diff --git a/corpus/frontend/ui-ux/dark-mode-principles.yaml b/corpus/frontend/ui-ux/dark-mode-principles.yaml
new file mode 100644
index 0000000..e45e6c1
--- /dev/null
+++ b/corpus/frontend/ui-ux/dark-mode-principles.yaml
@@ -0,0 +1,19 @@
+name: dark-mode-principles
+domain: color
+rule: "Dark mode: redesign, don't invert. Warm charcoal, not black."
+detail: >-
+  Background: oklch(0.13 0.008 265) - warm charcoal, not #000. Text:
+  oklch(0.94 0.008 265) - off-white, not #fff. Higher elevation = lighter bg
+  (shadows invisible on dark). Reduce saturation slightly (vivid on dark =
+  neon). Primary accents brighten: brand-600 → brand-400.
+cssExample: |
+  .dark {
+    --color-bg: oklch(0.13 0.008 265);   /* warm charcoal */
+    --color-text: oklch(0.94 0.008 265); /* off-white */
+    /* surface-1 */ --color-surface: oklch(0.16 0.008 265);
+    /* surface-2 */ --color-card: oklch(0.19 0.008 265);
+  }
+antiPatterns:
+  - Pure black background (#000)
+  - Pure white text (#fff) on dark
+  - Inverting light mode colors directly
diff --git a/corpus/frontend/ui-ux/index.yaml b/corpus/frontend/ui-ux/index.yaml
new file mode 100644
index 0000000..e2291f7
--- /dev/null
+++ b/corpus/frontend/ui-ux/index.yaml
@@ -0,0 +1,10 @@
+namespace: frontend.ui-ux
+principles:
+  type-scale:
+    file: type-scale.yaml
+  wcag-contrast:
+    file: wcag-contrast.yaml
+  dark-mode-principles:
+    file: dark-mode-principles.yaml
+  touch-targets:
+    file: touch-targets.yaml
diff --git a/corpus/frontend/ui-ux/touch-targets.yaml b/corpus/frontend/ui-ux/touch-targets.yaml
new file mode 100644
index 0000000..e4937cd
--- /dev/null
+++ b/corpus/frontend/ui-ux/touch-targets.yaml
@@ -0,0 +1,17 @@
+name: touch-targets
+domain: accessibility
+rule: "Touch targets: min 44×44px (WCAG), recommended 48×48px. Gap ≥ 8px between targets."
+detail: >-
+  Visual size ≠ touch target. A 34px button can have a 44px hit area via
+  padding or ::after pseudo-element. Gap prevents accidental taps on adjacent
+  targets.
+cssExample: |
+  /* Hit area expansion */
+  .btn-small::after {
+    content: '';
+    position: absolute;
+    inset: -8px;
+  }
+antiPatterns:
+  - "< 44px touch targets on mobile"
+  - Adjacent buttons with no gap
diff --git a/corpus/frontend/ui-ux/type-scale.yaml b/corpus/frontend/ui-ux/type-scale.yaml
new file mode 100644
index 0000000..77e81ac
--- /dev/null
+++ b/corpus/frontend/ui-ux/type-scale.yaml
@@ -0,0 +1,17 @@
+name: type-scale
+domain: typography
+rule: Use mathematical ratios for type scale, not arbitrary sizes.
+detail: >-
+  Common ratios: 1.25 (Major Third), 1.333 (Perfect Fourth), 1.414 (Augmented
+  Fourth). Recommended web scale: Display 48-72px, H1 40-56px, H2 28-40px,
+  H3 20-24px, Subtitle 16-20px, Body 16px, Body-sm 14px, Caption 13px,
+  Overline 12px.
+cssExample: |
+  /* fluid headings, fixed body */
+  font-size: clamp(2.5rem, 4vw, 3.5rem);  /* h1 */
+  font-size: clamp(1.75rem, 3vw, 2.5rem); /* h2 */
+  font-size: 1rem;                          /* body - fixed */
+antiPatterns:
+  - Random px values with no ratio
+  - Fluid body text (causes reflow)
+  - More than 2 type families
diff --git a/corpus/frontend/ui-ux/wcag-contrast.yaml b/corpus/frontend/ui-ux/wcag-contrast.yaml
new file mode 100644
index 0000000..c557c7e
--- /dev/null
+++ b/corpus/frontend/ui-ux/wcag-contrast.yaml
@@ -0,0 +1,12 @@
+name: wcag-contrast
+domain: color
+rule: "Body text: 4.5:1 (AA). Large text ≥18px bold or ≥24px: 3:1 (AA). UI components: 3:1."
