diff --git a/client/src/AGENTS.md b/client/src/AGENTS.md
index c2ef406c94..a64d26fe0f 100644
--- a/client/src/AGENTS.md
+++ b/client/src/AGENTS.md
@@ -6,7 +6,7 @@ These apply to React/Vite client code. Universal constraints (functional program
 
 - **No window.alert/confirm** - use inline confirmations or toast notifications
 - **Form labels need `htmlFor`/`id` pairing** - when adding a settings/config form field, wire `<label htmlFor="...">` to an `id="..."` on the input — screen readers and click-to-focus both depend on the association. The visual `block`/`mb-1` styling alone doesn't establish it.
-- **Mobile responsive** - all pages should be mobile responsive friendly
+- **Mobile responsive** - all pages should be mobile responsive friendly. **A `grid-cols-N` with N > 3 and no `…:grid-cols-*` anywhere in the same class string is a bug**, not a style nit: at 360px a four-up stat bar gives each card ~65px of outer width, so its label wraps to five lines and the whole above-the-fold region is unreadable. Give text-bearing cards a narrow phone default and let the viewport opt into the wide layout (`grid-cols-2 gap-3 sm:gap-4 lg:grid-cols-4`); a widened *gap* (`gap-4 sm:gap-6`) is not a column breakpoint. A grid of square thumbnails or heatmap cells may stay four-plus wide on a phone, but it still needs a column breakpoint so the desktop isn't stuck at the phone count. Enforced tree-wide by `client/src/responsiveGridConventions.test.js`, which carries the allowlist for the few grids that really are frozen N-across (a seven-day month grid, the fixed five-trait Big Five bars).
 - **Above the fold** - keep actionable content and info above the fold and design pages for maximum information and access without scrolling
 - **No hardcoded localhost** - use `window.location.hostname` for URLs; app accessed via Tailscale remotely
 - **Chrome floating over media uses `.port-media-overlay`** - a label, status bar, badge, or control panel sitting on top of an image, a video, or a 3D canvas takes its surface and ink from the theme (`.port-media-overlay`, or `.port-media-overlay-strong` for a small label directly on photography; `.port-media-overlay-item` for its buttons, `.port-media-overlay-divider` for its separators — all defined in `index.css`, which carries the mechanism and the rule for picking between this, `.always-dark`, and the OpenWorld HUD remap). **Don't write `bg-black/NN` + `text-white`/`text-gray-N00` for this** — day mode remaps the text to the theme's dark ink while the black scrim stays black, so the label renders dark-on-black. Older overlays outside the 3D/Three.js pages still carry that idiom (migration tracked in #3796), so match the new convention rather than the nearest neighbor.
diff --git a/client/src/components/IconPicker.jsx b/client/src/components/IconPicker.jsx
index d534ccc370..ca5abff7f2 100644
--- a/client/src/components/IconPicker.jsx
+++ b/client/src/components/IconPicker.jsx
@@ -37,7 +37,7 @@ export default function IconPicker({ value, onChange }) {
             aria-hidden="true"
           />
           <div className="absolute top-full left-0 mt-1 w-full bg-port-card border border-port-border rounded-lg shadow-xl z-50 p-2 max-h-64 overflow-auto">
-            <div className="grid grid-cols-5 gap-1">
+            <div className="grid grid-cols-3 gap-1 sm:grid-cols-5">
               {iconNames.map(name => (
                 <button
                   key={name}
diff --git a/client/src/components/agents/AgentList.jsx b/client/src/components/agents/AgentList.jsx
index 2d3dc272b5..dd0fa546c6 100644
--- a/client/src/components/agents/AgentList.jsx
+++ b/client/src/components/agents/AgentList.jsx
@@ -182,9 +182,9 @@ export default function AgentList() {
         </div>
       </div>
 
-      <div className="flex-1 overflow-auto p-6">
+      <div className="flex-1 overflow-auto p-4 md:p-6">
         {/* Stats Bar */}
-        <div className="grid grid-cols-4 gap-4 mb-6">
+        <div className="grid grid-cols-2 gap-3 sm:gap-4 lg:grid-cols-4 mb-6">
