fix(otel): fix thread leak in OpenTelemetry dynamic header path

Implemented TracerProvider caching in OpenTelemetry class to prevent linear thread growth when using dynamic headers (e.g., per-team Langfuse keys). Added formal unit test in tests/test_otel_thread_leak.py.

litellm/integrations/opentelemetry.py

@@ -140,6 +140,7 @@ class OpenTelemetry(CustomLogger):
         self.OTEL_EXPORTER = self.config.exporter
         self.OTEL_ENDPOINT = self.config.endpoint
         self.OTEL_HEADERS = self.config.headers
+        self._tracer_provider_cache: Dict[str, Any] = {}
         self._init_tracing(tracer_provider)
 
         _debug_otel = str(os.getenv("DEBUG_OTEL", "False")).lower()
@@ -611,12 +612,20 @@ class OpenTelemetry(CustomLogger):
         """Create a temporary tracer with dynamic headers for this request only."""
         from opentelemetry.sdk.trace import TracerProvider
 
+        # Prevents thread exhaustion by reusing providers for the same credential sets (e.g. per-team keys)
+        cache_key = str(sorted(dynamic_headers.items()))
+        if cache_key in self._tracer_provider_cache:
+            return self._tracer_provider_cache[cache_key].get_tracer(LITELLM_TRACER_NAME)
+
         # Create a temporary tracer provider with dynamic headers
         temp_provider = TracerProvider(resource=self._get_litellm_resource(self.config))
         temp_provider.add_span_processor(
             self._get_span_processor(dynamic_headers=dynamic_headers)
         )
 
+        # Store in cache for reuse
+        self._tracer_provider_cache[cache_key] = temp_provider
+
         return temp_provider.get_tracer(LITELLM_TRACER_NAME)
 
     def construct_dynamic_otel_headers(

tests/test_otel_thread_leak.py

@@ -0,0 +1,90 @@
+import sys
+import os
+import threading
+import time
+import pytest
+
+# Add the project root to the path
+sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), "..")))
+
+from litellm.integrations.opentelemetry import OpenTelemetry, OpenTelemetryConfig
+from litellm.types.utils import StandardCallbackDynamicParams
+
+def get_thread_count() -> int:
+    """Helper to get active thread count"""
+    return threading.active_count()
+
+@pytest.fixture
+def otel_logger():
+    """Fixture to provide a clean OTEL logger for each test"""
+    config = OpenTelemetryConfig(
+        exporter="console",
+        enable_metrics=False,
+        service_name="litellm-unit-test"
+    )
+    return OpenTelemetry(config=config)
+
+def test_otel_thread_leak_dynamic_headers(otel_logger):
+    """
+    Unit test to verify that calling get_tracer_to_use_for_request with 
+    dynamic headers doesn't cause a linear thread leak.
+    
+    This test reproduces the issue where each unique team/key credential
+    set causes a new TracerProvider (and its background threads) to be 
+    spawned but never closed.
+    """
+    
+    # 1. Setup dynamic header simulation (monkey-patch)
+    # This simulates what LangfuseOtelLogger does for per-team keys
+    def mock_construct_dynamic_headers(standard_callback_dynamic_params):
+        if standard_callback_dynamic_params:
+            return {"Authorization": "Bearer fake_token"}
+        return None
+    
+    otel_logger.construct_dynamic_otel_headers = mock_construct_dynamic_headers
+    
+    # 2. Establish Baseline
+    initial_threads = get_thread_count()
+    
+    # 3. Simulate requests
+    num_requests = 10
+    latencies = []
+    
+    print("\n🚀 Simulating requests with dynamic headers:")
+    for i in range(num_requests):
+        kwargs = {
+            "standard_callback_dynamic_params": StandardCallbackDynamicParams(
+                langfuse_public_key=f"key_{i}",
+                langfuse_secret_key=f"secret_{i}",
+            )
+        }
+        
+        # Measure latency
+        start_time = time.perf_counter()
+        tracer = otel_logger.get_tracer_to_use_for_request(kwargs)
+        end_time = time.perf_counter()
+        
+        latency_ms = (end_time - start_time) * 1000
+        latencies.append(latency_ms)
+        print(f"   Request {i+1:2d}: Latency = {latency_ms:6.2f} ms")
+        
+        # Verify a tracer was actually returned
+        assert tracer is not None
+    
+    avg_latency = sum(latencies) / len(latencies)
+    print(f"\n📊 Average Latency: {avg_latency:.2f} ms")
+        
+    # 4. Check for leaks
+    # Allow for a small constant increase (OTEL might start a few shared threads)
+    # but a linear leak would result in +10 or more threads here.
+    final_threads = get_thread_count()
+    thread_delta = final_threads - initial_threads
+    
+    print(f"\nThread growth: {thread_delta} threads across {num_requests} requests")
+    
+    # ASSERTION: The growth should be significantly less than 1 thread per request.
+    # If the bug exists, thread_delta will be >= num_requests.
+    assert thread_delta < (num_requests / 2), (
+        f"Thread leak detected! Threads grew by {thread_delta} over {num_requests} requests. "
+        "Each request with dynamic headers appears to be leaking background threads."
+    )