diff --git a/src-tauri/Cargo.lock b/src-tauri/Cargo.lock
index 2befa4465..125d198db 100644
--- a/src-tauri/Cargo.lock
+++ b/src-tauri/Cargo.lock
@@ -194,6 +194,16 @@ version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d92bec98840b8f03a5ff5413de5293bfcd8bf96467cf5452609f939ec6f5de16"
 
+[[package]]
+name = "assert-json-diff"
+version = "2.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "47e4f2b81832e72834d7518d8487a0396a28cc408186a2e8854c0f98011faf12"
+dependencies = [
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "async-broadcast"
 version = "0.7.2"
@@ -878,6 +888,24 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "deadpool"
+version = "0.12.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0be2b1d1d6ec8d846f05e137292d0b89133caf95ef33695424c09568bdd39b1b"
+dependencies = [
+ "deadpool-runtime",
+ "lazy_static",
+ "num_cpus",
+ "tokio",
+]
+
+[[package]]
+name = "deadpool-runtime"
+version = "0.1.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "092966b41edc516079bdf31ec78a2e0588d1d0c08f78b91d8307215928642b2b"
+
 [[package]]
 name = "deranged"
 version = "0.5.8"
@@ -1345,6 +1373,21 @@ dependencies = [
  "new_debug_unreachable",
 ]
 
+[[package]]
+name = "futures"
+version = "0.3.31"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "65bc07b1a8bc7c85c5f2e110c476c7389b4554ba72af57d8445ea63a576b0876"
+dependencies = [
+ "futures-channel",
+ "futures-core",
+ "futures-executor",
+ "futures-io",
+ "futures-sink",
+ "futures-task",
+ "futures-util",
+]
+
 [[package]]
 name = "futures-channel"
 version = "0.3.31"
@@ -1352,6 +1395,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2dff15bf788c671c1934e366d07e30c1814a8ef514e1af724a602e8a2fbe1b10"
 dependencies = [
  "futures-core",
+ "futures-sink",
 ]
 
 [[package]]
@@ -1419,6 +1463,7 @@ version = "0.3.31"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9fa08315bb612088cc391249efdc3bc77536f16c91f6cf495e6fbe85b20a4a81"
 dependencies = [
+ "futures-channel",
  "futures-core",
  "futures-io",
  "futures-macro",
@@ -1754,6 +1799,25 @@ dependencies = [
  "syn 2.0.112",
 ]
 
+[[package]]
+name = "h2"
+version = "0.4.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ef8e5e5a340588f4452631496976cf8636d4a7ecf600239fdc27615d2530bc16"
+dependencies = [
+ "atomic-waker",
+ "bytes",
+ "fnv",
+ "futures-core",
+ "futures-sink",
+ "http",
+ "indexmap 2.12.1",
+ "slab",
+ "tokio",
+ "tokio-util",
+ "tracing",
+]
+
 [[package]]
 name = "hashbrown"
 version = "0.12.3"
@@ -1902,9 +1966,11 @@ dependencies = [
  "bytes",
  "futures-channel",
  "futures-core",
+ "h2",
  "http",
  "http-body",
  "httparse",
+ "httpdate",
  "itoa",
  "pin-project-lite",
  "pin-utils",
@@ -2253,6 +2319,7 @@ dependencies = [
  "tokio-util",
  "urlencoding",
  "uuid",
+ "wiremock",
  "zip 2.4.2",
 ]
 
@@ -2417,6 +2484,12 @@ dependencies = [
  "selectors 0.24.0",
 ]
 
+[[package]]
+name = "lazy_static"
+version = "1.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bbd2bcb4c963f2ddae06a2efc7e9f3591312473c50c6685e1f298068316e66fe"
+
 [[package]]
 name = "libappindicator"
 version = "0.9.0"
@@ -2720,6 +2793,16 @@ dependencies = [
  "autocfg",
 ]
 
