diff --git a/backend/app/plex/imgcache.py b/backend/app/plex/imgcache.py
index 6f7ba2e0..8eed4e1b 100644
--- a/backend/app/plex/imgcache.py
+++ b/backend/app/plex/imgcache.py
@@ -27,7 +27,8 @@ _CT_EXT = {ct: ext for ext, ct in _EXT_CT}
 # page's ≤224px poster at 2× DPR, the full art (1280px) a hero backdrop. A card is much smaller: a grid
 # poster is ~144px and a shelf's landscape art ~212px, so the `_sm` pair serves those at 1.33× DPR
 # (E31 S4e: cards fetched the 400 poster, 42 KB, and the 1280 art for a 212px picture). The browser
-# picks between a pair itself (`srcset`, the frontend's `plexSrcSet`).
+# picks between a pair itself (`srcset`, built by the frontend's `lib/plexImage` cardImage, whose
+# widths plexImage.contract.test holds to these).
 SIZES = {
     "thumb": (400, 600),
     "thumb_sm": (240, 360),
diff --git a/frontend/src/components/PlexLibraryPicker.tsx b/frontend/src/components/PlexLibraryPicker.tsx
index aa480f8b..3e042133 100644
--- a/frontend/src/components/PlexLibraryPicker.tsx
+++ b/frontend/src/components/PlexLibraryPicker.tsx
@@ -5,6 +5,7 @@ import { qk } from "../lib/queryKeys";
 import { api, type PlexCard } from "../lib/api";
 import { useDebounced } from "../lib/useDebounced";
 import { LoadingState } from "./ui/QueryState";
+import { cardImage } from "../lib/plexImage";
 
 // A debounced library search-and-pick list, shared by the "Add title" panel (PlexCollectionView) and
 // the "New collection" first-title panel (plexBrowseLists) — both search the library the same way and
@@ -49,7 +50,7 @@ export default function PlexLibraryPicker({
       {results.map((c) => (
         <li key={c.id} className="flex items-center gap-3">
           <img
-            src={c.thumb}
+            {...cardImage(c.thumb)}
             alt=""
             className="h-12 w-8 shrink-0 rounded object-cover"
             loading="lazy"
diff --git a/frontend/src/components/PlexPlaylistView.tsx b/frontend/src/components/PlexPlaylistView.tsx
index 0285992d..d5fee5a5 100644
--- a/frontend/src/components/PlexPlaylistView.tsx
+++ b/frontend/src/components/PlexPlaylistView.tsx
@@ -55,6 +55,7 @@ import { SURFACE } from "./ui/surfaces";
 import { PageHeading } from "./ui/Headings";
 import { MEDIA_BADGE } from "./ui/mediaCardStyles";
 import { seasonTitle } from "../lib/plexDetail";
+import { cardImage } from "../lib/plexImage";
 
 // A playlist's items, grouped for readability: movies stay standalone, while a run of episodes from the
 // same show collapses into a season/show group so a whole series doesn't sprawl into an endless flat
@@ -157,7 +158,7 @@ function Poster({ src, className }: { src: string; className: string }) {
     <div
       className={`relative shrink-0 overflow-hidden rounded-sm border border-border bg-surface ${className}`}
     >
-      <img src={src} alt="" loading="lazy" className="h-full w-full object-cover" />
+      <img {...cardImage(src)} alt="" loading="lazy" className="h-full w-full object-cover" />
     </div>
   );
 }
@@ -200,7 +201,12 @@ function EpisodeRow({ ep, ctx, dense }: { ep: PlexCard; ctx: Ctx; dense: boolean
         <div
           className={`relative shrink-0 overflow-hidden rounded-sm border border-border bg-surface ${dense ? "aspect-video w-14" : "aspect-video w-20"}`}
         >
-          <img src={ep.thumb} alt="" loading="lazy" className="h-full w-full object-cover" />
+          <img
+            {...cardImage(ep.thumb)}
+            alt=""
+            loading="lazy"
+            className="h-full w-full object-cover"
+          />
           {ep.status === "watched" && <span className={`${MEDIA_BADGE} right-0.5 top-0.5`}>✓</span>}
           {inProgress && <div className="absolute inset-x-0 bottom-0 h-0.5 bg-accent" />}
           <div className="absolute inset-0 grid place-items-center bg-black/40 opacity-0 transition group-hover/thumb:opacity-100">
@@ -319,7 +325,7 @@ function ShowGroup({ block, ctx }: { block: ShowBlock; ctx: Ctx }) {
             {block.items.slice(0, 3).map((ep, i) => (
               <img
                 key={ep.id}
-                src={ep.thumb}
+                {...cardImage(ep.thumb)}
                 alt=""
                 loading="lazy"
                 className="aspect-2/3 w-8 rounded-sm border-2 border-surface object-cover"
diff --git a/frontend/src/components/PlexSearchBox.tsx b/frontend/src/components/PlexSearchBox.tsx
index 17c447bc..febb0383 100644
--- a/frontend/src/components/PlexSearchBox.tsx
+++ b/frontend/src/components/PlexSearchBox.tsx
@@ -12,6 +12,7 @@ import { availablePlexFields, healPlexField, plexFieldLabelKey } from "../lib/pl
 import { Pressable } from "./ui/Pressable";
 import { SURFACE } from "./ui/surfaces";
 import { ROVING_HIGHLIGHT } from "./ui/AnchoredPopover";
+import { cardImage } from "../lib/plexImage";
 
