diff --git a/crates/maestro-cli/src/sessions.rs b/crates/maestro-cli/src/sessions.rs
index 3d91685..cdeab8d 100644
--- a/crates/maestro-cli/src/sessions.rs
+++ b/crates/maestro-cli/src/sessions.rs
@@ -4,7 +4,8 @@ use std::collections::HashMap;
 use std::io::Write;
 
 use maestro_protocol::{
-    ConnectInfo, CreateSessionRequest, DaemonInfo, Session, SessionStatus, WorktreeSpec,
+    ActivityState, ConnectInfo, CreateSessionRequest, DaemonInfo, Session, SessionStatus,
+    WorktreeSpec,
 };
 use serde_json::Value;
 use uuid::Uuid;
@@ -126,8 +127,8 @@ fn print_session_table(out: &mut impl Write, sessions: &[Session], daemons: &[Da
         writeln!(out, "\n{daemon_id} ({hostname})").ok();
         writeln!(
             out,
-            "  {:<36}  {:<8}  {:<20}  {}",
-            "ID", "STATUS", "LABEL", "COMMAND"
+            "  {:<36}  {:<8}  {:<9}  {:<20}  {}",
+            "ID", "STATUS", "ACTIVITY", "LABEL", "COMMAND"
         )
         .ok();
         for s in group {
@@ -140,10 +141,21 @@ fn print_session_table(out: &mut impl Write, sessions: &[Session], daemons: &[Da
                 SessionStatus::Running => "running",
                 SessionStatus::Dead => "dead",
             };
+            let activity = match s.activity {
+                ActivityState::Active => "active",
+                ActivityState::Idle => "idle",
+                ActivityState::AwaitingInput => "awaiting",
+            };
+            // Show the live foreground command alongside the spawn command when
+            // the daemon has observed one that differs (NAP-340).
+            let command = match &s.foreground {
+                Some(fg) if fg != &s.command => format!("{} [{}]", s.command, fg),
+                _ => s.command.clone(),
+            };
             writeln!(
                 out,
-                "  {:<36}  {:<8}  {:<20}  {}",
-                s.id, status, label, s.command
+                "  {:<36}  {:<8}  {:<9}  {:<20}  {command}",
+                s.id, status, activity, label
             )
             .ok();
         }
@@ -156,7 +168,7 @@ mod tests {
     use std::path::PathBuf;
 
     use chrono::{TimeZone, Utc};
-    use maestro_protocol::{DaemonInfo, Session, SessionStatus};
+    use maestro_protocol::{ActivityState, DaemonInfo, Session, SessionStatus};
     use serde_json::Value;
     use uuid::Uuid;
 
@@ -172,6 +184,9 @@ mod tests {
             metadata: meta,
             status: SessionStatus::Running,
             created_at: Utc.with_ymd_and_hms(2026, 5, 25, 12, 0, 0).unwrap(),
+            foreground: None,
+            activity: Default::default(),
+            last_activity_at: chrono::Utc::now(),
         }
     }
 
@@ -205,6 +220,28 @@ mod tests {
         assert!(s.contains("vm1 (vm1.lan)"), "{s}");
         assert!(s.contains("personal/maestro"), "{s}");
         assert!(s.contains("running"), "{s}");
+        assert!(s.contains("ACTIVITY"), "{s}");
+        assert!(s.contains("idle"), "{s}");
+    }
+
+    #[test]
+    fn shows_activity_and_foreground() {
+        let mut s1 = session("laptop", "work", "bash", 1);
+        s1.activity = ActivityState::AwaitingInput;
+        s1.foreground = Some("claude".into()); // differs from spawn command
+        let daemons = vec![DaemonInfo {
+            daemon_id: "laptop".into(),
+            hostname: "h".into(),
+            version: "0.1.0".into(),
+            daemon_url: "ws://x".into(),
+            home_dir: String::new(),
+        }];
+        let mut buf = Vec::new();
+        super::print_session_table(&mut buf, &[s1], &daemons);
+        let out = String::from_utf8(buf).unwrap();
+        assert!(out.contains("awaiting"), "{out}");
+        // Live foreground surfaces next to the spawn command.
+        assert!(out.contains("bash [claude]"), "{out}");
     }
 
