zeph_core/

channel.rs

// SPDX-FileCopyrightText: 2026 Andrei G <bug-ops>
// SPDX-License-Identifier: MIT OR Apache-2.0

/// A single field in an elicitation form request.
///
/// Created by the MCP layer when a server sends an elicitation request; passed to
/// channels so they can render the field in a channel-appropriate way (CLI prompt,
/// Telegram inline keyboard, TUI form, etc.).
///
/// # Examples
///
/// ```
/// use zeph_core::channel::{ElicitationField, ElicitationFieldType};
///
/// let field = ElicitationField {
///     name: "username".to_owned(),
///     description: Some("Your login name".to_owned()),
///     field_type: ElicitationFieldType::String,
///     required: true,
/// };
/// assert_eq!(field.name, "username");
/// assert!(field.required);
/// ```
#[derive(Debug, Clone)]
pub struct ElicitationField {
    /// Field key as declared in the server's JSON Schema (sanitized before display).
    pub name: String,
    /// Optional human-readable description from the server (sanitized before display).
    pub description: Option<String>,
    /// Value type expected for this field.
    pub field_type: ElicitationFieldType,
    /// Whether the field must be filled before the form can be submitted.
    pub required: bool,
}

#[non_exhaustive]
/// Type of an elicitation form field.
///
/// # Examples
///
/// ```
/// use zeph_core::channel::ElicitationFieldType;
///
/// let enum_field = ElicitationFieldType::Enum(vec!["low".into(), "medium".into(), "high".into()]);
/// assert!(matches!(enum_field, ElicitationFieldType::Enum(_)));
/// ```
#[derive(Debug, Clone)]
pub enum ElicitationFieldType {
    String,
    Integer,
    Number,
    Boolean,
    /// Enum with allowed values (sanitized before display).
    Enum(Vec<String>),
}

/// An elicitation request from an MCP server.
///
/// Channels receive this struct and are responsible for rendering the form and
/// collecting user input. The `server_name` must be shown to help users identify
/// which server is requesting information (phishing prevention).
///
/// # Examples
///
/// ```
/// use zeph_core::channel::{ElicitationField, ElicitationFieldType, ElicitationRequest};
///
/// let req = ElicitationRequest {
///     server_name: "my-server".to_owned(),
///     message: "Please provide your credentials".to_owned(),
///     fields: vec![ElicitationField {
///         name: "api_key".to_owned(),
///         description: None,
///         field_type: ElicitationFieldType::String,
///         required: true,
///     }],
/// };
/// assert_eq!(req.server_name, "my-server");
/// assert_eq!(req.fields.len(), 1);
/// ```
#[derive(Debug, Clone)]
pub struct ElicitationRequest {
    /// Name of the MCP server making the request (shown for phishing prevention).
    pub server_name: String,
    /// Human-readable message from the server.
    pub message: String,
    /// Form fields to collect from the user.
    pub fields: Vec<ElicitationField>,
}

#[non_exhaustive]
/// User's response to an elicitation request.
///
/// Channels return this after the user interacts with the form. The MCP layer
/// maps `Declined` and `Cancelled` to the appropriate protocol responses.
///
/// # Examples
///
/// ```
/// use serde_json::json;
/// use zeph_core::channel::ElicitationResponse;
///
/// let accepted = ElicitationResponse::Accepted(json!({"username": "alice"}));
/// assert!(matches!(accepted, ElicitationResponse::Accepted(_)));
///
/// let declined = ElicitationResponse::Declined;
/// assert!(matches!(declined, ElicitationResponse::Declined));
/// ```
#[derive(Debug, Clone)]
pub enum ElicitationResponse {
    /// User filled in the form and submitted.
    Accepted(serde_json::Value),
    /// User actively declined to provide input.
    Declined,
    /// User cancelled (e.g. Escape, timeout).
    Cancelled,
}

/// Typed error for channel operations.
#[derive(Debug, thiserror::Error)]
#[non_exhaustive]
pub enum ChannelError {
    /// Underlying I/O failure.
    #[error("I/O error: {0}")]
    Io(#[from] std::io::Error),

    /// Channel closed (mpsc send/recv failure).
    #[error("channel closed")]
    ChannelClosed,

    /// Confirmation dialog cancelled.
    #[error("confirmation cancelled")]
    ConfirmCancelled,

    /// No active session is established yet (no message has been received).
    ///
    /// Occurs when `send` or related methods are called before any message has
    /// arrived on the channel (i.e., `recv` has never returned successfully).
    #[error("no active session")]
    NoActiveSession,

    /// A Telegram Bot API request failed.
    ///
    /// Wraps the teloxide `RequestError` as a string to avoid a direct
    /// `teloxide` dependency in `zeph-core`. The `zeph-channels` adapter
    /// constructs this variant before returning `ChannelError` to the agent.
    #[error("telegram error: {0}")]
    Telegram(String),

    /// Catch-all for third-party API errors that do not map to a more specific variant.
    #[error("{0}")]
    Other(String),
}

impl ChannelError {
    /// Create a `Telegram` error from any displayable teloxide error.
    ///
    /// # Examples
    ///
    /// ```ignore
    /// use zeph_core::channel::ChannelError;
    ///
    /// let err = ChannelError::telegram(teloxide_err);
    /// assert!(matches!(err, ChannelError::Telegram(_)));
    /// ```
    pub fn telegram(e: impl std::fmt::Display) -> Self {
        Self::Telegram(e.to_string())
    }

    /// Create a catch-all error from any displayable error.
    ///
    /// Converts the error message to a string and wraps it in the `Other` variant.
    /// Useful for wrapping provider-specific errors from third-party libraries.
    pub fn other(e: impl std::fmt::Display) -> Self {
        Self::Other(e.to_string())
    }
}

#[non_exhaustive]
/// Kind of binary attachment on an incoming message.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum AttachmentKind {
    Audio,
    Image,
    Video,
    File,
}

/// Binary attachment carried by a [`ChannelMessage`].
#[derive(Debug, Clone)]
pub struct Attachment {
    pub kind: AttachmentKind,
    pub data: Vec<u8>,
    pub filename: Option<String>,
}

