diff --git a/AGENTS.md b/AGENTS.md
index bdc9ab53f..59712966b 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -123,7 +123,7 @@ Fabro is an AI-powered workflow orchestration platform. Workflows are defined as
 - **fabro-util** — Shared utilities (redaction, terminal formatting)
 
 ### TypeScript (`apps/` and `lib/packages/`)
-- **apps/fabro-web** — React 19 + React Router + Vite + Tailwind CSS frontend
+- **apps/fabro-web** — React 19 + React Router + Tailwind CSS frontend, bundled by a custom Bun script (`apps/fabro-web/scripts/build.ts`), not Vite
 - **lib/packages/fabro-api-client** — Auto-generated TypeScript Axios client from OpenAPI spec
 
 ### Key design patterns
diff --git a/apps/fabro-web/app/components/playground/canvas/use-canvas-render.ts b/apps/fabro-web/app/components/playground/canvas/use-canvas-render.ts
index 688f27d8b..d11b7bb67 100644
--- a/apps/fabro-web/app/components/playground/canvas/use-canvas-render.ts
+++ b/apps/fabro-web/app/components/playground/canvas/use-canvas-render.ts
@@ -1,5 +1,7 @@
 import { useEffect, useRef, useState, type RefObject } from "react";
 
+import { importChunk } from "../../../lib/import-chunk";
+
 /**
  * Synchronizes a DOM container with a Graphviz-rendered SVG. Pipes the
  * supplied DOT string through `@viz-js/viz` (the same layout engine Fabro
@@ -28,7 +30,7 @@ export function useCanvasRender(
     let cancelled = false;
     (async () => {
       try {
-        const { instance } = await import("@viz-js/viz");
+        const { instance } = await importChunk(() => import("@viz-js/viz"));
         const viz = await instance();
         if (cancelled) return;
         const svg = viz.renderSVGElement(dot);
diff --git a/apps/fabro-web/app/components/toast.tsx b/apps/fabro-web/app/components/toast.tsx
index afa57fa90..02bfe9294 100644
--- a/apps/fabro-web/app/components/toast.tsx
+++ b/apps/fabro-web/app/components/toast.tsx
@@ -9,10 +9,17 @@ import { Toaster as SonnerToaster, toast as sonnerToast, useSonner } from "sonne
 
 export type ToastTone = "info" | "error";
 
+export interface ToastAction {
+  label: string;
+  onClick: () => void;
+}
+
 export interface ToastInput {
   message: string;
   tone?: ToastTone;
+  /** Pass `Infinity` for a toast that stays until dismissed or acted on. */
   autoDismissMs?: number;
+  action?: ToastAction;
 }
 
 interface ToastContextValue {
@@ -27,6 +34,7 @@ function push(toast: ToastInput): string {
   const id = `toast-${nextToastId++}`;
   const options = {
     id,
+    ...(toast.action ? { action: toast.action } : {}),
     ...(toast.tone === "error"
       ? { duration: Infinity }
       : toast.autoDismissMs != null
@@ -55,9 +63,9 @@ const toastApi: ToastContextValue = {
 /**
  * No-op wrapper retained so existing test harnesses and the standalone terminal
  * route can keep their <ToastProvider> mount points. In a browser the real
- * <Toaster /> is mounted globally in AppShell; in non-DOM test environments we
- * render an aria-live fallback that subscribes to the Sonner store so test
- * assertions can read the toast text.
+ * <Toaster /> is mounted globally at the entry point; in non-DOM test
+ * environments we render an aria-live fallback that subscribes to the Sonner
+ * store so test assertions can read the toast text.
  */
 export function ToastProvider({ children }: { children: ReactNode }) {
   if (typeof document !== "undefined") {
diff --git a/apps/fabro-web/app/entry.tsx b/apps/fabro-web/app/entry.tsx
index ff9d681f9..a3488b680 100644
--- a/apps/fabro-web/app/entry.tsx
+++ b/apps/fabro-web/app/entry.tsx
@@ -2,6 +2,8 @@ import { StrictMode } from "react";
 import { createRoot } from "react-dom/client";
 import { createBrowserRouter, RouterProvider } from "react-router";
 import { SWRConfig } from "swr";
+import { FabroToaster } from "./components/toast";
+import { useBuildVersionGuard } from "./hooks/use-build-version-guard";
 import { installRoutes } from "./install-router";
 import { resolveFabroMode } from "./mode";
 import { routes } from "./router";
@@ -21,6 +23,17 @@ if (!rootElement) {
   throw new Error("Missing #root element");
 }
 