     #[test]
diff --git a/crates/maestro-daemon/src/activity.rs b/crates/maestro-daemon/src/activity.rs
new file mode 100644
index 0000000..8c9bd9f
--- /dev/null
+++ b/crates/maestro-daemon/src/activity.rs
@@ -0,0 +1,56 @@
+//! Activity-state inference combine logic (NAP-340).
+//!
+//! Pure decision from three structured signals — PTY quiescence, foreground
+//! process, and `/proc` block-state — so it's unit-testable without tmux or a
+//! live session. The orchestration (gathering the signals each liveness tick
+//! and emitting `SessionUpdated` on change) lives in [`crate::session`].
+
+use std::time::Duration;
+
+use maestro_protocol::ActivityState;
+
+/// PTY output more recent than this keeps a session `Active`. It must exceed the
+/// liveness tick (~2s, [`crate::session::DEFAULT_LIVENESS_INTERVAL`]) so a
+/// session emitting a chunk every tick stays `Active` instead of flapping
+/// Active↔Idle. This window *is* the debounce the issue calls for.
+pub const ACTIVE_WINDOW: Duration = Duration::from_secs(3);
+
+/// Combine the observed signals into an [`ActivityState`]:
+/// recent output ⇒ `Active`; else blocked on a terminal read ⇒ `AwaitingInput`;
+/// else `Idle`.
+pub fn decide(idle_for: Duration, awaiting_input: bool) -> ActivityState {
+    if idle_for < ACTIVE_WINDOW {
+        ActivityState::Active
+    } else if awaiting_input {
+        ActivityState::AwaitingInput
+    } else {
+        ActivityState::Idle
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn recent_output_is_active_regardless_of_block_state() {
+        assert_eq!(decide(Duration::from_millis(100), false), ActivityState::Active);
+        assert_eq!(decide(Duration::from_millis(100), true), ActivityState::Active);
+    }
+
+    #[test]
+    fn quiescent_and_blocked_is_awaiting_input() {
+        assert_eq!(
+            decide(ACTIVE_WINDOW + Duration::from_secs(1), true),
+            ActivityState::AwaitingInput
+        );
+    }
+
+    #[test]
+    fn quiescent_and_not_blocked_is_idle() {
+        assert_eq!(
+            decide(ACTIVE_WINDOW + Duration::from_secs(1), false),
+            ActivityState::Idle
+        );
+    }
+}
diff --git a/crates/maestro-daemon/src/broker.rs b/crates/maestro-daemon/src/broker.rs
index 1ae6d73..5728f3f 100644
--- a/crates/maestro-daemon/src/broker.rs
+++ b/crates/maestro-daemon/src/broker.rs
@@ -571,6 +571,9 @@ mod tests {
             metadata: Default::default(),
             status: maestro_protocol::SessionStatus::Running,
             created_at: chrono::Utc::now(),
+            foreground: None,
+            activity: Default::default(),
+            last_activity_at: chrono::Utc::now(),
         };
         handle
             .complete_forward_create(
diff --git a/crates/maestro-daemon/src/catalog_mirror.rs b/crates/maestro-daemon/src/catalog_mirror.rs
index 3abe001..2dd3bad 100644
--- a/crates/maestro-daemon/src/catalog_mirror.rs
+++ b/crates/maestro-daemon/src/catalog_mirror.rs
@@ -122,6 +122,9 @@ mod tests {
             metadata: StdHashMap::new(),
             status: SessionStatus::Running,
             created_at: Utc::now(),
+            foreground: None,
+            activity: Default::default(),
+            last_activity_at: chrono::Utc::now(),
         }
     }
 
diff --git a/crates/maestro-daemon/src/lib.rs b/crates/maestro-daemon/src/lib.rs
index 8af0168..18618b0 100644
--- a/crates/maestro-daemon/src/lib.rs
+++ b/crates/maestro-daemon/src/lib.rs
@@ -6,6 +6,7 @@
 
 #![deny(missing_debug_implementations)]
 
