Select any of the transcript, and take a queued message back

Two things a reader could not do to what is on screen.

**Selection.** Nothing in the transcript was selectable at all, so a
command, a path or an error message could be read and not copied. One
`SelectionContainer` around the whole list rather than one per row: a
transcript is one body of text to a reader, and a selection has to be able
to run from a reply into the tool output under it. Per row it also could
not, and whatever was drawn without a container would have been silently
unselectable -- a state nothing on screen reports. Rows keep their tap
handlers; checked on the emulator that expanding a tool call, scrolling and
flinging are all unaffected, since a selection is a long press.

**Taking a message back.** A message sent into a running turn sits as a
bubble waiting to be read, and there was no way to change your mind: it is
tappable now, and the server answers `POST /sessions/{id}/unqueue`.

The answer has three states, and the middle one is the point. Claude's
driver writes a steer into the CLI's stdin the instant it arrives -- that
is what makes it reach the model at the next tool boundary rather than at
the end of the turn, and it was measured -- so the line is already gone and
`AlreadySent` is the only honest answer it can give. Holding the write
until a boundary would make the drop real and cost a steer one model call,
which is the latency the immediate write exists to remove; rejected on that
trade, with the reasoning in PLAN.md. The refusal is drawn on the bubble
that was pressed rather than in the error row under the header, a screen
away from it.

Where a driver really does hold its queue -- echo today -- the message goes
for good, and it goes as an `Event::MessageDropped` rather than as a return
value: every device watching the session loses the bubble, and a phone that
reconnects and replays the `messageQueued` does not put back one that was
cancelled with nothing left to resolve it.

server/src/routes.rs

@@ -17,6 +17,8 @@
 //!                                      `reset` frame plus the newest window)
 //! POST   /sessions/{id}/message        {text, attachmentIds?}
 //!                                     (starts the process first if it has exited)
+//! POST   /sessions/{id}/unqueue        {messageId} -- take back one not read yet
+//!                                     (409 when the session already has it)
 //! POST   /sessions/{id}/answer         {questionId, answers}  (questions and permissions)
 //! POST   /sessions/{id}/interrupt      stop the running turn; the process stays
 //! POST   /sessions/{id}/stop           end the process; the session and transcript stay
@@ -68,7 +70,7 @@ use tokio::sync::{broadcast, mpsc};
 use tokio_stream::StreamExt;
 use tokio_stream::wrappers::{BroadcastStream, ReceiverStream};
 
-use crate::session::driver::SessionCommand;
+use crate::session::driver::{SessionCommand, Unqueued};
 use crate::session::pending::Operation;
 use crate::session::transcript::{CATCH_UP_LIMIT, CatchUp, SeqEvent, catch_up};
 use crate::session::{LiveSession, SessionInfo, SessionManager, SpawnSpec};
@@ -93,6 +95,7 @@ pub fn router(manager: Arc<SessionManager>) -> Router {
         .route("/sessions/{id}/events", get(events))
         .route("/sessions/{id}/transcript", get(transcript))
         .route("/sessions/{id}/message", post(message))
+        .route("/sessions/{id}/unqueue", post(unqueue))
         .route("/sessions/{id}/answer", post(answer))
         .route("/sessions/{id}/interrupt", post(interrupt))
         .route("/sessions/{id}/stop", post(stop))
@@ -123,6 +126,10 @@ enum ApiError {
     UnknownRoute,
     #[error("{0}")]
     BadRequest(String),
+    /// The request was understood and the state it names has moved on --
+    /// distinct from `BadRequest`, which is a caller that got it wrong.
+    #[error("{0}")]
+    Conflict(String),
     #[error(transparent)]
     Internal(#[from] anyhow::Error),
 }
@@ -132,6 +139,7 @@ impl IntoResponse for ApiError {
         let status = match self {
             Self::NotFound(_) | Self::UnknownRoute => StatusCode::NOT_FOUND,
             Self::BadRequest(_) => StatusCode::BAD_REQUEST,
+            Self::Conflict(_) => StatusCode::CONFLICT,
             Self::Internal(err) => {
                 // The only variant whose real cause isn't safe to hand
                 // back verbatim, and the only one worth a log line.
@@ -900,6 +908,40 @@ async fn message(
     Ok(StatusCode::NO_CONTENT)
 }
 
