diff --git a/deploy/retire-jellyfin-library.py b/deploy/retire-jellyfin-library.py
index 98fb6bf..4bbe16a 100755
--- a/deploy/retire-jellyfin-library.py
+++ b/deploy/retire-jellyfin-library.py
@@ -143,9 +143,20 @@ def main() -> None:
         print(f"==> deleted {old['Name']!r}")
 
     if renaming:
+        # `name`, NOT `id`. This endpoint is the odd one out -- most of
+        # /Library/VirtualFolders/* takes an id, and passing one here returns a
+        # bare "HTTP 400: Error processing request." that says nothing about why.
+        # Verified against Jellyfin 10.11.4 on 2026-08-13: name -> 204.
         request(key, "POST", "/Library/VirtualFolders/Name",
-                {"id": new["ItemId"], "newName": FINAL_NAME})
-        print(f"==> renamed to {FINAL_NAME!r}")
+                {"name": new["Name"], "newName": FINAL_NAME})
+        print(f"==> renamed {new['Name']!r} to {FINAL_NAME!r}")
+        # A library's ItemId is derived from its name, so renaming re-ids it --
+        # measured, 98e74a0c… became 34f331a8…, which was the *deleted* library's
+        # id, because that one had this name. Nothing here caches an ItemId, and
+        # ytstream's own find_library() matches on path, so both survive it. Any
+        # future caller that stores an ItemId will not.
+        print("    note: the library's ItemId changed (Jellyfin derives it from "
+              "the name)")
 
     after = folders(key)
     print()
diff --git a/plan.md b/plan.md
index 6cdda48..a316a7e 100644
--- a/plan.md
+++ b/plan.md
@@ -1225,3 +1225,23 @@ Still worth keeping out of `rm`: `/var/lib/youtube-automate` (170 MB) holds the
 venv and `subs.db`, and `subs.db` is *not* in the repo — it is state, not code. It is
 already copied to `/var/lib/ytstream/youtube-automate-subs.db.archived-20260813`, so
 that directory is now safe to delete too, just not before checking that copy exists.
+
+### Jellyfin's rename endpoint takes a name, not an id — and re-ids the library
+
+`POST /Library/VirtualFolders/Name` is the odd one out in that controller: most of
+`/Library/VirtualFolders/*` takes an `id`, and this one takes `name`. Passing an id
+returns a bare `HTTP 400: Error processing request.` with nothing to say why. Verified
+against Jellyfin 10.11.4, 2026-08-13: `name=…&newName=…` → `204`.
+
+Renaming **changes the library's ItemId**, because Jellyfin derives it from the name:
+`98e74a0c…` became `34f331a8…` — which was the *deleted* library's id, since that one
+had the name we renamed to. Consequences, checked rather than assumed:
+
+* `find_library()` matches on **path**, so ytstream is unaffected. `doctor` reports
+  `library 'YouTube'` and `refresh-metadata` re-read all 257 NFOs with **0 proxy
+  requests** immediately after the rename.
+* Anything that ever caches a Jellyfin ItemId across a rename will break. Nothing
+  does today. Do not add one.
+
+End state: one library, `YouTube`, at `/disks/Plex/_ytstream`, 11 series and 257
+episodes. `/disks/Plex/YouTube` is no longer a library; its 9 files are still on disk.