diff --git a/apps/desktop/src/settings/DesktopClientSettings.test.ts b/apps/desktop/src/settings/DesktopClientSettings.test.ts
index 26c3127de5f84635d16793106c57c84b081840b4..c1cb8588b5ea45ddde0de063a885792c4df6e3cb 100644
--- a/apps/desktop/src/settings/DesktopClientSettings.test.ts
+++ b/apps/desktop/src/settings/DesktopClientSettings.test.ts
@@ -29,6 +29,7 @@ const clientSettings: ClientSettings = {
   fontSizeTerminal: 12,
   fontSmoothing: true,
   glassOpacity: 80,
+  planModeEnabled: false,
   providerModelPreferences: {},
   sidebarAutoSettleAfterDays: 3,
   sidebarProjectGroupingMode: "repository_path",
diff --git a/apps/web/src/components/ChatView.tsx b/apps/web/src/components/ChatView.tsx
index a4c9bcc7c5402dc716b55a63e23a5d676f6e1718..f0e25e644751214617277b380f4d32f971e49ef7 100644
--- a/apps/web/src/components/ChatView.tsx
+++ b/apps/web/src/components/ChatView.tsx
@@ -1481,8 +1481,13 @@ function ChatViewContent(props: ChatViewProps) {
     ? (localServerError ?? activeServerThread?.session?.lastError ?? null)
     : localDraftError;
   const runtimeMode = composerRuntimeMode ?? activeThread?.runtimeMode ?? DEFAULT_RUNTIME_MODE;
-  const interactionMode =
-    composerInteractionMode ?? activeThread?.interactionMode ?? DEFAULT_INTERACTION_MODE;
+  // Plan mode is legacy (Settings → Beta). With the flag off the effective
+  // mode is forced to "default" — even for threads with a stored plan mode —
+  // so nobody is trapped in plan mode while its toggle is hidden. The next
+  // send persists "default" back to the thread.
+  const interactionMode = settings.planModeEnabled
+    ? (composerInteractionMode ?? activeThread?.interactionMode ?? DEFAULT_INTERACTION_MODE)
+    : DEFAULT_INTERACTION_MODE;
   const isLocalDraftThread = !isServerThread && localDraftThread !== undefined;
   const canCheckoutPullRequestIntoThread = isLocalDraftThread;
   const activeThreadId = activeThread?.id ?? null;
@@ -4811,7 +4816,10 @@ function ChatViewContent(props: ChatViewProps) {
       });
       return;
     }
+    // Legacy plan mode: /plan and /default only act when the beta flag is on;
+    // otherwise they send as plain text like any other message.
     const standaloneSlashCommand =
+      settings.planModeEnabled &&
       composerImages.length === 0 &&
       sendableComposerTerminalContexts.length === 0 &&
       composerElementContexts.length === 0 &&
diff --git a/apps/web/src/components/chat/ChatComposer.tsx b/apps/web/src/components/chat/ChatComposer.tsx
index 6f3a6ec22cde684f505ddd2f54a097549509527d..795cb910227ae3725c35c44d626b15570221ea38 100644
--- a/apps/web/src/components/chat/ChatComposer.tsx
+++ b/apps/web/src/components/chat/ChatComposer.tsx
@@ -875,14 +875,16 @@ export const ChatComposer = memo(function ChatComposer(props: ChatComposerProps)
 
   const selectedPromptEffort = composerProviderState.promptEffort;
   const selectedModelOptionsForDispatch = composerProviderState.modelOptionsForDispatch;
+  // Plan mode is a legacy feature behind Settings → Beta. With the flag off,
+  // ChatView forces the effective mode to "default", so hiding the toggle
+  // can't trap anyone in plan mode.
+  const planModeUiEnabled = settings.planModeEnabled;
   const composerProviderControls = useMemo(
     () => ({
-      showInteractionModeToggle: getProviderInteractionModeToggle(
-        providerStatuses,
-        selectedProvider,
-      ),
+      showInteractionModeToggle:
+        planModeUiEnabled && getProviderInteractionModeToggle(providerStatuses, selectedProvider),
     }),
-    [providerStatuses, selectedProvider],
+    [planModeUiEnabled, providerStatuses, selectedProvider],
   );
   const selectedModelSelection = useMemo<ModelSelection>(
     () => createModelSelection(selectedInstanceId, selectedModel, selectedModelOptionsForDispatch),
@@ -1043,20 +1045,24 @@ export const ChatComposer = memo(function ChatComposer(props: ChatComposerProps)
           label: "/model",
           description: "Switch response model for this thread",
         },
-        {
-          id: "slash:plan",
-          type: "slash-command",
-          command: "plan",
-          label: "/plan",
-          description: "Switch this thread into plan mode",
-        },
-        {
-          id: "slash:default",
-          type: "slash-command",
-          command: "default",
-          label: "/default",
-          description: "Switch this thread back to normal build mode",
-        },
+        ...(planModeUiEnabled
+          ? ([
+              {
+                id: "slash:plan",
+                type: "slash-command",
+                command: "plan",
+                label: "/plan",
+                description: "Switch this thread into plan mode",
+              },
+              {
+                id: "slash:default",
+                type: "slash-command",
+                command: "default",
+                label: "/default",
+                description: "Switch this thread back to normal build mode",
+              },
+            ] as const)
+          : []),
       ] satisfies ReadonlyArray<Extract<ComposerCommandItem, { type: "slash-command" }>>;
       const providerSlashCommandItems = (selectedProviderStatus?.slashCommands ?? []).map(
         (command) => ({
@@ -1091,7 +1097,13 @@ export const ChatComposer = memo(function ChatComposer(props: ChatComposerProps)
       );
     }
     return [];
-  }, [composerTrigger, selectedProvider, selectedProviderStatus, workspaceEntries.entries]);
+  }, [
+    composerTrigger,
+    planModeUiEnabled,
+    selectedProvider,
+    selectedProviderStatus,
+    workspaceEntries.entries,
+  ]);
 
