feat: stream subagent token usage to header via terminal task events (#2882)

* feat: real-time subagent token usage display in header and per-turn

Backend:
- Persist subagent token usage to AIMessage.usage_metadata via
  TokenUsageMiddleware, so accumulateUsage() naturally includes
  subagent tokens without frontend state management
- Cache subagent usage by tool_call_id in task_tool, write back
  to the dispatching AIMessage on next model response
- Emit subagent token usage on all terminal task events
  (task_completed, task_failed, task_cancelled, task_timed_out)
- Report subagent usage to parent RunJournal for API totals
- Search backward from ToolMessage to find dispatching AIMessage
  for correct multi-tool-call attribution

Frontend:
- Remove subagentUsage state, custom event handling, and prop
  threading — subagent tokens are now embedded in message metadata
- Simplify selectHeaderTokenUsage (no subagentUsage parameter)
- Per-turn inline badges show turn-specific usage via message
  accumulation
- Remove isLoading guard from MessageTokenUsageList for dynamic
  updates during streaming

* fix: prevent header token double counting from baseline reset race

onFinish, onError, and thread-switch useEffect all reset
pendingUsageBaselineMessageIdsRef to an empty Set. If
thread.isLoading is still true on the next render, all messages
pass the getMessagesAfterBaseline filter and their tokens are
added to backendUsage (which already includes them), causing
the header to display up to 2× the actual token count.

Capture current message IDs instead of using an empty Set so
that getMessagesAfterBaseline correctly returns no pending
messages even if thread.isLoading lags behind the stream end.

* fix: write back subagent tokens for all concurrent task tool calls

TokenUsageMiddleware only processed messages[-2], so when a
single model response dispatched multiple task tool calls only
the last ToolMessage had its cached subagent usage written back
to the dispatch AIMessage.usage_metadata. Earlier tasks' usage
stayed in _subagent_usage_cache indefinitely (leak) and never
appeared in the per-turn inline token display.

Walk backward through all consecutive ToolMessages before the
new AIMessage, and accumulate updates targeting the same
dispatch message into one state update so overlapping writes
don't clobber each other.

* fix: clean up subagent usage cache entry on task cancellation

When a task_tool invocation is cancelled via CancelledError, any
cached subagent usage entry leaked because the TokenUsageMiddleware
writeback path never fires after cancellation. Pop the cache entry
before re-raising to prevent unbounded growth of the module-level
_subagent_usage_cache dict.

* fix: address token usage review feedback

* fix: handle missing config for subagent usage cache

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/packages/harness/deerflow/tools/builtins/task_tool.py

@@ -26,6 +26,28 @@ if TYPE_CHECKING:
 
 logger = logging.getLogger(__name__)
 
+# Cache subagent token usage by tool_call_id so TokenUsageMiddleware can
+# write it back to the triggering AIMessage's usage_metadata.
+_subagent_usage_cache: dict[str, dict[str, int]] = {}
+
+
+def _token_usage_cache_enabled(app_config: "AppConfig | None") -> bool:
+    if app_config is None:
+        try:
+            app_config = get_app_config()
+        except FileNotFoundError:
+            return False
+    return bool(getattr(getattr(app_config, "token_usage", None), "enabled", False))
+
+
+def _cache_subagent_usage(tool_call_id: str, usage: dict | None, *, enabled: bool = True) -> None:
+    if enabled and usage:
+        _subagent_usage_cache[tool_call_id] = usage
+
+
+def pop_cached_subagent_usage(tool_call_id: str) -> dict | None:
+    return _subagent_usage_cache.pop(tool_call_id, None)
+
 
 def _is_subagent_terminal(result: Any) -> bool:
     """Return whether a background subagent result is safe to clean up."""
@@ -92,6 +114,17 @@ def _find_usage_recorder(runtime: Any) -> Any | None:
     return None
 
 
+def _summarize_usage(records: list[dict] | None) -> dict | None:
+    """Summarize token usage records into a compact dict for SSE events."""
+    if not records:
+        return None
+    return {
+        "input_tokens": sum(r.get("input_tokens", 0) or 0 for r in records),
+        "output_tokens": sum(r.get("output_tokens", 0) or 0 for r in records),
+        "total_tokens": sum(r.get("total_tokens", 0) or 0 for r in records),
+    }
+
+
 def _report_subagent_usage(runtime: Any, result: Any) -> None:
     """Report subagent token usage to the parent RunJournal, if available.
 