+pub mod activity;
 pub mod attach;
 pub mod broker;
 pub mod catalog_mirror;
@@ -17,6 +18,7 @@ pub mod gitroot;
 pub mod jwt;
 pub mod local_api;
 pub mod local_events;
+pub mod proc;
 pub mod pty;
 pub mod registry_client;
 pub mod session;
diff --git a/crates/maestro-daemon/src/proc.rs b/crates/maestro-daemon/src/proc.rs
new file mode 100644
index 0000000..afe991b
--- /dev/null
+++ b/crates/maestro-daemon/src/proc.rs
@@ -0,0 +1,129 @@
+//! 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.
+//!
+//! `/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
+//! *unprivileged* signals instead:
+//!
+//! 1. process state `S` (interruptible sleep — where a blocking `read()` parks),
+//! 2. its `wchan` is not a pure timer sleep (so `sleep 30`, which also sits in
+//!    `S` with the terminal inherited, is excluded), and
+//! 3. one of its stdio fds is the pane's tty.
+//!
+//! 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);
+    // Interruptible sleep is where a blocking read parks; anything else
+    // (running, uninterruptible) is busy, not waiting on us.
+    if read_state(leaf) != 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) {
+        return false;
+    }
+    fd_points_at_tty(leaf, 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 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()
+}
+
+/// 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> {
+    let close = stat.rfind(')')?;
+    stat[close + 1..].trim_start().chars().next()
+}
+
+/// Whether `pid`'s `wchan` (the kernel symbol it's blocked in) is a timer
+/// sleep — `hrtimer_nanosleep`, `do_nanosleep`, etc. Those are quiescent but
+/// *not* waiting on the user.
+fn is_timer_sleep(pid: i32) -> bool {
+    std::fs::read_to_string(format!("/proc/{pid}/wchan"))
+        .map(|w| is_timer_wchan(&w))
+        .unwrap_or(false)
+}
+
+/// Classify a `wchan` symbol as a pure timer sleep. Kept pure for testing.
+fn is_timer_wchan(wchan: &str) -> bool {
+    wchan.contains("nanosleep")
+}
+
+/// Whether any of `pid`'s stdio fds (0/1/2) is the terminal `tty`.
+fn fd_points_at_tty(pid: i32, tty: &str) -> bool {
+    (0..3).any(|fd| {
+        std::fs::read_link(format!("/proc/{pid}/fd/{fd}"))
+            .map(|target| target.to_string_lossy() == tty)
+            .unwrap_or(false)
+    })
+}
+
+#[cfg(test)]
+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'));
+    }
+
+    #[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);
+    }
+
+    #[test]
+    fn timer_wchan_excludes_sleep_but_not_a_tty_wait() {
+        // `sleep` blocks here — excluded.
+        assert!(is_timer_wchan("hrtimer_nanosleep"));
+        assert!(is_timer_wchan("do_nanosleep"));
+        // A terminal read / poll parks here — not a timer.
+        assert!(!is_timer_wchan("wait_woken"));
+        assert!(!is_timer_wchan("n_tty_read"));
+        assert!(!is_timer_wchan("do_select"));
+        assert!(!is_timer_wchan("0"));
+    }
+
+    #[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"));
+    }
+}
diff --git a/crates/maestro-daemon/src/pty.rs b/crates/maestro-daemon/src/pty.rs
index ac73cf1..e29a47c 100644
--- a/crates/maestro-daemon/src/pty.rs
+++ b/crates/maestro-daemon/src/pty.rs
@@ -30,6 +30,7 @@ use std::os::unix::ffi::OsStrExt;
 use std::os::unix::fs::OpenOptionsExt;
 use std::path::{Path, PathBuf};
 use std::sync::{Arc, Mutex};
+use std::time::Instant;
 
 use tokio::io::unix::AsyncFd;
 use tokio::io::Interest;
@@ -59,6 +60,10 @@ pub struct PtyBridge {
     fifo_path: PathBuf,
     sender: broadcast::Sender<Vec<u8>>,
     replay: ReplayBuffer,
+    /// Monotonic timestamp of the last non-empty read off the FIFO, bumped by
+    /// `run_reader`. The activity watcher reads it to gauge PTY quiescence
+    /// without touching the byte stream (NAP-340).
+    last_activity: Arc<Mutex<Instant>>,
     task: JoinHandle<()>,
 }
 
@@ -88,17 +93,29 @@ impl PtyBridge {
         let replay = ReplayBuffer::new(replay_bytes);
         let tx = sender.clone();
         let replay_for_task = replay.clone();
+        let last_activity = Arc::new(Mutex::new(Instant::now()));
+        let last_activity_for_task = last_activity.clone();
         let task = tokio::spawn(async move {
-            run_reader(async_fd, tx, replay_for_task).await;
+            run_reader(async_fd, tx, replay_for_task, last_activity_for_task).await;
         });
         Ok(Self {
             fifo_path,
             sender,
             replay,
+            last_activity,
             task,
         })
     }
 
+    /// How long since the session last produced PTY output. The activity
+    /// watcher treats a short value as `Active` (NAP-340).
+    pub fn idle_for(&self) -> std::time::Duration {
+        self.last_activity
+            .lock()
+            .expect("last_activity poisoned")
+            .elapsed()
+    }
+
     pub fn fifo_path(&self) -> &Path {
         &self.fifo_path
     }
@@ -197,7 +214,12 @@ impl ReplayGuard<'_> {
     }
 }
 
