diff --git a/crates/chanora_audio/src/lib.rs b/crates/chanora_audio/src/lib.rs
index cc6ca9d..10aab84 100644
--- a/crates/chanora_audio/src/lib.rs
+++ b/crates/chanora_audio/src/lib.rs
@@ -29,6 +29,7 @@
 #![warn(missing_docs)]
 
 mod engine;
+pub mod mode_stack;
 pub mod ptt;
 pub mod ptt_backends;
 pub mod release_tail;
diff --git a/crates/chanora_audio/src/mode_stack.rs b/crates/chanora_audio/src/mode_stack.rs
new file mode 100644
index 0000000..799599b
--- /dev/null
+++ b/crates/chanora_audio/src/mode_stack.rs
@@ -0,0 +1,243 @@
+//! `ModeStack`: refcount-composable Android audio-mode acquire/release
+//! with prior-mode snapshot/restore.
+//!
+//! # Trace
+//!
+//! Sourced verbatim from SDD-108:
+//!
+//! - SDD-108 §1 (Engagement API): "`fn engage()`: increments the
+//!   internal reference count; on the 0 → 1 transition it shall
+//!   snapshot the current mode (`prior_mode = audioManager.mode`) and
+//!   call `audioManager.setMode(MODE_IN_COMMUNICATION)`. On counts ≥
+//!   1, it is a no-op success. `fn release()`: decrements the
+//!   reference count; on the 1 → 0 transition it shall call
+//!   `audioManager.setMode(prior_mode)` and clear `prior_mode`. On
+//!   counts ≥ 1 after decrement, it is a no-op. On underflow (release
+//!   without engage) it shall log and clamp at 0 without crashing."
+//! - SDD-108 §2 (Refcount semantics): "The count is an `AtomicI32`
+//!   guarded by a Kotlin `Mutex` for the snapshot/restore critical
+//!   section. P0 only ever observes counts in `{0, 1}` per SRS-189,
+//!   but the design must compose for future multi-session use. Any
+//!   two-engages-one-release sequence shall keep `prior_mode` from
+//!   the FIRST engage; a release at count 1 restores that original
+//!   mode."
+//! - SDD-108 §3 (Lifecycle binding): "`engage()` shall be called from
+//!   the audio engine's `ensure_running()` path … AFTER
+//!   `AndroidVoiceForegroundService.start` succeeds. `release()`
+//!   shall be called from `shutdown_if_idle()` BEFORE
+//!   `AndroidVoiceForegroundService.stop`."
+//!
+//! This module realises §1 + §2 as a pure data structure: the platform
+//! interaction (JNI getMode / setMode) is the engine integration's
+//! responsibility (SDD-108 §3); `ModeStack` here is panic-free pure
+//! Rust logic and is host-testable on all targets.
+
+/// Outcome of [`ModeStack::acquire`].
+///
+/// SDD-108 §1: the 0 → 1 transition is the one that must reach the
+/// platform; composed acquires return [`ModeAcquire::AlreadyHeld`]
+/// without re-snapshotting (SDD-108 §2).
+#[derive(Debug, PartialEq, Eq)]
+pub enum ModeAcquire {
+    /// First acquire from idle. Caller MUST platform-set the desired
+    /// target mode (e.g. `MODE_IN_COMMUNICATION`). `prior` is the
+    /// mode that will be restored on the matching last release.
+    FirstAcquire {
+        /// The system mode that was active before this acquire.
+        prior: i32,
+    },
+    /// Composed acquire: mode is already engaged, no platform write
+    /// required. Refcount has been incremented.
+    AlreadyHeld,
+}
+
+/// Outcome of [`ModeStack::release`].
+///
+/// SDD-108 §1: the 1 → 0 transition is the one that must restore the
+/// snapshot. Mid-stack releases return [`ModeRelease::StillHeld`].
+/// Releases against an empty stack return
+/// [`ModeRelease::AlreadyReleased`] (SDD-108 §1: "On underflow …
+/// clamp at 0 without crashing").
