diff --git a/Readme.md b/Readme.md
index 0d50e1f..73454fe 100644
--- a/Readme.md
+++ b/Readme.md
@@ -57,6 +57,65 @@ If no Tap is already connected at the OS level, `run()` will scan and wait for a
 
 Also make sure that you have updated your Tap device to the latest version.
 
+### BLE GATT reference
+
+Verified against firmware `tap_rd/ble-mcu-hw-v3-sdk-14.0` branch **`TAP_XR_develop`** (`ble_tap_service.c`, `services_init`, Nordic `ble_nus`). Also matches the other official Tap SDKs and [Tap BLE API Documentation](https://tapwithus.atlassian.net/wiki/spaces/FIR/pages/426803201/Tap+BLE+API+Documentation).
+
+Tap base UUID: `c3ff0000-1d8b-40fd-a56f-c7bd5d0f3370` (16-bit offsets below). NUS is the standard Nordic UART base `6e400000-b5a3-f393-e0a9-e50e24dcca9e`.
+
+**Advertising:** primary adv carries HID (`0x1812`); Tap service UUID is in the **scan response**. Active scan (or OS-paired reconnect) is needed to see `c3ff0001-…` in advertised service lists.
+
+#### Services the firmware always registers
+
+From `services_init()` / device mediator on `TAP_XR_develop`:
+
+| Service | UUID | Role |
+| --- | --- | --- |
+| HID | `00001812-0000-1000-8000-00805f9b34fb` | Keyboard / mouse HID over GATT |
+| Battery | `0000180f-0000-1000-8000-00805f9b34fb` | Battery Level |
+| Device Information | `0000180a-0000-1000-8000-00805f9b34fb` | FW / HW / serial strings |
+| Buttonless DFU | Nordic DFU | FOTA bootloader trigger |
+| **Tap** | `c3ff0001-1d8b-40fd-a56f-c7bd5d0f3370` | Proprietary events / commands |
+| **NUS** | `6e400001-b5a3-f393-e0a9-e50e24dcca9e` | Mode / raw-sensor channel |
+
+#### Required by this Python SDK
+
+Minimum used for discovery, mode control, and tap events (Windows SDK similarly requires Tap Data + NUS RX):
+
+| Role | UUID | FW properties | Purpose |
+| --- | --- | --- | --- |
+| Tap service | `c3ff0001-…` | (service; scan response) | Find / filter Tap |
+| Tap data | `c3ff0005-…` | Notify | Tap codes (`register_tap_events`) |
+| NUS service | `6e400001-…` | (service) | Mode channel |
+| NUS RX | `6e400002-…` | Write | Input mode / type (`set_input_mode`, `set_input_type`); refresh ~every 10s in controller/raw |
+
+#### Used optionally by this SDK
+
+Notify setup is best-effort if a characteristic is missing on older devices.
+
+| Characteristic | UUID | FW properties | Purpose |
+| --- | --- | --- | --- |
+| Mouse data | `c3ff0006-…` | Notify, Write, WriteWithoutResponse | Surface mouse (`register_mouse_events`) |
+| UI / haptic | `c3ff0009-…` | Write, WriteWithoutResponse | Vibration (`send_vibration_sequence`) |
+| Air gesture | `c3ff000a-…` | Notify, WriteWithoutResponse, Read | Air gestures / mouse-mode state (`register_air_gesture_*`) |
+| NUS TX | `6e400003-…` | Notify | Raw sensors (`register_raw_data_events`; Developer mode) |
+
+#### Other Tap-service characteristics (on device; unused here)
+
+Registered in `ble_tap_init()` on `TAP_XR_develop`:
+
+| Char | UUID | FW properties | Notes |
+| --- | --- | --- | --- |
+| Settings | `c3ff0002-…` | Read, Write | Device settings |
+| Name | `c3ff0003-…` | Read, Write | BLE name (Android SDK) |
+| Mapping RX | `c3ff0007-…` | Write, WriteWithoutResponse | Custom map upload |
+| Mapping TX | `c3ff0008-…` | Notify | Custom map download |
+| Data request | `c3ff000b-…` | Notify, WriteWithoutResponse, Read | State queries (Android SDK) |
+| Kneron model ver | `c3ff000c-…` | Notify, Read | TapXR NPU model |
+| Kneron FW ver | `c3ff000d-…` | Notify, Read | TapXR NPU firmware |
+
+Constants: [`tapsdk/tap.py`](tapsdk/tap.py).
+
 ### Features
 
 This SDK implements two basic interfaces with a Tap device.
diff --git a/tapsdk/tap.py b/tapsdk/tap.py
index 255d6c6..bbe0afd 100644
--- a/tapsdk/tap.py
+++ b/tapsdk/tap.py
@@ -11,14 +11,18 @@
 
 logger = logging.getLogger(__name__)
 
+# BLE GATT UUIDs — verified against tap_rd/ble-mcu-hw-v3-sdk-14.0 @ TAP_XR_develop
+# (ble_tap_service.c / Nordic ble_nus). Required for this SDK: Tap service (scan
+# response filter), Tap data (notify), NUS service, NUS RX (write input mode).
+# See README "BLE GATT reference".
 tap_service = 'c3ff0001-1d8b-40fd-a56f-c7bd5d0f3370'
 nus_service = '6e400001-b5a3-f393-e0a9-e50e24dcca9e'
 tap_data_characteristic = 'c3ff0005-1d8b-40fd-a56f-c7bd5d0f3370'
 mouse_data_characteristic = 'c3ff0006-1d8b-40fd-a56f-c7bd5d0f3370'
 ui_cmd_characteristic = 'c3ff0009-1d8b-40fd-a56f-c7bd5d0f3370'
 air_gesture_data_characteristic = 'c3ff000a-1d8b-40fd-a56f-c7bd5d0f3370'
-tap_mode_characteristic = '6e400002-b5a3-f393-e0a9-e50e24dcca9e'        # nus rx
-raw_sensors_characteristic = '6e400003-b5a3-f393-e0a9-e50e24dcca9e'     # nus tx
+tap_mode_characteristic = '6e400002-b5a3-f393-e0a9-e50e24dcca9e'        # NUS RX (write)
+raw_sensors_characteristic = '6e400003-b5a3-f393-e0a9-e50e24dcca9e'     # NUS TX (notify)
 
 
 if platform.system() == "Darwin":