diff --git a/.changeset/settings-locked-key-tolerant-read.md b/.changeset/settings-locked-key-tolerant-read.md
new file mode 100644
index 0000000000..aeb379fca9
--- /dev/null
+++ b/.changeset/settings-locked-key-tolerant-read.md
@@ -0,0 +1,26 @@
+---
+"@object-ui/console": patch
+---
+
+fix(settings): read the locked key from `error.details`, tolerating both wire shapes — objectstack#4224
+
+`SettingsView.onSave` rendered the `SETTINGS_LOCKED` toast from
+`err.payload.error.key`. That key was a SIBLING of `code`/`message` inside
+`error`, a position `ApiErrorSchema` never declared — it reached the console only
+because the schema is a plain `z.object` and silently strips what it does not
+declare, so nothing ever failed to flag it. objectstack#4224 moves it into
+`error.details`, the slot the contract does declare.
+
+This is the console's half, and it ships **first**: the read is now
+`error.details?.key ?? error.key`, so the toast keeps naming the locked key
+against servers on either side of that change rather than degrading to
+`Locked by environment: undefined` during the window where the two repos are on
+different versions. The fallback can go once the oldest supported server carries
+the fix.
+
+Also stops interpolating a missing key: when neither position carries one the
+toast now reads `Locked by environment` rather than appending `undefined`.
+
+This was the only in-console reader of the four keys objectstack#4224 relocated
+(`namespace`, `key`, `reason`, `fields`) — a repo-wide grep for the other three
+finds no consumer.
diff --git a/apps/console/src/pages/settings/SettingsView.tsx b/apps/console/src/pages/settings/SettingsView.tsx
index 16116dcbd6..5280f95f6a 100644
--- a/apps/console/src/pages/settings/SettingsView.tsx
+++ b/apps/console/src/pages/settings/SettingsView.tsx
@@ -15,6 +15,7 @@ import { getIcon } from '../../utils/getIcon';
 import { SettingsField } from './SettingsField';
 import {
   getSettingsNamespace,
+  lockedKeyOf,
   runSettingsAction,
   saveSettingsNamespace,
 } from './api';
@@ -128,8 +129,11 @@ export function SettingsView() {
       setDraft({});
       toast.success('Settings saved');
     } catch (err: any) {
-      if (err?.payload?.error?.code === 'SETTINGS_LOCKED') {
-        toast.error(`Locked by environment: ${err.payload.error.key}`);
+      const apiError = err?.payload?.error;
+      if (apiError?.code === 'SETTINGS_LOCKED') {
+        // `lockedKeyOf` reads both wire positions — see its note (objectstack#4224).
+        const key = lockedKeyOf(apiError);
+        toast.error(key ? `Locked by environment: ${key}` : 'Locked by environment');
       } else {
         toast.error(err?.message ?? 'Save failed');
       }
diff --git a/apps/console/src/pages/settings/api.test.ts b/apps/console/src/pages/settings/api.test.ts
new file mode 100644
index 0000000000..fa8e7ebd59
--- /dev/null
+++ b/apps/console/src/pages/settings/api.test.ts
@@ -0,0 +1,64 @@
+/**
+ * ObjectUI
+ * Copyright (c) 2024-present ObjectStack Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * `lockedKeyOf` — the two wire positions of a `SETTINGS_LOCKED` key
+ * (objectstack#4224).
+ *
+ * Why this is pinned rather than left as an inline `??`: it is a COMPAT read,
+ * and the failure mode of a compat read is that someone deletes the half they
+ * did not know was load-bearing. Both branches are asserted here, so removing
+ * either one is a red test that names the server version it breaks — which is
+ * also the signal for when the old branch may legitimately go.
+ *
+ * The old position (`error.key`) was never declared by `ApiErrorSchema`; it
+ * reached the console only because the schema is a plain `z.object` and strips
+ * undeclared keys instead of rejecting them. That is exactly why no test on
+ * either side of the wire ever caught it.
