I’m implementing a GET request to return a list of database records like the standard “index” method generated by phx.gen. But my method should filter the result, i.e. the client requests only a specific set of IDs. At first, I wanted to send the list of IDs in a JSON array in the request body, but …

Try option c, it should be supported by default in Phoenix IIRC. You should be able to find the array in the request params like %{"ids" => ["12bb23d2...", "654802aa..."]}.