exist"
-        env:
-          GH_TOKEN: ${{ github.token }}
-
-      - name: Save coverage build state
-        if: github.event_name == 'push' && github.ref == 'refs/heads/master'
-        uses: actions/cache/save@v6
-        with:
-          path: |
-            ~/.cache/dune
-            _build
-          key: ${{ steps.coverage-build-state-key.outputs.value }}
-
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v7
         with: