fix(langfuse): hand the SDK client a validated sample rate so an unusable LANGFUSE_SAMPLE_RATE no longer breaks the callback

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

litellm/integrations/langfuse/langfuse_sdk.py

@@ -395,6 +395,12 @@ def _parse_sample_rate(raw: str) -> float | None:
     return rate if 0.0 <= rate <= 1.0 else None
 
 
+def _usable_sample_rate() -> float:
+    raw: Final = os.environ.get("LANGFUSE_SAMPLE_RATE")
+    parsed: Final = _parse_sample_rate(raw) if raw is not None else 1.0
+    return 1.0 if parsed is None else parsed
+
+
 def configured_sample_rate() -> float:
     """``LANGFUSE_SAMPLE_RATE`` as a fraction, exporting everything when it is unset or unusable."""
     raw: Final = os.environ.get("LANGFUSE_SAMPLE_RATE")
@@ -633,6 +639,9 @@ def build_langfuse_client(
     That same cache keeps the first secret and host it saw for a public key, so the REST client
     behind ``get_prompt`` and ``auth_check`` is rebuilt from the credentials actually supplied.
     Without both keys the SDK disables the client, which has no REST client to rebuild.
+
+    The SDK reads ``LANGFUSE_SAMPLE_RATE`` itself and raises on anything it cannot parse, so it
+    gets the rate litellm already validated; the sampler that matters is on litellm's provider.
     """
     public_key: Final = parameters.get("public_key")
     secret_key: Final = parameters.get("secret_key")
@@ -641,6 +650,7 @@ def build_langfuse_client(
     credentialed: Final = isinstance(public_key, str) and isinstance(secret_key, str)
     client: Final = Langfuse(
         **parameters,  # pyright: ignore[reportArgumentType]  # kwargs-ok: dict mirrors the typed ctor, values resolved by the callers
+        sample_rate=_usable_sample_rate(),
         tracer_provider=TracerProvider(
             resource=_resource(environment=environment, release=release), shutdown_on_exit=False
         ),

tests/test_litellm/integrations/langfuse/test_langfuse_sdk.py

@@ -555,6 +555,33 @@ def test_sdk_client_without_keys_is_built_disabled_and_fails_auth_check(monkeypa
     assert client.auth_check() is False
 
 
+@pytest.mark.parametrize("raw", ["1.5", "-0.5", "abc"])
+def test_sdk_client_is_built_despite_an_unusable_sample_rate(monkeypatch: pytest.MonkeyPatch, raw: str):
+    """The SDK parses ``LANGFUSE_SAMPLE_RATE`` itself and would raise, which took the whole callback down."""
+    monkeypatch.setenv("LANGFUSE_SAMPLE_RATE", raw)
+    requests = []
+
+    def record(request: httpx.Request) -> httpx.Response:
+        requests.append(request)
+        return httpx.Response(401, json={"message": "unauthorized"})
+
+    client = build_langfuse_client(
+        parameters={
+            "public_key": "pk-sr-test-" + raw,
+            "secret_key": "sk",
+            "base_url": "http://127.0.0.1:1",
+            "httpx_client": httpx.Client(transport=httpx.MockTransport(record)),
+        },
+        environment=None,
+        release=None,
+        mock_mode=True,
+    )
+
+    with pytest.raises(UnauthorizedError):
+        client.auth_check()
+    assert requests[-1].headers["authorization"] == "Basic " + b64encode(f"pk-sr-test-{raw}:sk".encode()).decode()
+
+
 def test_sdk_client_does_not_take_over_the_process_tracer_provider():
     provider_before = otel_trace.get_tracer_provider()
     build_langfuse_client(