diff --git a/extensions/diffs/src/config.test.ts b/extensions/diffs/src/config.test.ts
index 8796ec5b061..cd621cad740 100644
--- a/extensions/diffs/src/config.test.ts
+++ b/extensions/diffs/src/config.test.ts
@@ -6,7 +6,7 @@ import {
   ResolvingThemes,
 } from "@pierre/diffs";
 import AjvPkg from "ajv";
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, vi } from "vitest";
 import {
   DEFAULT_DIFFS_PLUGIN_SECURITY,
   DEFAULT_DIFFS_TOOL_DEFAULTS,
@@ -17,7 +17,12 @@ import {
 } from "./config.js";
 import { renderDiffDocument } from "./render.js";
 import { buildViewerUrl, normalizeViewerBaseUrl } from "./url.js";
-import { getServedViewerAsset, VIEWER_LOADER_PATH, VIEWER_RUNTIME_PATH } from "./viewer-assets.js";
+import {
+  getServedViewerAsset,
+  resolveViewerRuntimeFileUrl,
+  VIEWER_LOADER_PATH,
+  VIEWER_RUNTIME_PATH,
+} from "./viewer-assets.js";
 import { parseViewerPayloadJson } from "./viewer-payload.js";
 
 const FULL_DEFAULTS = {
@@ -540,6 +545,47 @@ describe("renderDiffDocument", () => {
 });
 
 describe("viewer assets", () => {
+  it("prefers the built plugin asset layout when present", async () => {
+    const stat = vi.fn(async (path: string) => {
+      if (path === "/repo/dist/extensions/diffs/assets/viewer-runtime.js") {
+        return { mtimeMs: 1 };
+      }
+      const error = Object.assign(new Error(`missing: ${path}`), { code: "ENOENT" });
+      throw error;
+    });
+
+    await expect(
+      resolveViewerRuntimeFileUrl({
+        baseUrl: "file:///repo/dist/extensions/diffs/index.js",
+        stat,
+      }),
+    ).resolves.toMatchObject({
+      pathname: "/repo/dist/extensions/diffs/assets/viewer-runtime.js",
+    });
+    expect(stat).toHaveBeenCalledTimes(1);
+  });
+
+  it("falls back to the source asset layout when the built artifact is absent", async () => {
+    const stat = vi.fn(async (path: string) => {
+      if (path === "/repo/extensions/diffs/assets/viewer-runtime.js") {
+        return { mtimeMs: 1 };
+      }
+      const error = Object.assign(new Error(`missing: ${path}`), { code: "ENOENT" });
+      throw error;
+    });
+
+    await expect(
+      resolveViewerRuntimeFileUrl({
+        baseUrl: "file:///repo/extensions/diffs/src/viewer-assets.js",
+        stat,
+      }),
+    ).resolves.toMatchObject({
+      pathname: "/repo/extensions/diffs/assets/viewer-runtime.js",
+    });
+    expect(stat).toHaveBeenNthCalledWith(1, "/repo/extensions/diffs/src/assets/viewer-runtime.js");
+    expect(stat).toHaveBeenNthCalledWith(2, "/repo/extensions/diffs/assets/viewer-runtime.js");
+  });
+
   it("serves a stable loader that points at the current runtime bundle", async () => {
     const loader = await getServedViewerAsset(VIEWER_LOADER_PATH);
 
diff --git a/extensions/diffs/src/viewer-assets.ts b/extensions/diffs/src/viewer-assets.ts
index 83f1c0dc4af..7a7fc94f7d1 100644
--- a/extensions/diffs/src/viewer-assets.ts
+++ b/extensions/diffs/src/viewer-assets.ts
@@ -6,8 +6,10 @@ export const VIEWER_ASSET_PREFIX = "/plugins/diffs/assets/";
 export const VIEWER_LOADER_PATH = `${VIEWER_ASSET_PREFIX}viewer.js`;
 export const VIEWER_RUNTIME_PATH = `${VIEWER_ASSET_PREFIX}viewer-runtime.js`;
 const VIEWER_RUNTIME_RELATIVE_IMPORT_PATH = "./viewer-runtime.js";
-
-const VIEWER_RUNTIME_FILE_URL = new URL("../assets/viewer-runtime.js", import.meta.url);
+const VIEWER_RUNTIME_CANDIDATE_RELATIVE_PATHS = [
+  "./assets/viewer-runtime.js",
+  "../assets/viewer-runtime.js",
+] as const;
 