+#[derive(Deserialize)]
+#[serde(rename_all = "camelCase")]
+#[serde(deny_unknown_fields)]
+struct UnqueueRequest {
+    /// The id the `messageQueued` event carried, which is what the bubble
+    /// on screen is drawn from.
+    message_id: String,
+}
+
+/// Takes back a message the session has not read yet.
+///
+/// The two failures are separate answers rather than one refusal, because
+/// they are different things to whoever tapped: `409` means the session has
+/// already been told and the message is on its way into the conversation,
+/// and `404` means nothing is waiting under that id -- a bubble on screen
+/// that something else has already resolved. See [`Driver::unqueue`]; the
+/// Claude driver can only ever give the first, since it writes a steer into
+/// the CLI the moment it arrives.
+async fn unqueue(
+    State(manager): State<Arc<SessionManager>>,
+    UrlPath(id): UrlPath<String>,
+    axum::Json(body): axum::Json<UnqueueRequest>,
+) -> Result<StatusCode, ApiError> {
+    match lookup(&manager, &id)?.unqueue(&body.message_id) {
+        Unqueued::Dropped => Ok(StatusCode::NO_CONTENT),
+        Unqueued::AlreadySent => Err(ApiError::Conflict(
+            "the session has already been given this message".to_string(),
+        )),
+        Unqueued::Unknown => Err(ApiError::NotFound(
+            "this message is not waiting to be read".to_string(),
+        )),
+    }
+}
+
 #[derive(Deserialize)]
 #[serde(rename_all = "camelCase")]
 #[serde(deny_unknown_fields)]

server/src/session/claude.rs

@@ -57,7 +57,7 @@ use serde_json::{Value, json};
 use tokio::io::AsyncWriteExt;
 use tokio::sync::mpsc;
 
-use super::driver::{Driver, Event, EventSink, ImageRef, SessionStatus};
+use super::driver::{Driver, Event, EventSink, ImageRef, SessionStatus, Unqueued};
 use super::process;
 use super::transport::{Launch, Streams, Transport};
 use crate::config::{ProviderConfig, SessionConfig};
@@ -595,6 +595,24 @@ impl Driver for ClaudeDriver {
         self.send_line(line);
     }
 
+    /// Never droppable, and that is a property of the design rather than
+    /// an omission.
+    ///
+    /// A message queued here has already been written to the CLI's stdin
+    /// -- see [`Queue`], where only the *announcement* waits -- because
+    /// that is what makes a steer reach the model at the next tool
+    /// boundary instead of at the end of the turn. A line in the fifo
+    /// cannot be recalled, so the only honest answers are "the session has
+    /// already been told" and "nothing is waiting under that id".
+    fn unqueue(&self, id: &str) -> Unqueued {
+        let queue = self.queue.lock().unwrap();
+        if queue.awaiting.iter().any(|(waiting, ..)| waiting == id) {
+            Unqueued::AlreadySent
+        } else {
+            Unqueued::Unknown
+        }
+    }
+
     fn answer_question(&self, id: &str, answers: &[String]) {
         let response = {
             let mut state = self.state.lock().unwrap();

server/src/session/driver.rs

@@ -114,6 +114,22 @@ pub enum Event {
         #[serde(default, skip_serializing_if = "Vec::is_empty")]
         images: Vec<ImageRef>,
     },
+    /// A message taken out of the queue before the session read it, by
+    /// somebody tapping the bubble that was waiting for it.
+    ///
+    /// Recorded for the same reason `MessageQueued` is: the queue is the
+    /// server's, so what is waiting has to be answerable from the
+    /// transcript alone. Without it a phone that reconnects replays the
+    /// `MessageQueued` and puts back a bubble for a message that will
+    /// never arrive -- and nothing later would ever resolve it, since the
+    /// `UserMessage` that normally does is exactly what is not coming.
+    ///
+    /// Only ever sent for a message that had not been handed over. One
+    /// that has is not droppable and says so instead; see
+    /// [`Unqueued::AlreadySent`].
+    MessageDropped {
+        id: String,
+    },
     /// A driver has taken one of the user's messages and started reading
     /// it. The manager turns this into the `UserMessage` above, so it
     /// never reaches a phone itself.
@@ -444,6 +460,25 @@ pub enum SessionStatus {
     Unknown,
 }
 
