diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 4e2a565f72f..d807723a1d9 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -1736,6 +1736,9 @@ importers:
       dompurify:
         specifier: 3.4.3
         version: 3.4.3
+      highlight.js:
+        specifier: 10.7.3
+        version: 10.7.3
       json5:
         specifier: 2.2.3
         version: 2.2.3
diff --git a/ui/package.json b/ui/package.json
index fbc3054c10d..3fbe4e19a8f 100644
--- a/ui/package.json
+++ b/ui/package.json
@@ -12,6 +12,7 @@
     "@create-markdown/preview": "2.0.3",
     "@noble/ed25519": "3.1.0",
     "dompurify": "3.4.3",
+    "highlight.js": "10.7.3",
     "json5": "2.2.3",
     "lit": "3.3.3",
     "markdown-it": "14.1.1",
diff --git a/ui/src/styles/components.css b/ui/src/styles/components.css
index 12bc0a4cc6e..6714a9cfc9e 100644
--- a/ui/src/styles/components.css
+++ b/ui/src/styles/components.css
@@ -2004,6 +2004,159 @@
   background: var(--bg);
 }
 
+:is(.code-block .hljs, .code-block-wrapper pre code.hljs) {
+  color: var(--text);
+}
+
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-comment,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-quote {
+  color: var(--muted);
+  font-style: italic;
+}
+
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-keyword,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-selector-tag,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-subst {
+  color: #ff8a80;
+}
+
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-number,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-literal,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-variable,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-template-variable,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-tag .hljs-attr {
+  color: #79c0ff;
+}
+
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-string,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-doctag {
+  color: #a5d6ff;
+}
+
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-title,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-section,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-selector-id,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-function .hljs-title {
+  color: #d2a8ff;
+}
+
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-type,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-class .hljs-title,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-built_in,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-builtin-name {
+  color: #ffa657;
+}
+
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-attr,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-attribute,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-name,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-selector-class,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-selector-attr,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-selector-pseudo {
+  color: #7ee787;
+}
+
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-symbol,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-bullet,
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-link {
+  color: var(--accent-2);
+}
+
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-meta {
+  color: #c9d1d9;
+}
+
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-deletion {
+  color: #ffa198;
+  background: rgba(248, 81, 73, 0.12);
+}
+
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-addition {
+  color: #7ee787;
+  background: rgba(46, 160, 67, 0.12);
+}
+
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-emphasis {
+  font-style: italic;
+}
+
+:is(.code-block, .code-block-wrapper pre code.hljs) .hljs-strong {
+  font-weight: 600;
+}
+
+:root[data-theme-mode="light"] :is(.code-block .hljs, .code-block-wrapper pre code.hljs) {
+  color: var(--text);
+}
+
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-keyword,
+:root[data-theme-mode="light"]
+  :is(.code-block, .code-block-wrapper pre code.hljs)
+  .hljs-selector-tag,
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-subst {
+  color: #cf222e;
+}
+
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-number,
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-literal,