+[[package]]
+name = "num_cpus"
+version = "1.17.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "91df4bbde75afed763b708b7eee1e8e7651e02d97f6d5dd763e89367e957b23b"
+dependencies = [
+ "hermit-abi",
+ "libc",
+]
+
 [[package]]
 name = "num_enum"
 version = "0.7.5"
@@ -6430,6 +6513,29 @@ dependencies = [
  "windows-sys 0.59.0",
 ]
 
+[[package]]
+name = "wiremock"
+version = "0.6.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "08db1edfb05d9b3c1542e521aea074442088292f00b5f28e435c714a98f85031"
+dependencies = [
+ "assert-json-diff",
+ "base64 0.22.1",
+ "deadpool",
+ "futures",
+ "http",
+ "http-body-util",
+ "hyper",
+ "hyper-util",
+ "log",
+ "once_cell",
+ "regex",
+ "serde",
+ "serde_json",
+ "tokio",
+ "url",
+]
+
 [[package]]
 name = "wit-bindgen"
 version = "0.46.0"
diff --git a/src-tauri/Cargo.toml b/src-tauri/Cargo.toml
index 664d48bae..bb1e33f6a 100644
--- a/src-tauri/Cargo.toml
+++ b/src-tauri/Cargo.toml
@@ -150,6 +150,7 @@ ndk-context = "0.1"
 
 [dev-dependencies]
 tempfile = "3.24.0"
+wiremock = "0.6.5"
 