+function AppRuntime() {
+  useBuildVersionGuard();
+
+  return (
+    <>
+      <RouterProvider router={router} />
+      <FabroToaster />
+    </>
+  );
+}
+
 createRoot(rootElement).render(
   <StrictMode>
     <SWRConfig
@@ -30,7 +43,7 @@ createRoot(rootElement).render(
         shouldRetryOnError: false,
       }}
     >
-      <RouterProvider router={router} />
+      <AppRuntime />
     </SWRConfig>
   </StrictMode>,
 );
diff --git a/apps/fabro-web/app/hooks/dynamic-import-errors.test.tsx b/apps/fabro-web/app/hooks/dynamic-import-errors.test.tsx
new file mode 100644
index 000000000..1add46d1b
--- /dev/null
+++ b/apps/fabro-web/app/hooks/dynamic-import-errors.test.tsx
@@ -0,0 +1,95 @@
+import { afterEach, beforeEach, expect, mock, test } from "bun:test";
+import { useState } from "react";
+import TestRenderer, { act } from "react-test-renderer";
+
+import { setupReactTestEnv } from "../lib/test-utils";
+import type {
+  ConnectionStatus,
+  TerminalConnectionError,
+} from "./use-terminal-session";
+
+const importFailure = new Error("chunk unavailable");
+
+mock.module("../lib/import-chunk", () => ({
+  importChunk: async () => {
+    throw importFailure;
+  },
+}));
+
+const [{ useRenderedVizDiagram }, { useTerminalSession }] = await Promise.all([
+  import("./use-rendered-viz-diagram"),
+  import("./use-terminal-session"),
+]);
+
+let renderer: TestRenderer.ReactTestRenderer | null = null;
+let restoreReactTestEnv = () => {};
+
+beforeEach(() => {
+  restoreReactTestEnv = setupReactTestEnv();
+});
+
+afterEach(() => {
+  act(() => {
+    renderer?.unmount();
+  });
+  renderer = null;
+  restoreReactTestEnv();
+});
+
+async function renderAndFlush(element: React.ReactElement) {
+  await act(async () => {
+    renderer = TestRenderer.create(element);
+    await Promise.resolve();
+    await Promise.resolve();
+  });
+}
+
+const diagramContainerRef = { current: null };
+const diagramSvgRef = { current: null };
+const buildDot = () => "digraph {}";
+let diagramError: string | null = null;
+
+function DiagramHost() {
+  diagramError = useRenderedVizDiagram({
+    buildDot,
+    innerRef: diagramContainerRef,
+    identity: "diagram",
+    svgRef: diagramSvgRef,
+  });
+  return null;
+}
+
+test("diagram import failures populate the hook error instead of rejecting", async () => {
+  await renderAndFlush(<DiagramHost />);
+  expect(diagramError).toBe("chunk unavailable");
+});
+
+const terminalElementRef = {
+  current: {} as HTMLDivElement,
+};
+let terminalError: TerminalConnectionError | null = null;
+let terminalStatus: ConnectionStatus = "closed";
+
+function TerminalHost() {
+  const [error, setError] = useState<TerminalConnectionError | null>(null);
+  const [status, setStatus] = useState<ConnectionStatus>("closed");
+  terminalError = error;
+  terminalStatus = status;
+  useTerminalSession({
+    connectionKey: 0,
+    runId: "run_1",
+    setError,
+    setStatus,
+    terminalEl: terminalElementRef,
+  });
+  return null;
+}
+
+test("terminal import failures leave a recoverable error state", async () => {
+  await renderAndFlush(<TerminalHost />);
+  expect(terminalStatus).toBe("error");
+  expect(terminalError).toEqual({
+    message:     "Terminal initialization failed: chunk unavailable",
+    recoverable: true,
+  });
+});
diff --git a/apps/fabro-web/app/hooks/use-build-version-guard.test.tsx b/apps/fabro-web/app/hooks/use-build-version-guard.test.tsx
new file mode 100644
index 000000000..bec6a7d36
--- /dev/null
+++ b/apps/fabro-web/app/hooks/use-build-version-guard.test.tsx
@@ -0,0 +1,132 @@
+import { afterEach, beforeEach, expect, mock, test } from "bun:test";
+import TestRenderer, { act } from "react-test-renderer";
+import { toast as sonnerToast } from "sonner";
+
+import { setupReactTestEnv } from "../lib/test-utils";
+
+const loadedBuildId = "aaaaaaaa";
+let latestBuildId: string | null = loadedBuildId;
+
+mock.module("../lib/build-version", () => ({
+  documentBuildId: () => loadedBuildId,
+  isStaleBuild: (loaded: string | null, latest: string | null) =>
+    loaded != null && latest != null && loaded !== latest,
+  useLatestBuildId: () => latestBuildId,
+}));
+
+const { useBuildVersionGuard } = await import("./use-build-version-guard");
+
+let renderer: TestRenderer.ReactTestRenderer | null = null;
+let restoreReactTestEnv = () => {};
+let reloads = 0;
+let previousWindow: unknown;
+let hadWindow = false;
+let previousRequestAnimationFrame: typeof requestAnimationFrame | undefined;
+
+function GuardHost({ revision }: { revision: number }) {
+  void revision;
+  useBuildVersionGuard();
+  return null;
+}
+
+function activeToasts() {
+  return sonnerToast.getToasts().filter((toast) => !toast.delete);
+}
+
+function renderRevision(revision: number) {
+  act(() => {
+    if (renderer) {
+      renderer.update(<GuardHost revision={revision} />);
+    } else {
+      renderer = TestRenderer.create(<GuardHost revision={revision} />);
+    }
+  });
+}
+
+beforeEach(() => {
+  restoreReactTestEnv = setupReactTestEnv();
+  latestBuildId = loadedBuildId;
+  reloads = 0;
+  hadWindow = "window" in globalThis;
+  previousWindow = (globalThis as { window?: unknown }).window;
+  previousRequestAnimationFrame = globalThis.requestAnimationFrame;
+  globalThis.requestAnimationFrame = (callback) =>
+    setTimeout(callback, 0) as unknown as number;
+  Object.defineProperty(globalThis, "window", {
+    value: {
+      location: {
+        reload: () => {
+          reloads += 1;
+        },
+      },
+    },
+    writable:     true,
+    configurable: true,
+  });
+});
+
+afterEach(() => {
+  act(() => {
+    renderer?.unmount();
+  });
+  renderer = null;
+  sonnerToast.dismiss();
+  if (hadWindow) {
+    Object.defineProperty(globalThis, "window", {
+      value:        previousWindow,
+      writable:     true,
+      configurable: true,
+    });
+  } else {
+    delete (globalThis as { window?: unknown }).window;
+  }
+  if (previousRequestAnimationFrame) {
+    globalThis.requestAnimationFrame = previousRequestAnimationFrame;
+  } else {
+    delete (globalThis as { requestAnimationFrame?: typeof requestAnimationFrame })
+      .requestAnimationFrame;
+  }
+  restoreReactTestEnv();
+});
+
+test("keeps one actionable prompt synchronized with the latest stale build", () => {
+  renderRevision(0);
+  expect(activeToasts()).toHaveLength(0);
+
+  latestBuildId = "bbbbbbbb";
+  renderRevision(1);
+  const firstPrompt = activeToasts();
+  expect(firstPrompt).toHaveLength(1);
+  expect(firstPrompt[0]).toMatchObject({
+    duration: Infinity,
+    title:    "A new version of Fabro is available.",
+  });
+
+  const action = firstPrompt[0]?.action;
+  expect(action).toMatchObject({ label: "Reload" });
+  if (action && typeof action === "object" && "onClick" in action) {
+    action.onClick({} as never);
+  }
+  expect(reloads).toBe(1);
+
+  // Re-rendering the same poll result neither re-nags nor replaces the toast.
+  renderRevision(2);
+  expect(activeToasts()).toHaveLength(1);
+  expect(activeToasts()[0]?.id).toBe(firstPrompt[0]?.id);
+
+  // Later deploys replace the active prompt instead of accumulating forever.
+  latestBuildId = "cccccccc";
+  renderRevision(3);
+  expect(activeToasts()).toHaveLength(1);
+  expect(activeToasts()[0]?.id).not.toBe(firstPrompt[0]?.id);
+
+  latestBuildId = "bbbbbbbb";
+  renderRevision(4);
+  expect(activeToasts()).toHaveLength(1);
+
+  // If the server rolls back to the document's build, the claim is no longer
+  // true and the prompt disappears.
+  latestBuildId = loadedBuildId;
+  renderRevision(5);
+  expect(activeToasts()).toHaveLength(0);
+});
diff --git a/apps/fabro-web/app/hooks/use-build-version-guard.ts b/apps/fabro-web/app/hooks/use-build-version-guard.ts
new file mode 100644
index 000000000..31367c164
--- /dev/null
+++ b/apps/fabro-web/app/hooks/use-build-version-guard.ts
@@ -0,0 +1,57 @@
+import { useEffect, useRef, useState } from "react";
+
+import { useToast } from "../components/toast";
+import {
+  documentBuildId,
+  isStaleBuild,
+  useLatestBuildId,
+} from "../lib/build-version";
+
+const NEW_VERSION_MESSAGE = "A new version of Fabro is available.";
+
+/**
+ * Synchronizes a reload prompt with the build id the server is publishing.
+ *
+ * Client-side routing never re-fetches `index.html`, and hashed bundles are
+ * served `immutable`, so a tab left open across a deploy keeps running the
+ * previous build's JavaScript indefinitely with nothing to reveal it. This
+ * offers a reload when that happens; it never reloads on its own.
+ */
+export function useBuildVersionGuard(): void {
+  const { dismiss, push } = useToast();
+  // Read once. It describes the document this tab loaded, which cannot change
+  // without a full page load — and that remounts the hook anyway.
+  const [loadedBuildId] = useState<string | null>(documentBuildId);
+  const latestBuildId = useLatestBuildId();
+  const promptRef = useRef<{ buildId: string; toastId: string } | null>(null);
+
+  const stale = isStaleBuild(loadedBuildId, latestBuildId);
+
+  useEffect(() => {
+    if (!latestBuildId) return;
+    if (!stale) {
+      // A rollback to the document's own build makes an existing prompt false.
+      if (promptRef.current) {
+        dismiss(promptRef.current.toastId);
+        promptRef.current = null;
+      }
+      return;
+    }
+    if (promptRef.current?.buildId === latestBuildId) return;
+    if (promptRef.current) {
+      dismiss(promptRef.current.toastId);
+    }
+
+    const toastId = push({
+      message: NEW_VERSION_MESSAGE,
+      // Persistent by design: a prompt that vanishes after a few seconds is one
+      // the user will miss, which is the whole failure this exists to fix.
+      autoDismissMs: Infinity,
+      action: {
+        label: "Reload",
+        onClick: () => window.location.reload(),
+      },
+    });
+    promptRef.current = { buildId: latestBuildId, toastId };
+  }, [dismiss, latestBuildId, push, stale]);
+}
diff --git a/apps/fabro-web/app/hooks/use-rendered-viz-diagram.ts b/apps/fabro-web/app/hooks/use-rendered-viz-diagram.ts
index 9733f335e..b9627b712 100644
--- a/apps/fabro-web/app/hooks/use-rendered-viz-diagram.ts
+++ b/apps/fabro-web/app/hooks/use-rendered-viz-diagram.ts
@@ -1,5 +1,7 @@
 import { useEffect, useState } from "react";
 
