diff --git a/crates/spacesh-core/src/detect.rs b/crates/spacesh-core/src/detect.rs
new file mode 100644
index 0000000..d86f3e4
--- /dev/null
+++ b/crates/spacesh-core/src/detect.rs
@@ -0,0 +1,179 @@
+//! Pure status detectors over terminal output. No I/O.
+use spacesh_proto::status::SurfaceState;
+
+/// Scans a byte stream for OSC 133 semantic-prompt markers and yields the
+/// status each marker implies. Robust to escape sequences split across feeds:
+/// an incomplete trailing marker is buffered until the next feed.
+///
+/// Markers: ESC ] 133 ; A ST (prompt) → Idle; ; C ST (command output) → Work;
+///          ; D [;exit] ST (command end) → Done (exit 0) / Error (exit != 0).
+/// ST is BEL (0x07) or ESC \ (0x1b 0x5c). The `B` marker (input start) is ignored.
+#[derive(Default)]
+pub struct Osc133Scanner {
+    buf: Vec<u8>,
+}
+
+impl Osc133Scanner {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn feed(&mut self, bytes: &[u8]) -> Vec<SurfaceState> {
+        self.buf.extend_from_slice(bytes);
+        let mut out = Vec::new();
+        let prefix: &[u8] = b"\x1b]133;";
+        loop {
+            // Find the next marker start.
+            let Some(start) = find(&self.buf, prefix) else {
+                // No marker start. Keep only a possible partial prefix at the tail.
+                self.buf = keep_partial_tail(&self.buf, prefix);
+                break;
+            };
+            // Drop anything before the marker start.
+            if start > 0 {
+                self.buf.drain(0..start);
+            }
+            // After the prefix, find the terminator (BEL or ESC \).
+            let body_start = prefix.len();
+            let Some((body_end, term_len)) = find_terminator(&self.buf, body_start) else {
+                break; // incomplete marker; wait for more bytes
+            };
+            let body = &self.buf[body_start..body_end];
+            if let Some(state) = classify(body) {
+                out.push(state);
+            }
+            // Consume through the terminator.
+            self.buf.drain(0..body_end + term_len);
+        }
+        out
+    }
+}
+
+/// Classify the `133;` body (e.g. `A`, `C`, `D`, `D;0`, `D;1`).
+fn classify(body: &[u8]) -> Option<SurfaceState> {
+    let s = std::str::from_utf8(body).ok()?;
+    let mut parts = s.split(';');
+    match parts.next()? {
+        "C" => Some(SurfaceState::Work),
+        "A" => Some(SurfaceState::Idle),
+        "D" => {
+            // exit code is the next part, if present.
+            match parts.next() {
+                Some(code) if code != "0" && !code.is_empty() => Some(SurfaceState::Error),
+                _ => Some(SurfaceState::Done),
+            }
+        }
+        _ => None, // B and others: no status
+    }
+}
+
+fn find(hay: &[u8], needle: &[u8]) -> Option<usize> {
+    if needle.is_empty() || hay.len() < needle.len() {
+        return None;
+    }
+    hay.windows(needle.len()).position(|w| w == needle)
+}
+
+/// Terminator search from `from`: returns (index_of_terminator, terminator_len).
+fn find_terminator(hay: &[u8], from: usize) -> Option<(usize, usize)> {
+    let mut i = from;
+    while i < hay.len() {
+        if hay[i] == 0x07 {
+            return Some((i, 1));
+        }
+        if hay[i] == 0x1b && i + 1 < hay.len() && hay[i + 1] == 0x5c {
+            return Some((i, 2));
+        }
+        i += 1;
+    }
+    None
+}
+
+/// Keep only the longest suffix of `buf` that is a strict prefix of `needle`
+/// (a possibly-incomplete marker start), so it can complete on the next feed.
+fn keep_partial_tail(buf: &[u8], needle: &[u8]) -> Vec<u8> {
+    let max = needle.len().saturating_sub(1).min(buf.len());
+    for n in (1..=max).rev() {
+        let tail = &buf[buf.len() - n..];
+        if needle.starts_with(tail) {
+            return tail.to_vec();
+        }
+    }
+    Vec::new()
+}
+
+/// Stateless best-effort heuristics over a window of recent terminal text.
+pub struct FallbackScanner;
+
+impl FallbackScanner {
+    /// Returns a status implied by the tail text, or None for "no change".
+    pub fn scan(text: &str) -> Option<SurfaceState> {
+        let tail = text.trim_end();
+        let last_line = tail.lines().last().unwrap_or("");
+        // Confirmation / input prompts → waiting for the user.