 // The Plex header search box: the shared SearchBar enriched with the S2 search scope-pill (a
 // GitHub-style field selector inside the box) and an instant typeahead of the top matches attached
@@ -160,7 +161,7 @@ export default function PlexSearchBox() {
                   className={`flex w-full items-center gap-3 rounded-lg p-1.5 transition ${ROVING_HIGHLIGHT}`}
                 >
                   <img
-                    src={c.thumb}
+                    {...cardImage(c.thumb)}
                     alt=""
                     loading="lazy"
                     className="h-12 w-8 shrink-0 rounded-md object-cover bg-border/40"
diff --git a/frontend/src/lib/plexImage.contract.test.ts b/frontend/src/lib/plexImage.contract.test.ts
new file mode 100644
index 00000000..9315f0d2
--- /dev/null
+++ b/frontend/src/lib/plexImage.contract.test.ts
@@ -0,0 +1,75 @@
+import { readFileSync } from "node:fs";
+import { expect, it } from "vitest";
+
+import { cardImage } from "./plexImage";
+
+// A card's srcset names each Plex cut by its width, and the browser picks one by that width. The
+// widths are the image proxy's (backend `app/plex/imgcache.py` SIZES), with no wire format between
+// the two: a backend-only change would have the browser pick a cut by a width it does not have (E31
+// S5). Read from source, as plexFinishMargin.contract does.
+const source = readFileSync(
+  new URL("../../../backend/app/plex/imgcache.py", import.meta.url),
+  "utf8",
+);
+const backendWidth = (variant: string) => {
+  const m = new RegExp(`"${variant}": \\((\\d+), \\d+\\)`).exec(source);
+  // If this fails the variant was renamed or moved: re-point this test, the invariant stayed.
+  expect(m, `${variant} not in imgcache.SIZES`).not.toBeNull();
+  return Number(m![1]);
+};
+
+const srcsetWidths = (url: string) =>
+  Object.fromEntries(
+    cardImage(url)
+      .srcSet!.split(", ")
+      .map((c) => {
+        const [u, w] = c.split(" ");
+        return [
+          new URL(u!, "http://x").searchParams.get("variant") ?? "thumb",
+          Number(w!.slice(0, -1)),
+        ];
+      }),
+  );
+
+it("names each poster cut by the width the proxy cuts it at", () => {
+  expect(srcsetWidths("/api/plex/image/1")).toEqual({
+    thumb_sm: backendWidth("thumb_sm"),
+    thumb: backendWidth("thumb"),
+  });
+});
+
+it("names each backdrop cut by the width the proxy cuts it at", () => {
+  expect(srcsetWidths("/api/plex/image/1?variant=art")).toEqual({
+    art_sm: backendWidth("art_sm"),
+    art: backendWidth("art"),
+  });
+});
+
+// Every small Plex picture asks for its card-size cut through cardImage (E31 S4e, swept in S5: the
+// playlist rows, the search box and the collection picker still fetched the 400px poster for a 32px
+// picture). A bare `src={…thumb}` is allowed only where the full cut is the point or the URL is not
+// the proxy's; the SET is pinned, so a new bare one fails here.
+const plexFiles = import.meta.glob(
+  [
+    "../components/{Plex,plex}*.tsx",
+    "../components/{Home,PosterTile,UpNextCard}.tsx",
+    "!../components/**/*.test.tsx",
+  ],
+  { query: "?raw", import: "default", eager: true },
+);
+const ALLOWED = new Set([
+  "plexCards.tsx c.thumb", // cast photos: the person-image route, not the image proxy
+  "PlexInfo.tsx c.thumb", // the same cast photos on the detail page
+  "PlexInfo.tsx detail.thumb", // the detail page's own poster: the full cut is the point
+  "plexDetailUi.tsx thumb", // the show/season header poster: the same
+  "Home.tsx card.thumb", // the hero (full width) and the shelf's own tiles (YouTube channel art)
+]);
+
+it("leaves no small Plex picture on the full cut", () => {
+  expect(Object.keys(plexFiles).length).toBeGreaterThan(5);
+  const bare = new Set<string>();
+  for (const [path, src] of Object.entries(plexFiles))
+    for (const m of (src as string).matchAll(/<img\s+src=\{([^}]*thumb[^}]*)\}/g))
+      bare.add(`${path.split("/").pop()} ${m[1]!.trim()}`);
+  expect([...bare].filter((b) => !ALLOWED.has(b))).toEqual([]);
+});