diff --git a/ui/src/styles/chat/mermaid.css b/ui/src/styles/chat/mermaid.css
index f24852e393f..9445873126b 100644
--- a/ui/src/styles/chat/mermaid.css
+++ b/ui/src/styles/chat/mermaid.css
@@ -9,6 +9,7 @@
 .chat-text :where(.mermaid-block__render) {
   overflow-x: auto;
   padding: 12px;
+  cursor: zoom-in;
 }
 
 .chat-text :where(.mermaid-block__render svg) {
@@ -17,6 +18,11 @@
   height: auto;
 }
 
+.chat-text :where(.mermaid-block__render:focus-visible) {
+  outline: 2px solid var(--accent);
+  outline-offset: -2px;
+}
+
 .chat-text :where(.mermaid-block__source) {
   border-top: 1px solid var(--border);
 }
@@ -33,3 +39,55 @@
 .chat-text :where(.mermaid-block__source > summary::-webkit-details-marker) {
   display: none;
 }
+
+.mermaid-block__dialog {
+  width: min(92vw, 1200px);
+  max-width: 1200px;
+  max-height: 88vh;
+  padding: 0;
+  border: 1px solid var(--border);
+  border-radius: calc(var(--radius-lg) + 4px);
+  background: var(--panel);
+  color: var(--text);
+  box-shadow: 0 20px 60px rgba(0, 0, 0, 0.35);
+}
+
+.mermaid-block__dialog::backdrop {
+  background: rgba(15, 23, 42, 0.55);
+  backdrop-filter: blur(4px);
+}
+
+.mermaid-block__dialog-panel {
+  display: flex;
+  flex-direction: column;
+  max-height: 88vh;
+}
+
+.mermaid-block__dialog-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 12px;
+  padding: 14px 16px;
+  border-bottom: 1px solid var(--border);
+}
+
+.mermaid-block__dialog-title {
+  font-size: 14px;
+  font-weight: 600;
+}
+
+.mermaid-block__dialog-body {
+  overflow: auto;
+  padding: 20px;
+  background: color-mix(in srgb, var(--secondary) 65%, transparent);
+}
+
+.mermaid-block__dialog-body svg {
+  display: block;
+  min-width: min(960px, 100%);
+  width: max-content;
+  max-width: none;
+  height: auto;
+  margin: 0 auto;
+}
diff --git a/ui/src/ui/code-block-copy.test.ts b/ui/src/ui/code-block-copy.test.ts
new file mode 100644
index 00000000000..297b3d8853c
--- /dev/null
+++ b/ui/src/ui/code-block-copy.test.ts
@@ -0,0 +1,36 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { handleCodeBlockCopyClick } from "./code-block-copy.ts";
+
+describe("handleCodeBlockCopyClick", () => {
+  beforeEach(() => {
+    document.body.innerHTML = "";
+  });
+
+  it("copies code from a clicked code-block button", async () => {
+    const writeText = vi.fn().mockResolvedValue(undefined);
+    Object.defineProperty(navigator, "clipboard", {
+      configurable: true,
+      value: { writeText },
+    });
+
+    document.body.innerHTML = `
+      <div class="sidebar-markdown">
+        <button type="button" class="code-block-copy" data-code="console.log('copied')">
+          <span>Copy</span>
+        </button>
+      </div>
+    `;
+
+    const button = document.querySelector<HTMLButtonElement>(".code-block-copy");
+    expect(button).not.toBeNull();
+
+    const event = new MouseEvent("click", { bubbles: true });
+    button?.dispatchEvent(event);
+    handleCodeBlockCopyClick(event);
+
+    await Promise.resolve();
+
+    expect(writeText).toHaveBeenCalledWith("console.log('copied')");
+    expect(button?.classList.contains("copied")).toBe(true);
+  });
+});
diff --git a/ui/src/ui/code-block-copy.ts b/ui/src/ui/code-block-copy.ts
new file mode 100644
index 00000000000..99c0b7aa093
--- /dev/null
+++ b/ui/src/ui/code-block-copy.ts
@@ -0,0 +1,26 @@
+const COPIED_FOR_MS = 1500;
+
+export function handleCodeBlockCopyClick(e: Event): void {
+  const btn = (e.target as HTMLElement | null)?.closest<HTMLButtonElement>(".code-block-copy");
+  if (!btn) {
+    return;
+  }
+
+  const code = btn.dataset.code ?? "";
+  if (!code) {
+    return;
+  }
+
+  void navigator.clipboard.writeText(code).then(
+    () => {
+      btn.classList.add("copied");
+      window.setTimeout(() => {
+        if (!btn.isConnected) {
+          return;
+        }
+        btn.classList.remove("copied");
+      }, COPIED_FOR_MS);
+    },
+    () => {},
+  );
+}
diff --git a/ui/src/ui/markdown.test.ts b/ui/src/ui/markdown.test.ts
index fb626009065..86ee6503717 100644
--- a/ui/src/ui/markdown.test.ts
+++ b/ui/src/ui/markdown.test.ts
@@ -36,8 +36,10 @@ describe("toSanitizedMarkdownHtml", () => {
     );
     expect(html).toContain('class="mermaid-block"');
     expect(html).toContain('class="mermaid-block__render"');
+    expect(html).toContain('class="mermaid-block__dialog"');
     expect(html).toContain('class="language-mermaid"');
     expect(html).toContain("Mermaid source");
+    expect(html).toContain("Click to enlarge");
   });
 
