diff --git a/.github/workflows/nix-action.yml b/.github/workflows/nix-action.yml
index 5adc150..38435c7 100644
--- a/.github/workflows/nix-action.yml
+++ b/.github/workflows/nix-action.yml
@@ -7,8 +7,9 @@ on:
     branches:
       - master
   pull_request:
-    branches:
-      - '**'
+    paths:
+    - .github/workflows/**
+  pull_request_target:
 
 jobs:
   build:
@@ -19,6 +20,18 @@ jobs:
           - 'coq = "master"'
       fail-fast: false
     steps:
+      - name: Determine which ref to test
+        run: |
+          if [ ${{ github.event_name }} = "push" ]; then
+            echo "tested_ref=${{ github.ref }}" >> $GITHUB_ENV
+          else
+            merge_commit=$(git ls-remote ${{ github.event.repository.html_url }} refs/pull/${{ github.event.number }}/merge | cut -f1)
+            if [ -z "$merge_commit" ]; then
+              echo "tested_ref=refs/pull/${{ github.event.number }}/head" >> $GITHUB_ENV
+            else
+              echo "tested_ref=refs/pull/${{ github.event.number }}/merge" >> $GITHUB_ENV
+            fi
+          fi
       - uses: cachix/install-nix-action@v12
         with:
           nix_path: nixpkgs=channel:nixpkgs-unstable
@@ -33,5 +46,7 @@ jobs:
         with:
           name: math-comp
       - uses: actions/checkout@v2
+        with:
+          ref: ${{ env.tested_ref }}
       - run: >
           nix-build https://coq.inria.fr/nix/toolbox --argstr job aac-tactics --arg override '{ ${{ matrix.overrides }}; aac-tactics = builtins.filterSource (path: _: baseNameOf path != ".git") ./.; }'