Windows 11 (WinRT): GattSession becomes CLOSED shortly after connect (is_connected flips True→False) – HCI disconnect reason 0x3E

[lahlouEl-Mehdi]

Hello,
I’m trying to connect from a Windows 11 PC to a u‑blox ANNA‑B412 peripheral using Bleak (WinRT backend). The device is discovered correctly, client.is_connected becomes True right after connect(), but within ~0.2–2 seconds the GATT session is closed and client.is_connected becomes False.

Environment

• OS: Windows 11
• BLE peripheral: u‑blox ANNA‑B412
• Bleak version: 3.0.0

Expected behavior

• await client.connect() should keep the connection stable (at least a few seconds)
• client.is_connected should remain True

Actual behavior

• GattSessionStatus becomes ACTIVE then quickly switches to CLOSED
• client.is_connected becomes False shortly after connecting
• In Wireshark HCI logs, a Disconnect Complete appears with reason 0x3E (Connection Failed to be Established)

Bleak logs (WinRT)

2026-05-19 12:04:20,857 | DEBUG    | bleak.backends.winrt.scanner | 20 devices found. Watcher status: <BluetoothLEAdvertisementWatcherStatus.STOPPED: 3>.
2026-05-19 12:04:20,859 | DEBUG    | bleak.backends.winrt.client | Connecting to BLE device @ ******
2026-05-19 12:04:21,089 | DEBUG    | bleak.backends.winrt.client | getting services (service_cache_mode=<BluetoothCacheMode.CACHED: 0>, cache_mode=<BluetoothCacheMode.CACHED: 0>)...
2026-05-19 12:04:21,347 | DEBUG    | bleak.backends.winrt.client | session_status_changed_event_handler: id: BluetoothLE#BluetoothLE , error: <BluetoothError.SUCCESS: 0>, status: <GattSessionStatus.ACTIVE: 1>
2026-05-19 12:04:21,359 | INFO     | APP |  Connect� en 1.145 s
2026-05-19 12:04:21,599 | DEBUG    | bleak.backends.winrt.client | session_status_changed_event_handler: id: BluetoothLE#BluetoothLE , error: <BluetoothError.SUCCESS: 0>, status: <GattSessionStatus.CLOSED: 0>
2026-05-19 12:04:21,602 | DEBUG    | bleak.backends.winrt.client | closing requester
2026-05-19 12:04:21,606 | DEBUG    | bleak.backends.winrt.client | closing session
2026-05-19 12:04:22,358 | ERROR    | APP | ========== EXCEPTION ==========
2026-05-19 12:04:22,358 | ERROR    | APP | [CONNECT ERROR] attempt=1
2026-05-19 12:04:22,358 | ERROR    | APP | Type : RuntimeError
2026-05-19 12:04:22,360 | ERROR    | APP | Message :Client disconnect after connect
2026-05-19 12:04:22,364 | ERROR    | APP | Traceback:
Traceback (most recent call last):
  File "****\code.py", line 133, in _connect_with_policy
    raise RuntimeError("Client disconnect after connect")
RuntimeError: Client disconnect after connect
2026-05-19 12:04:22,366 | ERROR    | APP | ==============================

Minimal code

    async def _connect_with_policy(self):
        global DEBUG_FALSE
        last_err = None
        total_time = 0.0

        for attempt in range(1, self.max_attempts + 1):
            t0 = time.perf_counter()

            client = BleakClient(self.address, timeout=self.connect_timeout_s,winrt={"use_cached_services": True})

            try:
                await client.connect()

                conn_time = time.perf_counter() - t0
                total_time += conn_time

                logger.info(" Connecté en %.3f s", conn_time)

                await asyncio.sleep(1)

                if not client.is_connected:
                    raise RuntimeError("Client disconnect after connect")

                return True, total_time, None, client

            except Exception as e:
                last_err = e
                dump_exception(e, prefix=f"[CONNECT ERROR] attempt={attempt}")

                await self._safe_disconnect(client)
                DEBUG_FALSE=1
                break #debug

                if attempt < self.max_attempts:
                    delay = 1.5 * (2 ** (attempt - 1))
                    logger.warning("Retry après %.1f s", delay)

                    total_time += delay
                    await asyncio.sleep(delay)
                    continue

                logger.error("ECHEC après %d tentatives", attempt)
                return False, None, str(e), None

        return False, None, str(last_err), None

Wireshark HCI observations (bad case)

