diff --git a/README.md b/README.md
index 90a181cd0..972114801 100644
--- a/README.md
+++ b/README.md
@@ -77,7 +77,7 @@ The chat director adds context management on top of the reactor:
 
 ## Permissions and auto mode
 
-Corbits Code defaults to **auto mode** (`auto = true`). Workspace file writes/edits/deletes and unconstrained shell commands run without per-action prompts. Pass `--no-auto` to start in ask-on-every-consequential-action mode, or press **SHIFT+TAB** in the TUI to toggle at any time. Enabling auto prints a one-line reminder of the envelope below.
+Corbits Code defaults to **auto mode** (`auto = true`). Workspace file writes/edits/deletes and unconstrained shell commands run without per-action prompts. Pass `--no-auto` to start in ask-on-every-consequential-action mode (there is currently no in-session key to toggle auto). Press **Shift+Tab** in the TUI to cycle reasoning effort for the current model. Enabling auto prints a one-line reminder of the envelope below.
 
 ### What auto allows
 
diff --git a/docs/IMPLEMENTATION.md b/docs/IMPLEMENTATION.md
index a5b7f4452..8180e248b 100644
--- a/docs/IMPLEMENTATION.md
+++ b/docs/IMPLEMENTATION.md
@@ -154,6 +154,10 @@ When auto is on, the gate auto-allows workspace file tools in `AUTO_ALLOWED_TOOL
 
 Unmatched shell auto-allows. Writes under the session state root (`~/.corbits/projects/<project-key>/…`, and legacy in-repo `.agent-state` during dual-read), mutating MCP, and unknown built-ins still prompt. Authorization hard-denies (catastrophic commands, open-ended shell search) remain independent of auto mode.
 
+### Reasoning Effort
+
+**Shift+Tab** in the TUI cycles reasoning effort for the live model (`cycleReasoningEffort` in `src/provider/reasoning-effort.ts`); the runner rebuilds inference sources and the prompt-border `profile · model · effort` label so the next turn picks it up. Plain Tab still toggles focus.
+
 ### Interrupt and Queue Steering
 