/// Incoming message from a channel.
#[derive(Debug, Clone)]
pub struct ChannelMessage {
    pub text: String,
    pub attachments: Vec<Attachment>,
    /// `true` when the message originated from a Telegram guest mention (`guest_message` update).
    pub is_guest_context: bool,
    /// `true` when the sender is a Telegram bot (`from.is_bot = true`).
    pub is_from_bot: bool,
    /// Cross-thread store owner key (spec-080 §10 OQ-1, GitHub #6389) derived by the
    /// originating dispatch path from its own caller identity — the gateway webhook
    /// forwarder derives it from `WebhookPayload.sender`, the A2A task processor from
    /// `Message.context_id`. `None` for CLI/TUI/Telegram, which intentionally collapse to
    /// the default local owner bucket (single-user deployment model, unchanged by this
    /// field).
    pub owner_key: Option<String>,
}

/// Upper bound on [`Channel::send_status_best_effort`]. Status sends are a UX nicety, not a
/// value the agent turn depends on, so a slow or rate-limited channel (see issue #6094 — Discord
/// and Slack's 429 retry loop can otherwise take minutes) must never stall the turn loop past
/// this bound.
///
/// Deliberately much shorter than the full retry-loop worst case (~180-255s — see
/// `common::http_retry`/`common::teloxide_retry`'s `# Timing` docs): a single status ping (e.g.
/// "thinking...") is stale within seconds of being superseded by the next one, so there is no UX
/// value in waiting anywhere near the full retry budget for it. 10s is long enough for one
/// `Retry-After` backoff sleep to complete (typical values are 1-5s) but short enough that even a
/// turn with several status transitions cannot accumulate more than a few tens of seconds of
/// aggregate stall. `send`/`flush_chunks` (the actual response content) are NOT wrapped in this
/// timeout — those are worth retrying to completion, unlike an ephemeral status label.
const STATUS_SEND_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(10);

/// Bidirectional communication channel for the agent.
///
/// # TODO (A3 — deferred: split monolithic Channel into focused sub-traits)
///
/// `Channel` currently has 16+ methods with 12 default no-op bodies. This makes it easy to
/// accidentally ignore capabilities (e.g., streaming, elicitation) on a new channel
/// implementation without a compile error. The planned split:
///
/// - `MessageChannel` — `send` / `recv` (required for all channels)
/// - `StreamingChannel` — `send_streaming_chunk` / `finish_stream` (opt-in)
/// - `ElicitationChannel` — `request_elicitation` (opt-in)
/// - `StatusChannel` — `set_status` / `clear_status` (opt-in)
///
/// **Blocked by:** workspace-wide breaking change affecting CLI, Telegram, TUI, gateway, JSON,
/// Discord, Slack, loopback channels, and all integration tests. Must be migrated channel by
/// channel across ≥5 PRs. Requires its own SDD spec. See critic review §S4.
pub trait Channel: Send {
    /// Receive the next message. Returns `None` on EOF or shutdown.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn recv(&mut self)
    -> impl Future<Output = Result<Option<ChannelMessage>, ChannelError>> + Send;

    /// Non-blocking receive. Returns `None` if no message is immediately available.
    fn try_recv(&mut self) -> Option<ChannelMessage> {
        None
    }

    /// Whether `/exit` and `/quit` commands should terminate the agent loop.
    ///
    /// Returns `false` for persistent server-side channels (e.g. Telegram) where
    /// breaking the loop would not meaningfully exit from the user's perspective.
    fn supports_exit(&self) -> bool {
        true
    }

    /// Whether messages from this channel are raw external input that must be sanitized
    /// (`ContentTrustLevel::ExternalUntrusted`) before the residual, non-command text reaches
    /// the LLM context.
    ///
    /// Returns `true` for direct bot-adapter channels (Telegram, Discord, Slack) whose input
    /// comes from arbitrary remote users. Returns `false` (default) for local/operator-trusted
    /// channels (CLI, TUI) and for [`LoopbackChannel`] — gateway webhooks and A2A messages are
    /// already sanitized by their respective forwarders before being injected as a
    /// [`ChannelMessage`], so sanitizing again here would double-wrap them.
    ///
    /// Sanitization is applied downstream of all command dispatch (`Agent::run`'s registries and
    /// `dispatch_slash_command`), not at `recv`/`try_recv`, so recognized commands still dispatch
    /// on raw text — only the text that actually reaches the LLM is wrapped.
    ///
    /// Also reused as an "is this channel display-owning" proxy by the resume-banner call sites
    /// in `Agent::load_history` and `Agent::load_and_resume_conversation` (spec-068 §13.2,
    /// #6420): `false` gates the banner in; `true` excludes chat channels from it. `JsonCli`
    /// does not override this (correctly local/operator-trusted for sanitization purposes), so
    /// it instead overrides `Channel::send_resume_banner` directly to stay excluded from the
    /// banner without being excluded from sanitization semantics — the two concerns are related
    /// but not identical; check both when adding a new banner-adjacent call site.
    fn requires_input_sanitization(&self) -> bool {
        false
    }

    /// Send a text response.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send(&mut self, text: &str) -> impl Future<Output = Result<(), ChannelError>> + Send;

    /// Send a partial chunk of streaming response.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send_chunk(&mut self, chunk: &str) -> impl Future<Output = Result<(), ChannelError>> + Send;

    /// Flush any buffered chunks.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn flush_chunks(&mut self) -> impl Future<Output = Result<(), ChannelError>> + Send;

    /// Send a typing indicator. No-op by default.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send_typing(&mut self) -> impl Future<Output = Result<(), ChannelError>> + Send {
        async { Ok(()) }
    }

    /// Send a status label (shown as spinner text in TUI). No-op by default.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send_status(
        &mut self,
        _text: &str,
    ) -> impl Future<Output = Result<(), ChannelError>> + Send {
        async { Ok(()) }
    }

    /// Send a bounded transcript slice for `/history` backfill (spec-068 §13.6-§13.7).
    ///
    /// Default: renders `entries` into one flat string via
    /// [`zeph_commands::TranscriptFormatter::render_flat`] and forwards through
    /// [`Channel::send`] — correct for every channel with no structured display buffer of
    /// its own (CLI, Telegram, Discord, Slack). `TuiChannel` overrides this to backfill
    /// per-entry into its own display buffer instead of flattening, keeping the backfill
    /// path split from `input_history`/up-arrow recall (INV-SP-6, §13.7, AC-20).
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send_transcript_backfill(
        &mut self,
        entries: &[zeph_commands::TranscriptEntry],
    ) -> impl Future<Output = Result<(), ChannelError>> + Send {
        let text = zeph_commands::TranscriptFormatter::render_flat(entries);
        async move { self.send(&text).await }
    }