+/// What became of a request to take a queued message back.
+///
+/// Three states rather than a bool because the two failures are not the
+/// same fact. A driver that writes into its session the moment a message
+/// arrives -- which is what `ClaudeDriver` does, so that a steer reaches
+/// the model at the next tool boundary rather than at the end of the turn
+/// -- can never take one back, and a phone that was told only "no" would
+/// have to guess whether it had asked too late or asked about nothing.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Unqueued {
+    /// Out of the queue; the session will never read it.
+    Dropped,
+    /// Already handed to the session, so there is nothing left to take
+    /// back. The message is on its way into the conversation.
+    AlreadySent,
+    /// Nothing is waiting under that id.
+    Unknown,
+}
+
 /// Where a driver reports events. Unbounded because producers are child
 /// processes a slow phone must never be able to stall; the transcript file
 /// is the backpressure-free buffer of record.
@@ -463,6 +498,19 @@ pub trait Driver: Send + Sync {
     /// message in the transcript, so a driver that never sends it drops
     /// the message from the conversation entirely.
     fn send_user_message(&self, text: String, images: Vec<ImageRef>);
+    /// Takes back a message that is still waiting, named by the id its
+    /// [`Event::MessageQueued`] carried.
+    ///
+    /// Answering is the whole of the contract: a driver that drops the
+    /// message owes an [`Event::MessageDropped`], and one that cannot must
+    /// say which of the two reasons it is, because they are different
+    /// things to a reader -- "the session has already been told" is worth
+    /// knowing, and "there is nothing under that id" means the bubble on
+    /// screen is stale. The default is the honest answer for a driver with
+    /// no queue at all: nothing of yours is waiting.
+    fn unqueue(&self, _id: &str) -> Unqueued {
+        Unqueued::Unknown
+    }
     /// Answers one question with everything that was chosen, in the order
     /// it was offered. One answer is a list of one; a driver whose dialect
     /// takes a single value joins them where it writes it.

server/src/session/echo.rs

@@ -53,7 +53,7 @@ use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
 use std::sync::{Arc, Mutex};
 use std::time::Duration;
 
-use super::driver::{Driver, Event, EventSink, ImageRef, QuestionOption, SessionStatus};
+use super::driver::{Driver, Event, EventSink, ImageRef, QuestionOption, SessionStatus, Unqueued};
 
 /// Delay between streamed deltas -- long enough that streaming is visibly
 /// streaming in the UI, short enough that tests waiting on a full turn
@@ -796,6 +796,23 @@ impl Driver for EchoDriver {
         !self.busy.load(Ordering::SeqCst)
     }
 
+    /// Really droppable, which is what makes this the rig for the phone's
+    /// side of it: the held message is this driver's own and nothing has
+    /// been written anywhere, so a tap here exercises the whole path
+    /// through to the bubble disappearing on every device. The Claude
+    /// driver can only ever refuse -- see its own `unqueue` -- so it
+    /// cannot exercise the case where the drop succeeds.
+    fn unqueue(&self, id: &str) -> Unqueued {
+        let mut queued = self.queued.lock().unwrap();
+        let Some(at) = queued.iter().position(|(waiting, ..)| waiting == id) else {
+            return Unqueued::Unknown;
+        };
+        queued.remove(at);
+        drop(queued);
+        self.emit(Event::MessageDropped { id: id.to_string() });
+        Unqueued::Dropped
+    }
+
     fn send_user_message(&self, text: String, images: Vec<ImageRef>) {
         // Announced, because this is a message: every driver owes exactly
         // one `MessageTaken` per message, and one that quietly vanishes

server/src/session/mod.rs

@@ -32,7 +32,9 @@ use crate::config::{
     Config, DriverKind, ProviderConfig, SessionConfig, SetupConfig, SshConfig, TokenEntry,
 };
 use claude::ClaudeDriver;
-use driver::{Driver, Event, EventSink, ImageRef, SessionCommand, SessionStatus, context_after};
+use driver::{
+    Driver, Event, EventSink, ImageRef, SessionCommand, SessionStatus, Unqueued, context_after,
+};
 use echo::EchoDriver;
 use llama::LlamaDriver;
 use transcript::{SeqEvent, Transcript};
@@ -440,6 +442,21 @@ impl LiveSession {
         self.ask("be interrupted", |driver| driver.interrupt());
     }
 
