py: improve firmware loader UX

### Makefile
@@ -171,7 +171,7 @@ run-valgrind-on-unit-tests:
 	$(MAKE) unit-test
 	bash -ec 'for exe in build-build/bin/test_*; do  valgrind --leak-check=yes --track-origins=yes --error-exitcode=1 --exit-on-first-error=yes $$exe; done'
 flash-dev-firmware:
-	./py/load_firmware.py build/bin/firmware.bin --debug
+	./py/load_firmware.py build/bin/firmware.bin --yes
 
 # Per-product development stage0/stage1 J-Link wrappers flash already-built images.
 jlink-flash-bootloader-stage0-bitbox02-btconly-development: | build

### py/README.md
@@ -75,25 +75,36 @@ This is a one-time action.
 ## Flash the firmware.bin
 
 Use the following script to flash the firmware.bin onto the BitBox.
-The script will prompt to enter the bootloader on the device before flashing.
-
-Production devices only accept `./firmware.signed.bin` signed by BitBox.
+The script prompts to enter the bootloader when necessary and confirms the detected firmware and
+device types before flashing.
 
 ```bash
 python ./load_firmware.py ./firmware.signed.bin
 ```
 
-Please note:
-On production devices the bootloader only accepts newer signed
-firmware versions and
-[prevents downgrades](https://bitbox.swiss/bitbox02/security-features/#secure-bootloader).
+Signed firmware is detected by its header; all other input is treated as raw unsigned firmware.
+The file name is not used. A recognized but malformed signed-firmware container is rejected.
+
+The supported combinations are:
+
+| Firmware input | Production device | Development device |
+| --- | --- | --- |
+| Signed | Flash firmware and signature data; all errors are fatal | Flash firmware, attempt signature data, warn if the signature data is rejected, and reboot anyway |
+| Raw unsigned | Warn that firmware verification will fail, then flash without signature data | Flash firmware without signature data |
 
-On dev-devices use the `--debug` flag to flash unsigned `./firmware.bin`.
+On production devices the bootloader only accepts newer signed firmware versions and
+[prevents downgrades](https://bitbox.swiss/bitbox02/security-features/#secure-bootloader). On a
+production device, unsigned firmware cannot boot. A signed firmware for a different product or
+edition is allowed after a warning, but installing its signature data is expected to fail.
+
+Every flash requires confirmation. Use `-y` or `--yes` to skip the prompt for non-interactive use:
 
 ```bash
-python ./load_firmware.py --debug ./firmware.bin
+python ./load_firmware.py --yes ./firmware.bin
 ```
 
+The deprecated `--debug` option is accepted for backwards compatibility but has no effect.
+
 Contributors that don't have a dev-devices please refer to the
 [simulator](https://github.com/BitBoxSwiss/bitbox02-firmware?tab=readme-ov-file#simulator).
 

### py/bitbox02/bitbox02/__init__.py
@@ -31,4 +31,5 @@
     eth,
     system,
     Bootloader,
+    BootloaderError,
 )

### py/bitbox02/bitbox02/bitbox02/__init__.py
@@ -49,4 +49,4 @@
     eth,
     system,
 )
-from .bootloader import Bootloader
+from .bootloader import Bootloader, BootloaderError

### py/bitbox02/bitbox02/bitbox02/bootloader.py
@@ -38,6 +38,12 @@
 SIGDATA_MAGIC_BITBOX02_BTCONLY = struct.pack(">I", 0x11233B0B)
 SIGDATA_MAGIC_BITBOX02PLUS_MULTI = struct.pack(">I", 0x5B648CEB)
 SIGDATA_MAGIC_BITBOX02PLUS_BTCONLY = struct.pack(">I", 0x48714774)
+SIGDATA_MAGICS = (
+    SIGDATA_MAGIC_BITBOX02_MULTI,
+    SIGDATA_MAGIC_BITBOX02_BTCONLY,
+    SIGDATA_MAGIC_BITBOX02PLUS_MULTI,
+    SIGDATA_MAGIC_BITBOX02PLUS_BTCONLY,
+)
 
 PRODUCT_ID_BITBOX02_MULTI = 1
 PRODUCT_ID_BITBOX02_BTCONLY = 2
@@ -65,23 +71,32 @@ class Hardware(TypedDict):
     secure_chip_model: SecureChipModel
 
 
+class BootloaderError(Exception):
+    """A nonzero status returned by the bootloader API."""
+
+    def __init__(self, code: int) -> None:
+        self.code = code
+        super().__init__(f"bootloader API error: code={code}")
+
+
+class InvalidFirmwareMagic(ValueError):
+    """The input does not start with a signed-firmware magic."""
+
+
 def parse_signed_firmware(firmware: bytes) -> typing.Tuple[bytes, bytes, bytes]:
-    """
-    Split raw firmware bytes into magic, sigdata and firmware
-    """
+    """Parse and validate signed firmware into magic, signature data, and payload."""
 
