fix(llmobs): move listener hooks to enable instead of on init

ddtrace/llmobs/_llmobs.py

@@ -109,9 +109,9 @@ def __init__(self, tracer=None):
         self._annotations = []
         self._annotation_context_lock = forksafe.RLock()
 
-        # Register hooks for span events
-        core.on("trace.span_start", self._do_annotations)
-        core.on("trace.span_finish", self._on_span_finish)
+    def _on_span_start(self, span):
+        if self.enabled and span.span_type == SpanTypes.LLM:
+            self._do_annotations(span)
 
     def _on_span_finish(self, span):
         if self.enabled and span.span_type == SpanTypes.LLM:
@@ -270,6 +270,10 @@ def _start_service(self) -> None:
             log.debug("Error starting evaluator runner")
 
     def _stop_service(self) -> None:
+        # Remove listener hooks for span events
+        core.reset_listeners("trace.span_start", self._on_span_start)
+        core.reset_listeners("trace.span_finish", self._on_span_finish)
+
         try:
             self._evaluator_runner.stop()
             # flush remaining evaluation spans & evaluations
@@ -364,6 +368,10 @@ def enable(
         cls.enabled = True
         cls._instance.start()
 
+        # Register hooks for span events
+        core.on("trace.span_start", cls._instance._on_span_start)
+        core.on("trace.span_finish", cls._instance._on_span_finish)
+
         atexit.register(cls.disable)
         telemetry_writer.product_activated(TELEMETRY_APM_PRODUCT.LLMOBS, True)
 

releasenotes/notes/fix-llmobs-default-writer-hooks-5e456c2f7dfd4381.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    LLM Observability: Resolves an issue where enabling LLM Observability in agentless mode would result in traces also being sent to the agent proxy endpoint.

tests/llmobs/test_llmobs_service.py

@@ -1319,6 +1319,34 @@ def test_activate_distributed_headers_activates_context(llmobs, mock_llmobs_logs
             mock_activate.assert_called_once_with(dummy_context)
 
 
+def test_listener_hooks_enqueue_correct_writer(run_python_code_in_subprocess):
+    """
+    Regression test that ensures that listener hooks enqueue span events to the correct writer,
+    not the default writer created at startup.
+    """
+    env = os.environ.copy()
+    pypath = [os.path.dirname(os.path.dirname(os.path.dirname(os.path.dirname(__file__))))]
+    if "PYTHONPATH" in env:
+        pypath.append(env["PYTHONPATH"])
+    env.update({"PYTHONPATH": ":".join(pypath), "DD_TRACE_ENABLED": "0"})
+    out, err, status, pid = run_python_code_in_subprocess(
+        """
+from ddtrace.llmobs import LLMObs
+
+LLMObs.enable(ml_app="repro-issue", agentless_enabled=True, api_key="foobar.baz", site="datad0g.com")
+with LLMObs.agent("dummy"):
+    pass
+""",
+        env=env,
+    )
+    assert status == 0, err
+    assert out == b""
+    agentless_writer_log = b"failed to send traces to intake at https://llmobs-intake.datad0g.com/api/v2/llmobs: HTTP error status 403, reason Forbidden\n"  # noqa: E501
+    agent_proxy_log = b"failed to send, dropping 1 traces to intake at http://localhost:8126/evp_proxy/v2/api/v2/llmobs after 5 retries"  # noqa: E501
+    assert err == agentless_writer_log
+    assert agent_proxy_log not in err
+
+
 def test_llmobs_fork_recreates_and_restarts_span_writer():
     """Test that forking a process correctly recreates and restarts the LLMObsSpanWriter."""
     with mock.patch("ddtrace.internal.writer.HTTPWriter._send_payload"):