diff --git a/bun.lock b/bun.lock
index 34f00be6e44..e2b8abdfafc 100644
--- a/bun.lock
+++ b/bun.lock
@@ -730,7 +730,7 @@
         "@solid-primitives/resize-observer": "2.1.3",
         "@solidjs/meta": "catalog:",
         "diff": "catalog:",
-        "dompurify": "3.3.1",
+        "dompurify": "3.4.14",
         "effect": "catalog:",
         "fuzzysort": "catalog:",
         "luxon": "catalog:",
@@ -934,7 +934,7 @@
         "@solid-primitives/media": "2.3.3",
         "@solid-primitives/resize-observer": "2.1.3",
         "diff": "catalog:",
-        "dompurify": "3.3.1",
+        "dompurify": "3.4.14",
         "fuzzysort": "catalog:",
         "katex": "0.16.47",
         "luxon": "catalog:",
@@ -1136,7 +1136,7 @@
     "ai": "6.0.168",
     "cross-spawn": "7.0.6",
     "diff": "8.0.4",
-    "dompurify": "3.3.1",
+    "dompurify": "3.4.14",
     "drizzle-kit": "1.0.0-rc.5-ab785fc",
     "drizzle-orm": "1.0.0-rc.5-169397b",
     "effect": "4.0.0-rc.111",
@@ -3852,7 +3852,7 @@
 
     "domhandler": ["domhandler@5.0.3", "", { "dependencies": { "domelementtype": "^2.3.0" } }, "sha512-cgwlv/1iFQiFnU96XXgROh8xTeetsnJiDsTc7TYCLFd9+/WNkIqPTxiM/8pSd8VIrhXGTf1Ny1q1hquVqDJB5w=="],
 
-    "dompurify": ["dompurify@3.3.1", "", { "optionalDependencies": { "@types/trusted-types": "^2.0.7" } }, "sha512-qkdCKzLNtrgPFP1Vo+98FRzJnBRGe4ffyCea9IwHB1fyxPOeNTHpLKYGd4Uk9xvNoH0ZoOjwZxNptyMwqrId1Q=="],
+    "dompurify": ["dompurify@3.4.14", "", { "optionalDependencies": { "@types/trusted-types": "^2.0.7" } }, "sha512-dVoH9z+MY+C9IilgGCk3YfFqjLi3fChm2OiKJMzh6axrJ5qwxqWaZamgmHrpv22CN/KdbZJuGEGgfQoL00LTdg=="],
 
     "domutils": ["domutils@3.2.2", "", { "dependencies": { "dom-serializer": "^2.0.0", "domelementtype": "^2.3.0", "domhandler": "^5.0.3" } }, "sha512-6kZKyUajlDuqlHKVX1w7gyslj9MPIXzIFiz/rGu35uC1wMi+kMhQwGhl4lt9unC9Vb9INnY9Z3/ZA3+FhASLaw=="],
 
@@ -6606,8 +6606,6 @@
 
     "md-to-react-email/marked": ["marked@7.0.4", "", { "bin": { "marked": "bin/marked.js" } }, "sha512-t8eP0dXRJMtMvBojtkcsA7n48BkauktUKzfkPSCq85ZMTJ0v76Rke4DYz01omYpPTUh4p/f7HePgRo3ebG8+QQ=="],
 
-    "mermaid/dompurify": ["dompurify@3.4.14", "", { "optionalDependencies": { "@types/trusted-types": "^2.0.7" } }, "sha512-dVoH9z+MY+C9IilgGCk3YfFqjLi3fChm2OiKJMzh6axrJ5qwxqWaZamgmHrpv22CN/KdbZJuGEGgfQoL00LTdg=="],
-
     "mermaid/marked": ["marked@16.4.2", "", { "bin": { "marked": "bin/marked.js" } }, "sha512-TI3V8YYWvkVf3KJe1dRkpnjs68JUPyEa5vjKrp1XEEJUAOaQc+Qj+L1qWbPd0SJuAdQkFU0h73sXXqwDYxsiDA=="],
 
