diff --git a/.github/workflows/nix-action-coq-8.20.yml b/.github/workflows/nix-action-coq-8.20.yml
index b40e380e4..cffab76ce 100644
--- a/.github/workflows/nix-action-coq-8.20.yml
+++ b/.github/workflows/nix-action-coq-8.20.yml
@@ -1,4 +1,61 @@
 jobs:
+  autosubst:
+    needs:
+    - coq
+    - mathcomp-ssreflect
+    runs-on: ubuntu-latest
+    steps:
+    - name: Determine which commit to initially checkout
+      run: "if [ ${{ github.event_name }} = \"push\" ]; then\n  echo \"target_commit=${{\
+        \ github.sha }}\" >> $GITHUB_ENV\nelse\n  echo \"target_commit=${{ github.event.pull_request.head.sha\
+        \ }}\" >> $GITHUB_ENV\nfi\n"
+    - name: Git checkout
+      uses: actions/checkout@v4
+      with:
+        fetch-depth: 0
+        ref: ${{ env.target_commit }}
+    - name: Determine which commit to test
+      run: "if [ ${{ github.event_name }} = \"push\" ]; then\n  echo \"tested_commit=${{\
+        \ github.sha }}\" >> $GITHUB_ENV\nelse\n  merge_commit=$(git ls-remote ${{\
+        \ github.event.repository.html_url }} refs/pull/${{ github.event.number }}/merge\
+        \ | cut -f1)\n  mergeable=$(git merge --no-commit --no-ff ${{ github.event.pull_request.base.sha\
+        \ }} > /dev/null 2>&1; echo $?; git merge --abort > /dev/null 2>&1 || true)\n\
+        \  if [ -z \"$merge_commit\" -o \"x$mergeable\" != \"x0\" ]; then\n    echo\
+        \ \"tested_commit=${{ github.event.pull_request.head.sha }}\" >> $GITHUB_ENV\n\
+        \  else\n    echo \"tested_commit=$merge_commit\" >> $GITHUB_ENV\n  fi\nfi\n"
+    - name: Git checkout
+      uses: actions/checkout@v4
+      with:
+        fetch-depth: 0
+        ref: ${{ env.tested_commit }}
+    - name: Cachix install
+      uses: cachix/install-nix-action@v27
+      with:
+        nix_path: nixpkgs=channel:nixpkgs-unstable
+    - name: Cachix setup coq-elpi
+      uses: cachix/cachix-action@v15
+      with:
+        authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
+        extraPullNames: coq, coq-community, math-comp
+        name: coq-elpi
+    - id: stepCheck
+      name: Checking presence of CI target autosubst
+      run: "nb_dry_run=$(NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link \\\n   --argstr\
+        \ bundle \"coq-8.20\" --argstr job \"autosubst\" \\\n   --dry-run 2>&1 > /dev/null)\n\
+        echo $nb_dry_run\necho status=$(echo $nb_dry_run | grep \"built:\" | sed \"\
+        s/.*/built/\") >> $GITHUB_OUTPUT\n"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: coq'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "coq-8.20"
+        --argstr job "coq"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: mathcomp-ssreflect'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "coq-8.20"
+        --argstr job "mathcomp-ssreflect"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: Building/fetching current CI target
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "coq-8.20"
+        --argstr job "autosubst"
   coq:
     needs: []
     runs-on: ubuntu-latest
diff --git a/.nix/coq-nix-toolbox.nix b/.nix/coq-nix-toolbox.nix
index 9b872206b..02b5e945e 100644
--- a/.nix/coq-nix-toolbox.nix
+++ b/.nix/coq-nix-toolbox.nix
@@ -1 +1 @@
-"24e96b4870378d5e87fd2d0dd46405b471c286ab"
+"b472641a6662a06901b280845b1473e7d6b08ab5"