how to make function in RSS feed like it does in posts

Here’s a link to a code snippet that you can put in functions.php to do what you want.

By default, the RSS feed either shows the entire article when set to show “Full Text” in the Reading Settings or it shows the Excerpt field when set to “Summary.”

Support for the tag can only be done through a plugin or theme snippet.

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EDIT: I wasn’t thrilled with how that snippet above worked and so went searching more. I’ll leave it for posterity, but I’d look into using this snippet instead. It actually enables the “more” feature rather than trying to recreate it. I haven’t tried it, but it looks right.