   const composerMenuOpen = Boolean(composerTrigger);
   const composerMenuSearchKey = composerTrigger
@@ -1857,6 +1869,7 @@ export const ChatComposer = memo(function ChatComposer(props: ChatComposerProps)
     event: KeyboardEvent,
   ) => {
     if (key === "Tab" && event.shiftKey) {
+      if (!planModeUiEnabled) return false;
       toggleInteractionMode();
       return true;
     }
diff --git a/apps/web/src/components/settings/BetaSettingsPanel.tsx b/apps/web/src/components/settings/BetaSettingsPanel.tsx
index 740d3048f0e38a203aaedba68f3d22a9855f4f24..4b96fb15398d2d79d9f5ba666ef5d8be9753b4de 100644
--- a/apps/web/src/components/settings/BetaSettingsPanel.tsx
+++ b/apps/web/src/components/settings/BetaSettingsPanel.tsx
@@ -60,6 +60,7 @@ export function BetaSettingsPanel() {
   const sidebarAutoSettleAfterDays = useClientSettings(
     (settings) => settings.sidebarAutoSettleAfterDays,
   );
+  const planModeEnabled = useClientSettings((settings) => settings.planModeEnabled);
   const updateSettings = useUpdateClientSettings();
 
   return (
@@ -114,6 +115,17 @@ export function BetaSettingsPanel() {
             ) : null}
           </>
         ) : null}
+        <SettingsRow
+          {...searchableSetting("restore-plan-mode")}
+          description="Legacy feature. Brings back the Build/Plan toggle in the composer along with the /plan and /default commands and the Shift+Tab shortcut. While off, every thread runs in build mode."
+          control={
+            <Switch
+              checked={planModeEnabled}
+              onCheckedChange={(checked) => updateSettings({ planModeEnabled: Boolean(checked) })}
+              aria-label="Restore plan mode (legacy)"
+            />
+          }
+        />
       </SettingsSection>
     </SettingsPageContainer>
   );
diff --git a/apps/web/src/components/settings/settingsSearch.ts b/apps/web/src/components/settings/settingsSearch.ts
index 69d16147ac7efe1aa1a5ad94db8d2d41e0df9329..3b3a8c220ac47077348d554baf6c59be9898be2f 100644
--- a/apps/web/src/components/settings/settingsSearch.ts
+++ b/apps/web/src/components/settings/settingsSearch.ts
@@ -177,6 +177,11 @@ export const SETTINGS_SEARCH_ITEMS = [
     to: "/settings/beta",
     targetId: "sidebar-v2",
   },
+  {
+    id: "restore-plan-mode",
+    title: "Restore plan mode (legacy)",
+    to: "/settings/beta",
+  },
   {
     id: "archive",
     title: "Archived threads",
diff --git a/packages/contracts/src/settings.ts b/packages/contracts/src/settings.ts
index 7679ab6e4929ecadf28ca69d988c57fa6d476e72..4b477227f26e263e01f1027a602fd05119f6c0ac 100644
--- a/packages/contracts/src/settings.ts
+++ b/packages/contracts/src/settings.ts
@@ -167,6 +167,10 @@ export const ClientSettingsSchema = Schema.Struct({
       modelOrder: Schema.Array(Schema.String).pipe(Schema.withDecodingDefault(Effect.succeed([]))),
     }),
   ).pipe(Schema.withDecodingDefault(Effect.succeed({}))),
+  // Legacy plan mode. The composer's Build/Plan toggle was removed from the
+  // default UI; this beta flag restores it (plus the /plan and /default slash
+  // commands) for users who still rely on the old workflow.
+  planModeEnabled: Schema.Boolean.pipe(Schema.withDecodingDefault(Effect.succeed(false))),
   sidebarAutoSettleAfterDays: Schema.NullOr(SidebarAutoSettleAfterDays).pipe(
     Schema.withDecodingDefault(Effect.succeed(DEFAULT_SIDEBAR_AUTO_SETTLE_AFTER_DAYS)),
   ),
@@ -781,6 +785,7 @@ export const ClientSettingsPatch = Schema.Struct({
       }),
     ),
   ),
+  planModeEnabled: Schema.optionalKey(Schema.Boolean),
   sidebarAutoSettleAfterDays: Schema.optionalKey(Schema.NullOr(SidebarAutoSettleAfterDays)),
   sidebarProjectGroupingMode: Schema.optionalKey(SidebarProjectGroupingMode),
   sidebarProjectGroupingOverrides: Schema.optionalKey(