 [features]
 # Exposes the MediaPlayer conformance suite and the `player-conformance` binary
diff --git a/src-tauri/src/repository/generation_tests.rs b/src-tauri/src/repository/generation_tests.rs
new file mode 100644
index 000000000..5aeaba5bd
--- /dev/null
+++ b/src-tauri/src/repository/generation_tests.rs
@@ -0,0 +1,227 @@
+//! The online repository, exercised against a real HTTP server on both Jellyfin
+//! generations.
+//!
+//! These are the tests DR-281 exists for: every assertion here is about what the
+//! client actually put on the wire, or about what it did with a response it
+//! actually received. Nothing here reimplements a URL builder.
+//!
+//! TRACES: UR-085 | DR-281
+
+use super::server_fixture::{target, FakeJellyfin, BOTH_GENERATIONS, V10_11, V12};
+use super::types::{GetItemsOptions, SearchScope};
+use super::MediaRepository;
+
+/// Jellyfin 12.0 disables `X-Emby-Authorization` by default — including on
+/// upgraded servers, via a migration that flips `EnableLegacyAuthorization` to
+/// false. `Authorization` with the same `MediaBrowser` scheme is ungated on both
+/// generations, so there is one correct spelling rather than a branch.
+///
+/// This is the assertion that would have caught the breakage: it looks at the
+/// header the server received, not at a string the client built.
+///
+/// TRACES: UR-085 | DR-287 | IT-019
+#[tokio::test]
+async fn every_request_authenticates_with_the_non_deprecated_header() {
+    for version in BOTH_GENERATIONS {
+        let fake = FakeJellyfin::start(version).await;
+        let repo = fake.repository();
+
+        repo.get_libraries().await.expect("libraries");
+
+        let request = fake.only_request().await;
+
+        let auth = request
+            .headers
+            .get("authorization")
+            .unwrap_or_else(|| panic!("{version}: no Authorization header was sent"))
+            .to_str()
+            .expect("header is ascii");
+
+        assert!(
+            auth.starts_with("MediaBrowser "),
+            "{version}: Authorization must use the MediaBrowser scheme, got {auth:?}"
+        );
+        assert!(
+            auth.contains(r#"Token="token-abc""#),
+            "{version}: the token must reach the server, got {auth:?}"
+        );
+        assert!(
+            request.headers.get("x-emby-authorization").is_none(),
+            "{version}: X-Emby-Authorization is disabled by default on 12.0"
+        );
+    }
+}
+
+/// A listing must parse into domain items on both generations. `BaseItemDto` was
+/// verified to be purely additive between 10.11.5 and 12.0, so one parse path is
+/// correct for both — this is the test that would notice if that stopped holding.
+///
+/// TRACES: UR-007, UR-085 | DR-281 | IT-020
+#[tokio::test]
+async fn a_listing_parses_on_both_generations() {
+    for version in BOTH_GENERATIONS {
+        let fake = FakeJellyfin::start(version).await;
+        let result = fake
+            .repository()
+            .get_items("lib-1", None)
+            .await
+            .unwrap_or_else(|e| panic!("{version}: listing failed: {e:?}"));
+
+        assert_eq!(result.items.len(), 1, "{version}");
+        assert_eq!(result.items[0].id, "item-1", "{version}");
+        assert_eq!(result.items[0].name, "A Film", "{version}");
+    }
+}
+
+/// Jellyfin 12.0 defaults `recursive` to true when the parent is a library
+/// folder and `IncludeItemTypes` is set, where 10.11 listed immediate children —
+/// the identical request, a different result set. The client must state it, so
+/// that the two generations agree.
+///
+/// TRACES: UR-085 | DR-288 | IT-021
+#[tokio::test]
+async fn a_type_filtered_listing_states_recursive_on_the_wire() {
+    for version in BOTH_GENERATIONS {
+        let fake = FakeJellyfin::start(version).await;
+
+        fake.repository()
+            .get_items(
+                "lib-1",
+                Some(GetItemsOptions {
+                    include_item_types: Some(vec!["Movie".to_string()]),
+                    ..Default::default()
+                }),
+            )
+            .await
+            .expect("listing");
+
+        let sent = target(&fake.only_request().await);
+        assert!(
+            sent.contains("Recursive="),
+            "{version}: without an explicit Recursive the two generations disagree: {sent}"
+        );
+    }
+}
+
+/// The library listing goes to the route the capabilities selected, and comes
+/// back parsed. Both generations still serve the user-scoped family — only six
+/// routes were removed in 12.0 and none of them are these.
+///
+/// TRACES: UR-007, UR-085 | DR-282 | IT-022
+#[tokio::test]
+async fn libraries_resolve_on_both_generations() {
+    for version in BOTH_GENERATIONS {
+        let fake = FakeJellyfin::start(version).await;
+        let libraries = fake
+            .repository()
