(bug): fix to queue drain

src/agentex/lib/core/tracing/processors/agentex_tracing_processor.py

@@ -79,6 +79,9 @@ def __init__(self, config: AgentexTracingProcessorConfig):  # noqa: ARG002
             ),
         )
 
+    # TODO(AGX1-199): Add batch create/update endpoints to Agentex API and use
+    # them here instead of one HTTP call per span.
+    # https://linear.app/scale-epd/issue/AGX1-199/add-agentex-batch-endpoint-for-traces
     @override
     async def on_span_start(self, span: Span) -> None:
         await self.client.spans.create(

src/agentex/lib/core/tracing/processors/sgp_tracing_processor.py

@@ -141,6 +141,9 @@ async def on_span_start(self, span: Span) -> None:
         if self.disabled:
             logger.warning("SGP is disabled, skipping span upsert")
             return
+        # TODO(AGX1-198): Batch multiple spans into a single upsert_batch call
+        # instead of one span per HTTP request.
+        # https://linear.app/scale-epd/issue/AGX1-198/actually-use-sgp-batching-for-spans
         await self.sgp_async_client.spans.upsert_batch(  # type: ignore[union-attr]
             items=[sgp_span.to_request_params()]
         )
@@ -155,6 +158,7 @@ async def on_span_end(self, span: Span) -> None:
             return
 
         self._add_source_to_span(span)
+        sgp_span.input = span.input  # type: ignore[assignment]
         sgp_span.output = span.output  # type: ignore[assignment]
         sgp_span.metadata = span.data  # type: ignore[assignment]
         sgp_span.end_time = span.end_time.isoformat()  # type: ignore[union-attr]

src/agentex/lib/core/tracing/span_queue.py

@@ -12,6 +12,8 @@
 
 logger = make_logger(__name__)
 
+_DEFAULT_BATCH_SIZE = 50
+
 
 class SpanEventType(str, Enum):
     START = "start"
@@ -28,15 +30,18 @@ class _SpanQueueItem:
 class AsyncSpanQueue:
     """Background FIFO queue for async span processing.
 
-    Span events are enqueued synchronously (non-blocking) and processed
-    sequentially by a background drain task. This keeps tracing HTTP calls
-    off the critical request path while preserving start-before-end ordering.
+    Span events are enqueued synchronously (non-blocking) and drained by a
+    background task.  Items are processed in batches: all START events in a
+    batch are flushed concurrently, then all END events, so that per-span
+    start-before-end ordering is preserved while HTTP calls for independent
+    spans execute in parallel.
     """
 
-    def __init__(self) -> None:
+    def __init__(self, batch_size: int = _DEFAULT_BATCH_SIZE) -> None:
         self._queue: asyncio.Queue[_SpanQueueItem] = asyncio.Queue()
         self._drain_task: asyncio.Task[None] | None = None
         self._stopping = False
+        self._batch_size = batch_size
 
     def enqueue(
         self,
@@ -54,9 +59,45 @@ def _ensure_drain_running(self) -> None:
         if self._drain_task is None or self._drain_task.done():
             self._drain_task = asyncio.create_task(self._drain_loop())
 
+    # ------------------------------------------------------------------
+    # Drain loop
+    # ------------------------------------------------------------------
+
     async def _drain_loop(self) -> None:
         while True:
-            item = await self._queue.get()
+            # Block until at least one item is available.
+            first = await self._queue.get()
+            batch: list[_SpanQueueItem] = [first]
+
+            # Opportunistically grab more ready items (non-blocking).
+            while len(batch) < self._batch_size:
+                try:
+                    batch.append(self._queue.get_nowait())
+                except asyncio.QueueEmpty:
+                    break
+
+            try:
+                # Separate START and END events.  Processing all STARTs before
+                # ENDs ensures that on_span_start completes before on_span_end
+                # for any span whose both events land in the same batch.
+                starts = [i for i in batch if i.event_type == SpanEventType.START]
+                ends = [i for i in batch if i.event_type == SpanEventType.END]
+
+                if starts:
+                    await self._process_items(starts)
+                if ends:
+                    await self._process_items(ends)
+            finally:
+                for _ in batch:
+                    self._queue.task_done()
+                # Release span data for GC.
+                batch.clear()
+
+    @staticmethod
+    async def _process_items(items: list[_SpanQueueItem]) -> None:
+        """Process a list of span events concurrently."""
+
+        async def _handle(item: _SpanQueueItem) -> None:
             try:
                 if item.event_type == SpanEventType.START:
                     coros = [p.on_span_start(item.span) for p in item.processors]
@@ -72,9 +113,15 @@ async def _drain_loop(self) -> None:
                             exc_info=result,
                         )
             except Exception:
-                logger.exception("Unexpected error in span queue drain loop for span %s", item.span.id)
-            finally:
-                self._queue.task_done()
+                logger.exception(
+                    "Unexpected error in span queue for span %s", item.span.id
+                )
+
+        await asyncio.gather(*[_handle(item) for item in items])
+
+    # ------------------------------------------------------------------
+    # Shutdown
+    # ------------------------------------------------------------------
 