   it("flattens remote markdown images into alt text", () => {
diff --git a/ui/src/ui/markdown.ts b/ui/src/ui/markdown.ts
index 12e813c7204..a1ca49e723d 100644
--- a/ui/src/ui/markdown.ts
+++ b/ui/src/ui/markdown.ts
@@ -11,6 +11,7 @@ const allowedTags = [
   "code",
   "del",
   "details",
+  "dialog",
   "div",
   "em",
   "h1",
@@ -48,7 +49,9 @@ const allowedAttrs = [
   "data-code",
   "type",
   "aria-label",
+  "aria-modal",
   "role",
+  "tabindex",
 ];
 const sanitizeOptions = {
   ALLOWED_TAGS: allowedTags,
@@ -199,7 +202,7 @@ htmlEscapeRenderer.code = ({
   const header = `<div class="code-block-header">${langLabel}${copyBtn}</div>`;
 
   if (normalizedLang === "mermaid") {
-    return `<div class="mermaid-block"><div class="mermaid-block__render" aria-label="Mermaid diagram" role="img"></div><details class="mermaid-block__source"><summary>Mermaid source</summary><div class="code-block-wrapper">${header}${codeBlock}</div></details></div>`;
+    return `<div class="mermaid-block"><div class="mermaid-block__render" aria-label="Mermaid diagram. Click to enlarge." role="button" tabindex="0" title="Click to enlarge"></div><dialog class="mermaid-block__dialog" aria-modal="true"><div class="mermaid-block__dialog-panel"><div class="mermaid-block__dialog-header"><div class="mermaid-block__dialog-title">Mermaid diagram</div><button type="button" class="btn btn--sm mermaid-block__dialog-close" aria-label="Close Mermaid preview">Close</button></div><div class="mermaid-block__dialog-body"></div></div></dialog><details class="mermaid-block__source"><summary>Mermaid source</summary><div class="code-block-wrapper">${header}${codeBlock}</div></details></div>`;
   }
 
   const trimmed = text.trim();
diff --git a/ui/src/ui/mermaid.test.ts b/ui/src/ui/mermaid.test.ts
new file mode 100644
index 00000000000..5f978dc0134
--- /dev/null
+++ b/ui/src/ui/mermaid.test.ts
@@ -0,0 +1,35 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { installMermaidInteractions } from "./mermaid.ts";
+
+describe("installMermaidInteractions", () => {
+  beforeEach(() => {
+    document.body.innerHTML = "";
+  });
+
+  it("opens the mermaid preview dialog when the rendered diagram is clicked", () => {
+    installMermaidInteractions(document);
+
+    document.body.innerHTML = `
+      <div class="mermaid-block" data-mermaid-status="ready">
+        <div class="mermaid-block__render" role="button" tabindex="0">
+          <svg><text>Diagram</text></svg>
+        </div>
+        <dialog class="mermaid-block__dialog">
+          <div class="mermaid-block__dialog-body"></div>
+        </dialog>
+      </div>
+    `;
+
+    const dialog = document.querySelector<HTMLDialogElement>(".mermaid-block__dialog");
+    const showModal = vi.fn();
+    Object.defineProperty(dialog!, "showModal", {
+      configurable: true,
+      value: showModal,
+    });
+
+    document.querySelector<HTMLElement>(".mermaid-block__render")?.click();
+
+    expect(showModal).toHaveBeenCalledOnce();
+    expect(dialog?.querySelector(".mermaid-block__dialog-body")?.innerHTML).toContain("<svg");
+  });
+});
diff --git a/ui/src/ui/mermaid.ts b/ui/src/ui/mermaid.ts
index 7178a57ed2a..3d7a96d5389 100644
--- a/ui/src/ui/mermaid.ts
+++ b/ui/src/ui/mermaid.ts
@@ -8,6 +8,7 @@ type MermaidApi = {
 let mermaidApiPromise: Promise<MermaidApi> | null = null;
 let renderScheduled = false;
 let renderCounter = 0;
+const interactionDocs = new WeakSet<Document>();
 