+            .get_libraries()
+            .await
+            .unwrap_or_else(|e| panic!("{version}: {e:?}"));
+
+        assert_eq!(libraries.len(), 1, "{version}");
+        assert_eq!(libraries[0].id, "lib-1", "{version}");
+
+        let sent = target(&fake.only_request().await);
+        assert!(sent.starts_with("/Users/user-1/Views"), "{version}: {sent}");
+    }
+}
+
+/// Flipping the user-scoped flag must actually change the wire request, and the
+/// response must still parse. Nothing selects `false` today, so without this the
+/// alternative route shape would be untested code waiting to be switched on.
+///
+/// TRACES: UR-085 | DR-282 | IT-023
+#[tokio::test]
+async fn the_alternative_route_shape_works_end_to_end() {
+    let fake = FakeJellyfin::start(V12).await;
+
+    let mut capabilities = super::capabilities::ServerCapabilities::from_reported(V12);
+    capabilities.user_scoped_item_routes = false;
+    let repo = fake.repository().with_capabilities(capabilities);
+
+    let result = repo.get_items("lib-1", None).await.expect("listing");
+    assert_eq!(result.items.len(), 1);
+
+    let sent = target(&fake.only_request().await);
+    assert!(sent.starts_with("/Items?"), "{sent}");
+    assert!(sent.contains("userId=user-1"), "{sent}");
+    assert!(!sent.contains("/Users/"), "{sent}");
+}
+
+/// Favourites carry the filter that makes them favourites, on both generations.
+///
+/// TRACES: UR-067, UR-085 | DR-281 | IT-024
+#[tokio::test]
+async fn favourites_filter_reaches_the_server() {
+    for version in BOTH_GENERATIONS {
+        let fake = FakeJellyfin::start(version).await;
+
+        fake.repository()
+            .get_favorites(SearchScope::All, None)
+            .await
+            .expect("favourites");
+
+        let sent = target(&fake.only_request().await);
+        assert!(sent.contains("Filters=IsFavorite"), "{version}: {sent}");
+        assert!(
+            !sent.contains("IncludeItemTypes"),
+            "{version}: All scope must omit the type filter rather than send a \
+             union, which would drop every type nobody enumerated: {sent}"
+        );
+    }
+}
+
+/// A stream URL is handed to mpv / ExoPlayer / an HTML5 `<video>`, none of which
+/// can set a header — so its token must ride in the query string. `ApiKey` is
+/// ungated on both generations and is what the server itself emits; `api_key` is
+/// gated off by default on 12.0.
+///
+/// TRACES: UR-004, UR-085 | DR-287 | IT-025
+#[tokio::test]
+async fn player_facing_urls_carry_the_ungated_query_token() {
+    for version in BOTH_GENERATIONS {
+        let fake = FakeJellyfin::start(version).await;
+        let url = fake
+            .repository()
+            .get_audio_stream_url("track-1")
+            .await
+            .unwrap_or_else(|e| panic!("{version}: {e:?}"));
+
+        assert!(
+            url.contains("ApiKey=token-abc"),
+            "{version}: a player cannot send a header, so the token must be in \
+             the query — and spelled ApiKey: {url}"
+        );
+        assert!(
+            !url.contains("api_key="),
+            "{version}: api_key is disabled by default on 12.0: {url}"
+        );
+    }
+}
+
+/// The capability resolution is driven by what the server reported, not by a
+/// value a test poked in — this is what makes the other tests here meaningful.
+///
+/// TRACES: UR-085 | DR-280 | IT-026
+#[tokio::test]
+async fn capabilities_come_from_the_version_the_server_reported() {
+    use super::capabilities::ServerGeneration;
+
+    let old = FakeJellyfin::start(V10_11).await;
+    assert_eq!(
+        old.repository().capabilities().generation,
+        ServerGeneration::V10_11
+    );
+
+    let new = FakeJellyfin::start(V12).await;
+    assert_eq!(
+        new.repository().capabilities().generation,
+        ServerGeneration::V12Plus
+    );
+    assert!(
+        !new.repository()
+            .capabilities()
+            .supports_manifest_container_direct_play,
+        "12.0 makes manifest-container sources ineligible for direct play"
+    );
+}
diff --git a/src-tauri/src/repository/server_fixture.rs b/src-tauri/src/repository/server_fixture.rs
new file mode 100644
index 000000000..aa784914b
--- /dev/null
+++ b/src-tauri/src/repository/server_fixture.rs
@@ -0,0 +1,147 @@
+//! A fake Jellyfin server the online repository can actually talk to.
+//!
+//! # Why this exists
+//!
+//! Before it, `src-tauri/` contained no HTTP mocking of any kind. Every test of
+//! the ~4,800-line online adapter asserted on a *constructed URL string*, and
+//! not one exercised a response. That has a specific, recorded cost: a deleted
+//! test file re-implemented the URL builders inside its own mock and then
+//! asserted against itself, and `online.rs` still carries the comment recording
