diff --git a/crates/maestro-daemon/src/attach.rs b/crates/maestro-daemon/src/attach.rs
index 84a83dc..c53fb89 100644
--- a/crates/maestro-daemon/src/attach.rs
+++ b/crates/maestro-daemon/src/attach.rs
@@ -6,29 +6,26 @@
 use axum::extract::ws::{Message, WebSocket};
 use futures_util::{SinkExt, StreamExt};
 use maestro_protocol::ClientControl;
+use tokio::sync::broadcast;
 use tokio::sync::broadcast::error::RecvError;
 use uuid::Uuid;
 
 use crate::session::SessionManager;
 
 /// Pump a WebSocket against a session's PTY, behind `WS /api/attach/:id`.
+///
+/// The subscription is acquired by the handler *before* the upgrade and handed
+/// in here, so there is no window in which the session can disappear between
+/// the existence check and the subscribe.
 pub(crate) async fn serve(
     socket: WebSocket,
     manager: SessionManager,
     id: Uuid,
+    replay: Vec<u8>,
+    mut rx: broadcast::Receiver<Vec<u8>>,
 ) -> Result<(), axum::Error> {
     let (mut sink, mut stream) = socket.split();
 
-    let (replay, mut rx) = match manager.subscribe_pty_with_replay(id).await {
-        Some(pair) => pair,
-        None => {
-            // session was killed between the existence check and now — let the
-            // client see a clean close instead of a dropped socket.
-            let _ = sink.send(Message::Close(None)).await;
-            return Ok(());
-        }
-    };
-
     // NAP-229: send buffered scrollback up-front so the client sees the last
     // screenful immediately instead of a blank terminal. The replay lock the
     // bridge held during snapshot+subscribe guarantees no byte appears both
diff --git a/crates/maestro-daemon/src/local_api.rs b/crates/maestro-daemon/src/local_api.rs
index c456364..d7bb438 100644
--- a/crates/maestro-daemon/src/local_api.rs
+++ b/crates/maestro-daemon/src/local_api.rs
@@ -177,21 +177,25 @@ pub fn state_for(
 // ---------------------------------------------------------------------------
 
 #[allow(clippy::result_large_err)]
+/// Subscribing *before* the upgrade makes the 404 and the subscription one
+/// atomic step. Checking existence and then subscribing inside the callback
+/// left a window in which the liveness watcher could reap the session, so a
+/// client that passed the check still got a socket that immediately closed.
 async fn attach_ws(
     Path(id): Path<Uuid>,
     State(s): State<LocalApiState>,
     ws: axum::extract::WebSocketUpgrade,
 ) -> Result<Response, Response> {
-    if !s.manager.has(id).await {
+    let Some((replay, rx)) = s.manager.subscribe_pty_with_replay(id).await else {
         return Err(error_response(
             StatusCode::NOT_FOUND,
             "unknown_session",
             "no such session on this daemon",
         ));
-    }
+    };
     let manager = s.manager.clone();
     Ok(ws.on_upgrade(move |socket| async move {
-        let _ = crate::attach::serve(socket, manager, id).await;
+        let _ = crate::attach::serve(socket, manager, id, replay, rx).await;
     }))
 }
 
diff --git a/crates/maestro-daemon/src/proc.rs b/crates/maestro-daemon/src/proc.rs
index afe991b..28cba42 100644
--- a/crates/maestro-daemon/src/proc.rs
+++ b/crates/maestro-daemon/src/proc.rs
@@ -1,8 +1,8 @@
 //! Best-effort `/proc` inspection for the activity watcher (NAP-340, Linux).
 //!
-//! Decides whether a session is **blocked waiting on the user**: walk from the
-//! pane's process down to the deepest descendant (the agent/shell actually on
-//! the terminal) and check it's parked waiting on terminal input.
+//! Decides whether a session is **blocked waiting on the user**: ask the kernel
+//! which process group owns the terminal, then check whether any live member of
+//! that group is parked on the terminal.
 //!
 //! `/proc/<pid>/syscall` would say so exactly, but it's ptrace-gated and
 //! unreadable for our own sessions on a default kernel, so we combine three
@@ -16,55 +16,120 @@
 //! Every step degrades to `false` on a read failure, so a non-Linux host or a
 //! denied `/proc` read falls back to `Idle`.
 