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-variable,
+:root[data-theme-mode="light"]
+  :is(.code-block, .code-block-wrapper pre code.hljs)
+  .hljs-template-variable,
+:root[data-theme-mode="light"]
+  :is(.code-block, .code-block-wrapper pre code.hljs)
+  .hljs-tag
+  .hljs-attr {
+  color: #0550ae;
+}
+
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-string,
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-doctag {
+  color: #0a3069;
+}
+
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-title,
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-section,
+:root[data-theme-mode="light"]
+  :is(.code-block, .code-block-wrapper pre code.hljs)
+  .hljs-selector-id,
+:root[data-theme-mode="light"]
+  :is(.code-block, .code-block-wrapper pre code.hljs)
+  .hljs-function
+  .hljs-title {
+  color: #8250df;
+}
+
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-type,
+:root[data-theme-mode="light"]
+  :is(.code-block, .code-block-wrapper pre code.hljs)
+  .hljs-class
+  .hljs-title,
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-built_in,
+:root[data-theme-mode="light"]
+  :is(.code-block, .code-block-wrapper pre code.hljs)
+  .hljs-builtin-name {
+  color: #953800;
+}
+
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-attr,
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-attribute,
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-name,
+:root[data-theme-mode="light"]
+  :is(.code-block, .code-block-wrapper pre code.hljs)
+  .hljs-selector-class,
+:root[data-theme-mode="light"]
+  :is(.code-block, .code-block-wrapper pre code.hljs)
+  .hljs-selector-attr,
+:root[data-theme-mode="light"]
+  :is(.code-block, .code-block-wrapper pre code.hljs)
+  .hljs-selector-pseudo {
+  color: #116329;
+}
+
+:root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-meta {
+  color: #57606a;
+}
+
 .markdown-plain-text-fallback {
   display: block;
   white-space: pre-wrap;
diff --git a/ui/src/styles/components.test.ts b/ui/src/styles/components.test.ts
index 49df92d12a3..4037480d132 100644
--- a/ui/src/styles/components.test.ts
+++ b/ui/src/styles/components.test.ts
@@ -5,6 +5,18 @@ function readComponentsCss(): string {
   return readStyleSheet("ui/src/styles/components.css");
 }
 
+describe("code block highlight styles", () => {
+  it("targets the markdown renderer's generated code block wrapper", () => {
+    const css = readComponentsCss();
+
+    expect(css).toContain(":is(.code-block .hljs, .code-block-wrapper pre code.hljs)");
+    expect(css).toContain(":is(.code-block, .code-block-wrapper pre code.hljs) .hljs-keyword");
+    expect(css).toContain(
+      ':root[data-theme-mode="light"] :is(.code-block, .code-block-wrapper pre code.hljs) .hljs-string',
+    );
+  });
+});
+
 describe("agent fallback chip styles", () => {
   it("styles the chip remove control inside the agent model input", () => {
     const css = readComponentsCss();
diff --git a/ui/src/types/highlight-js-subpaths.d.ts b/ui/src/types/highlight-js-subpaths.d.ts
new file mode 100644
index 00000000000..535d78b37fd
--- /dev/null
+++ b/ui/src/types/highlight-js-subpaths.d.ts
@@ -0,0 +1,9 @@
+declare module "highlight.js/lib/core.js" {
+  import hljs = require("highlight.js");
+
+  export default hljs;
+}
+
+declare module "highlight.js/lib/languages/*.js" {
+  export default function language(hljs?: HLJSApi): LanguageDetail;
+}
diff --git a/ui/src/ui/markdown.test.ts b/ui/src/ui/markdown.test.ts
index 0549558f6cf..c2645ed978d 100644
--- a/ui/src/ui/markdown.test.ts
+++ b/ui/src/ui/markdown.test.ts
@@ -332,9 +332,14 @@ describe("toSanitizedMarkdownHtml", () => {
   describe("code blocks", () => {
     it("renders fenced code blocks", () => {
       const html = toSanitizedMarkdownHtml("```ts\nconsole.log(1)\n```");
-      expect(html).toBe(
-        '<div class="code-block-wrapper"><div class="code-block-header"><span class="code-block-lang">ts</span><button type="button" class="code-block-copy" data-code="console.log(1)" aria-label="Copy code"><span class="code-block-copy__idle">Copy</span><span class="code-block-copy__done">Copied!</span></button></div><pre><code class="language-ts">console.log(1)\n</code></pre></div>',
