Kigi-CLI/crates/codegen/kigi-tools/src/implementations/grok_build/task/backend.rs

//! Backend trait abstracting how subagent operations are dispatched.
//!
//! `SubagentBackend` decouples the tool implementations (`TaskTool`,
//! `TaskOutputTool`, `KillTaskTool`) from the transport mechanism used to
//! communicate with the subagent coordinator.
//!
//! Two implementations are planned:
//!
//! - [`ChannelBackend`] — wraps in-process `tokio::mpsc` channels used by
//!   the local host shell. This is the only implementation today.
//! - `RemoteBackend` (future) — dispatches over a remote transport to an
//!   out-of-process spawner.

use std::sync::Arc;

use tokio::sync::{mpsc, oneshot};

use super::types::{
    SubagentCancelOutcome, SubagentCancelRequest, SubagentCancelTarget, SubagentDescribeOutcome,
    SubagentDescribeRequest, SubagentEvent, SubagentQueryRequest, SubagentRequest, SubagentResult,
    SubagentSnapshot, SubagentValidateTypeOutcome, SubagentValidateTypeRequest,
};
use crate::register_resource;
use kigi_tool_runtime::ToolError;

/// Abstraction over the mechanism used to spawn, query, and cancel subagents.
///
/// Injected into `Resources` as [`SubagentBackendResource`] so that
/// `TaskTool`, `TaskOutputTool`, and `KillTaskTool` can operate
/// identically regardless of the underlying transport.
#[async_trait::async_trait]
pub trait SubagentBackend: Send + Sync + 'static {
    /// Spawn a subagent and await its result.
    ///
    /// For blocking mode the caller awaits the returned future directly.
    /// For background mode the caller spawns a tokio task around this call
    /// and drops the receiver immediately.
    async fn spawn(&self, request: SubagentRequest) -> Result<SubagentResult, ToolError>;

    /// Query the current state of a subagent by ID.
    ///
    /// When `block` is true the backend waits (up to `timeout_ms`) for the
    /// subagent to reach a terminal state before responding.
    async fn query(
        &self,
        id: &str,
        block: bool,
        timeout_ms: Option<u64>,
    ) -> Option<SubagentSnapshot>;

    /// Request cancellation of a subagent by ID.
    async fn cancel(&self, id: &str) -> SubagentCancelOutcome;

    /// Validate a subagent type synchronously before spawning.
    /// Returns `ValidationUnavailable` on channel close / responder drop / timeout.
    async fn validate_type(
        &self,
        subagent_type: &str,
        parent_session_id: &str,
    ) -> SubagentValidateTypeOutcome;

    /// Describe a subagent type's resolved toolset (tool names + capability
    /// flags) before spawning. Read-only: builds the agent definition and
    /// applies the same parent-dependent toolset re-selection a spawn would,
    /// then reports the result without starting a child session.
    ///
    /// Returns [`SubagentDescribeOutcome::Unavailable`] on channel close /
    /// responder drop / timeout (modeled exactly on [`Self::validate_type`]).
    ///
    /// `harness_agent_type` is the `/goal`-only harness override (see
    /// [`super::types::SubagentRuntimeOverrides::harness_agent_type`]); the
    /// coordinator resolves the toolset for `(subagent_type,
    /// harness_agent_type)`. `None` (every non-goal caller) defers the flavor
    /// to the parent agent.
    async fn describe_subagent_type(
        &self,
        subagent_type: &str,
        harness_agent_type: Option<&str>,
        parent_session_id: &str,
    ) -> SubagentDescribeOutcome;
}

/// Resource wrapper injected into every session's `Resources`.
///
/// Wraps an `Arc<dyn SubagentBackend>` so the backend can be shared across
/// concurrent tool invocations within the same session.
#[derive(Clone)]
pub struct SubagentBackendResource(pub Arc<dyn SubagentBackend>);

impl SubagentBackendResource {
    pub fn backend(&self) -> &dyn SubagentBackend {
        self.0.as_ref()
    }
}

impl std::fmt::Debug for SubagentBackendResource {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("SubagentBackendResource").finish()
    }
}

register_resource!(
    "grok_build",
    "SubagentBackendResource",
    SubagentBackendResource
);