-async fn run_reader(async_fd: AsyncFd<File>, tx: broadcast::Sender<Vec<u8>>, replay: ReplayBuffer) {
+async fn run_reader(
+    async_fd: AsyncFd<File>,
+    tx: broadcast::Sender<Vec<u8>>,
+    replay: ReplayBuffer,
+    last_activity: Arc<Mutex<Instant>>,
+) {
     let mut buf = [0u8; READ_BUF_BYTES];
     loop {
         let mut guard = match async_fd.readable().await {
@@ -219,6 +241,9 @@ async fn run_reader(async_fd: AsyncFd<File>, tx: broadcast::Sender<Vec<u8>>, rep
                 tokio::task::yield_now().await;
             }
             Ok(Ok(n)) => {
+                // Mark the session as having just produced output, for the
+                // activity watcher's quiescence check (NAP-340).
+                *last_activity.lock().expect("last_activity poisoned") = Instant::now();
                 // Hold the replay lock across both the buffer push and the
                 // broadcast send so `subscribe_with_replay` either sees this
                 // chunk in its snapshot *or* receives it on the broadcast —
diff --git a/crates/maestro-daemon/src/registry_client.rs b/crates/maestro-daemon/src/registry_client.rs
index f485ae4..da7cf12 100644
--- a/crates/maestro-daemon/src/registry_client.rs
+++ b/crates/maestro-daemon/src/registry_client.rs
@@ -558,6 +558,9 @@ mod tests {
             metadata: Default::default(),
             status: maestro_protocol::SessionStatus::Running,
             created_at: chrono::Utc::now(),
+            foreground: None,
+            activity: Default::default(),
+            last_activity_at: chrono::Utc::now(),
         }
     }
 
diff --git a/crates/maestro-daemon/src/session.rs b/crates/maestro-daemon/src/session.rs
index b6b104b..3a16032 100644
--- a/crates/maestro-daemon/src/session.rs
+++ b/crates/maestro-daemon/src/session.rs
@@ -11,7 +11,9 @@ use std::sync::Arc;
 use std::time::Duration;
 
 use chrono::Utc;
-use maestro_protocol::{CreateSessionRequest, ErrorInfo, Session, SessionStatus, WorktreeSpec};
+use maestro_protocol::{
+    ActivityState, CreateSessionRequest, ErrorInfo, Session, SessionStatus, WorktreeSpec,
+};
 use tokio::sync::{broadcast, Mutex};
 use uuid::Uuid;
 
@@ -290,6 +292,11 @@ impl SessionManager {
             metadata: req.metadata,
             status: SessionStatus::Running,
             created_at: Utc::now(),
+            // A freshly-spawned session is producing its initial output; the
+            // activity watcher refines `foreground`/`activity` on its next tick.
+            foreground: None,
+            activity: ActivityState::Active,
+            last_activity_at: Utc::now(),
         };
 
         if let Err(e) = self.inner.sidecar.write(&session) {
@@ -384,6 +391,9 @@ impl SessionManager {
                     return;
                 };
                 run_liveness_pass(&inner).await;
+                // Same tick, no new timer (NAP-340): refresh activity for the
+                // survivors and emit SessionUpdated on any transition.
+                run_activity_pass(&inner).await;
             }
         });
     }
@@ -414,6 +424,10 @@ impl SessionManager {
                     metadata: Default::default(),
                     status: SessionStatus::Running,
                     created_at: Utc::now(),
+                    // Activity is unknown on adoption; the watcher fills it in.
+                    foreground: None,
+                    activity: ActivityState::Idle,
+                    last_activity_at: Utc::now(),
                 },
             };
             // The sidecar persists whatever daemon_id was current when the
@@ -529,6 +543,63 @@ async fn run_liveness_pass(inner: &Arc<Inner>) {
     }
 }
 
