Interop research: every major platform's wire shapes, gotchas, and what PrivaPub still drops

docs/INTEROP.md collects five research passes from 2026-10-01: Mastodon 4.7 and GoToSocial 0.22, the Misskey and
Pleroma families, the threadiverse (Lemmy 0.19/1.0, PieFed, Mbin, NodeBB), media and long-form (PeerTube, Loops,
Pixelfed, WordPress, Ghost, events, audio, books, Threads, Flipboard, Bridgy Fed), and cross-cutting FEPs and
signatures. Each claim was checked against source code or live fetches, with dates and versions.

It is checked against our own code: what is already right, three cheap things that are wrong today (summary read as a
CW on every type, unchecked usernames, an undefined context term), what the rich client needs kept (a post kind with
typed payloads, raw capture and provenance for the details view), and the six privacy choices the owner has to make.

The roadmap's P5 becomes P5 to P8, ordered by that evidence.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

CLAUDE.md

@@ -1,7 +1,9 @@
 # CLAUDE.md
 
 Guidance for working in this repository. `docs/ROADMAP.md` holds the owner's decisions and the phased plan to full
-ActivityPub interop; read it before changing anything federation-, privacy- or API-shaped.
+ActivityPub interop; read it before changing anything federation-, privacy- or API-shaped. `docs/INTEROP.md` is the
+per-platform evidence behind phases P5 to P8: what each peer sends, what it expects, and what PrivaPub still drops.
+Check the platform's section there before writing a parser or a renderer, and add to it whatever you learn.
 
 ## What this is