     async def shutdown(self, timeout: float = 30.0) -> None:
         self._stopping = True

tests/lib/core/tracing/processors/test_sgp_tracing_processor.py

@@ -162,3 +162,29 @@ async def test_span_end_for_unknown_span_is_noop(self):
         await processor.on_span_end(span)
 
         assert len(processor._spans) == 0
+
+    async def test_sgp_span_input_updated_on_end(self):
+        """on_span_end should update sgp_span.input from the incoming span."""
+        processor, _ = self._make_processor()
+
+        with patch(f"{MODULE}.create_span", side_effect=lambda **kw: _make_mock_sgp_span()):
+            span = _make_span()
+            span.input = {"messages": [{"role": "user", "content": "hello"}]}
+            await processor.on_span_start(span)
+
+        assert len(processor._spans) == 1
+
+        # Simulate modified input at end time
+        updated_input: dict[str, object] = {"messages": [
+            {"role": "user", "content": "hello"},
+            {"role": "assistant", "content": "hi"},
+        ]}
+        span.input = updated_input
+        span.output = {"response": "hi"}
+        span.end_time = datetime.now(UTC)
+        await processor.on_span_end(span)
+
+        # Span should be removed after end
+        assert len(processor._spans) == 0
+        # The end upsert should have been called
+        assert processor.sgp_async_client.spans.upsert_batch.call_count == 2  # start + end

tests/lib/core/tracing/test_span_queue.py

@@ -3,6 +3,7 @@
 import time
 import uuid
 import asyncio
+from typing import cast
 from datetime import UTC, datetime
 from unittest.mock import AsyncMock, MagicMock, patch
 
@@ -52,7 +53,8 @@ async def slow_start(span: Span) -> None:
 
 
 class TestAsyncSpanQueueOrdering:
-    async def test_fifo_ordering_preserved(self):
+    async def test_per_span_start_before_end(self):
+        """START always completes before END for the same span, even with batching."""
         call_log: list[tuple[str, str]] = []
 
         async def record_start(span: Span) -> None:
@@ -77,12 +79,19 @@ async def record_end(span: Span) -> None:
 
         await queue.shutdown()
 
-        assert call_log == [
-            ("start", "span-a"),
-            ("end", "span-a"),
-            ("start", "span-b"),
-            ("end", "span-b"),
-        ]
+        # All 4 events should fire
+        assert len(call_log) == 4
+
+        # Per-span invariant: START before END
+        for span_id in ("span-a", "span-b"):
+            start_idx = next(i for i, (ev, sid) in enumerate(call_log) if ev == "start" and sid == span_id)
+            end_idx = next(i for i, (ev, sid) in enumerate(call_log) if ev == "end" and sid == span_id)
+            assert start_idx < end_idx, f"START should come before END for {span_id}"
+
+        # All STARTs before all ENDs within a batch
+        start_indices = [i for i, (ev, _) in enumerate(call_log) if ev == "start"]
+        end_indices = [i for i, (ev, _) in enumerate(call_log) if ev == "end"]
+        assert max(start_indices) < min(end_indices), "All STARTs should complete before any END"
 
 
 class TestAsyncSpanQueueErrorHandling:
@@ -154,6 +163,61 @@ async def test_enqueue_after_shutdown_is_dropped(self):
         proc.on_span_start.assert_not_called()
 