+import { importChunk } from "../lib/import-chunk";
+
 /**
  * Synchronizes a DOT source with the imperative @viz-js SVG renderer and a DOM
  * container. Async renders are ignored after identity changes or unmount.
@@ -26,12 +28,13 @@ export function useRenderedVizDiagram<TIdentity>({
 
     async function render() {
       setError(null);
-      onRenderStart?.();
-      const { instance } = await import("@viz-js/viz");
-      const viz = await instance();
-      if (cancelled) return;
 
       try {
+        onRenderStart?.();
+        const { instance } = await importChunk(() => import("@viz-js/viz"));
+        const viz = await instance();
+        if (cancelled) return;
+
         const svg = viz.renderSVGElement(buildDot(identity));
         prepareSvg?.(svg);
 
@@ -40,7 +43,9 @@ export function useRenderedVizDiagram<TIdentity>({
           innerRef.current.replaceChildren(svg);
         }
       } catch (e) {
-        setError(e instanceof Error ? e.message : "Failed to render diagram");
+        if (!cancelled) {
+          setError(e instanceof Error ? e.message : "Failed to render diagram");
+        }
       }
     }
 
diff --git a/apps/fabro-web/app/hooks/use-terminal-session.ts b/apps/fabro-web/app/hooks/use-terminal-session.ts
index c8476f4e8..963f00a0d 100644
--- a/apps/fabro-web/app/hooks/use-terminal-session.ts
+++ b/apps/fabro-web/app/hooks/use-terminal-session.ts
@@ -6,6 +6,7 @@ import {
   buildTerminalWebSocketUrl,
   parseTerminalServerMessage,
 } from "../components/terminal-view-helpers";
+import { importChunk } from "../lib/import-chunk";
 
 export type ConnectionStatus = "connecting" | "ready" | "closed" | "error";
 
@@ -88,13 +89,32 @@ export function useTerminalSession({
     const textEncoder = new TextEncoder();
     const disposables: Array<{ dispose: () => void }> = [];
 
+    function disposeResources() {
+      resizeObserver?.disconnect();
+      resizeObserver = null;
+      for (const disposable of disposables.splice(0)) disposable.dispose();
+
+      const socket = socketRef.current;
+      if (socket) {
+        if (socket.readyState === WebSocket.OPEN) {
+          socket.send(JSON.stringify({ type: "close" }));
+        }
+        socket.close();
+        socketRef.current = null;
+      }
+
+      terminalRef.current?.dispose();
+      terminalRef.current = null;
+      fitRef.current = null;
+    }
+
     async function connect() {
       setStatus("connecting");
       setError(null);
 
       const [{ Terminal }, { FitAddon }] = await Promise.all([
-        import("@xterm/xterm"),
-        import("@xterm/addon-fit"),
+        importChunk(() => import("@xterm/xterm")),
+        importChunk(() => import("@xterm/addon-fit")),
       ]);
       if (disposed || !terminalEl.current) return;
 
@@ -108,12 +128,12 @@ export function useTerminalSession({
         theme: TERMINAL_THEME,
       });
       const fitAddon = new FitAddon();
+      terminalRef.current = terminal;
+      fitRef.current = fitAddon;
       terminal.loadAddon(fitAddon);
       terminal.open(terminalEl.current);
       fitAddon.fit();
       terminal.focus();
-      terminalRef.current = terminal;
-      fitRef.current = fitAddon;
 
       const socket = new WebSocket(buildTerminalWebSocketUrl(window.location, runId));
       socket.binaryType = "arraybuffer";
@@ -190,18 +210,21 @@ export function useTerminalSession({
       }
     }
 
-    void connect();
+    void connect().catch((error: unknown) => {
+      if (disposed) return;
+      disposeResources();
+      setStatus("error");
+      setError({
+        message: error instanceof Error
+          ? `Terminal initialization failed: ${error.message}`
+          : "Terminal initialization failed.",
+        recoverable: true,
+      });
+    });
 
