diff --git a/.github/workflows/push.yml b/.github/workflows/push.yml
index 593b0ad822..4f39093beb 100644
--- a/.github/workflows/push.yml
+++ b/.github/workflows/push.yml
@@ -137,6 +137,21 @@ jobs:
         with:
           cache-key: test-${{ matrix.deployment }}
 
+      # Make origin/main available with enough history for `git diff --merge-base
+      # origin/main`, which the acceptance subset selector uses to always run the
+      # tests this PR adds/changes on the windows/macOS cells. actions/checkout does
+      # a shallow single-ref clone by default, so origin/main is otherwise absent and
+      # the diff fails, silently disabling change detection. This must run AFTER
+      # setup-build-environment, whose own checkout would re-shallow the repo. Only
+      # PRs subset tests, so the fetch is unnecessary elsewhere.
+      - name: Fetch origin/main for change detection
+        if: ${{ github.event_name == 'pull_request' }}
+        run: |
+          git fetch --no-tags origin +refs/heads/main:refs/remotes/origin/main
+          if [ "$(git rev-parse --is-shallow-repository)" = "true" ]; then
+            git fetch --no-tags --unshallow origin
+          fi
+
       - name: Run tests
         env:
           ENVFILTER: DATABRICKS_BUNDLE_ENGINE=${{ matrix.deployment }}
diff --git a/acceptance/acceptance_test.go b/acceptance/acceptance_test.go
index ae650a19dd..2c9c2b31a1 100644
--- a/acceptance/acceptance_test.go
+++ b/acceptance/acceptance_test.go
@@ -450,7 +450,7 @@ func testAccept(t *testing.T, inprocessMode bool, singleTest string) int {
 	// subset selector keep these tests, so detect them at most once here.
 	var changedTests map[string][]string
 	if skipLocalWithChanged || subset.enabled {
-		changedTests = selectChangedLocalTests(testDirsSet)
+		changedTests = selectChangedLocalTests(t, testDirsSet)
 	}
 	subset.changed = changedTests
 
diff --git a/acceptance/skiplocal_test.go b/acceptance/skiplocal_test.go
index c72f7c39a3..a16de1f0ef 100644
--- a/acceptance/skiplocal_test.go
+++ b/acceptance/skiplocal_test.go
@@ -1,10 +1,12 @@
 package acceptance_test
 
 import (
+	"errors"
 	"os/exec"
 	"path/filepath"
 	"slices"
 	"strings"
+	"testing"
 )
 
 // DATABRICKS_TEST_SKIPLOCAL controls skipping of Local acceptance tests.
@@ -61,8 +63,21 @@ func testDirForFile(repoRelPath string, testDirs map[string]bool) string {
 // committed. The three-dot form origin/main...HEAD only covers committed
 // changes and misses unstaged edits, which breaks the "touch a config, run
 // the test" local dev workflow (same reason lintdiff.py uses --merge-base).
-func selectChangedLocalTests(testDirs map[string]bool) map[string][]string {
-	out, _ := exec.Command("git", "diff", "--name-status", "--merge-base", "-M", "origin/main").Output()
+func selectChangedLocalTests(t *testing.T, testDirs map[string]bool) map[string][]string {
+	out, err := exec.Command("git", "diff", "--name-status", "--merge-base", "-M", "origin/main").Output()
+	if err != nil {
+		// A failed diff (most commonly a missing origin/main in a shallow CI
+		// checkout) disables change detection, so newly added tests fall back to
+		// the seeded subset and may not run. Log loudly but continue: failing the
+		// test here breaks integration runs whose checkout has no origin/main. The
+		// push.yml PR cells fetch full history so origin/main resolves there.
+		stderr := ""
+		if exitErr, ok := errors.AsType[*exec.ExitError](err); ok {
+			stderr = strings.TrimSpace(string(exitErr.Stderr))
+		}
+		t.Logf("WARNING: change detection disabled: git diff --merge-base origin/main failed: %v\n%s", err, stderr)
+		return nil
+	}
 	diff := strings.TrimSpace(string(out))
 
 	// result accumulates dirs with their filters; added tracks brand-new dirs.