    /// Send a resume banner (spec-068 §13.5) from a live mid-session conversation swap
    /// (`/conv resume`, `/conv fork` — see `Agent::load_and_resume_conversation`), not just the
    /// process-startup path.
    ///
    /// Default: forwards through [`Channel::send`] like any other message — correct for CLI
    /// (prints the line) and any channel with no persistent-banner concept. `TuiChannel`
    /// overrides this to emit `AgentEvent::ResumeBanner` into its persistent header instead of
    /// a scrolling chat line.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send_resume_banner(
        &mut self,
        text: &str,
    ) -> impl Future<Output = Result<(), ChannelError>> + Send {
        async move { self.send(text).await }
    }

    /// Best-effort variant of [`send_status`](Channel::send_status) for the many call sites
    /// where a status update is a UX nicety, not a value the turn depends on.
    ///
    /// Bounds the send to `STATUS_SEND_TIMEOUT` and logs the outcome (`tracing::debug!` on
    /// success, `tracing::warn!` on error or timeout) instead of returning a `Result`. Callers
    /// that used to write `let _ = channel.send_status(...).await;` should call this instead:
    /// failures become visible in logs, and a slow/rate-limited channel (see #6094) can no
    /// longer stall the agent turn loop.
    fn send_status_best_effort(&mut self, text: &str) -> impl Future<Output = ()> + Send {
        async move {
            match tokio::time::timeout(STATUS_SEND_TIMEOUT, self.send_status(text)).await {
                Ok(Ok(())) => tracing::debug!(text, "channel status sent"),
                Ok(Err(error)) => tracing::warn!(%error, text, "channel status send failed"),
                Err(_) => tracing::warn!(
                    text,
                    timeout_secs = STATUS_SEND_TIMEOUT.as_secs(),
                    "channel status send timed out"
                ),
            }
        }
    }

    /// Send a thinking/reasoning token chunk. No-op by default.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send_thinking_chunk(
        &mut self,
        _chunk: &str,
    ) -> impl Future<Output = Result<(), ChannelError>> + Send {
        async { Ok(()) }
    }

    /// Notify channel of queued message count. No-op by default.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send_queue_count(
        &mut self,
        _count: usize,
    ) -> impl Future<Output = Result<(), ChannelError>> + Send {
        async { Ok(()) }
    }

    /// Send the projected context token count to the channel after context assembly.
    ///
    /// The value is an approximation; non-TUI channels may ignore it. No-op by default.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send_context_estimate(
        &mut self,
        _tokens: usize,
    ) -> impl Future<Output = Result<(), ChannelError>> + Send {
        async { Ok(()) }
    }

    /// Send token usage after an LLM call. No-op by default.
    ///
    /// `cost_cents` is the **cumulative** session cost in USD cents as tracked by the
    /// internal cost tracker (already-cumulative value — do not sum across calls).
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send_usage(
        &mut self,
        _input_tokens: u64,
        _output_tokens: u64,
        _context_window: u64,
        _cache_read_tokens: u64,
        _cache_write_tokens: u64,
        _cost_cents: f64,
    ) -> impl Future<Output = Result<(), ChannelError>> + Send {
        async { Ok(()) }
    }

    /// Send diff data for a tool result. No-op by default (TUI overrides).
    ///
    /// `tool_call_id` identifies which tool call produced the diff so it can
    /// be attached to the correct `ChatMessage`.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send_diff(
        &mut self,
        _diff: crate::DiffData,
        _tool_call_id: &str,
    ) -> impl Future<Output = Result<(), ChannelError>> + Send {
        async { Ok(()) }
    }

    /// Announce that a tool call is starting.
    ///
    /// Emitted before execution begins so the transport layer can send an
    /// `InProgress` status to the peer before the result arrives.
    /// No-op by default.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send_tool_start(
        &mut self,
        _event: ToolStartEvent,
    ) -> impl Future<Output = Result<(), ChannelError>> + Send {
        async { Ok(()) }
    }

    /// Send a complete tool output with optional diff and filter stats atomically.
    ///
    /// `display` is the formatted tool output. The default implementation forwards to
    /// [`Channel::send`]. Structured channels (e.g. `LoopbackChannel`) override this to
    /// emit a typed event so consumers can access `tool_name` and `display` as separate fields.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send_tool_output(
        &mut self,
        event: ToolOutputEvent,
    ) -> impl Future<Output = Result<(), ChannelError>> + Send {
        let formatted = crate::agent::format_tool_output(event.tool_name.as_str(), &event.display);
        async move { self.send(&formatted).await }
    }

    /// Request user confirmation for a destructive action. Returns `true` if confirmed.
    /// Default: auto-confirm (for headless/test scenarios).
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn confirm(
        &mut self,
        _prompt: &str,
    ) -> impl Future<Output = Result<bool, ChannelError>> + Send {
        async { Ok(true) }
    }

    /// Request structured input from the user for an MCP elicitation.
    ///
    /// Always displays `request.server_name` to prevent phishing by malicious servers.
    /// Default: auto-decline (for headless/daemon/non-interactive scenarios).
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn elicit(
        &mut self,
        _request: ElicitationRequest,
    ) -> impl Future<Output = Result<ElicitationResponse, ChannelError>> + Send {
        async { Ok(ElicitationResponse::Declined) }
    }

    /// Signal the non-default stop reason to the consumer before flushing.
    ///
    /// Called by the agent loop immediately before `flush_chunks()` when a
    /// truncation or turn-limit condition is detected. No-op by default.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn send_stop_hint(
        &mut self,
        _hint: StopHint,
    ) -> impl Future<Output = Result<(), ChannelError>> + Send {
        async { Ok(()) }
    }

    /// Notify channel that a foreground subagent has started. No-op by default.
    ///
    /// Called after the subagent is spawned and before polling begins. Channels
    /// that support subagent views (e.g. TUI) should switch to the subagent
    /// transcript view on receipt.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn notify_foreground_subagent_started(
        &mut self,
        _id: &str,
        _name: &str,
    ) -> impl Future<Output = Result<(), ChannelError>> + Send {
        async { Ok(()) }
    }