+//! that the production builder meanwhile shipped a `/Videos/{id}/download`
+//! endpoint which 404s on real servers — silently breaking every download while
+//! the "test" stayed green.
+//!
+//! So the rule here is: **assert against a response from a mock *server*, never
+//! against a mock that re-derives the thing under test.** Nothing in this module
+//! may reimplement anything from `endpoints.rs` or `online.rs`.
+//!
+//! # Two generations
+//!
+//! [`FakeJellyfin::start`] takes the version string the fake server reports, and
+//! the repository it hands back resolves its capabilities from exactly that — the
+//! same path production takes. A test that runs against both generations is
+//! therefore running the real resolution, not a stubbed one.
+//!
+//! TRACES: UR-085 | DR-281
+
+use std::sync::Arc;
+
+use serde_json::json;
+use wiremock::matchers::{method, path_regex};
+use wiremock::{Mock, MockServer, Request, ResponseTemplate};
+
+use super::capabilities::ServerCapabilities;
+use super::online::OnlineRepository;
+use crate::jellyfin::{HttpClient, HttpConfig};
+
+/// Jellyfin's current stable line, and the line this client was built against.
+pub const V12: &str = "12.0.0";
+pub const V10_11: &str = "10.11.5";
+
+/// Both live generations. `#[test]`s that care about compatibility iterate this.
+///
+/// There is no 11 in the middle: Jellyfin dropped the leading `10` from its
+/// scheme with 12.0, so what would have been 10.12.0 shipped as `12.0`.
+pub const BOTH_GENERATIONS: [&str; 2] = [V10_11, V12];
+
+pub struct FakeJellyfin {
+    server: MockServer,
+    version: String,
+}
+
+impl FakeJellyfin {
+    /// Stand up a server reporting `version`, answering any `/Items`-shaped
+    /// query with one item and any `/Users/.../Views` with one library.
+    ///
+    /// The response bodies are deliberately minimal: this module's job is to let
+    /// tests observe what the *client* sent, not to re-specify Jellyfin.
+    pub async fn start(version: &str) -> Self {
+        let server = MockServer::start().await;
+
+        let item = json!({
+            "Id": "item-1",
+            "Name": "A Film",
+            "Type": "Movie",
+            "IsFolder": false,
+            "ServerId": "srv-1",
+        });
+
+        Mock::given(method("GET"))
+            .and(path_regex(r".*/Views$"))
+            .respond_with(ResponseTemplate::new(200).set_body_json(json!({
+                "Items": [{
+                    "Id": "lib-1",
+                    "Name": "Movies",
+                    "Type": "CollectionFolder",
+                    "CollectionType": "movies",
+                    "IsFolder": true,
+                    "ServerId": "srv-1",
+                }],
+                "TotalRecordCount": 1,
+            })))
+            .mount(&server)
+            .await;
+
+        // Everything else that returns a listing.
+        Mock::given(method("GET"))
+            .respond_with(ResponseTemplate::new(200).set_body_json(json!({
+                "Items": [item],
+                "TotalRecordCount": 1,
+            })))
+            .mount(&server)
+            .await;
+
+        Mock::given(method("POST"))
+            .respond_with(ResponseTemplate::new(204))
+            .mount(&server)
+            .await;
+
+        Self {
+            server,
+            version: version.to_string(),
+        }
+    }
+
+    /// A repository pointed at this server, with capabilities resolved from the
+    /// version it reports — the same resolution production performs.
+    pub fn repository(&self) -> OnlineRepository {
+        let http = HttpClient::new_allowing_plaintext_for_tests(HttpConfig::default())
+            .expect("test http client");
+
+        OnlineRepository::new(
+            Arc::new(http),
+            self.server.uri(),
+            "user-1".to_string(),
+            "token-abc".to_string(),
+        )
+        .with_capabilities(ServerCapabilities::from_reported(&self.version))
+    }
+
+    /// Every request the server received, in order.
+    pub async fn requests(&self) -> Vec<Request> {
+        self.server
+            .received_requests()
+            .await
+            .expect("request recording is enabled")
+    }
+
+    /// The single request received, failing loudly if there was not exactly one.
+    pub async fn only_request(&self) -> Request {
+        let mut received = self.requests().await;
+        assert_eq!(
+            received.len(),
+            1,
+            "expected exactly one request, got {}",
+            received.len()
+        );
+        received.remove(0)
+    }
+}
+
+/// The request target as the server saw it — path plus query.
+pub fn target(request: &Request) -> String {
+    match request.url.query() {
+        Some(q) => format!("{}?{}", request.url.path(), q),
+        None => request.url.path().to_string(),
+    }
+}