diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 1e7898bfb..e5a2a415c 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -3,8 +3,9 @@ name: Tests
 on:
   push:
     branches: [main]
+  # Pull requests into any base branch: a stacked pull request targets
+  # another pull request's branch and needs the same jobs.
   pull_request:
-    branches: [main]
 
 # Cancel outdated PR runs; give each main push its own independent group.
 concurrency:
diff --git a/changelog.d/1321-ci-pull-requests-into-any-base.changed.md b/changelog.d/1321-ci-pull-requests-into-any-base.changed.md
new file mode 100644
index 000000000..40534bcbc
--- /dev/null
+++ b/changelog.d/1321-ci-pull-requests-into-any-base.changed.md
@@ -0,0 +1 @@
+The Tests workflow (`.github/workflows/test.yml`) now runs for pull requests into any base branch, not only `main` (microcosm#1321). A stacked pull request, whose base is another pull request's branch, previously got no CI at all. Pushes still run it only for `main`, and the jobs, the country selection, the concurrency group and the secrets are unchanged.
diff --git a/docs/agent-guide.md b/docs/agent-guide.md
index b14084800..b6b043db8 100644
--- a/docs/agent-guide.md
+++ b/docs/agent-guide.md
@@ -41,7 +41,13 @@ or `uv sync --all-packages --all-extras`. To keep both countries set up, point
 
 PR CI (`.github/workflows/test.yml`) has `lint`, `engine-free`, `engine-us`,
 `engine-uk`, `integration-uk`, and `wheels` jobs, plus the
-`select-countries` orchestration job.
+`select-countries` orchestration job. It runs for pull requests into any base
+branch, so a stacked pull request, whose base is another pull request's
+branch, gets the same jobs; pushes run it only for `main`. That costs runner
+time: every push to a pull request's branch, whatever its base, runs the full
+workflow on the organization's shared runners, with the jobs and country
+selection a pull request into `main` would get. Merging one change up a stack
+of N pull requests therefore starts N full runs.
 `tools/ci_test_plan.py` is the only authority for test-directory ownership, CI
 job assignment, country ownership, US timing-report categories, and
 changed-path country selection. Its `TEST_GROUPS` registry defines every valid
diff --git a/packages/microcosm-build/tests/engine_free/shared/test_ci_integration_job.py b/packages/microcosm-build/tests/engine_free/shared/test_ci_integration_job.py
index bcaac635a..70caa2549 100644
--- a/packages/microcosm-build/tests/engine_free/shared/test_ci_integration_job.py
+++ b/packages/microcosm-build/tests/engine_free/shared/test_ci_integration_job.py
@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+from typing import Any
+
 import yaml
 
 from test_support.paths import paths_for
@@ -71,6 +73,30 @@ def test_integration_job_is_uk_selected_and_read_only() -> None:
     assert "run: bash tools/run_integration_tests.sh" in integration
 
 
+def _workflow_triggers() -> dict[str, Any]:
+    workflow = yaml.safe_load(_TEST_WORKFLOW.read_text(encoding="utf-8"))
+    # PyYAML follows YAML 1.1, which reads the bare key ``on`` as boolean true.
+    return workflow[True]
+
+
+def test_pull_requests_into_any_base_branch_run_the_tests_workflow() -> None:
+    """A stacked pull request targets another pull request's branch, not main,
+    and must get the same jobs.
+
+    GitHub matches ``branches`` and ``branches-ignore`` entries as glob
+    patterns, so checking particular branch names against them proves nothing
+    about other bases. The trigger must have no base-branch filter at all.
+    """
+    pull_request_trigger = _workflow_triggers()["pull_request"] or {}
+
+    base_branch_filters = {"branches", "branches-ignore"} & pull_request_trigger.keys()
+    assert sorted(base_branch_filters) == []
+
+
+def test_only_pushes_to_main_run_the_tests_workflow() -> None:
+    assert _workflow_triggers()["push"] == {"branches": ["main"]}
+
+
 def test_every_workflow_job_caps_its_runtime() -> None:
     """A hung job must not hold an org-shared runner for GitHub's 6-hour default."""
     jobs = yaml.safe_load(_TEST_WORKFLOW.read_text(encoding="utf-8"))["jobs"]