           <div className="p-4 bg-port-card border border-port-border rounded-lg">
             <div className="text-2xl font-bold text-white">{stats.agents}</div>
             <div className="text-sm text-gray-400">Total Agents</div>
diff --git a/client/src/components/cos/DailyTrendsChart.jsx b/client/src/components/cos/DailyTrendsChart.jsx
index 0f0aa9bfb4..74cfec48c5 100644
--- a/client/src/components/cos/DailyTrendsChart.jsx
+++ b/client/src/components/cos/DailyTrendsChart.jsx
@@ -103,7 +103,7 @@ export default function DailyTrendsChart({ days = 30, initialExpanded = true })
       {expanded && (
         <div className="p-4 pt-0">
           {/* Summary Stats */}
-          <div className="grid grid-cols-4 gap-2 mb-4">
+          <div className="grid grid-cols-2 gap-2 sm:grid-cols-4 mb-4">
             <div className="text-center">
               <div className="text-lg font-bold text-white">{summary.totalTasks}</div>
               <div className="text-xs text-gray-500">Total Tasks</div>
diff --git a/client/src/components/cos/tabs/LearningTab.jsx b/client/src/components/cos/tabs/LearningTab.jsx
index ec96b4bd61..c6f54336ee 100644
--- a/client/src/components/cos/tabs/LearningTab.jsx
+++ b/client/src/components/cos/tabs/LearningTab.jsx
@@ -65,7 +65,7 @@ export function FeedbackSummary({ feedback }) {
             Durable ratings across live and archived agent runs
           </p>
         </div>
-        <div className="grid grid-cols-4 gap-4 sm:gap-6 text-center">
+        <div className="grid grid-cols-2 gap-4 sm:grid-cols-4 sm:gap-6 text-center">
           <div>
             <div className={`text-lg font-bold ${satisfactionClass}`}>{satisfaction}%</div>
             <div className="text-[11px] text-gray-500">helpful</div>
diff --git a/client/src/components/digital-twin/InterviewAnalysisCard.jsx b/client/src/components/digital-twin/InterviewAnalysisCard.jsx
index 7fabf012b6..8af8f86a90 100644
--- a/client/src/components/digital-twin/InterviewAnalysisCard.jsx
+++ b/client/src/components/digital-twin/InterviewAnalysisCard.jsx
@@ -69,10 +69,10 @@ export default function InterviewAnalysisCard({ analysisResult }) {
               {traitsUpdated.bigFive && (
                 <div>
                   <p className="text-gray-400 mb-1">Big Five:</p>
-                  <div className="grid grid-cols-5 gap-2">
+                  <div className="grid grid-cols-5 gap-1 sm:gap-2">
                     {Object.entries(traitsUpdated.bigFive).filter(([k]) => k !== 'notes').map(([k, v]) => (
                       <div key={k} className="text-center">
-                        <div className="text-xs text-gray-500">{k}</div>
+                        <div className="text-[10px] sm:text-xs text-gray-500">{k}</div>
                         <div className="text-white font-medium">{typeof v === 'number' ? v.toFixed(2) : v}</div>
                       </div>
                     ))}
diff --git a/client/src/components/digital-twin/tabs/ImportTab.jsx b/client/src/components/digital-twin/tabs/ImportTab.jsx
index d30aab114a..852d434e9b 100644
--- a/client/src/components/digital-twin/tabs/ImportTab.jsx
+++ b/client/src/components/digital-twin/tabs/ImportTab.jsx
@@ -435,12 +435,12 @@ export default function ImportTab() {
               {analysisResult.insights.personalityInferences.bigFive && (
                 <div className="mb-6">
                   <h4 className="text-sm font-medium text-gray-400 mb-3">Big Five Traits</h4>
-                  <div className="grid grid-cols-5 gap-2">
+                  <div className="grid grid-cols-5 gap-1 sm:gap-2">
                     {Object.entries(analysisResult.insights.personalityInferences.bigFive).map(([trait, score]) => {
                       const labels = { O: 'Open', C: 'Consc', E: 'Extra', A: 'Agree', N: 'Neuro' };
                       return (
                         <div key={trait} className="text-center">
-                          <div className="text-xs text-gray-500 mb-1">{labels[trait] || trait}</div>
