[WEBJS] - POST /api/{session}/groups/join fails with joinGroupViaInvite is undefined #2266

Describe the bug

Joining a WhatsApp group using the documented endpoint fails with HTTP 500
on the WEBJS engine.

The invite code is valid because groups/join-info successfully returns
the group information, but groups/join fails inside whatsapp-web.js.

The issue reproduces on both WAHA 2026.7.1 and 2026.8.2.

Version

Get the WAHA version by calling GET /api/version

{
    "version": "2026.8.2",
    "engine": "WEBJS",
    "tier": "CORE",
    "browser": "/usr/bin/chromium",
    "platform": "linux/x64",
    "worker": {
        "id": null
    }
}

Try to update to
the latest version before
creating an issue!

Steps

To Reproduce Steps to reproduce the behavior:

1. Start an authenticated WEBJS session.

2. Obtain a valid WhatsApp group invite link.

3. Verify the invite:

   GET /api/{session}/groups/join-info?code=REDACTED

4. The endpoint returns HTTP 200 with valid group information.

5. Attempt to join:

   POST /api/{session}/groups/join

The endpoint returns HTTP 500:

Cannot read properties of undefined (reading 'joinGroupViaInvite')

Expected behavior

The account should join the group, or a membership approval request should
be created when admin approval is enabled

Docker Logs

{
"level": 30,
"req": {
"method": "GET",
"url": "/api/{session}/groups/join-info?code=<REDACTED_INVITE_CODE>",
"query": {
"code": "<REDACTED_INVITE_CODE>"
}
},
"res": {
"statusCode": 200
},
"responseTime": 426,
"msg": "request completed"
}

{
"level": 30,
"req": {
"method": "GET",
"url": "/api/sessions/{session}/me"
},
"res": {
"statusCode": 200
},
"responseTime": 1,
"msg": "request completed"
}

{
"level": 30,
"req": {
"method": "GET",
"url": "/api/{session}/lids/pn/<REDACTED_PHONE>%40c.us"
},
"res": {
"statusCode": 200
},
"responseTime": 4,
"msg": "request completed"
}

{
"level": 30,
"req": {
"method": "POST",
"url": "/api/{session}/groups/join"
},
"res": {
"statusCode": 500
},
"err": {
"type": "Error",
"message": "Cannot read properties of undefined (reading 'joinGroupViaInvite')",
"stack": "TypeError: Cannot read properties of undefined (reading 'joinGroupViaInvite')\n at WebjsClientCore.acceptInvite (/app/node_modules/whatsapp-web.js/src/Client.js:2134:21)\n at GroupsController.joinGroup (/app/dist/api/groups.controller.js:43:20)"
},
"responseTime": 13,
"msg": "request errored"
}