host	controller	HCI_CMD	10	Sent LE Set Extended Scan Enable
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Enable)
host	controller	HCI_CMD	6	Sent Vendor Command 0x001E (opcode 0xFC1E)
controller	host	HCI_EVT	8	Rcvd Command Complete (Vendor Command 0x001E [opcode 0xFC1E])
host	controller	HCI_CMD	12	Sent LE Set Extended Scan Parameters
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Parameters)
host	controller	HCI_CMD	10	Sent LE Set Extended Scan Enable
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Enable)
host	controller	HCI_CMD	10	Sent LE Set Extended Scan Enable
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Enable)
host	controller	HCI_CMD	11	Sent LE Add Device To Filter Accept List
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Add Device To Filter Accept List)
host	controller	HCI_CMD	26	Sent Vendor Command 0x001E (opcode 0xFC1E)
controller	host	HCI_EVT	9	Rcvd Command Complete (Vendor Command 0x001E [opcode 0xFC1E])
host	controller	HCI_CMD	12	Sent LE Set Extended Scan Parameters
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Parameters)
host	controller	HCI_CMD	10	Sent LE Set Extended Scan Enable
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Enable)
controller	host	HCI_EVT	59	Rcvd LE Meta (LE Extended Advertising Report)
host	controller	HCI_CMD	30	Sent LE Extended Create Connection [v1]
controller	host	HCI_EVT	7	Rcvd Command Status (LE Extended Create Connection [v1])
controller	host	HCI_EVT	34	Rcvd LE Meta (LE Enhanced Connection Complete [v1])
host	controller	HCI_CMD	6	Sent LE Read Channel Map
controller	host	HCI_EVT	36	Rcvd Vendor-Specific
controller	host	HCI_EVT	14	Rcvd Command Complete (LE Read Channel Map)
host	controller	HCI_CMD	32	Sent LE Enable Encryption
controller	host	HCI_EVT	7	Rcvd Command Status (LE Enable Encryption)
host	controller	HCI_CMD	10	Sent LE Set Extended Scan Enable
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Enable)
host	controller	HCI_CMD	11	Sent LE Remove Device From Filter Accept List
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Remove Device From Filter Accept List)
host	controller	HCI_CMD	6	Sent Vendor Command 0x001E (opcode 0xFC1E)
controller	host	HCI_EVT	8	Rcvd Command Complete (Vendor Command 0x001E [opcode 0xFC1E])
host	controller	HCI_CMD	12	Sent LE Set Extended Scan Parameters
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Parameters)
host	controller	HCI_CMD	10	Sent LE Set Extended Scan Enable
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Enable)
controller	host	HCI_EVT	7	Rcvd Disconnect Complete

with

Bluetooth HCI Event - Disconnect Complete
    Event Code: Disconnect Complete (0x05)
    Parameter Total Length: 4
    Status: Success (0x00)
    Connection Handle: 0x0800
    Reason: Connection Failed to be Established (0x3e)

Wireshark “good case”

Sometimes the connection proceeds further:

