Hello Anthony

The decode error inside wolfSSL is a BUFFER_ERROR inside DoTls13SupportedVersions during the ClientHello check for extensions.

/* TLS 1.3 must have extensions */
if (i < helloSz) {
    if (i + OPAQUE16_LEN > helloSz)
        return BUFFER_ERROR;
    ato16(&input[i], &totalExtSz);
    i += OPAQUE16_LEN;
    if (totalExtSz != helloSz - i)    // failure here
        return BUFFER_ERROR;
...

I have appended a wireshark capture of a tried connection.
The extension length field is correct
The overall handshake length field is off by 5 zeroed bytes at the very end of the handshake which correspond exactly to my custom extension:
- 2 bytes extension type
- 2 bytes extension length
- 1 byte extension data

Thank you for trying to help me!

Hello Anthony

I tried using the debugger and I don't see it being written to the extension data.
That's what I am confused about as I thought I'd have written enough code for that to happen.
Unless wolfSSL does smth. unexpected somewhere.

I readded the file, apparently I had to  press two buttons for the upload :-)

After introducing more debug statements I found out that

TLSX_Write()

skips my extension because some semaphore is being held.
I have no idea why this semaphore is switched on, however. I didn't switch it on myself.
It comes from

TLSX_WriteRequest()

I really put hours into trying to fix this one single behaviour, because it's difficult to navigate the source code of wolfSSL without much documentation.