/// In-process channel-based backend for the local host shell.
///
/// Wraps a single `mpsc::UnboundedSender<SubagentEvent>` that carries
/// spawn, query, and cancel messages to the coordinator. The oneshot for
/// `spawn` is created inside the backend so callers never manage it.
pub struct ChannelBackend {
    tx: mpsc::UnboundedSender<SubagentEvent>,
}

impl ChannelBackend {
    pub fn new(tx: mpsc::UnboundedSender<SubagentEvent>) -> Self {
        Self { tx }
    }
}

#[async_trait::async_trait]
impl SubagentBackend for ChannelBackend {
    async fn spawn(&self, request: SubagentRequest) -> Result<SubagentResult, ToolError> {
        let (result_tx, result_rx) = oneshot::channel();

        // Replace the dummy oneshot with our fresh one. Using struct update
        // syntax (`..request`) ensures new fields added to `SubagentRequest`
        // are forwarded automatically — a field-by-field copy would silently
        // drop them.
        self.tx
            .send(SubagentEvent::Spawn(Box::new(SubagentRequest {
                result_tx,
                ..request
            })))
            .map_err(|_| {
                ToolError::custom(
                    "channel_closed",
                    "Subagent coordinator channel closed — cannot spawn subagent",
                )
            })?;

        result_rx.await.map_err(|_| {
            ToolError::custom(
                "channel_closed",
                "Subagent result channel dropped — child session may have crashed",
            )
        })
    }

    async fn query(
        &self,
        id: &str,
        block: bool,
        timeout_ms: Option<u64>,
    ) -> Option<SubagentSnapshot> {
        let (respond_to, response_rx) = oneshot::channel();
        let sent = self.tx.send(SubagentEvent::Query(SubagentQueryRequest {
            subagent_id: id.to_string(),
            block,
            timeout_ms,
            respond_to,
        }));
        if sent.is_err() {
            return None;
        }
        response_rx.await.ok().flatten()
    }

    async fn cancel(&self, id: &str) -> SubagentCancelOutcome {
        let (respond_to, response_rx) = oneshot::channel();
        let sent = self.tx.send(SubagentEvent::Cancel(SubagentCancelRequest {
            target: SubagentCancelTarget::SubagentId(id.to_string()),
            respond_to,
        }));
        if sent.is_err() {
            return SubagentCancelOutcome::NotFound;
        }
        response_rx.await.unwrap_or(SubagentCancelOutcome::NotFound)
    }

    async fn validate_type(
        &self,
        subagent_type: &str,
        parent_session_id: &str,
    ) -> SubagentValidateTypeOutcome {
        let (respond_to, response_rx) = oneshot::channel();
        if self
            .tx
            .send(SubagentEvent::ValidateType(SubagentValidateTypeRequest {
                subagent_type: subagent_type.to_string(),
                parent_session_id: parent_session_id.to_string(),
                respond_to,
            }))
            .is_err()
        {
            tracing::warn!(
                subagent_type,
                "coordinator validation channel closed, treating as ValidationUnavailable",
            );
            return SubagentValidateTypeOutcome::ValidationUnavailable;
        }
        let timeout = validate_type_timeout();
        match tokio::time::timeout(timeout, response_rx).await {
            Ok(Ok(outcome)) => outcome,
            Ok(Err(_)) => {
                tracing::warn!(
                    subagent_type,
                    "coordinator validation responder dropped, treating as ValidationUnavailable",
                );
                SubagentValidateTypeOutcome::ValidationUnavailable
            }
            Err(_) => {
                tracing::warn!(
                    subagent_type,
                    timeout_ms = timeout.as_millis() as u64,
                    "coordinator validation timed out, treating as ValidationUnavailable",
                );
                SubagentValidateTypeOutcome::ValidationUnavailable
            }
        }
    }

