diff --git a/mergify_cli/ci/detector.py b/mergify_cli/ci/detector.py
index cea90202..e27ca6d2 100644
--- a/mergify_cli/ci/detector.py
+++ b/mergify_cli/ci/detector.py
@@ -178,7 +178,7 @@ def get_github_pull_request_number() -> int | None:
     match get_ci_provider():
         case "github_actions":
             try:
-                event = utils.get_github_event()
+                _, event = utils.get_github_event()
             except utils.GitHubEventNotFoundError:
                 return None
             pr = event.get("pull_request")
diff --git a/mergify_cli/ci/scopes/base_detector.py b/mergify_cli/ci/scopes/base_detector.py
index ac3316bf..8ee4e14b 100644
--- a/mergify_cli/ci/scopes/base_detector.py
+++ b/mergify_cli/ci/scopes/base_detector.py
@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 import dataclasses
-import os
 import typing
 
 import yaml
@@ -69,32 +68,70 @@ def _detect_base_from_event(ev: dict[str, typing.Any]) -> str | None:
     return None
 
 
+def _detect_default_branch_from_event(ev: dict[str, typing.Any]) -> str | None:
+    repo = ev.get("repository")
+    if isinstance(repo, dict):
+        sha = repo.get("default_branch")
+        if isinstance(sha, str) and sha:
+            return sha
+    return None
+
+
+def _detect_base_from_push_event(ev: dict[str, typing.Any]) -> str | None:
+    sha = ev.get("before")
+    if isinstance(sha, str) and sha:
+        return sha
+    return None
+
+
 @dataclasses.dataclass
 class Base:
     ref: str
     is_merge_queue: bool
 
 
+PULL_REQUEST_EVENTS = {
+    "pull_request",
+    "pull_request_review",
+    "pull_request_review_comment",
+    "pull_request_target",
+}
+
+
 def detect() -> Base:
     try:
-        event = utils.get_github_event()
+        event_name, event = utils.get_github_event()
     except utils.GitHubEventNotFoundError:
-        pass
+        # fallback to last commit
+        return Base("HEAD^", is_merge_queue=False)
     else:
-        # 0) merge-queue PR override
-        mq_sha = _detect_base_from_merge_queue_payload(event)
-        if mq_sha:
-            return Base(mq_sha, is_merge_queue=True)
-
-        # 1) standard event payload
-        event_sha = _detect_base_from_event(event)
-        if event_sha:
-            return Base(event_sha, is_merge_queue=False)
-
-    # 2) base ref (e.g., PR target branch)
-    base_ref = os.environ.get("GITHUB_BASE_REF")
-    if base_ref:
-        return Base(base_ref, is_merge_queue=False)
-
-    msg = "Could not detect base SHA. Provide GITHUB_EVENT_PATH / GITHUB_BASE_REF."
+        if event_name in PULL_REQUEST_EVENTS:
+            # 0) merge-queue PR override
+            mq_sha = _detect_base_from_merge_queue_payload(event)
+            if mq_sha:
+                return Base(mq_sha, is_merge_queue=True)
+
+            # 1) standard event payload
+            event_sha = _detect_base_from_event(event)
+            if event_sha:
+                return Base(event_sha, is_merge_queue=False)
+
+            # 2) standard event payload
+            event_sha = _detect_default_branch_from_event(event)
+            if event_sha:
+                return Base(event_sha, is_merge_queue=False)
+
+        elif event_name == "push":
+            event_sha = _detect_base_from_push_event(event)
+            if event_sha:
+                return Base(event_sha, is_merge_queue=False)
+
+            event_sha = _detect_default_branch_from_event(event)
+            if event_sha:
+                return Base(event_sha, is_merge_queue=False)
+        else:
+            msg = "Unhandled GITHUB_EVENT_NAME"
+            raise BaseNotFoundError(msg)
+
+    msg = "Could not detect base SHA. Provide GITHUB_EVENT_NAME / GITHUB_EVENT_PATH."
     raise BaseNotFoundError(msg)
diff --git a/mergify_cli/tests/ci/scopes/test_base_detector.py b/mergify_cli/tests/ci/scopes/test_base_detector.py
index 4fa6a23a..ddb384fa 100644
--- a/mergify_cli/tests/ci/scopes/test_base_detector.py
+++ b/mergify_cli/tests/ci/scopes/test_base_detector.py
@@ -6,18 +6,41 @@
 from mergify_cli.ci.scopes import base_detector
 