 `ChatInputProps` carries `isProcessing?: boolean` and `onInterrupt?: (message: string) => void`. When `isProcessing` is true:
diff --git a/docs/PRODUCT.md b/docs/PRODUCT.md
index 06502643c..77d58cdf0 100644
--- a/docs/PRODUCT.md
+++ b/docs/PRODUCT.md
@@ -48,7 +48,7 @@ The evidence is in how the product fails today: the personas already produce exc
 $ corbits "Add JWT auth to the API"
 ```
 
-A full-screen terminal interface: a pinned header (session title and workflow progress), a scrollable event log, modals for permission prompts and operator questions, and a chat input for follow-up turns.
+A full-screen terminal interface: a pinned header (session title and workflow progress), a scrollable event log, modals for permission prompts and operator questions, and a chat input for follow-up turns. Press **Shift+Tab** to cycle reasoning effort for the current model; the prompt border shows the active level. Plain **Tab** still toggles focus between the prompt and the transcript.
 
 **Behavior spec** (OpenTUI is the shipping shell): `docs/TUI.md` — layout,
 chrome budget, overlays, selectors, the `/` command list, prompt box, and
diff --git a/docs/TUI.md b/docs/TUI.md
index cea6b44c9..79eb5afe5 100644
--- a/docs/TUI.md
+++ b/docs/TUI.md
@@ -61,7 +61,8 @@ the pad is part of the bubble itself, not an extra turn-boundary gap, and
 assistant/tool rows are unchanged.
 
 The prompt box's border carries the metadata that would otherwise cost a
-titlebar row: the model label sits right-aligned in the top rule; the brand
+titlebar row: the model label sits right-aligned in the top rule as
+`profile · model · effort` (empty segments omitted); the brand
 lockup sits at the left of the bottom rule with the working directory and git
 branch at its right (`AppShell.promptTopRule` / `promptBottomRule`,
 `src/tui/shell.ts`). Both rules cost zero transcript rows because they
@@ -601,6 +602,10 @@ holding the current scroll lease responds to them.
 
 `Ctrl+G` (the Emacs/readline "abort" chord) cancels the most recently queued
 mid-run message. `Tab` toggles focus between the prompt and the transcript.
+`Shift+Tab` cycles reasoning effort for the current model (wrapping the
+supported ladder) and flashes the new level; the prompt-border effort
+segment updates immediately. A model with no effort levels flashes instead
+of mutating the session. Unshifted `Tab` still toggles focus.
 `e` (with Alt/Option) expands a collapsed row — a collapsed permission
 payload while an overlay owns focus, or a collapsed transcript row (tool
 output, a long diff) while the transcript does — one expand idiom shared
diff --git a/src/config/index.ts b/src/config/index.ts
index 4d05a3b81..37f295742 100644
--- a/src/config/index.ts
+++ b/src/config/index.ts
@@ -468,7 +468,8 @@ export async function loadConfig(
   // writes/edits and unconstrained shell) run without prompting, while shell
   // file-mutation stays denied and installs / recursive rm / worktree /
   // sensitive-path / opaque-wrapper shell still ask. Pass --no-auto to revert
-  // to ask-on-every-write, or toggle live in the TUI with SHIFT+TAB.
+  // to ask-on-every-write. There is currently no in-session key to toggle auto;
+  // Shift+Tab in the TUI cycles reasoning effort instead.
   let auto = true;
   let configPath: string | undefined;
   let provider: string | undefined;
diff --git a/src/permission/gate.ts b/src/permission/gate.ts
index 738bdb469..501e3a164 100644
--- a/src/permission/gate.ts
+++ b/src/permission/gate.ts
@@ -253,8 +253,9 @@ export type PermissionGate = {
   // Whether auto mode is currently on. Auto mode auto-approves non-destructive
   // consequential actions (file writes/edits, unconstrained shell) without prompting.
   getAuto: () => boolean;
-  // Turn auto mode on or off for the rest of the session. SHIFT+TAB in the TUI
-  // wires the toggle here so a switch takes effect on the next tool call.
+  // Turn auto mode on or off for the rest of the session. Live callers (slash
+  // commands, settings) wire the toggle here so a switch takes effect on the
+  // next tool call. There is currently no in-session key chord for this.
   setAuto: (value: boolean) => void;
   // Whether --dangerously-skip-permissions is active for this session. Immutable
   // after gate construction; pre-gate sandboxes (path-escape, shell cwd bounds)
diff --git a/src/provider/reasoning-effort.test.ts b/src/provider/reasoning-effort.test.ts
index 34724ee92..297fc871f 100644
--- a/src/provider/reasoning-effort.test.ts
+++ b/src/provider/reasoning-effort.test.ts
@@ -5,6 +5,7 @@ import {
   isReasoningEffort,
   supportedEfforts,
   validateEffort,
+  cycleReasoningEffort,
   setModelReasoningCapabilities,
   modelReasoningCapability,