 export type ServedViewerAsset = {
   body: string | Buffer;
@@ -22,6 +24,43 @@ type RuntimeAssetCache = {
 
 let runtimeAssetCache: RuntimeAssetCache | null = null;
 
+type ViewerRuntimeFileUrlParams = {
+  baseUrl?: string | URL;
+  stat?: (path: string) => Promise<unknown>;
+};
+
+function isMissingFileError(error: unknown): error is NodeJS.ErrnoException {
+  return error instanceof Error && "code" in error && error.code === "ENOENT";
+}
+
+export async function resolveViewerRuntimeFileUrl(
+  params: ViewerRuntimeFileUrlParams = {},
+): Promise<URL> {
+  const baseUrl = params.baseUrl ?? import.meta.url;
+  const stat = params.stat ?? ((path: string) => fs.stat(path));
+  let missingFileError: NodeJS.ErrnoException | null = null;
+
+  for (const relativePath of VIEWER_RUNTIME_CANDIDATE_RELATIVE_PATHS) {
+    const candidateUrl = new URL(relativePath, baseUrl);
+    try {
+      await stat(fileURLToPath(candidateUrl));
+      return candidateUrl;
+    } catch (error) {
+      if (isMissingFileError(error)) {
+        missingFileError = error;
+        continue;
+      }
+      throw error;
+    }
+  }
+
+  if (missingFileError) {
+    throw missingFileError;
+  }
+
+  throw new Error("viewer runtime asset candidates were not checked");
+}
+
 export async function getServedViewerAsset(pathname: string): Promise<ServedViewerAsset | null> {
   if (pathname !== VIEWER_LOADER_PATH && pathname !== VIEWER_RUNTIME_PATH) {
     return null;
@@ -46,7 +85,8 @@ export async function getServedViewerAsset(pathname: string): Promise<ServedView
 }
 
 async function loadViewerAssets(): Promise<RuntimeAssetCache> {
-  const runtimePath = fileURLToPath(VIEWER_RUNTIME_FILE_URL);
+  const runtimeUrl = await resolveViewerRuntimeFileUrl();
+  const runtimePath = fileURLToPath(runtimeUrl);
   const runtimeStat = await fs.stat(runtimePath);
   if (runtimeAssetCache && runtimeAssetCache.mtimeMs === runtimeStat.mtimeMs) {
     return runtimeAssetCache;
diff --git a/scripts/release-check.ts b/scripts/release-check.ts
index a370068e610..9ae6e063ab0 100755
--- a/scripts/release-check.ts
+++ b/scripts/release-check.ts
@@ -11,6 +11,7 @@ import {
 } from "./lib/bundled-extension-manifest.ts";
 import { listBundledPluginPackArtifacts } from "./lib/bundled-plugin-build-entries.mjs";
 import { listPluginSdkDistArtifacts } from "./lib/plugin-sdk-entries.mjs";
+import { listStaticExtensionAssetOutputs } from "./runtime-postbuild.mjs";
 import { sparkleBuildFloorsFromShortVersion, type SparkleBuildFloors } from "./sparkle-build.ts";
 
 export { collectBundledExtensionManifestErrors } from "./lib/bundled-extension-manifest.ts";
@@ -23,6 +24,7 @@ const requiredPathGroups = [
   ["dist/entry.js", "dist/entry.mjs"],
   ...listPluginSdkDistArtifacts(),
   ...listBundledPluginPackArtifacts(),
+  ...listStaticExtensionAssetOutputs(),
   "scripts/npm-runner.mjs",
   "scripts/postinstall-bundled-plugins.mjs",
   "dist/plugin-sdk/compat.js",
@@ -169,7 +171,7 @@ export function collectMissingPackPaths(paths: Iterable<string>): string[] {
       }
       return available.has(group) ? [] : [group];
     })
-    .toSorted();
+    .toSorted((left, right) => left.localeCompare(right));
 }
 
