diff --git a/posthog/ai/prompts.py b/posthog/ai/prompts.py
index 0e5ef38b..a3d4eba2 100644
--- a/posthog/ai/prompts.py
+++ b/posthog/ai/prompts.py
@@ -532,6 +532,16 @@ def _fetch_prompt_from_api(
             if response.status_code == 404:
                 raise Exception(f"[PostHog Prompts] {prompt_title} not found")
 
+            if response.status_code == 401:
+                raise Exception(
+                    f"[PostHog Prompts] Authentication failed for {prompt_reference}. "
+                    "The key may be missing, expired, or the wrong type. Prompt fetches "
+                    "require a personal API key (starts with 'phx_'); a project secret "
+                    "key is not accepted. Pass this key as personal_api_key when you "
+                    "construct Prompts directly, or as secret_key when you configure the "
+                    "PostHog client."
+                )
+
             if response.status_code == 403:
                 raise Exception(
                     f"[PostHog Prompts] Access denied for {prompt_reference}. "
diff --git a/posthog/test/ai/test_prompts.py b/posthog/test/ai/test_prompts.py
index 15d4b911..774646d1 100644
--- a/posthog/test/ai/test_prompts.py
+++ b/posthog/test/ai/test_prompts.py
@@ -402,6 +402,22 @@ def test_handle_403_response(self, mock_get_session):
             'Access denied for prompt "restricted-prompt"', str(context.exception)
         )
 
+    @patch("posthog.ai.prompts._get_session")
+    def test_handle_401_response(self, mock_get_session):
+        """Should explain the likely auth causes on a 401 response."""
+        mock_get = mock_get_session.return_value.get
+        mock_get.return_value = MockResponse(status_code=401, ok=False)
+
+        posthog = self.create_mock_posthog()
+        prompts = Prompts(posthog)
+
+        with self.assertRaises(Exception) as context:
+            prompts.get("restricted-prompt", with_metadata=False)
+
+        message = str(context.exception)
+        self.assertIn('Authentication failed for prompt "restricted-prompt"', message)
+        self.assertIn("personal API key", message)
+
     def test_throw_when_no_personal_api_key_configured(self):
         """Should throw when no personal_api_key is configured."""
         posthog = self.create_mock_posthog(personal_api_key=None)