-/// Whether the process tree under `pane_pid` looks blocked on terminal input
-/// from `tty` (e.g. `/dev/pts/3`) — the signal for `AwaitingInput`.
-pub fn awaiting_input(pane_pid: i32, tty: &str) -> bool {
-    let leaf = deepest_descendant(pane_pid);
+use std::collections::HashMap;
+
+/// One walk of `/proc`, shared by every session in an activity pass.
+///
+/// The foreground process group is the unit that matters, and its members are
+/// not reliably reachable from the pane: the group leader can exit while others
+/// run on, and a stage that forks a terminal-reading child and exits leaves
+/// that child reparented to init. Walking the pane's process tree misses both.
+///
+/// Snapshotting once per pass costs one `/proc` walk however many sessions the
+/// daemon owns — cheaper than the per-session `tmux list-panes` fork that
+/// already runs on the same tick.
+#[derive(Debug, Default)]
+pub struct ProcGroups {
+    stats: HashMap<i32, Stat>,
+    by_pgid: HashMap<i32, Vec<i32>>,
+}
+
+impl ProcGroups {
+    pub fn snapshot() -> Self {
+        let mut stats = HashMap::new();
+        let mut by_pgid: HashMap<i32, Vec<i32>> = HashMap::new();
+        let Ok(entries) = std::fs::read_dir("/proc") else {
+            return Self::default();
+        };
+        for pid in entries
+            .flatten()
+            .filter_map(|e| e.file_name().to_str()?.parse::<i32>().ok())
+        {
+            // A process can exit mid-walk; skipping it is the correct answer.
+            if let Some(stat) = read_stat(pid) {
+                by_pgid.entry(stat.pgrp).or_default().push(pid);
+                stats.insert(pid, stat);
+            }
+        }
+        Self { stats, by_pgid }
+    }
+
+    /// The process group the terminal considers foreground, per `tpgid`.
+    fn foreground_pgid(&self, pane_pid: i32) -> Option<i32> {
+        let tpgid = self.stats.get(&pane_pid)?.tpgid;
+        // -1 when the process has no controlling terminal.
+        (tpgid > 0).then_some(tpgid)
+    }
+
+    fn members(&self, pgid: i32) -> &[i32] {
+        self.by_pgid.get(&pgid).map_or(&[], Vec::as_slice)
+    }
+
+    fn state(&self, pid: i32) -> Option<char> {
+        self.stats.get(&pid).map(|s| s.state)
+    }
+}
+
+/// Whether the session on `tty` (e.g. `/dev/pts/3`) is blocked waiting on the
+/// user — the signal for `AwaitingInput`.
+///
+/// True when *any* member of the terminal's foreground group is parked on it: a
+/// group can hold a busy or timer-sleeping stage alongside the one actually
+/// reading, and either can be the reparented one.
+pub fn awaiting_input(pane_pid: i32, tty: &str, groups: &ProcGroups) -> bool {
+    let Some(pgid) = groups.foreground_pgid(pane_pid) else {
+        return false;
+    };
+    groups
+        .members(pgid)
+        .iter()
+        .any(|pid| parked_on_tty(*pid, tty, groups))
+}
+
+/// Whether `pid` is parked on `tty` rather than busy or sleeping on a timer.
+fn parked_on_tty(pid: i32, tty: &str, groups: &ProcGroups) -> bool {
     // Interruptible sleep is where a blocking read parks; anything else
     // (running, uninterruptible) is busy, not waiting on us.
-    if read_state(leaf) != Some('S') {
+    if groups.state(pid) != Some('S') {
         return false;
     }
     // A pure timer sleep (`sleep`, `usleep`) also sits in 'S' with the terminal
     // inherited — exclude it so it reads as Idle, not AwaitingInput.
-    if is_timer_sleep(leaf) {
+    if is_timer_sleep(pid) {
         return false;
     }
-    fd_points_at_tty(leaf, tty)
+    fd_points_at_tty(pid, tty)
 }
 