-    if len(firmware) < MAGIC_LEN + SIGDATA_LEN:
-        raise ValueError("firmware too small")
+    if len(firmware) < MAGIC_LEN:
+        raise InvalidFirmwareMagic("invalid magic")
     magic, firmware = firmware[:MAGIC_LEN], firmware[MAGIC_LEN:]
-    if magic not in (
-        SIGDATA_MAGIC_BITBOX02_MULTI,
-        SIGDATA_MAGIC_BITBOX02_BTCONLY,
-        SIGDATA_MAGIC_BITBOX02PLUS_MULTI,
-        SIGDATA_MAGIC_BITBOX02PLUS_BTCONLY,
-    ):
-        raise ValueError("invalid magic")
+    if magic not in SIGDATA_MAGICS:
+        raise InvalidFirmwareMagic("invalid magic")
+    if len(firmware) <= SIGDATA_LEN:
+        raise ValueError("firmware too small")
 
     sigdata, firmware = firmware[:SIGDATA_LEN], firmware[SIGDATA_LEN:]
+    if len(firmware) > MAX_FIRMWARE_SIZE:
+        raise ValueError("firmware too big")
     return magic, sigdata, firmware
 
 
@@ -104,20 +119,31 @@ def __init__(self, transport: TransportLayer, device_info: DeviceInfo):
             BITBOX02PLUS_MULTI_BOOTLOADER: PRODUCT_ID_BITBOX02PLUS_MULTI,
             BITBOX02PLUS_BTC_BOOTLOADER: PRODUCT_ID_BITBOX02PLUS_BTCONLY,
         }.get(device_info["product_string"])
-        self.version = parse_device_version(device_info["serial_number"])
+        version = parse_device_version(device_info["serial_number"])
+        version_identifiers = [
+            identifier
+            for metadata in (version.prerelease, version.build)
+            if metadata is not None
+            for identifier in metadata.split(".")
+        ]
+        self._is_devdevice = "dev" in version_identifiers
         # Delete the prelease part, as it messes with the comparison (e.g. 3.0.0-pre < 3.0.0 is
         # True, but the 3.0.0-pre has already the same API breaking changes like 3.0.0...).
-        self.version = self.version.replace(prerelease=None)
+        self.version = version.replace(prerelease=None)
         assert self.expected_magic
         assert self.product_id
 
+    def is_devdevice(self) -> bool:
+        """Returns whether this is a development-device bootloader."""
+        return self._is_devdevice
+
     def _query(self, msg: bytes) -> bytes:
         cid = self._transport.generate_cid()
         response = self._transport.query(msg, BOOTLOADER_CMD, cid)
         if response[0] != msg[0]:
             raise Exception("bootloader api error, expected {}, got {}".format(msg[0], response[0]))
         if response[1] != 0:
-            raise Exception("bootloader api error: code={}".format(response[1]))
+            raise BootloaderError(response[1])
         return response[2:]
 
     def versions(self) -> typing.Tuple[int, int]:
@@ -222,6 +248,12 @@ def flash_signed_firmware(
         if magic != self.expected_magic:
             raise ValueError("wrong firmware edition")
         self.flash_unsigned_firmware(firmware, progress_callback=progress_callback)
+        self.flash_sigdata(sigdata)
+
+    def flash_sigdata(self, sigdata: bytes) -> None:
+        """Flashes firmware signature data."""
+        if len(sigdata) != SIGDATA_LEN:
+            raise ValueError(f"signature data must be {SIGDATA_LEN} bytes")
         self._query(b"s" + sigdata)
 
     def erase(self) -> None:

### py/load_firmware.py
@@ -1,24 +1,96 @@
 #!/usr/bin/env python3
 # SPDX-License-Identifier: Apache-2.0
 
-"""TODO: document"""
+"""Flash signed or unsigned firmware onto a BitBox device.
 
-import argparse
-import sys
+The input type is detected from the signed-firmware header. The supported combinations are:
+
+* Production device, signed firmware: flash the firmware and signature data.
+* Production device, unsigned firmware: warn that it will not boot, then flash the firmware.
+* Development device, signed firmware: flash the firmware, attempt to flash the signature data,
+  warn if the signature data is rejected, and reboot into the firmware anyway.
+* Development device, unsigned firmware: flash the firmware without signature data.
 
+Every flash requires confirmation unless ``-y``/``--yes`` is used. Edition mismatches and
+combinations that are expected to fail are called out before the confirmation.
+"""
+
+import argparse
 import pprint
-from typing import Callable, Any, Tuple
-from time import sleep
+import sys
+import time
+from pathlib import Path
+from typing import Any, Callable, Dict, NamedTuple, Optional, Sequence, Tuple
 
 import hid
 
 from bitbox02.communication import devices, TransportLayer, u2fhid, bitbox_api_protocol
 from bitbox02.communication.devices import TooManyFoundException, NoneFoundException
 
-from bitbox02.bitbox02 import Bootloader, BitBox02
+from bitbox02.bitbox02 import (
+    BitBox02,
+    Bootloader,
+    BootloaderError,
+    bootloader as bootloader_api,
+)
 from bitbox02 import util
 