+        let wait_markers = ["(y/n)", "(Y/n)", "(y/N)", "[y/N]", "[Y/n]", "Press enter", "press enter", "❯ 1.", "? "];
+        if wait_markers.iter().any(|m| last_line.contains(m)) {
+            return Some(SurfaceState::Wait);
+        }
+        // Spinner glyphs at the tail → working.
+        let spinners = ['⠋', '⠙', '⠹', '⠸', '⠼', '⠴', '⠦', '⠧', '⠇', '⠏', '|', '/', '-', '\\'];
+        if let Some(c) = last_line.chars().rev().find(|c| !c.is_whitespace()) {
+            if spinners.contains(&c) {
+                return Some(SurfaceState::Work);
+            }
+        }
+        None
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn osc133_c_then_d0_gives_work_then_done() {
+        let mut s = Osc133Scanner::new();
+        let evs = s.feed(b"\x1b]133;C\x07hello\x1b]133;D;0\x07");
+        assert_eq!(evs, vec![SurfaceState::Work, SurfaceState::Done]);
+    }
+
+    #[test]
+    fn osc133_d_nonzero_is_error() {
+        let mut s = Osc133Scanner::new();
+        assert_eq!(s.feed(b"\x1b]133;D;1\x07"), vec![SurfaceState::Error]);
+    }
+
+    #[test]
+    fn osc133_a_is_idle_and_st_can_be_esc_backslash() {
+        let mut s = Osc133Scanner::new();
+        assert_eq!(s.feed(b"\x1b]133;A\x1b\\"), vec![SurfaceState::Idle]);
+    }
+
+    #[test]
+    fn osc133_split_across_feeds_is_buffered() {
+        let mut s = Osc133Scanner::new();
+        assert_eq!(s.feed(b"\x1b]133;C"), vec![]); // no terminator yet
+        assert_eq!(s.feed(b"\x07"), vec![SurfaceState::Work]);
+    }
+
+    #[test]
+    fn osc133_split_in_prefix_is_buffered() {
+        let mut s = Osc133Scanner::new();
+        assert_eq!(s.feed(b"text\x1b]13"), vec![]); // partial prefix retained
+        assert_eq!(s.feed(b"3;C\x07"), vec![SurfaceState::Work]);
+    }
+
+    #[test]
+    fn osc133_ignores_plain_text() {
+        let mut s = Osc133Scanner::new();
+        assert_eq!(s.feed(b"just some output\n"), vec![]);
+        assert!(s.feed(b"more\n").is_empty());
+    }
+
+    #[test]
+    fn fallback_detects_confirmation_and_spinner() {
+        assert_eq!(FallbackScanner::scan("Apply changes? (y/n)"), Some(SurfaceState::Wait));
+        assert_eq!(FallbackScanner::scan("building ⠹"), Some(SurfaceState::Work));
+        assert_eq!(FallbackScanner::scan("normal output"), None);
+    }
+}
diff --git a/crates/spacesh-core/src/grid.rs b/crates/spacesh-core/src/grid.rs
index 1f5e361..a4a514d 100644
--- a/crates/spacesh-core/src/grid.rs
+++ b/crates/spacesh-core/src/grid.rs
@@ -59,6 +59,23 @@ impl GridSurface {
     pub fn term(&self) -> &Term<VoidListener> {
         &self.term
     }
+
+    /// The visible grid as text — the last `lines` rows, trailing blanks trimmed.
+    /// Used by the fallback detector.
+    pub fn tail_text(&self, lines: usize) -> String {
+        let size = self.size();
+        let start = size.lines.saturating_sub(lines);
+        let mut out = String::new();
+        for line in start..size.lines {
+            let mut row = String::new();
+            for col in 0..size.cols {
+                row.push(self.char_at(line, col));
+            }
+            out.push_str(row.trim_end());
+            out.push('\n');
+        }
+        out
+    }
 }
 
 #[cfg(test)]
diff --git a/crates/spacesh-core/src/lib.rs b/crates/spacesh-core/src/lib.rs
index 04b0164..ed2711e 100644
--- a/crates/spacesh-core/src/lib.rs
+++ b/crates/spacesh-core/src/lib.rs
@@ -1,7 +1,9 @@
+pub mod detect;
 pub mod grid;
 pub mod ops;
 pub mod presets;
 pub mod snapshot;
 
+pub use detect::{FallbackScanner, Osc133Scanner};
 pub use grid::GridSurface;
 pub use snapshot::Snapshot;