+detail: >-
+  AAA (enhanced): 7:1 for body, 4.5:1 for large. Fix: reduce OKLCH Lightness
+  (L) of status colors from ~0.63 to ~0.55. Keep C and H unchanged.
+examples:
+  - "Run: npm install wcag-contrast"
+  - "Online: https://webaim.org/resources/contrastchecker/"
+antiPatterns:
+  - Testing only in light mode
+  - Assuming brand colors pass without checking
diff --git a/corpus/index.yaml b/corpus/index.yaml
index 2f1978e..91bffc9 100644
--- a/corpus/index.yaml
+++ b/corpus/index.yaml
@@ -11,3 +11,5 @@ namespaces:
     index: frontend/design-tokens/index.yaml
   frontend.shadcn:
     index: frontend/shadcn/index.yaml
+  frontend.ui-ux:
+    index: frontend/ui-ux/index.yaml
diff --git a/src/plugins/ui-ux/tools/get-principle.ts b/src/plugins/ui-ux/tools/get-principle.ts
index 45594ef..d1309be 100644
--- a/src/plugins/ui-ux/tools/get-principle.ts
+++ b/src/plugins/ui-ux/tools/get-principle.ts
@@ -1,6 +1,101 @@
 import type { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
+import { readFileSync } from "node:fs";
+import { dirname, join } from "node:path";
+import { fileURLToPath } from "node:url";
+import YAML from "yaml";
 import { z } from "zod";
-import { PRINCIPLES } from "../data.js";
+import { PRINCIPLES, type Principle } from "../data.js";
+
+type CorpusIndex = {
+  namespaces?: {
+    "frontend.ui-ux"?: {
+      index?: string;
+    };
+  };
+};
+
+type CorpusNamespaceIndex = {
+  namespace?: string;
+  principles?: Record<string, { file?: string }>;
+};
+
+type CorpusPrincipleEntry = {
+  name?: string;
+  domain?: string;
+  rule?: string;
+  detail?: string;
+  examples?: string[];
+  antiPatterns?: string[];
+  cssExample?: string;
+};
+
+type LoadedCorpusPrinciple = { principle: Principle; source: string };
+
+const moduleDir = dirname(fileURLToPath(import.meta.url));
+const corpusRoot = join(moduleDir, "../../../../corpus");
+const corpusNamespace = "frontend.ui-ux";
+
+const cachedCorpusPrinciples = new Map<string, LoadedCorpusPrinciple | null>();
+
+function normalize(name: string): string {
+  return name.toLowerCase().trim();
+}
+
+function loadCorpusPrinciple(name: string): LoadedCorpusPrinciple | null {
+  const key = normalize(name);
+  if (cachedCorpusPrinciples.has(key)) {
+    return cachedCorpusPrinciples.get(key) ?? null;
+  }
+
+  try {
+    const indexRaw = readFileSync(join(corpusRoot, "index.yaml"), "utf8");
+    const index = YAML.parse(indexRaw) as CorpusIndex | null;
+    const namespaceIndexPath = index?.namespaces?.[corpusNamespace]?.index;
+    if (!namespaceIndexPath) {
+      cachedCorpusPrinciples.set(key, null);
+      return null;
+    }
+
+    const namespaceRaw = readFileSync(join(corpusRoot, namespaceIndexPath), "utf8");
+    const namespaceIndex = YAML.parse(namespaceRaw) as CorpusNamespaceIndex | null;
+    const principlePath = namespaceIndex?.principles?.[key]?.file;
+    if (!principlePath) {
+      cachedCorpusPrinciples.set(key, null);
+      return null;
+    }
+
+    const raw = readFileSync(join(corpusRoot, "frontend/ui-ux", principlePath), "utf8");
+    const entry = YAML.parse(raw) as CorpusPrincipleEntry | null;
+    if (
+      !entry ||
+      normalize(entry.name ?? "") !== key ||
+      !entry.domain ||
+      !entry.rule ||
+      !entry.detail
+    ) {
+      cachedCorpusPrinciples.set(key, null);
+      return null;
+    }
+
+    const loaded: LoadedCorpusPrinciple = {
+      principle: {
+        name: entry.name ?? name,
+        domain: entry.domain as Principle["domain"],
+        rule: entry.rule,
+        detail: entry.detail,
+        examples: entry.examples,
+        antiPatterns: entry.antiPatterns,
+        cssExample: entry.cssExample,
+      },
+      source: corpusNamespace,
+    };
+    cachedCorpusPrinciples.set(key, loaded);
+    return loaded;
+  } catch {
+    cachedCorpusPrinciples.set(key, null);
+    return null;
+  }
+}
 
 export function register(server: McpServer): void {
   server.tool(
@@ -10,9 +105,10 @@ export function register(server: McpServer): void {
       name: z.string().describe("Principle name (e.g. 'type-scale', 'wcag-contrast', 'dark-mode-principles', 'touch-targets', 'easing-rules')"),
     },
     async ({ name }) => {
-      const principle = PRINCIPLES.find(
-        (p) => p.name.toLowerCase() === name.toLowerCase()
-      );
+      const corpusEntry = loadCorpusPrinciple(name);
+      const principle =
+        corpusEntry?.principle ??
