diff --git a/docs/DECISIONS.md b/docs/DECISIONS.md
index 283273e..84d7a19 100644
--- a/docs/DECISIONS.md
+++ b/docs/DECISIONS.md
@@ -1214,3 +1214,37 @@ filter now offers a **"none"** option alongside its values.
   byte-identical copies; this extracted them into `src/client/FilterSelect.tsx`
   with a `nullable` flag that adds the "none" option, so both surfaces stay in
   step.
+
+### PROG-79 — mobile top nav becomes a bottom tab bar
+
+The header packed the "Progress" logo + six text nav links + the New menu + the
+avatar into one row; on a phone that overflowed and scrolled sideways (the issue
+is filed under the **Mobile functionality** arc). Fixed by swapping the *pattern*
+on small screens, not by trimming destinations.
+
+- **Bottom tab bar on mobile, inline nav on desktop.** Below Tailwind's `sm`
+  breakpoint the header's inline `<nav>` is hidden and a fixed bottom tab bar
+  (`MobileTabBar.tsx`, `sm:hidden`) takes over; at `sm`+ the bar is hidden and the
+  original inline nav returns unchanged. The mobile header is now just logo +
+  New + avatar, so it can't overflow.
+- **The iOS-standard 5-slot split.** Four primary surfaces get their own tab
+  (Board · Outline · Agenda · Search); the rest (Structure · Archive) live behind
+  a **More** tab that opens a small sheet above the bar. Chosen over cramming all
+  six text tabs across a 375 px phone. Archive was already deemed a rare
+  destination (D49) and Structure is curation, so they're the natural pair to
+  demote.
+- **Active state is always visible** (the issue's explicit ask). The current
+  tab's icon + label are lit in the adobe accent; the **More** tab lights when
+  its sheet holds the current page (e.g. on `/structure`), so you can always see
+  where you are without opening anything. Verified with a real phone-viewport
+  browser pass (header 390 px = viewport 390 px, no horizontal scroll).
+- **One source of destinations.** The list moved to a shared `nav.tsx` (href,
+  label, `match`, an `icon` used only by the bar, and a `primary` flag) imported
+  by both the desktop header and the tab bar, so the two can't drift — same
+  reasoning as the shared `FilterSelect` (PROG-76).
+- **Bottom-anchored chrome lifts above the bar on mobile.** `main` gains
+  `pb-24` (cleared at `sm`), and the toast stack + PWA install card float above
+  the bar on phones; all use the existing `pwa-safe-bottom` inset so they clear
+  the iOS home indicator. *Rejected:* a hamburger menu (hides the active state
+  the issue wants kept visible) and a 6-tab bar (too cramped with text labels on
+  a narrow phone).
diff --git a/docs/REFERENCE.md b/docs/REFERENCE.md
index 6fb39fd..5de2936 100644
--- a/docs/REFERENCE.md
+++ b/docs/REFERENCE.md
@@ -405,7 +405,13 @@ canonical key — entirely client-side from the loaded workspace (D22).
   signed-in identity avatar. The always-available structure-creation entry point
   (SPEC v2 §4). The avatar dropdown holds the profile + **Sign out**, plus an
   **Admin** link for super-admins (D44) — Admin lives here, not in the top nav,
-  as a rare destination.
+  as a rare destination. The inline nav is **desktop-only**: below `sm` it is
+  hidden and a fixed **bottom tab bar** (`MobileTabBar.tsx`) takes over — Board ·
+  Outline · Agenda · Search as tabs and a **More** tab (sheet) for Structure ·
+  Archive, the active tab lit in the adobe accent (More included when its sheet's
+  page is current), clear of the iOS home indicator. This stops the header from
+  overflowing and scrolling sideways on a phone (PROG-79). Both surfaces read
+  their destinations from one shared `nav.tsx` list so they can't drift.
 - **Agenda (`/agenda`)** — the time-driven cut: every issue with a due date
   that isn't done/canceled, sorted by due date ascending and grouped **Overdue ·
   Today · This week · Later** (computed from the owner's local day; "this week"
diff --git a/src/client/App.tsx b/src/client/App.tsx
index f94a990..9c5af18 100644
--- a/src/client/App.tsx
+++ b/src/client/App.tsx
@@ -1,6 +1,7 @@
 import { Link, Route, Switch } from "wouter";
 import CommandLayer from "./commands/CommandLayer";
 import Header from "./Header";
+import MobileTabBar from "./MobileTabBar";
 import InstallPrompt from "./pwa/InstallPrompt";
 import SignIn from "./SignIn";
 import { UnauthenticatedError, useWorkspace } from "./store";
@@ -36,8 +37,9 @@ export default function App() {
       {/* Wide shell for the board; narrow pages re-constrain themselves.
