connections.ev3: Fix flashing on Windows.

Windows requires a reportid byte at the front of each sent
message for an HID device, even if the device doesn't use
reportids. Adding this synthetic 0x00 to the front of the
message enables successful EV3 flashing on Windows.

Author: jaguilar

CHANGELOG.md

@@ -12,6 +12,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - Fixed compiling multi-file projects with implicit namespace packages.
+- Fixed EV3 firmware upload on Windows ([pybricksdev#128]).
+
+[pybricksdev#128]: https://github.com/pybricks/pybricksdev/pull/128
 
 ## [2.3.0] - 2025-10-31
 

pybricksdev/connections/ev3.py

@@ -84,10 +84,17 @@ def _send_command(self, command: Command, payload: bytes | None = None) -> int:
 
             length += len(payload)
 
+        # report_id is not used by the EV3 but is required by HIDAPI on Windows.
+        # It does no harm on Linux, so we include it unconditionally. Note that
+        # the report ID is automatically stripped from incoming messages by
+        # HIDAPI on all platforms.
+        report_id = 0
+
         message_number = next(self._msg_count)
 
         message = struct.pack(
-            "<HHBB",
+            "<BHHBB",
+            report_id,
             length,
             message_number,
             MessageType.SYSTEM_COMMAND_REPLY,