     "micromark-extension-mdxjs/acorn": ["acorn@8.18.0", "", { "bin": { "acorn": "bin/acorn" } }, "sha512-lGq+9yr1/GuAWaVYIHRjvvySG5/4VfKIvC8EWxStPdcDh/Ka7FG3twP6v4d5BkravUilhIAsG4Qj83t02LWUPQ=="],
diff --git a/package.json b/package.json
index eb48e504aaf..85808e57252 100644
--- a/package.json
+++ b/package.json
@@ -70,7 +70,7 @@
       "@solid-primitives/storage": "4.3.3",
       "@tailwindcss/vite": "4.1.11",
       "diff": "8.0.4",
-      "dompurify": "3.3.1",
+      "dompurify": "3.4.14",
       "drizzle-kit": "1.0.0-rc.5-ab785fc",
       "drizzle-orm": "1.0.0-rc.5-169397b",
       "effect": "4.0.0-rc.111",
diff --git a/packages/session-ui/component-tests/markdown.fixture.tsx b/packages/session-ui/component-tests/markdown.fixture.tsx
index c31096f901b..0b534470efe 100644
--- a/packages/session-ui/component-tests/markdown.fixture.tsx
+++ b/packages/session-ui/component-tests/markdown.fixture.tsx
@@ -3,6 +3,9 @@ import { render } from "solid-js/web"
 import { Markdown } from "../src/components/markdown"
 import { preloadMarkdown } from "../src/components/markdown-cache"
 
+export { sanitizeMarkdown } from "../src/components/markdown-cache"
+export { renderMermaidSvg } from "../src/components/markdown-mermaid"
+
 export async function mountMarkdown(options: { text: string; streaming?: boolean; cached?: boolean }) {
   if (options.cached) await preloadMarkdown(options.text, "markdown-test")
   const host = document.createElement("div")
diff --git a/packages/session-ui/component-tests/markdown.spec.ts b/packages/session-ui/component-tests/markdown.spec.ts
index 850fbd0ed1c..707f0581c40 100644
--- a/packages/session-ui/component-tests/markdown.spec.ts
+++ b/packages/session-ui/component-tests/markdown.spec.ts
@@ -8,6 +8,79 @@ story.beforeEach(async ({ mount }) => {
   await expect(root.locator('[data-component="markdown"]')).toHaveAttribute("data-markdown-ready", "")
 })
 