    async fn describe_subagent_type(
        &self,
        subagent_type: &str,
        harness_agent_type: Option<&str>,
        parent_session_id: &str,
    ) -> SubagentDescribeOutcome {
        let (respond_to, response_rx) = oneshot::channel();
        if self
            .tx
            .send(SubagentEvent::DescribeType(SubagentDescribeRequest {
                subagent_type: subagent_type.to_string(),
                harness_agent_type: harness_agent_type.map(str::to_string),
                parent_session_id: parent_session_id.to_string(),
                respond_to,
            }))
            .is_err()
        {
            tracing::warn!(
                subagent_type,
                "coordinator describe channel closed, treating as Unavailable",
            );
            return SubagentDescribeOutcome::Unavailable;
        }
        let timeout = validate_type_timeout();
        match tokio::time::timeout(timeout, response_rx).await {
            Ok(Ok(outcome)) => outcome,
            Ok(Err(_)) => {
                tracing::warn!(
                    subagent_type,
                    "coordinator describe responder dropped, treating as Unavailable",
                );
                SubagentDescribeOutcome::Unavailable
            }
            Err(_) => {
                tracing::warn!(
                    subagent_type,
                    timeout_ms = timeout.as_millis() as u64,
                    "coordinator describe timed out, treating as Unavailable",
                );
                SubagentDescribeOutcome::Unavailable
            }
        }
    }
}

/// Default `validate_type` timeout. Override via [`VALIDATE_TYPE_TIMEOUT_ENV_VAR`].
pub const VALIDATE_TYPE_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(2);

/// Env-var override for [`VALIDATE_TYPE_TIMEOUT`] (positive milliseconds).
pub const VALIDATE_TYPE_TIMEOUT_ENV_VAR: &str = "XAI_VALIDATE_TYPE_TIMEOUT_MS";

/// Validation timeout, honoring the env-var override.
pub(crate) fn validate_type_timeout() -> std::time::Duration {
    let raw = std::env::var(VALIDATE_TYPE_TIMEOUT_ENV_VAR).ok();
    parse_timeout_ms(raw.as_deref())
        .map(std::time::Duration::from_millis)
        .unwrap_or(VALIDATE_TYPE_TIMEOUT)
}

/// Parse a positive `u64` millisecond value; `None` for unset, invalid, or zero.
pub(crate) fn parse_timeout_ms(value: Option<&str>) -> Option<u64> {
    value?.parse::<u64>().ok().filter(|&ms| ms > 0)
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::sync::Arc;
    use tokio::sync::mpsc;

    /// Helper: receive the next event, match the expected variant, or panic.
    macro_rules! recv_event {
        ($rx:expr, Spawn) => {{
            let event = $rx.recv().await.unwrap();
            match event {
                SubagentEvent::Spawn(inner) => *inner,
                _ => panic!("Expected SubagentEvent::Spawn, got different variant"),
            }
        }};
        ($rx:expr, $variant:ident) => {{
            let event = $rx.recv().await.unwrap();
            match event {
                SubagentEvent::$variant(inner) => inner,
                _ => panic!(
                    "Expected SubagentEvent::{}, got different variant",
                    stringify!($variant)
                ),
            }
        }};
    }