+                          <div className="text-[10px] sm:text-xs text-gray-500 mb-1">{labels[trait] || trait}</div>
                           <div className="h-16 bg-port-bg rounded-lg relative overflow-hidden">
                             <div
                               className="absolute bottom-0 left-0 right-0 bg-port-accent/50"
diff --git a/client/src/components/sync/SyncDetailDrawer.jsx b/client/src/components/sync/SyncDetailDrawer.jsx
index 7cd54d4961..6d129422ba 100644
--- a/client/src/components/sync/SyncDetailDrawer.jsx
+++ b/client/src/components/sync/SyncDetailDrawer.jsx
@@ -124,7 +124,7 @@ function CollectionPreview({ collection, loading, error }) {
         <p className="text-gray-500 text-xs">{items.length} item{items.length !== 1 ? 's' : ''}</p>
       </div>
       {imageItems.length > 0 && (
-        <div className="grid grid-cols-4 gap-1">
+        <div className="grid grid-cols-2 gap-1 sm:grid-cols-4">
           {imageItems.map((it) => (
             <MediaImage
               key={`${it.kind}:${it.ref}`}
diff --git a/client/src/pages/ChiefOfStaff.jsx b/client/src/pages/ChiefOfStaff.jsx
index d20ab7ea04..378e739853 100644
--- a/client/src/pages/ChiefOfStaff.jsx
+++ b/client/src/pages/ChiefOfStaff.jsx
@@ -1026,7 +1026,7 @@ export default function ChiefOfStaff() {
       <div className="flex-1 min-h-0 min-w-0 overflow-y-auto overflow-x-hidden p-3 lg:p-4">
         {/* Stats Bar - hidden for SVG/canvas modes (now integrated into CoS sidebar);
             ascii/terminal mode keeps it because TerminalCoSPanel doesn't host the cards. */}
-        <div className={`grid grid-cols-5 gap-1.5 sm:gap-2 lg:gap-3 mb-3 sm:mb-4 lg:mb-6 ${avatarStyle !== 'ascii' ? 'hidden' : ''}`}>
+        <div className={`grid grid-cols-3 gap-1.5 sm:grid-cols-5 sm:gap-2 lg:gap-3 mb-3 sm:mb-4 lg:mb-6 ${avatarStyle !== 'ascii' ? 'hidden' : ''}`}>
           <StatCard
             label="Active"
             value={activeAgentCount}
diff --git a/client/src/responsiveGridConventions.test.js b/client/src/responsiveGridConventions.test.js
new file mode 100644
index 0000000000..7ef6b0fe09
--- /dev/null
+++ b/client/src/responsiveGridConventions.test.js
@@ -0,0 +1,179 @@
+/**
+ * Repo-wide responsive-grid convention.
+ *
+ * `client/src/AGENTS.md` requires every page to be mobile responsive, and the
+ * overwhelming majority of grids in the tree already carry a column breakpoint
+ * (`grid-cols-1 sm:grid-cols-2 xl:grid-cols-3`). The failure mode this guard
+ * catches is the one that keeps slipping through review: a stat/summary bar
+ * hard-coding four or five columns with no breakpoint prefix at all. At a 360px
+ * viewport that leaves each card ~65px of outer width — after its own padding,
+ * ~34px of content — so a label like "Platform Accounts" wraps to five lines and
+ * the whole above-the-fold region becomes unreadable (issue #5679).
+ *
+ * The rule: a `className` string holding a bare `grid-cols-N` for N >= 4 must
+ * also hold at least one *prefixed* `…:grid-cols-*` in the same string, so the
+ * column count is conditional on available width instead of frozen at every
+ * viewport. Any variant prefix counts — a media breakpoint (`sm:`), a container
+ * query (`@3xl:`), or a custom variant.
+ *
+ * Deliberately NOT "the unprefixed default must be <= 3 columns": a grid of
+ * square thumbnails or heatmap cells is legitimately four to six wide on a
+ * phone (`grid-cols-4 sm:grid-cols-8 lg:grid-cols-12` over sprite frames,
+ * `grid-cols-6 sm:grid-cols-12` over a 24-hour heatmap) because the cell holds
+ * no text to wrap. The defect this guard exists for is the *frozen* count — a
+ * grid that never learned the viewport changes — and the author who writes a
+ * breakpoint at all has demonstrably thought about the narrow case.
+ *
+ * Scoped to git-tracked non-test sources under `client/src` so an untracked
+ * scratch file can't fail the suite. Comments are masked first: a doc comment
+ * quoting an example class string is documentation, not markup.