host	controller	HCI_CMD	10	Sent LE Set Extended Scan Enable
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Enable)
host	controller	HCI_CMD	6	Sent Vendor Command 0x001E (opcode 0xFC1E)
controller	host	HCI_EVT	8	Rcvd Command Complete (Vendor Command 0x001E [opcode 0xFC1E])
host	controller	HCI_CMD	12	Sent LE Set Extended Scan Parameters
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Parameters)
host	controller	HCI_CMD	10	Sent LE Set Extended Scan Enable
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Enable)
host	controller	HCI_CMD	10	Sent LE Set Extended Scan Enable
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Enable)
host	controller	HCI_CMD	11	Sent LE Add Device To Filter Accept List
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Add Device To Filter Accept List)
host	controller	HCI_CMD	26	Sent Vendor Command 0x001E (opcode 0xFC1E)
controller	host	HCI_EVT	9	Rcvd Command Complete (Vendor Command 0x001E [opcode 0xFC1E])
host	controller	HCI_CMD	12	Sent LE Set Extended Scan Parameters
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Parameters)
host	controller	HCI_CMD	10	Sent LE Set Extended Scan Enable
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Enable)
controller	host	HCI_EVT	59	Rcvd LE Meta (LE Extended Advertising Report)
host	controller	HCI_CMD	30	Sent LE Extended Create Connection [v1]
controller	host	HCI_EVT	7	Rcvd Command Status (LE Extended Create Connection [v1])
controller	host	HCI_EVT	34	Rcvd LE Meta (LE Enhanced Connection Complete [v1])
host	controller	HCI_CMD	6	Sent LE Read Channel Map
controller	host	HCI_EVT	36	Rcvd Vendor-Specific
controller	host	HCI_EVT	14	Rcvd Command Complete (LE Read Channel Map)
host	controller	HCI_CMD	32	Sent LE Enable Encryption
controller	host	HCI_EVT	7	Rcvd Command Status (LE Enable Encryption)
host	controller	HCI_CMD	10	Sent LE Set Extended Scan Enable
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Enable)
host	controller	HCI_CMD	11	Sent LE Remove Device From Filter Accept List
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Remove Device From Filter Accept List)
host	controller	HCI_CMD	6	Sent Vendor Command 0x001E (opcode 0xFC1E)
controller	host	HCI_EVT	8	Rcvd Command Complete (Vendor Command 0x001E [opcode 0xFC1E])
host	controller	HCI_CMD	12	Sent LE Set Extended Scan Parameters
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Parameters)
host	controller	HCI_CMD	10	Sent LE Set Extended Scan Enable
controller	host	HCI_EVT	7	Rcvd Command Complete (LE Set Extended Scan Enable)
ublox_0f:27:c3 (ANNA-B4-0F27C3)	localhost ()	SMP	11	Rcvd Security Request: AuthReq: Bonding
controller	host	HCI_EVT	7	Rcvd Encryption Change [v1]
localhost ()	ublox_0f:27:c3 (ANNA-B4-0F27C3)	ATT	12	Sent Exchange MTU Request, Client Rx MTU: 527
controller	host	HCI_EVT	8	Rcvd Number of Completed Packets
controller	host	HCI_EVT	14	Rcvd LE Meta (LE Data Length Change)
ublox_0f:27:c3 (ANNA-B4-0F27C3)	localhost ()	ATT	12	Rcvd Exchange MTU Response, Server Rx MTU: 247
localhost ()	ublox_0f:27:c3 (ANNA-B4-0F27C3)	ATT	14	Sent Write Request, Handle: 0x001a (Unknown)
controller	host	HCI_EVT	8	Rcvd Number of Completed Packets
ublox_0f:27:c3 (ANNA-B4-0F27C3)	localhost ()	ATT	10	Rcvd Write Response, Handle: 0x001a (Unknown)
localhost ()	ublox_0f:27:c3 (ANNA-B4-0F27C3)	ATT	19	Sent Write Command, Handle: 0x0019 (Unknown)
ublox_0f:27:c3 (ANNA-B4-0F27C3)	localhost ()	ATT	19	Rcvd Handle Value Notification, Handle: 0x0019 (Unknown)
controller	host	HCI_EVT	9	Rcvd LE Meta (LE PHY Update Complete)
controller	host	HCI_EVT	8	Rcvd Number of Completed Packets
ublox_0f:27:c3 (ANNA-B4-0F27C3)	localhost ()	ATT	29	Rcvd Handle Value Notification, Handle: 0x0019 (Unknown)
localhost ()	ublox_0f:27:c3 (ANNA-B4-0F27C3)	ATT	14	Sent Write Command, Handle: 0x0019 (Unknown)
localhost ()	ublox_0f:27:c3 (ANNA-B4-0F27C3)	ATT	14	Sent Write Command, Handle: 0x0019 (Unknown)
localhost ()	ublox_0f:27:c3 (ANNA-B4-0F27C3)	ATT	14	Sent Write Request, Handle: 0x001a (Unknown)
controller	host	HCI_EVT	8	Rcvd Number of Completed Packets
controller	host	HCI_EVT	8	Rcvd Number of Completed Packets
controller	host	HCI_EVT	8	Rcvd Number of Completed Packets
ublox_0f:27:c3 (ANNA-B4-0F27C3)	localhost ()	ATT	10	Rcvd Write Response, Handle: 0x001a (Unknown)
host	controller	HCI_CMD	7	Sent Disconnect
controller	host	HCI_EVT	7	Rcvd Command Status (Disconnect)
controller	host	HCI_EVT	7	Rcvd Disconnect Complete

Questions

1. On Windows/WinRT, is it expected that a BLE connection briefly becomes ACTIVE and then immediately transitions to CLOSED, even though connect() returns successfully?

2. What could be the underlying reason for the controller terminating the connection with HCI error 0x3E ("Connection Failed to be Established") in this case, despite the connection being reported as established initially?

Thanks!

[dlech]

2. What could be the underlying reason for the controller terminating the connection with HCI error 0x3E ("Connection Failed to be Established") in this case, despite the connection being reported as established initially?

It sounds like a hardware issue. I believe this error can also mean "Synchronization Timeout error code", so that might explain why it connected but failed later.

[lahlouEl-Mehdi]

Thanks for your reply, @dlech. I really appreciate it. Since I’m still getting familiar with Bluetooth protocols and BLE, I’m trying to understand what might be causing this issue.

When comparing the Wireshark traces, I noticed that in the good test we receive the "Security Request: AuthReq: Bonding," but this is missing in the bad test. I’m not sure if this difference is what you meant by a synchronization timeout.

Could you clarify what you think? Does this missing security step usually point to a hardware-related cause, or could it still be something else, such as a timing or firmware issue? I just want to make sure I’m interpreting it correctly.

Thanks again!

[dlech]

Without the actual wireshark log files, it it hard to guess since we can't see most of the details here.