"""Subagent execution engine."""

import asyncio
import atexit
import logging
import threading
import uuid
from collections.abc import Callable, Coroutine
from concurrent.futures import Future, ThreadPoolExecutor
from concurrent.futures import TimeoutError as FuturesTimeoutError
from contextvars import Context, copy_context
from dataclasses import dataclass, field
from datetime import datetime
from enum import Enum
from typing import Any

from langchain.agents import create_agent
from langchain.tools import BaseTool
from langchain_core.messages import AIMessage, HumanMessage, SystemMessage
from langchain_core.runnables import RunnableConfig

from deerflow.agents.thread_state import SandboxState, ThreadDataState, ThreadState
from deerflow.config import get_app_config
from deerflow.config.app_config import AppConfig
from deerflow.models import create_chat_model
from deerflow.skills.tool_policy import filter_tools_by_skill_allowed_tools
from deerflow.skills.types import Skill
from deerflow.subagents.config import SubagentConfig, resolve_subagent_model_name
from deerflow.subagents.token_collector import SubagentTokenCollector

logger = logging.getLogger(__name__)


_previous_shutdown_isolated_subagent_loop = globals().get("_shutdown_isolated_subagent_loop")
if callable(_previous_shutdown_isolated_subagent_loop):
    atexit.unregister(_previous_shutdown_isolated_subagent_loop)
    _previous_shutdown_isolated_subagent_loop()


class SubagentStatus(Enum):
    """Status of a subagent execution."""

    PENDING = "pending"
    RUNNING = "running"
    COMPLETED = "completed"
    FAILED = "failed"
    CANCELLED = "cancelled"
    TIMED_OUT = "timed_out"

    @property
    def is_terminal(self) -> bool:
        return self in {
            type(self).COMPLETED,
            type(self).FAILED,
            type(self).CANCELLED,
            type(self).TIMED_OUT,
        }


@dataclass
class SubagentResult:
    """Result of a subagent execution.

    Attributes:
        task_id: Unique identifier for this execution.
        trace_id: Trace ID for distributed tracing (links parent and subagent logs).
        status: Current status of the execution.
        result: The final result message (if completed).
        error: Error message (if failed).
        started_at: When execution started.
        completed_at: When execution completed.
        ai_messages: List of complete AI messages (as dicts) generated during execution.
    """

    task_id: str
    trace_id: str
    status: SubagentStatus
    result: str | None = None
    error: str | None = None
    started_at: datetime | None = None
    completed_at: datetime | None = None
    ai_messages: list[dict[str, Any]] | None = None
    token_usage_records: list[dict[str, int | str]] = field(default_factory=list)
    usage_reported: bool = False
    cancel_event: threading.Event = field(default_factory=threading.Event, repr=False)
    _state_lock: threading.Lock = field(default_factory=threading.Lock, init=False, repr=False)

    def __post_init__(self):
        """Initialize mutable defaults."""
        if self.ai_messages is None:
            self.ai_messages = []

    def try_set_terminal(
        self,
        status: SubagentStatus,
        *,
        result: str | None = None,
        error: str | None = None,
        completed_at: datetime | None = None,
        ai_messages: list[dict[str, Any]] | None = None,
        token_usage_records: list[dict[str, int | str]] | None = None,
    ) -> bool:
        """Set a terminal status exactly once.

        Background timeout/cancellation and the execution worker can race on the
        same result holder.  The first terminal transition wins; late terminal
        writes must not change status or payload fields.
        """
        if not status.is_terminal:
            raise ValueError(f"Status {status} is not terminal")

        with self._state_lock:
            if self.status.is_terminal:
                return False

            if result is not None:
                self.result = result
            if error is not None:
                self.error = error
            if ai_messages is not None:
                self.ai_messages = ai_messages
            if token_usage_records is not None:
                self.token_usage_records = token_usage_records
            self.completed_at = completed_at or datetime.now()
            self.status = status
            return True


# Global storage for background task results
_background_tasks: dict[str, SubagentResult] = {}
_background_tasks_lock = threading.Lock()

# Thread pool for background task scheduling and orchestration
_scheduler_pool = ThreadPoolExecutor(max_workers=3, thread_name_prefix="subagent-scheduler-")