-      );
+      const fragment = htmlFragment(html);
+      const code = fragment.querySelector("pre code");
+      const copy = fragment.querySelector<HTMLButtonElement>(".code-block-copy");
+
+      expect(fragment.querySelector(".code-block-lang")?.textContent).toBe("ts");
+      expect(copy?.dataset.code).toBe("console.log(1)");
+      expect(code?.classList.contains("language-ts")).toBe(true);
+      expect(code?.textContent).toBe("console.log(1)\n");
     });
 
     it("renders indented code blocks", () => {
@@ -352,6 +357,52 @@ describe("toSanitizedMarkdownHtml", () => {
       );
     });
 
+    it("highlights fenced code blocks while preserving copy text", () => {
+      const source = 'const answer = "yes";\nconsole.log(answer);\n';
+      const html = toSanitizedMarkdownHtml(`\`\`\`js\n${source}\`\`\``);
+      const fragment = htmlFragment(html);
+      const code = fragment.querySelector("pre code");
+      const copy = fragment.querySelector<HTMLButtonElement>(".code-block-copy");
+
+      expect(fragment.querySelector(".code-block-lang")?.textContent).toBe("js");
+      expect(copy?.dataset.code).toBe(source.trimEnd());
+      expect(code?.textContent).toBe(source);
+      expect(code?.querySelector(".hljs-keyword")?.textContent).toBe("const");
+      expect(code?.querySelector(".hljs-string")?.textContent).toBe('"yes"');
+    });
+
+    it("highlights collapsed JSON code blocks", () => {
+      const html = toSanitizedMarkdownHtml('```json\n{"ok": true}\n```');
+      const fragment = htmlFragment(html);
+      const details = fragment.querySelector("details.json-collapse");
+      const code = details?.querySelector("pre code");
+
+      expect(details?.querySelector("summary")?.textContent).toBe("JSON · 2 lines");
+      expect(code?.textContent).toBe('{"ok": true}\n');
+      expect(code?.innerHTML).toContain("hljs-");
+    });
+
+    it("auto-highlights unlabeled code blocks only when detection is confident", () => {
+      const html = toSanitizedMarkdownHtml("```\n#include <vector>\nstd::vector<int> nums;\n```");
+      const fragment = htmlFragment(html);
+      const code = fragment.querySelector("pre code");
+
+      expect(code?.classList.contains("hljs")).toBe(true);
+      expect(code?.textContent).toBe("#include <vector>\nstd::vector<int> nums;\n");
+      expect(code?.innerHTML).toContain("hljs-meta");
+      expect(code?.innerHTML).toContain("hljs-keyword");
+    });
+
+    it("keeps highlighted HTML code escaped", () => {
+      const html = toSanitizedMarkdownHtml("```html\n<script>alert(1)</script>\n```");
+      const fragment = htmlFragment(html);
+      const code = fragment.querySelector("pre code");
+
+      expect(code?.querySelector("script")).toBeNull();
+      expect(code?.textContent).toBe("<script>alert(1)</script>\n");
+      expect(code?.innerHTML).not.toContain("<script>");
+    });
+
     it("keeps localized copy labels fresh after locale changes", async () => {
       const markdown = "```ts\nconst localizedCopy = true;\n```";
       await i18n.setLocale("en");
@@ -360,12 +411,25 @@ describe("toSanitizedMarkdownHtml", () => {
       try {
         await i18n.setLocale("zh-CN");
         const chinese = toSanitizedMarkdownHtml(markdown);
+        const englishFragment = htmlFragment(english);
+        const chineseFragment = htmlFragment(chinese);
+        const englishCopy = englishFragment.querySelector<HTMLButtonElement>(".code-block-copy");
+        const chineseCopy = chineseFragment.querySelector<HTMLButtonElement>(".code-block-copy");
 
-        expect(english).toBe(
-          '<div class="code-block-wrapper"><div class="code-block-header"><span class="code-block-lang">ts</span><button type="button" class="code-block-copy" data-code="const localizedCopy = true;" aria-label="Copy code"><span class="code-block-copy__idle">Copy</span><span class="code-block-copy__done">Copied!</span></button></div><pre><code class="language-ts">const localizedCopy = true;\n</code></pre></div>',
+        expect(englishCopy?.dataset.code).toBe("const localizedCopy = true;");