+story("sanitizes raw HTML while preserving supported Markdown markup", async ({ page }) => {
+  const result = await page.evaluate(async (fixture) => {
+    const { sanitizeMarkdown } = await import(fixture)
+    return [
+      "<p><strong>Safe</strong> <em>formatting</em> <code>const x = 1</code></p>",
+      '<script>alert(1)</script><style>body { display: none }</style><img src="safe.png" onerror="alert(2)"><a href="java&#x73;cript:alert(3)">unsafe</a>',
+      '<a href="https://example.com" target="_blank" rel="nofollow">external</a><a href="/local">local</a>',
+      '<form id="location" name="document"><input name="cookie"></form>',
+      "<math><mrow><mi>x</mi><mo>+</mo><mn>1</mn></mrow></math>",
+      '<svg viewBox="0 0 10 10"><path d="M0 0L10 10" onload="alert(4)"></path><script>alert(5)</script></svg>',
+    ].map(sanitizeMarkdown)
+  }, fixture)
+  expect(result).toEqual([
+    "<p><strong>Safe</strong> <em>formatting</em> <code>const x = 1</code></p>",
+    '<img src="safe.png"><a>unsafe</a>',
+    '<a href="https://example.com" target="_blank" rel="nofollow noopener noreferrer">external</a><a href="/local">local</a>',
+    '<form name="user-content-document" id="user-content-location"><input name="user-content-cookie"></form>',
+    "<math><mrow><mi>x</mi><mo>+</mo><mn>1</mn></mrow></math>",
+    '<svg viewBox="0 0 10 10"><path d="M0 0L10 10"></path></svg>',
+  ])
+})
+
+story("keeps Markdown sanitization and link protections after Mermaid renders", async ({ page }) => {
+  const result = await page.evaluate(async (fixture) => {
+    const { renderMermaidSvg, sanitizeMarkdown } = await import(fixture)
+    const html =
+      '<a href="https://example.com" target="_blank" rel="nofollow">external</a><img src="safe.png" onerror="alert(1)">'
+    const before = sanitizeMarkdown(html)
+    const renders = []
+    for (const source of [
+      "flowchart LR\n A[Start] --> B[End]",
+      "sequenceDiagram\n Alice->>Bob: Hello",
+      '%%{init: {"flowchart": {"htmlLabels": true}}}%%\nflowchart LR\n A["<a href=\'https://example.com\' target=\'_blank\'>External</a>"] --> B[End]',
+    ]) {
+      const svg = await renderMermaidSvg(source)
+      const document = new DOMParser().parseFromString(svg, "image/svg+xml")
+      renders.push({
+        root: document.documentElement.localName,
+        text: document.documentElement.textContent,
+        unsafe: document.querySelectorAll('script, [onerror], [onload], [href^="javascript:"]').length,
+        links: Array.from(document.querySelectorAll("a")).map((link) => ({
+          href: link.getAttribute("href"),
+          target: link.getAttribute("target"),
+          rel: link.getAttribute("rel"),
+        })),
+        markdown: sanitizeMarkdown(html),
+      })
+    }
+    return {
+      before,
+      renders,
+      invalid: await renderMermaidSvg("not a diagram"),
+      afterInvalid: sanitizeMarkdown(html),
+    }
+  }, fixture)
+  expect(result.before).toBe(
+    '<a href="https://example.com" target="_blank" rel="nofollow noopener noreferrer">external</a><img src="safe.png">',
+  )
+  expect(result.renders).toEqual([
+    { root: "svg", text: expect.stringContaining("Start"), unsafe: 0, links: [], markdown: result.before },
+    { root: "svg", text: expect.stringContaining("Hello"), unsafe: 0, links: [], markdown: result.before },
+    {
+      root: "svg",
+      text: expect.stringContaining("External"),
+      unsafe: 0,
+      links: [{ href: "https://example.com", target: "_blank", rel: "noopener" }],
+      markdown: result.before,
+    },
+  ])
+  expect(result.invalid).toBeUndefined()
+  expect(result.afterInvalid).toBe(result.before)
+})
+
 story("mounts cached completed Markdown with sanitized HTML and decorations", async ({ page }) => {
   await page.evaluate(
     async ({ fixture, text }) => {
@@ -36,6 +109,7 @@ story("mounts cached completed Markdown with sanitized HTML and decorations", as
     "noopener noreferrer",
   )
   await expect(markdown.locator("pre code")).toContainText("const answer = 42")
+  await expect(markdown.getByRole("button", { name: "Copy", exact: true })).toBeVisible()
   await expect(markdown.locator("[data-markdown-word]")).toHaveCount(0)
 
   await harness.getByLabel("Markdown text").fill("## Replacement\n\n`new/file.ts`")
@@ -53,6 +127,25 @@ story("mounts cached completed Markdown with sanitized HTML and decorations", as
   await expect(markdown).toHaveAttribute("data-markdown-ready", "")
 })
 