     return () => {
       disposed = true;
-      resizeObserver?.disconnect();
-      for (const disposable of disposables) disposable.dispose();
-      socketRef.current?.send(JSON.stringify({ type: "close" }));
-      socketRef.current?.close();
-      socketRef.current = null;
-      terminalRef.current?.dispose();
-      terminalRef.current = null;
-      fitRef.current = null;
+      disposeResources();
     };
   }, [connectionKey, runId, setError, setStatus, terminalEl]);
 }
diff --git a/apps/fabro-web/app/layouts/app-shell.tsx b/apps/fabro-web/app/layouts/app-shell.tsx
index c8a758018..6b610f941 100644
--- a/apps/fabro-web/app/layouts/app-shell.tsx
+++ b/apps/fabro-web/app/layouts/app-shell.tsx
@@ -12,7 +12,6 @@ import {
   XMarkIcon,
 } from "@heroicons/react/24/outline";
 import { Link, Outlet, useLocation, useMatches } from "react-router";
-import { FabroToaster } from "../components/toast";
 import { ErrorState } from "../components/state";
 import { TooltipProvider } from "../components/ui";
 import { DemoModeProvider } from "../lib/demo-mode";
@@ -248,9 +247,6 @@ export default function AppShell() {
       )}
       <ShellMain fullHeight={fullHeight} maxWidth={maxWidth} />
     </div>
-    {typeof document !== "undefined" && (
-      <FabroToaster />
-    )}
     </TooltipProvider>
     </DemoModeProvider>
   );
diff --git a/apps/fabro-web/app/lib/build-version-contract.ts b/apps/fabro-web/app/lib/build-version-contract.ts
new file mode 100644
index 000000000..f9fb07059
--- /dev/null
+++ b/apps/fabro-web/app/lib/build-version-contract.ts
@@ -0,0 +1,22 @@
+import { getString } from "./unknown";
+
+export const BUILD_ID_FILE_NAME = "build-id.json";
+export const BUILD_ID_URL = `/${BUILD_ID_FILE_NAME}`;
+export const BUILD_ID_META_NAME = "fabro-build-id";
+export const BUILD_ID_FIELD = "buildId";
+
+const BUILD_ID_PATTERN = /^[a-z0-9]{8}$/;
+
+export function parseBuildId(value: unknown): string | null {
+  if (typeof value !== "string") return null;
+  const normalized = value.trim();
+  return BUILD_ID_PATTERN.test(normalized) ? normalized : null;
+}
+
+export function parseBuildIdDocument(value: unknown): string | null {
+  return parseBuildId(getString(value, BUILD_ID_FIELD));
+}
+
+export function buildIdDocument(buildId: string): Record<typeof BUILD_ID_FIELD, string> {
+  return { [BUILD_ID_FIELD]: buildId };
+}
diff --git a/apps/fabro-web/app/lib/build-version.test.ts b/apps/fabro-web/app/lib/build-version.test.ts
new file mode 100644
index 000000000..59f10f9de
--- /dev/null
+++ b/apps/fabro-web/app/lib/build-version.test.ts
@@ -0,0 +1,73 @@
+import { afterEach, describe, expect, test } from "bun:test";
+
+import { fetchBuildId, isStaleBuild } from "./build-version";
+import { parseBuildId } from "./build-version-contract";
+
+describe("isStaleBuild", () => {
+  test("reports stale only when both ids are known and differ", () => {
+    expect(isStaleBuild("aaaaaaaa", "bbbbbbbb")).toBe(true);
+    expect(isStaleBuild("aaaaaaaa", "aaaaaaaa")).toBe(false);
+  });
+
+  // A false "new version" claim is worse than a missed one: it trains people to
+  // ignore the toast. Anything unknown must stay silent.
+  test("stays silent when either side is unknown", () => {
+    expect(isStaleBuild(null, "bbbbbbbb")).toBe(false);
+    expect(isStaleBuild("aaaaaaaa", null)).toBe(false);
+    expect(isStaleBuild(null, null)).toBe(false);
+    expect(isStaleBuild("", "bbbbbbbb")).toBe(false);
+  });
+});
+
+describe("fetchBuildId", () => {
+  const realFetch = globalThis.fetch;
+  afterEach(() => {
+    globalThis.fetch = realFetch;
+  });
+
+  function stubFetch(response: Response) {
+    globalThis.fetch = async () => response;
+  }
+
+  test("returns the published build id", async () => {
+    stubFetch(Response.json({ buildId: "8f2yqj8q" }));
+    expect(await fetchBuildId("/build-id.json")).toBe("8f2yqj8q");
+  });
+
+  test("returns null for a non-ok response", async () => {
+    stubFetch(new Response(null, { status: 503 }));
+    expect(await fetchBuildId("/build-id.json")).toBeNull();
+  });
+
+  // A server that returns something unexpected must not be read as "a new
+  // build shipped" — that would fire the toast on every poll.
+  test("returns null for a malformed body", async () => {
+    stubFetch(Response.json({ buildId: 42 }));
+    expect(await fetchBuildId("/build-id.json")).toBeNull();
+
+    stubFetch(Response.json({}));
+    expect(await fetchBuildId("/build-id.json")).toBeNull();
+
+    stubFetch(Response.json(null));
+    expect(await fetchBuildId("/build-id.json")).toBeNull();
+
+    stubFetch(Response.json({ buildId: "" }));
+    expect(await fetchBuildId("/build-id.json")).toBeNull();
+
+    stubFetch(Response.json({ buildId: "not-a-build-id" }));
+    expect(await fetchBuildId("/build-id.json")).toBeNull();
+
+    stubFetch(new Response("<!doctype html>"));
+    expect(await fetchBuildId("/build-id.json")).toBeNull();
+  });
+});
+
+describe("parseBuildId", () => {
+  test("normalizes valid ids and rejects values outside the wire format", () => {
+    expect(parseBuildId(" 8f2yqj8q ")).toBe("8f2yqj8q");
+    expect(parseBuildId("        ")).toBeNull();
+    expect(parseBuildId("8F2YQJ8Q")).toBeNull();
+    expect(parseBuildId("abc123")).toBeNull();
+    expect(parseBuildId(null)).toBeNull();
+  });
+});
diff --git a/apps/fabro-web/app/lib/build-version.ts b/apps/fabro-web/app/lib/build-version.ts
new file mode 100644
index 000000000..36cf54fc5
--- /dev/null
+++ b/apps/fabro-web/app/lib/build-version.ts
@@ -0,0 +1,68 @@
+import useSWR from "swr";
+
+import {
+  BUILD_ID_META_NAME,
+  BUILD_ID_URL,
+  parseBuildId,
+  parseBuildIdDocument,
+} from "./build-version-contract";
+
+/**
+ * How often a visible tab re-checks. SWR does not poll while the document is
+ * hidden (`refreshWhenHidden` defaults to false), so background tabs stay
+ * silent without any extra gating, and a hidden tab revalidates on focus.
+ */
+const POLL_INTERVAL_MS = 60_000;
+
+/**
+ * The build this document loaded, from the meta tag `scripts/build.ts` writes
+ * into `index.html`.
+ *
+ * The meta tag is the honest source for "what is this tab running": client-side
+ * routing never re-fetches `index.html`, so it stays pinned to the build the
+ * tab actually started with, however long the tab lives.
+ */
+export function documentBuildId(): string | null {
+  if (typeof document === "undefined") return null;
+  const content = document
+    .querySelector(`meta[name="${BUILD_ID_META_NAME}"]`)
+    ?.getAttribute("content");
+  return parseBuildId(content);
+}
+
+export async function fetchBuildId(url: string): Promise<string | null> {
+  // Served `no-cache` with an ETag, so the browser revalidates and normally
+  // gets a 304 rather than a fresh body.
+  const response = await fetch(url);
+  if (!response.ok) return null;
+  const body: unknown = await response.json().catch(() => null);
+  return parseBuildIdDocument(body);
+}
+
+/**
+ * True only when the running document is provably behind what the server is
+ * serving now.
+ *
+ * Unknown on either side means "claim nothing". A missing meta tag (a build
+ * predating this feature, or a non-DOM test environment) or a failed fetch must
+ * never produce a reload prompt — a false "new version" claim is worse than a
+ * missed one, because it teaches people to ignore the real ones.
+ */
+export function isStaleBuild(
+  loaded: string | null,
+  latest: string | null,
+): boolean {
+  if (!loaded || !latest) return false;
+  return loaded !== latest;
+}
+
+/** Synchronizes React with the build id the server is currently publishing. */
+export function useLatestBuildId(): string | null {
+  const { data } = useSWR(BUILD_ID_URL, fetchBuildId, {
+    refreshInterval: POLL_INTERVAL_MS,
+    revalidateOnFocus: true,
+    // A failed check is not worth retry storms; the next poll covers it.
+    shouldRetryOnError: false,
+  });
+  return data ?? null;
+}
diff --git a/apps/fabro-web/app/lib/import-chunk.test.ts b/apps/fabro-web/app/lib/import-chunk.test.ts
new file mode 100644
index 000000000..06a24ee5e