           Tighter padding on phones — the board needs the width. Top gap is
           kept small so content sits just under the sticky header, with roomier
-          bottom padding for scroll breathing room (PROG-69). */}
-      <main className="mx-auto max-w-screen-2xl px-3 pb-5 pt-3 sm:px-6 sm:pb-10 sm:pt-4">
+          bottom padding for scroll breathing room (PROG-69). The extra mobile
+          bottom padding (pb-24) clears the fixed bottom tab bar (PROG-79). */}
+      <main className="mx-auto max-w-screen-2xl px-3 pb-24 pt-3 sm:px-6 sm:pb-10 sm:pt-4">
         {/* Initial app load: the only permitted loading state (SPEC §8.2). */}
         {isPending && <p className="text-ink-faint">Loading workspace…</p>}
         {error && <p className="text-danger">{String(error)}</p>}
@@ -87,6 +89,7 @@ export default function App() {
           </Switch>
         )}
       </main>
+      {workspace && <MobileTabBar />}
       <Toasts />
       <InstallPrompt />
     </div>
diff --git a/src/client/Header.tsx b/src/client/Header.tsx
index 72a3ecf..654feb7 100644
--- a/src/client/Header.tsx
+++ b/src/client/Header.tsx
@@ -7,6 +7,7 @@
 import { useState } from "react";
 import { Link, useLocation } from "wouter";
 import { openCreateContainer, openCreateIssue, type ContainerDialogRequest } from "./commands/controller";
+import { NAV } from "./nav";
 import { useWorkspaceSlice } from "./store";
 
 // End the session, then reload — an unauthenticated load bounces to sign-in.
@@ -18,17 +19,6 @@ async function signOut() {
   }
 }
 
-type NavItem = { href: string; label: string; match: (path: string) => boolean };
-
-const NAV: NavItem[] = [
-  { href: "/", label: "Board", match: (p) => p === "/" },
-  { href: "/outline", label: "Outline", match: (p) => p.startsWith("/outline") },
-  { href: "/agenda", label: "Agenda", match: (p) => p.startsWith("/agenda") },
-  { href: "/search", label: "Search", match: (p) => p.startsWith("/search") },
-  { href: "/structure", label: "Structure", match: (p) => p.startsWith("/structure") },
-  { href: "/archive", label: "Archive", match: (p) => p.startsWith("/archive") },
-];
-
 export default function Header() {
   const [path] = useLocation();
   const [menuOpen, setMenuOpen] = useState(false);
@@ -53,7 +43,10 @@ export default function Header() {
         <Link href="/" className="mr-2 font-semibold tracking-tight text-ink">
           Progress
         </Link>
-        <nav className="flex items-center gap-1 text-sm">
+        {/* Inline nav is desktop-only; on phones the bottom tab bar
+            (MobileTabBar) carries navigation so the header can't overflow and
+            scroll sideways (PROG-79). */}
+        <nav className="hidden items-center gap-1 text-sm sm:flex">
           {NAV.map((item) => (
             <Link
               key={item.href}
diff --git a/src/client/MobileTabBar.tsx b/src/client/MobileTabBar.tsx
new file mode 100644
index 0000000..4a36736
--- /dev/null
+++ b/src/client/MobileTabBar.tsx
@@ -0,0 +1,86 @@
+// Bottom tab bar — the mobile-only primary navigation (PROG-79). On phones the
+// header's inline nav is hidden (it overflowed and scrolled sideways), and this
+// fixed bar takes over: the four primary surfaces get a tab each, the rest sit
+// behind a "More" sheet (the iOS-standard 5-slot pattern). The active tab is
+// always lit (icon + label in the adobe accent), so you can see where you are at
+// a glance without opening anything. Hidden at `sm` and up, where the inline nav
+// returns. Destinations come from the shared NAV list so the two can't drift.
+
+import { useState } from "react";
+import { Link, useLocation } from "wouter";
+import { MoreIcon, NAV, type NavItem } from "./nav";
+
+export default function MobileTabBar() {
+  const [path] = useLocation();
+  const [moreOpen, setMoreOpen] = useState(false);
+  const primary = NAV.filter((i) => i.primary);
+  const secondary = NAV.filter((i) => !i.primary);
+  // "More" reads as active whenever a destination it holds is the current page,
+  // so the bar still shows where you are even when the surface isn't a top tab.