 async function loadMermaidApi(): Promise<MermaidApi> {
   if (!mermaidApiPromise) {
@@ -17,6 +18,10 @@ async function loadMermaidApi(): Promise<MermaidApi> {
         api.initialize({
           startOnLoad: false,
           securityLevel: "strict",
+          // Keep labels as native SVG text instead of foreignObject-backed HTML.
+          // The rendered SVG is sanitized before insertion, and HTML labels can
+          // lose their contents during that pass, leaving empty shapes behind.
+          htmlLabels: false,
         });
         return api;
       })
@@ -30,7 +35,90 @@ async function loadMermaidApi(): Promise<MermaidApi> {
   return mermaidApiPromise;
 }
 
+export function installMermaidInteractions(root: ParentNode = document): void {
+  const doc = root instanceof Document ? root : root.ownerDocument;
+  if (!doc || interactionDocs.has(doc)) {
+    return;
+  }
+  interactionDocs.add(doc);
+
+  doc.addEventListener("click", (e) => {
+    const target = e.target;
+    if (!(target instanceof Element)) {
+      return;
+    }
+
+    const closeButton = target.closest<HTMLButtonElement>(".mermaid-block__dialog-close");
+    if (closeButton) {
+      closeMermaidDialog(closeButton.closest(".mermaid-block__dialog"));
+      return;
+    }
+
+    const dialog = target.closest<HTMLDialogElement>(".mermaid-block__dialog");
+    if (dialog && target === dialog) {
+      closeMermaidDialog(dialog);
+      return;
+    }
+
+    const renderTarget = target.closest<HTMLElement>(".mermaid-block__render");
+    if (!renderTarget) {
+      return;
+    }
+
+    const block = renderTarget.closest<HTMLElement>(".mermaid-block");
+    if (!block || block.dataset.mermaidStatus !== "ready") {
+      return;
+    }
+
+    openMermaidDialog(block);
+  });
+
+  doc.addEventListener("keydown", (e) => {
+    const target = e.target;
+    if (!(target instanceof HTMLElement) || !target.classList.contains("mermaid-block__render")) {
+      return;
+    }
+    if (e.key !== "Enter" && e.key !== " ") {
+      return;
+    }
+    const block = target.closest<HTMLElement>(".mermaid-block");
+    if (!block || block.dataset.mermaidStatus !== "ready") {
+      return;
+    }
+    e.preventDefault();
+    openMermaidDialog(block);
+  });
+}
+
+function openMermaidDialog(block: HTMLElement): void {
+  const dialog = block.querySelector<HTMLDialogElement>(".mermaid-block__dialog");
+  const body = block.querySelector<HTMLElement>(".mermaid-block__dialog-body");
+  const renderTarget = block.querySelector<HTMLElement>(".mermaid-block__render");
+  if (!dialog || !body || !renderTarget) {
+    return;
+  }
+
+  body.innerHTML = renderTarget.innerHTML;
+  if (typeof dialog.showModal === "function") {
+    dialog.showModal();
+    return;
+  }
+  dialog.setAttribute("open", "");
+}
+
+function closeMermaidDialog(dialog: HTMLDialogElement | null): void {
+  if (!dialog) {
+    return;
+  }
+  if (typeof dialog.close === "function") {
+    dialog.close();
+    return;
+  }
+  dialog.removeAttribute("open");
+}
+
 export function scheduleMermaidRender(root: ParentNode = document): void {
+  installMermaidInteractions(root);
   if (renderScheduled) {
     return;
   }
diff --git a/ui/src/ui/views/agents-panels-status-files.ts b/ui/src/ui/views/agents-panels-status-files.ts
index bff74f0523b..360e8c3bcb8 100644
--- a/ui/src/ui/views/agents-panels-status-files.ts
+++ b/ui/src/ui/views/agents-panels-status-files.ts
@@ -1,5 +1,6 @@
 import { html, nothing } from "lit";
 import { unsafeHTML } from "lit/directives/unsafe-html.js";
+import { handleCodeBlockCopyClick } from "../code-block-copy.ts";
 import { formatRelativeTimestamp } from "../format.ts";
 import { icons } from "../icons.ts";
 import { toSanitizedMarkdownHtml } from "../markdown.ts";
@@ -485,7 +486,10 @@ export function renderAgentFiles(params: {
                                   }}
                                 >${icons.x} Close</button>
                               </div>
-                              <div class="md-preview-dialog__body sidebar-markdown">
+                              <div
+                                class="md-preview-dialog__body sidebar-markdown"
+                                @click=${handleCodeBlockCopyClick}
+                              >
                                 ${unsafeHTML(toSanitizedMarkdownHtml(draft))}
                               </div>
                             </div>
diff --git a/ui/src/ui/views/chat.ts b/ui/src/ui/views/chat.ts
index 88a712706f0..31ccfb5c997 100644
--- a/ui/src/ui/views/chat.ts
+++ b/ui/src/ui/views/chat.ts
@@ -26,6 +26,7 @@ import {
   type SlashCommandDef,
 } from "../chat/slash-commands.ts";
 import { isSttSupported, startStt, stopStt } from "../chat/speech.ts";
+import { handleCodeBlockCopyClick } from "../code-block-copy.ts";
 import { icons } from "../icons.ts";
 import { detectTextDirection } from "../text-direction.ts";
 import type { GatewaySessionRow, SessionsListResult } from "../types.ts";
@@ -834,21 +835,6 @@ export function renderChat(props: ChatProps) {
   const splitRatio = props.splitRatio ?? 0.6;
   const sidebarOpen = Boolean(props.sidebarOpen && props.onCloseSidebar);
 
