diff --git a/.github/workflows/unittest.yml b/.github/workflows/unittest.yml
index 392f5ece6270..e59727813fc4 100644
--- a/.github/workflows/unittest.yml
+++ b/.github/workflows/unittest.yml
@@ -59,21 +59,33 @@ jobs:
     steps:
     - name: Checkout
       uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
-      # Use a fetch-depth of 2 to avoid error `fatal: origin/main...HEAD: no merge base`
-      # See https://github.com/googleapis/google-cloud-python/issues/12013
-      # and https://github.com/actions/checkout#checkout-head.
       with:
-        fetch-depth: 2
+        # Use fetch-depth of 0 (full history) so Git can find the merge-base with the
+        # target branch and calculate the exact list of modified packages.
+        fetch-depth: 0
         persist-credentials: false
+      # We must explicitly fetch and set the target branch reference (main, etc.)
+      # to perform a relative diff (target_branch...HEAD). Comparing against a generic "HEAD~1"
+      # is fragile and fails if developers merge main back into their PR, if the PR has
+      # multiple commits, or if running inside a Merge Queue (merge_group) environment.
+    - name: Set target branch ref
+      id: target
+      env:
+        TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref || 'main' }}
+      run: |
+        git fetch origin $TARGET_BRANCH
+        echo "branch=origin/$TARGET_BRANCH" >> "$GITHUB_OUTPUT"
     - name: Setup Python
       uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5
       with:
         python-version: "3.10"
     - name: Set number of files changes in packages and preview-packages directories
       id: packages
+      env:
+        TARGET_BRANCH_REF: ${{ steps.target.outputs.branch }}
       run: |
-        git diff HEAD~1 -- packages preview-packages > /dev/null
-        num_files_changed=$(git diff HEAD~1 -- packages preview-packages | wc -l | tr -d ' ')
+        git diff "$TARGET_BRANCH_REF"...HEAD -- packages preview-packages > /dev/null
+        num_files_changed=$(git diff "$TARGET_BRANCH_REF"...HEAD -- packages preview-packages | wc -l | tr -d ' ')
         echo "num_files_changed=${num_files_changed}" >> "$GITHUB_OUTPUT"
     - name: Install coverage
       if: ${{ steps.packages.outputs.num_files_changed > 0 }}
@@ -91,9 +103,11 @@ jobs:
         # TODO: default to 100% coverage after next gapic-generator release
         # https://github.com/googleapis/google-cloud-python/issues/17459
         DEFAULT_FAIL_UNDER: 99
+        TARGET_BRANCH_REF: ${{ steps.target.outputs.branch }}
       run: |
+        target_ref="$TARGET_BRANCH_REF"
         # Find all modified packages
-        modified_packages=$(git diff --name-only HEAD~1 -- packages preview-packages | cut -d/ -f1,2 | sort -u)
+        modified_packages=$(git diff --name-only ${target_ref}...HEAD -- packages preview-packages | cut -d/ -f1,2 | sort -u)
 
         existing_modified_packages=()
         for pkg in ${modified_packages}; do
@@ -122,7 +136,7 @@ jobs:
           fi
 
           # Find all modified packages
-          modified_packages=$(git diff --name-only HEAD~1 -- packages preview-packages | cut -d/ -f1,2 | sort -u)
+          modified_packages=$(git diff --name-only ${target_ref}...HEAD -- packages preview-packages | cut -d/ -f1,2 | sort -u)
 
           failed_packages=()
           passed_packages=()
diff --git a/packages/bigquery-magics/MANIFEST.in b/packages/bigquery-magics/MANIFEST.in
index d6814cd60037..2257d978dbf2 100644
--- a/packages/bigquery-magics/MANIFEST.in
+++ b/packages/bigquery-magics/MANIFEST.in
@@ -23,3 +23,4 @@ global-exclude __pycache__
 
 # Exclude scripts for samples readmegen
 prune scripts/readme-gen
+
diff --git a/packages/db-dtypes/pytest.ini b/packages/db-dtypes/pytest.ini
index dbe13ba38624..6c1de90e70e0 100644
--- a/packages/db-dtypes/pytest.ini
+++ b/packages/db-dtypes/pytest.ini
@@ -5,3 +5,4 @@ filterwarnings =
     # Remove once https://github.com/googleapis/python-db-dtypes-pandas/issues/227 is fixed
     ignore:.*any.*with datetime64 dtypes is deprecated and will raise in a future version:FutureWarning
     ignore:.*all.*with datetime64 dtypes is deprecated and will raise in a future version:FutureWarning 
+