diff --git a/frontend/src/api/design.ts b/frontend/src/api/design.ts
new file mode 100644
index 0000000..cbdfe09
--- /dev/null
+++ b/frontend/src/api/design.ts
@@ -0,0 +1,9 @@
+import { apiGet } from "@/api/client";
+import type { ExpectationResponse } from "@/utils/designDrift";
+
+/** Checkable claims from the rulebook this install designated as its design system.
+ *
+ *  `rulebook_id: null` means none has been designated — the normal state for a
+ *  fresh install, not an error. The caller shows an explanatory empty state. */
+export const fetchDesignExpectations = () =>
+  apiGet<ExpectationResponse>("/api/design/expectations");
diff --git a/frontend/src/utils/designDrift.ts b/frontend/src/utils/designDrift.ts
new file mode 100644
index 0000000..453e84f
--- /dev/null
+++ b/frontend/src/utils/designDrift.ts
@@ -0,0 +1,169 @@
+/**
+ * Drift comparison — what the rulebook claims vs what the stylesheet does.
+ *
+ * Milestone #251 step 5. Deliberately thin: the hard half (turning rulebook
+ * prose into claims) is server-side in `services/design_system.py`, where pytest
+ * can assert on it. What's left here is set arithmetic over live token values,
+ * which is the one thing the browser knows and the server doesn't.
+ *
+ * SCOPE, and it is a real limit rather than an omission. This compares the
+ * rulebook against the TOKENS. It cannot see the third category of drift — a
+ * literal hardcoded in a component where a token should be referenced (#2275,
+ * 67 occurrences of `color: #fff` against a rule that forbids pure white). That
+ * drift isn't in the tokens at all, so no amount of inspecting them finds it.
+ *
+ * Catching it needs the component sources, which would mean bundling every SFC
+ * into the app to read at runtime — a large cost for a panel. It belongs in CI,
+ * as a lint-shaped check, and is tracked there (#2277). Saying so in the panel
+ * matters: a drift report that silently omits a category invites the reader to
+ * conclude the category is clean.
+ */
+import type { DesignToken } from "@/utils/designTokens";
+
+export type ExpectationKind = "token" | "color" | "prohibited_color";
+
+export interface Expectation {
+  kind: ExpectationKind;
+  value: string;
+  rule_id: number;
+  rule_title: string;
+  context: string;
+}
+
+export interface ExpectationResponse {
+  rulebook_id: number | null;
+  expectations: Expectation[];
+}
+
+export type FindingStatus = "ok" | "missing" | "violated";
+
+export interface Finding {
+  expectation: Expectation;
+  status: FindingStatus;
+  /** Tokens that satisfy (or, for a prohibition, breach) the expectation. */
+  matches: string[];
+}
+
+/**
+ * Normalise a colour for comparison — the client-side twin of
+ * `normalize_hex` in services/design_system.py.
+ *
+ * These two MUST agree. The rulebook writes `#FFFFFF`, `theme.css` writes
+ * `#fff`, and getComputedStyle hands back `rgb(255, 255, 255)` — three
+ * spellings of one colour, and a comparison that misses any of them under-reports
+ * rather than erroring. The rgb() case is browser-specific and therefore has no
+ * server-side counterpart, which is exactly why it is handled here.
+ */
+export function normalizeColour(value: string): string | null {
+  const raw = value.trim().toLowerCase();
+
+  const hex = /^#([0-9a-f]{3,8})$/.exec(raw);
+  if (hex) {
+    let digits = hex[1];
+    if (digits.length === 3 || digits.length === 4) {
+      digits = digits.split("").map((c) => c + c).join("");
+    }
+    return digits.length === 6 || digits.length === 8 ? `#${digits}` : null;
+  }
+
+  // getComputedStyle always reports colours as rgb()/rgba(), never as authored.
+  const rgb = /^rgba?\(([^)]+)\)$/.exec(raw);
+  if (rgb) {
+    const parts = rgb[1].split(/[,\s/]+/).filter(Boolean);
+    if (parts.length < 3) return null;
+    const channels = parts.slice(0, 3).map((p) => Number(p));
+    if (channels.some((n) => !Number.isFinite(n))) return null;
+    const hexOf = (n: number) => Math.round(n).toString(16).padStart(2, "0");
+    const base = `#${channels.map(hexOf).join("")}`;
+    if (parts.length === 3) return base;
+    const alpha = Number(parts[3]);
+    if (!Number.isFinite(alpha) || alpha >= 1) return base;
+    return `${base}${hexOf(alpha * 255)}`;
+  }
+
+  return null;
+}
+
+/** Every distinct colour the stylesheet actually resolves to, mapped to its tokens. */
+export function colourIndex(tokens: DesignToken[]): Map<string, string[]> {
+  const index = new Map<string, string[]>();
+  for (const token of tokens) {
+    const colour = normalizeColour(token.value);
+    if (!colour) continue;
+    const names = index.get(colour);
+    if (names) names.push(token.name);
+    else index.set(colour, [token.name]);
+  }
+  return index;
+}
+
+/**
+ * Compare claims against the live tokens.
+ *
+ * A `token` claim asks whether a custom property of that name exists.
+ * A `color` claim asks whether any token resolves to that value.
+ * A `prohibited_color` claim INVERTS the test — present is the failure.
+ */
+export function compareToTokens(
+  expectations: Expectation[],
+  tokens: DesignToken[],
+): Finding[] {
+  const names = new Set(tokens.map((t) => t.name));
+  const colours = colourIndex(tokens);
+
+  return expectations.map((expectation) => {
+    if (expectation.kind === "token") {
+      const present = names.has(expectation.value);
+      return {
+        expectation,
+        status: present ? "ok" : "missing",
+        matches: present ? [expectation.value] : [],
+      };
+    }
+
+    const matches = colours.get(expectation.value) ?? [];
+    if (expectation.kind === "prohibited_color") {
+      return {
+        expectation,
+        status: matches.length ? "violated" : "ok",
+        matches,
+      };
+    }
+    return {
+      expectation,
+      status: matches.length ? "ok" : "missing",
+      matches,
+    };
+  });
+}
+
+export interface DriftSummary {
+  ok: number;
+  missing: number;
+  violated: number;
+  total: number;
+}
+
+export function summarise(findings: Finding[]): DriftSummary {
+  const summary: DriftSummary = { ok: 0, missing: 0, violated: 0, total: findings.length };
+  for (const finding of findings) summary[finding.status] += 1;
+  return summary;
+}
+
+/**
+ * Findings worth leading with.
+ *
+ * A panel that opens with every row gets closed and never reopened — the same
+ * principle the auto-inject menu is built on: a short list that gets read beats
+ * a complete one that doesn't. Violations first (something is actively wrong),
+ * then missing (something was never built), and `ok` rows are not "findings" at
+ * all — they belong behind an expansion.
+ */
+export function rankFindings(findings: Finding[]): Finding[] {
+  const order: Record<FindingStatus, number> = { violated: 0, missing: 1, ok: 2 };
+  return [...findings].sort((a, b) => {
+    const byStatus = order[a.status] - order[b.status];
+    if (byStatus !== 0) return byStatus;
+    return a.expectation.rule_id - b.expectation.rule_id;
+  });
+}
diff --git a/frontend/src/views/DesignView.vue b/frontend/src/views/DesignView.vue
index 39cfd05..0e86b8d 100644
--- a/frontend/src/views/DesignView.vue
+++ b/frontend/src/views/DesignView.vue
@@ -20,21 +20,52 @@
  */
 import { computed, onMounted, ref } from "vue";
 