    /// Notify channel that a foreground subagent has completed. No-op by default.
    ///
    /// Called after `poll_subagent_until_done` returns. Channels that support
    /// subagent views should switch back to the main view and show a status
    /// notification.
    ///
    /// # Errors
    ///
    /// Returns an error if the underlying I/O fails.
    fn notify_foreground_subagent_completed(
        &mut self,
        _id: &str,
        _name: &str,
        _success: bool,
    ) -> impl Future<Output = Result<(), ChannelError>> + Send {
        async { Ok(()) }
    }
}

pub use zeph_common::StopHint;

/// Event carrying data for a tool call start, emitted before execution begins.
///
/// Passed by value to [`Channel::send_tool_start`] and carried by
/// [`LoopbackEvent::ToolStart`]. All fields are owned — no lifetime parameters.
#[derive(Debug, Clone)]
pub struct ToolStartEvent {
    /// Name of the tool being invoked.
    pub tool_name: zeph_common::ToolName,
    /// Opaque tool call ID assigned by the LLM.
    pub tool_call_id: String,
    /// Raw input parameters passed to the tool (e.g. `{"command": "..."}` for bash).
    pub params: Option<serde_json::Value>,
    /// Set when this tool call is made by a subagent; identifies the parent's `tool_call_id`.
    pub parent_tool_use_id: Option<String>,
    /// Wall-clock instant when the tool call was initiated; used to compute elapsed time.
    pub started_at: std::time::Instant,
    /// True when this tool call was speculatively dispatched before LLM finished decoding.
    ///
    /// TUI renders a `[spec]` prefix; Telegram suppresses unless `chat_visibility = verbose`.
    pub speculative: bool,
    /// OS sandbox profile applied to this tool call, if any.
    ///
    /// `None` means no sandbox was applied (not configured or not a subprocess executor).
    pub sandbox_profile: Option<zeph_tools::SandboxProfile>,
    /// True when this tool call originates from an MCP server rather than a native tool.
    pub is_mcp: bool,
}

/// Event carrying data for a completed tool output, emitted after execution.
///
/// Passed by value to [`Channel::send_tool_output`] and carried by
/// [`LoopbackEvent::ToolOutput`]. All fields are owned — no lifetime parameters.
#[derive(Debug, Clone)]
pub struct ToolOutputEvent {
    /// Name of the tool that produced this output.
    pub tool_name: zeph_common::ToolName,
    /// Human-readable output text.
    pub display: String,
    /// Optional diff for file-editing tools.
    pub diff: Option<crate::DiffData>,
    /// Optional filter statistics from output filtering.
    pub filter_stats: Option<String>,
    /// Kept line indices after filtering (for display).
    pub kept_lines: Option<Vec<usize>>,
    /// Source locations for code search results.
    pub locations: Option<Vec<String>>,
    /// Opaque tool call ID matching the corresponding `ToolStartEvent`.
    pub tool_call_id: String,
    /// Whether this output represents an error.
    pub is_error: bool,
    /// Terminal ID for shell tool calls routed through the IDE terminal.
    pub terminal_id: Option<String>,
    /// Set when this tool output belongs to a subagent; identifies the parent's `tool_call_id`.
    pub parent_tool_use_id: Option<String>,
    /// Structured tool response payload for ACP intermediate `tool_call_update` notifications.
    pub raw_response: Option<serde_json::Value>,
    /// Wall-clock instant when the corresponding `ToolStartEvent` was emitted.
    pub started_at: Option<std::time::Instant>,
}

/// Backward-compatible alias for [`ToolStartEvent`].
///
/// Kept for use in the ACP layer. Prefer [`ToolStartEvent`] in new code.
pub type ToolStartData = ToolStartEvent;

/// Backward-compatible alias for [`ToolOutputEvent`].
///
/// Kept for use in the ACP layer. Prefer [`ToolOutputEvent`] in new code.
pub type ToolOutputData = ToolOutputEvent;

#[non_exhaustive]
/// Events emitted by the agent side toward the A2A caller.
#[derive(Debug, Clone)]
pub enum LoopbackEvent {
    Chunk(String),
    Flush,
    FullMessage(String),
    Status(String),
    /// Emitted immediately before tool execution begins.
    ToolStart(Box<ToolStartEvent>),
    ToolOutput(Box<ToolOutputEvent>),
    /// Token usage from the last LLM call.
    ///
    /// `cost_cents` is the **cumulative** session cost in USD cents at the time of emission;
    /// receivers should overwrite (not sum) their stored cost field.
    ///
    /// This variant is only produced by `Agent::emit_usage_event` in `metrics_compact.rs`
    /// after a verified LLM response — it is never constructed from external input.
    Usage {
        input_tokens: u64,
        output_tokens: u64,
        context_window: u64,
        /// Cache read tokens for this LLM call.
        cache_read_tokens: u64,
        /// Cache write tokens for this LLM call.
        cache_write_tokens: u64,
        /// Cumulative session cost in USD cents (overwrite, do not sum).
        cost_cents: f64,
    },
    /// Generated session title (emitted after the first agent response).
    SessionTitle(String),
    /// Execution plan update.
    Plan(Vec<(String, PlanItemStatus)>),
    /// Thinking/reasoning token chunk from the LLM.
    ThinkingChunk(String),
    /// Non-default stop condition detected by the agent loop.
    ///
    /// Emitted immediately before `Flush`. When absent, the stop reason is `EndTurn`.
    Stop(StopHint),
}

#[non_exhaustive]
/// Status of a plan item, mirroring `acp::PlanEntryStatus`.
#[derive(Debug, Clone)]
pub enum PlanItemStatus {
    Pending,
    InProgress,
    Completed,
}

/// Caller-side handle for sending input and receiving agent output.
pub struct LoopbackHandle {
    pub input_tx: tokio::sync::mpsc::Sender<ChannelMessage>,
    pub output_rx: tokio::sync::mpsc::Receiver<LoopbackEvent>,
    /// Shared cancel signal: notify to interrupt the agent's current operation.
    pub cancel_signal: std::sync::Arc<tokio::sync::Notify>,
}