@@ -177,6 +210,7 @@ async def task_tool(
         subagent_type: The type of subagent to use. ALWAYS PROVIDE THIS PARAMETER THIRD.
     """
     runtime_app_config = _get_runtime_app_config(runtime)
+    cache_token_usage = _token_usage_cache_enabled(runtime_app_config)
     available_subagent_names = get_available_subagent_names(app_config=runtime_app_config) if runtime_app_config is not None else get_available_subagent_names()
 
     # Get subagent configuration
@@ -312,27 +346,32 @@ async def task_tool(
                 last_message_count = current_message_count
 
             # Check if task completed, failed, or timed out
+            usage = _summarize_usage(getattr(result, "token_usage_records", None))
             if result.status == SubagentStatus.COMPLETED:
+                _cache_subagent_usage(tool_call_id, usage, enabled=cache_token_usage)
                 _report_subagent_usage(runtime, result)
-                writer({"type": "task_completed", "task_id": task_id, "result": result.result})
+                writer({"type": "task_completed", "task_id": task_id, "result": result.result, "usage": usage})
                 logger.info(f"[trace={trace_id}] Task {task_id} completed after {poll_count} polls")
                 cleanup_background_task(task_id)
                 return f"Task Succeeded. Result: {result.result}"
             elif result.status == SubagentStatus.FAILED:
+                _cache_subagent_usage(tool_call_id, usage, enabled=cache_token_usage)
                 _report_subagent_usage(runtime, result)
-                writer({"type": "task_failed", "task_id": task_id, "error": result.error})
+                writer({"type": "task_failed", "task_id": task_id, "error": result.error, "usage": usage})
                 logger.error(f"[trace={trace_id}] Task {task_id} failed: {result.error}")
                 cleanup_background_task(task_id)
                 return f"Task failed. Error: {result.error}"
             elif result.status == SubagentStatus.CANCELLED:
+                _cache_subagent_usage(tool_call_id, usage, enabled=cache_token_usage)
                 _report_subagent_usage(runtime, result)
-                writer({"type": "task_cancelled", "task_id": task_id, "error": result.error})
+                writer({"type": "task_cancelled", "task_id": task_id, "error": result.error, "usage": usage})
                 logger.info(f"[trace={trace_id}] Task {task_id} cancelled: {result.error}")
                 cleanup_background_task(task_id)
                 return "Task cancelled by user."
             elif result.status == SubagentStatus.TIMED_OUT:
+                _cache_subagent_usage(tool_call_id, usage, enabled=cache_token_usage)
                 _report_subagent_usage(runtime, result)
-                writer({"type": "task_timed_out", "task_id": task_id, "error": result.error})
+                writer({"type": "task_timed_out", "task_id": task_id, "error": result.error, "usage": usage})
                 logger.warning(f"[trace={trace_id}] Task {task_id} timed out: {result.error}")
                 cleanup_background_task(task_id)
                 return f"Task timed out. Error: {result.error}"
@@ -351,7 +390,9 @@ async def task_tool(
                 timeout_minutes = config.timeout_seconds // 60
                 logger.error(f"[trace={trace_id}] Task {task_id} polling timed out after {poll_count} polls (should have been caught by thread pool timeout)")
                 _report_subagent_usage(runtime, result)
-                writer({"type": "task_timed_out", "task_id": task_id})
+                usage = _summarize_usage(getattr(result, "token_usage_records", None))
+                _cache_subagent_usage(tool_call_id, usage, enabled=cache_token_usage)
+                writer({"type": "task_timed_out", "task_id": task_id, "usage": usage})
                 return f"Task polling timed out after {timeout_minutes} minutes. This may indicate the background task is stuck. Status: {result.status.value}"
     except asyncio.CancelledError:
         # Signal the background subagent thread to stop cooperatively.
@@ -374,4 +415,8 @@ async def task_tool(
             cleanup_background_task(task_id)
         else:
             _schedule_deferred_subagent_cleanup(task_id, trace_id, max_poll_count)
+        _subagent_usage_cache.pop(tool_call_id, None)
+        raise
+    except Exception:
+        _subagent_usage_cache.pop(tool_call_id, None)
         raise