--- /dev/null
+++ b/apps/fabro-web/app/lib/import-chunk.test.ts
@@ -0,0 +1,170 @@
+import { afterEach, describe, expect, test } from "bun:test";
+
+import { importChunk } from "./import-chunk";
+
+interface WindowStub {
+  buildId: string | null;
+  reloads: number;
+  store: Map<string, string>;
+  restore: () => void;
+}
+
+/**
+ * bun:test runs without a DOM, so install a `window` carrying just the surface
+ * `importChunk` touches. Follows the descriptor save/restore pattern used by
+ * stage-insights-sidebar.test.tsx so other test files can install their own.
+ */
+function installWindow({
+  buildId = "aaaaaaaa",
+  throwOnStorage = false,
+}: {
+  buildId?: string | null;
+  throwOnStorage?: boolean;
+} = {}): WindowStub {
+  const store = new Map<string, string>();
+  const stub: WindowStub = {
+    buildId,
+    reloads: 0,
+    store,
+    restore: () => undefined,
+  };
+
+  const windowStub = {
+    sessionStorage: {
+      getItem: (key: string) => {
+        if (throwOnStorage) throw new Error("storage disabled");
+        return store.get(key) ?? null;
+      },
+      setItem: (key: string, value: string) => {
+        if (throwOnStorage) throw new Error("storage disabled");
+        store.set(key, value);
+      },
+    },
+    location: {
+      reload: () => {
+        stub.reloads += 1;
+      },
+    },
+  };
+
+  const had = "window" in globalThis;
+  const prev = (globalThis as { window?: unknown }).window;
+  const hadDocument = "document" in globalThis;
+  const previousDocument = (globalThis as { document?: unknown }).document;
+  Object.defineProperty(globalThis, "window", {
+    value:        windowStub,
+    writable:     true,
+    configurable: true,
+  });
+  Object.defineProperty(globalThis, "document", {
+    value: {
+      querySelector: () => stub.buildId == null
+        ? null
+        : {
+            getAttribute: () => stub.buildId,
+          },
+    },
+    writable:     true,
+    configurable: true,
+  });
+  stub.restore = () => {
+    if (had) {
+      Object.defineProperty(globalThis, "window", {
+        value:        prev,
+        writable:     true,
+        configurable: true,
+      });
+    } else {
+      delete (globalThis as { window?: unknown }).window;
+    }
+    if (hadDocument) {
+      Object.defineProperty(globalThis, "document", {
+        value:        previousDocument,
+        writable:     true,
+        configurable: true,
+      });
+    } else {
+      delete (globalThis as { document?: unknown }).document;
+    }
+  };
+  return stub;
+}
+
+let installed: WindowStub | null = null;
+afterEach(() => {
+  installed?.restore();
+  installed = null;
+});
+
+describe("importChunk", () => {
+  test("passes a successful import through untouched", async () => {
+    installed = installWindow();
+    await expect(importChunk(async () => "loaded")).resolves.toBe("loaded");
+    expect(installed.reloads).toBe(0);
+  });
+
+  test("reloads once and rethrows when a chunk fails to load", async () => {
+    installed = installWindow();
+    const failure = new Error("Failed to fetch dynamically imported module");
+
+    await expect(
+      importChunk(async () => {
+        throw failure;
+      }),
+    ).rejects.toThrow(failure);
+
+    expect(installed.reloads).toBe(1);
+  });
+
+  // Without this, a chunk that fails for a reason a reload cannot fix would
+  // reload forever.
+  test("does not reload again for the same build", async () => {
+    installed = installWindow();
+    const load = async () => {
+      throw new Error("Failed to fetch dynamically imported module");
+    };
+
+    await expect(importChunk(load)).rejects.toThrow();
+    await expect(importChunk(load)).rejects.toThrow();
+    await expect(importChunk(load)).rejects.toThrow();
+
+    expect(installed.reloads).toBe(1);
+  });
+
+  // The marker is keyed by build id, so a tab that recovers from one deploy
+  // still has a reload available for the next.
+  test("allows one recovery reload for each loaded build id", async () => {
+    installed = installWindow();
+    const load = async () => {
+      throw new Error("boom");
+    };
+
+    await expect(importChunk(load)).rejects.toThrow();
+    await expect(importChunk(load)).rejects.toThrow();
+    expect(installed.reloads).toBe(1);
+
+    installed.buildId = "bbbbbbbb";
+    await expect(importChunk(load)).rejects.toThrow();
+    await expect(importChunk(load)).rejects.toThrow();
+
+    expect(installed.reloads).toBe(2);
+    expect([...installed.store.keys()]).toEqual([
+      "fabro:chunk-reload:aaaaaaaa",
+      "fabro:chunk-reload:bbbbbbbb",
+    ]);
+  });
+
+  // No durable marker means no way to promise "only once", and a reload loop is
+  // far worse than a surfaced error.
+  test("does not reload when session storage is unavailable", async () => {
+    installed = installWindow({ throwOnStorage: true });
+
+    await expect(
+      importChunk(async () => {
+        throw new Error("boom");
+      }),
+    ).rejects.toThrow();
+
+    expect(installed.reloads).toBe(0);
+  });
+});
diff --git a/apps/fabro-web/app/lib/import-chunk.ts b/apps/fabro-web/app/lib/import-chunk.ts
new file mode 100644
index 000000000..fe56eaaf9
--- /dev/null
+++ b/apps/fabro-web/app/lib/import-chunk.ts
@@ -0,0 +1,54 @@
+import { documentBuildId } from "./build-version";
+
+const RELOAD_MARKER_PREFIX = "fabro:chunk-reload:";
+
+/**
+ * Loads a lazily-imported chunk, reloading the page once if it cannot be
+ * fetched.
+ *
+ * Each deploy replaces the served assets and the previous build's hashed
+ * filenames stop existing, so a tab open across a deploy can request a chunk
+ * that now 404s. Static route imports mean most of the graph is already in
+ * memory, but the handful of genuinely lazy imports — the terminal, Graphviz
+ * rendering, the file tree — are loaded on demand and can land in that window.
+ *
+ * A failed chunk means the feature is already broken, so reloading is recovery
+ * rather than an interruption. This is the one place the app reloads without an
+ * explicit click; the build-version toast never does.
+ */
+export function importChunk<T>(load: () => Promise<T>): Promise<T> {
+  return load().catch((error: unknown) => {
+    reloadOnceForStaleChunk();
+    // Rethrow rather than returning a never-settling promise. The reload
+    // normally replaces the document before this surfaces; if it doesn't, an
+    // error boundary is a better outcome than a spinner that hangs forever.
+    throw error;
+  });
+}
+
+/**
+ * Reloads at most once per build.
+ *
+ * Keyed by build id rather than a bare flag so a tab that recovers from one
+ * deploy still has a reload available for the next one. Without the key, a
+ * single chunk failure would disarm the backstop for the rest of the session.
+ *
+ * A module that loads fine but throws while evaluating is indistinguishable
+ * here from a missing chunk, so it also spends the reload. The per-build key
+ * bounds the cost at one wasted reload, after which the real error surfaces.
+ */
+function reloadOnceForStaleChunk(): void {
+  if (typeof window === "undefined") return;
+
+  const key = `${RELOAD_MARKER_PREFIX}${documentBuildId() ?? "unknown"}`;
+  try {
+    if (window.sessionStorage.getItem(key)) return;
+    window.sessionStorage.setItem(key, "1");
+  } catch {
+    // Storage disabled or full. Without a durable marker we can't guarantee
+    // "only once", and a reload loop is far worse than a surfaced error.
+    return;
+  }
+
+  window.location.reload();
+}
diff --git a/apps/fabro-web/app/routes/run-files.tsx b/apps/fabro-web/app/routes/run-files.tsx
index a2f3a1a25..aba185f1f 100644
--- a/apps/fabro-web/app/routes/run-files.tsx
+++ b/apps/fabro-web/app/routes/run-files.tsx
@@ -16,6 +16,7 @@ import {
   type FileContents,
 } from "@pierre/diffs/react";
 import { useToast } from "../components/toast";
+import { importChunk } from "../lib/import-chunk";
 import type {
   FileDiff as ApiFileDiff,
   PaginatedRunFileList,
@@ -58,9 +59,11 @@ import { useTickingNow } from "../lib/time";
 export { extractRequestId };
 