+  const moreActive = secondary.some((i) => i.match(path));
+
+  return (
+    // pwa-safe-bottom/x: clear the iOS home indicator and rounded corners; inert
+    // in a desktop browser. backdrop-blur + translucent paper matches the header.
+    <nav className="pwa-safe-bottom pwa-safe-x fixed inset-x-0 bottom-0 z-40 border-t border-line bg-paper/95 backdrop-blur sm:hidden">
+      {moreOpen && (
+        <>
+          {/* Tap anywhere outside to dismiss the sheet. */}
+          <div className="fixed inset-0 z-40" onClick={() => setMoreOpen(false)} />
+          <div className="absolute inset-x-0 bottom-full z-50 border-t border-line bg-card pb-1 shadow-xl">
+            {secondary.map((item) => {
+              const active = item.match(path);
+              return (
+                <Link
+                  key={item.href}
+                  href={item.href}
+                  onClick={() => setMoreOpen(false)}
+                  aria-current={active ? "page" : undefined}
+                  className={`flex items-center gap-3 px-5 py-3 text-sm ${
+                    active ? "bg-adobe-wash/40 text-adobe-deep" : "text-ink-soft hover:bg-line"
+                  }`}
+                >
+                  <span className="text-ink-faint">{item.icon}</span>
+                  {item.label}
+                </Link>
+              );
+            })}
+          </div>
+        </>
+      )}
+
+      <div className="mx-auto flex max-w-screen-2xl items-stretch">
+        {primary.map((item) => (
+          <Tab key={item.href} item={item} active={item.match(path)} />
+        ))}
+        <button
+          type="button"
+          onClick={() => setMoreOpen((o) => !o)}
+          aria-expanded={moreOpen}
+          aria-current={moreActive ? "page" : undefined}
+          className={`flex flex-1 flex-col items-center justify-center gap-0.5 py-1.5 text-[10px] font-medium ${
+            moreActive || moreOpen ? "text-adobe-deep" : "text-ink-faint"
+          }`}
+        >
+          {MoreIcon}
+          More
+        </button>
+      </div>
+    </nav>
+  );
+}
+
+function Tab({ item, active }: { item: NavItem; active: boolean }) {
+  return (
+    <Link
+      href={item.href}
+      aria-current={active ? "page" : undefined}
+      className={`flex flex-1 flex-col items-center justify-center gap-0.5 py-1.5 text-[10px] font-medium ${
+        active ? "text-adobe-deep" : "text-ink-faint"
+      }`}
+    >
+      {item.icon}
+      {item.label}
+    </Link>
+  );
+}
diff --git a/src/client/nav.tsx b/src/client/nav.tsx
new file mode 100644
index 0000000..ad5aa8b
--- /dev/null
+++ b/src/client/nav.tsx
@@ -0,0 +1,95 @@
+// The top-level navigation destinations, shared by the desktop header's inline
+// nav (Header.tsx) and the mobile bottom tab bar (MobileTabBar.tsx) so the two
+// can't drift (PROG-79). Each item carries an `icon` (used only by the tab bar;
+// the desktop nav is text-only) and a `primary` flag: the four primary surfaces
+// get their own bottom tab, the rest live behind a "More" tab — the standard
+// iOS 5-slot pattern. Desktop shows all six inline.
+
+import type { ReactNode } from "react";
+
+export type NavItem = {
+  href: string;
+  label: string;
+  match: (path: string) => boolean;
+  icon: ReactNode;
+  // True ⇒ gets its own bottom tab on mobile; false ⇒ lives in the "More" sheet.
+  primary: boolean;
+};
+
+// Shared SVG props: line icons on `currentColor`, sized for the tab bar.