/// Headless channel bridging an A2A `TaskProcessor` with the agent loop.
pub struct LoopbackChannel {
    input_rx: tokio::sync::mpsc::Receiver<ChannelMessage>,
    output_tx: tokio::sync::mpsc::Sender<LoopbackEvent>,
}

impl LoopbackChannel {
    /// Create a linked `(LoopbackChannel, LoopbackHandle)` pair.
    #[must_use]
    pub fn pair(buffer: usize) -> (Self, LoopbackHandle) {
        let (input_tx, input_rx) = tokio::sync::mpsc::channel(buffer);
        let (output_tx, output_rx) = tokio::sync::mpsc::channel(buffer);
        let cancel_signal = std::sync::Arc::new(tokio::sync::Notify::new());
        (
            Self {
                input_rx,
                output_tx,
            },
            LoopbackHandle {
                input_tx,
                output_rx,
                cancel_signal,
            },
        )
    }
}

impl Channel for LoopbackChannel {
    fn supports_exit(&self) -> bool {
        false
    }

    async fn recv(&mut self) -> Result<Option<ChannelMessage>, ChannelError> {
        Ok(self.input_rx.recv().await)
    }

    async fn send(&mut self, text: &str) -> Result<(), ChannelError> {
        self.output_tx
            .send(LoopbackEvent::FullMessage(text.to_owned()))
            .await
            .map_err(|_| ChannelError::ChannelClosed)
    }

    async fn send_chunk(&mut self, chunk: &str) -> Result<(), ChannelError> {
        self.output_tx
            .send(LoopbackEvent::Chunk(chunk.to_owned()))
            .await
            .map_err(|_| ChannelError::ChannelClosed)
    }

    async fn flush_chunks(&mut self) -> Result<(), ChannelError> {
        self.output_tx
            .send(LoopbackEvent::Flush)
            .await
            .map_err(|_| ChannelError::ChannelClosed)
    }

    async fn send_status(&mut self, text: &str) -> Result<(), ChannelError> {
        self.output_tx
            .send(LoopbackEvent::Status(text.to_owned()))
            .await
            .map_err(|_| ChannelError::ChannelClosed)
    }

    async fn send_thinking_chunk(&mut self, chunk: &str) -> Result<(), ChannelError> {
        self.output_tx
            .send(LoopbackEvent::ThinkingChunk(chunk.to_owned()))
            .await
            .map_err(|_| ChannelError::ChannelClosed)
    }

    async fn send_tool_start(&mut self, event: ToolStartEvent) -> Result<(), ChannelError> {
        self.output_tx
            .send(LoopbackEvent::ToolStart(Box::new(event)))
            .await
            .map_err(|_| ChannelError::ChannelClosed)
    }

    async fn send_tool_output(&mut self, event: ToolOutputEvent) -> Result<(), ChannelError> {
        self.output_tx
            .send(LoopbackEvent::ToolOutput(Box::new(event)))
            .await
            .map_err(|_| ChannelError::ChannelClosed)
    }

    async fn confirm(&mut self, _prompt: &str) -> Result<bool, ChannelError> {
        Ok(true)
    }

    async fn send_stop_hint(&mut self, hint: StopHint) -> Result<(), ChannelError> {
        self.output_tx
            .send(LoopbackEvent::Stop(hint))
            .await
            .map_err(|_| ChannelError::ChannelClosed)
    }

    async fn send_usage(
        &mut self,
        input_tokens: u64,
        output_tokens: u64,
        context_window: u64,
        cache_read_tokens: u64,
        cache_write_tokens: u64,
        cost_cents: f64,
    ) -> Result<(), ChannelError> {
        self.output_tx
            .send(LoopbackEvent::Usage {
                input_tokens,
                output_tokens,
                context_window,
                cache_read_tokens,
                cache_write_tokens,
                cost_cents,
            })
            .await
            .map_err(|_| ChannelError::ChannelClosed)
    }
}

/// Adapter that wraps a [`Channel`] reference and implements [`zeph_commands::ChannelSink`].
///
/// Used at command dispatch time to coerce `&mut C` into `&mut dyn ChannelSink` without
/// a blanket impl (which would violate Rust's orphan rules).
pub(crate) struct ChannelSinkAdapter<'a, C: Channel>(pub &'a mut C);

impl<C: Channel> zeph_commands::ChannelSink for ChannelSinkAdapter<'_, C> {
    fn send<'a>(
        &'a mut self,
        msg: &'a str,
    ) -> std::pin::Pin<
        Box<dyn std::future::Future<Output = Result<(), zeph_commands::CommandError>> + Send + 'a>,
    > {
        Box::pin(async move {
            self.0
                .send(msg)
                .await
                .map_err(zeph_commands::CommandError::new)
        })
    }

    fn flush_chunks<'a>(
        &'a mut self,
    ) -> std::pin::Pin<
        Box<dyn std::future::Future<Output = Result<(), zeph_commands::CommandError>> + Send + 'a>,
    > {
        Box::pin(async move {
            self.0
                .flush_chunks()
                .await
                .map_err(zeph_commands::CommandError::new)
        })
    }

    fn send_queue_count<'a>(
        &'a mut self,
        count: usize,
    ) -> std::pin::Pin<
        Box<dyn std::future::Future<Output = Result<(), zeph_commands::CommandError>> + Send + 'a>,
    > {
        Box::pin(async move {
            self.0
                .send_queue_count(count)
                .await
                .map_err(zeph_commands::CommandError::new)
        })
    }

    fn supports_exit(&self) -> bool {
        self.0.supports_exit()
    }

    fn send_transcript<'a>(
        &'a mut self,
        entries: &'a [zeph_commands::TranscriptEntry],
    ) -> std::pin::Pin<
        Box<dyn std::future::Future<Output = Result<(), zeph_commands::CommandError>> + Send + 'a>,
    > {
        Box::pin(async move {
            self.0
                .send_transcript_backfill(entries)
                .await
                .map_err(zeph_commands::CommandError::new)
        })
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::assert_matches;