 
+BOOTLOADER_REBOOT_TIMEOUT_SECONDS = 30.0
+
+FLASHING_HELP = """\
+Flashing behavior:
+  signed firmware   + production device  flash firmware and signatures
+  signed firmware   + development device flash firmware, attempt signatures, and treat a
+                                         signature error as a warning
+  unsigned firmware + production device  flash firmware, but it will not boot
+  unsigned firmware + development device flash firmware without signatures
+
+The input type is detected from the signed-firmware header. Recognized but malformed signed
+firmware is rejected. Every flash asks for confirmation; -y/--yes skips the prompt. Edition
+mismatches and combinations that are expected to fail are called out before confirmation.
+"""
+
+BOOTLOADER_ERROR_MESSAGES: Dict[int, str] = {
+    ord("Z"): (
+        "The bootloader rejected the signature data. The signatures may be invalid, corrupt, or "
+        "for a different device."
+    ),
+    ord("V"): (
+        "The bootloader rejected a firmware or signing-key downgrade. Use a signed firmware with "
+        "versions at least as new as those already stored on the device."
+    ),
+    ord("N"): "The bootloader rejected an invalid firmware length or chunk number.",
+    ord("M"): (
+        "The loader and bootloader disagree about the firmware chunk size. Update the Python "
+        "package and retry."
+    ),
+    ord("W"): "The device could not write the firmware to flash memory.",
+    ord("C"): "The device could not verify flash memory after writing or erasing it.",
+    ord("A"): "The device aborted the firmware operation.",
+    ord("E"): "The device could not erase flash memory.",
+    ord("L"): ("The bootloader was not ready for this operation. Reconnect the device and retry."),
+    ord("I"): (
+        "The bootloader does not support this command. Check that the Python package is compatible "
+        "with the bootloader."
+    ),
+    ord("U"): "The device could not unlock flash memory for writing.",
+    ord("K"): "The device could not lock flash memory after writing.",
+}
+
+
+class FirmwareInput(NamedTuple):
+    """A validated firmware input and its parsed signed-container fields."""
+
+    payload: bytes
+    magic: Optional[bytes]
+    sigdata: Optional[bytes]
+
+
+class FirmwareInputError(Exception):
+    """The selected firmware file is invalid."""
+
+
 def eprint(*args: Any, **kwargs: Any) -> None:
     """
     Like print, but defaults to stderr.
@@ -27,6 +99,35 @@ def eprint(*args: Any, **kwargs: Any) -> None:
     print(*args, **kwargs)
 
 
+def _bootloader_error_message(error: BootloaderError) -> str:
+    """Return an actionable description of a bootloader status."""
+    return BOOTLOADER_ERROR_MESSAGES.get(
+        error.code, f"The bootloader returned an unknown error status (0x{error.code:02x})."
+    )
+
+
+def _read_firmware(filename: str) -> FirmwareInput:
+    """Read, classify, and validate a signed container or raw firmware."""
+    try:
+        firmware = Path(filename).read_bytes()
+    except OSError as error:
+        raise FirmwareInputError(f"Could not read firmware file '{filename}': {error}") from error
+
+    if not firmware:
+        raise FirmwareInputError("The firmware file is empty.")
+
+    try:
+        magic, sigdata, payload = bootloader_api.parse_signed_firmware(firmware)
+    except bootloader_api.InvalidFirmwareMagic:
+        payload = firmware
+        magic = None
+        sigdata = None
+    except ValueError as error:
+        raise FirmwareInputError(f"Invalid signed firmware: {error}") from error
+
+    return FirmwareInput(payload, magic, sigdata)
+
+
 def _get_bitbox_and_reboot(use_cache: bool) -> devices.DeviceInfo:
     """Search for a bitbox and then reboot it into bootloader"""
     device = devices.get_any_bitbox02()
@@ -62,14 +163,26 @@ def show_pairing(self, code: str, device_response: Callable[[], bool]) -> bool:
         raise RuntimeError("User aborted")
 
     # wait for it to reboot
+    deadline = time.monotonic() + BOOTLOADER_REBOOT_TIMEOUT_SECONDS
+    waiting = False
     while True:
         try:
             bootloader_device = devices.get_any_bitbox02_bootloader()
         except NoneFoundException:
+            if time.monotonic() >= deadline:
+                if waiting:
+                    print()
+                raise TimeoutError(
+                    "The device did not enter bootloader mode within 30 seconds. "
+                    "Reconnect it and retry."
+                )
             sys.stdout.write(".")
             sys.stdout.flush()
-            sleep(1)
+            waiting = True
+            time.sleep(1)
             continue
+        if waiting:
+            print()
         return bootloader_device
 
 
@@ -106,46 +219,140 @@ def _find_and_open_usb_bitbox02(use_cache: bool) -> Tuple[devices.DeviceInfo, Tr
     return bootloader_device, u2fhid.U2FHid(hid_device)
 