# Persistent event loop for isolated subagent executions triggered from an
# already-running parent loop. Reusing one long-lived loop avoids creating a
# fresh loop per execution and then closing async resources bound to it.
_isolated_subagent_loop: asyncio.AbstractEventLoop | None = None
_isolated_subagent_loop_thread: threading.Thread | None = None
_isolated_subagent_loop_started: threading.Event | None = None
_isolated_subagent_loop_lock = threading.Lock()


def _run_isolated_subagent_loop(
    loop: asyncio.AbstractEventLoop,
    started_event: threading.Event,
) -> None:
    """Run the persistent isolated subagent loop in a dedicated daemon thread."""
    asyncio.set_event_loop(loop)
    loop.call_soon(started_event.set)
    try:
        loop.run_forever()
    finally:
        started_event.clear()


def _shutdown_isolated_subagent_loop() -> None:
    """Stop and close the persistent isolated subagent loop."""
    global _isolated_subagent_loop, _isolated_subagent_loop_thread, _isolated_subagent_loop_started

    with _isolated_subagent_loop_lock:
        loop = _isolated_subagent_loop
        thread = _isolated_subagent_loop_thread
        _isolated_subagent_loop = None
        _isolated_subagent_loop_thread = None
        _isolated_subagent_loop_started = None

    if loop is None:
        return

    if loop.is_running():
        loop.call_soon_threadsafe(loop.stop)

    if thread is not None and thread.is_alive() and thread is not threading.current_thread():
        thread.join(timeout=1)

    thread_stopped = thread is None or not thread.is_alive()
    loop_stopped = not loop.is_running()

    if not loop.is_closed():
        if thread_stopped and loop_stopped:
            loop.close()
        else:
            logger.warning(
                "Skipping close of isolated subagent loop because shutdown did not complete within timeout (thread_alive=%s, loop_running=%s)",
                thread is not None and thread.is_alive(),
                loop.is_running(),
            )


atexit.register(_shutdown_isolated_subagent_loop)


def _get_isolated_subagent_loop() -> asyncio.AbstractEventLoop:
    """Return the persistent event loop used by isolated subagent executions."""
    global _isolated_subagent_loop, _isolated_subagent_loop_thread, _isolated_subagent_loop_started
    with _isolated_subagent_loop_lock:
        thread_is_alive = _isolated_subagent_loop_thread is not None and _isolated_subagent_loop_thread.is_alive()
        loop_is_usable = _isolated_subagent_loop is not None and not _isolated_subagent_loop.is_closed() and _isolated_subagent_loop.is_running() and thread_is_alive

        if not loop_is_usable:
            loop = asyncio.new_event_loop()
            started_event = threading.Event()
            thread = threading.Thread(
                target=_run_isolated_subagent_loop,
                args=(loop, started_event),
                name="subagent-persistent-loop",
                daemon=True,
            )
            thread.start()
            if not started_event.wait(timeout=5):
                loop.call_soon_threadsafe(loop.stop)
                thread.join(timeout=1)
                loop.close()
                raise RuntimeError("Timed out starting isolated subagent event loop")
            _isolated_subagent_loop = loop
            _isolated_subagent_loop_thread = thread
            _isolated_subagent_loop_started = started_event

        if _isolated_subagent_loop is None:
            raise RuntimeError("Isolated subagent event loop is not initialized")
        return _isolated_subagent_loop


def _submit_to_isolated_loop_in_context(
    context: Context,
    coro_factory: Callable[[], Coroutine[Any, Any, SubagentResult]],
) -> Future[SubagentResult]:
    """Submit a coroutine to the isolated loop while preserving ContextVar state."""
    return context.run(
        lambda: asyncio.run_coroutine_threadsafe(
            coro_factory(),
            _get_isolated_subagent_loop(),
        )
    )


def _filter_tools(
    all_tools: list[BaseTool],
    allowed: list[str] | None,
    disallowed: list[str] | None,
) -> list[BaseTool]:
    """Filter tools based on subagent configuration.

    Args:
        all_tools: List of all available tools.
        allowed: Optional allowlist of tool names. If provided, only these tools are included.
        disallowed: Optional denylist of tool names. These tools are always excluded.