+ */
+
+import { describe, it, expect } from 'vitest';
+import { lockedKeyOf } from './api';
+
+describe('lockedKeyOf', () => {
+  it('reads the declared slot — a server carrying objectstack#4224', () => {
+    expect(
+      lockedKeyOf({
+        code: 'SETTINGS_LOCKED',
+        message: "Setting 'branding.workspace_name' is locked (locked-by-env).",
+        details: { namespace: 'branding', key: 'workspace_name', reason: 'locked-by-env' },
+      }),
+    ).toBe('workspace_name');
+  });
+
+  it('falls back to the pre-#4224 sibling — a server without it', () => {
+    expect(
+      lockedKeyOf({
+        code: 'SETTINGS_LOCKED',
+        message: "Setting 'branding.workspace_name' is locked (locked-by-env).",
+        namespace: 'branding',
+        key: 'workspace_name',
+        reason: 'locked-by-env',
+      }),
+    ).toBe('workspace_name');
+  });
+
+  it('prefers the declared slot when a body somehow carries both', () => {
+    expect(lockedKeyOf({ key: 'old_key', details: { key: 'new_key' } })).toBe('new_key');
+  });
+
+  it('yields undefined rather than a string when no position carries a key', () => {
+    // The toast branches on this: no key means "Locked by environment" rather
+    // than "Locked by environment: undefined".
+    expect(lockedKeyOf({ code: 'SETTINGS_LOCKED', message: 'Locked.' })).toBeUndefined();
+    expect(lockedKeyOf({ details: {} })).toBeUndefined();
+    expect(lockedKeyOf({ details: null })).toBeUndefined();
+    expect(lockedKeyOf({ key: '' })).toBeUndefined();
+    expect(lockedKeyOf({ key: 42 })).toBeUndefined();
+    expect(lockedKeyOf(undefined)).toBeUndefined();
+    expect(lockedKeyOf('not an object')).toBeUndefined();
+  });
+});
diff --git a/apps/console/src/pages/settings/api.ts b/apps/console/src/pages/settings/api.ts
index 90a4af98f4..82783928ba 100644
--- a/apps/console/src/pages/settings/api.ts
+++ b/apps/console/src/pages/settings/api.ts
@@ -28,6 +28,35 @@ const jsonHeaders = (): HeadersInit => ({
   Accept: 'application/json',
 });
 
+/**
+ * The env-locked key named by a `SETTINGS_LOCKED` error, read from whichever
+ * position the serving version puts it in (objectstack#4224).
+ *
+ * It used to arrive as `error.key` — a SIBLING of `code`/`message` that
+ * `ApiErrorSchema` never declared. That body was accepted only because the
+ * schema is a plain `z.object` and strips undeclared keys rather than rejecting
+ * them, so nothing on either side ever flagged it. objectstack#4224 moves it to
+ * `error.details.key`, the slot the contract does declare.
+ *
+ * Declared home first, old position second, so the console names the key against
+ * servers on either side of that change instead of rendering `undefined` for the
+ * duration of the window. Drop the second read once the oldest supported server
+ * carries the fix.
+ *
+ * Lives here rather than inline in the view because this module already owns
+ * what the server's error body looks like (`jsonOrThrow` is what builds
+ * `err.payload`), and because a compat shim that no test exercises is a compat
+ * shim that gets deleted by the next person who reads only one of the two
+ * shapes.
+ */
+export function lockedKeyOf(apiError: unknown): string | undefined {
+  if (!apiError || typeof apiError !== 'object') return undefined;
+  const e = apiError as { key?: unknown; details?: { key?: unknown } | null };
+  const declared = e.details && typeof e.details === 'object' ? e.details.key : undefined;
+  const found = declared ?? e.key;
+  return typeof found === 'string' && found.length > 0 ? found : undefined;
+}
+
 export async function listSettingsManifests(): Promise<SettingsListResponse> {
   const res = await fetch(BASE, { credentials: 'include', headers: jsonHeaders() });
   return jsonOrThrow<SettingsListResponse>(res);