diff --git a/client/state.py b/client/state.py
new file mode 100644
index 0000000..ff0465e
--- /dev/null
+++ b/client/state.py
@@ -0,0 +1,70 @@
+"""Assistant client state machine.
+
+States flow: IDLE -> WAKE_PENDING -> LISTENING <-> ASSISTANT_SPEAKING
+                                       \\         /
+                                        session_ended
+                                                 \\
+                                                  IDLE_PENDING -> IDLE
+"""
+from enum import Enum, auto
+
+from client.log import get_logger
+
+_log = get_logger("state")
+
+
+class State(Enum):
+    IDLE = auto()
+    WAKE_PENDING = auto()
+    LISTENING = auto()
+    ASSISTANT_SPEAKING = auto()
+    IDLE_PENDING = auto()
+
+
+class StateMachine:
+    def __init__(self) -> None:
+        self._state = State.IDLE
+
+    @property
+    def state(self) -> State:
+        return self._state
+
+    @property
+    def wakeword_enabled(self) -> bool:
+        return self._state is State.IDLE
+
+    @property
+    def uplink_enabled(self) -> bool:
+        return self._state is State.LISTENING
+
+    def _transition(self, target: State) -> None:
+        if target is self._state:
+            return
+        _log.info("state %s -> %s", self._state.name, target.name)
+        self._state = target
+
+    def wake_fired(self) -> bool:
+        if self._state is not State.IDLE:
+            return False
+        self._transition(State.WAKE_PENDING)
+        return True
+
+    def session_started(self) -> None:
+        if self._state in (State.WAKE_PENDING, State.IDLE):
+            self._transition(State.LISTENING)
+
+    def assistant_audio_arrived(self) -> None:
+        if self._state is State.LISTENING:
+            self._transition(State.ASSISTANT_SPEAKING)
+
+    def assistant_done(self) -> None:
+        if self._state is State.ASSISTANT_SPEAKING:
+            self._transition(State.LISTENING)
+
+    def session_ended(self) -> None:
+        if self._state in (State.LISTENING, State.ASSISTANT_SPEAKING, State.WAKE_PENDING):
+            self._transition(State.IDLE_PENDING)
+
+    def sleep_played(self) -> None:
+        if self._state is State.IDLE_PENDING:
+            self._transition(State.IDLE)
diff --git a/client/tests/test_state.py b/client/tests/test_state.py
new file mode 100644
index 0000000..8a5e88b
--- /dev/null
+++ b/client/tests/test_state.py
@@ -0,0 +1,102 @@
+from client.state import State, StateMachine
+
+
+def test_starts_idle_with_wakeword_enabled_and_uplink_disabled():
+    sm = StateMachine()
+    assert sm.state is State.IDLE
+    assert sm.wakeword_enabled
+    assert not sm.uplink_enabled
+
+
+def test_wake_fired_goes_idle_to_wake_pending_and_disables_wakeword():
+    sm = StateMachine()
+    assert sm.wake_fired() is True
+    assert sm.state is State.WAKE_PENDING
+    assert not sm.wakeword_enabled
+    assert not sm.uplink_enabled
+
+
+def test_wake_fired_is_idempotent_when_not_idle():
+    sm = StateMachine()
+    sm.wake_fired()
+    assert sm.wake_fired() is False
+    assert sm.state is State.WAKE_PENDING
+
+
+def test_session_started_goes_wake_pending_to_listening_and_unmutes_uplink():
+    sm = StateMachine()
+    sm.wake_fired()
+    sm.session_started()
+    assert sm.state is State.LISTENING
+    assert sm.uplink_enabled
+    assert not sm.wakeword_enabled
+
+
+def test_assistant_audio_arrived_mutes_uplink():
+    sm = StateMachine()
+    sm.wake_fired()
+    sm.session_started()
+    sm.assistant_audio_arrived()
+    assert sm.state is State.ASSISTANT_SPEAKING
+    assert not sm.uplink_enabled
+
+
+def test_assistant_audio_arrived_is_noop_outside_listening():
+    sm = StateMachine()
+    sm.assistant_audio_arrived()
+    assert sm.state is State.IDLE  # ignored from IDLE
+    sm.wake_fired()
+    sm.assistant_audio_arrived()
+    assert sm.state is State.WAKE_PENDING  # ignored from WAKE_PENDING
+
+
+def test_assistant_done_returns_to_listening_from_speaking():
+    sm = StateMachine()
+    sm.wake_fired()
+    sm.session_started()
+    sm.assistant_audio_arrived()
+    sm.assistant_done()
+    assert sm.state is State.LISTENING
+    assert sm.uplink_enabled
+
+
+def test_assistant_done_is_noop_when_not_speaking():
+    sm = StateMachine()
+    sm.wake_fired()
+    sm.session_started()  # LISTENING; never went to ASSISTANT_SPEAKING
+    sm.assistant_done()
+    assert sm.state is State.LISTENING
+
+
+def test_session_ended_routes_through_idle_pending_then_idle():
+    sm = StateMachine()
+    sm.wake_fired()
+    sm.session_started()
+    sm.session_ended()
+    assert sm.state is State.IDLE_PENDING
+    assert not sm.wakeword_enabled
+    assert not sm.uplink_enabled
+    sm.sleep_played()
+    assert sm.state is State.IDLE
+    assert sm.wakeword_enabled
+
+
+def test_session_ended_works_from_assistant_speaking():
+    sm = StateMachine()
+    sm.wake_fired()
+    sm.session_started()
+    sm.assistant_audio_arrived()
+    sm.session_ended()
+    assert sm.state is State.IDLE_PENDING
+
+
+def test_session_ended_from_idle_is_noop():
+    sm = StateMachine()
+    sm.session_ended()
+    assert sm.state is State.IDLE
+
+
+def test_sleep_played_outside_idle_pending_is_noop():
+    sm = StateMachine()
+    sm.sleep_played()
+    assert sm.state is State.IDLE