I'm getting a C++ error Assertion failed! Expression: false, File: Packet.cpp, Line: 425

The problem is that LegacyCrossPlay connects to Java servers with minecraft-protocol as version "1.8.9" which doesn't have profile key signing. But I'm using ViaProxy too, so, if the target server is 1.19+ and enforces secure profiles, the server will reject the connection.

javaclienthandler.js has authConfig with version: "1.8.9". mc.createClient(authConfig) is how you connect.

For servers enforcing secure profiles (1.19+), the client should:

Use a version with support for profile keys (1.19+)
ProfileKeys set (certificates fetched)
Send signature in login_start packet

In setProtocol.js , when client.profileKeys is set and the version doesn't support chat sessions (1.19-1.19.2), the signature is sent in login_start
1.19.3+ (useChatSessions): sends profile keys after encryption (chat_session_update)
Currently the auth.js only fetches the Minecraft Java token, not certificates. The getMinecraftJavaToken call is missing fetchCertificates: true.

In microsoftAuth.js (built-in to minecraft-protocol) it calls getMinecraftJavaToken({ fetchProfile: true, fetchCertificates:!options.disableChatSigning }) followed by Object.assign(client, certificates) which sets client.profileKeys.

However, in LegacyCrossPlay's javaclienthandler.js, the auth is done manually through auth.js which only grabs the token, not certificates. mc.createClient is called with auth: "microsoft" and a session object, but no profile.Keys. If you are using "microsoft" it will use microsoftAuth.js which calls getMinecraftJavaToken({ fetchProfile: true, fetchCertificates: !options.disableChatSigning). So it should get certificate automatically.

But in LegacyCrossPlay the auth is done differently, pre-auths via auth.js and then passes the session to mc.createClient auth: "microsoft"

EDIT: The error in Packet.cpp line 425 is an assert on "Bad packet id". The LCE client receives a packet with an unknown ID. The problem is that javaclienthandler.js sends packets with Java ID (for example, 0x03, 0x04), but the LCE client expects LCE ID.

EDIT 2: Legacy Console Edition (Xbox/PS4) uses Little Endian for numbers and UTF-16LE for strings, while your PacketWriter writes in Big Endian (like Java). Because of this, the string length is read incorrectly (for example, 5 becomes 1280), and the game crashes.

Idea

can you make a plugin for ViaProxy to support any versions of Java server or just create the same way as ViaProxy did