 
+class TestAsyncSpanQueueBatchConcurrency:
+    async def test_batch_processes_multiple_items_concurrently(self):
+        """Items in the same batch should run concurrently, not serially."""
+        concurrency = 0
+        max_concurrency = 0
+        lock = asyncio.Lock()
+
+        async def slow_start(span: Span) -> None:
+            nonlocal concurrency, max_concurrency
+            async with lock:
+                concurrency += 1
+                max_concurrency = max(max_concurrency, concurrency)
+            await asyncio.sleep(0.05)
+            async with lock:
+                concurrency -= 1
+
+        proc = _make_processor(on_span_start=AsyncMock(side_effect=slow_start))
+        queue = AsyncSpanQueue()
+
+        # Enqueue 10 START events before the drain loop runs — they should
+        # all land in the same batch and be processed concurrently.
+        for i in range(10):
+            queue.enqueue(SpanEventType.START, _make_span(f"span-{i}"), [proc])
+
+        await queue.shutdown()
+
+        assert max_concurrency > 1, (
+            f"Expected concurrent processing, but max concurrency was {max_concurrency}"
+        )
+
+    async def test_batch_faster_than_serial(self):
+        """Batched drain should be significantly faster than serial for slow processors."""
+        n_items = 10
+        per_item_delay = 0.05  # 50ms per processor call
+
+        async def slow_start(span: Span) -> None:
+            await asyncio.sleep(per_item_delay)
+
+        proc = _make_processor(on_span_start=AsyncMock(side_effect=slow_start))
+        queue = AsyncSpanQueue()
+
+        for i in range(n_items):
+            queue.enqueue(SpanEventType.START, _make_span(f"span-{i}"), [proc])
+
+        start = time.monotonic()
+        await queue.shutdown()
+        elapsed = time.monotonic() - start
+
+        serial_time = n_items * per_item_delay
+        assert elapsed < serial_time * 0.5, (
+            f"Batch drain took {elapsed:.3f}s — serial would be {serial_time:.3f}s. "
+            f"Expected at least 2x speedup from concurrency."
+        )
+
+
 class TestAsyncSpanQueueIntegration:
     async def test_integration_with_async_trace(self):
         call_log: list[tuple[str, str]] = []
@@ -196,3 +260,55 @@ async def record_end(span: Span) -> None:
         assert call_log[1][0] == "end"
         # Same span ID for both events
         assert call_log[0][1] == call_log[1][1]
+
+    async def test_end_event_preserves_modified_input(self):
+        """END event should carry span.input so modifications after start are preserved."""
+        start_spans: list[Span] = []
+        end_spans: list[Span] = []
+
+        async def capture_start(span: Span) -> None:
+            start_spans.append(span)
+
+        async def capture_end(span: Span) -> None:
+            end_spans.append(span)
+
+        proc = _make_processor(
+            on_span_start=AsyncMock(side_effect=capture_start),
+            on_span_end=AsyncMock(side_effect=capture_end),
+        )
+        queue = AsyncSpanQueue()
+
+        from agentex.lib.core.tracing.trace import AsyncTrace
+
+        mock_client = MagicMock()
+        trace = AsyncTrace(
+            processors=[proc],
+            client=mock_client,
+            trace_id="test-trace",
+            span_queue=queue,
+        )
+
+        initial_input: dict[str, object] = {"messages": [{"role": "user", "content": "hello"}]}
+        async with trace.span("llm-call", input=initial_input) as span:
+            # Simulate modifying input after start (e.g. chatbot appending messages)
+            messages = cast(list[dict[str, str]], cast(dict[str, object], span.input)["messages"])
+            messages.append({"role": "assistant", "content": "hi there"})
+            messages.append({"role": "user", "content": "how are you?"})
+            span.output = cast(dict[str, object], {"response": "I'm good!"})
+
+        await queue.shutdown()
+
+        assert len(start_spans) == 1
+        assert len(end_spans) == 1
+
+        # START should carry the original input (serialized at start time)
+        assert start_spans[0].input is not None
+        assert len(cast(dict[str, list[object]], start_spans[0].input)["messages"]) == 1  # only the original message
+
+        # END should carry the modified input (re-serialized at end time)
+        assert end_spans[0].input is not None
+        assert len(cast(dict[str, list[object]], end_spans[0].input)["messages"]) == 3  # all three messages
+
+        # END should still carry output and end_time
+        assert end_spans[0].output is not None
+        assert end_spans[0].end_time is not None