diff --git a/framework.manifest.json b/framework.manifest.json
index eb4d93ac..703645af 100644
--- a/framework.manifest.json
+++ b/framework.manifest.json
@@ -1,6 +1,6 @@
 {
   "version": 1,
-  "buildId": "93daaafcf078589bc4501699e1881a0f64181bfbabedc2188f42c6f98102182b",
+  "buildId": "4a212e87b080759b7b19828bd37e5a82d813f61898c4307a02cbd29f46e25dde",
   "tiers": {
     "0": [
       "core",
@@ -402,6 +402,11 @@
       "owner": "cli",
       "at": "packages/cli/src/error-codes.ts"
     },
+    {
+      "code": "X_ASYNC_CONTEXT_UNAVAILABLE",
+      "owner": "core",
+      "at": "packages/core/src/error-codes.ts"
+    },
     {
       "code": "X_AUDIT_SINK_FAILED",
       "owner": "action",
diff --git a/packages/core/src/async-context.test.ts b/packages/core/src/async-context.test.ts
new file mode 100644
index 00000000..6ed4edeb
--- /dev/null
+++ b/packages/core/src/async-context.test.ts
@@ -0,0 +1,126 @@
+import { describe, expect, test } from 'bun:test';
+import { mkdtemp } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { asyncContext } from './async-context';
+
+describe('asyncContext, on a server', () => {
+  test('is undefined outside a scope and the value inside one', () => {
+    const scope = asyncContext<string>('the thing');
+    expect(scope.get()).toBeUndefined();
+    expect(scope.run('inside', () => scope.get())).toBe('inside');
+    expect(scope.get()).toBeUndefined();
+  });
+
+  test('propagates across an await, which is the whole reason for AsyncLocalStorage', async () => {
+    const scope = asyncContext<number>('the thing');
+    await scope.run(7, async () => {
+      await Bun.sleep(1);
+      expect(scope.get()).toBe(7);
+    });
+  });
+
+  test('stays isolated across interleaved async tasks', async () => {
+    const scope = asyncContext<string>('the thing');
+    const task = async (value: string, delayMs: number): Promise<string> =>
+      scope.run(value, async () => {
+        await Bun.sleep(delayMs);
+        return scope.get() ?? 'lost';
+      });
+    expect(await Promise.all([task('a', 20), task('b', 5), task('c', 10)])).toEqual([
+      'a',
+      'b',
+      'c',
+    ]);
+  });
+
+  test('nests, innermost wins, and the outer value survives the inner scope', () => {
+    const scope = asyncContext<string>('the thing');
+    scope.run('outer', () => {
+      expect(scope.run('inner', () => scope.get())).toBe('inner');
+      expect(scope.get()).toBe('outer');
+    });
+  });
+
+  test('two scopes cannot see each other', () => {
+    const left = asyncContext<string>('the left');
+    const right = asyncContext<string>('the right');
+    left.run('L', () => {
+      expect(right.get()).toBeUndefined();
+    });
+  });
+});
+
+/**
+ * The mechanical guard for this whole defect class, one level up from the seam: the BARREL is the
+ * entry `@ultimat3/ui` reaches (`packages/ui/src/errors.ts` imports `@ultimat3/core`, not a file),
+ * so a fourth module-scope `new AsyncLocalStorage()` added anywhere in the package turns this red
+ * on the day it lands. The bundler's stub is the subject and cannot be faked by a test.
+ */
+describe('a browser bundle of @ultimat3/core', () => {
+  interface BrowserBarrel {
+    readonly hasContext: () => boolean;
+    readonly currentSpan: () => unknown;
+    readonly isImpersonating: () => boolean;
+    readonly createContext: () => unknown;
+    readonly anonymousActor: () => unknown;
+    readonly runWithContext: (ctx: unknown, fn: () => unknown) => unknown;
+    readonly withSpan: (name: string, fn: () => unknown) => unknown;
+    readonly impersonate: (actor: unknown, reason: string, fn: () => unknown) => unknown;
+  }
+
+  let built: Promise<BrowserBarrel> | undefined;
+
+  function barrel(): Promise<BrowserBarrel> {
+    built ??= (async (): Promise<BrowserBarrel> => {
+      const output = await Bun.build({
+        entrypoints: [join(import.meta.dir, 'index.ts')],
+        target: 'browser',
+      });
+      expect(output.success).toBe(true);
+      const chunk = output.outputs[0] ?? expect.unreachable('the browser build emitted no chunk');