+const ICON = {
+  viewBox: "0 0 24 24",
+  fill: "none",
+  stroke: "currentColor",
+  strokeWidth: 1.7,
+  strokeLinecap: "round" as const,
+  strokeLinejoin: "round" as const,
+  className: "h-6 w-6",
+  "aria-hidden": true,
+};
+
+const BoardIcon = (
+  <svg {...ICON}>
+    <rect x="3" y="4" width="5" height="16" rx="1" />
+    <rect x="9.5" y="4" width="5" height="11" rx="1" />
+    <rect x="16" y="4" width="5" height="7" rx="1" />
+  </svg>
+);
+
+const OutlineIcon = (
+  <svg {...ICON}>
+    <circle cx="5" cy="6" r="1.3" fill="currentColor" stroke="none" />
+    <circle cx="5" cy="12" r="1.3" fill="currentColor" stroke="none" />
+    <circle cx="5" cy="18" r="1.3" fill="currentColor" stroke="none" />
+    <path d="M9 6h11M9 12h11M9 18h11" />
+  </svg>
+);
+
+const AgendaIcon = (
+  <svg {...ICON}>
+    <rect x="3.5" y="5" width="17" height="15" rx="2" />
+    <path d="M3.5 9.5h17M8 3.5v3M16 3.5v3" />
+  </svg>
+);
+
+const SearchIcon = (
+  <svg {...ICON}>
+    <circle cx="11" cy="11" r="6" />
+    <line x1="15.5" y1="15.5" x2="20" y2="20" />
+  </svg>
+);
+
+const StructureIcon = (
+  <svg {...ICON}>
+    <rect x="9" y="3.5" width="6" height="4" rx="1" />
+    <rect x="3.5" y="16.5" width="6" height="4" rx="1" />
+    <rect x="14.5" y="16.5" width="6" height="4" rx="1" />
+    <path d="M12 7.5V12M6.5 16.5V13h11v3.5" />
+  </svg>
+);
+
+const ArchiveIcon = (
+  <svg {...ICON}>
+    <rect x="3.5" y="4.5" width="17" height="4" rx="1" />
+    <path d="M5 8.5V19a1 1 0 0 0 1 1h12a1 1 0 0 0 1-1V8.5" />
+    <line x1="10" y1="12.5" x2="14" y2="12.5" />
+  </svg>
+);
+
+// Three filled dots — the "More" tab glyph.
+export const MoreIcon = (
+  <svg {...ICON}>
+    <circle cx="5" cy="12" r="1.6" fill="currentColor" stroke="none" />
+    <circle cx="12" cy="12" r="1.6" fill="currentColor" stroke="none" />
+    <circle cx="19" cy="12" r="1.6" fill="currentColor" stroke="none" />
+  </svg>
+);
+
+export const NAV: NavItem[] = [
+  { href: "/", label: "Board", match: (p) => p === "/", icon: BoardIcon, primary: true },
+  { href: "/outline", label: "Outline", match: (p) => p.startsWith("/outline"), icon: OutlineIcon, primary: true },
+  { href: "/agenda", label: "Agenda", match: (p) => p.startsWith("/agenda"), icon: AgendaIcon, primary: true },
+  { href: "/search", label: "Search", match: (p) => p.startsWith("/search"), icon: SearchIcon, primary: true },
+  { href: "/structure", label: "Structure", match: (p) => p.startsWith("/structure"), icon: StructureIcon, primary: false },
+  { href: "/archive", label: "Archive", match: (p) => p.startsWith("/archive"), icon: ArchiveIcon, primary: false },
+];
diff --git a/src/client/pwa/InstallPrompt.tsx b/src/client/pwa/InstallPrompt.tsx
index 60c4e45..c2c725e 100644
--- a/src/client/pwa/InstallPrompt.tsx
+++ b/src/client/pwa/InstallPrompt.tsx
@@ -48,7 +48,7 @@ function PlusSquareIcon({ className }: { className?: string }) {
 // on phones without spanning a desktop window.
 function Card({ children, onDismiss }: { children: React.ReactNode; onDismiss: () => void }) {
   return (
-    <div className="pwa-safe-bottom pwa-safe-x fixed inset-x-0 bottom-0 z-50 flex justify-center px-3 pb-3">
+    <div className="pwa-safe-bottom pwa-safe-x fixed inset-x-0 bottom-16 z-50 flex justify-center px-3 pb-3 sm:bottom-0">
       <div className="relative w-full max-w-md rounded-lg border border-line bg-paper p-4 shadow-xl">
         <button
           onClick={onDismiss}
diff --git a/src/client/toast.tsx b/src/client/toast.tsx
index c2851f8..dc21d15 100644
--- a/src/client/toast.tsx
+++ b/src/client/toast.tsx
@@ -60,8 +60,10 @@ function subscribe(cb: () => void) {
 export function Toasts() {
   const current = useSyncExternalStore(subscribe, () => toasts);
   if (current.length === 0) return null;
+  // bottom-24 on mobile floats toasts above the fixed bottom tab bar (PROG-79);
+  // back to bottom-4 once the bar is gone at sm+.
   return (
-    <div className="fixed bottom-4 right-4 z-50 flex flex-col gap-2">
+    <div className="fixed bottom-24 right-4 z-50 flex flex-col gap-2 sm:bottom-4">
       {current.map((t) => (
         <div
           key={t.id}