 
-def main() -> int:
+def _confirm_flash(signed: bool, devdevice: bool, product_string: str) -> bool:
+    """Ask the user to confirm the detected firmware and device combination."""
+    firmware_kind = "signed" if signed else "unsigned"
+    device_kind = "development" if devdevice else "production"
+    try:
+        response = input(
+            f"Flash {firmware_kind} firmware to this {device_kind} device "
+            f"({product_string})? [Y/n] "
+        )
+    except EOFError:
+        return False
+    return response.strip().lower() in ("", "y", "yes")
+
+
+def _flash_firmware(
+    bootloader: Bootloader,
+    bootloader_device: devices.DeviceInfo,
+    firmware: FirmwareInput,
+    yes: bool,
+    progress: Callable[[float], None],
+) -> bool:
+    """Flash a validated firmware input. Returns false if the user declines."""
+    devdevice = bootloader.is_devdevice()
+    signed = firmware.sigdata is not None
+
+    if not signed and not devdevice:
+        eprint(
+            "WARNING: Unsigned firmware cannot boot on a production device; rebooting is "
+            "expected to show a firmware verification error."
+        )
+    if signed and firmware.magic != bootloader.expected_magic:
+        eprint(
+            "WARNING: The signed firmware edition does not match the connected device; "
+            "installing its signature data is expected to fail."
+        )
+    if signed and devdevice:
+        eprint(
+            "WARNING: Development device detected. Signature data will be attempted, but a "
+            "rejection will not prevent the firmware from booting."
+        )
+
+    if not yes and not _confirm_flash(signed, devdevice, bootloader_device["product_string"]):
+        return False
+
+    bootloader.flash_unsigned_firmware(firmware.payload, progress)
+    if not signed:
+        return True
+
+    assert firmware.sigdata is not None
+    try:
+        bootloader.flash_sigdata(firmware.sigdata)
+    except Exception as error:  # pylint: disable=broad-exception-caught
+        if not devdevice:
+            raise
+        if isinstance(error, BootloaderError):
+            message = _bootloader_error_message(error)
+        else:
+            message = f"Could not install the signature data: {error}"
+        eprint(f"WARNING: {message}")
+        eprint("The firmware payload was flashed and the development device will be rebooted.")
+    return True
+
+
+def main(argv: Optional[Sequence[str]] = None) -> int:
     """Main function"""
     parser = argparse.ArgumentParser(
-        description="Tool for flashing a new firmware on BitBox devices."
+        description="Tool for flashing a new firmware on BitBox devices.",
+        epilog=FLASHING_HELP,
+        formatter_class=argparse.RawDescriptionHelpFormatter,
     )
     parser.add_argument(
         "--no-cache", action="store_true", help="Don't use cached or store noise keys"
     )
-    parser.add_argument("--debug", action="store_true", help="Flash a debug (unsigned) firmware.")
-    parser.add_argument("firmware", nargs=1, help="Firmware to flash.")
-    args = parser.parse_args()
+    parser.add_argument(
+        "-y",
+        "--yes",
+        action="store_true",
+        help="Skip the confirmation prompt.",
+    )
+    parser.add_argument(
+        "--debug",
+        action="store_true",
+        help="Deprecated no-op; firmware type is detected automatically.",
+    )
+    parser.add_argument("firmware", help="Firmware to flash.")
+    args = parser.parse_args(argv)
+
+    if args.debug:
+        eprint(
+            "WARNING: --debug is deprecated and has no effect; firmware type is detected "
+            "automatically."
+        )
 
-    if not args.debug and ".signed.bin" not in args.firmware[0]:
-        eprint("Expecting firmware to end with '.signed.bin'")
+    try:
+        firmware = _read_firmware(args.firmware)
+    except FirmwareInputError as error:
+        eprint(f"Error: {error}")
         return 1
 
-    bootloader_device, transport = _find_and_open_usb_bitbox02(not args.no_cache)
+    try:
+        bootloader_device, transport = _find_and_open_usb_bitbox02(not args.no_cache)
+    except TimeoutError as error:
+        eprint(f"Error: {error}")
+        return 1
     bootloader = Bootloader(transport, bootloader_device)
 
-    with open(args.firmware[0], "rb") as file:
-        firmware = file.read()
-
     def progress(perc: float) -> None:
         sys.stdout.write(f"{perc*100:.02f}%\r")
 
-    if bootloader.erased():
-        print("device contains NO firmware")
-    else:
-        print("firmware version: %d\nsigning pubkeys version: %d" % bootloader.versions())
-        firmware_hash, signing_keydata_hash = bootloader.get_hashes()
-        print("firmware hash:", firmware_hash.hex())
-        print("signing keydata hash:", signing_keydata_hash.hex())
+    try:
+        if bootloader.erased():
+            print("device contains NO firmware")
+        else:
+            print("firmware version: %d\nsigning pubkeys version: %d" % bootloader.versions())
+            firmware_hash, signing_keydata_hash = bootloader.get_hashes()
+            print("firmware hash:", firmware_hash.hex())
+            print("signing keydata hash:", signing_keydata_hash.hex())
 
-    if args.debug:
-        bootloader.flash_unsigned_firmware(firmware, progress)
-    else:
-        bootloader.flash_signed_firmware(firmware, progress)
-    print()  # print a newline
+        try:
+            flashed = _flash_firmware(bootloader, bootloader_device, firmware, args.yes, progress)
+        finally:
+            print()  # Finish the progress line, including when flashing fails.
+    except BootloaderError as error:
+        eprint(f"Error: {_bootloader_error_message(error)}")
+        return 1
+    except ValueError as error:
+        eprint(f"Error: {error}")
+        return 1
+
+    if not flashed:
+        eprint("Firmware flashing aborted.")
+        return 1
 
-    sleep(1)  # Pause to show the upgrade finished at 100%
+    time.sleep(1)  # Pause to show the upgrade finished at 100%
     bootloader.reboot()
     return 0
 