+      // A fresh path per run: the module cache would otherwise serve a chunk built before a fix.
+      const file = join(await mkdtemp(join(tmpdir(), 'ultimate-core-')), 'barrel.mjs');
+      await Bun.write(file, await chunk.text());
+      return import(file) as Promise<BrowserBarrel>;
+    })();
+    return built;
+  }
+
+  test('evaluates instead of throwing at module scope', async () => {
+    const core = await barrel();
+    expect(typeof core.hasContext).toBe('function');
+  }, 60_000);
+
+  test('answers every ambient read with a definite no, not an exception', async () => {
+    const core = await barrel();
+    expect(core.hasContext()).toBe(false);
+    expect(core.currentSpan()).toBeUndefined();
+    expect(core.isImpersonating()).toBe(false);
+  }, 60_000);
+
+  /**
+   * Also the guard that keeps the two tests above from being vacuous: had the bundler inlined the
+   * REAL `node:async_hooks`, these would open a scope instead of refusing, and evaluating the
+   * chunk would have proved nothing about the stub.
+   */
+  test('refuses to OPEN a scope, with a code and a fix, on every write path', async () => {
+    const core = await barrel();
+    expect(() => core.runWithContext(core.createContext(), () => 1)).toThrow(
+      /X_ASYNC_CONTEXT_UNAVAILABLE/,
+    );
+    expect(() => core.withSpan('work', () => 1)).toThrow(/X_ASYNC_CONTEXT_UNAVAILABLE/);
+  }, 60_000);
+
+  /**
+   * `impersonate()` needs no refusal of its own and deliberately has none: it reads the parent
+   * context first, and there is no parent here. A second code for the same fact would be two
+   * answers to one question.
+   */
+  test('reports a missing parent, not a missing runtime, when impersonating', async () => {
+    const core = await barrel();
+    expect(() => core.impersonate(core.anonymousActor(), 'ticket 4821', () => 1)).toThrow(
+      /X_NO_CONTEXT/,
+    );
+  }, 60_000);
+});
diff --git a/packages/core/src/async-context.ts b/packages/core/src/async-context.ts
new file mode 100644
index 00000000..190c679a
--- /dev/null
+++ b/packages/core/src/async-context.ts
@@ -0,0 +1,77 @@
+// Single responsibility: the one lazily-constructed `AsyncLocalStorage` in the framework. Every
+// ambient value core carries — the request context, the active span, the impersonation reason —
+// opens its scope through this seam, so "what happens where there is no async context" has one
+// answer instead of one per module.
+
+// `node:async_hooks` is unavoidable and deliberate: nothing else in Bun makes a value ambient
+// across an `await` without threading it through every signature in the framework.
+import { AsyncLocalStorage } from 'node:async_hooks';
+import { UltimateError } from './errors';
+
+export interface AsyncContext<T> {
+  /** The value in flight, or `undefined` — outside a scope, and in a runtime that has none. */
+  get(): T | undefined;
+  /** Run `fn` with `value` in flight. `X_ASYNC_CONTEXT_UNAVAILABLE` where that is impossible. */
+  run<R>(value: T, fn: () => R): R;
+}
+
+/**
+ * The storage is constructed on first `run()`, never at module scope. That is the whole point of
+ * this file: a browser bundler stubs `node:async_hooks` to `{}` — Bun's `target: 'browser'` emits
+ * `var { AsyncLocalStorage } = (() => ({}))` — so a module-scope `new` threw
+ * `TypeError: undefined is not a constructor` at module EVALUATION, and every package that
+ * transitively imports core was dead on arrival in a client bundle. `@ultimat3/ui` calls itself a
+ * SolidJS design system and could not be put on a client by the only client bundler the framework
+ * has, for this reason and no other.
+ *
+ * The server pays nothing: `getStore()` before any `run()` answers `undefined` whether the storage
+ * was ever constructed or not, so deferring the construction changes no observable behaviour.
+ *
+ * **Reads degrade, writes throw**, and that split is the doctrine rather than a convenience.
+ * `get()` answers `undefined` in a browser because that is TRUE — nothing is in flight there, so
