Hi,

thanks for your suggestions. Looking at what happens on the web client network, when requesting a template signature, it looks nothing the CRUD endpoints listed for the JSON API here: https://www.odoo.com/documentation/19.0/developer/reference/external_api.html#api

Web client 1st sends a "web_save" method, model "sign.send.request", 
then a "send_request" method, model "sign.send.request",

Trying to add most of the args in my create endpoint generate  INTERNAL SERVER ERROR.

Very frustrating to not get answers from Odoo support... and not having any documentation for this.

Hi,

The 422 Unprocessable Entity error indicates that the Signature request is reaching Odoo correctly, but the JSON payload is missing required values or does not match what the Signature app expects. The sign.request/create endpoint requires more data than just template_id and message_partner_ids, and the exact structure of vals_list depends on how the Signature template is configured (roles, signers, and required fields). Since the Signature API is not fully documented, the recommended approach is to enable Developer Mode, create a Signature request manually from the Odoo UI using the same template, and inspect the Network tab in your browser to capture the /sign/request/create payload. That request shows the exact fields and structure required for your template. Most 422 errors occur because a required signer role or assignment is missing or mismatched, so ensuring the template is valid, published, and that all signer roles are properly provided will resolve the issue.

Hope it helps