+        PRINCIPLES.find((p) => p.name.toLowerCase() === name.toLowerCase());
 
       if (!principle) {
         const available = PRINCIPLES.map((p) => p.name).join(", ");
@@ -41,6 +137,10 @@ export function register(server: McpServer): void {
         for (const ap of principle.antiPatterns) text += `- ❌ ${ap}\n`;
       }
 
+      if (corpusEntry) {
+        text += `\n**Corpus Source:** ${corpusEntry.source}`;
+      }
+
       return { content: [{ type: "text", text }] };
     }
   );
diff --git a/tests/ui-ux-principle-corpus-backed-tools-behaviour.test.ts b/tests/ui-ux-principle-corpus-backed-tools-behaviour.test.ts
new file mode 100644
index 0000000..2249517
--- /dev/null
+++ b/tests/ui-ux-principle-corpus-backed-tools-behaviour.test.ts
@@ -0,0 +1,49 @@
+import { expect, test } from "bun:test";
+import { captureTool, extractTextContent } from "./helpers";
+import { register as registerUiUxGetPrinciple } from "../src/plugins/ui-ux/tools/get-principle.ts";
+
+const uiUxGetPrinciple = captureTool(registerUiUxGetPrinciple);
+
+test("ui_ux_get_principle prefers corpus metadata for type-scale", async () => {
+  const result = await uiUxGetPrinciple.invoke({ name: "type-scale" });
+  const text = extractTextContent(result);
+
+  expect(text).toContain("# type-scale [typography]");
+  expect(text).toContain("**Corpus Source:** frontend.ui-ux");
+  expect(text).toContain("clamp(2.5rem, 4vw, 3.5rem)");
+});
+
+test("ui_ux_get_principle prefers corpus metadata for wcag-contrast", async () => {
+  const result = await uiUxGetPrinciple.invoke({ name: "wcag-contrast" });
+  const text = extractTextContent(result);
+
+  expect(text).toContain("# wcag-contrast [color]");
+  expect(text).toContain("**Corpus Source:** frontend.ui-ux");
+  expect(text).toContain("4.5:1 (AA)");
+});
+
+test("ui_ux_get_principle prefers corpus metadata for dark-mode-principles", async () => {
+  const result = await uiUxGetPrinciple.invoke({ name: "dark-mode-principles" });
+  const text = extractTextContent(result);
+
+  expect(text).toContain("# dark-mode-principles [color]");
+  expect(text).toContain("**Corpus Source:** frontend.ui-ux");
+  expect(text).toContain("oklch(0.13 0.008 265)");
+});
+
+test("ui_ux_get_principle prefers corpus metadata for touch-targets", async () => {
+  const result = await uiUxGetPrinciple.invoke({ name: "touch-targets" });
+  const text = extractTextContent(result);
+
+  expect(text).toContain("# touch-targets [accessibility]");
+  expect(text).toContain("**Corpus Source:** frontend.ui-ux");
+  expect(text).toContain("min 44×44px");
+});
+
+test("ui_ux_get_principle falls back to in-file data for non-corpus principles", async () => {
+  const result = await uiUxGetPrinciple.invoke({ name: "easing-rules" });
+  const text = extractTextContent(result);
+
+  expect(text).toContain("# easing-rules [motion]");
+  expect(text).not.toContain("**Corpus Source:** frontend.ui-ux");
+});