+#[derive(Debug, PartialEq, Eq)]
+pub enum ModeRelease {
+    /// Last release. Caller MUST platform-set the system mode back to
+    /// `prior`. Snapshot has been cleared.
+    LastRelease {
+        /// The mode that was snapshotted on the matching first acquire.
+        prior: i32,
+    },
+    /// Refcount remains > 0 after this release; mode stays engaged.
+    StillHeld,
+    /// Release called against an idle stack. Programmer error; the
+    /// stack remains idle (no panic, no underflow — SDD-108 §1).
+    AlreadyReleased,
+}
+
+/// Refcount-composable audio-mode controller (SDD-108 §1, §2).
+///
+/// `ModeStack` does NOT call the platform. The caller is responsible
+/// for passing the current platform mode into [`Self::acquire`] and
+/// for executing the platform write (setMode) when the outcome
+/// indicates a transition is required. This separation keeps
+/// `ModeStack` host-testable and `#[cfg]`-free.
+///
+/// `ModeStack` is not internally synchronised; callers must serialise
+/// access (e.g. behind a `Mutex`), as SDD-108 §2 specifies for the
+/// Kotlin side.
+#[derive(Debug, Default)]
+pub struct ModeStack {
+    refcount: usize, // `usize` (not `AtomicI32` as in SDD-108 §2) — synchronisation is the caller's responsibility; saturating_add prevents wrap.
+    snapshot: Option<i32>,
+}
+
+impl ModeStack {
+    /// Construct an idle stack (refcount 0, no snapshot).
+    pub fn new() -> Self {
+        Self {
+            refcount: 0,
+            snapshot: None,
+        }
+    }
+
+    /// Acquire the audio-mode lock (SDD-108 §1).
+    ///
+    /// `current_platform_mode` is the mode that
+    /// `AudioManager.getMode()` returned at the call site. On the 0 →
+    /// 1 transition this value is snapshotted as the restore target.
+    /// On composed acquires (count ≥ 1 prior), the snapshot is
+    /// preserved verbatim per SDD-108 §2 ("Any two-engages-one-release
+    /// sequence shall keep `prior_mode` from the FIRST engage").
+    pub fn acquire(&mut self, current_platform_mode: i32) -> ModeAcquire {
+        if self.refcount == 0 {
+            self.snapshot = Some(current_platform_mode);
+            self.refcount = 1;
+            ModeAcquire::FirstAcquire {
+                prior: current_platform_mode,
+            }
+        } else {
+            // Saturating add guards against pathological refcount
+            // overflow without panicking; on usize this would require
+            // ~2^64 composed acquires which is not reachable in
+            // practice but keeps the panic-free contract honest.
+            self.refcount = self.refcount.saturating_add(1);
+            ModeAcquire::AlreadyHeld
+        }
+    }
+
+    /// Release the audio-mode lock (SDD-108 §1).
+    ///
+    /// On the 1 → 0 transition, returns [`ModeRelease::LastRelease`]
+    /// carrying the snapshotted prior mode that the caller must
+    /// platform-restore. The snapshot is cleared as part of the
+    /// transition.
+    pub fn release(&mut self) -> ModeRelease {
+        match self.refcount {
+            0 => ModeRelease::AlreadyReleased,
+            1 => {
+                self.refcount = 0;
+                debug_assert!(
+                    self.snapshot.is_some(),
+                    "ModeStack invariant: refcount==1 implies snapshot.is_some()"
+                );
+                // `0` corresponds to Android `MODE_NORMAL` — a safe restore target.
+                let prior = self.snapshot.take().unwrap_or(0);
+                ModeRelease::LastRelease { prior }
+            }
+            n => {
+                self.refcount = n - 1;
+                ModeRelease::StillHeld
+            }
+        }
+    }
+
+    /// Current refcount (diagnostics only, per SDD-108 §1
+    /// `is_engaged()` rationale).
