I sent multiple notifications via Slack without any problem, but I just sent one that got truncated. It contains some URLs that are pretty long. The URLs are embedded in Markdown Links, so our users will actually see a pretty short message, but the overall content after converted to Slack’s mrkdwn has over four thousand characters. My message got truncated in char 4036.
Found the problem. Here’s some detail in case anyone else bump into the same issue.
Slack API for sending messages has a limit of 3k characters per Block. When you use Courier’s send providing a body, Courier translates your whole message into a single Block when sending to Slack. It’s possible to workaround that by using Courier’s Elements feature. You can send an array of those in in the Send API, and each element will be translated to a single Block in Slack. That way you can break a huge message into smaller pieces.