flow – Power automate: “send an http request to Sharepoint” token error

I’m trying to use a Flow to create news links in a modern site when a new post is published to an external site’s rss feed.
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It appears to get the JSON content correctly but the “send an http request to SP” action fails with this error:

{"odata.error":{"code":"-1, Microsoft.SharePoint.Client.InvalidClientQueryException","message": 
{"lang":"en-US","value":"Invalid JSON. A token was not recognized in the JSON content."}}}
clientRequestId: 5aba3c13-ac95-432e-acb7-139895cadcd9
serviceRequestId: 4c448d9f-408d-b000-94d2-48ad01427372