-  const handleCodeBlockCopy = (e: Event) => {
-    const btn = (e.target as HTMLElement).closest(".code-block-copy");
-    if (!btn) {
-      return;
-    }
-    const code = (btn as HTMLElement).dataset.code ?? "";
-    navigator.clipboard.writeText(code).then(
-      () => {
-        btn.classList.add("copied");
-        setTimeout(() => btn.classList.remove("copied"), 1500);
-      },
-      () => {},
-    );
-  };
-
   const chatItems = buildChatItems(props);
   const isEmpty = chatItems.length === 0 && !props.loading;
 
@@ -858,7 +844,7 @@ export function renderChat(props: ChatProps) {
       role="log"
       aria-live="polite"
       @scroll=${props.onChatScroll}
-      @click=${handleCodeBlockCopy}
+      @click=${handleCodeBlockCopyClick}
     >
       <div class="chat-thread-inner">
       ${
diff --git a/ui/src/ui/views/markdown-sidebar.ts b/ui/src/ui/views/markdown-sidebar.ts
index 006bd6ac44b..6c9805a6f25 100644
--- a/ui/src/ui/views/markdown-sidebar.ts
+++ b/ui/src/ui/views/markdown-sidebar.ts
@@ -1,5 +1,6 @@
 import { html } from "lit";
 import { unsafeHTML } from "lit/directives/unsafe-html.js";
+import { handleCodeBlockCopyClick } from "../code-block-copy.ts";
 import { icons } from "../icons.ts";
 import { toSanitizedMarkdownHtml } from "../markdown.ts";
 
@@ -19,7 +20,7 @@ export function renderMarkdownSidebar(props: MarkdownSidebarProps) {
           ${icons.x}
         </button>
       </div>
-      <div class="sidebar-content">
+      <div class="sidebar-content" @click=${handleCodeBlockCopyClick}>
         ${
           props.error
             ? html`