+import { fetchDesignExpectations } from "@/api/design";
 import PriorityBadge from "@/components/PriorityBadge.vue";
 import StatusBadge from "@/components/StatusBadge.vue";
 import TagPill from "@/components/TagPill.vue";
+import {
+  compareToTokens,
+  rankFindings,
+  summarise,
+  type Expectation,
+  type Finding,
+} from "@/utils/designDrift";
 import { groupTokens, readTokens, type DesignToken, type TokenGroup } from "@/utils/designTokens";
 
 const tokens = ref<DesignToken[]>([]);
+const expectations = ref<Expectation[]>([]);
+const designRulebookId = ref<number | null>(null);
+const driftLoaded = ref(false);
+const showCleanRows = ref(false);
 
 /**
  * Read on mount, not at module scope: the values depend on the live cascade,
  * which needs the app's stylesheets applied and the theme attribute set.
  */
-onMounted(() => {
+onMounted(async () => {
   tokens.value = readTokens();
+  try {
+    const response = await fetchDesignExpectations();
+    designRulebookId.value = response.rulebook_id;
+    expectations.value = response.expectations;
+  } catch {
+    // The gallery is useful without the panel, so a failed fetch degrades to
+    // "no drift data" rather than taking the page down with it.
+    designRulebookId.value = null;
+  } finally {
+    driftLoaded.value = true;
+  }
 });
 