### test/scripts/test_load_firmware.py
@@ -0,0 +1,467 @@
+# SPDX-License-Identifier: Apache-2.0
+
+"""Tests for py/load_firmware.py and its bootloader helpers."""
+
+import contextlib
+import importlib.util
+import io
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+from unittest import mock
+
+
+REPOSITORY_ROOT = Path(__file__).resolve().parents[2]
+PYTHON_PACKAGE_ROOT = REPOSITORY_ROOT / "py" / "bitbox02"
+sys.path.insert(0, str(PYTHON_PACKAGE_ROOT))
+
+MODULE_PATH = REPOSITORY_ROOT / "py" / "load_firmware.py"
+SPEC = importlib.util.spec_from_file_location("load_firmware_under_test", MODULE_PATH)
+if SPEC is None or SPEC.loader is None:
+    raise RuntimeError(f"Could not load {MODULE_PATH}")
+LOAD_FIRMWARE = importlib.util.module_from_spec(SPEC)
+sys.modules[SPEC.name] = LOAD_FIRMWARE
+SPEC.loader.exec_module(LOAD_FIRMWARE)
+
+import bitbox02 as bitbox02_package  # noqa: E402
+from bitbox02.bitbox02 import bootloader as bootloader_module  # noqa: E402
+
+
+MAGIC = bootloader_module.SIGDATA_MAGIC_BITBOX02_MULTI
+OTHER_MAGIC = bootloader_module.SIGDATA_MAGIC_BITBOX02_BTCONLY
+SIGDATA = bytes(bootloader_module.SIGDATA_LEN)
+PAYLOAD = b"firmware payload"
+
+
+def device_info(serial_number: str = "v1.2.2") -> dict:
+    """Return minimal HID device information for a Multi bootloader."""
+    return {
+        "serial_number": serial_number,
+        "path": b"test-path",
+        "product_string": bootloader_module.BB02MULTI_BOOTLOADER,
+    }
+
+
+def signed_input(magic: bytes = MAGIC) -> LOAD_FIRMWARE.FirmwareInput:
+    """Build a parsed signed-firmware input."""
+    return LOAD_FIRMWARE.FirmwareInput(PAYLOAD, magic, SIGDATA)
+
+
+class DummyTransport:
+    """Minimal bootloader transport for package-level tests."""
+
+    def __init__(self, response_code: int = 0) -> None:
+        self.response_code = response_code
+        self.queries = []
+
+    def generate_cid(self) -> int:
+        return 1
+
+    def query(self, data: bytes, _endpoint: int, _cid: int) -> bytes:
+        self.queries.append(data)
+        return bytes((data[0], self.response_code))
+
+    def write(self, _data: bytes, _endpoint: int, _cid: int) -> None:
+        pass
+
+    def close(self) -> None:
+        pass
+
+
+class BootloaderPackageTests(unittest.TestCase):
+    """Test the public bootloader helpers used by the loader."""
+
+    def test_compat_package_exports_bootloader_module(self) -> None:
+        self.assertIs(bitbox02_package.bootloader, bootloader_module)
+
+    def test_parse_signed_firmware_wrong_magic(self) -> None:
+        with self.assertRaises(bootloader_module.InvalidFirmwareMagic):
+            bootloader_module.parse_signed_firmware(PAYLOAD)
+
+    def test_parse_signed_firmware_rejects_invalid_payload_size(self) -> None:
+        for payload in (b"", b"x" * (bootloader_module.MAX_FIRMWARE_SIZE + 1)):
+            with self.subTest(size=len(payload)), self.assertRaises(ValueError):
+                bootloader_module.parse_signed_firmware(MAGIC + SIGDATA + payload)
+
+    def test_is_devdevice(self) -> None:
+        for serial_number in (
+            "bb02.bl:v1.0.0-dev",
+            "v1.2.2+dev",
+            "v1.2.2+git.123.dev",
+        ):
+            with self.subTest(serial_number=serial_number):
+                bootloader = bootloader_module.Bootloader(
+                    DummyTransport(), device_info(serial_number)
+                )
+                self.assertTrue(bootloader.is_devdevice())
+
+    def test_is_devdevice_rejects_other_version_identifiers(self) -> None:
+        for serial_number in (
+            "v1.2.2",
+            "v1.2.2+git.123",
+            "v1.2.2+device",
+            "v1.2.2-development",
+        ):
+            with self.subTest(serial_number=serial_number):
+                bootloader = bootloader_module.Bootloader(
+                    DummyTransport(), device_info(serial_number)
+                )
+                self.assertFalse(bootloader.is_devdevice())
+
+    def test_is_devdevice_preserves_normalized_version(self) -> None:
+        bootloader = bootloader_module.Bootloader(
+            DummyTransport(), device_info("bb02.bl:v1.0.0-dev")
+        )
+        self.assertEqual(str(bootloader.version), "1.0.0")
+
+    def test_query_raises_structured_bootloader_error(self) -> None:
+        bootloader = bootloader_module.Bootloader(DummyTransport(ord("V")), device_info())
+        with self.assertRaises(bootloader_module.BootloaderError) as raised:
+            bootloader.versions()
+        self.assertEqual(raised.exception.code, ord("V"))
+
+    def test_flash_sigdata(self) -> None:
+        transport = DummyTransport()
+        bootloader = bootloader_module.Bootloader(transport, device_info())
+        bootloader.flash_sigdata(SIGDATA)
+        self.assertEqual(transport.queries, [b"s" + SIGDATA])
+
+    def test_flash_sigdata_rejects_wrong_length(self) -> None:
+        bootloader = bootloader_module.Bootloader(DummyTransport(), device_info())
+        with self.assertRaises(ValueError):
+            bootloader.flash_sigdata(SIGDATA[:-1])
+
+    def test_flash_unsigned_firmware_rejects_oversized_payload(self) -> None:
+        transport = DummyTransport()
+        bootloader = bootloader_module.Bootloader(transport, device_info())
+        with self.assertRaises(ValueError):
+            bootloader.flash_unsigned_firmware(b"x" * (bootloader_module.MAX_FIRMWARE_SIZE + 1))
+        self.assertEqual(transport.queries, [])
+
+
+class FirmwareInputTests(unittest.TestCase):
+    """Test input classification before a device is touched."""
+
+    def _write(self, data: bytes) -> str:
+        temporary = tempfile.NamedTemporaryFile(delete=False)
+        self.addCleanup(Path(temporary.name).unlink, missing_ok=True)
+        temporary.write(data)
+        temporary.close()
+        return temporary.name
+
+    def test_read_signed_firmware(self) -> None:
+        filename = self._write(MAGIC + SIGDATA + PAYLOAD)
+        firmware = LOAD_FIRMWARE._read_firmware(filename)
+        self.assertEqual(firmware, signed_input())
+
+    def test_read_unsigned_firmware(self) -> None:
+        for payload in (b"x", PAYLOAD):
+            with self.subTest(size=len(payload)):
+                filename = self._write(payload)
+                firmware = LOAD_FIRMWARE._read_firmware(filename)
+                self.assertEqual(firmware, LOAD_FIRMWARE.FirmwareInput(payload, None, None))
+
+    def test_read_truncated_signed_firmware_is_rejected(self) -> None:
+        filename = self._write(MAGIC + b"truncated")
+        with self.assertRaises(LOAD_FIRMWARE.FirmwareInputError):
+            LOAD_FIRMWARE._read_firmware(filename)
+
+    def test_read_firmware_rejects_empty_and_oversized_payloads(self) -> None:
+        for data in (
+            b"",
+            MAGIC + SIGDATA,
+            MAGIC + SIGDATA + b"x" * (bootloader_module.MAX_FIRMWARE_SIZE + 1),
+        ):
+            with self.subTest(size=len(data)):
+                filename = self._write(data)
+                with self.assertRaises(LOAD_FIRMWARE.FirmwareInputError):
+                    LOAD_FIRMWARE._read_firmware(filename)
+
+    def test_main_reads_firmware_before_device_discovery(self) -> None:
+        with mock.patch.object(LOAD_FIRMWARE, "_find_and_open_usb_bitbox02") as find_device:
+            with contextlib.redirect_stderr(io.StringIO()):
+                result = LOAD_FIRMWARE.main(["does-not-exist.bin"])
+        self.assertEqual(result, 1)
+        find_device.assert_not_called()
+
+
+class FlashingMatrixTests(unittest.TestCase):
+    """Test all supported device and firmware combinations."""
+
+    def setUp(self) -> None:
+        self.bootloader = mock.Mock()
+        self.bootloader.expected_magic = MAGIC
+        self.device = device_info()
+        self.progress = mock.Mock()
+
+    def test_unsigned_development_firmware(self) -> None:
+        self.bootloader.is_devdevice.return_value = True
+        firmware = LOAD_FIRMWARE.FirmwareInput(PAYLOAD, None, None)
+        self.assertTrue(
+            LOAD_FIRMWARE._flash_firmware(
+                self.bootloader, self.device, firmware, True, self.progress
+            )
+        )
+        self.bootloader.flash_unsigned_firmware.assert_called_once_with(PAYLOAD, self.progress)
+        self.bootloader.flash_sigdata.assert_not_called()
+
+    def test_unsigned_production_firmware_is_allowed_with_warning(self) -> None:
+        self.bootloader.is_devdevice.return_value = False
+        firmware = LOAD_FIRMWARE.FirmwareInput(PAYLOAD, None, None)
+        stderr = io.StringIO()
+        with contextlib.redirect_stderr(stderr):
+            result = LOAD_FIRMWARE._flash_firmware(
+                self.bootloader, self.device, firmware, True, self.progress
+            )
+        self.assertTrue(result)
+        self.assertIn("cannot boot", stderr.getvalue())
+        self.bootloader.flash_unsigned_firmware.assert_called_once_with(PAYLOAD, self.progress)
+
+    def test_signed_production_firmware(self) -> None:
+        self.bootloader.is_devdevice.return_value = False
+        firmware = signed_input()
+        self.assertTrue(
+            LOAD_FIRMWARE._flash_firmware(
+                self.bootloader, self.device, firmware, True, self.progress
+            )
+        )
+        self.bootloader.flash_unsigned_firmware.assert_called_once_with(PAYLOAD, self.progress)
+        self.bootloader.flash_sigdata.assert_called_once_with(SIGDATA)
+
+    def test_mismatched_signed_production_firmware_is_attempted(self) -> None:
+        self.bootloader.is_devdevice.return_value = False
+        stderr = io.StringIO()
+        with contextlib.redirect_stderr(stderr):
+            result = LOAD_FIRMWARE._flash_firmware(
+                self.bootloader, self.device, signed_input(OTHER_MAGIC), True, self.progress
+            )
+        self.assertTrue(result)
+        self.assertIn("expected to fail", stderr.getvalue())
+        self.bootloader.flash_unsigned_firmware.assert_called_once_with(PAYLOAD, self.progress)
+        self.bootloader.flash_sigdata.assert_called_once_with(SIGDATA)
+
+    def test_signed_development_firmware_attempts_sigdata(self) -> None:
+        self.bootloader.is_devdevice.return_value = True