+        expect(englishCopy?.getAttribute("aria-label")).toBe("Copy code");
+        expect(englishCopy?.querySelector(".code-block-copy__idle")?.textContent).toBe("Copy");
+        expect(englishCopy?.querySelector(".code-block-copy__done")?.textContent).toBe("Copied!");
+        expect(englishFragment.querySelector("pre code")?.textContent).toBe(
+          "const localizedCopy = true;\n",
         );
-        expect(chinese).toBe(
-          '<div class="code-block-wrapper"><div class="code-block-header"><span class="code-block-lang">ts</span><button type="button" class="code-block-copy" data-code="const localizedCopy = true;" aria-label="复制代码"><span class="code-block-copy__idle">复制</span><span class="code-block-copy__done">已复制！</span></button></div><pre><code class="language-ts">const localizedCopy = true;\n</code></pre></div>',
+
+        expect(chineseCopy?.dataset.code).toBe("const localizedCopy = true;");
+        expect(chineseCopy?.getAttribute("aria-label")).toBe("复制代码");
+        expect(chineseCopy?.querySelector(".code-block-copy__idle")?.textContent).toBe("复制");
+        expect(chineseCopy?.querySelector(".code-block-copy__done")?.textContent).toBe("已复制！");
+        expect(chineseFragment.querySelector("pre code")?.textContent).toBe(
+          "const localizedCopy = true;\n",
         );
       } finally {
         await i18n.setLocale("en");
@@ -374,9 +438,16 @@ describe("toSanitizedMarkdownHtml", () => {
 
     it("collapses JSON code blocks", () => {
       const html = toSanitizedMarkdownHtml('```json\n{"key": "value"}\n```');
-      expect(html).toBe(
-        '<details class="json-collapse"><summary>JSON · 2 lines</summary><div class="code-block-wrapper"><div class="code-block-header"><span class="code-block-lang">json</span><button type="button" class="code-block-copy" data-code="{&quot;key&quot;: &quot;value&quot;}" aria-label="Copy code"><span class="code-block-copy__idle">Copy</span><span class="code-block-copy__done">Copied!</span></button></div><pre><code class="language-json">{"key": "value"}\n</code></pre></div></details>',
-      );
+      const fragment = htmlFragment(html);
+      const details = fragment.querySelector("details.json-collapse");
+      const code = details?.querySelector("pre code");
+      const copy = details?.querySelector<HTMLButtonElement>(".code-block-copy");
+
+      expect(details?.querySelector("summary")?.textContent).toBe("JSON · 2 lines");
+      expect(details?.querySelector(".code-block-lang")?.textContent).toBe("json");
+      expect(copy?.dataset.code).toBe('{"key": "value"}');
+      expect(code?.classList.contains("language-json")).toBe(true);
+      expect(code?.textContent).toBe('{"key": "value"}\n');
     });
   });
 
diff --git a/ui/src/ui/markdown.ts b/ui/src/ui/markdown.ts
index 5efac70871d..124e2ceabc9 100644
--- a/ui/src/ui/markdown.ts
+++ b/ui/src/ui/markdown.ts
@@ -1,4 +1,19 @@
 import DOMPurify from "dompurify";
+import hljs from "highlight.js/lib/core.js";
+import bash from "highlight.js/lib/languages/bash.js";
+import cpp from "highlight.js/lib/languages/cpp.js";
+import css from "highlight.js/lib/languages/css.js";
+import diff from "highlight.js/lib/languages/diff.js";
+import go from "highlight.js/lib/languages/go.js";
+import java from "highlight.js/lib/languages/java.js";
+import javascript from "highlight.js/lib/languages/javascript.js";
+import json from "highlight.js/lib/languages/json.js";
+import markdown from "highlight.js/lib/languages/markdown.js";
+import python from "highlight.js/lib/languages/python.js";
+import rust from "highlight.js/lib/languages/rust.js";
+import typescript from "highlight.js/lib/languages/typescript.js";
+import xml from "highlight.js/lib/languages/xml.js";
+import yaml from "highlight.js/lib/languages/yaml.js";
 import MarkdownIt from "markdown-it";
 import markdownItTaskLists from "markdown-it-task-lists";
 import { i18n, t } from "../i18n/index.ts";
@@ -151,6 +166,90 @@ function normalizeMarkdownImageLabel(text?: string | null): string {
   return trimmed ? trimmed : "image";
 }
 
+for (const [language, definition, aliases] of [
+  ["bash", bash, ["sh", "shell"]],