+ */
+
+import { describe, it, expect } from 'vitest';
+import { readFileSync } from 'fs';
+import { join, dirname } from 'path';
+import { fileURLToPath } from 'url';
+import { trackedSourceFiles } from './test/trackedFiles.js';
+
+const CLIENT_ROOT = join(dirname(fileURLToPath(import.meta.url)), '..');
+
+/**
+ * Grids that are legitimately N-across on a phone, with the reason they are.
+ * Adding a row here is a deliberate design call, not a way past the guard — the
+ * cell has to stay readable at 360px on its own merits.
+ */
+const ALLOWED = new Map([
+  [
+    'src/components/calendar/MonthView.jsx',
+    'A month grid is inherently seven days wide; collapsing the columns would stop it being a calendar.',
+  ],
+  [
+    'src/components/digital-twin/tabs/ImportTab.jsx',
+    'Big Five traits are a fixed five-across bar chart; each cell is one bar plus a five-character label, kept legible with a reduced gap and a smaller label below `sm`.',
+  ],
+  [
+    'src/components/digital-twin/InterviewAnalysisCard.jsx',
+    'Same Big Five five-across readout, in the interview-analysis summary.',
+  ],
+]);
+
+/** Blank out `//` and block comments so a quoted example class isn't scanned as markup. */
+function maskComments(source) {
+  let out = '';
+  let i = 0;
+  while (i < source.length) {
+    const ch = source[i];
+    if (ch === '/' && source[i + 1] === '/') {
+      while (i < source.length && source[i] !== '\n') {
+        out += ' ';
+        i += 1;
+      }
+      continue;
+    }
+    if (ch === '/' && source[i + 1] === '*') {
+      while (i < source.length && !(source[i] === '*' && source[i + 1] === '/')) {
+        out += source[i] === '\n' ? '\n' : ' ';
+        i += 1;
+      }
+      out += '  ';
+      i += 2;
+      continue;
+    }
+    if (ch === '"' || ch === "'" || ch === '`') {
+      out += ch;
+      i += 1;
+      while (i < source.length && source[i] !== ch) {
+        if (source[i] === '\\') {
+          out += source.slice(i, i + 2);
+          i += 2;
+          continue;
+        }
+        out += source[i];
+        i += 1;
+      }
+      out += source[i] ?? '';
+      i += 1;
+      continue;
+    }
+    out += ch;
+    i += 1;
+  }
+  return out;
+}
+
+const STRING_LITERAL = /(['"`])((?:\\.|(?!\1)[\s\S])*?)\1/g;
+// A column count that is the phone default: no `variant:` in front of it. Global,
+// because a single class string can hold more than one bare token and the widest
+// one is the one that decides whether the layout is legible.
+const BARE_GRID = /(?:^|\s)grid-cols-(\d+)(?=\s|$)/g;
+// Any variant prefix — `sm:`, `lg:`, `@3xl:`, `roomy-viewport:` — makes it conditional.
+const PREFIXED_GRID = /[\w@[\]().\-/]+:grid-cols-/;
+const MIN_WIDE_COLUMNS = 4;
+
+function widestBareColumnCount(value) {
+  BARE_GRID.lastIndex = 0;
+  let widest = 0;
+  let bare;
+  while ((bare = BARE_GRID.exec(value))) widest = Math.max(widest, Number(bare[1]));
+  return widest;
+}
+
+function lineOf(source, index) {
+  return source.slice(0, index).split('\n').length;
+}
+
+function violationsIn(rawSource, file) {
+  const source = maskComments(rawSource);
+  const found = [];
+  let match;
+  STRING_LITERAL.lastIndex = 0;
+  while ((match = STRING_LITERAL.exec(source))) {
+    const value = match[2];
+    if (widestBareColumnCount(value) < MIN_WIDE_COLUMNS) continue;
+    if (PREFIXED_GRID.test(value)) continue;
+    found.push(`${file}:${lineOf(source, match.index)} — "${value.trim()}"`);
+  }
+  return found;
+}
+
+const findViolations = (file) =>
+  violationsIn(readFileSync(join(CLIENT_ROOT, file), 'utf8'), file);
+
+describe('responsive grid conventions', () => {
+  const files = trackedSourceFiles(CLIENT_ROOT);
+
+  it('scans a populated client tree', () => {
+    expect(files.length).toBeGreaterThan(100);
+  });
+
+  // Without this the suite would still pass if the detector silently stopped
+  // matching anything — a green tree-wide guard proves nothing on its own.