+/// Refresh each live session's inferred activity (NAP-340) and emit
+/// `SessionUpdated` only when `activity` or `foreground` actually changes — so a
+/// steady state produces no diffs. Runs on the existing liveness tick.
+async fn run_activity_pass(inner: &Arc<Inner>) {
+    let ids: Vec<Uuid> = {
+        let sessions = inner.sessions.lock().await;
+        if sessions.is_empty() {
+            return;
+        }
+        sessions.keys().copied().collect()
+    };
+
+    for id in ids {
+        // The tmux pane lookup is a subprocess — keep it off the runtime worker
+        // and outside the sessions lock.
+        let pane = {
+            let tmux = inner.tmux.clone();
+            match tmux_blocking(move || tmux.pane_info(id)).await {
+                Ok(Some(p)) => p,
+                // Session/pane vanished — the liveness pass reaps it; skip.
+                Ok(None) => continue,
+                Err(e) => {
+                    tracing::debug!(%id, error = ?e, "activity: pane_info failed");
+                    continue;
+                }
+            }
+        };
+        // `/proc` inspection is cheap, but still done off the async lock.
+        let awaiting = crate::proc::awaiting_input(pane.pane_pid, &pane.tty);
+
+        let updated = {
+            let mut sessions = inner.sessions.lock().await;
+            let Some(rec) = sessions.get_mut(&id) else {
+                continue;
+            };
+            let idle = rec.bridge.idle_for();
+            let activity = crate::activity::decide(idle, awaiting);
+            let foreground = Some(pane.command.clone());
+            // Change-detection on (activity, foreground) is the debounce: nothing
+            // emitted at steady state.
+            if rec.session.activity == activity && rec.session.foreground == foreground {
+                None
+            } else {
+                rec.session.activity = activity;
+                rec.session.foreground = foreground;
+                // Wall-clock of the last output ≈ now − idle.
+                rec.session.last_activity_at = Utc::now()
+                    - chrono::Duration::from_std(idle).unwrap_or_else(|_| chrono::Duration::zero());
+                Some(rec.session.clone())
+            }
+        };
+        if let Some(session) = updated {
+            let _ = inner.events.send(SessionEvent::Updated(session));
+        }
+    }
+}
+
 /// Remove a session whose tmux session is already gone: drop the in-memory
 /// record, delete the sidecar + FIFO, clean up its worktree if requested, and
 /// fire `SessionEvent::Removed`. No tmux kill — there is nothing left to kill.
diff --git a/crates/maestro-daemon/src/tmux.rs b/crates/maestro-daemon/src/tmux.rs
index 9ede484..d063974 100644
--- a/crates/maestro-daemon/src/tmux.rs
+++ b/crates/maestro-daemon/src/tmux.rs
@@ -155,6 +155,39 @@ impl Tmux {
         Ok(ids)
     }
 
+    /// `tmux list-panes -t maestro-<id> -F '#{pane_pid} #{pane_current_command}
+    /// #{pane_tty}'` for the session's (single) pane. Used by the activity
+    /// watcher (NAP-340) to learn the foreground command, the pane's process,
+    /// and its tty. Returns `Ok(None)` when the session/pane is gone or tmux
+    /// isn't running, so the watcher degrades to "unknown" rather than erroring.
+    pub fn pane_info(&self, id: Uuid) -> Result<Option<PaneInfo>, TmuxError> {
+        let name = session_name(id);
+        let out = self
+            .cmd()
+            .args([
+                "list-panes",
+                "-t",
+                &name,
+                "-F",
+                "#{pane_pid} #{pane_current_command} #{pane_tty}",
+            ])
+            .output()?;
+        if !out.status.success() {
+            let stderr = String::from_utf8_lossy(&out.stderr);
+            // No server, or the session/pane vanished between the liveness check
+            // and now — both are "no info", not a hard error.
+            if is_no_server_error(&stderr) || stderr.contains("can't find") {
+                return Ok(None);
+            }
+            return Err(TmuxError::NonZero {
+                code: out.status.code().unwrap_or(-1),
+                stderr: stderr.into_owned(),
+            });
+        }
+        let stdout = String::from_utf8_lossy(&out.stdout);
+        Ok(stdout.lines().next().and_then(parse_pane_line))
+    }
+
     /// `tmux send-keys -t maestro-<id> -l <bytes>`. The `-l` flag tells tmux to
     /// treat the payload as literal input (no key-name translation), which is
     /// what we want for forwarded PTY input.
@@ -229,6 +262,34 @@ fn is_no_server_error(stderr: &str) -> bool {
         || stderr.contains("error connecting")
 }
 
+/// A session pane's process id, foreground command, and controlling tty, as
+/// reported by `tmux list-panes` (NAP-340).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PaneInfo {
+    pub pane_pid: i32,
+    pub command: String,
+    pub tty: String,
+}
+
+/// Parse one `#{pane_pid} #{pane_current_command} #{pane_tty}` line. `pane_pid`
+/// is the leading integer and `pane_tty` the trailing token; everything between
+/// is the command (which is normally a single word, but is rejoined defensively
+/// in case tmux ever emits a spaced value).
+fn parse_pane_line(line: &str) -> Option<PaneInfo> {
+    let mut parts = line.split_whitespace();
+    let pane_pid: i32 = parts.next()?.parse().ok()?;
+    let rest: Vec<&str> = parts.collect();
+    let (tty, command) = rest.split_last()?;
+    if command.is_empty() {
+        return None;
+    }
+    Some(PaneInfo {
+        pane_pid,
+        command: command.join(" "),
+        tty: (*tty).to_string(),
+    })
+}
+
 pub fn session_name(id: Uuid) -> String {
     format!("{SESSION_PREFIX}{id}")
 }
@@ -289,6 +350,22 @@ fn check(out: Output) -> Result<(), TmuxError> {
 mod tests {
     use super::*;
 