+    pub fn refcount(&self) -> usize {
+        self.refcount
+    }
+
+    /// Current snapshot (diagnostics only). `None` when idle.
+    pub fn snapshot(&self) -> Option<i32> {
+        self.snapshot
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// SWE4-UV-045: First acquire from idle snapshots prior mode.
+    #[test]
+    fn swe4_uv_045_acquire_from_idle_snapshots_prior_mode() {
+        let mut s = ModeStack::new();
+        assert_eq!(s.acquire(0), ModeAcquire::FirstAcquire { prior: 0 });
+        assert_eq!(s.refcount(), 1);
+        assert_eq!(s.snapshot(), Some(0));
+    }
+
+    /// SWE4-UV-045: Second acquire returns AlreadyHeld; snapshot is
+    /// preserved from the FIRST acquire (SDD-108 §2).
+    #[test]
+    fn swe4_uv_045_acquire_twice_returns_already_held() {
+        let mut s = ModeStack::new();
+        let _ = s.acquire(7);
+        // Even if a different "current" mode is reported on the
+        // composed acquire, the snapshot must remain the first one.
+        assert_eq!(s.acquire(99), ModeAcquire::AlreadyHeld);
+        assert_eq!(s.refcount(), 2);
+        assert_eq!(s.snapshot(), Some(7));
+    }
+
+    /// SWE4-UV-045: Last release returns the snapshotted prior mode.
+    #[test]
+    fn swe4_uv_045_release_last_returns_prior() {
+        let mut s = ModeStack::new();
+        let _ = s.acquire(7);
+        assert_eq!(s.release(), ModeRelease::LastRelease { prior: 7 });
+        assert_eq!(s.refcount(), 0);
+        assert_eq!(s.snapshot(), None);
+    }
+
+    /// SWE4-UV-045: Release at refcount 2 keeps the mode engaged.
+    #[test]
+    fn swe4_uv_045_release_with_refcount_two_keeps_held() {
+        let mut s = ModeStack::new();
+        let _ = s.acquire(3);
+        let _ = s.acquire(3);
+        assert_eq!(s.release(), ModeRelease::StillHeld);
+        assert_eq!(s.refcount(), 1);
+        // Snapshot still pinned to first acquire.
+        assert_eq!(s.snapshot(), Some(3));
+    }
+
+    /// SWE4-UV-045: Release on idle stack signals programmer error
+    /// without panicking (SDD-108 §1 "clamp at 0 without crashing").
+    #[test]
+    fn swe4_uv_045_release_on_idle_signals_error() {
+        let mut s = ModeStack::new();
+        assert_eq!(s.release(), ModeRelease::AlreadyReleased);
+        assert_eq!(s.refcount(), 0);
+        assert_eq!(s.snapshot(), None);
+    }
+
+    /// SWE4-UV-045: Balanced acquire/release pairs return to idle, and
+    /// a subsequent acquire snapshots the (potentially new) prior
+    /// mode — proving the snapshot is cleared on last release.
+    #[test]
+    fn swe4_uv_045_compose_acquire_release_pairs() {
+        let mut s = ModeStack::new();
+        // Pair 1: snapshot 0, restore 0.
+        assert_eq!(s.acquire(0), ModeAcquire::FirstAcquire { prior: 0 });
+        let _ = s.acquire(0); // composed
+        assert_eq!(s.release(), ModeRelease::StillHeld);
+        assert_eq!(s.release(), ModeRelease::LastRelease { prior: 0 });
+        assert_eq!(s.refcount(), 0);
+        assert_eq!(s.snapshot(), None);
+
+        // Pair 2: the platform mode is now different; the new
+        // snapshot must reflect THAT mode, not the prior pair's.
+        assert_eq!(s.acquire(2), ModeAcquire::FirstAcquire { prior: 2 });
+        assert_eq!(s.snapshot(), Some(2));
+        assert_eq!(s.release(), ModeRelease::LastRelease { prior: 2 });
+    }
+}