 const FileTreeSidebar = lazy(() =>
-  import("./run-files/file-tree-sidebar").then((module) => ({
-    default: module.FileTreeSidebar,
-  })),
+  importChunk(() =>
+    import("./run-files/file-tree-sidebar").then((module) => ({
+      default: module.FileTreeSidebar,
+    })),
+  ),
 );
 
 export const handle = { wide: true, fullHeight: true };
diff --git a/apps/fabro-web/app/routes/run-terminal.tsx b/apps/fabro-web/app/routes/run-terminal.tsx
index 6e4bddd93..9c6327ddc 100644
--- a/apps/fabro-web/app/routes/run-terminal.tsx
+++ b/apps/fabro-web/app/routes/run-terminal.tsx
@@ -1,5 +1,5 @@
 import TerminalView from "../components/terminal-view";
-import { FabroToaster, ToastProvider } from "../components/toast";
+import { ToastProvider } from "../components/toast";
 import { useDocumentTitle } from "../hooks/effects";
 
 export default function RunTerminal({ params }: { params: { id: string } }) {
@@ -10,7 +10,6 @@ export default function RunTerminal({ params }: { params: { id: string } }) {
       <div className="h-screen w-screen overflow-hidden">
         <TerminalView runId={params.id} chromeless />
       </div>
-      {typeof document !== "undefined" && <FabroToaster />}
     </ToastProvider>
   );
 }
diff --git a/apps/fabro-web/index.template.html b/apps/fabro-web/index.template.html
index 5ebdb65b9..eab794756 100644
--- a/apps/fabro-web/index.template.html
+++ b/apps/fabro-web/index.template.html
@@ -14,6 +14,7 @@
       href="https://fonts.googleapis.com/css2?family=Geist:wght@100..900&family=JetBrains+Mono:wght@400;500;600&display=swap"
     />
     {{styles}}
+    {{buildMeta}}
   </head>
   <body class="h-full font-sans antialiased">
     <div id="root"></div>
diff --git a/apps/fabro-web/scripts/build.test.ts b/apps/fabro-web/scripts/build.test.ts
index e3e852a71..1a2b9cd44 100644
--- a/apps/fabro-web/scripts/build.test.ts
+++ b/apps/fabro-web/scripts/build.test.ts
@@ -1,10 +1,30 @@
 import { test, expect } from "bun:test";
 import { existsSync } from "node:fs";
-import { lstat, readdir, readlink } from "node:fs/promises";
-import { basename, join } from "node:path";
+import { lstat, readFile, readdir, readlink } from "node:fs/promises";
+import { basename, join, relative, sep } from "node:path";
+
+import {
+  BUILD_ID_FILE_NAME,
+  BUILD_ID_META_NAME,
+  parseBuildIdDocument,
+} from "../app/lib/build-version-contract";
+import { localBundlerInputPaths } from "./build";
 
 const root = Bun.fileURLToPath(new URL("..", import.meta.url));
 
+test("resolved build inputs retain workspace sources and omit installed packages", () => {
+  const inputs = localBundlerInputPaths([
+    "app/entry.tsx",
+    "../../lib/packages/fabro-api-client/src/index.ts",
+    "../../node_modules/example/index.js",
+  ]).map((path) => relative(root, path).split(sep).join("/"));
+
+  expect(inputs).toEqual([
+    "app/entry.tsx",
+    "../../lib/packages/fabro-api-client/src/index.ts",
+  ]);
+});
+
 async function runBuild() {
   const process = Bun.spawn(["bun", "run", "scripts/build.ts"], {
     cwd:    root,
@@ -22,10 +42,15 @@ async function runBuild() {
   }
 }
 
-test("production build copies Pierre worker assets", async () => {
+test("production builds publish a stable asset set and prune the previous build", async () => {
   await runBuild();
 
-  const workerDist = join(root, "dist", "assets", "pierre-diffs-worker");
+  const distPath = join(root, "dist");
+  const firstTarget = await readlink(distPath);
+  expect((await lstat(distPath)).isSymbolicLink()).toBe(true);
+  expect(firstTarget.startsWith(".dist-builds/")).toBe(true);
+
+  const workerDist = join(distPath, "assets", "pierre-diffs-worker");
   expect(existsSync(join(workerDist, "worker-portable.js"))).toBe(true);
 
   const upstreamWorkerDir = join(
@@ -43,26 +68,43 @@ test("production build copies Pierre worker assets", async () => {
   for (const wasmFile of wasmFiles) {
     expect(existsSync(join(workerDist, wasmFile))).toBe(true);
   }
-}, 60000);
 
-test("dist is a symlink into .dist-builds and old builds are pruned", async () => {
-  await runBuild();
+  const published = JSON.parse(
+    await readFile(join(distPath, BUILD_ID_FILE_NAME), "utf8"),
+  );
+  const firstBuildId = parseBuildIdDocument(published);
+  expect(firstBuildId).not.toBeNull();
+
+  const html = await readFile(join(distPath, "index.html"), "utf8");
+  expect(html).toContain(
+    `<meta name="${BUILD_ID_META_NAME}" content="${firstBuildId}" />`,
+  );
+
+  const assets = await readdir(join(distPath, "assets"));
+  const stylesheets = assets.filter((file) => /^app-.*\.css$/.test(file));
+  expect(stylesheets).toHaveLength(1);
+  expect(stylesheets[0]).toMatch(/^app-[a-z0-9]{8}\.css$/);
+  expect(assets).not.toContain("app.css");
+  expect(html).toContain(`href="/assets/${stylesheets[0]}"`);
+  expect(html).not.toContain('href="/assets/app.css"');
+
+  // The id is derived from source inputs rather than emitted filenames. Bun's
+  // minified identifiers are nondeterministic, so output hashes can move even
+  // when the source graph is unchanged.
   await runBuild();
 
-  const distPath = join(root, "dist");
-  const stat = await lstat(distPath);
-  expect(stat.isSymbolicLink()).toBe(true);
-
-  const target = await readlink(distPath);
-  expect(target.startsWith(".dist-builds/")).toBe(true);
-
-  const buildId = target.slice(".dist-builds/".length);
-  const buildsRoot = join(root, ".dist-builds");
-  const remaining = await readdir(buildsRoot);
-  expect(remaining).toEqual([buildId]);
+  const secondTarget = await readlink(distPath);
+  expect(secondTarget.startsWith(".dist-builds/")).toBe(true);
+  expect(secondTarget).not.toBe(firstTarget);
+  const secondBuildId = parseBuildIdDocument(
+    JSON.parse(await readFile(join(distPath, BUILD_ID_FILE_NAME), "utf8")),
+  );
+  expect(secondBuildId).toBe(firstBuildId);
 