    #[test]
    fn channel_message_creation() {
        let msg = ChannelMessage {
            text: "hello".to_string(),
            attachments: vec![],
            is_guest_context: false,
            is_from_bot: false,
            owner_key: None,
        };
        assert_eq!(msg.text, "hello");
        assert!(msg.attachments.is_empty());
    }

    struct StubChannel;

    impl Channel for StubChannel {
        async fn recv(&mut self) -> Result<Option<ChannelMessage>, ChannelError> {
            Ok(None)
        }

        async fn send(&mut self, _text: &str) -> Result<(), ChannelError> {
            Ok(())
        }

        async fn send_chunk(&mut self, _chunk: &str) -> Result<(), ChannelError> {
            Ok(())
        }

        async fn flush_chunks(&mut self) -> Result<(), ChannelError> {
            Ok(())
        }
    }

    #[tokio::test]
    async fn send_chunk_default_is_noop() {
        let mut ch = StubChannel;
        ch.send_chunk("partial").await.unwrap();
    }

    #[tokio::test]
    async fn flush_chunks_default_is_noop() {
        let mut ch = StubChannel;
        ch.flush_chunks().await.unwrap();
    }

    #[tokio::test]
    async fn stub_channel_confirm_auto_approves() {
        let mut ch = StubChannel;
        let result = ch.confirm("Delete everything?").await.unwrap();
        assert!(result);
    }

    #[tokio::test]
    async fn stub_channel_send_typing_default() {
        let mut ch = StubChannel;
        ch.send_typing().await.unwrap();
    }

    #[tokio::test]
    async fn stub_channel_recv_returns_none() {
        let mut ch = StubChannel;
        let msg = ch.recv().await.unwrap();
        assert!(msg.is_none());
    }

    #[tokio::test]
    async fn stub_channel_send_ok() {
        let mut ch = StubChannel;
        ch.send("hello").await.unwrap();
    }

    #[tokio::test]
    async fn send_status_best_effort_succeeds_silently() {
        let mut ch = StubChannel;
        // Must not panic even though the return type carries no `Result`.
        ch.send_status_best_effort("hello").await;
    }

    struct ErroringStatusChannel;

    impl Channel for ErroringStatusChannel {
        async fn recv(&mut self) -> Result<Option<ChannelMessage>, ChannelError> {
            Ok(None)
        }

        async fn send(&mut self, _text: &str) -> Result<(), ChannelError> {
            Ok(())
        }

        async fn send_chunk(&mut self, _chunk: &str) -> Result<(), ChannelError> {
            Ok(())
        }

        async fn flush_chunks(&mut self) -> Result<(), ChannelError> {
            Ok(())
        }

        async fn send_status(&mut self, _text: &str) -> Result<(), ChannelError> {
            Err(ChannelError::ChannelClosed)
        }
    }

    #[tokio::test]
    async fn send_status_best_effort_swallows_errors() {
        let mut ch = ErroringStatusChannel;
        // Must not propagate or panic — errors are logged, not surfaced.
        ch.send_status_best_effort("hello").await;
    }

    // The two tests below use `tracing_test::traced_test` to assert on the actual log
    // output of `send_status_best_effort`, not just its return type — closing the gap
    // flagged in the #6106 handoff where only "doesn't panic" was verified.

    #[tokio::test]
    #[tracing_test::traced_test]
    async fn send_status_best_effort_warns_on_error() {
        let mut ch = ErroringStatusChannel;
        ch.send_status_best_effort("hello").await;
        assert!(
            logs_contain("channel status send failed"),
            "expected a tracing::warn! logging the send_status error"
        );
    }

    #[tokio::test]
    #[tracing_test::traced_test]
    async fn send_status_best_effort_debug_logs_on_success() {
        let mut ch = StubChannel;
        ch.send_status_best_effort("hello").await;
        assert!(
            logs_contain("channel status sent"),
            "expected a tracing::debug! logging the successful send_status"
        );
    }

    struct HangingStatusChannel;

    impl Channel for HangingStatusChannel {
        async fn recv(&mut self) -> Result<Option<ChannelMessage>, ChannelError> {
            Ok(None)
        }

        async fn send(&mut self, _text: &str) -> Result<(), ChannelError> {
            Ok(())
        }

        async fn send_chunk(&mut self, _chunk: &str) -> Result<(), ChannelError> {
            Ok(())
        }

        async fn flush_chunks(&mut self) -> Result<(), ChannelError> {
            Ok(())
        }

        async fn send_status(&mut self, _text: &str) -> Result<(), ChannelError> {
            std::future::pending().await
        }
    }

    // Regression test for #6094: a channel whose `send_status` never resolves (e.g. stuck in
    // a retry-with-backoff loop under sustained 429s) must not stall the caller past
    // `STATUS_SEND_TIMEOUT`. Uses paused tokio time so the test itself completes instantly.
    #[tokio::test(start_paused = true)]
    async fn send_status_best_effort_times_out_instead_of_hanging() {
        let mut ch = HangingStatusChannel;
        let call = ch.send_status_best_effort("hello");
        tokio::pin!(call);

        // Not ready before the timeout elapses.
        assert!(
            futures::poll!(&mut call).is_pending(),
            "expected send_status_best_effort to still be pending immediately"
        );

        tokio::time::advance(STATUS_SEND_TIMEOUT + std::time::Duration::from_secs(1)).await;

        // Now the timeout has elapsed and the future must resolve (returns `()`, not stuck).
        tokio::time::timeout(std::time::Duration::from_secs(1), call)
            .await
            .expect("send_status_best_effort must resolve once STATUS_SEND_TIMEOUT elapses");
    }

    #[tokio::test(start_paused = true)]
    #[tracing_test::traced_test]
    async fn send_status_best_effort_warns_on_timeout() {
        let mut ch = HangingStatusChannel;
        let call = ch.send_status_best_effort("hello");
        tokio::pin!(call);
        let _ = futures::poll!(&mut call);

        tokio::time::advance(STATUS_SEND_TIMEOUT + std::time::Duration::from_secs(1)).await;
        call.await;

        assert!(
            logs_contain("channel status send timed out"),
            "expected a tracing::warn! logging the send_status timeout"
        );
    }

    #[test]
    fn channel_message_clone() {
        let msg = ChannelMessage {
            text: "test".to_string(),
            attachments: vec![],
            is_guest_context: false,
            is_from_bot: false,
            owner_key: None,
        };
        let cloned = msg.clone();
        assert_eq!(cloned.text, "test");
    }

