diff --git a/backend/app/routes/_common.py b/backend/app/routes/_common.py
index d70a12aa..51cd22c4 100644
--- a/backend/app/routes/_common.py
+++ b/backend/app/routes/_common.py
@@ -23,9 +23,10 @@ M = TypeVar("M", bound=Base)
 # Resume-position thresholds of a player that only keeps a resume point (the YouTube player, the
 # library's): positions below this are "didn't really start" and within this of the end are
 # "basically finished" — neither is worth storing, so the position is cleared instead (no progress
-# bar). The client's local resume (`lib/resume.ts`) keeps the same two numbers. NB: the Plex player
-# intentionally uses a LARGER finish margin (30s) because it marks long-form items watched, not just
-# clears a resume bar — see routes/plex/playback.py.
+# bar). The client's local resume (`lib/resume.ts`) keeps the same two numbers, held equal by
+# `resume.contract.test.ts` beside it. NB: the Plex player intentionally uses a LARGER finish margin
+# (30s) because it marks long-form items watched, not just clears a resume bar — see
+# routes/plex/playback.py.
 PROGRESS_MIN_SECONDS = 5
 FINISH_MARGIN_SECONDS = 10
 
diff --git a/frontend/src/components/PlexPlayer.session.dom.test.tsx b/frontend/src/components/PlexPlayer.session.dom.test.tsx
index 2a2b3603..7bfe4579 100644
--- a/frontend/src/components/PlexPlayer.session.dom.test.tsx
+++ b/frontend/src/components/PlexPlayer.session.dom.test.tsx
@@ -731,3 +731,38 @@ it("a session answer that lands while the next item's detail is still loading is
   await tick();
   expect(video().getAttribute("src")).not.toBe("/media/B");
 });
+
+describe("the server session the player no longer wants", () => {
+  const hlsA = { mode: "hls", url: "/hls/A/index.m3u8", start_s: 0, id: "sid-A" } as Awaited<
+    ReturnType<typeof api.plexSession>
+  >;
+
+  it("is stopped by its item and its own id when the page is unloaded", async () => {
+    const stop = vi.spyOn(api, "plexStopSession").mockImplementation(() => {});
+    mount({ details: { A: item("A") }, sessions: { A: () => Promise.resolve(hlsA) } });
+    await waitFor(() => expect(video()?.getAttribute("src")).toBe("/hls/A/index.m3u8"));
+    act(
+      () => void window.dispatchEvent(Object.assign(new Event("pagehide"), { persisted: false })),
+    );
+    expect(stop).toHaveBeenCalledWith("A", "sid-A");
+  });
+
+  it("is stopped the same way when the player closes", async () => {
+    const stop = vi.spyOn(api, "plexStopSession").mockImplementation(() => {});
+    const { unmount } = mount({
+      details: { A: item("A") },
+      sessions: { A: () => Promise.resolve(hlsA) },
+    });
+    await waitFor(() => expect(video()?.getAttribute("src")).toBe("/hls/A/index.m3u8"));
+    unmount();
+    expect(stop).toHaveBeenCalledWith("A", "sid-A");
+  });
+
+  it("is kept when the page only goes into the back/forward cache", async () => {
+    const stop = vi.spyOn(api, "plexStopSession").mockImplementation(() => {});
+    mount({ details: { A: item("A") }, sessions: { A: () => Promise.resolve(hlsA) } });
+    await waitFor(() => expect(video()?.getAttribute("src")).toBe("/hls/A/index.m3u8"));
+    act(() => void window.dispatchEvent(Object.assign(new Event("pagehide"), { persisted: true })));
+    expect(stop).not.toHaveBeenCalled();
+  });
+});
diff --git a/frontend/src/lib/api/plexStream.contract.test.ts b/frontend/src/lib/api/plexStream.contract.test.ts
new file mode 100644
index 00000000..3565d73b
--- /dev/null
+++ b/frontend/src/lib/api/plexStream.contract.test.ts
@@ -0,0 +1,36 @@
+// @vitest-environment jsdom
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
+import { afterEach, expect, it, vi } from "vitest";
+import { api } from "../api";
+
+// The session stop is a keepalive beacon: nothing answers it on screen, so a URL that no longer
+// matches the server's route fails silently — every transcode the player leaves runs on until the
+// idle reaper. The route is read from the server's source, as the other contract tests do.