+    /// Takes back a message the session has not read yet, named by the id
+    /// its `MessageQueued` carried. See [`Driver::unqueue`] for why the
+    /// answer has three states.
+    ///
+    /// A session with no process answers `Unknown` rather than being
+    /// reported as a failure, and that is the true answer: a driver on its
+    /// way out already said what it was holding (`Queue::close`), so there
+    /// is nothing waiting to take back.
+    pub fn unqueue(&self, message_id: &str) -> Unqueued {
+        match self.driver() {
+            Some(driver) => driver.unqueue(message_id),
+            None => Unqueued::Unknown,
+        }
+    }
+
     /// Leaves this session's process running and stops attending to it,
     /// for a server that is going away and means to come back. See
     /// [`Driver::detach`].
@@ -2716,6 +2733,86 @@ mod tests {
         );
     }
 
+    /// A queued message can be taken back until the driver has handed it
+    /// over, and the taking back is an event rather than a return value --
+    /// which is what makes the bubble disappear on every device watching,
+    /// and stay gone when one of them reconnects and replays.
+    ///
+    /// Exercised on echo because echo really holds its queue. The Claude
+    /// driver writes a steer into the CLI the moment it arrives, so it can
+    /// only ever answer `AlreadySent`; the case where a drop *succeeds*
+    /// has no other driver to be tested against.
+    #[tokio::test]
+    async fn a_queued_message_can_be_taken_back_until_the_session_has_it() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        seed_echo_only(&dir.path().join("config.ron"));
+        let manager = SessionManager::new(
+            dir.path().join("config.ron"),
+            dir.path().join("sessions"),
+            dir.path().join("models"),
+        )
+        .expect("manager");
+        let info = manager.spawn_session(echo_spec()).expect("spawn");
+        let session = manager.session(&info.id).expect("live session");
+        let mut rx = session.subscribe();
+
+        // A turn long enough that the next message has to wait behind it.
+        session.send_message("/slow 1".to_string(), Vec::new());
+        collect_until(&mut rx, |event| {
+            matches!(
+                event,
+                Event::Status {
+                    state: SessionStatus::Running
+                }
+            )
+        })
+        .await;
+
+        session.send_message("second thoughts".to_string(), Vec::new());
+        let seen = collect_until(&mut rx, |event| {
+            matches!(event, Event::MessageQueued { .. })
+        })
+        .await;
+        let Some(Event::MessageQueued { id, .. }) =
+            seen.iter().map(|entry| entry.event.clone()).next_back()
+        else {
+            panic!("expected the message to be queued: {seen:?}");
+        };
+
+        assert_eq!(session.unqueue(&id), Unqueued::Dropped);
+        let seen = collect_until(&mut rx, |event| {
+            matches!(event, Event::MessageDropped { .. })
+        })
+        .await;
+        assert!(
+            seen.iter().any(|entry| matches!(
+                &entry.event,
+                Event::MessageDropped { id: dropped } if *dropped == id
+            )),
+            "the drop has to be recorded, not merely returned: {seen:?}"
+        );
+
+        // Gone for good: the turn ends without the message ever entering
+        // the conversation, and asking again says there is nothing there
+        // rather than dropping it twice.
+        assert_eq!(session.unqueue(&id), Unqueued::Unknown);
+        let seen = collect_until(&mut rx, |event| {
+            matches!(
+                event,
+                Event::Status {
+                    state: SessionStatus::Idle
+                }
+            )
+        })
+        .await;
+        assert!(
+            !seen
+                .iter()
+                .any(|entry| matches!(entry.event, Event::UserMessage { .. })),
+            "a message taken back must never be read: {seen:?}"
+        );
+    }
+
     #[tokio::test]
     async fn a_command_on_an_idle_session_goes_straight_out() {
         let dir = tempfile::tempdir().expect("tempdir");