    #[test]
    fn channel_message_debug() {
        let msg = ChannelMessage {
            text: "debug".to_string(),
            attachments: vec![],
            is_guest_context: false,
            is_from_bot: false,
            owner_key: None,
        };
        let debug = format!("{msg:?}");
        assert!(debug.contains("debug"));
    }

    #[test]
    fn attachment_kind_equality() {
        assert_eq!(AttachmentKind::Audio, AttachmentKind::Audio);
        assert_ne!(AttachmentKind::Audio, AttachmentKind::Image);
    }

    #[test]
    fn attachment_construction() {
        let a = Attachment {
            kind: AttachmentKind::Audio,
            data: vec![0, 1, 2],
            filename: Some("test.wav".into()),
        };
        assert_eq!(a.kind, AttachmentKind::Audio);
        assert_eq!(a.data.len(), 3);
        assert_eq!(a.filename.as_deref(), Some("test.wav"));
    }

    #[test]
    fn channel_message_with_attachments() {
        let msg = ChannelMessage {
            text: String::new(),
            attachments: vec![Attachment {
                kind: AttachmentKind::Audio,
                data: vec![42],
                filename: None,
            }],
            is_guest_context: false,
            is_from_bot: false,
            owner_key: None,
        };
        assert_eq!(msg.attachments.len(), 1);
        assert_eq!(msg.attachments[0].kind, AttachmentKind::Audio);
    }

    #[test]
    fn stub_channel_try_recv_returns_none() {
        let mut ch = StubChannel;
        assert!(ch.try_recv().is_none());
    }

    #[tokio::test]
    async fn stub_channel_send_queue_count_noop() {
        let mut ch = StubChannel;
        ch.send_queue_count(5).await.unwrap();
    }

    // LoopbackChannel tests

    #[test]
    fn loopback_pair_returns_linked_handles() {
        let (channel, handle) = LoopbackChannel::pair(8);
        // Both sides exist and channels are connected via their sender capacity
        drop(channel);
        drop(handle);
    }

    #[tokio::test]
    async fn loopback_cancel_signal_can_be_notified_and_awaited() {
        let (_channel, handle) = LoopbackChannel::pair(8);
        let signal = std::sync::Arc::clone(&handle.cancel_signal);
        // Notify from one side, await on the other.
        let notified = signal.notified();
        handle.cancel_signal.notify_one();
        notified.await; // resolves immediately after notify_one()
    }

    #[tokio::test]
    async fn loopback_cancel_signal_shared_across_clones() {
        let (_channel, handle) = LoopbackChannel::pair(8);
        let signal_a = std::sync::Arc::clone(&handle.cancel_signal);
        let signal_b = std::sync::Arc::clone(&handle.cancel_signal);
        let notified = signal_b.notified();
        signal_a.notify_one();
        notified.await;
    }

    #[tokio::test]
    async fn loopback_send_recv_round_trip() {
        let (mut channel, handle) = LoopbackChannel::pair(8);
        handle
            .input_tx
            .send(ChannelMessage {
                text: "hello".to_owned(),
                attachments: vec![],
                is_guest_context: false,
                is_from_bot: false,
                owner_key: None,
            })
            .await
            .unwrap();
        let msg = channel.recv().await.unwrap().unwrap();
        assert_eq!(msg.text, "hello");
    }

    #[tokio::test]
    async fn loopback_recv_returns_none_when_handle_dropped() {
        let (mut channel, handle) = LoopbackChannel::pair(8);
        drop(handle);
        let result = channel.recv().await.unwrap();
        assert!(result.is_none());
    }

    #[tokio::test]
    async fn loopback_send_produces_full_message_event() {
        let (mut channel, mut handle) = LoopbackChannel::pair(8);
        channel.send("world").await.unwrap();
        let event = handle.output_rx.recv().await.unwrap();
        assert_matches!(event, LoopbackEvent::FullMessage(t) if t == "world");
    }

    #[tokio::test]
    async fn loopback_send_chunk_then_flush() {
        let (mut channel, mut handle) = LoopbackChannel::pair(8);
        channel.send_chunk("part1").await.unwrap();
        channel.flush_chunks().await.unwrap();
        let ev1 = handle.output_rx.recv().await.unwrap();
        let ev2 = handle.output_rx.recv().await.unwrap();
        assert_matches!(ev1, LoopbackEvent::Chunk(t) if t == "part1");
        assert_matches!(ev2, LoopbackEvent::Flush);
    }

    #[tokio::test]
    async fn loopback_send_tool_output() {
        let (mut channel, mut handle) = LoopbackChannel::pair(8);
        channel
            .send_tool_output(ToolOutputEvent {
                tool_name: "bash".into(),
                display: "exit 0".into(),
                diff: None,
                filter_stats: None,
                kept_lines: None,
                locations: None,
                tool_call_id: String::new(),
                terminal_id: None,
                is_error: false,
                parent_tool_use_id: None,
                raw_response: None,
                started_at: None,
            })
            .await
            .unwrap();
        let event = handle.output_rx.recv().await.unwrap();
        match event {
            LoopbackEvent::ToolOutput(data) => {
                assert_eq!(data.tool_name, "bash");
                assert_eq!(data.display, "exit 0");
                assert!(data.diff.is_none());
                assert!(data.filter_stats.is_none());
                assert!(data.kept_lines.is_none());
                assert!(data.locations.is_none());
                assert_eq!(data.tool_call_id, "");
                assert!(!data.is_error);
                assert!(data.terminal_id.is_none());
                assert!(data.parent_tool_use_id.is_none());
                assert!(data.raw_response.is_none());
            }
            _ => panic!("expected ToolOutput event"),
        }
    }

    #[tokio::test]
    async fn loopback_confirm_auto_approves() {
        let (mut channel, _handle) = LoopbackChannel::pair(8);
        let result = channel.confirm("are you sure?").await.unwrap();
        assert!(result);
    }

    #[tokio::test]
    async fn loopback_send_error_when_output_closed() {
        let (mut channel, handle) = LoopbackChannel::pair(8);
        // Drop only the output_rx side by dropping the handle
        drop(handle);
        let result = channel.send("too late").await;
        assert_matches!(result, Err(ChannelError::ChannelClosed));
    }