    Returns:
        Filtered list of tools.
    """
    filtered = all_tools

    # Apply allowlist if specified
    if allowed is not None:
        allowed_set = set(allowed)
        filtered = [t for t in filtered if t.name in allowed_set]

    # Apply denylist
    if disallowed is not None:
        disallowed_set = set(disallowed)
        filtered = [t for t in filtered if t.name not in disallowed_set]

    return filtered


class SubagentExecutor:
    """Executor for running subagents."""

    def __init__(
        self,
        config: SubagentConfig,
        tools: list[BaseTool],
        app_config: AppConfig | None = None,
        parent_model: str | None = None,
        sandbox_state: SandboxState | None = None,
        thread_data: ThreadDataState | None = None,
        thread_id: str | None = None,
        trace_id: str | None = None,
    ):
        """Initialize the executor.

        Args:
            config: Subagent configuration.
            tools: List of all available tools (will be filtered).
            app_config: Resolved AppConfig. When None, ``_create_agent`` falls
                back to ``get_app_config()`` (matches the lead-agent factory's
                pattern).
            parent_model: The parent agent's model name for inheritance.
            sandbox_state: Sandbox state from parent agent.
            thread_data: Thread data from parent agent.
            thread_id: Thread ID for sandbox operations.
            trace_id: Trace ID from parent for distributed tracing.
        """
        self.config = config
        self.app_config = app_config
        self.parent_model = parent_model
        # Resolve eagerly only when it does not require loading config.yaml; otherwise defer
        # to _create_agent (which already loads app_config) so unit tests can construct
        # executors without a config file present.
        if config.model != "inherit" or parent_model is not None or app_config is not None:
            self.model_name: str | None = resolve_subagent_model_name(config, parent_model, app_config=app_config)
        else:
            self.model_name = None
        self.sandbox_state = sandbox_state
        self.thread_data = thread_data
        self.thread_id = thread_id
        # Generate trace_id if not provided (for top-level calls)
        self.trace_id = trace_id or str(uuid.uuid4())[:8]

        self._base_tools = _filter_tools(
            tools,
            config.tools,
            config.disallowed_tools,
        )
        self.tools = self._base_tools

        logger.info(f"[trace={self.trace_id}] SubagentExecutor initialized: {config.name} with {len(self.tools)} tools")

    def _create_agent(self, tools: list[BaseTool] | None = None):
        """Create the agent instance."""
        app_config = self.app_config or get_app_config()
        if self.model_name is None:
            self.model_name = resolve_subagent_model_name(self.config, self.parent_model, app_config=app_config)
        model = create_chat_model(name=self.model_name, thinking_enabled=False, app_config=app_config)

        from deerflow.agents.middlewares.tool_error_handling_middleware import build_subagent_runtime_middlewares

        # Reuse shared middleware composition with lead agent.
        middlewares = build_subagent_runtime_middlewares(app_config=app_config, model_name=self.model_name, lazy_init=True)

        # system_prompt is included in initial state messages (see _build_initial_state)
        # to avoid multiple SystemMessages which some LLM APIs don't support.
        return create_agent(
            model=model,
            tools=tools if tools is not None else self.tools,
            middleware=middlewares,
            system_prompt=None,
            state_schema=ThreadState,
        )

    async def _load_skills(self) -> list[Skill]:
        """Load enabled skill metadata based on config.skills."""
        if self.config.skills is not None and len(self.config.skills) == 0:
            logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} skills=[] — skipping skill loading")
            return []

        try:
            from deerflow.skills.storage import get_or_new_skill_storage

            storage_kwargs = {"app_config": self.app_config} if self.app_config is not None else {}
            storage = await asyncio.to_thread(get_or_new_skill_storage, **storage_kwargs)
            # Use asyncio.to_thread to avoid blocking the event loop (LangGraph ASGI requirement)
            all_skills = await asyncio.to_thread(storage.load_skills, enabled_only=True)
            logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} loaded {len(all_skills)} enabled skills from disk")
        except Exception:
            logger.exception(f"[trace={self.trace_id}] Failed to load skills for subagent {self.config.name}")
            raise

        if not all_skills:
            logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} no enabled skills found")
            return []

        # Filter by config.skills whitelist
        if self.config.skills is not None:
            allowed = set(self.config.skills)
            return [s for s in all_skills if s.name in allowed]
        return all_skills

    def _apply_skill_allowed_tools(self, skills: list[Skill]) -> list[BaseTool]:
        return filter_tools_by_skill_allowed_tools(self._base_tools, skills)

    async def _load_skill_messages(self, skills: list[Skill]) -> list[SystemMessage]:
        """Load skill content as conversation items based on config.skills.

