diff --git a/web/src/catalog.ts b/web/src/catalog.ts
index febb706..2f1a4f2 100644
--- a/web/src/catalog.ts
+++ b/web/src/catalog.ts
@@ -1,6 +1,7 @@
 import { escapeHtml } from "./html.js";
 import { getDaemonInfo, killSession, renameSession } from "./api.js";
 import { openContextMenu } from "./contextMenu.js";
+import { toggleHelp } from "./help.js";
 import { CatalogStore } from "./events.js";
 import type { Session } from "./types.js";
 import {
@@ -305,6 +306,14 @@ export function renderTopbar(store: CatalogStore, onToggleSidebar?: () => void):
     kit.title = "Open the design kit (DEV)";
     actions.appendChild(kit);
   }
+  const helpBtn = document.createElement("button");
+  helpBtn.type = "button";
+  helpBtn.className = "btn btn-ghost btn-icon help-btn";
+  helpBtn.textContent = "?";
+  helpBtn.title = "Keyboard shortcuts (?)";
+  helpBtn.setAttribute("aria-label", "Keyboard shortcuts");
+  helpBtn.addEventListener("click", () => toggleHelp());
+  actions.appendChild(helpBtn);
   actions.appendChild(createThemeToggle());
   const spawnBtn = document.createElement("button");
   spawnBtn.type = "button";
diff --git a/web/src/help.test.ts b/web/src/help.test.ts
new file mode 100644
index 0000000..660e06d
--- /dev/null
+++ b/web/src/help.test.ts
@@ -0,0 +1,112 @@
+// @vitest-environment happy-dom
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+
+import {
+  SHORTCUTS,
+  closeHelp,
+  installHelpKeybind,
+  isHelpOpen,
+  isTypingTarget,
+  openHelp,
+} from "./help.js";
+
+beforeEach(() => {
+  installHelpKeybind();
+});
+
+afterEach(() => {
+  closeHelp();
+  document.body.replaceChildren();
+});
+
+function press(key: string, init: KeyboardEventInit = {}): void {
+  window.dispatchEvent(new KeyboardEvent("keydown", { key, bubbles: true, ...init }));
+}
+
+function rows(): string[] {
+  return [...document.querySelectorAll(".help-row")].map((r) =>
+    [...r.querySelectorAll("kbd")].map((k) => k.textContent).join(" "),
+  );
+}
+
+describe("help overlay", () => {
+  it("renders every registered shortcut, grouped", () => {
+    openHelp();
+    const groups = [...document.querySelectorAll(".help-group-title")].map((h) => h.textContent);
+    expect(groups).toEqual(SHORTCUTS.map((g) => g.title));
+    expect(rows()).toEqual(SHORTCUTS.flatMap((g) => g.items.map((s) => s.keys.join(" "))));
+    expect(document.querySelectorAll(".help-desc")).toHaveLength(
+      SHORTCUTS.reduce((n, g) => n + g.items.length, 0),
+    );
+  });
+
+  it("opens on ? and closes on a second ?", () => {
+    press("?", { shiftKey: true });
+    expect(isHelpOpen()).toBe(true);
+    press("?", { shiftKey: true });
+    expect(isHelpOpen()).toBe(false);
+  });
+
+  it("closes on Escape, on the close button, and on a click outside the panel", () => {
+    openHelp();
+    press("Escape");
+    expect(isHelpOpen()).toBe(false);
+
+    openHelp();
+    (document.querySelector(".help-close") as HTMLButtonElement).click();
+    expect(isHelpOpen()).toBe(false);
+
+    openHelp();
+    (document.querySelector(".help-overlay") as HTMLElement).dispatchEvent(
+      new MouseEvent("click", { bubbles: true }),
+    );
+    expect(isHelpOpen()).toBe(false);
+  });
+
+  it("stays open for a click inside the panel", () => {
+    openHelp();
+    (document.querySelector(".help-panel") as HTMLElement).dispatchEvent(
+      new MouseEvent("click", { bubbles: true }),
+    );
+    expect(isHelpOpen()).toBe(true);
+  });
+
+  it("opening twice leaves one overlay", () => {
+    openHelp();
+    openHelp();
+    expect(document.querySelectorAll(".help-overlay")).toHaveLength(1);
+  });
+