   clampEffort,
@@ -98,6 +99,27 @@ describe("validateEffort", () => {
   });
 });
 
+describe("cycleReasoningEffort", () => {
+  afterEach(() => setModelReasoningCapabilities({}));
+
+  test("walks the gpt-5 ladder and wraps", () => {
+    expect(cycleReasoningEffort("gpt-5", undefined)).toBe("minimal");
+    expect(cycleReasoningEffort("gpt-5", "minimal")).toBe("low");
+    expect(cycleReasoningEffort("gpt-5", "low")).toBe("medium");
+    expect(cycleReasoningEffort("gpt-5", "medium")).toBe("high");
+    expect(cycleReasoningEffort("gpt-5", "high")).toBe("minimal");
+  });
+
+  test("starts at the first supported level when current is unsupported", () => {
+    expect(cycleReasoningEffort("gpt-5", "xhigh")).toBe("minimal");
+  });
+
+  test("returns undefined for a non-reasoning model", () => {
+    setModelReasoningCapabilities({ "chat-only-model": false });
+    expect(cycleReasoningEffort("chat-only-model", "medium")).toBeUndefined();
+  });
+});
+
 describe("reasoning capability gate", () => {
   afterEach(() => setModelReasoningCapabilities({}));
 
diff --git a/src/provider/reasoning-effort.ts b/src/provider/reasoning-effort.ts
index 38425d303..bf742530e 100644
--- a/src/provider/reasoning-effort.ts
+++ b/src/provider/reasoning-effort.ts
@@ -96,6 +96,25 @@ export function validateEffort(
   };
 }
 
+/**
+ * Next effort on the model's supported ladder (wraps around). Returns undefined
+ * when the model supports no reasoning effort — callers flash a status and leave
+ * the session config alone.
+ */
+export function cycleReasoningEffort(
+  model: string,
+  current: ReasoningEffort | undefined,
+  isCodex = false,
+): ReasoningEffort | undefined {
+  const supported = supportedEfforts(model, undefined, isCodex);
+  if (supported.length === 0) return undefined;
+  if (current === undefined || !supported.includes(current)) {
+    return supported[0];
+  }
+  const idx = supported.indexOf(current);
+  return supported[(idx + 1) % supported.length];
+}
+
 // ---------------------------------------------------------------------------
 // Role-based product defaults (CL-5162)
 //
diff --git a/src/tui/keybindings.test.ts b/src/tui/keybindings.test.ts
index 9e18937ba..5a68e64a2 100644
--- a/src/tui/keybindings.test.ts
+++ b/src/tui/keybindings.test.ts
@@ -37,6 +37,7 @@ import {
   setSentMessageHistory,
   setShellBridgeHooks,
   setShellExitHandler,
+  setEffortCycleHandler,
   clearShellBridgeHooks,
   setShellRunState,
   shellFocusPrompt,
@@ -98,7 +99,7 @@ function chordsOf(keys: string): readonly (string | null)[] {
     const bytes = chordBytes(token.trim())
     // A token nothing can encode and that is not the known kitty-only chord is
     // a typo in the catalog, not an untestable chord.
-    if (bytes === null && token.trim() !== "Ctrl+Enter") {
+    if (bytes === null && token.trim() !== "Ctrl+Enter" && token.trim() !== "Shift+Tab") {
       throw new Error(`catalog row "${keys}" has unreadable chord "${token.trim()}"`)
     }
     return bytes
@@ -371,6 +372,22 @@ const PROBES: Readonly<Record<string, { readonly group: Group; readonly probe: P
       expect(focusOwner(shell.focus)).toBe("prompt")
     },
   },
+  "Shift+Tab": {
+    group: "surfaces",
+    probe: ({ h, shell }) => {
+      let cycles = 0
+      setEffortCycleHandler(shell, () => {
+        cycles++
+      })
+      shellFocusPrompt(shell)
+      const before = focusOwner(shell.focus)
+      // Classic terminals often emit CSI Z for Shift+Tab; the harness can also
+      // inject name:"tab" with shift:true, which is what the shell handler reads.
+      h.pressKey("Tab", { shift: true })
+      expect(cycles).toBe(1)
+      expect(focusOwner(shell.focus)).toBe(before)
+    },
+  },
   Esc: {
     group: "surfaces",
     probe: async ({ h, shell, chords }) => {
diff --git a/src/tui/keybindings.ts b/src/tui/keybindings.ts
index 97ac0adae..6037d1350 100644
--- a/src/tui/keybindings.ts
+++ b/src/tui/keybindings.ts
@@ -30,6 +30,7 @@ export const SHELL_SHORTCUTS: readonly ShellShortcut[] = [
   { keys: "Alt+T", description: "show or hide the task list above the prompt" },
   { keys: "Alt+O", description: "observe a live subagent session; a system row says so when there is none" },
   { keys: "Tab", description: "move focus between the prompt and the transcript" },
+  { keys: "Shift+Tab", description: "cycle reasoning effort for the current model" },
   { keys: "Esc", description: "close the open overlay, or leave subagent observe" },
   { keys: "Ctrl+B / Ctrl+F", description: "move the cursor back / forward one character" },
   { keys: "Ctrl+D", description: "delete the character under the cursor" },
diff --git a/src/tui/runner.ts b/src/tui/runner.ts