    #[tokio::test]
    async fn channel_backend_spawn_success() {
        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);

        let handle = tokio::spawn(async move {
            let req = recv_event!(rx, Spawn);
            assert_eq!(req.id, "test-id");
            assert_eq!(req.prompt, "do something");
            req.result_tx
                .send(SubagentResult {
                    success: true,
                    output: Arc::from("done"),
                    subagent_id: "test-id".to_string(),
                    child_session_id: "test-id".to_string(),
                    tool_calls: 3,
                    turns: 1,
                    duration_ms: 500,
                    ..Default::default()
                })
                .unwrap();
        });

        let (dummy_tx, _dummy_rx) = oneshot::channel();
        let request = SubagentRequest {
            id: "test-id".to_string(),
            prompt: "do something".to_string(),
            description: "test".to_string(),
            subagent_type: "general-purpose".to_string(),
            parent_session_id: "parent".to_string(),
            parent_prompt_id: None,
            resume_from: None,
            cwd: None,
            runtime_overrides: Default::default(),
            run_in_background: false,
            surface_completion: true,
            fork_context: false,
            result_tx: dummy_tx,
        };

        let result = backend.spawn(request).await.unwrap();
        assert!(result.success);
        assert_eq!(result.subagent_id, "test-id");
        assert_eq!(result.tool_calls, 3);

        handle.await.unwrap();
    }

    #[tokio::test]
    async fn channel_backend_spawn_closed_channel() {
        let (tx, rx) = mpsc::unbounded_channel::<SubagentEvent>();
        drop(rx);

        let backend = ChannelBackend::new(tx);

        let (dummy_tx, _dummy_rx) = oneshot::channel();
        let request = SubagentRequest {
            id: "test-id".to_string(),
            prompt: "do something".to_string(),
            description: "test".to_string(),
            subagent_type: "general-purpose".to_string(),
            parent_session_id: "parent".to_string(),
            parent_prompt_id: None,
            resume_from: None,
            cwd: None,
            runtime_overrides: Default::default(),
            run_in_background: false,
            surface_completion: true,
            fork_context: false,
            result_tx: dummy_tx,
        };

        let err = backend.spawn(request).await.unwrap_err();
        assert!(err.to_string().contains("channel closed"));
    }

    #[tokio::test]
    async fn channel_backend_query_found() {
        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);

        let handle = tokio::spawn(async move {
            let req = recv_event!(rx, Query);
            assert_eq!(req.subagent_id, "sub-1");
            assert!(req.block);
            assert_eq!(req.timeout_ms, Some(5000));
            req.respond_to
                .send(Some(SubagentSnapshot {
                    subagent_id: "sub-1".to_string(),
                    description: "find bugs".to_string(),
                    subagent_type: "explore".to_string(),
                    status: super::super::types::SubagentSnapshotStatus::Completed {
                        output: "result".to_string(),
                        tool_calls: 2,
                        turns: 1,
                        worktree_path: None,
                    },
                    started_at_epoch_ms: 1000,
                    duration_ms: 200,
                    persona: Some("reviewer".to_string()),
                }))
                .unwrap();
        });

        let snap = backend.query("sub-1", true, Some(5000)).await;
        let snap = snap.expect("snapshot should be present");
        assert_eq!(snap.subagent_id, "sub-1");
        assert_eq!(snap.description, "find bugs");
        assert_eq!(snap.subagent_type, "explore");
        assert_eq!(snap.started_at_epoch_ms, 1000);
        assert_eq!(snap.duration_ms, 200);
        assert_eq!(snap.persona.as_deref(), Some("reviewer"));
        match &snap.status {
            super::super::types::SubagentSnapshotStatus::Completed {
                output,
                tool_calls,
                turns,
                worktree_path,
            } => {
                assert_eq!(output, "result");
                assert_eq!(*tool_calls, 2);
                assert_eq!(*turns, 1);
                assert!(worktree_path.is_none());
            }
            other => panic!("Expected Completed, got {:?}", other),
        }

        handle.await.unwrap();
    }

    #[tokio::test]
    async fn channel_backend_query_non_blocking_passes_through() {
        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);

        let handle = tokio::spawn(async move {
            let req = recv_event!(rx, Query);
            assert_eq!(req.subagent_id, "sub-nb");
            assert!(!req.block, "block should be false");
            assert_eq!(req.timeout_ms, None, "timeout_ms should be None");
            req.respond_to.send(None).unwrap();
        });

        let snap = backend.query("sub-nb", false, None).await;
        assert!(snap.is_none());