+  it("ignores ? while typing and when a non-shift modifier is held", () => {
+    const input = document.createElement("input");
+    document.body.appendChild(input);
+    input.dispatchEvent(new KeyboardEvent("keydown", { key: "?", bubbles: true, shiftKey: true }));
+    expect(isHelpOpen()).toBe(false);
+
+    press("?", { ctrlKey: true });
+    expect(isHelpOpen()).toBe(false);
+  });
+
+  it("installing the keybind twice still toggles once", () => {
+    installHelpKeybind();
+    press("?", { shiftKey: true });
+    expect(isHelpOpen()).toBe(true);
+    expect(document.querySelectorAll(".help-overlay")).toHaveLength(1);
+  });
+});
+
+describe("isTypingTarget", () => {
+  it("is true for form fields and editable nodes, false otherwise", () => {
+    const el = (html: string) => {
+      const d = document.createElement("div");
+      d.innerHTML = html;
+      return d.firstElementChild as HTMLElement;
+    };
+    expect(isTypingTarget(el("<input>"))).toBe(true);
+    expect(isTypingTarget(el("<textarea></textarea>"))).toBe(true);
+    expect(isTypingTarget(el("<select></select>"))).toBe(true);
+    expect(isTypingTarget(el('<div contenteditable="true"></div>'))).toBe(true);
+    expect(isTypingTarget(el("<button></button>"))).toBe(false);
+    expect(isTypingTarget(null)).toBe(false);
+  });
+});
diff --git a/web/src/help.ts b/web/src/help.ts
new file mode 100644
index 0000000..2437aec
--- /dev/null
+++ b/web/src/help.ts
@@ -0,0 +1,159 @@
+/** Keyboard-shortcut overlay (`?`), the web counterpart of the TUI's help
+ *  overlay. One registry drives both the rendering and what we claim is bound,
+ *  so a binding added to `main.ts` without an entry here reads as missing. */
+
+export interface Shortcut {
+  /** Keycaps, rendered left to right as separate <kbd>. */
+  keys: string[];
+  description: string;
+}
+
+export interface ShortcutGroup {
+  title: string;
+  items: Shortcut[];
+}
+
+export const SHORTCUTS: ShortcutGroup[] = [
+  {
+    title: "Sessions",
+    items: [
+      { keys: ["n"], description: "Open a spawn draft pane" },
+      { keys: ["N"], description: "Spawn a session with every default resolved" },
+    ],
+  },
+  {
+    title: "Help",
+    items: [
+      { keys: ["?"], description: "Show or hide this help" },
+      { keys: ["Esc"], description: "Close the help overlay or an open menu" },
+    ],
+  },
+  {
+    title: "Debug",
+    items: [{ keys: ["Ctrl", "Shift", "D"], description: "Toggle the debug HUD" }],
+  },
+];
+
+/** True when the event target swallows plain keystrokes — a form field, or the
+ *  hidden textarea xterm types into. Shared with `main.ts` so every global
+ *  single-key binding is deaf in the same places. */
+export function isTypingTarget(target: EventTarget | null): boolean {
+  const el = target as HTMLElement | null;
+  if (!el || typeof el.tagName !== "string") return false;
+  return (
+    el.tagName === "INPUT" ||
+    el.tagName === "TEXTAREA" ||
+    el.tagName === "SELECT" ||
+    el.isContentEditable
+  );
+}
+
+let open: { el: HTMLElement; dispose: () => void } | null = null;
+
+export function isHelpOpen(): boolean {
+  return open !== null;
+}
+
+export function closeHelp(): void {
+  open?.dispose();
+}
+
+export function toggleHelp(): void {
+  if (open) closeHelp();
+  else openHelp();
+}
+
+/** Mount the overlay. Opening while already open is a no-op re-render, so a
+ *  stray second call can never stack two dialogs. */
+export function openHelp(): HTMLElement {
+  closeHelp();
+
+  const el = document.createElement("div");
+  el.className = "help-overlay";
+  el.setAttribute("role", "dialog");
+  el.setAttribute("aria-modal", "true");
