diff --git a/README.md b/README.md
index 5e759dc..76bb4bd 100644
--- a/README.md
+++ b/README.md
@@ -137,10 +137,77 @@ PY
 Don't send `wake` from the smoke — it would open a real OpenAI session and
 burn budget. Plan 3 exercises that end-to-end.
 
+## Plan 3: Pi client
+
+The Python client lives in `client/`. It is shipped to the Pi inside
+`client.tar.gz` (built into the backend Docker image) and installed by
+`install.sh`. Modules:
+
+- `main.py` — entry point. Sets `PA_ALSA_PLUGHW=1` before importing
+  sounddevice. Wires audio, state, wakeword, session, and playback.
+- `state.py` — `IDLE → WAKE_PENDING → LISTENING ↔ ASSISTANT_SPEAKING →
+  IDLE_PENDING → IDLE`. `wakeword_enabled` is true only in `IDLE`;
+  `uplink_enabled` is true only in `LISTENING`.
+- `audio.py` — USB device discovery, persistent 24 kHz/mono/int16 InputStream
+  and OutputStream, and the 24 → 16 kHz resample for the wakeword feed.
+- `wakeword.py` — openwakeword `alexa` model, threshold + cooldown.
+- `playback.py` — single worker thread that owns the OutputStream and
+  implements `set_volume` as a software gain (the USB Speaker Phone has no
+  hardware playback volume control; only a `PCM Playback Switch`).
+- `session.py` — backend WS client, dispatch, tool round-trip,
+  exponential-backoff reconnect.
+- `config.py` — `~/assistant/state/config.json` (mode 0600).
+- `pair.py` — `python -m client.pair --backend <url> [--code <code>]`.
+
+### Updating the Pi
+
+For a fast iteration loop:
+
+```sh
+sshpass -p 'assistant' rsync -az --delete --exclude __pycache__ --exclude tests \
+  client/ pi@192.168.50.115:assistant/code/client/
+sshpass -p 'assistant' ssh pi@192.168.50.115 'systemctl --user restart assistant'
+sshpass -p 'assistant' ssh pi@192.168.50.115 \
+  'journalctl --user -u assistant -n 50 --no-pager'
+```
+
+For a production update (after a backend deploy):
+
+```sh
+sshpass -p 'assistant' ssh pi@192.168.50.115 \
+  'curl -fsSL https://assistant.volcanic.tes.gd/install.sh | bash'
+```
+
+### Local tests
+
+```sh
+python3 -m venv .venv
+.venv/bin/pip install -r requirements-dev.txt requests websockets numpy scipy
+.venv/bin/python -m pytest client/tests
+```
+
+Hardware-touching modules (`audio.py` streams, `wakeword.py`, `playback.py`,
+`session.run_forever`) are smoke-tested via the live deploy. Deterministic
+modules (`state.py`, `config.py`, `pair.py`, the resample helper, the session
+dispatcher) are covered by the pytest suite.
+
+### End-to-end smoke
+
+With the unit running on the Pi:
+
+```sh
+sshpass -p 'assistant' ssh pi@192.168.50.115 'journalctl --user -u assistant -f'
+```
+
+…then say `alexa` near the Speaker Phone. Expected log sequence: `wakeword
+fired`, `state IDLE -> WAKE_PENDING`, `session_started conversation_id=…`,
+`state WAKE_PENDING -> LISTENING`. Speak a question; the assistant replies
+through the speaker (`state LISTENING -> ASSISTANT_SPEAKING`, then
+`assistant_done`). Say "bye" → `session_ended reason=tool`, then `state
+LISTENING -> IDLE_PENDING -> IDLE`. The conversation row + turns are visible
+via the deployed backend's DB (admin UI in Plan 4).
+
 ## What's next
 
-- **Plan 3** — full Python client on the Pi (wakeword, VAD, audio plumbing,
-  state machine, install + pair CLIs). This is the production caller of
-  `/device` and the integration test for everything above.
 - **Plan 4** — React management UI for the admin dashboard, per-device
   config editor, and conversation history viewer.