+afterEach(() => vi.restoreAllMocks());
+
+function serverRoute(): RegExp {
+  // From the frontend root (vitest's cwd): this file runs under jsdom — `api` boots i18n, which reads
+  // localStorage — and there `import.meta.url` is not a file URL `readFileSync` takes.
+  const source = readFileSync(
+    join(process.cwd(), "../backend/app/routes/plex/playback.py"),
+    "utf8",
+  );
+  const prefix = /APIRouter\(prefix="([^"]+)"/.exec(source)?.[1];
+  const path = /@router\.post\("(\/stream\/[^"]*\/stop)"\)/.exec(source)?.[1];
+  // If either is missing the route was renamed or moved — re-point this test, the rule stands.
+  expect(prefix, "router prefix").toBeDefined();
+  expect(path, "the session stop route").toBeDefined();
+  const pattern = (prefix! + path!).replace(/\{[^}]+\}/g, "[^/]+");
+  return new RegExp(`^${pattern}$`);
+}
+
+it("sends the session stop to the route the server serves it on", () => {
+  const fetch = vi.spyOn(globalThis, "fetch").mockResolvedValue(new Response(null));
+  api.plexStopSession("12345", "a1b2c3");
+  const url = String(fetch.mock.calls[0]![0]);
+  expect(url).toMatch(serverRoute());
+  expect(url).toContain("/12345/");
+  expect(url).toContain("/a1b2c3/");
+  expect(fetch.mock.calls[0]![1]).toMatchObject({ method: "POST", keepalive: true });
+});
diff --git a/frontend/src/lib/resume.contract.test.ts b/frontend/src/lib/resume.contract.test.ts
new file mode 100644
index 00000000..4ab31850
--- /dev/null
+++ b/frontend/src/lib/resume.contract.test.ts
@@ -0,0 +1,31 @@
+import { readFileSync } from "node:fs";
+import { expect, it } from "vitest";
+
+// One rule, two languages, no shared wire format — so a test is the only thing that holds them
+// together. Which point a library item keeps is the server's (`routes/_common.kept_position`); the
+// client repeats it (`lib/resume.ts`) for the shared link, which has no server side, and for the
+// library row it patches before the server has answered. A server-only change left the row showing
+// a Continue point the server had already cleared, and a link resuming at points the library no
+// longer keeps. Both must be EQUAL: neither direction is merely conservative here.
+//
+// Read from source rather than imported, as in plexFinishMargin.contract.test.ts.
+function constantIn(path: string, pattern: RegExp): number {
+  const source = readFileSync(new URL(path, import.meta.url), "utf8");
+  const m = pattern.exec(source);
+  // If this fails the constant was renamed or moved — find it and re-point this test, because the
+  // rule it guards did not go away.
+  expect(m, `${pattern} not found in ${path}`).not.toBeNull();
+  return Number(m![1]);
+}
+
+const SERVER = "../../../backend/app/routes/_common.py";
+
+it.each([
+  ["the least position that counts as started", "PROGRESS_MIN_SECONDS", "PROGRESS_MIN_S"],
+  ["the margin before the end that counts as finished", "FINISH_MARGIN_SECONDS", "FINISH_MARGIN_S"],
+])("keeps %s the same as the server", (_what, server, client) => {
+  const backend = constantIn(SERVER, new RegExp(`^${server}\\s*=\\s*(\\d+)`, "m"));
+  const frontend = constantIn("./resume.ts", new RegExp(`^const ${client}\\s*=\\s*(\\d+)`, "m"));
+  expect(backend).toBeGreaterThan(0);
+  expect(frontend).toBe(backend);
+});
diff --git a/frontend/src/lib/resume.ts b/frontend/src/lib/resume.ts
index c0f49428..20d2e093 100644
--- a/frontend/src/lib/resume.ts
+++ b/frontend/src/lib/resume.ts
@@ -6,8 +6,8 @@
 // Where the point is kept is the caller's: the library's on the server (`library_states`), a shared
 // link's in this browser (the viewer has no account). The rule for which point is worth keeping is
 // the server's own (`routes/_common.kept_position`), repeated here for the link that has no server
-// side: too early to count as started, or close enough to the end to count as finished, is "from
-// the start".
+// side (`resume.contract.test.ts` holds the two numbers equal): too early to count as started, or
+// close enough to the end to count as finished, is "from the start".
 import { useEffect, useEffectEvent } from "react";
 
 import { LS, readJSON, writeJSON } from "./storage";