+    #[test]
+    fn parse_pane_line_splits_pid_command_tty() {
+        let p = parse_pane_line("12345 claude /dev/pts/3").unwrap();
+        assert_eq!(p.pane_pid, 12345);
+        assert_eq!(p.command, "claude");
+        assert_eq!(p.tty, "/dev/pts/3");
+    }
+
+    #[test]
+    fn parse_pane_line_rejects_malformed() {
+        assert!(parse_pane_line("").is_none());
+        assert!(parse_pane_line("notapid bash /dev/pts/0").is_none());
+        // pid + tty but no command between them.
+        assert!(parse_pane_line("123 /dev/pts/0").is_none());
+    }
+
     #[test]
     fn new_session_args_pass_256color_env_and_default_terminal() {
         let args = new_session_args("maestro-abc", Path::new("/tmp/proj"), "sleep 30", None);
diff --git a/crates/maestro-daemon/tests/sidecar.rs b/crates/maestro-daemon/tests/sidecar.rs
index b8a58a0..c853af2 100644
--- a/crates/maestro-daemon/tests/sidecar.rs
+++ b/crates/maestro-daemon/tests/sidecar.rs
@@ -24,6 +24,9 @@ fn sample(id: Uuid, daemon_id: &str) -> Session {
         created_at: DateTime::parse_from_rfc3339("2026-05-24T12:00:00Z")
             .unwrap()
             .with_timezone(&Utc),
+        foreground: None,
+        activity: Default::default(),
+        last_activity_at: chrono::Utc::now(),
     }
 }
 
diff --git a/crates/maestro-e2e/tests/e2e.rs b/crates/maestro-e2e/tests/e2e.rs
index dc7e77e..5de053e 100644
--- a/crates/maestro-e2e/tests/e2e.rs
+++ b/crates/maestro-e2e/tests/e2e.rs
@@ -522,6 +522,65 @@ async fn spawn_records_history_and_infers_gitroot_for_form() {
     client.kill_session(s.id).await.expect("kill");
 }
 
+#[tokio::test(flavor = "multi_thread")]
+async fn activity_reflects_foreground_and_input_blocking() {
+    // NAP-340 (#18): the watcher infers each session's foreground command and
+    // activity from structured tmux/proc state. `cat` parks in read() on the
+    // pty → AwaitingInput; `sleep` is quiescent but not reading the tty → Idle
+    // (the distinction the /proc/<pid>/syscall check buys us). Both report the
+    // right foreground. Exercised against real tmux.
+    use maestro_protocol::ActivityState;
+    let (_reg, _daemons, client) = bring_up(&["host"]).await;
+    let cwd = TempDir::new().unwrap();
+
+    let cat = client
+        .spawn_session(&create_req("host", cwd.path(), "cat"))
+        .await
+        .expect("spawn cat");
+    let slp = client
+        .spawn_session(&create_req("host", cwd.path(), "sleep 300"))
+        .await
+        .expect("spawn sleep");
+
+    // Initial state is Active; the watcher needs a couple ticks past the
+    // quiescence window (3s) to settle into the steady states.
+    wait_for_async(
+        "cat → AwaitingInput, sleep → Idle, with correct foreground",
+        Duration::from_secs(20),
+        || {
+            let client = client.clone();
+            let (cat_id, sleep_id) = (cat.id, slp.id);
+            async move {
+                let sessions = match client.list_sessions().await {
+                    Ok(s) => s,
+                    Err(_) => return false,
+                };
+                let cat_ok = sessions
+                    .iter()
+                    .find(|s| s.id == cat_id)
+                    .map(|s| {
+                        s.activity == ActivityState::AwaitingInput
+                            && s.foreground.as_deref() == Some("cat")
+                    })
+                    .unwrap_or(false);
+                let sleep_ok = sessions
+                    .iter()
+                    .find(|s| s.id == sleep_id)
+                    .map(|s| {
+                        s.activity == ActivityState::Idle
+                            && s.foreground.as_deref() == Some("sleep")
+                    })
+                    .unwrap_or(false);
+                cat_ok && sleep_ok
+            }
+        },
+    )
+    .await;
+
+    client.kill_session(cat.id).await.expect("kill cat");
+    client.kill_session(slp.id).await.expect("kill sleep");
+}
+
 // ---------------------------------------------------------------------------
 // Helpers
 // ---------------------------------------------------------------------------
diff --git a/crates/maestro-protocol/src/lib.rs b/crates/maestro-protocol/src/lib.rs
index e58a4e4..614b27a 100644
--- a/crates/maestro-protocol/src/lib.rs
+++ b/crates/maestro-protocol/src/lib.rs
@@ -34,6 +34,28 @@ pub struct Session {
     pub metadata: HashMap<String, Value>,
     pub status: SessionStatus,
     pub created_at: DateTime<Utc>,
+    /// Current foreground command in the session's pane (e.g. `claude`,
+    /// `bash`), as tmux reports it — distinct from the `command` it was spawned
+    /// with. Daemon-derived (NAP-340); `None` until the daemon's activity
+    /// watcher has observed it. All three activity fields default on the wire so
+    /// older daemons/payloads stay forward-compatible.
+    #[serde(default)]
+    pub foreground: Option<String>,
+    /// Inferred activity dimension, orthogonal to [`SessionStatus`]
+    /// (NAP-340). Derived from structured tmux/`/proc` state, never from the
+    /// byte stream.
+    #[serde(default)]
+    pub activity: ActivityState,
+    /// Wall-clock time of the session's most recent observed PTY output.
+    /// Drives "idle for Ns" displays. Defaults to the Unix epoch on payloads
+    /// from daemons that predate the field.
+    #[serde(default = "epoch")]
+    pub last_activity_at: DateTime<Utc>,
+}
+
+/// Default for [`Session::last_activity_at`] when an older payload omits it.
+fn epoch() -> DateTime<Utc> {
+    DateTime::from_timestamp(0, 0).expect("unix epoch is a valid timestamp")
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
@@ -42,6 +64,20 @@ pub enum SessionStatus {
     Dead,
 }
 
