diff --git a/CI.md b/CI.md
index cc90a6a..1dce3f9 100644
--- a/CI.md
+++ b/CI.md
@@ -48,7 +48,8 @@ workflow has all of them.
 3. **Drafts skip.** Every job has `github.event.pull_request.draft != true`
    in its `if`, or needs a job that has it.
 4. **A `changes` job** runs `cocoindex-io/.github/actions/ci-mode@v1` (it
-   needs `checks: read`) and the path filter. Expensive jobs run when
+   needs `checks: read`, and `pull-requests: read` in a repository with a
+   merge queue) and the path filter. Expensive jobs run when
    `full == 'true'` and the filter output is not `'false'`. The filter is
    skipped for a stack layer, and so is any other diff against the pull
    request's base branch: a stack layer's base is the layer below it, but
@@ -110,6 +111,13 @@ requests directly. Run the full tier on its branch first with
 `gh pr merge --merge --admin`. Apart from an incident (below), nothing else
 bypasses the queue.
 
+Label such a pull request `merge-commit` when you open it. `ci-mode` fails
+the merge group of a pull request with that label before any test runs, so
+the queue removes it instead of squashing it. When `ci-mode` can't read the
+labels, the merge group fails too: queueing a pull request again costs one
+build, and a squashed merge commit costs a repair merge. The label has to be
+on the pull request before it enters the queue.
+
 ## Urgent changes
 
 An urgent fix jumps the queue: it goes to the front, is built next on top of
diff --git a/actions/ci-mode/action.yml b/actions/ci-mode/action.yml
index 2a4692d..6c1299e 100644
--- a/actions/ci-mode/action.yml
+++ b/actions/ci-mode/action.yml
@@ -1,8 +1,10 @@
 name: CI mode
 description: >-
-  Decides whether this run executes the full CI tier or only the fast one.
+  Decides whether this run executes the full CI tier or only the fast one,
+  and fails a merge group whose pull request must land as a merge commit.
   See CI.md in cocoindex-io/.github for the tiers. The calling job needs
-  `checks: read` and `contents: read`.
+  `checks: read` and `contents: read`, and in a repository with a merge
+  queue also `pull-requests: read`.
 outputs:
   full:
     description: "'true' when this run executes the full tier, 'false' for the fast tier."
@@ -22,6 +24,7 @@ runs:
         BASE: ${{ github.event.pull_request.stack.base.ref || github.event.pull_request.base.ref }}
         STACK_POSITION: ${{ github.event.pull_request.stack.position }}
         STACK_SIZE: ${{ github.event.pull_request.stack.size }}
+        HEAD_REF: ${{ github.event.merge_group.head_ref }}
       run: |
         full=true
         reason="a $EVENT run executes the full tier"
@@ -50,6 +53,27 @@ runs:
           else
             echo "::warning::Could not read the checks of $SHA, so this run executes the full tier."
           fi
+        elif [ "$EVENT" = merge_group ]; then
+          # The queue squashes. A pull request labelled merge-commit, such as
+          # an upstream merge, must keep its merge commit, so its merge group
+          # fails here, before any expensive job starts, and the queue removes
+          # it. A failed lookup fails too: queueing a pull request again costs
+          # one build, and a squashed merge commit costs a repair. The queue's
+          # branch names the pull request: gh-readonly-queue/<base>/pr-<n>-<sha>.
+          pr=${HEAD_REF##*/pr-}
+          pr=${pr%%-*}
+          if ! [[ $pr =~ ^[0-9]+$ ]]; then
+            echo "::error::Could not find the pull request number in the merge group ref $HEAD_REF."
+            exit 1
+          fi
+          if ! labelled=$(gh api "repos/$REPO/pulls/$pr" --jq 'any(.labels[]; .name == "merge-commit")'); then
+            echo "::error::Could not read the labels of #$pr, so its merge group fails. Queue it again; if this repeats, check that the job that runs ci-mode has \`pull-requests: read\`."
+            exit 1
+          fi
+          if [ "$labelled" = true ]; then
+            echo "::error::#$pr is labelled merge-commit, so it must land as a merge commit, and the queue squashes. Merge it around the queue after a full run on its branch passes: gh pr merge $pr --merge --admin (CI.md, Merge commits)."
+            exit 1
+          fi
         fi
         echo "full=$full" >> "$GITHUB_OUTPUT"
         tier=fast