+  el.setAttribute("aria-label", "Keyboard shortcuts");
+
+  const panel = document.createElement("div");
+  panel.className = "help-panel";
+
+  const head = document.createElement("header");
+  head.className = "help-head";
+  const title = document.createElement("h2");
+  title.className = "help-title";
+  title.textContent = "Keyboard shortcuts";
+  const close = document.createElement("button");
+  close.type = "button";
+  close.className = "help-close";
+  close.setAttribute("aria-label", "Close");
+  close.textContent = "✕";
+  close.addEventListener("click", closeHelp);
+  head.append(title, close);
+  panel.appendChild(head);
+
+  for (const group of SHORTCUTS) {
+    const section = document.createElement("section");
+    section.className = "help-group";
+    const heading = document.createElement("h3");
+    heading.className = "help-group-title";
+    heading.textContent = group.title;
+    const list = document.createElement("dl");
+    list.className = "help-list";
+    for (const item of group.items) {
+      const row = document.createElement("div");
+      row.className = "help-row";
+      const keys = document.createElement("dt");
+      keys.className = "help-keys";
+      for (const key of item.keys) {
+        const cap = document.createElement("kbd");
+        cap.textContent = key;
+        keys.appendChild(cap);
+      }
+      const desc = document.createElement("dd");
+      desc.className = "help-desc";
+      desc.textContent = item.description;
+      row.append(keys, desc);
+      list.appendChild(row);
+    }
+    section.append(heading, list);
+    panel.appendChild(section);
+  }
+
+  // A press on the scrim closes; one inside the panel is left alone.
+  el.addEventListener("click", (ev) => {
+    if (!panel.contains(ev.target as Node)) closeHelp();
+  });
+  el.appendChild(panel);
+  document.body.appendChild(el);
+
+  const dispose = () => {
+    if (open?.el !== el) return;
+    open = null;
+    el.remove();
+  };
+  open = { el, dispose };
+  close.focus();
+  return el;
+}
+
+let installed = false;
+
+/** Global keybind: `?` toggles the overlay, Escape closes it. Idempotent, so a
+ *  second call during a hot reload doesn't double-toggle. */
+export function installHelpKeybind(): void {
+  if (installed) return;
+  installed = true;
+  window.addEventListener("keydown", (e) => {
+    if (e.key === "Escape" && open) {
+      e.preventDefault();
+      closeHelp();
+      return;
+    }
+    // Shift is how `?` is typed on most layouts; the others would be a
+    // different binding entirely.
+    if (e.key !== "?" || e.ctrlKey || e.altKey || e.metaKey) return;
+    if (isTypingTarget(e.target)) return;
+    e.preventDefault();
+    toggleHelp();
+  });
+}
diff --git a/web/src/main.ts b/web/src/main.ts
index 11130bd..857c96f 100644
--- a/web/src/main.ts
+++ b/web/src/main.ts
@@ -1,6 +1,7 @@
 import { renderSidebar, renderTopbar } from "./catalog.js";
 import { ConnectionPool } from "./connections.js";
 import { installHudKeybind } from "./debug.js";
+import { installHelpKeybind, isHelpOpen, isTypingTarget } from "./help.js";
 import { CatalogStore } from "./events.js";
 import { shouldRetryClose } from "./reconnect.js";
 import {
@@ -28,6 +29,7 @@ import "./styles.css";
 initTheme();
 installViewportSizing();
 installHudKeybind();
+installHelpKeybind();
 registerServiceWorker();
 
 // Register the PWA service worker so the app is installable and the shell loads
@@ -219,6 +221,7 @@ function renderWelcome(): HTMLElement {
       Pick a session from the list, or start a new one. Open sessions keep
       streaming in the background — switching back is instant.
