Is there any way to trigger a post_redirect action when clicking on the image of a frame, instead of just opening the post_url?

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No, clicking the image should open the embed URL.

i don't think image click behaviour is even specified in the frames spec, so it's up to the client. if you follow standard opengraph behaviour then opening the embed url makes sense

there are browser redirects, but limited in what you can do programmatically

frog.fm/concepts/browser-redirects