+const findings = computed<Finding[]>(() =>
+  rankFindings(compareToTokens(expectations.value, tokens.value)),
+);
+const driftSummary = computed(() => summarise(findings.value));
+const visibleFindings = computed(() =>
+  showCleanRows.value ? findings.value : findings.value.filter((f) => f.status !== "ok"),
+);
+
 const grouped = computed(() => groupTokens(tokens.value));
 
 const GROUP_ORDER: TokenGroup[] = ["color", "radius", "glow", "gradient", "focus", "layout", "other"];
@@ -81,6 +112,82 @@ const TYPE_SPECIMENS = [
       </p>
     </header>
 
+    <!-- Drift: what the rulebook claims vs what the tokens do. -->
+    <section class="design-section">
+      <h2>Rulebook drift</h2>
+
+      <p v-if="!driftLoaded" class="muted">Checking against the design rulebook…</p>
+
+      <div v-else-if="designRulebookId === null" class="gap-notice">
+        <strong>No design rulebook designated.</strong>
+        <p>
+          This install hasn't said which rulebook describes its design system, so
+          there is nothing to check the tokens against. Designate one in
+          <router-link to="/settings">Settings</router-link> and this panel will
+          compare every colour and token the rulebook names against what the
+          stylesheet actually resolves to.
+        </p>
+      </div>
+
+      <template v-else>
+        <p class="section-note">
+          <strong>{{ driftSummary.violated }}</strong> violated ·
+          <strong>{{ driftSummary.missing }}</strong> missing ·
+          {{ driftSummary.ok }} matching, from {{ driftSummary.total }} checkable
+          claims in rulebook #{{ designRulebookId }}.
+        </p>
+
+        <div class="gap-notice">
+          <strong>This compares the rulebook against the TOKENS only.</strong>
+          <p>
+            A value hardcoded in a component — where a token should have been
+            referenced — is invisible here, because the drift isn't in the tokens
+            at all. Reading it would mean bundling every component's source into
+            the app. That check belongs in CI and is tracked separately, so treat
+            a clean panel as "the tokens agree", not "the app agrees".
+          </p>
+        </div>
+
+        <p v-if="!findings.length" class="muted">
+          The rulebook names nothing this panel can check. Rules that state values
+          — colours, token names — produce claims; rules that state judgement
+          don't, by design.
+        </p>
+
+        <ul v-else class="spec-list">
+          <li v-for="finding in visibleFindings" :key="`${finding.expectation.kind}:${finding.expectation.value}`">
+            <span class="spec-name">
+              <span
+                v-if="finding.expectation.kind !== 'token'"
+                class="swatch"
+                :style="{ background: finding.expectation.value }"
+                aria-hidden="true"
+              />
+              <code>{{ finding.expectation.value }}</code>
+            </span>
+            <span class="spec-detail">
+              rule #{{ finding.expectation.rule_id }} — {{ finding.expectation.rule_title }}
+              <span v-if="finding.matches.length" class="matches">
+                · {{ finding.matches.join(", ") }}
+              </span>
+            </span>
+            <span class="spec-status" :class="finding.status">
+              {{ finding.status === "violated" ? "forbidden, but present"
+                 : finding.status === "missing" ? "not in the stylesheet" : "ok" }}
+            </span>
+          </li>
+        </ul>
+
+        <button
+          v-if="findings.length && driftSummary.ok"
+          class="reveal-toggle"
+          @click="showCleanRows = !showCleanRows"
+        >
+          {{ showCleanRows ? "Hide" : "Show" }} the {{ driftSummary.ok }} matching claims
+        </button>
+      </template>
+    </section>
+
     <!-- Real components: these are imported, not recreated. -->
     <section class="design-section">
       <h2>Components</h2>
@@ -307,6 +414,41 @@ const TYPE_SPECIMENS = [
   color: var(--color-priority-medium);
 }
 
+.spec-status.violated {
+  background: var(--color-priority-high-bg);
+  color: var(--color-priority-high);
+}
+
+.spec-status.ok {
+  background: var(--color-status-done-bg);
+  color: var(--color-status-done);
+}
+
+.spec-name .swatch {
+  vertical-align: middle;
+  margin-right: 0.4rem;
+}
+
+.matches {
+  color: var(--color-text-muted);
+}
+
+.reveal-toggle {
+  margin-top: 0.75rem;
+  padding: 0.35rem 0.75rem;
+  background: transparent;
+  color: var(--color-text-secondary);
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-sm);
+  cursor: pointer;
+  font-family: inherit;
+  font-size: 0.8rem;
+}
+
+.reveal-toggle:hover {
+  border-color: var(--color-text-muted);
+}
+
 /* Tokens ----------------------------------------------------------------- */
 