+/// Inferred session activity, orthogonal to [`SessionStatus`] (NAP-340). The
+/// daemon derives this from tmux pane state, PTY quiescence, and `/proc`
+/// inspection — never by parsing the terminal byte stream.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
+pub enum ActivityState {
+    /// Producing output, or the foreground process is busy.
+    Active,
+    /// Foreground process alive but quiescent. The forward-compatible default.
+    #[default]
+    Idle,
+    /// Blocked on a `read()` from the pty — waiting on the user.
+    AwaitingInput,
+}
+
 /// Optional worktree request attached to a `CreateSessionRequest`.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct WorktreeSpec {
@@ -632,9 +668,41 @@ mod tests {
             created_at: DateTime::parse_from_rfc3339("2026-05-24T12:00:00Z")
                 .unwrap()
                 .with_timezone(&Utc),
+            foreground: Some("claude".into()),
+            activity: ActivityState::Active,
+            last_activity_at: DateTime::parse_from_rfc3339("2026-05-24T12:05:00Z")
+                .unwrap()
+                .with_timezone(&Utc),
         }
     }
 
+    #[test]
+    fn session_activity_fields_default_on_legacy_payload() {
+        // A pre-NAP-340 daemon omits the activity fields; they must deserialize
+        // to the forward-compat defaults rather than failing.
+        let json = serde_json::json!({
+            "id": Uuid::from_u128(1),
+            "daemon_id": "laptop",
+            "cwd": "/tmp",
+            "command": "bash",
+            "status": "Running",
+            "created_at": "2026-05-24T12:00:00Z"
+        });
+        let s: Session = serde_json::from_value(json).unwrap();
+        assert!(s.foreground.is_none());
+        assert_eq!(s.activity, ActivityState::Idle);
+        assert_eq!(s.last_activity_at, epoch());
+    }
+
+    #[test]
+    fn activity_state_serializes_pascal_case_and_defaults_idle() {
+        assert_eq!(
+            serde_json::to_value(ActivityState::AwaitingInput).unwrap(),
+            serde_json::json!("AwaitingInput")
+        );
+        assert_eq!(ActivityState::default(), ActivityState::Idle);
+    }
+
     #[test]
     fn session_round_trips() {
         let s = sample_session();
diff --git a/crates/maestro-registry/src/state.rs b/crates/maestro-registry/src/state.rs
index 447e357..8511b98 100644
--- a/crates/maestro-registry/src/state.rs
+++ b/crates/maestro-registry/src/state.rs
@@ -929,6 +929,9 @@ mod tests {
             metadata: HashMap::new(),
             status: SessionStatus::Running,
             created_at: Utc::now(),
+            foreground: None,
+            activity: Default::default(),
+            last_activity_at: chrono::Utc::now(),
         }
     }
 
diff --git a/crates/maestro-registry/tests/daemon_ws.rs b/crates/maestro-registry/tests/daemon_ws.rs
index 70e48a4..aa19c54 100644
--- a/crates/maestro-registry/tests/daemon_ws.rs
+++ b/crates/maestro-registry/tests/daemon_ws.rs
@@ -105,6 +105,9 @@ fn sample_session(id: Uuid, daemon_id: &str) -> Session {
         created_at: DateTime::parse_from_rfc3339("2026-05-24T12:00:00Z")
             .unwrap()
             .with_timezone(&Utc),
+        foreground: None,
+        activity: Default::default(),
+        last_activity_at: chrono::Utc::now(),
     }
 }
 