        Aligned with Codex's pattern: each subagent loads its own skills
        per-session and injects them as conversation items (developer messages),
        not as system prompt text. The config.skills whitelist controls which
        skills are loaded:
        - None: load all enabled skills
        - []: no skills
        - ["skill-a", "skill-b"]: only these skills

        Returns:
            List of SystemMessages containing skill content.
        """
        if not skills:
            return []

        # Read each skill's SKILL.md content and create conversation items
        messages = []
        for skill in skills:
            try:
                content = await asyncio.to_thread(skill.skill_file.read_text, encoding="utf-8")
                content = content.strip()
                if content:
                    messages.append(SystemMessage(content=f'<skill name="{skill.name}">\n{content}\n</skill>'))
                    logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} loaded skill: {skill.name}")
            except Exception:
                logger.debug(f"[trace={self.trace_id}] Failed to read skill {skill.name}", exc_info=True)

        return messages

    async def _build_initial_state(self, task: str) -> tuple[dict[str, Any], list[BaseTool]]:
        """Build the initial state for agent execution.

        Args:
            task: The task description.

        Returns:
            Initial state dictionary and tools filtered by loaded skill metadata.
        """

        # Load skills as conversation items (Codex pattern)
        skills = await self._load_skills()
        filtered_tools = self._apply_skill_allowed_tools(skills)
        skill_messages = await self._load_skill_messages(skills)

        # Combine system_prompt and skills into a single SystemMessage.
        # Some LLM APIs reject multiple SystemMessages with
        # "System message must be at the beginning."
        system_parts: list[str] = []
        if self.config.system_prompt:
            system_parts.append(self.config.system_prompt)
        for skill_msg in skill_messages:
            system_parts.append(skill_msg.content)

        messages: list[Any] = []
        if system_parts:
            messages.append(SystemMessage(content="\n\n".join(system_parts)))

        # Then the actual task
        messages.append(HumanMessage(content=task))

        state: dict[str, Any] = {
            "messages": messages,
        }

        # Pass through sandbox and thread data from parent
        if self.sandbox_state is not None:
            state["sandbox"] = self.sandbox_state
        if self.thread_data is not None:
            state["thread_data"] = self.thread_data

        return state, filtered_tools

    async def _aexecute(self, task: str, result_holder: SubagentResult | None = None) -> SubagentResult:
        """Execute a task asynchronously.

        Args:
            task: The task description for the subagent.
            result_holder: Optional pre-created result object to update during execution.

        Returns:
            SubagentResult with the execution result.
        """
        if result_holder is not None:
            # Use the provided result holder (for async execution with real-time updates)
            result = result_holder
        else:
            # Create a new result for synchronous execution
            task_id = str(uuid.uuid4())[:8]
            result = SubagentResult(
                task_id=task_id,
                trace_id=self.trace_id,
                status=SubagentStatus.RUNNING,
                started_at=datetime.now(),
            )
        ai_messages = result.ai_messages
        if ai_messages is None:
            ai_messages = []
            result.ai_messages = ai_messages

        collector: SubagentTokenCollector | None = None
        try:
            state, filtered_tools = await self._build_initial_state(task)
            agent = self._create_agent(filtered_tools)

            # Token collector for subagent LLM calls
            collector_caller = f"subagent:{self.config.name}"
            collector = SubagentTokenCollector(caller=collector_caller)

            # Build config with thread_id for sandbox access and recursion limit
            run_config: RunnableConfig = {
                "recursion_limit": self.config.max_turns,
                "callbacks": [collector],
                "tags": [collector_caller],
            }
            context: dict[str, Any] = {}
            if self.thread_id:
                run_config["configurable"] = {"thread_id": self.thread_id}
                context["thread_id"] = self.thread_id
            if self.app_config is not None:
                context["app_config"] = self.app_config

            logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} starting async execution with max_turns={self.config.max_turns}")

