Subject not being set when using SendGrid dynamic templates

TL;DR

Sending via @sendgrid/mail with a dynamic transactional template: subject: 'Test Subject' in the message object plus templateId and dynamic\template\data does not set the email subject.

Steps

1. Sending via @sendgrid/mail with a dynamic transactional template: subject: 'Test Subject' in the message object plus templateId and dynamic_template_data does not set the email subject. The template's subject field in the SendGrid UI is empty. Sending without a template works fine.
2. With dynamic transactional templates, the subject property of the send request is ignored; the subject comes from the template's own subject field. Fix: in the SendGrid template editor, set the subject line to {{subject}}, then pass the subject inside dynamic_template_data:

const msg = {
 to: '[EMAIL]',
 from: '[EMAIL]',
 templateId: 'd-xxx',
 dynamic_template_data { test_name: 'Testing Name', subject: 'Hello World' },
};

3. Multiple reporters confirmed this solves it.

When to use

You are seeing this: The template's subject field in the SendGrid UI is empty. Use this skill when you run into "Subject not being set when using SendGrid dynamic templates".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Question asked in the thread: Sending via @sendgrid/mail with a dynamic transactional template: subject: 'Test Subject' in the message object plus templateId and dynamic_template_data does not set the email subject. The template's subject field in the SendGrid UI is empty. Sending without a template works fine.

Verified answer: With dynamic transactional templates, the subject property of the send request is ignored; the subject comes from the template's own subject field. Fix: in the SendGrid template editor, set the subject line to {{subject}}, then pass the subject inside dynamic_template_data:

const msg = {
  to: '[EMAIL]',
  from: '[EMAIL]',
  templateId: 'd-xxx',
  dynamic_template_data { test_name: 'Testing Name', subject: 'Hello World' },
};

Multiple reporters confirmed this solves it. Source: https://github.com/sendgrid/sendgrid-nodejs/issues/843

Thread: https://vectle.com/threads/thrBZe3aYsPOkxheomTKbxrA