diff --git a/crates/maestro-registry/tests/http_api.rs b/crates/maestro-registry/tests/http_api.rs
index 87f9df7..16378c5 100644
--- a/crates/maestro-registry/tests/http_api.rs
+++ b/crates/maestro-registry/tests/http_api.rs
@@ -219,6 +219,9 @@ fn mock_session(daemon_id: &str, request: &CreateSessionRequest) -> Session {
         metadata: request.metadata.clone(),
         status: SessionStatus::Running,
         created_at: Utc::now(),
+        foreground: None,
+        activity: Default::default(),
+        last_activity_at: chrono::Utc::now(),
     }
 }
 
diff --git a/crates/maestro-tui/src/app.rs b/crates/maestro-tui/src/app.rs
index b76038e..b5475a7 100644
--- a/crates/maestro-tui/src/app.rs
+++ b/crates/maestro-tui/src/app.rs
@@ -549,6 +549,9 @@ mod tests {
                 metadata: Default::default(),
                 status: SessionStatus::Running,
                 created_at: Utc::now(),
+                foreground: None,
+                activity: Default::default(),
+                last_activity_at: chrono::Utc::now(),
             }],
         });
         app.dispatch(Action::KillHighlighted).await;
diff --git a/crates/maestro-tui/src/catalog.rs b/crates/maestro-tui/src/catalog.rs
index 55f0a77..c194d3d 100644
--- a/crates/maestro-tui/src/catalog.rs
+++ b/crates/maestro-tui/src/catalog.rs
@@ -219,6 +219,9 @@ mod tests {
             metadata: Default::default(),
             status: SessionStatus::Running,
             created_at: Utc::now(),
+            foreground: None,
+            activity: Default::default(),
+            last_activity_at: chrono::Utc::now(),
         }
     }
 
diff --git a/crates/maestro-tui/src/sidebar.rs b/crates/maestro-tui/src/sidebar.rs
index b1117f5..4baf0d3 100644
--- a/crates/maestro-tui/src/sidebar.rs
+++ b/crates/maestro-tui/src/sidebar.rs
@@ -109,6 +109,9 @@ mod tests {
             metadata,
             status: SessionStatus::Running,
             created_at: Utc::now(),
+            foreground: None,
+            activity: Default::default(),
+            last_activity_at: chrono::Utc::now(),
         }
     }
 
@@ -203,6 +206,9 @@ mod tests {
             metadata: HashMap::new(),
             status: SessionStatus::Running,
             created_at: Utc::now(),
+            foreground: None,
+            activity: Default::default(),
+            last_activity_at: chrono::Utc::now(),
         };
         let sb = Sidebar {
             daemons: vec![daemon("konishi")],
diff --git a/crates/maestro-tui/tests/catalog_ws.rs b/crates/maestro-tui/tests/catalog_ws.rs
index 8c5e27b..87dedbc 100644
--- a/crates/maestro-tui/tests/catalog_ws.rs
+++ b/crates/maestro-tui/tests/catalog_ws.rs
@@ -33,6 +33,9 @@ fn session(id: u128, daemon_id: &str) -> Session {
         metadata: Default::default(),
         status: SessionStatus::Running,
         created_at: Utc::now(),
+        foreground: None,
+        activity: Default::default(),
+        last_activity_at: chrono::Utc::now(),
     }
 }
 
diff --git a/web/src/types.ts b/web/src/types.ts
index 6f43503..c5d274e 100644
--- a/web/src/types.ts
+++ b/web/src/types.ts
@@ -6,6 +6,10 @@ export type IsoTimestamp = string;
 
 export type SessionStatus = "Running" | "Dead";
 
+/** Daemon-inferred activity dimension, orthogonal to `SessionStatus` (NAP-340).
+ * Absent on payloads from daemons that predate the field. */
+export type ActivityState = "Active" | "Idle" | "AwaitingInput";
+
 export interface Session {
   id: Uuid;
   daemon_id: string;
@@ -15,6 +19,13 @@ export interface Session {
   metadata: Record<string, unknown>;
   status: SessionStatus;
   created_at: IsoTimestamp;
+  /** Current foreground command in the pane (e.g. `claude`, `bash`), as the
+   * daemon's activity watcher observed it. Absent until observed (NAP-340). */
+  foreground?: string | null;
+  /** Inferred activity. Defaults to `Idle` on the wire when omitted. */
+  activity?: ActivityState;
+  /** Wall-clock of the most recent observed PTY output (NAP-340). */
+  last_activity_at?: IsoTimestamp;
 }
 
 export interface DaemonInfo {