            # Use stream instead of invoke to get real-time updates
            # This allows us to collect AI messages as they are generated
            final_state = None

            # Pre-check: bail out immediately if already cancelled before streaming starts
            if result.cancel_event.is_set():
                logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} cancelled before streaming")
                result.try_set_terminal(
                    SubagentStatus.CANCELLED,
                    error="Cancelled by user",
                    token_usage_records=collector.snapshot_records(),
                )
                return result

            async for chunk in agent.astream(state, config=run_config, context=context, stream_mode="values"):  # type: ignore[arg-type]
                # Cooperative cancellation: check if parent requested stop.
                # Note: cancellation is only detected at astream iteration boundaries,
                # so long-running tool calls within a single iteration will not be
                # interrupted until the next chunk is yielded.
                if result.cancel_event.is_set():
                    logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} cancelled by parent")
                    result.try_set_terminal(
                        SubagentStatus.CANCELLED,
                        error="Cancelled by user",
                        token_usage_records=collector.snapshot_records(),
                    )
                    return result

                final_state = chunk

                # Extract AI messages from the current state
                messages = chunk.get("messages", [])
                if messages:
                    last_message = messages[-1]
                    # Check if this is a new AI message
                    if isinstance(last_message, AIMessage):
                        # Convert message to dict for serialization
                        message_dict = last_message.model_dump()
                        # Only add if it's not already in the list (avoid duplicates)
                        # Check by comparing message IDs if available, otherwise compare full dict
                        message_id = message_dict.get("id")
                        is_duplicate = False
                        if message_id:
                            is_duplicate = any(msg.get("id") == message_id for msg in ai_messages)
                        else:
                            is_duplicate = message_dict in ai_messages

                        if not is_duplicate:
                            ai_messages.append(message_dict)
                            logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} captured AI message #{len(ai_messages)}")

            logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} completed async execution")
            token_usage_records = collector.snapshot_records()
            final_result: str | None = None

            if final_state is None:
                logger.warning(f"[trace={self.trace_id}] Subagent {self.config.name} no final state")
                final_result = "No response generated"
            else:
                # Extract the final message - find the last AIMessage
                messages = final_state.get("messages", [])
                logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} final messages count: {len(messages)}")

                # Find the last AIMessage in the conversation
                last_ai_message = None
                for msg in reversed(messages):
                    if isinstance(msg, AIMessage):
                        last_ai_message = msg
                        break

                if last_ai_message is not None:
                    content = last_ai_message.content
                    # Handle both str and list content types for the final result
                    if isinstance(content, str):
                        final_result = content
                    elif isinstance(content, list):
                        # Extract text from list of content blocks for final result only.
                        # Concatenate raw string chunks directly, but preserve separation
                        # between full text blocks for readability.
                        text_parts = []
                        pending_str_parts = []
                        for block in content:
                            if isinstance(block, str):
                                pending_str_parts.append(block)
                            elif isinstance(block, dict):
                                if pending_str_parts:
                                    text_parts.append("".join(pending_str_parts))
                                    pending_str_parts.clear()
                                text_val = block.get("text")
                                if isinstance(text_val, str):
                                    text_parts.append(text_val)
                        if pending_str_parts:
                            text_parts.append("".join(pending_str_parts))
                        final_result = "\n".join(text_parts) if text_parts else "No text content in response"
                    else:
                        final_result = str(content)
                elif messages:
                    # Fallback: use the last message if no AIMessage found
                    last_message = messages[-1]
                    logger.warning(f"[trace={self.trace_id}] Subagent {self.config.name} no AIMessage found, using last message: {type(last_message)}")
                    raw_content = last_message.content if hasattr(last_message, "content") else str(last_message)
                    if isinstance(raw_content, str):
                        final_result = raw_content
                    elif isinstance(raw_content, list):
                        parts = []
                        pending_str_parts = []
                        for block in raw_content:
                            if isinstance(block, str):
                                pending_str_parts.append(block)
                            elif isinstance(block, dict):
                                if pending_str_parts:
                                    parts.append("".join(pending_str_parts))
                                    pending_str_parts.clear()
                                text_val = block.get("text")
                                if isinstance(text_val, str):
                                    parts.append(text_val)
                        if pending_str_parts:
                            parts.append("".join(pending_str_parts))
                        final_result = "\n".join(parts) if parts else "No text content in response"
                    else:
                        final_result = str(raw_content)
                else:
                    logger.warning(f"[trace={self.trace_id}] Subagent {self.config.name} no messages in final state")
                    final_result = "No response generated"

