diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 52499d2b..6bbdcb0a 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -3,8 +3,9 @@ name: CI
 on:
   push:
     branches: [main]
-  pull_request:
-    branches: [main]
+  # Every pull request target, including immutable stack branches, receives the
+  # same exact-head, synthetic-merge, and buyer-readiness acceptance evidence.
+  pull_request: {}
 
 permissions:
   contents: read
diff --git a/scripts/test_ci_workflow_stack_coverage.py b/scripts/test_ci_workflow_stack_coverage.py
new file mode 100644
index 00000000..8a3af8b7
--- /dev/null
+++ b/scripts/test_ci_workflow_stack_coverage.py
@@ -0,0 +1,24 @@
+from pathlib import Path
+
+
+REPOSITORY_ROOT = Path(__file__).resolve().parents[1]
+CI_WORKFLOW = REPOSITORY_ROOT / ".github" / "workflows" / "ci.yml"
+
+
+def test_ci_runs_for_every_pull_request_base() -> None:
+    """Stacked pull requests must receive the same exact-head CI as main-bound PRs."""
+    workflow = CI_WORKFLOW.read_text(encoding="utf-8")
+
+    assert "  pull_request: {}" in workflow
+    assert "  pull_request:\n    branches: [main]" not in workflow
+
+
+def test_ci_preserves_exact_head_and_synthetic_merge_evidence() -> None:
+    """Broadening PR coverage must not weaken exact-head or merge verification."""
+    workflow = CI_WORKFLOW.read_text(encoding="utf-8")
+
+    exact_head_expression = "github.event.pull_request.head.sha || github.sha"
+    assert workflow.count(exact_head_expression) >= 4
+    assert 'test "$(git rev-parse HEAD)" = "$EXPECTED_SHA"' in workflow
+    assert "name: Maven merge compatibility" in workflow
+    assert "name: Buyer-readiness script tests" in workflow