        handle.await.unwrap();
    }

    #[tokio::test]
    async fn channel_backend_query_not_found() {
        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);

        let handle = tokio::spawn(async move {
            let req = recv_event!(rx, Query);
            req.respond_to.send(None).unwrap();
        });

        let snap = backend.query("nonexistent", false, None).await;
        assert!(snap.is_none());

        handle.await.unwrap();
    }

    #[tokio::test]
    async fn channel_backend_cancel_success() {
        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);

        let handle = tokio::spawn(async move {
            let req = recv_event!(rx, Cancel);
            match &req.target {
                SubagentCancelTarget::SubagentId(id) => assert_eq!(id, "sub-cancel"),
                other => panic!("Expected SubagentId, got {:?}", other),
            }
            req.respond_to
                .send(SubagentCancelOutcome::Cancelled)
                .unwrap();
        });

        let outcome = backend.cancel("sub-cancel").await;
        assert!(matches!(outcome, SubagentCancelOutcome::Cancelled));

        handle.await.unwrap();
    }

    #[tokio::test]
    async fn channel_backend_cancel_closed_channel() {
        let (tx, rx) = mpsc::unbounded_channel::<SubagentEvent>();
        drop(rx);

        let backend = ChannelBackend::new(tx);

        let outcome = backend.cancel("sub-cancel").await;
        assert!(matches!(outcome, SubagentCancelOutcome::NotFound));
    }

    #[tokio::test]
    async fn channel_backend_spawn_result_dropped() {
        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);

        let handle = tokio::spawn(async move {
            let req = recv_event!(rx, Spawn);
            drop(req.result_tx);
        });

        let (dummy_tx, _dummy_rx) = oneshot::channel();
        let request = SubagentRequest {
            id: "drop-test".to_string(),
            prompt: "test".to_string(),
            description: "test".to_string(),
            subagent_type: "general-purpose".to_string(),
            parent_session_id: "parent".to_string(),
            parent_prompt_id: None,
            resume_from: None,
            cwd: None,
            runtime_overrides: Default::default(),
            run_in_background: false,
            surface_completion: true,
            fork_context: false,
            result_tx: dummy_tx,
        };

        let err = backend.spawn(request).await.unwrap_err();
        assert!(
            err.to_string().contains("result channel dropped"),
            "error: {err}"
        );

        handle.await.unwrap();
    }

    #[tokio::test]
    async fn channel_backend_query_closed_channel() {
        let (tx, rx) = mpsc::unbounded_channel::<SubagentEvent>();
        drop(rx);

        let backend = ChannelBackend::new(tx);

        let snap = backend.query("sub-1", false, None).await;
        assert!(snap.is_none());
    }

    // ── validate_type ────────────────────────────────────────────────

    #[tokio::test]
    async fn channel_backend_validate_type_round_trips_outcome() {
        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);

        let handle = tokio::spawn(async move {
            let event = rx.recv().await.unwrap();
            match event {
                SubagentEvent::ValidateType(req) => {
                    assert_eq!(req.subagent_type, "explore");
                    assert_eq!(req.parent_session_id, "parent-1");
                    req.respond_to
                        .send(SubagentValidateTypeOutcome::Ok)
                        .unwrap();
                }
                _ => panic!("Expected ValidateType event"),
            }
        });

        let outcome = backend.validate_type("explore", "parent-1").await;
        assert!(matches!(outcome, SubagentValidateTypeOutcome::Ok));
        handle.await.unwrap();
    }

    #[tokio::test]
    async fn channel_backend_validate_type_propagates_unknown_outcome() {
        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);

        let handle = tokio::spawn(async move {
            if let Some(SubagentEvent::ValidateType(req)) = rx.recv().await {
                req.respond_to
                    .send(SubagentValidateTypeOutcome::Unknown {
                        available: vec!["explore".into(), "plan".into()],
                    })
                    .unwrap();
            }
        });

        let outcome = backend.validate_type("invented", "p").await;
        match outcome {
            SubagentValidateTypeOutcome::Unknown { available } => {
                assert_eq!(available, vec!["explore".to_string(), "plan".to_string()]);
            }
            other => panic!("expected Unknown, got {other:?}"),
        }
        handle.await.unwrap();
    }