-/// Follow the (last) child link down the process tree to the deepest
-/// descendant. Bounded so a pathological/cyclic tree can't loop.
-fn deepest_descendant(pid: i32) -> i32 {
-    let mut cur = pid;
-    for _ in 0..64 {
-        match last_child(cur) {
-            Some(child) if child != cur => cur = child,
-            _ => break,
-        }
-    }
-    cur
+/// The fields of `/proc/<pid>/stat` this module needs.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+struct Stat {
+    state: char,
+    /// The process group this process belongs to.
+    pgrp: i32,
+    /// Foreground process group of the controlling terminal.
+    tpgid: i32,
 }
 
-/// The most-recently-created child of `pid`, via
-/// `/proc/<pid>/task/<pid>/children` (a space-separated pid list). The last
-/// entry is the newest child — typically the foreground one an agent spawned.
-fn last_child(pid: i32) -> Option<i32> {
-    let contents = std::fs::read_to_string(format!("/proc/{pid}/task/{pid}/children")).ok()?;
-    contents.split_whitespace().last()?.parse().ok()
+fn read_stat(pid: i32) -> Option<Stat> {
+    parse_stat(&std::fs::read_to_string(format!("/proc/{pid}/stat")).ok()?)
 }
 
-/// Read the process state char from `/proc/<pid>/stat`.
-fn read_state(pid: i32) -> Option<char> {
-    parse_stat_state(&std::fs::read_to_string(format!("/proc/{pid}/stat")).ok()?)
-}
-
-/// Extract the state field from a `/proc/<pid>/stat` line. The format is
-/// `pid (comm) STATE ...`; `comm` can itself contain spaces and parens, so the
-/// state is the first non-space char *after the final `)`*.
-fn parse_stat_state(stat: &str) -> Option<char> {
+/// Parse the fields after `comm` from a `/proc/<pid>/stat` line.
+///
+/// The format is `pid (comm) state ppid pgrp session tty_nr tpgid ...`, and
+/// `comm` can itself contain spaces and parens — so everything is indexed from
+/// the *final* `)`, never by splitting the whole line.
+fn parse_stat(stat: &str) -> Option<Stat> {
     let close = stat.rfind(')')?;
-    stat[close + 1..].trim_start().chars().next()
+    let mut rest = stat[close + 1..].split_whitespace();
+    let state = rest.next()?.chars().next()?;
+    let _ppid = rest.next()?;
+    let pgrp = rest.next()?.parse().ok()?;
+    let _session = rest.next()?;
+    let _tty_nr = rest.next()?;
+    let tpgid = rest.next()?.parse().ok()?;
+    Some(Stat { state, pgrp, tpgid })
 }
 
 /// Whether `pid`'s `wchan` (the kernel symbol it's blocked in) is a timer
@@ -94,19 +159,51 @@ fn fd_points_at_tty(pid: i32, tty: &str) -> bool {
 mod tests {
     use super::*;
 
-    #[test]
-    fn parse_stat_state_handles_simple_and_tricky_comm() {
-        assert_eq!(parse_stat_state("1234 (bash) S 1 1234 ..."), Some('S'));
-        assert_eq!(parse_stat_state("1234 (sleep) S 1 ..."), Some('S'));
-        assert_eq!(parse_stat_state("99 (cat) R 1 ..."), Some('R'));
-        // comm with spaces and a paren — only the final ')' delimits it.
-        assert_eq!(parse_stat_state("42 (weird (name)) D 1 42 ..."), Some('D'));
+    /// `pid (comm) state ppid pgrp session tty_nr tpgid …`
+    fn stat_line(comm: &str, state: char, tpgid: i32) -> String {
+        stat_line_pgrp(comm, state, 1234, tpgid)
+    }
+
+    fn stat_line_pgrp(comm: &str, state: char, pgrp: i32, tpgid: i32) -> String {
+        format!("1234 ({comm}) {state} 1000 {pgrp} 1234 34816 {tpgid} 4194304 …")
     }
 