     </p>
+    <p class="empty-hint">Press <kbd>?</kbd> for keyboard shortcuts.</p>
     <div class="empty-actions"></div>`;
   const spawn = document.createElement("button");
   spawn.type = "button";
@@ -327,16 +330,9 @@ async function requestFastForward(): Promise<void> {
 // textarea, search) so an attached session's keys are never hijacked.
 document.addEventListener("keydown", (e) => {
   if (e.ctrlKey || e.altKey || e.metaKey) return;
-  const target = e.target as HTMLElement | null;
-  if (
-    target &&
-    (target.tagName === "INPUT" ||
-      target.tagName === "TEXTAREA" ||
-      target.tagName === "SELECT" ||
-      target.isContentEditable)
-  ) {
-    return;
-  }
+  if (isTypingTarget(e.target)) return;
+  // The help overlay owns the keyboard while it is up.
+  if (isHelpOpen()) return;
   if (e.key === "n") {
     e.preventDefault();
     requestOpenSpawn();
diff --git a/web/src/styles.css b/web/src/styles.css
index 67fabfb..ddb2e04 100644
--- a/web/src/styles.css
+++ b/web/src/styles.css
@@ -876,6 +876,113 @@ input.leaf-rename:focus {
   background: var(--danger-tint);
 }
 
+/* ---------- help overlay (`?`) ---------- */
+.help-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: var(--z-modal);
+  display: grid;
+  place-items: center;
+  padding: var(--sp-4);
+  background: oklch(0 0 0 / 0.5);
+  backdrop-filter: blur(4px);
+}
+
+.help-panel {
+  width: min(440px, 100%);
+  max-height: min(80vh, 640px);
+  overflow-y: auto;
+  padding: var(--sp-4);
+  background: var(--surface);
+  border: 1px solid var(--border-strong);
+  border-radius: var(--r-3);
+  box-shadow: var(--shadow-2);
+}
+
+.help-head {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: var(--sp-2);
+  margin-bottom: var(--sp-3);
+}
+
+.help-title {
+  margin: 0;
+  font-family: var(--font-mono);
+  font-size: var(--fs-15);
+  color: var(--ink);
+}
+
+.help-close {
+  appearance: none;
+  background: none;
+  border: 0;
+  border-radius: var(--r-2);
+  color: var(--muted);
+  font: inherit;
+  line-height: 1;
+  padding: var(--sp-1) var(--sp-2);
+  cursor: pointer;
+}
+
+.help-close:hover,
+.help-close:focus-visible {
+  background: var(--surface-3);
+  color: var(--ink);
+  outline: none;
+}
+
+.help-group + .help-group {
+  margin-top: var(--sp-4);
+}
+
+.help-group-title {
+  margin: 0 0 var(--sp-2);
+  font-size: var(--fs-11);
+  font-weight: 600;
+  letter-spacing: 0.08em;
+  text-transform: uppercase;
+  color: var(--faint);
+}
+
+.help-list {
+  margin: 0;
+}
+
+/* Keycaps in a fixed column so the descriptions line up down the list. */
+.help-row {
+  display: grid;
+  grid-template-columns: 14ch 1fr;
+  align-items: baseline;
+  gap: var(--sp-3);
+  padding: var(--sp-1) 0;
+}
+
+.help-keys {
+  display: flex;
+  flex-wrap: nowrap;
+  gap: var(--sp-1);
+  margin: 0;
+}
+
+.help-keys kbd {
+  font-family: var(--font-mono);
+  font-size: var(--fs-11);
+  background: var(--surface-2);
+  border: 1px solid var(--border);
+  border-bottom-width: 2px;
+  border-radius: var(--r-1);
+  padding: 1px 6px;
+  color: var(--ink);
+}
+
+.help-desc {
+  margin: 0;
+  font-size: var(--fs-13);
+  color: var(--muted);
+}
+
 /* Line 2 — dot + duration in the state's colour, then muted fragments. The
    state word itself is never spelled out here; the colour carries it. */
 .card-line2 {
@@ -2106,7 +2213,7 @@ input.leaf-rename:focus {
 .mv-canvas > .mv-split,
 .mv-canvas > .mv-pane {
   position: absolute;
-  inset: var(--sp-2);
+  inset: 0;
 }
 .mv-canvas > .content-welcome {
   position: absolute;
@@ -2346,7 +2453,6 @@ input.leaf-rename:focus {
 
 .mv-mobile {
   position: absolute;
-  /* Flush to the edges — a single full-screen pane needs no canvas inset. */
   inset: 0;
   display: flex;
   flex-direction: column;