diff --git a/scripts/ci/slack_notification_state.py b/scripts/ci/slack_notification_state.py
index f6dc0a035c234..3bedf4545b5e4 100644
--- a/scripts/ci/slack_notification_state.py
+++ b/scripts/ci/slack_notification_state.py
@@ -54,10 +54,15 @@
 
 def download_previous_state(artifact_name: str, repo: str) -> dict | None:
     """Download previous notification state artifact from GitHub Actions."""
+    # `gh api` defaults to POST when -f/-F parameters are passed, which makes
+    # the artifacts list endpoint return 404. Force GET so the parameters are
+    # encoded as query string.
     result = subprocess.run(
         [
             "gh",
             "api",
+            "--method",
+            "GET",
             f"repos/{repo}/actions/artifacts",
             "-f",
             f"name={artifact_name}",
diff --git a/scripts/tests/ci/test_slack_notification_state.py b/scripts/tests/ci/test_slack_notification_state.py
new file mode 100644
index 0000000000000..6400dd1fdf4c5
--- /dev/null
+++ b/scripts/tests/ci/test_slack_notification_state.py
@@ -0,0 +1,71 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+from __future__ import annotations
+
+import importlib.util
+import subprocess
+import sys
+from pathlib import Path
+from unittest.mock import patch
+
+import pytest
+
+MODULE_PATH = Path(__file__).resolve().parents[3] / "scripts" / "ci" / "slack_notification_state.py"
+
+
+@pytest.fixture
+def slack_state_module():
+    module_name = "test_slack_notification_state_module"
+    sys.modules.pop(module_name, None)
+    spec = importlib.util.spec_from_file_location(module_name, MODULE_PATH)
+    assert spec is not None
+    assert spec.loader is not None
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+class TestDownloadPreviousState:
+    def test_uses_explicit_get_method(self, slack_state_module):
+        """`gh api` defaults to POST when -f is passed; we must force GET to avoid 404."""
+        completed = subprocess.CompletedProcess(args=[], returncode=0, stdout="null", stderr="")
+        with patch.object(subprocess, "run", return_value=completed) as mock_run:
+            slack_state_module.download_previous_state("artifact", "apache/airflow")
+        args = mock_run.call_args[0][0]
+        assert "--method" in args
+        method_index = args.index("--method")
+        assert args[method_index + 1] == "GET"
+
+
+class TestDetermineAction:
+    def test_no_failures_no_prev_state(self, slack_state_module):
+        assert slack_state_module.determine_action([], None) == "skip"
+
+    def test_no_failures_no_prev_failures(self, slack_state_module):
+        prev = {"failures": [], "last_notified": "2025-01-01T00:00:00+00:00"}
+        assert slack_state_module.determine_action([], prev) == "skip"
+
+    def test_recovery_when_clear_after_failures(self, slack_state_module):
+        prev = {"failures": ["job-a"], "last_notified": "2025-01-01T00:00:00+00:00"}
+        assert slack_state_module.determine_action([], prev) == "notify_recovery"
+
+    def test_notify_new_on_first_failure(self, slack_state_module):
+        assert slack_state_module.determine_action(["job-a"], None) == "notify_new"
+
+    def test_notify_new_on_changed_failures(self, slack_state_module):
+        prev = {"failures": ["job-a"], "last_notified": "2025-01-01T00:00:00+00:00"}
+        assert slack_state_module.determine_action(["job-b"], prev) == "notify_new"