    #[tokio::test]
    async fn channel_backend_validate_type_returns_validation_unavailable_when_channel_closed() {
        let (tx, rx) = mpsc::unbounded_channel::<SubagentEvent>();
        drop(rx);
        let backend = ChannelBackend::new(tx);
        let outcome = backend.validate_type("explore", "p").await;
        assert!(matches!(
            outcome,
            SubagentValidateTypeOutcome::ValidationUnavailable
        ));
    }

    #[tokio::test]
    async fn channel_backend_validate_type_returns_validation_unavailable_when_responder_dropped() {
        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);
        let handle = tokio::spawn(async move {
            if let Some(SubagentEvent::ValidateType(req)) = rx.recv().await {
                drop(req.respond_to);
            }
        });
        let outcome = backend.validate_type("explore", "p").await;
        assert!(matches!(
            outcome,
            SubagentValidateTypeOutcome::ValidationUnavailable,
        ));
        handle.await.unwrap();
    }

    use super::super::types::test_capture;

    #[tokio::test(start_paused = true)]
    async fn channel_backend_validate_type_logs_warn_on_timeout() {
        let captured = test_capture::capture();
        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);

        // Coordinator receives but never replies; keeps the responder
        // alive so the timeout arm fires (not responder-dropped).
        let holder = tokio::spawn(async move {
            if let Some(SubagentEvent::ValidateType(req)) = rx.recv().await {
                std::mem::forget(req.respond_to);
                std::future::pending::<()>().await;
            }
        });

        let validate = tokio::spawn(async move { backend.validate_type("explore", "p").await });
        tokio::time::advance(VALIDATE_TYPE_TIMEOUT + std::time::Duration::from_millis(1)).await;
        let outcome = validate.await.unwrap();
        assert!(matches!(
            outcome,
            SubagentValidateTypeOutcome::ValidationUnavailable
        ));

        let mut events_rx = captured.events_rx;
        let mut saw_timeout_warn = false;
        while let Ok(event) = events_rx.try_recv() {
            if event.level == tracing::Level::WARN
                && event.fields.contains("coordinator validation timed out")
                && event.fields.contains("subagent_type=explore")
                && event.fields.contains("timeout_ms=")
            {
                saw_timeout_warn = true;
                break;
            }
        }
        assert!(saw_timeout_warn, "must emit WARN with timeout_ms field");

        holder.abort();
    }

    // ── describe_subagent_type ───────────────────────────────────────

    #[tokio::test]
    async fn channel_backend_describe_round_trips_summary() {
        use super::super::types::{SubagentDescribeOutcome, SubagentTypeSummary};
        use crate::types::tool::ToolKind;

        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);

        let handle = tokio::spawn(async move {
            match rx.recv().await.unwrap() {
                SubagentEvent::DescribeType(req) => {
                    assert_eq!(req.subagent_type, "explore");
                    assert_eq!(req.harness_agent_type.as_deref(), Some("cursor"));
                    assert_eq!(req.parent_session_id, "parent-1");
                    let mut summary = SubagentTypeSummary {
                        can_read: true,
                        can_search: true,
                        ..Default::default()
                    };
                    summary
                        .tool_names
                        .insert(ToolKind::Read, "read_file".to_string());
                    req.respond_to
                        .send(SubagentDescribeOutcome::Ok(summary))
                        .unwrap();
                }
                _ => panic!("Expected DescribeType event"),
            }
        });

        let outcome = backend
            .describe_subagent_type("explore", Some("cursor"), "parent-1")
            .await;
        match outcome {
            SubagentDescribeOutcome::Ok(summary) => {
                assert!(summary.can_read && summary.can_search && !summary.can_execute);
                assert_eq!(
                    summary.tool_names.get(&ToolKind::Read).unwrap(),
                    "read_file"
                );
            }
            other => panic!("expected Ok, got {other:?}"),
        }
        handle.await.unwrap();
    }