+ * "am I inside a scope" has a definite no for an answer and does not deserve an exception. It is
+ * the same call `@ultimat3/ui`'s `solid()` makes: inert where the capability is genuinely absent,
+ * throwing only where a caller asked for something the runtime cannot deliver. `run()` is that
+ * second case, so it names itself with a code and a fix rather than leaving a bare `TypeError`
+ * from a stack that mentions no file the caller wrote.
+ *
+ * **A synchronous save/restore fallback is not the answer here**, and this note exists so that it
+ * is not re-proposed: a module-level `current` swapped in a `try`/`finally` serves sync code and
+ * is silently WRONG across an `await` — two overlapping scopes interleave and the second one's
+ * `finally` restores a value the first is still inside. That is the `jobs: { driver: 'redis' }`
+ * failure mode this repo already paid for once: accepted, unwarned, and wrong in the dangerous
+ * direction. An error a caller can read beats an ambient value that is occasionally somebody
+ * else's.
+ *
+ * `subject` names what could not be opened, and is a `string` by construction — never an
+ * `unknown` reaching a `cause:`, which `bun run error-render` refuses.
+ */
+export function asyncContext<T>(subject: string): AsyncContext<T> {
+  let storage: AsyncLocalStorage<T> | undefined;
+
+  function open(): AsyncLocalStorage<T> | undefined {
+    if (storage !== undefined) return storage;
+    // The stub is an object with no `AsyncLocalStorage` key, so the binding reads `undefined`.
+    if (typeof AsyncLocalStorage !== 'function') return undefined;
+    storage = new AsyncLocalStorage<T>();
+    return storage;
+  }
+
+  return {
+    get(): T | undefined {
+      return open()?.getStore();
+    },
+    run<R>(value: T, fn: () => R): R {
+      const store = open();
+      if (store === undefined) {
+        throw new UltimateError({
+          code: 'X_ASYNC_CONTEXT_UNAVAILABLE',
+          cause: `${subject} needs AsyncLocalStorage, and node:async_hooks is stubbed to {} in this runtime`,
+          fix: `${subject} is server-only — open it in apps/web/server.ts or a route handler, and keep every module that opens one out of the import graph of a client island entry`,
+          meta: { subject },
+        });
+      }
+      return store.run(value, fn);
+    },
+  };
+}
diff --git a/packages/core/src/context.test.ts b/packages/core/src/context.test.ts
index d4760326..ec6dd137 100644
--- a/packages/core/src/context.test.ts
+++ b/packages/core/src/context.test.ts
@@ -181,3 +181,30 @@ describe('request-scoped log fields', () => {
     expect(JSON.parse(lines[0] ?? '{}')).not.toHaveProperty('orgId');
   });
 });
+
+/**
+ * The browser-bundle guard for the whole package — including this module — lives in
+ * `async-context.test.ts`, beside the seam that owns the defect. What is asserted here is the
+ * half that seam must not have changed: the server.
+ */
+describe('the server path is unchanged by the lazy storage', () => {
+  test('resolves the same context object inside the scope, and none outside it', () => {
+    const ctx = createContext({ locale: 'de-DE' });
+    expect(hasContext()).toBe(false);
+    const seen = runWithContext(ctx, () => {
+      expect(hasContext()).toBe(true);
+      return useContext();
+    });
+    expect(seen).toBe(ctx);
+    expect(hasContext()).toBe(false);
+    expect(tryUseContext()).toBeUndefined();
+  });
+
+  test('propagates across an await, which is the whole reason for AsyncLocalStorage', async () => {
+    const ctx = createContext({ locale: 'fr-FR' });
+    await runWithContext(ctx, async () => {
+      await Bun.sleep(1);
+      expect(useContext().locale).toBe('fr-FR');
+    });
+  });
+});
diff --git a/packages/core/src/context.ts b/packages/core/src/context.ts
index 627934ba..2cbb97a6 100644
--- a/packages/core/src/context.ts
+++ b/packages/core/src/context.ts
@@ -2,8 +2,8 @@
 // service bag reach every layer through AsyncLocalStorage instead of being threaded as
 // parameters — otherwise every signature in the framework grows a `ctx` argument twice.
 