+  const currentDirName = secondTarget.slice(".dist-builds/".length);
+  expect(await readdir(join(root, ".dist-builds"))).toEqual([currentDirName]);
   expect(existsSync(join(distPath, "index.html"))).toBe(true);
-}, 60000);
+}, 120000);
 
 test("watch mode keeps running until interrupted", async () => {
   const process = Bun.spawn([
diff --git a/apps/fabro-web/scripts/build.ts b/apps/fabro-web/scripts/build.ts
index 78e520d29..360726fae 100644
--- a/apps/fabro-web/scripts/build.ts
+++ b/apps/fabro-web/scripts/build.ts
@@ -1,3 +1,4 @@
+import { createHash } from "node:crypto";
 import { watch as fsWatch } from "node:fs";
 import {
   cp,
@@ -10,7 +11,13 @@ import {
   symlink,
   writeFile,
 } from "node:fs/promises";
-import { dirname, join, relative } from "node:path";
+import { dirname, join, relative, resolve, sep } from "node:path";
+
+import {
+  BUILD_ID_FILE_NAME,
+  BUILD_ID_META_NAME,
+  buildIdDocument,
+} from "../app/lib/build-version-contract";
 
 declare const Bun: any;
 
@@ -34,59 +41,228 @@ const tailwindCliBin = join(
   JSON.parse(await readFile(tailwindCliPackageJsonPath, "utf8")).bin.tailwindcss,
 );
 
-function newBuildId(): string {
+// Names the `.dist-builds/` staging directory only. Time-ordered so builds sort
+// chronologically on disk, and unique so concurrent builds never collide. This
+// is deliberately NOT the id published to browsers: see `publishedBuildId`.
+function newBuildDirName(): string {
   return `${Date.now()}-${Math.random().toString(36).slice(2, 10)}`;
 }
 
+/**
+ * Lowercase-alphanumeric 8-char digest, matching the `[a-z0-9]{8}` shape the
+ * bundler uses for its own content hashes — and which the server's cache
+ * classifier (`is_content_hashed` in `static_files.rs`) keys on to decide
+ * between `immutable` and `no-cache`.
+ */
+function toShortId(hex: string): string {
+  return BigInt(`0x${hex.slice(0, 32)}`)
+    .toString(36)
+    .padStart(8, "0")
+    .slice(0, 8);
+}
+
+function contentHash8(content: string | Uint8Array): string {
+  return toShortId(createHash("sha256").update(content).digest("hex"));
+}
+
+// Inputs outside Bun's JavaScript module graph. Tailwind scans `app/` for class
+// names, `public/` is copied verbatim, and the remaining files control template
+// rendering, module resolution, dependency versions, or the build itself.
+const BUILD_INPUT_DIRS = ["app", "public"];
+const BUILD_INPUT_FILES = [
+  "index.template.html",
+  "package.json",
+  "scripts/build.ts",
+  "tsconfig.json",
+  "../../bun.lock",
+];
+const FILE_HASH_BATCH_SIZE = 32;
+
+/**
+ * The build id published to browsers, derived from the bundle's *source inputs*.
+ *
+ * The obvious implementation — hash the emitted asset filenames, which already
+ * embed content hashes — does not work, because **Bun's minified identifier
+ * naming is not deterministic**. Building this app twice from an unchanged tree
+ * produces byte-different output roughly one run in three: same length, ~100k
+ * differing bytes, all of it mangled names (`var Gr=C3((Pl5,qq)=>` in one run,
+ * `var yr=C3((Uc5,Oq)=>` in the next). Output hashes therefore change without
+ * any source change.
+ *
+ * That matters because the client shows a "new version" toast on mismatch. An id
+ * that flips at random would fire the toast on redeploys of identical code and
+ * train people to ignore it, which is worse than having no toast at all. Hashing
+ * the inputs makes the id change if and only if something we actually control
+ * changed.
+ *
+ * The tradeoff: when Bun emits a different permutation for the same source, the
+ * asset filenames change while the build id does not, so an open tab isn't told
+ * to reload. That is the correct call — the two builds are the same program —
+ * and `importChunk` covers the case where such a tab later needs a chunk whose
+ * name moved.
+ */
+async function publishedBuildId(bundlerInputs: Iterable<string>): Promise<string> {
+  const files = new Set<string>();
+  for (const dir of BUILD_INPUT_DIRS) {
+    for (const file of await collectFiles(join(rootPath, dir))) {
+      files.add(file);
+    }
+  }
+  for (const file of BUILD_INPUT_FILES) {
+    files.add(resolve(rootPath, file));
+  }
+  // Bun's metafile is the source of truth for resolved production modules. In
+  // particular, it captures workspace sources reached through tsconfig path
+  // aliases, which a hand-maintained app-local file list would miss.
+  for (const file of localBundlerInputPaths(bundlerInputs)) {
+    files.add(file);
+  }
+
+  const digest = createHash("sha256");
+  // The bundler itself is an input: a Bun upgrade can change output semantics.
+  digest.update(`bun:${Bun.version}\n`);
+  const inputs = [...files]
+    .map((file) => ({
+      file,
+      name: toUrlPath(relative(rootPath, file)),
+    }))
+    .sort((left, right) =>
+      left.name < right.name ? -1 : left.name > right.name ? 1 : 0
+    );
+
+  // Bound parallel reads so hashing stays off the rebuild critical path without
+  // exhausting low per-process file-descriptor limits on macOS.
+  for (let start = 0; start < inputs.length; start += FILE_HASH_BATCH_SIZE) {
+    const batch = inputs.slice(start, start + FILE_HASH_BATCH_SIZE);
+    const hashes = await Promise.all(
+      batch.map(async ({ file, name }) => ({
+        name,
+        hash: createHash("sha256").update(await readFile(file)).digest(),
+      })),
+    );
+    for (const { name, hash } of hashes) {
+      // Hash the repo-relative path, not the absolute one, so the id doesn't
+      // depend on where the repo is checked out.
+      digest.update(name);
+      digest.update("\0");
+      digest.update(hash);
+    }
+  }
+  return toShortId(digest.digest("hex"));
+}
+
+async function collectFiles(dir: string): Promise<string[]> {
+  const glob = new Bun.Glob("**/*");
+  const files: string[] = [];
+  for await (const file of glob.scan({ cwd: dir, dot: true, onlyFiles: true })) {
+    files.push(join(dir, file));
+  }
+  return files;
+}
+
+export function localBundlerInputPaths(inputs: Iterable<string>): string[] {
+  return [...inputs]
+    .map((input) => resolve(rootPath, input))
+    .filter((path) => !isInstalledDependency(path));
+}
+
+function isInstalledDependency(path: string): boolean {
+  return toUrlPath(relative(rootPath, path))
+    .split("/")
+    .includes("node_modules");
+}
+
+function toUrlPath(path: string): string {
+  return path.split(sep).join("/");
+}
+
+function assetHref(path: string): string {
+  return `/${toUrlPath(path)}`;
+}
+
 async function buildOnce() {
-  const buildId = newBuildId();
-  const buildDir = join(buildsRootDir, buildId);
+  const buildDirName = newBuildDirName();
+  const buildDir = join(buildsRootDir, buildDirName);
   const buildAssetsDir = join(buildDir, "assets");
   await mkdir(buildAssetsDir, { recursive: true });
 