 export function collectForbiddenPackPaths(paths: Iterable<string>): string[] {
diff --git a/scripts/runtime-postbuild.mjs b/scripts/runtime-postbuild.mjs
index 6c50667a00a..094e4165b35 100644
--- a/scripts/runtime-postbuild.mjs
+++ b/scripts/runtime-postbuild.mjs
@@ -25,8 +25,21 @@ export const STATIC_EXTENSION_ASSETS = [
     src: "extensions/acpx/src/runtime-internals/mcp-proxy.mjs",
     dest: "dist/extensions/acpx/mcp-proxy.mjs",
   },
+  // diffs viewer runtime bundle — co-deployed inside the plugin package so the
+  // built bundle can resolve `./assets/viewer-runtime.js` from dist.
+  {
+    src: "extensions/diffs/assets/viewer-runtime.js",
+    dest: "dist/extensions/diffs/assets/viewer-runtime.js",
+  },
 ];
 
+export function listStaticExtensionAssetOutputs(params = {}) {
+  const assets = params.assets ?? STATIC_EXTENSION_ASSETS;
+  return assets
+    .map(({ dest }) => dest.replace(/\\/g, "/"))
+    .toSorted((left, right) => left.localeCompare(right));
+}
+
 export function copyStaticExtensionAssets(params = {}) {
   const rootDir = params.rootDir ?? ROOT;
   const assets = params.assets ?? STATIC_EXTENSION_ASSETS;
diff --git a/test/release-check.test.ts b/test/release-check.test.ts
index 90ec2b79b03..912ec135c7a 100644
--- a/test/release-check.test.ts
+++ b/test/release-check.test.ts
@@ -263,6 +263,7 @@ describe("collectMissingPackPaths", () => {
         "dist/control-ui/index.html",
         "scripts/npm-runner.mjs",
         "scripts/postinstall-bundled-plugins.mjs",
+        bundledDistPluginFile("diffs", "assets/viewer-runtime.js"),
         bundledDistPluginFile("matrix", "helper-api.js"),
         bundledDistPluginFile("matrix", "runtime-api.js"),
         bundledDistPluginFile("matrix", "thread-bindings-runtime.js"),
@@ -282,6 +283,8 @@ describe("collectMissingPackPaths", () => {
         "dist/index.js",
         "dist/entry.js",
         "dist/control-ui/index.html",
+        "dist/extensions/acpx/mcp-proxy.mjs",
+        bundledDistPluginFile("diffs", "assets/viewer-runtime.js"),
         ...requiredBundledPluginPackPaths,
         ...requiredPluginSdkPackPaths,
         "scripts/npm-runner.mjs",
diff --git a/test/scripts/runtime-postbuild.test.ts b/test/scripts/runtime-postbuild.test.ts
index 81d3a325580..bab69773634 100644
--- a/test/scripts/runtime-postbuild.test.ts
+++ b/test/scripts/runtime-postbuild.test.ts
@@ -4,6 +4,7 @@ import path from "node:path";
 import { afterEach, describe, expect, it, vi } from "vitest";
 import {
   copyStaticExtensionAssets,
+  listStaticExtensionAssetOutputs,
   writeStableRootRuntimeAliases,
 } from "../../scripts/runtime-postbuild.mjs";
 
@@ -22,6 +23,12 @@ async function createTempRoot() {
 }
 
 describe("runtime postbuild static assets", () => {
+  it("tracks plugin-owned static assets that release packaging must ship", () => {
+    expect(listStaticExtensionAssetOutputs()).toContain(
+      "dist/extensions/diffs/assets/viewer-runtime.js",
+    );
+  });
+
   it("copies declared static assets into dist", async () => {
     const rootDir = await createTempRoot();
     const src = "extensions/acpx/src/runtime-internals/mcp-proxy.mjs";