diff --git a/.github/scripts/fetch-validation-history.sh b/.github/scripts/fetch-validation-history.sh
new file mode 100755
index 0000000..65fddc1
--- /dev/null
+++ b/.github/scripts/fetch-validation-history.sh
@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+base=${1:?Usage: fetch-validation-history.sh <base-sha>}
+head=$(git rev-parse HEAD)
+
+if ! git cat-file -e "$base^{commit}" 2>/dev/null; then
+  git fetch --no-tags --depth=1 origin "$base"
+fi
+
+base=$(git rev-parse "$base^{commit}")
+shallow_file=$(git rev-parse --git-path shallow)
+
+# Deepening is needed until no range commit can reach $base through unfetched history.
+range_needs_deepening() {
+  [[ -s $shallow_file ]] || return 1
+
+  local roots joins join
+  roots=$(git rev-list --max-parents=0 "$base..$head") || exit "$?"
+  [[ -z $roots ]] || return 0
+
+  joins=$(git rev-list --boundary "$base..$head" | sed -n 's/^-//p') || exit "$?"
+  for join in $joins; do
+    roots=$(git rev-list --max-parents=0 "$base" --not "$join") || exit "$?"
+    [[ -z $roots ]] || return 0
+  done
+  return 1
+}
+
+deepen=8
+while range_needs_deepening; do
+  git fetch --no-tags --deepen="$deepen" origin "$head" "$base"
+  deepen=$((deepen * 2))
+done
diff --git a/.github/workflows/checks.yml b/.github/workflows/checks.yml
index 0c5abcb..d76b99b 100644
--- a/.github/workflows/checks.yml
+++ b/.github/workflows/checks.yml
@@ -1,6 +1,8 @@
 name: Flake checks
 on:
   pull_request:
+  merge_group:
+    types: [checks_requested]
 permissions:
   actions: read # lets hestia detect evicted cache entries upfront
   contents: read
@@ -8,14 +10,25 @@ concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true
 jobs:
-  check:
+  validate-commits:
     runs-on: ubuntu-latest
-    timeout-minutes: 30
+    env:
+      BASE_SHA: ${{ github.event.pull_request.base.sha || github.event.merge_group.base_sha }}
     steps:
+      - name: Compute initial fetch depth
+        id: fetch-depth
+        env:
+          PR_COMMITS: ${{ github.event.pull_request.commits || 0 }}
+        run: echo "depth=$((PR_COMMITS + 2))" >> "$GITHUB_OUTPUT"
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          fetch-depth: ${{ steps.fetch-depth.outputs.depth }}
+      - name: Fetch validation history
+        run: bash .github/scripts/fetch-validation-history.sh "$BASE_SHA"
       - uses: DeterminateSystems/nix-installer-action@3138316df39ed29be04236d7ffc686fa525866aa # v23
       - uses: Mic92/hestia@dfed9ced335d28978ba74e513939a10db1f71025 # v3.1.0
         with:
           # Do not upload paths that cache.nixos.org already serves.
           upstream-cache-filter: true
-      - run: nix flake check --no-update-lock-file
+      - name: Validate commits
+        run: nix run --no-update-lock-file '.?shallow=1#validate-commits' -- "$BASE_SHA..HEAD"
diff --git a/flake.nix b/flake.nix
index e5e1f62..c0e75f6 100644
--- a/flake.nix
+++ b/flake.nix
@@ -19,6 +19,7 @@
         ./nix/site.nix
         ./nix/devshell.nix
         ./nix/spellcheck.nix
+        ./nix/apps/validate-commits
       ];
     };
 }
diff --git a/nix/apps/validate-commits/default.nix b/nix/apps/validate-commits/default.nix
new file mode 100644
index 0000000..3cf5a77
--- /dev/null
+++ b/nix/apps/validate-commits/default.nix
@@ -0,0 +1,22 @@
+{
+  perSystem =
+    { pkgs, ... }:
+    let
+      validate-commits = pkgs.writeShellApplication {
+        name = "validate-commits";
+        runtimeInputs = [
+          pkgs.git
+          pkgs.nix
+        ];
+        text = builtins.readFile ./validate-commits.sh;
+      };
+    in
+    {
+      packages.validate-commits = validate-commits;
+      apps.validate-commits = {
+        type = "app";
+        program = "${validate-commits}/bin/validate-commits";
+        meta.description = "Validate the flake across pull request commits";
+      };
+    };
+}
diff --git a/nix/apps/validate-commits/validate-commits.sh b/nix/apps/validate-commits/validate-commits.sh
new file mode 100644
index 0000000..cc13aac
--- /dev/null
+++ b/nix/apps/validate-commits/validate-commits.sh
@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+usage() {
+  echo "Usage: validate-commits <revision-range>"
+}
+
+if [[ ${1:-} == --help ]]; then
+  usage
+  exit 0
+fi
+
+if [[ $# -ne 1 ]]; then
+  usage >&2
+  exit 2
+fi
+
+repo_root=$(git rev-parse --show-toplevel)
+revision_range=$1
+commits=$(git rev-list --reverse "$revision_range")
+
+if [[ -z $commits ]]; then
+  echo "No commits in range."
+  exit 0
+fi
+
+while read -r commit; do
+  echo "Validating $commit"
+  nix flake check --no-update-lock-file --print-build-logs \
+    "git+file://$repo_root?rev=$commit&shallow=1"
+done <<<"$commits"