-import { AsyncLocalStorage } from 'node:async_hooks';
 import { type Actor, anonymousActor } from './actor';
+import { asyncContext } from './async-context';
 import { type Clock, systemClock } from './clock';
 import { UltimateError } from './errors';
 import { traceId as newTraceId, uuid } from './ids';
@@ -72,7 +72,12 @@ export interface CtxInit {
 
 export type CtxPatch = Omit<CtxInit, 'requestId'>;
 
-const storage = new AsyncLocalStorage<Ctx>();
+/**
+ * `async-context.ts` owns why this is a lazily-opened seam rather than a module-scope
+ * `new AsyncLocalStorage()`, and why a browser gets `undefined` from a read and an error from a
+ * write. It is the same seam `telemetry.ts` and `impersonate.ts` open, on purpose: one answer.
+ */
+const requestContext = asyncContext<Ctx>('the request context');
 
 const neverAborted = new AbortController().signal;
 
@@ -133,16 +138,16 @@ export function createContext(init: CtxInit = {}): Ctx {
 }
 
 export function runWithContext<T>(ctx: Ctx, fn: () => T): T {
-  return storage.run(ctx, fn);
+  return requestContext.run(ctx, fn);
 }
 
 /** The context, or `undefined` outside a request. Prefer `useContext()` in app code. */
 export function tryUseContext(): Ctx | undefined {
-  return storage.getStore();
+  return requestContext.get();
 }
 
 export function useContext(): Ctx {
-  const ctx = storage.getStore();
+  const ctx = tryUseContext();
   if (ctx === undefined) {
     throw new UltimateError({
       code: 'X_NO_CONTEXT',
@@ -154,7 +159,7 @@ export function useContext(): Ctx {
 }
 
 export function hasContext(): boolean {
-  return storage.getStore() !== undefined;
+  return tryUseContext() !== undefined;
 }
 
 /**
@@ -183,7 +188,7 @@ export function withChildContext<T>(patch: CtxPatch, fn: () => T): T {
     signal: patch.signal ?? parent.signal,
     services: { ...carried, ...(patch.services ?? {}) },
   });
-  return storage.run(child, fn);
+  return requestContext.run(child, fn);
 }
 
 /** Resolve a late-bound service. Throws `X_SERVICE_MISSING` rather than returning undefined. */
@@ -222,7 +227,7 @@ export function throwIfAborted(ctx: Ctx = useContext()): void {
  * never mistaken for the customer's own.
  */
 setLoggerContextFields(() => {
-  const ctx = storage.getStore();
+  const ctx = tryUseContext();
   if (ctx === undefined) return undefined;
   const { actor } = ctx;
   return {
diff --git a/packages/core/src/error-codes.ts b/packages/core/src/error-codes.ts
index 0cf383ed..66ecb4d1 100644
--- a/packages/core/src/error-codes.ts
+++ b/packages/core/src/error-codes.ts
@@ -28,6 +28,7 @@ export function errorDocsUrl(code: string): string {
 /** Codes owned by `@ultimat3/core`. Every other package calls `registerErrorCodes()`. */
 const CORE_CODE_TITLES = {
   X_ABORTED: 'operation aborted',
+  X_ASYNC_CONTEXT_UNAVAILABLE: 'async context unavailable',
   X_CONFIG_INVALID: 'app.config.ts is invalid',
   X_CURSOR_INVALID: 'pagination cursor is malformed, tampered with or from another query',
   X_CURSOR_SECRET_DEV: 'cursors are signed with the shipped development key',
diff --git a/packages/core/src/impersonate.ts b/packages/core/src/impersonate.ts
index 50794fc2..f04bb5ca 100644
--- a/packages/core/src/impersonate.ts
+++ b/packages/core/src/impersonate.ts
@@ -2,13 +2,16 @@
 // the mechanism; this is the ONE door through it, because a swap with no reason and no origin is
 // indistinguishable in an audit trail from the customer doing it themselves.
 
-import { AsyncLocalStorage } from 'node:async_hooks';
 import { type Actor, actorLabel, actorOrigin } from './actor';
 import { assert } from './assert';
+import { asyncContext } from './async-context';
 import { useContext, withChildContext } from './context';
 import { currentSpan } from './telemetry';
 
-const storage = new AsyncLocalStorage<string>();
+// The same lazily-opened seam `context.ts` uses. Its `run()` cannot be reached in a runtime with
+// no async context: `impersonate()` calls `useContext()` first, and that throws `X_NO_CONTEXT`
+// there — so no second refusal is written here for symmetry's sake.
+const impersonation = asyncContext<string>('the impersonation reason');
 
 /**
  * Run `fn` as `actor`, recording who asked and why.
@@ -45,7 +48,7 @@ export function impersonate<T>(actor: Actor, reason: string, fn: () => T): T {
     'actor.label': label,
     'impersonation.reason': reason,
   });
-  return withChildContext({ actor: impersonated }, () => storage.run(reason, fn));
+  return withChildContext({ actor: impersonated }, () => impersonation.run(reason, fn));
 }
 