+        firmware = signed_input()
+        events = []
+        self.bootloader.flash_unsigned_firmware.side_effect = lambda *_args: events.append(
+            "firmware"
+        )
+        self.bootloader.flash_sigdata.side_effect = lambda *_args: events.append("sigdata")
+        with contextlib.redirect_stderr(io.StringIO()):
+            result = LOAD_FIRMWARE._flash_firmware(
+                self.bootloader, self.device, firmware, True, self.progress
+            )
+        self.assertTrue(result)
+        self.assertEqual(events, ["firmware", "sigdata"])
+
+    def test_signed_development_sigdata_error_is_warning(self) -> None:
+        self.bootloader.is_devdevice.return_value = True
+        self.bootloader.flash_sigdata.side_effect = bootloader_module.BootloaderError(ord("V"))
+        stderr = io.StringIO()
+        with contextlib.redirect_stderr(stderr):
+            result = LOAD_FIRMWARE._flash_firmware(
+                self.bootloader, self.device, signed_input(), True, self.progress
+            )
+        self.assertTrue(result)
+        self.assertIn("WARNING", stderr.getvalue())
+        self.assertIn("downgrade", stderr.getvalue())
+
+    def test_signed_development_payload_error_is_fatal(self) -> None:
+        self.bootloader.is_devdevice.return_value = True
+        self.bootloader.flash_unsigned_firmware.side_effect = bootloader_module.BootloaderError(
+            ord("W")
+        )
+        with contextlib.redirect_stderr(io.StringIO()), self.assertRaises(
+            bootloader_module.BootloaderError
+        ):
+            LOAD_FIRMWARE._flash_firmware(
+                self.bootloader, self.device, signed_input(), True, self.progress
+            )
+        self.bootloader.flash_sigdata.assert_not_called()
+
+    def test_signed_development_transport_error_is_warning(self) -> None:
+        self.bootloader.is_devdevice.return_value = True
+        self.bootloader.flash_sigdata.side_effect = Exception("transport failed")
+        stderr = io.StringIO()
+        with contextlib.redirect_stderr(stderr):
+            result = LOAD_FIRMWARE._flash_firmware(
+                self.bootloader, self.device, signed_input(), True, self.progress
+            )
+        self.assertTrue(result)
+        self.assertIn("transport failed", stderr.getvalue())
+
+    def test_flash_can_be_declined(self) -> None:
+        self.bootloader.is_devdevice.return_value = True
+        with mock.patch.object(LOAD_FIRMWARE, "_confirm_flash", return_value=False) as confirm:
+            with contextlib.redirect_stderr(io.StringIO()):
+                result = LOAD_FIRMWARE._flash_firmware(
+                    self.bootloader,
+                    self.device,
+                    signed_input(),
+                    False,
+                    self.progress,
+                )
+        self.assertFalse(result)
+        confirm.assert_called_once_with(True, True, self.device["product_string"])
+        self.bootloader.flash_unsigned_firmware.assert_not_called()
+        self.bootloader.flash_sigdata.assert_not_called()
+
+    def test_flash_can_be_confirmed(self) -> None:
+        self.bootloader.is_devdevice.return_value = True
+        with mock.patch.object(LOAD_FIRMWARE, "_confirm_flash", return_value=True) as confirm:
+            with contextlib.redirect_stderr(io.StringIO()):
+                result = LOAD_FIRMWARE._flash_firmware(
+                    self.bootloader,
+                    self.device,
+                    signed_input(),
+                    False,
+                    self.progress,
+                )
+        self.assertTrue(result)
+        confirm.assert_called_once_with(True, True, self.device["product_string"])
+        self.bootloader.flash_unsigned_firmware.assert_called_once()
+        self.bootloader.flash_sigdata.assert_called_once_with(SIGDATA)
+
+    def test_yes_skips_confirmation(self) -> None:
+        self.bootloader.is_devdevice.return_value = True
+        firmware = LOAD_FIRMWARE.FirmwareInput(PAYLOAD, None, None)
+        with mock.patch.object(LOAD_FIRMWARE, "_confirm_flash") as confirm:
+            LOAD_FIRMWARE._flash_firmware(
+                self.bootloader, self.device, firmware, True, self.progress
+            )
+        confirm.assert_not_called()
+
+
+class LoaderUxTests(unittest.TestCase):
+    """Test user-visible failures, help, rebooting, and timeouts."""
+
+    def test_known_bootloader_errors_are_human_readable(self) -> None:
+        self.assertEqual(
+            set(LOAD_FIRMWARE.BOOTLOADER_ERROR_MESSAGES),
+            {ord(code) for code in "ZVNMWCAELIUK"},
+        )
+        for code in LOAD_FIRMWARE.BOOTLOADER_ERROR_MESSAGES:
+            with self.subTest(code=code):
+                message = LOAD_FIRMWARE._bootloader_error_message(
+                    bootloader_module.BootloaderError(code)
+                )
+                self.assertNotIn("code=", message)
+                self.assertTrue(message)
+
+    def test_unknown_bootloader_error_has_fallback(self) -> None:
+        message = LOAD_FIRMWARE._bootloader_error_message(bootloader_module.BootloaderError(0xFE))
