fix: raise LangfuseAuthCheckError instead of bare Exception in auth_check()

auth_check() previously raised a bare Exception when no project was found
for the provided credentials, which is easy to swallow with a broad
`except Exception` at the call site and hides real bugs (issue #906).

Author: mittalpk

langfuse/__init__.py

@@ -96,9 +96,11 @@
 )
 
 Langfuse = _client_module.Langfuse
+LangfuseAuthCheckError = _client_module.LangfuseAuthCheckError
 
 __all__ = [
     "Langfuse",
+    "LangfuseAuthCheckError",
     "LangfuseMedia",
     "LangfuseMediaReference",
     "get_client",

langfuse/_client/client.py

@@ -176,6 +176,10 @@ def _serialize_evaluations(evaluations: List[Evaluation]) -> List[Dict[str, Any]
     ]
 
 
+class LangfuseAuthCheckError(Exception):
+    """Raised by `Langfuse.auth_check()` when no project is found for the provided credentials."""
+
+
 class Langfuse:
     """Main client for Langfuse tracing and platform features.
 
@@ -3474,7 +3478,7 @@ def auth_check(self) -> bool:
         """Check if the provided credentials (public and secret key) are valid.
 
         Raises:
-            Exception: If no projects were found for the provided credentials.
+            LangfuseAuthCheckError: If no projects were found for the provided credentials.
 
         Note:
             This method is blocking. It is discouraged to use it in production code.
@@ -3485,9 +3489,10 @@ def auth_check(self) -> bool:
                 f"Auth check successful, found {len(projects.data)} projects"
             )
             if len(projects.data) == 0:
-                raise Exception(
+                no_project_message = (
                     "Auth check failed, no project found for the keys provided."
                 )
+                raise LangfuseAuthCheckError(no_project_message)
             return True
 
         except AttributeError as e:

tests/unit/test_auth_check.py

@@ -0,0 +1,39 @@
+"""Tests for Langfuse.auth_check() error behavior.
+
+See https://github.com/langfuse/langfuse/issues/906 - auth_check() used to
+raise a bare `Exception` when no project was found for the provided
+credentials, which is easy to swallow with a broad `except Exception` at the
+call site.
+"""
+
+from unittest.mock import Mock
+
+import pytest
+
+from langfuse import Langfuse, LangfuseAuthCheckError
+from langfuse._client.resource_manager import LangfuseResourceManager
+
+
+@pytest.fixture
+def langfuse():
+    langfuse_instance = Langfuse()
+
+    if langfuse_instance._resources is None:
+        langfuse_instance._resources = Mock(spec=LangfuseResourceManager)
+
+    langfuse_instance.api = Mock()
+
+    return langfuse_instance
+
+
+def test_auth_check_raises_langfuse_auth_check_error_when_no_projects(langfuse):
+    langfuse.api.projects.get.return_value = Mock(data=[])
+
+    with pytest.raises(LangfuseAuthCheckError):
+        langfuse.auth_check()
+
+
+def test_auth_check_returns_true_when_projects_found(langfuse):
+    langfuse.api.projects.get.return_value = Mock(data=[Mock()])
+
+    assert langfuse.auth_check() is True