     #[test]
-    fn parse_stat_state_rejects_garbage() {
-        assert_eq!(parse_stat_state("not a stat line"), None);
-        assert_eq!(parse_stat_state("123 (comm)"), None);
+    fn parse_stat_reads_state_pgrp_and_tpgid() {
+        let s = parse_stat(&stat_line_pgrp("bash", 'S', 500, 4321)).unwrap();
+        assert_eq!(s.state, 'S');
+        assert_eq!(s.pgrp, 500);
+        assert_eq!(s.tpgid, 4321);
+
+        let r = parse_stat(&stat_line_pgrp("cat", 'R', 7, 99)).unwrap();
+        assert_eq!(r.state, 'R');
+        assert_eq!(r.pgrp, 7);
+        assert_eq!(r.tpgid, 99);
+    }
+
+    /// `comm` is attacker-adjacent: it is the process name, it can contain
+    /// spaces and parens, and splitting the whole line on whitespace would put
+    /// every later field at the wrong index.
+    #[test]
+    fn parse_stat_indexes_from_the_final_paren() {
+        let s = parse_stat(&stat_line_pgrp("weird (name) with spaces", 'D', 42, 777)).unwrap();
+        assert_eq!(s.state, 'D');
+        assert_eq!(s.pgrp, 42);
+        assert_eq!(s.tpgid, 777);
+    }
+
+    #[test]
+    fn parse_stat_carries_the_no_terminal_sentinel() {
+        // -1 means no controlling terminal; `foreground_pid` rejects it.
+        assert_eq!(parse_stat(&stat_line("daemon", 'S', -1)).unwrap().tpgid, -1);
+    }
+
+    #[test]
+    fn parse_stat_rejects_garbage() {
+        assert!(parse_stat("not a stat line").is_none());
+        assert!(parse_stat("123 (comm)").is_none());
+        // Truncated before tpgid.
+        assert!(parse_stat("123 (comm) S 1 1 1").is_none());
     }
 
     #[test]
@@ -121,9 +218,46 @@ mod tests {
         assert!(!is_timer_wchan("0"));
     }
 
+    /// The snapshot indexes by process group, not by parentage — that is the
+    /// whole point, since a group member need not be under the pane. Checked
+    /// against this process's own group, which certainly has a live member.
+    #[test]
+    fn snapshot_indexes_this_process_under_its_group() {
+        // SAFETY: argument-less query of the caller's own process group.
+        let own = unsafe { libc::getpgid(0) };
+        assert!(own > 0, "expected a real process group, got {own}");
+
+        let groups = ProcGroups::snapshot();
+        let me = std::process::id() as i32;
+        assert!(
+            groups.members(own).contains(&me),
+            "snapshot missed this process ({me}) in group {own}: {:?}",
+            groups.members(own)
+        );
+        assert!(
+            groups.state(me).is_some(),
+            "a process in the snapshot must carry its state"
+        );
+    }
+
+    #[test]
+    fn snapshot_has_no_members_for_a_group_that_cannot_exist() {
+        assert!(ProcGroups::snapshot().members(i32::MAX).is_empty());
+    }
+
+    #[test]
+    fn a_pid_with_no_controlling_terminal_has_no_foreground_group() {
+        let groups = ProcGroups::snapshot();
+        assert_eq!(groups.foreground_pgid(i32::MAX), None);
+    }
+
     #[test]
     fn awaiting_input_is_false_for_nonexistent_pid() {
-        // A pid that can't exist → every read fails → false (fall back to Idle).
-        assert!(!awaiting_input(i32::MAX, "/dev/pts/0"));
+        // A pid that can't exist → not in the snapshot → false (falls back to Idle).
+        assert!(!awaiting_input(
+            i32::MAX,
+            "/dev/pts/0",
+            &ProcGroups::snapshot()
+        ));
     }
 }
diff --git a/crates/maestro-daemon/src/session.rs b/crates/maestro-daemon/src/session.rs
index 1019dcb..2372aef 100644
--- a/crates/maestro-daemon/src/session.rs
+++ b/crates/maestro-daemon/src/session.rs
@@ -595,6 +595,11 @@ async fn run_activity_pass(inner: &Arc<Inner>) {
         return;
     }
 
+    // One `/proc` walk for the whole pass, rather than one per session. Reading
+    // it is pure filesystem work, so it goes on the blocking pool alongside the
+    // tmux calls below.
+    let groups = tmux_blocking(crate::proc::ProcGroups::snapshot).await;
+
     for id in ids {
         // The tmux pane lookup is a subprocess — keep it off the runtime worker
         // and outside the sessions lock.
@@ -611,7 +616,7 @@ async fn run_activity_pass(inner: &Arc<Inner>) {
             }
         };
         // `/proc` inspection is cheap, but still done off the async lock.
-        let awaiting = crate::proc::awaiting_input(pane.pane_pid, &pane.tty);
+        let awaiting = crate::proc::awaiting_input(pane.pane_pid, &pane.tty, &groups);
 