            if final_result is None:
                final_result = "No response generated"

            result.try_set_terminal(
                SubagentStatus.COMPLETED,
                result=final_result,
                token_usage_records=token_usage_records,
            )

        except Exception as e:
            logger.exception(f"[trace={self.trace_id}] Subagent {self.config.name} async execution failed")
            result.try_set_terminal(
                SubagentStatus.FAILED,
                error=str(e),
                token_usage_records=collector.snapshot_records() if collector is not None else None,
            )

        return result

    def _execute_in_isolated_loop(self, task: str, result_holder: SubagentResult | None = None) -> SubagentResult:
        """Execute the subagent on the persistent isolated event loop.

        This method is used by the sync ``execute()`` path when the caller is
        already running inside an event loop. Because ``execute()`` is a sync
        API, this path blocks the caller while the actual coroutine runs on the
        long-lived isolated loop. Reusing that loop keeps shared async clients
        from being tied to a short-lived loop that gets closed per execution.
        """
        future: Future[SubagentResult] | None = None
        parent_context = copy_context()
        try:
            future = _submit_to_isolated_loop_in_context(
                parent_context,
                lambda: self._aexecute(task, result_holder),
            )
            return future.result(timeout=self.config.timeout_seconds)
        except FuturesTimeoutError:
            if result_holder is not None:
                result_holder.cancel_event.set()
            if future is not None:
                future.cancel()
            raise
        except Exception:
            if future is None:
                logger.debug(
                    f"[trace={self.trace_id}] Failed to submit subagent {self.config.name} to the isolated event loop",
                    exc_info=True,
                )
            else:
                logger.debug(
                    f"[trace={self.trace_id}] Subagent {self.config.name} failed while executing on the isolated event loop",
                    exc_info=True,
                )
            raise

    def execute(self, task: str, result_holder: SubagentResult | None = None) -> SubagentResult:
        """Execute a task synchronously (wrapper around async execution).

        This method runs the async execution in a new event loop, allowing
        asynchronous tools (like MCP tools) to be used within the thread pool.

        When called from within an already-running event loop (e.g., when the
        parent agent is async), this method synchronously waits on the
        persistent isolated loop to avoid event loop conflicts with shared
        async primitives like httpx clients.

        Args:
            task: The task description for the subagent.
            result_holder: Optional pre-created result object to update during execution.

        Returns:
            SubagentResult with the execution result.
        """
        try:
            try:
                loop = asyncio.get_running_loop()
            except RuntimeError:
                loop = None

            if loop is not None and loop.is_running():
                logger.debug(f"[trace={self.trace_id}] Subagent {self.config.name} detected running event loop, using isolated loop")
                return self._execute_in_isolated_loop(task, result_holder)

            # Standard path: no running event loop, use asyncio.run
            return asyncio.run(self._aexecute(task, result_holder))
        except Exception as e:
            logger.exception(f"[trace={self.trace_id}] Subagent {self.config.name} execution failed")
            # Create a result with error if we don't have one
            if result_holder is not None:
                result = result_holder
            else:
                result = SubagentResult(
                    task_id=str(uuid.uuid4())[:8],
                    trace_id=self.trace_id,
                    status=SubagentStatus.RUNNING,
                )
            result.try_set_terminal(SubagentStatus.FAILED, error=str(e))
            return result

    def execute_async(self, task: str, task_id: str | None = None) -> str:
        """Start a task execution in the background.

        Args:
            task: The task description for the subagent.
            task_id: Optional task ID to use. If not provided, a random UUID will be generated.

