diff --git a/gateway/platforms/api_server.py b/gateway/platforms/api_server.py
index a1b066a85b4a..0c22ae3b9230 100644
--- a/gateway/platforms/api_server.py
+++ b/gateway/platforms/api_server.py
@@ -5975,7 +5975,43 @@ class APIServerAdapter(BasePlatformAdapter):
                     "timestamp": ts,
                     "text": preview or "",
                 })
-            # _thinking and subagent_progress are intentionally not forwarded
+            elif event_type in {"subagent.start", "subagent.complete"}:
+                event = {
+                    "event": event_type,
+                    "run_id": run_id,
+                    "timestamp": ts,
+                }
+                if preview is not None:
+                    event["preview"] = preview
+                for key in (
+                    "goal",
+                    "task_count",
+                    "task_index",
+                    "subagent_id",
+                    "parent_id",
+                    "depth",
+                    "model",
+                    "tool_count",
+                    "status",
+                    "summary",
+                    "duration_seconds",
+                    "input_tokens",
+                    "output_tokens",
+                    "reasoning_tokens",
+                    "api_calls",
+                    "cost_usd",
+                    "files_read",
+                    "files_written",
+                    "output_tail",
+                ):
+                    value = kwargs.get(key)
+                    if value is not None:
+                        event[key] = value
+                _push(event)
+            # _thinking, subagent.tool, and subagent_progress are intentionally
+            # not forwarded on the /v1/runs stream: they are high-volume UI
+            # noise. Lifecycle boundaries (start/complete) still need to land
+            # so clients can observe delegate_task timeouts and failures.
 
         return _callback
 
diff --git a/tests/gateway/test_api_server.py b/tests/gateway/test_api_server.py
index 347dc423a86d..37eb3f35f343 100644
--- a/tests/gateway/test_api_server.py
+++ b/tests/gateway/test_api_server.py
@@ -891,6 +891,57 @@ class TestAgentExecution:
         assert captured["service_tier"] == "priority"
 
 
+class TestRunEventCallback:
+    @pytest.mark.asyncio
+    async def test_forwards_subagent_lifecycle_events(self, adapter):
+        run_id = "run_subagent_events"
+        loop = asyncio.get_running_loop()
+        queue = asyncio.Queue()
+        adapter._run_streams[run_id] = queue
+        adapter._run_statuses.pop(run_id, None)
+
+        callback = adapter._make_run_event_callback(run_id, loop)
+
+        callback(
+            "subagent.start",
+            preview="research candidate issue",
+            goal="research candidate issue",
+            task_index=0,
+            task_count=1,
+            subagent_id="deleg_123",
+        )
+        callback(
+            "subagent.complete",
+            preview="Timed out after 300s",
+            goal="research candidate issue",
+            task_index=0,
+            task_count=1,
+            subagent_id="deleg_123",
+            status="timeout",
+            duration_seconds=300.0,
+            summary="Subagent timed out after 300s with 2 API call(s) completed.",
+            api_calls=2,
+        )
+
+        start_event = await asyncio.wait_for(queue.get(), timeout=1.0)
+        complete_event = await asyncio.wait_for(queue.get(), timeout=1.0)
+
+        assert start_event["event"] == "subagent.start"
+        assert start_event["preview"] == "research candidate issue"
+        assert start_event["task_index"] == 0
+        assert start_event["task_count"] == 1
+        assert start_event["subagent_id"] == "deleg_123"
+
+        assert complete_event["event"] == "subagent.complete"
+        assert complete_event["preview"] == "Timed out after 300s"
+        assert complete_event["status"] == "timeout"
+        assert complete_event["duration_seconds"] == 300.0
+        assert complete_event["api_calls"] == 2
+        assert "timed out after 300s" in complete_event["summary"]
+
+        assert adapter._run_statuses[run_id]["last_event"] == "subagent.complete"
+
+
 # ---------------------------------------------------------------------------
 # /health endpoint
 # ---------------------------------------------------------------------------