+  it('flags a bare wide grid and clears every conditional form', () => {
+    const flagged = (markup) => violationsIn(`<div className="${markup}" />`, 'probe.jsx').length;
+    expect(flagged('grid grid-cols-4 gap-4 mb-6')).toBe(1);
+    expect(flagged('grid grid-cols-5 gap-2')).toBe(1);
+    // A widened gap is not a column breakpoint — it leaves four columns frozen.
+    expect(flagged('grid grid-cols-4 gap-4 sm:gap-6 text-center')).toBe(1);
+    // The widest bare token decides, not the first one seen.
+    expect(flagged('grid grid-cols-2 gap-2 grid-cols-6')).toBe(1);
+    expect(flagged('grid grid-cols-2 gap-3 sm:gap-4 lg:grid-cols-4')).toBe(0);
+    expect(flagged('grid grid-cols-4 sm:grid-cols-8 lg:grid-cols-12 gap-1.5')).toBe(0);
+    expect(flagged('grid grid-cols-2 gap-3 @3xl:grid-cols-4')).toBe(0);
+    // Narrow grids are fine unprefixed — three stat cards still fit at 360px.
+    expect(flagged('grid grid-cols-3 gap-2')).toBe(0);
+    // A doc comment quoting an example class string is not markup.
+    expect(violationsIn('// e.g. "grid grid-cols-4 gap-4"', 'probe.jsx')).toEqual([]);
+  });
+
+  it('never hard-codes four or more columns without a breakpoint', () => {
+    const violations = files
+      .filter((file) => !ALLOWED.has(file))
+      .flatMap((file) => findViolations(file));
+    expect(violations).toEqual([]);
+  });
+
+  it('keeps the allowlist honest — every exempt file still has the wide grid it was exempted for', () => {
+    const stale = [...ALLOWED.keys()].filter((file) => findViolations(file).length === 0);
+    expect(stale).toEqual([]);
+  });
+});
diff --git a/scripts/ci-test-plan.js b/scripts/ci-test-plan.js
index 03097ce3cc..0b7d57a299 100644
--- a/scripts/ci-test-plan.js
+++ b/scripts/ci-test-plan.js
@@ -238,13 +238,15 @@ const structuralTestsFor = (changedFiles, trackedSet) => {
   if (changedFiles.some((path) => /^client\/src\/.*\.jsx$/.test(path))) {
     add('client/src/a11yConventions.test.js');
   }
-  // Both `.js` and `.jsx`: the StrictMode mounted-ref bug this guards against
+  // Both `.js` and `.jsx`: the StrictMode mounted-ref bug the first guard covers
   // reached its widest blast radius through a plain-`.js` hook (`useAsyncAction`),
-  // so a `.jsx`-only trigger would miss the case that matters most. Nothing else
-  // selects this file — it has no source sibling and imports no app module, so
-  // without this entry it only ever runs on a full suite.
+  // so a `.jsx`-only trigger would miss the case that matters most, and the
+  // responsive-grid guard reads class strings out of both extensions. Neither
+  // file has a source sibling or imports an app module, so nothing else selects
+  // them — without this entry they only ever run on a full suite.
   if (changedFiles.some((path) => /^client\/src\/.*\.jsx?$/.test(path))) {
     add('client/src/hooks/mountedRefConventions.test.js');
+    add('client/src/responsiveGridConventions.test.js');
   }
 
   return selected;
diff --git a/scripts/repo-scan-guards.test.js b/scripts/repo-scan-guards.test.js
index b231955ffc..30f70375ad 100644
--- a/scripts/repo-scan-guards.test.js
+++ b/scripts/repo-scan-guards.test.js
@@ -39,6 +39,7 @@ const STRUCTURALLY_SELECTED = new Map([
   // the only way either can start failing.
   ['client/src/a11yConventions.test.js', 'structuralTestsFor: client/src/**.jsx'],
   ['client/src/hooks/mountedRefConventions.test.js', 'structuralTestsFor: client/src/**.js(x)'],
+  ['client/src/responsiveGridConventions.test.js', 'structuralTestsFor: client/src/**.js(x)'],
   // `.ps1` is not in EXECUTABLE_RE, so touching one is an "unclassified changed
   // file" and forces the complete suite. The guard also rides the Windows
   // contract list.