fix: preserve zero sample rate

langfuse/_client/client.py

@@ -351,7 +351,8 @@ def __init__(
             or get_common_release_envs()
         )
         self._project_id: Optional[str] = None
-        sample_rate = sample_rate or float(os.environ.get(LANGFUSE_SAMPLE_RATE, 1.0))
+        if sample_rate is None:
+            sample_rate = float(os.environ.get(LANGFUSE_SAMPLE_RATE, 1.0))
         if not 0.0 <= sample_rate <= 1.0:
             raise ValueError(
                 f"Sample rate must be between 0.0 and 1.0, got {sample_rate}"

tests/unit/test_initialization.py

@@ -109,6 +109,22 @@ def test_default_base_url(self, cleanup_env_vars):
 
         assert client._base_url == "https://cloud.langfuse.com"
 
+    def test_zero_sample_rate_parameter_is_preserved(
+        self, cleanup_env_vars, monkeypatch
+    ):
+        """Test that an explicit zero sample rate overrides the environment."""
+        monkeypatch.setenv("LANGFUSE_SAMPLE_RATE", "1.0")
+
+        client = Langfuse(
+            public_key="test_pk_zero_sample_rate",
+            secret_key="test_sk",
+            tracing_enabled=False,
+            sample_rate=0.0,
+        )
+
+        assert client._resources is not None
+        assert client._resources.sample_rate == 0.0
+
     def test_base_url_env_var(self, cleanup_env_vars):
         """Test that LANGFUSE_BASE_URL environment variable is used correctly."""
         os.environ["LANGFUSE_BASE_URL"] = "http://test-base-url.com"