diff --git a/CHANGELOG.md b/CHANGELOG.md
index b438aac29..8880f82fc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,96 @@ generated trace matrix lives in [docs/traceability.md](docs/traceability.md).
 For how long each fixed defect had been shipping before it was found, see
 [docs/defect-windows.md](docs/defect-windows.md).
 
+## v0.12.0
+
+JellyTau works against Jellyfin 12. Jellyfin 12.0 shipped on 2026-09-08 and
+turns off, by default, the two ways every earlier JellyTau build identified
+itself to a server — including on servers that were upgraded rather than freshly
+installed. An app that was not changed for it stops signing in the day the server
+updates. This release is changed for it, and still works against 10.11, so the
+app can be updated first and the server whenever it suits.
+
+There is no Jellyfin 11. The project dropped the leading `10` from its version
+scheme: what would have been 10.12.0 shipped as 12.0. Anything that compares
+Jellyfin version numbers needed to learn that, and this app now has.
+
+### ✨ Changes
+
+- **Signing in survives a server upgrade to Jellyfin 12.** The server used to be
+  told who was asking through a header and a URL parameter that 12.0 disables by
+  default — a migration disables them on upgraded servers too, so nothing in an
+  admin's hands changes the outcome. The replacement spellings are accepted by
+  10.11 and 12 alike, so this is one way of identifying the app that works
+  everywhere, not a switch between two. The URL half matters more than it
+  sounds: video and audio are streamed by the device's media player, which cannot
+  send headers at all, so the URL parameter is the only way playback can
+  authenticate. A test now refuses any request built with the old spellings,
+  because the failure is silent right up until a server upgrades.
+  (UR-085 → DR-287)
+
+- **Browsing a library returns the same things on both server versions.** 12.0
+  changed what a filtered library listing means — asked for the films in a
+  library, 10.11 returned the folder's immediate contents and 12.0 returns
+  everything beneath it, and nothing in the reply says which rule applied. The
+  app now says which it wants, so both servers answer the same way, and the
+  answer is the one it always had. (UR-085 → DR-288)
+
+- **The app knows what server it is talking to, once, and adapts.** The server
+  version was already fetched at sign-in and then thrown away. It is now
+  resolved into a small set of named capabilities that every version-dependent
+  decision reads from, rather than the version number being compared wherever
+  somebody needed it — which is unreadable by the second occurrence and cannot
+  express a backport. A server newer than this build is treated as the newest
+  one it knows and keeps working; refusing it would make every release expire
+  the moment the server updated. Only a server older than 10.10 is refused, and
+  the sign-in screen says so and names the minimum. (UR-085 → IR-035, DR-280,
+  DR-286)
+
+- **The cached library re-fetches itself after a server upgrade.** Nothing had
+  recorded which server version wrote the cached catalog, so a server upgraded
+  underneath the app kept serving rows read under the old rules. The generation
+  is now recorded, and a change clears the cache so it fills back under the new
+  one. The first launch of this version records and clears nothing — an
+  existing install is not charged a full re-download to defend against an
+  upgrade that has not happened. (UR-085 → DR-284)
+
+### 🛠 Development
+
+- **Every route the app speaks lives in one place.** Fifty-seven inline URL
+  strings across the server adapter became one module of route functions, each
+  taking the resolved capabilities. Both shapes of the item routes Jellyfin has
+  deprecated are built and tested, though nothing selects the second yet — the
+  family still works on 12.0, and 12.0's written policy that unlisted endpoints
+  may go in any major release is why having the alternative ready costs less
+  than needing it. (UR-085 → DR-279, DR-282)
+
+- **The server adapter is tested against a server.** There was no HTTP mocking
+  in the Rust tree at all: every test of the adapter asserted on a URL string it
+  had built, and none exercised a reply. A fake Jellyfin now answers over real
+  HTTP and reports whichever version a test asks for, so the same assertions run
+  against both generations through the production resolution path. The one
+  file that had tried this before reimplemented the URL builders inside its own
+  mock and asserted against itself — and had never compiled, and had once
+  stayed green while the real code shipped a download endpoint that 404s. It is
+  deleted, and the rule it teaches is written at the top of its replacement.
+  (UR-085 → DR-281)
+
+- **A Jellyfin URL was being built in the interface layer** — the last one,
+  and, it turned out, unused. Deleted rather than moved. (UR-085 → DR-285)
+
+### ⚠️ Known limits
+
+Every cross-version assertion runs against a fake server built from a
+source-level diff of the two Jellyfin releases, not against a running 12.0. Two
+things that diff could not settle: whether remote control and casting behave
+identically, and whether the audio-codec check that forces a transcode on 10.11
+is still needed on 12 — it is left on, which errs toward an unnecessary
+transcode rather than silent playback. Both resolve with a real 12.0 server;
+reports welcome.
+
+**Upgrading:** install this version *before* upgrading the server, not after.
+It works against both; an older JellyTau does not work against 12.
+
 ## v0.11.6
 
 Found by an audit of the stack's most fragile seams rather than by hitting them,
diff --git a/package.json b/package.json
index d6fdaec3d..c4c6c5e8b 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "jellytau",
-  "version": "0.11.6",
+  "version": "0.12.0",
   "description": "A cross-platform Jellyfin client built with Tauri, SvelteKit and Rust.",
   "author": "Duncan Tourolle <duncan@tourolle.paris>",
   "license": "MIT",
diff --git a/src-tauri/Cargo.lock b/src-tauri/Cargo.lock
index 125d198db..b9d238854 100644
--- a/src-tauri/Cargo.lock
+++ b/src-tauri/Cargo.lock
@@ -2275,7 +2275,7 @@ dependencies = [
 
 [[package]]
 name = "jellytau"
-version = "0.11.6"
+version = "0.12.0"
 dependencies = [
  "aes-gcm",
  "argon2",
diff --git a/src-tauri/Cargo.toml b/src-tauri/Cargo.toml
index bb1e33f6a..9a8d14bce 100644
--- a/src-tauri/Cargo.toml
+++ b/src-tauri/Cargo.toml
@@ -4,7 +4,7 @@ name = "jellytau"
 # `player-conformance`, and a second binary makes a bare `cargo run` —
 # which `tauri dev` issues — ambiguous.
 default-run = "jellytau"
-version = "0.11.6"
+version = "0.12.0"
 description = "A cross-platform Jellyfin client"
 authors = ["Duncan Tourolle <duncan@tourolle.paris>"]
 license = "MIT"
diff --git a/src-tauri/tauri.conf.json b/src-tauri/tauri.conf.json
index 822f2a616..c38d29e50 100644
--- a/src-tauri/tauri.conf.json
+++ b/src-tauri/tauri.conf.json
@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "JellyTau",
-  "version": "0.11.6",
+  "version": "0.12.0",
   "identifier": "com.dtourolle.jellytau",
   "build": {
     "beforeDevCommand": "bun run dev",