 
-def test_detect_base_github_base_ref(
+@pytest.mark.parametrize("event_name", ["pull_request", "pull_request_review", "push"])
+def test_detect_base_from_repository_default_branch(
+    event_name: str,
     monkeypatch: pytest.MonkeyPatch,
+    tmp_path: pathlib.Path,
 ) -> None:
-    monkeypatch.setenv("GITHUB_BASE_REF", "main")
-    monkeypatch.delenv("GITHUB_EVENT_PATH", raising=False)
+    event_data = {"repository": {"default_branch": "main"}}
+    event_file = tmp_path / "event.json"
+    event_file.write_text(json.dumps(event_data))
+
+    monkeypatch.setenv("GITHUB_EVENT_NAME", event_name)
+    monkeypatch.setenv("GITHUB_EVENT_PATH", str(event_file))
 
     result = base_detector.detect()
 
     assert result == base_detector.Base("main", is_merge_queue=False)
 
 
-def test_detect_base_from_event_path(
+def test_detect_base_from_push_event(
+    monkeypatch: pytest.MonkeyPatch,
+    tmp_path: pathlib.Path,
+) -> None:
+    event_data = {"before": "abc123"}
+    event_file = tmp_path / "event.json"
+    event_file.write_text(json.dumps(event_data))
+
+    monkeypatch.setenv("GITHUB_EVENT_NAME", "push")
+    monkeypatch.setenv("GITHUB_EVENT_PATH", str(event_file))
+
+    result = base_detector.detect()
+
+    assert result == base_detector.Base("abc123", is_merge_queue=False)
+
+
+def test_detect_base_from_pull_request_event_path(
     monkeypatch: pytest.MonkeyPatch,
     tmp_path: pathlib.Path,
 ) -> None:
@@ -29,8 +52,8 @@ def test_detect_base_from_event_path(
     event_file = tmp_path / "event.json"
     event_file.write_text(json.dumps(event_data))
 
+    monkeypatch.setenv("GITHUB_EVENT_NAME", "pull_request")
     monkeypatch.setenv("GITHUB_EVENT_PATH", str(event_file))
-    monkeypatch.delenv("GITHUB_BASE_REF", raising=False)
 
     result = base_detector.detect()
 
@@ -51,6 +74,7 @@ def test_detect_base_merge_queue_override(
     event_file = tmp_path / "event.json"
     event_file.write_text(json.dumps(event_data))
 
+    monkeypatch.setenv("GITHUB_EVENT_NAME", "pull_request")
     monkeypatch.setenv("GITHUB_EVENT_PATH", str(event_file))
 
     result = base_detector.detect()
@@ -58,9 +82,16 @@ def test_detect_base_merge_queue_override(
     assert result == base_detector.Base("xyz789", is_merge_queue=True)
 
 
-def test_detect_base_no_info(monkeypatch: pytest.MonkeyPatch) -> None:
-    monkeypatch.delenv("GITHUB_EVENT_PATH", raising=False)
-    monkeypatch.delenv("GITHUB_BASE_REF", raising=False)
+def test_detect_base_no_info(
+    tmp_path: pathlib.Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    event_data: dict[str, str] = {}
+    event_file = tmp_path / "event.json"
+    event_file.write_text(json.dumps(event_data))
+
+    monkeypatch.setenv("GITHUB_EVENT_NAME", "pull_request")
+    monkeypatch.setenv("GITHUB_EVENT_PATH", str(event_file))
 
     with pytest.raises(
         base_detector.BaseNotFoundError,
diff --git a/mergify_cli/tests/test_utils.py b/mergify_cli/tests/test_utils.py
index 3f33a2b5..a075e6e0 100644
--- a/mergify_cli/tests/test_utils.py
+++ b/mergify_cli/tests/test_utils.py
@@ -117,9 +117,11 @@ def test_get_github_event_success(
     event_file = tmp_path / "event.json"
     event_file.write_text(json.dumps(event_data))
 
+    monkeypatch.setenv("GITHUB_EVENT_NAME", "pull_request")
     monkeypatch.setenv("GITHUB_EVENT_PATH", str(event_file))
-    result = utils.get_github_event()
-    assert result == event_data
+    name, event = utils.get_github_event()
+    assert name == "pull_request"
+    assert event == event_data
 
 
 def test_get_github_event_not_found(monkeypatch: pytest.MonkeyPatch) -> None:
diff --git a/mergify_cli/utils.py b/mergify_cli/utils.py
index 55a207f5..726de3fe 100644
--- a/mergify_cli/utils.py
+++ b/mergify_cli/utils.py
@@ -321,12 +321,15 @@ class GitHubEventNotFoundError(Exception):
     pass
 
 
-def get_github_event() -> typing.Any:  # noqa: ANN401
+def get_github_event() -> tuple[str, typing.Any]:
+    event_name = os.environ.get("GITHUB_EVENT_NAME")
+    if not event_name:
+        raise GitHubEventNotFoundError
     event_path = os.environ.get("GITHUB_EVENT_PATH")
     if event_path and pathlib.Path(event_path).is_file():
         try:
             with pathlib.Path(event_path).open("r", encoding="utf-8") as f:
-                return json.load(f)
+                return event_name, json.load(f)
         except FileNotFoundError:
             pass
     raise GitHubEventNotFoundError