 /**
@@ -53,10 +56,10 @@ export function impersonate<T>(actor: Actor, reason: string, fn: () => T): T {
  * an app where nobody is impersonating. Read by an audit sink, and by nothing else.
  */
 export function impersonationReason(): string | undefined {
-  return storage.getStore();
+  return impersonation.get();
 }
 
 /** Is the caller acting as somebody else right now? */
 export function isImpersonating(): boolean {
-  return storage.getStore() !== undefined;
+  return impersonation.get() !== undefined;
 }
diff --git a/packages/core/src/telemetry.ts b/packages/core/src/telemetry.ts
index 4ad62261..29fc1517 100644
--- a/packages/core/src/telemetry.ts
+++ b/packages/core/src/telemetry.ts
@@ -2,7 +2,7 @@
 // is a no-op so unconfigured apps pay nothing, and trace context is serialised explicitly
 // (`traceparent`) so a trace survives HTTP -> job -> live query.
 
-import { AsyncLocalStorage } from 'node:async_hooks';
+import { asyncContext } from './async-context';
 import { type Clock, systemClock } from './clock';
 import { tryUseContext } from './context';
 import { renderThrowable } from './error-render';
@@ -118,7 +118,10 @@ export function memoryExporter(): MemoryExporter {
   };
 }
 
-const activeSpan = new AsyncLocalStorage<Span>();
+// The same lazily-opened seam `context.ts` uses, and for the same reason: a module-scope
+// `new AsyncLocalStorage()` throws at EVALUATION in a browser bundle, taking every importer of
+// `@ultimat3/core` down with it. `async-context.ts` owns the argument.
+const activeSpan = asyncContext<Span>('the active span');
 
 let exporter: SpanExporter = noopExporter;
 let clock: Clock = systemClock;
@@ -161,12 +164,12 @@ export function serviceResource(): SpanResource {
 }
 
 export function currentSpan(): Span | undefined {
-  return activeSpan.getStore();
+  return activeSpan.get();
 }
 
 /** The trace the caller is inside: active span, else the request context, else a fresh trace. */
 export function currentSpanContext(): SpanContext | undefined {
-  const span = activeSpan.getStore();
+  const span = activeSpan.get();
   if (span !== undefined) return span.context;
   const ctx = tryUseContext();
   if (ctx === undefined) return undefined;
diff --git a/scripts/error-map-backlog.ts b/scripts/error-map-backlog.ts
index c9b196ff..05278393 100644
--- a/scripts/error-map-backlog.ts
+++ b/scripts/error-map-backlog.ts
@@ -25,6 +25,7 @@ export const ERROR_STATUS_BACKLOG: Readonly<Record<string, readonly string[]>> =
   // by an image/secrets path that answers nothing. `X_CURSOR_INVALID` left on purpose: it IS a
   // request, and it has a row.
   core: [
+    'X_ASYNC_CONTEXT_UNAVAILABLE',
     'X_CONFIG_INVALID',
     'X_CURSOR_SECRET_DEV',
     'X_ENVIRONMENT_INVALID',
diff --git a/wiki/Error-Codes.md b/wiki/Error-Codes.md
index 92af9b3b..71dd0df4 100644
--- a/wiki/Error-Codes.md
+++ b/wiki/Error-Codes.md
@@ -34,6 +34,7 @@ X_DB_DRIFT: schema differs from migrations
 | `X_INVARIANT` | invariant violated | framework state that "cannot happen" happened | report it with `x doctor --json` output attached |
 | `X_UNREACHABLE` | unreachable branch was reached | an exhaustive `switch` met a value the type says cannot exist | narrow the union at the call site |
 | `X_NOT_IMPLEMENTED` | this driver does not implement the requested feature | an interface-complete driver whose remote half is unwritten | use the default driver, or implement the named method |
+| `X_ASYNC_CONTEXT_UNAVAILABLE` | async context unavailable | `node:async_hooks` is stubbed to `{}` in this runtime, so nothing can be made ambient — a browser bundle | the scope is server-only: open it in `apps/web/server.ts` or a route handler, and keep every module that opens one out of the import graph of a client island entry |
 | `X_NO_CONTEXT` | no request context is active | framework code called outside the ALS context | `runWithContext(createContext({ … }), fn)` |
 | `X_SERVICE_MISSING` | service is not registered on the request context | `ctx.<service>` used without providing it | pass it in `createContext({ services: { … } })` |
 | `X_SERVICE_DUPLICATE` | a service name is registered twice | two `defineService('name', ...)` calls used the same name | rename one of the two declarations |