+  ["cpp", cpp, ["c++", "cxx"]],
+  ["css", css, []],
+  ["diff", diff, ["patch"]],
+  ["go", go, ["golang"]],
+  ["java", java, []],
+  ["javascript", javascript, ["js", "jsx"]],
+  ["json", json, []],
+  ["markdown", markdown, ["md"]],
+  ["python", python, ["py"]],
+  ["rust", rust, ["rs"]],
+  ["typescript", typescript, ["ts", "tsx"]],
+  ["xml", xml, ["html", "svg"]],
+  ["yaml", yaml, ["yml"]],
+] as const) {
+  hljs.registerLanguage(language, definition);
+  if (aliases.length > 0) {
+    hljs.registerAliases([...aliases], { languageName: language });
+  }
+}
+
+function normalizeHighlightLanguage(lang: string): string {
+  const normalized = lang.trim().toLowerCase();
+  if (!normalized) {
+    return "";
+  }
+  const aliases: Record<string, string> = {
+    "c++": "cpp",
+    cxx: "cpp",
+    js: "javascript",
+    jsx: "javascript",
+    md: "markdown",
+    sh: "bash",
+    shell: "bash",
+    ts: "typescript",
+    tsx: "typescript",
+  };
+  return aliases[normalized] ?? normalized;
+}
+
+const autoHighlightLanguages = [
+  "bash",
+  "cpp",
+  "css",
+  "diff",
+  "go",
+  "java",
+  "javascript",
+  "json",
+  "markdown",
+  "python",
+  "rust",
+  "typescript",
+  "xml",
+  "yaml",
+];
+
+function highlightCode(text: string, lang: string): string {
+  const language = normalizeHighlightLanguage(lang);
+  try {
+    if (language && hljs.getLanguage(language)) {
+      return hljs.highlight(text, { language, ignoreIllegals: true }).value;
+    }
+    if (!language && text.trim()) {
+      const result = hljs.highlightAuto(text, autoHighlightLanguages);
+      if (result.relevance >= 2) {
+        return result.value;
+      }
+    }
+  } catch {
+    // Fall back to escaped plaintext; malformed input should not break chat rendering.
+  }
+  return escapeHtml(text);
+}
+
+function codeClassAttribute(lang: string, highlighted: string): string {
+  const classes = [
+    highlighted.includes("hljs-") ? "hljs" : "",
+    lang ? `language-${lang}` : "",
+  ].filter(Boolean);
+  return classes.length > 0 ? ` class="${escapeHtml(classes.join(" "))}"` : "";
+}
+
 export const md = new MarkdownIt({
   html: true, // Enable HTML recognition so html_block/html_inline overrides can escape it
   breaks: true,
@@ -427,9 +526,9 @@ md.renderer.rules.fence = (tokens, idx) => {
   // extract only the first whitespace-separated token as the language.
   const lang = token.info.trim().split(/\s+/)[0] || "";
   const text = token.content;
-  const langClass = lang ? ` class="language-${escapeHtml(lang)}"` : "";
-  const safeText = escapeHtml(text);
-  const codeBlock = `<pre><code${langClass}>${safeText}</code></pre>`;
+  const highlighted = highlightCode(text, lang);
+  const classAttr = codeClassAttribute(lang, highlighted);
+  const codeBlock = `<pre><code${classAttr}>${highlighted}</code></pre>`;
   const langLabel = lang ? `<span class="code-block-lang">${escapeHtml(lang)}</span>` : "";
   const attrSafe = escapeHtml(text);
   const copyBtn = `<button type="button" class="code-block-copy" data-code="${attrSafe}" aria-label="${escapeHtml(t("common.copyCode"))}"><span class="code-block-copy__idle">${escapeHtml(t("common.copy"))}</span><span class="code-block-copy__done">${escapeHtml(t("common.copied"))}</span></button>`;
@@ -455,8 +554,9 @@ md.renderer.rules.fence = (tokens, idx) => {
 md.renderer.rules.code_block = (tokens, idx) => {
   const token = tokens[idx];
   const text = token.content;
-  const safeText = escapeHtml(text);
-  const codeBlock = `<pre><code>${safeText}</code></pre>`;
+  const highlighted = highlightCode(text, "");
+  const classAttr = codeClassAttribute("", highlighted);
+  const codeBlock = `<pre><code${classAttr}>${highlighted}</code></pre>`;
   const attrSafe = escapeHtml(text);
   const copyBtn = `<button type="button" class="code-block-copy" data-code="${attrSafe}" aria-label="${escapeHtml(t("common.copyCode"))}"><span class="code-block-copy__idle">${escapeHtml(t("common.copy"))}</span><span class="code-block-copy__done">${escapeHtml(t("common.copied"))}</span></button>`;
   const header = `<div class="code-block-header">${copyBtn}</div>`;