+        self.assertIn("unknown", message)
+        self.assertIn("0xfe", message)
+
+    def test_help_documents_flashing_matrix(self) -> None:
+        stdout = io.StringIO()
+        with contextlib.redirect_stdout(stdout), self.assertRaises(SystemExit) as raised:
+            LOAD_FIRMWARE.main(["--help"])
+        self.assertEqual(raised.exception.code, 0)
+        help_text = stdout.getvalue()
+        self.assertIn("signed firmware   + production device", help_text)
+        self.assertIn("signed firmware   + development device", help_text)
+        self.assertIn("unsigned firmware + development device", help_text)
+        self.assertIn("unsigned firmware + production device", help_text)
+        self.assertIn("--yes", help_text)
+        self.assertNotIn("--unsigned", help_text)
+        self.assertIn("-y, --yes", help_text)
+        self.assertIn("--debug", help_text)
+
+    def test_debug_option_is_noop_with_warning(self) -> None:
+        stderr = io.StringIO()
+        with mock.patch.object(LOAD_FIRMWARE, "_find_and_open_usb_bitbox02") as find_device:
+            with contextlib.redirect_stderr(stderr):
+                result = LOAD_FIRMWARE.main(["--debug", "does-not-exist.bin"])
+        self.assertEqual(result, 1)
+        self.assertIn("--debug is deprecated and has no effect", stderr.getvalue())
+        find_device.assert_not_called()
+
+    def test_confirmation_defaults_to_yes(self) -> None:
+        with mock.patch("builtins.input", return_value="") as prompt:
+            self.assertTrue(LOAD_FIRMWARE._confirm_flash(True, False, "BitBox02 Multi bootloader"))
+        self.assertIn("signed firmware", prompt.call_args.args[0])
+        self.assertIn("production device", prompt.call_args.args[0])
+
+    def test_confirmation_rejects_eof(self) -> None:
+        with mock.patch("builtins.input", side_effect=EOFError):
+            self.assertFalse(LOAD_FIRMWARE._confirm_flash(False, True, "BitBox02 Multi bootloader"))
+
+    def test_sigdata_warning_still_reboots(self) -> None:
+        bootloader = mock.Mock()
+        bootloader.erased.return_value = True
+        bootloader.is_devdevice.return_value = True
+        bootloader.expected_magic = MAGIC
+        bootloader.flash_sigdata.side_effect = bootloader_module.BootloaderError(ord("V"))
+        with mock.patch.object(
+            LOAD_FIRMWARE, "_read_firmware", return_value=signed_input()
+        ), mock.patch.object(
+            LOAD_FIRMWARE,
+            "_find_and_open_usb_bitbox02",
+            return_value=(device_info(), mock.Mock()),
+        ), mock.patch.object(
+            LOAD_FIRMWARE, "Bootloader", return_value=bootloader
+        ), mock.patch.object(
+            LOAD_FIRMWARE.time, "sleep"
+        ), contextlib.redirect_stdout(
+            io.StringIO()
+        ), contextlib.redirect_stderr(
+            io.StringIO()
+        ):
+            result = LOAD_FIRMWARE.main(["-y", "firmware.signed.bin"])
+        self.assertEqual(result, 0)
+        bootloader.reboot.assert_called_once_with()
+
+    def test_production_bootloader_error_is_fatal(self) -> None:
+        bootloader = mock.Mock()
+        bootloader.erased.return_value = True
+        bootloader.is_devdevice.return_value = False
+        bootloader.expected_magic = MAGIC
+        bootloader.flash_sigdata.side_effect = bootloader_module.BootloaderError(ord("V"))
+        stderr = io.StringIO()
+        with mock.patch.object(
+            LOAD_FIRMWARE, "_read_firmware", return_value=signed_input()
+        ), mock.patch.object(
+            LOAD_FIRMWARE,
+            "_find_and_open_usb_bitbox02",
+            return_value=(device_info(), mock.Mock()),
+        ), mock.patch.object(
+            LOAD_FIRMWARE, "Bootloader", return_value=bootloader
+        ), contextlib.redirect_stdout(
+            io.StringIO()
+        ), contextlib.redirect_stderr(
+            stderr
+        ):
+            result = LOAD_FIRMWARE.main(["--yes", "firmware.signed.bin"])
+        self.assertEqual(result, 1)
+        self.assertIn("downgrade", stderr.getvalue())
+        bootloader.reboot.assert_not_called()
+
+    def test_wait_for_bootloader_times_out(self) -> None:
+        bitbox = mock.Mock()
+        bitbox.reboot.return_value = True
+        with mock.patch.object(
+            LOAD_FIRMWARE.devices, "get_any_bitbox02", return_value=device_info()
+        ), mock.patch.object(
+            LOAD_FIRMWARE.devices,
+            "get_any_bitbox02_bootloader",
+            side_effect=LOAD_FIRMWARE.NoneFoundException(),
+        ), mock.patch.object(
+            LOAD_FIRMWARE.hid, "device", return_value=mock.Mock()
+        ), mock.patch.object(
+            LOAD_FIRMWARE, "BitBox02", return_value=bitbox
+        ), mock.patch.object(
+            LOAD_FIRMWARE.time, "monotonic", side_effect=(0.0, 30.0)
+        ), mock.patch.object(
+            LOAD_FIRMWARE.time, "sleep"
+        ), contextlib.redirect_stdout(
+            io.StringIO()
+        ):
+            with self.assertRaises(TimeoutError):
+                LOAD_FIRMWARE._get_bitbox_and_reboot(False)
+
+
+if __name__ == "__main__":
+    unittest.main()