         let updated = {
             let mut sessions = inner.sessions.lock().await;
diff --git a/crates/maestro-daemon/tests/activity.rs b/crates/maestro-daemon/tests/activity.rs
new file mode 100644
index 0000000..d7fe201
--- /dev/null
+++ b/crates/maestro-daemon/tests/activity.rs
@@ -0,0 +1,196 @@
+//! Activity detection against real processes on a real tty.
+//!
+//! The review noted this pipeline had no end-to-end coverage — `activity::decide`
+//! was unit-tested, but nothing exercised the `/proc` inspection that feeds it.
+//! These use a real tmux pane because that is the only way to get a controlling
+//! terminal with a genuine foreground process group.
+
+mod common;
+
+use std::time::Duration;
+
+use common::TmuxFixture;
+use maestro_daemon::proc::{awaiting_input, ProcGroups};
+use tempfile::TempDir;
+use uuid::Uuid;
+
+/// Poll `awaiting_input` until it reports `want`, so we don't race the shell
+/// reaching its blocking read. Reports what it last saw — the useful half of
+/// the failure.
+fn wait_for_awaiting(fx: &TmuxFixture, id: Uuid, want: bool, label: &str) {
+    let deadline = std::time::Instant::now() + Duration::from_secs(5);
+    let mut last = None;
+    while std::time::Instant::now() < deadline {
+        if let Ok(Some(pane)) = fx.tmux.pane_info(id) {
+            // Fresh each poll: the tree is still settling.
+            let got = awaiting_input(pane.pane_pid, &pane.tty, &ProcGroups::snapshot());
+            if got == want {
+                return;
+            }
+            last = Some(got);
+        }
+        std::thread::sleep(Duration::from_millis(25));
+    }
+    panic!("{label}: wanted awaiting_input={want}, last saw {last:?}");
+}
+
+#[test]
+fn an_idle_shell_is_awaiting_input() {
+    let fx = TmuxFixture::new();
+    let cwd = TempDir::new().unwrap();
+    let id = Uuid::new_v4();
+    fx.tmux
+        .new_session(id, cwd.path(), "bash --noprofile --norc")
+        .expect("new_session");
+
+    wait_for_awaiting(&fx, id, true, "an idle shell sits in read()");
+}
+
+/// The case the old heuristic got wrong. It walked `children` to the deepest
+/// descendant, so it landed on the background `sleep` — which is not on the
+/// terminal — and reported the session as *not* awaiting input, while the
+/// foreground process was blocked on `read`.
+#[test]
+fn a_foreground_reader_with_a_background_helper_is_awaiting_input() {
+    let fx = TmuxFixture::new();
+    let cwd = TempDir::new().unwrap();
+    let script = cwd.path().join("agentish.sh");
+    std::fs::write(&script, "#!/bin/bash\nsleep 300 &\nread -r line\n").unwrap();
+    #[cfg(unix)]
+    {
+        use std::os::unix::fs::PermissionsExt;
+        std::fs::set_permissions(&script, std::fs::Permissions::from_mode(0o755)).unwrap();
+    }
+
+    let id = Uuid::new_v4();
+    fx.tmux
+        .new_session(
+            id,
+            cwd.path(),
+            &format!("bash --noprofile --norc {}", script.display()),
+        )
+        .expect("new_session");
+
+    // Wait for the helper itself. Asserting before it exists would pass
+    // against a deepest-descendant walk too, because until then the newest
+    // process in the tree *is* the one blocked on read.
+    let pane = fx.tmux.pane_info(id).expect("pane_info").expect("pane");
+    wait_until(
+        || subtree_has_comm(pane.pane_pid, "sleep"),
+        "the background helper to be spawned",
+    );
+
+    wait_for_awaiting(
+        &fx,
+        id,
+        true,
+        "a foreground reader must win over a newer background child",
+    );
+}
+
+/// A pure timer sleep also parks in 'S' with the terminal inherited; it must
+/// read as idle, not as waiting on the user.
+#[test]
+fn a_foreground_sleep_is_not_awaiting_input() {
+    let fx = TmuxFixture::new();
+    let cwd = TempDir::new().unwrap();
+    let id = Uuid::new_v4();
+    fx.tmux
+        .new_session(id, cwd.path(), "sleep 300")