-  const result = await Bun.build({
-    entrypoints: [join(rootPath, "app", "entry.tsx")],
-    outdir: buildAssetsDir,
-    naming: "[name]-[hash].[ext]",
-    minify: true,
-    splitting: true,
-    target: "browser",
-  });
-
-  if (!result.success) {
-    throw new Error(result.logs.map((log: any) => log.message).join("\n"));
-  }
-
-  const cssResult = await Bun.spawn([
-    process.execPath,
-    tailwindCliBin,
-    "-i",
-    "app/app.css",
-    "-o",
-    relative(rootPath, join(buildAssetsDir, "app.css")),
-    "--minify",
-  ], {
-    cwd: rootPath,
-    stdout: "inherit",
-    stderr: "inherit",
-  }).exited;
+  const [result, cssResult] = await Promise.all([
+    Bun.build({
+      entrypoints: [join(rootPath, "app", "entry.tsx")],
+      outdir: buildAssetsDir,
+      naming: "[name]-[hash].[ext]",
+      minify: true,
+      splitting: true,
+      target: "browser",
+      metafile: true,
+      root: rootPath,
+    }),
+    Bun.spawn([
+      process.execPath,
+      tailwindCliBin,
+      "-i",
+      "app/app.css",
+      "-o",
+      relative(rootPath, join(buildAssetsDir, "app.css")),
+      "--minify",
+    ], {
+      cwd: rootPath,
+      stdout: "inherit",
+      stderr: "inherit",
+    }).exited,
+  ]);
 
   if (cssResult !== 0) {
     throw new Error("Tailwind build failed");
   }
 
-  await cp(publicDir, buildDir, { recursive: true });
-  await copyPierreWorkerAssets(join(buildAssetsDir, "pierre-diffs-worker"));
-  await writeIndexHtml(
-    buildDir,
-    result.outputs.map((output: any) => ({
+  if (!result.success) {
+    throw new Error(result.logs.map((log: any) => log.message).join("\n"));
+  }
+
+  const [stylesheetPath, buildId] = await Promise.all([
+    hashStylesheet(buildAssetsDir),
+    publishedBuildId(Object.keys(result.metafile.inputs)),
+    cp(publicDir, buildDir, { recursive: true }),
+    copyPierreWorkerAssets(join(buildAssetsDir, "pierre-diffs-worker")),
+  ]);
+
+  const outputs: IndexHtmlOutput[] = [
+    { kind: "asset", path: stylesheetPath },
+    ...result.outputs.map((output: any) => ({
       kind: output.kind,
       path: relative(buildDir, output.path),
     })),
+  ];
+
+  await writeIndexHtml(buildDir, outputs, buildId);
+  // Served with `no-cache` + ETag (it doesn't match the server's content-hash
+  // pattern), so a polling client revalidates it as a cheap 304.
+  await writeFile(
+    join(buildDir, BUILD_ID_FILE_NAME),
+    `${JSON.stringify(buildIdDocument(buildId), null, 2)}\n`,
+    "utf8",
   );
 
   await publishBuild(buildDir);
-  await pruneOldBuilds(buildId);
+  await pruneOldBuilds(buildDirName);
+}
+
+/**
+ * Renames Tailwind's stable-named `app.css` to `app-<hash>.css`.
+ *
+ * A stable name forces `no-cache`, which lets a tab revalidate into the new
+ * stylesheet while still running the previous build's JavaScript. Tailwind
+ * purges unused classes per build, so classes the old JS still emits can vanish
+ * from the new CSS and elements silently render unstyled. Hashing pins the two
+ * together and lets the server cache the stylesheet immutably.
+ */
+async function hashStylesheet(buildAssetsDir: string): Promise<string> {
+  const source = join(buildAssetsDir, "app.css");
+  const css = await readFile(source);
+  const hashedName = `app-${contentHash8(css)}.css`;
+  await rename(source, join(buildAssetsDir, hashedName));
+  return join("assets", hashedName);
 }
 
 async function copyPierreWorkerAssets(targetDir: string) {
@@ -110,7 +286,11 @@ type IndexHtmlOutput = {
   path: string;
 };
 
-async function writeIndexHtml(buildDir: string, outputs: IndexHtmlOutput[]) {
+async function writeIndexHtml(
+  buildDir: string,
+  outputs: IndexHtmlOutput[],
+  buildId: string,
+) {
   const template = await readFile(templatePath, "utf8");
   // Only entry points get <script> tags. Bun's `splitting: true` emits
   // hundreds of chunks reachable from the entry through static and dynamic
@@ -120,20 +300,24 @@ async function writeIndexHtml(buildDir: string, outputs: IndexHtmlOutput[]) {
   // import() chunks load on demand.
   const scripts = outputs
     .filter((output) => output.kind === "entry-point" && output.path.endsWith(".js"))
-    .map((output) => `<script type="module" src="/${output.path.replaceAll("\\\\", "/")}"></script>`)
+    .map((output) => `<script type="module" src="${assetHref(output.path)}"></script>`)
     .join("\n    ");
-  const styles = [
-    "/assets/app.css",
-    ...outputs
-      .filter((output) => output.path.endsWith(".css"))
-      .map((output) => `/${output.path.replaceAll("\\\\", "/")}`),
-  ]
+  const styles = outputs
+    .filter((output) => output.path.endsWith(".css"))
+    .map((output) => assetHref(output.path))
     .filter((value, index, array) => array.indexOf(value) === index)
     .map((path) => `<link rel="stylesheet" href="${path}" />`)
     .join("\n    ");
 
+  // Records which build this document loaded. A tab reads it back at runtime
+  // and compares against /build-id.json; the meta tag is the honest answer
+  // because client-side routing never re-fetches index.html, so it stays
+  // pinned to the build the tab actually started with.
+  const buildMeta = `<meta name="${BUILD_ID_META_NAME}" content="${buildId}" />`;
+
   const html = template
     .replace("{{styles}}", styles)
+    .replace("{{buildMeta}}", buildMeta)
     .replace("{{scripts}}", scripts);
 
   await writeFile(join(buildDir, "index.html"), html, "utf8");
@@ -264,7 +448,9 @@ async function main() {
   });
 }
 
-main().catch((error) => {
-  console.error(error);
-  process.exit(1);
-});
+if (import.meta.main) {
+  main().catch((error) => {
+    console.error(error);
+    process.exit(1);
+  });
+}
diff --git a/lib/apps/fabro-server/src/static_files.rs b/lib/apps/fabro-server/src/static_files.rs
index b0a9e623e..67db55a4e 100644
--- a/lib/apps/fabro-server/src/static_files.rs
+++ b/lib/apps/fabro-server/src/static_files.rs
@@ -473,6 +473,10 @@ mod tests {
             "assets/entry-0sv53bs3.js",
             "assets/chunk-4tr91ktd.js",
             "assets/chunk-x912wb67.css",
+            // Tailwind's stylesheet is hashed by the build so it moves with the
+            // bundle; a stable name would let a tab revalidate into new CSS
+            // while still running the previous build's JavaScript.
+            "assets/app-381qtfxr.css",
         ] {
             assert!(is_content_hashed(path), "{path} should be content-hashed");
         }
@@ -485,6 +489,9 @@ mod tests {
         // pinned stale in browsers for a year if marked immutable.
         for path in [
             "index.html",
+            // Clients poll this to learn whether their tab is running a stale
+            // build, so it must revalidate rather than be pinned for a year.
+            "build-id.json",
             "assets/app.css",
             "assets/pierre-diffs-worker/worker-portable.js",
             "images/apple-touch-icon.png",