igpapi — Instagram private API Simulator for Node.js (API reference) 4.14.0
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    Variable ReelsViewerSessionSchemaConst

    ReelsViewerSessionSchema: Struct<
        {
            chainingSessionId: optionalKey<String>;
            mediaPctWatched: optionalKey<Union<readonly [Number, String]>>;
            rankingSessionId: optionalKey<String>;
        },
    > = ...

    What a caller opening the viewer on a list declares about the session: the two ids the app mints when the viewer opens and the watched percentage it measured. A caller that holds such values (a scene) declares them; one that holds none declares nothing and the viewer mints the ids its like and comment must carry. The per-reel facts are not declared: the seed is the tapped reel's container id and the positions are its index in the list.