index 5145af14b..38c22caf9 100644
--- a/src/tui/runner.ts
+++ b/src/tui/runner.ts
@@ -48,11 +48,12 @@ import { modelOptionId } from "./model-catalog.js";
 import type { SessionModeScope } from "./command-surfaces.js";
 import { resolveWaitForApproval, type ToolWatchdogConfig } from "./tool-execution-watchdog.js";
 import { attachApprovalBudget, createGateRequestApproval } from "./request-approval.js";
-import { codexProfileFromProviderName } from "../config/codex-providers.js";
+import { codexProfileFromProviderName, isCodexProviderName } from "../config/codex-providers.js";
 import { xaiProfileFromProviderName } from "../config/xai-providers.js";
 import type { PluginsAdmin, PluginDescriptor } from "../plugins/admin.js";
 import type { PluginManifest } from "../plugins/manifest.js";
 import { createInferenceDependencies } from "../provider/inference-dependencies.js";
+import { cycleReasoningEffort } from "../provider/reasoning-effort.js";
 import { getValidCodexToken } from "../auth/codex/session.js";
 import { getValidXaiToken } from "../auth/xai/session.js";
 import { refreshCodexInstructions } from "../auth/codex/instructions.js";
@@ -164,11 +165,14 @@ import { mountRunnerHost } from "./runner-host.js";
 import {
   applyFocus,
   attachClipboardImage,
+  setEffortCycleHandler,
   setMentionSuggestionSource,
+  setPromptModelLabel,
   setPromptRecognitionSource,
   setSentMessageHistory,
   setShellInputSuspended,
   setShellRunState,
+  setStatusFlash,
   surfaceSystemNotice,
 } from "./shell.js";
 import {
@@ -2409,6 +2413,29 @@ export async function runTUI(initialConfig: Config): Promise<number> {
     agentNames: liveAgentProfiles.map((profile) => profile.id),
   }));
 
+  // Shift+Tab: cycle reasoning effort for the live model and rebuild sources so
+  // the next inference turn picks up the new providerOptions.reasoning_effort.
+  setEffortCycleHandler(host.shell, () => {
+    const next = cycleReasoningEffort(
+      config.model,
+      config.reasoningEffort,
+      isCodexProviderName(config.providerName),
+    );
+    if (next === undefined) {
+      setStatusFlash(host.shell, "this model has no reasoning effort levels");
+      return;
+    }
+    config = { ...config, reasoningEffort: next };
+    const bundle = buildSessionSources();
+    agentProxy.setSources(bundle.sources, bundle.defaultSource);
+    setPromptModelLabel(host.shell, {
+      profile: config.providerName,
+      model: config.model,
+      effort: next,
+    });
+    setStatusFlash(host.shell, `reasoning effort: ${next}`);
+  });
+
   // Recall spans the whole session, including what was sent before a resume.
   void loadSentMessages(config.cwd, sessionId)
     .then((sent) => setSentMessageHistory(host.shell, sent))
diff --git a/src/tui/shell.ts b/src/tui/shell.ts
index 024072720..b160de37f 100644
--- a/src/tui/shell.ts
+++ b/src/tui/shell.ts
@@ -223,6 +223,17 @@ export function clearShellExitHandler(shell: AppShell): void {
   shellExitHandlers.delete(shell)
 }
 
+const effortCycleHandlers = new WeakMap<AppShell, () => void>()
+
+/** Shift+Tab host callback: cycle reasoning effort for the live session. */
+export function setEffortCycleHandler(shell: AppShell, onCycle: () => void): void {
+  effortCycleHandlers.set(shell, onCycle)
+}
+
+export function clearEffortCycleHandler(shell: AppShell): void {
+  effortCycleHandlers.delete(shell)
+}
+
 /** Optional Wave-4 bridge hooks (runtime-bridge attaches exclusively). */
 export type ShellBridgeHooks = {
   onSubmit: (
@@ -3874,6 +3885,7 @@ export function handleOverlayAnswerKey(
 
   if (
     key.name === "tab" &&
+    !key.shift &&
     !key.ctrl &&
     !key.meta &&
     !key.option &&
@@ -5093,7 +5105,7 @@ export function handleSlashPopupKey(shell: AppShell, key: KeyEvent): boolean {
 
   const active = shell.paletteCommands[shell.overlayList.activeIndex]
 
-  if (key.name === "tab" && !key.ctrl && !key.meta && !key.option) {
+  if (key.name === "tab" && !key.shift && !key.ctrl && !key.meta && !key.option) {
     if (active) setPromptText(shell, `/${active.id} `)
     closeSlashPopup(shell)
     return true
@@ -5820,7 +5832,18 @@ export function createAppShell(
       shell.sentHistory = sentHistoryOnEdit(shell.sentHistory)
     }
 
-    if (key.name === "tab" && !key.ctrl && !key.meta && !key.option) {
+    if (
+      ((key.name === "tab" && key.shift) || key.name === "backtab") &&
+      !key.ctrl &&
+      !key.meta &&
+      !key.option
+    ) {
+      key.preventDefault()
+      effortCycleHandlers.get(shell)?.()
+      return
+    }
+
+    if (key.name === "tab" && !key.ctrl && !key.meta && !key.option && !key.shift) {
       key.preventDefault()
       toggleShellFocus(shell)
       return