 .token-list {
diff --git a/frontend/src/views/SettingsView.vue b/frontend/src/views/SettingsView.vue
index 4c7c8d8..fbcf524 100644
--- a/frontend/src/views/SettingsView.vue
+++ b/frontend/src/views/SettingsView.vue
@@ -4,6 +4,7 @@ import { useSettingsStore } from "@/stores/settings";
 import { useAuthStore } from "@/stores/auth";
 import { useToastStore } from "@/stores/toast";
 import { apiGet, apiPost, apiPut, apiDelete, listGroups, createGroup, deleteGroup, listGroupMembers, addGroupMember, removeGroupMember, searchUsers, listApiKeys, createApiKey as apiCreateApiKey, revokeApiKey as apiRevokeApiKey, getProfile, updateProfile, type ApiKeyEntry, type GroupEntry, type GroupMember, type UserSearchResult, type UserProfile } from "@/api/client";
+import { listRulebooks } from "@/api/rulebooks";
 import type { User } from "@/types/auth";
 import PaginationBar from "@/components/PaginationBar.vue";
 import TagInput from "@/components/TagInput.vue";
@@ -31,6 +32,11 @@ const kbWritePathThreshold = ref("0.68");
 // gate: that one BLOCKS a create and must be unforgiving of noise, this one only
 // suggests a merge the operator reviews (services/dedup.py).
 const kbDuplicateThreshold = ref("0.82");
+// Which rulebook describes this install's design system, for the /design drift
+// panel. Empty = none designated, which is the normal state for a fresh install
+// rather than a misconfiguration — the panel explains itself when unset.
+const designRulebookId = ref("");
+const designRulebooks = ref<{ id: number; title: string }[]>([]);
 const savingKbInject = ref(false);
 const kbInjectSaved = ref(false);
 
@@ -100,6 +106,9 @@ async function saveKbInject() {
       kb_writepath_enabled: kbWritePathEnabled.value ? 'true' : 'false',
       kb_writepath_threshold: String(wpT),
       kb_duplicate_threshold: String(dupT),
+      // Empty string DELETES the setting (see routes/settings.py), which is
+      // exactly right for "no design rulebook" — absent rather than zero.
+      design_rulebook_id: designRulebookId.value,
     });
     kbInjectSaved.value = true;
     setTimeout(() => (kbInjectSaved.value = false), 2000);
@@ -490,6 +499,14 @@ onMounted(async () => {
   if (allSettings.kb_duplicate_threshold !== undefined) {
     kbDuplicateThreshold.value = allSettings.kb_duplicate_threshold;
   }
+  designRulebookId.value = allSettings.design_rulebook_id ?? "";
+  // Best-effort: the picker degrades to "none available" rather than blocking
+  // the whole settings page if rulebooks can't be listed.
+  try {
+    designRulebooks.value = (await listRulebooks()).map((r) => ({ id: r.id, title: r.title }));
+  } catch {
+    designRulebooks.value = [];
+  }
   if (allSettings.notify_task_reminders !== undefined) {
     notifyTaskReminders.value = allSettings.notify_task_reminders !== "false";
   }
@@ -1260,6 +1277,22 @@ function formatUserDate(iso: string): string {
             location, not by resemblance.
           </p>
         </div>
+        <div class="field">
+          <label for="design-rulebook">Design-system rulebook</label>
+          <select id="design-rulebook" v-model="designRulebookId" class="input" style="max-width: 22rem">
+            <option value="">None — don't check for design drift</option>
+            <option v-for="rb in designRulebooks" :key="rb.id" :value="String(rb.id)">
+              {{ rb.title }}
+            </option>
+          </select>
+          <p class="field-hint">
+            Which rulebook describes how this app should look. Once set, the
+            <router-link to="/design">Design</router-link> page compares every colour
+            and token your rules name against what the stylesheet actually resolves
+            to, and reports where they disagree. Leave it as None if your rules
+            don't describe a design system — nothing else depends on this.
+          </p>
+        </div>
         <div class="field">
           <label for="kb-duplicate-threshold">Near-duplicate report threshold</label>
           <input