Movable Type text filters vs comments

I wrestled with a strange problem of a text filter not working correctly with comments in some servers, but working OK in others. It gave completely inconsistent results and I was going pretty nuts. But as usual, turns out there’s a perfectly logical explanation.

When comments are processed with a text filter, the text filter in the processing chain comes BEFORE the comments get sanitized and HTML tags limited. So, for example, if you have a text filter that replaces some text tokens with images, you need to make sure that the image tag is also allowed in the “Limit HTML Tags” option of general settings. Here’s the value that I’m using (the only change from default is the added image tag with the two parameters.

a href,b,i,br/,p,strong,em,ul,ol,li,blockquote,pre,img/ src alt

And why it was working on some servers and not on others was, of course, because I had configured this HTML limiting setting differently.