+        .expect("new_session");
+
+    wait_for_awaiting(&fx, id, false, "a timer sleep is not user input");
+}
+
+/// Whether any process under `root` (inclusive) has this `comm`.
+fn subtree_has_comm(root: i32, comm: &str) -> bool {
+    let mut queue = vec![root];
+    while let Some(pid) = queue.pop() {
+        if std::fs::read_to_string(format!("/proc/{pid}/comm")).is_ok_and(|c| c.trim() == comm) {
+            return true;
+        }
+        queue.extend(
+            std::fs::read_to_string(format!("/proc/{pid}/task/{pid}/children"))
+                .map(|s| {
+                    s.split_whitespace()
+                        .filter_map(|x| x.parse::<i32>().ok())
+                        .collect::<Vec<_>>()
+                })
+                .unwrap_or_default(),
+        );
+    }
+    false
+}
+
+/// Block until `cond` holds, so a test asserts on the state it means to
+/// exercise rather than on whatever the tree looked like first.
+fn wait_until(cond: impl Fn() -> bool, what: &str) {
+    let deadline = std::time::Instant::now() + Duration::from_secs(5);
+    while std::time::Instant::now() < deadline {
+        if cond() {
+            return;
+        }
+        std::thread::sleep(Duration::from_millis(20));
+    }
+    panic!("timed out waiting for {what}");
+}
+
+/// Read `pgrp` and `tpgid` for a pid straight from `/proc`, so the test can
+/// wait for the exact condition it means to exercise rather than a proxy.
+fn pgrp_and_tpgid(pid: i32) -> Option<(i32, i32)> {
+    let stat = std::fs::read_to_string(format!("/proc/{pid}/stat")).ok()?;
+    let close = stat.rfind(')')?;
+    let f: Vec<&str> = stat[close + 1..].split_whitespace().collect();
+    Some((f.get(2)?.parse().ok()?, f.get(5)?.parse().ok()?))
+}
+
+/// `tpgid` names a process *group*, and its leader can exit while another
+/// member still holds the terminal — a foreground pipeline whose first stage
+/// finished. Reading `/proc/<tpgid>` directly then finds nothing and reports
+/// idle, so the group has to be searched for a live member.
+///
+/// Needs an interactive shell: job control is what puts a pipeline in its own
+/// process group, and that is how the daemon spawns commands
+/// (`$SHELL -lic '<cmd>'`).
+#[test]
+fn a_foreground_pipeline_whose_leader_exited_is_still_awaiting_input() {
+    let fx = TmuxFixture::new();
+    let cwd = TempDir::new().unwrap();
+    let id = Uuid::new_v4();
+    fx.tmux
+        .new_session(id, cwd.path(), "bash --noprofile --norc -i")
+        .expect("new_session");
+    wait_for_awaiting(&fx, id, true, "the shell must settle first");
+
+    // `true` leads the group and exits immediately; the right-hand stage
+    // survives, blocked reading the terminal.
+    fx.tmux
+        .send_keys(id, b"true | { read -r line < /dev/tty; }\n")
+        .expect("send_keys");
+
+    // Wait for the state this test exists for: the foreground group is no
+    // longer the shell's own, and its leader is gone. Asserting before that
+    // would pass against a leader-only lookup, because the shell is still
+    // foreground and alive.
+    let pane = fx.tmux.pane_info(id).expect("pane_info").expect("pane");
+    let deadline = std::time::Instant::now() + Duration::from_secs(5);
+    let mut reached = false;
+    while std::time::Instant::now() < deadline {
+        if let Some((pgrp, tpgid)) = pgrp_and_tpgid(pane.pane_pid) {
+            if tpgid > 0 && tpgid != pgrp && pgrp_and_tpgid(tpgid).is_none() {
+                reached = true;
+                break;
+            }
+        }
+        std::thread::sleep(Duration::from_millis(20));
+    }
+    assert!(
+        reached,
+        "never observed a foreground group whose leader had exited"
+    );
+
+    assert!(
+        awaiting_input(pane.pane_pid, &pane.tty, &ProcGroups::snapshot()),
+        "a surviving pipeline member still holds the tty"
+    );
+}