    #[tokio::test]
    async fn loopback_send_chunk_error_when_output_closed() {
        let (mut channel, handle) = LoopbackChannel::pair(8);
        drop(handle);
        let result = channel.send_chunk("chunk").await;
        assert_matches!(result, Err(ChannelError::ChannelClosed));
    }

    #[tokio::test]
    async fn loopback_flush_error_when_output_closed() {
        let (mut channel, handle) = LoopbackChannel::pair(8);
        drop(handle);
        let result = channel.flush_chunks().await;
        assert_matches!(result, Err(ChannelError::ChannelClosed));
    }

    #[tokio::test]
    async fn loopback_send_status_event() {
        let (mut channel, mut handle) = LoopbackChannel::pair(8);
        channel.send_status("working...").await.unwrap();
        let event = handle.output_rx.recv().await.unwrap();
        assert_matches!(event, LoopbackEvent::Status(s) if s == "working...");
    }

    #[tokio::test]
    async fn loopback_send_usage_produces_usage_event() {
        let (mut channel, mut handle) = LoopbackChannel::pair(8);
        channel
            .send_usage(100, 50, 200_000, 10, 5, 1.5)
            .await
            .unwrap();
        let event = handle.output_rx.recv().await.unwrap();
        match event {
            LoopbackEvent::Usage {
                input_tokens,
                output_tokens,
                context_window,
                cache_read_tokens,
                cache_write_tokens,
                cost_cents,
            } => {
                assert_eq!(input_tokens, 100);
                assert_eq!(output_tokens, 50);
                assert_eq!(context_window, 200_000);
                assert_eq!(cache_read_tokens, 10);
                assert_eq!(cache_write_tokens, 5);
                assert!((cost_cents - 1.5).abs() < f64::EPSILON);
            }
            _ => panic!("expected Usage event"),
        }
    }

    #[tokio::test]
    async fn loopback_send_usage_error_when_closed() {
        let (mut channel, handle) = LoopbackChannel::pair(8);
        drop(handle);
        let result = channel.send_usage(1, 2, 3, 0, 0, 0.0).await;
        assert_matches!(result, Err(ChannelError::ChannelClosed));
    }

    #[test]
    fn plan_item_status_variants_are_distinct() {
        assert!(!matches!(
            PlanItemStatus::Pending,
            PlanItemStatus::InProgress
        ));
        assert!(!matches!(
            PlanItemStatus::InProgress,
            PlanItemStatus::Completed
        ));
        assert!(!matches!(
            PlanItemStatus::Completed,
            PlanItemStatus::Pending
        ));
    }

    #[test]
    fn loopback_event_session_title_carries_string() {
        let event = LoopbackEvent::SessionTitle("hello".to_owned());
        assert_matches!(event, LoopbackEvent::SessionTitle(s) if s == "hello");
    }

    #[test]
    fn loopback_event_plan_carries_entries() {
        let entries = vec![
            ("step 1".to_owned(), PlanItemStatus::Pending),
            ("step 2".to_owned(), PlanItemStatus::InProgress),
        ];
        let event = LoopbackEvent::Plan(entries);
        match event {
            LoopbackEvent::Plan(e) => {
                assert_eq!(e.len(), 2);
                assert_matches!(e[0].1, PlanItemStatus::Pending);
                assert_matches!(e[1].1, PlanItemStatus::InProgress);
            }
            _ => panic!("expected Plan event"),
        }
    }

    #[tokio::test]
    async fn loopback_send_tool_start_produces_tool_start_event() {
        let (mut channel, mut handle) = LoopbackChannel::pair(8);
        channel
            .send_tool_start(ToolStartEvent {
                tool_name: "shell".into(),
                tool_call_id: "tc-001".into(),
                params: Some(serde_json::json!({"command": "ls"})),
                parent_tool_use_id: None,
                started_at: std::time::Instant::now(),
                speculative: false,
                sandbox_profile: None,
                is_mcp: false,
            })
            .await
            .unwrap();
        let event = handle.output_rx.recv().await.unwrap();
        match event {
            LoopbackEvent::ToolStart(data) => {
                assert_eq!(data.tool_name.as_str(), "shell");
                assert_eq!(data.tool_call_id.as_str(), "tc-001");
                assert!(data.params.is_some());
                assert!(data.parent_tool_use_id.is_none());
            }
            _ => panic!("expected ToolStart event"),
        }
    }

    #[tokio::test]
    async fn loopback_send_tool_start_with_parent_id() {
        let (mut channel, mut handle) = LoopbackChannel::pair(8);
        channel
            .send_tool_start(ToolStartEvent {
                tool_name: "web".into(),
                tool_call_id: "tc-002".into(),
                params: None,
                parent_tool_use_id: Some("parent-123".into()),
                started_at: std::time::Instant::now(),
                speculative: false,
                sandbox_profile: None,
                is_mcp: false,
            })
            .await
            .unwrap();
        let event = handle.output_rx.recv().await.unwrap();
        assert_matches!(
            event,
            LoopbackEvent::ToolStart(ref data) if data.parent_tool_use_id.as_deref() == Some("parent-123")
        );
    }

    #[tokio::test]
    async fn loopback_send_tool_start_error_when_output_closed() {
        let (mut channel, handle) = LoopbackChannel::pair(8);
        drop(handle);
        let result = channel
            .send_tool_start(ToolStartEvent {
                tool_name: "shell".into(),
                tool_call_id: "tc-003".into(),
                params: None,
                parent_tool_use_id: None,
                started_at: std::time::Instant::now(),
                speculative: false,
                sandbox_profile: None,
                is_mcp: false,
            })
            .await;
        assert_matches!(result, Err(ChannelError::ChannelClosed));
    }

    #[tokio::test]
    async fn default_send_tool_output_formats_message() {
        let mut ch = StubChannel;
        // Default impl calls self.send() which is a no-op in StubChannel — just verify it doesn't panic.
        ch.send_tool_output(ToolOutputEvent {
            tool_name: "bash".into(),
            display: "hello".into(),
            diff: None,
            filter_stats: None,
            kept_lines: None,
            locations: None,
            tool_call_id: "id".into(),
            terminal_id: None,
            is_error: false,
            parent_tool_use_id: None,
            raw_response: None,
            started_at: None,
        })
        .await
        .unwrap();
    }
}