        Returns:
            Task ID that can be used to check status later.
        """
        # Use provided task_id or generate a new one
        if task_id is None:
            task_id = str(uuid.uuid4())[:8]

        # Create initial pending result
        result = SubagentResult(
            task_id=task_id,
            trace_id=self.trace_id,
            status=SubagentStatus.PENDING,
        )

        logger.info(f"[trace={self.trace_id}] Subagent {self.config.name} starting async execution, task_id={task_id}, timeout={self.config.timeout_seconds}s")

        with _background_tasks_lock:
            _background_tasks[task_id] = result

        parent_context = copy_context()

        # Submit to scheduler pool
        def run_task():
            with _background_tasks_lock:
                _background_tasks[task_id].status = SubagentStatus.RUNNING
                _background_tasks[task_id].started_at = datetime.now()
                result_holder = _background_tasks[task_id]

            try:
                # Submit execution directly to the persistent isolated loop so the
                # background path does not create a temporary loop via execute().
                execution_future = _submit_to_isolated_loop_in_context(
                    parent_context,
                    lambda: self._aexecute(task, result_holder),
                )
                try:
                    # Wait for execution with timeout
                    execution_future.result(timeout=self.config.timeout_seconds)
                except FuturesTimeoutError:
                    logger.error(f"[trace={self.trace_id}] Subagent {self.config.name} execution timed out after {self.config.timeout_seconds}s")
                    # Signal cooperative cancellation and cancel the future
                    result_holder.cancel_event.set()
                    result_holder.try_set_terminal(
                        SubagentStatus.TIMED_OUT,
                        error=f"Execution timed out after {self.config.timeout_seconds} seconds",
                    )
                    execution_future.cancel()
            except Exception as e:
                logger.exception(f"[trace={self.trace_id}] Subagent {self.config.name} async execution failed")
                with _background_tasks_lock:
                    task_result = _background_tasks[task_id]
                task_result.try_set_terminal(SubagentStatus.FAILED, error=str(e))

        _scheduler_pool.submit(run_task)
        return task_id


MAX_CONCURRENT_SUBAGENTS = 3


def request_cancel_background_task(task_id: str) -> None:
    """Signal a running background task to stop.

    Sets the cancel_event on the task, which is checked cooperatively
    by ``_aexecute`` during ``agent.astream()`` iteration.  This allows
    subagent threads — which cannot be force-killed via ``Future.cancel()``
    — to stop at the next iteration boundary.

    Args:
        task_id: The task ID to cancel.
    """
    with _background_tasks_lock:
        result = _background_tasks.get(task_id)
        if result is not None:
            result.cancel_event.set()
            logger.info("Requested cancellation for background task %s", task_id)


def get_background_task_result(task_id: str) -> SubagentResult | None:
    """Get the result of a background task.

    Args:
        task_id: The task ID returned by execute_async.

    Returns:
        SubagentResult if found, None otherwise.
    """
    with _background_tasks_lock:
        return _background_tasks.get(task_id)


def list_background_tasks() -> list[SubagentResult]:
    """List all background tasks.

    Returns:
        List of all SubagentResult instances.
    """
    with _background_tasks_lock:
        return list(_background_tasks.values())


def cleanup_background_task(task_id: str) -> None:
    """Remove a completed task from background tasks.

    Should be called by task_tool after it finishes polling and returns the result.
    This prevents memory leaks from accumulated completed tasks.

    Only removes tasks that are in a terminal state (COMPLETED/FAILED/TIMED_OUT)
    to avoid race conditions with the background executor still updating the task entry.

    Args:
        task_id: The task ID to remove.
    """
    with _background_tasks_lock:
        result = _background_tasks.get(task_id)
        if result is None:
            # Nothing to clean up; may have been removed already.
            logger.debug("Requested cleanup for unknown background task %s", task_id)
            return

        # Only clean up tasks that are in a terminal state to avoid races with
        # the background executor still updating the task entry.
        if result.status.is_terminal or result.completed_at is not None:
            del _background_tasks[task_id]
            logger.debug("Cleaned up background task: %s", task_id)
        else:
            logger.debug(
                "Skipping cleanup for non-terminal background task %s (status=%s)",
                task_id,
                result.status.value if hasattr(result.status, "value") else result.status,
            )