    #[tokio::test]
    async fn channel_backend_describe_propagates_not_allowed_outcome() {
        use super::super::types::SubagentDescribeOutcome;

        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);

        let handle = tokio::spawn(async move {
            if let Some(SubagentEvent::DescribeType(req)) = rx.recv().await {
                req.respond_to
                    .send(SubagentDescribeOutcome::NotAllowed {
                        allowed: vec!["explore".into()],
                    })
                    .unwrap();
            }
        });

        match backend.describe_subagent_type("plan", None, "p").await {
            SubagentDescribeOutcome::NotAllowed { allowed } => {
                assert_eq!(allowed, vec!["explore".to_string()]);
            }
            other => panic!("expected NotAllowed, got {other:?}"),
        }
        handle.await.unwrap();
    }

    #[tokio::test]
    async fn channel_backend_describe_returns_unavailable_when_channel_closed() {
        use super::super::types::SubagentDescribeOutcome;
        let (tx, rx) = mpsc::unbounded_channel::<SubagentEvent>();
        drop(rx);
        let backend = ChannelBackend::new(tx);
        assert!(matches!(
            backend.describe_subagent_type("explore", None, "p").await,
            SubagentDescribeOutcome::Unavailable
        ));
    }

    #[tokio::test]
    async fn channel_backend_describe_returns_unavailable_when_responder_dropped() {
        use super::super::types::SubagentDescribeOutcome;
        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);
        let handle = tokio::spawn(async move {
            if let Some(SubagentEvent::DescribeType(req)) = rx.recv().await {
                drop(req.respond_to);
            }
        });
        assert!(matches!(
            backend.describe_subagent_type("explore", None, "p").await,
            SubagentDescribeOutcome::Unavailable
        ));
        handle.await.unwrap();
    }

    #[tokio::test(start_paused = true)]
    async fn channel_backend_describe_returns_unavailable_on_timeout() {
        use super::super::types::SubagentDescribeOutcome;
        let (tx, mut rx) = mpsc::unbounded_channel::<SubagentEvent>();
        let backend = ChannelBackend::new(tx);

        let holder = tokio::spawn(async move {
            if let Some(SubagentEvent::DescribeType(req)) = rx.recv().await {
                std::mem::forget(req.respond_to);
                std::future::pending::<()>().await;
            }
        });

        let describe =
            tokio::spawn(async move { backend.describe_subagent_type("explore", None, "p").await });
        tokio::time::advance(VALIDATE_TYPE_TIMEOUT + std::time::Duration::from_millis(1)).await;
        assert!(matches!(
            describe.await.unwrap(),
            SubagentDescribeOutcome::Unavailable
        ));
        holder.abort();
    }

    #[test]
    fn parse_timeout_ms_returns_none_for_unset() {
        assert_eq!(parse_timeout_ms(None), None);
    }

    #[test]
    fn parse_timeout_ms_returns_none_for_unparseable() {
        assert_eq!(parse_timeout_ms(Some("not-a-number")), None);
        assert_eq!(parse_timeout_ms(Some("")), None);
        assert_eq!(parse_timeout_ms(Some("3.14")), None);
        assert_eq!(parse_timeout_ms(Some("-100")), None);
    }

    #[test]
    fn parse_timeout_ms_returns_none_for_zero() {
        assert_eq!(parse_timeout_ms(Some("0")), None);
    }

    #[test]
    fn parse_timeout_ms_returns_value_for_positive_integer() {
        assert_eq!(parse_timeout_ms(Some("5000")), Some(5000));
        assert_eq!(parse_timeout_ms(Some("1")), Some(1));
    }
}