diff --git a/tensorrt_llm/serve/harmony_adapter.py b/tensorrt_llm/serve/harmony_adapter.py
index f326fc74360e..6a37f28a26b5 100644
--- a/tensorrt_llm/serve/harmony_adapter.py
+++ b/tensorrt_llm/serve/harmony_adapter.py
@@ -417,6 +417,18 @@ def get_debug_info(self) -> dict[str, Any]:
             self.should_filter_tools
         }
 
+    def active_tool_call_names(self) -> list[str]:
+        """Names of tool calls still 'active' at end-of-request.
+
+        A tool call that was started but never closed (a recipient or channel
+        transition did not occur) means the stream ended inside an incomplete
+        tool invocation, which would otherwise be returned to the caller as an
+        empty or truncated tool_calls (see #17437 / #16377)."""
+        return [
+            info["name"] for info in self.tool_calls.values()
+            if info.get("active", True)
+        ]
+
     def finalize_request(self) -> dict[str, Any] | None:
         """
         Finalize the request and return any remaining closing token delta.
@@ -1584,6 +1596,17 @@ def cleanup_stream_state(self, request_id: str) -> None:
         Call this when a request finishes to free memory.
         """
         if request_id in self._stream_states:
+            state = self._stream_states[request_id]
+            incomplete = state.active_tool_call_names()
+            if incomplete:
+                logger.warning(
+                    "Request %s finished with incomplete (never-closed) tool "
+                    "call(s): %s. Often caused by speculative-decode "
+                    "acceptance truncating a control-token prelude; the tool "
+                    "call was silently dropped (see #17437 / #16377).",
+                    request_id,
+                    incomplete,
+                )
             del self._stream_states[request_id]
             logger.debug(f"Cleaned up stream state for request {request_id}")
 
diff --git a/tests/unittest/llmapi/apps/test_harmony_parsing.py b/tests/unittest/llmapi/apps/test_harmony_parsing.py
index 8689454a9e14..d2d9500ecdc9 100644
--- a/tests/unittest/llmapi/apps/test_harmony_parsing.py
+++ b/tests/unittest/llmapi/apps/test_harmony_parsing.py
@@ -1335,5 +1335,84 @@ def test_none_tokenizer_num_postprocess_workers():
         _logit_bias_to_embedding_bias({"0": 1.0}, vocab_size=None)
 
 
+# ===========================================================================
+# Fix 3  -  incomplete (never-closed) tool call guard (see #17437 / #16377)
+# ===========================================================================
+
+
+class TestIncompleteToolCallGuard:
+    """Verify the end-of-request guardrail for unclosed tool calls.
+
+    A tool call that was started but never closed (for example a
+    speculative-decode acceptance truncated a control prelude) must be
+    reported instead of silently dropped.
+    """
+
+    def _make_adapter_state(self):
+        from tensorrt_llm.serve.harmony_adapter import HarmonyAdapter
+
+        adapter = HarmonyAdapter(harmony_input=False, harmony_output=False)
+        request_id = "test-guard-tool"
+        state = adapter.create_stream_state(
+            request_id=request_id, available_tools=None, tool_choice=None
+        )
+        return adapter, request_id, state
+
+    def test_active_tool_call_reported(self):
+        adapter, request_id, state = self._make_adapter_state()
+        try:
+            state.tool_calls["call_1"] = {
+                "id": "call_1",
+                "name": "get_weather",
+                "arguments": '{"city":',
+                "index": 0,
+            }
+            assert state.active_tool_call_names() == ["get_weather"]
+        finally:
+            adapter.cleanup_stream_state(request_id)
+
+    def test_closed_tool_call_not_reported(self):
+        adapter, request_id, state = self._make_adapter_state()
+        try:
+            state.tool_calls["call_1"] = {
+                "id": "call_1",
+                "name": "get_weather",
+                "arguments": '{"city":"SF"}',
+                "index": 0,
+                "active": False,
+            }
+            assert state.active_tool_call_names() == []
+        finally:
+            adapter.cleanup_stream_state(request_id)
+
+    def test_cleanup_warns_on_incomplete_tool_call(self):
+        adapter, request_id, state = self._make_adapter_state()
+        state.tool_calls["call_1"] = {
+            "id": "call_1",
+            "name": "search",
+            "arguments": "",
+            "index": 0,
+        }
+        with patch("tensorrt_llm.serve.harmony_adapter.logger.warning") as mock_warn:
+            adapter.cleanup_stream_state(request_id)
+        assert mock_warn.call_count == 1
+        args = mock_warn.call_args.args
+        assert request_id in args[0]
+        assert "search" in args[1]
+
+    def test_cleanup_no_warning_when_complete(self):
+        adapter, request_id, state = self._make_adapter_state()
+        state.tool_calls["call_1"] = {
+            "id": "call_1",
+            "name": "get_weather",
+            "arguments": '{"city":"SF"}',
+            "index": 0,
+            "active": False,
+        }
+        with patch("tensorrt_llm.serve.harmony_adapter.logger.warning") as mock_warn:
+            adapter.cleanup_stream_state(request_id)
+        mock_warn.assert_not_called()
+
+
 if __name__ == "__main__":
     pytest.main([__file__, "-v"])