+story("renders cached Mermaid blocks and falls back to code for invalid diagrams", async ({ page }) => {
+  await page.evaluate(async (fixture) => {
+    const { mountMarkdown } = await import(fixture)
+    await mountMarkdown({ text: "```mermaid\nflowchart LR\n A[Start] --> B[End]\n```", cached: true })
+  }, fixture)
+  const harness = page.getByTestId("markdown-fixture")
+  const markdown = harness.locator('[data-component="markdown"]')
+  await expect(markdown.locator('[data-component="markdown-mermaid"] > svg')).toBeVisible()
+  await harness.getByRole("button", { name: "Toggle Markdown" }).click()
+  await expect(markdown).toHaveCount(0)
+  await harness.getByRole("button", { name: "Toggle Markdown" }).click()
+  await expect(markdown.locator('[data-component="markdown-mermaid"] > svg')).toBeVisible()
+  await harness.getByLabel("Markdown text").fill("```mermaid\nnot a diagram\n```")
+  await expect(markdown.locator('[data-component="markdown-mermaid"]')).toHaveCount(0)
+  await expect(markdown.locator("pre code")).toBeVisible()
+  await expect(markdown.locator("pre code")).toHaveText("not a diagram")
+  await expect(markdown.getByRole("button", { name: "Copy", exact: true })).toBeVisible()
+})
+
 story("keeps live elements and selection when a stream completes and later changes", async ({ page }) => {
   await page.evaluate(async (fixture) => {
     const { mountMarkdown } = await import(fixture)
diff --git a/packages/session-ui/component-tests/session-timeline.spec.ts b/packages/session-ui/component-tests/session-timeline.spec.ts
index e7007d8830a..378eb8e63fa 100644
--- a/packages/session-ui/component-tests/session-timeline.spec.ts
+++ b/packages/session-ui/component-tests/session-timeline.spec.ts
@@ -1,5 +1,33 @@
 import { expect, story } from "../../storybook/playwright/story"
 
+for (const width of [1280, 390]) {
+  for (const streaming of [false, true]) {
+    story(
+      `renders Mermaid in the ${streaming ? "streaming" : "completed"} timeline at ${width}px`,
+      async ({ mount, page }) => {
+        await page.setViewportSize({ width, height: 900 })
+        const root = await mount("current-session-mermaid--diagrams", { args: { streaming } })
+        const timeline = root.locator('[data-component="session-timeline"]')
+        const diagrams = timeline.locator('[data-component="markdown-mermaid"] > svg')
+        await expect(diagrams).toHaveCount(2)
+        await expect(diagrams.nth(0)).toBeVisible()
+        await expect(diagrams.nth(0)).toContainText("Client")
+        await expect(diagrams.nth(1)).toBeVisible()
+        await expect(diagrams.nth(1)).toContainText("Send prompt")
+        await expect(timeline.locator('[data-mermaid-ready="true"]')).toHaveCount(2)
+        await expect(timeline.locator('[data-mermaid-ready="true"] > pre:visible')).toHaveCount(0)
+        if (streaming) {
+          await root.getByRole("button", { name: "Complete response" }).click()
+          await expect(timeline.locator('[data-markdown-complete="true"]')).toHaveCount(2)
+          await expect(diagrams).toHaveCount(2)
+          await expect(diagrams.nth(0)).toBeVisible()
+          await expect(diagrams.nth(1)).toBeVisible()
+        }
+      },
+    )
+  }
+}
+
 story("renders streamed reasoning without starting the app", async ({ mount }) => {
   const timeline = await mount("current-session-timeline-rows--streaming-reasoning-and-text")
   await expect(timeline.locator('[data-component="session-timeline"]')).toBeVisible()
diff --git a/packages/session-ui/package.json b/packages/session-ui/package.json
index 1a6b0f9ee1d..1051d4ac564 100644
--- a/packages/session-ui/package.json
+++ b/packages/session-ui/package.json
@@ -74,7 +74,7 @@
     "@solidjs/meta": "catalog:",
     "diff": "catalog:",
     "effect": "catalog:",
-    "dompurify": "3.3.1",
+    "dompurify": "3.4.14",
     "fuzzysort": "catalog:",
     "luxon": "catalog:",
     "marked": "catalog:",
diff --git a/packages/session-ui/src/components/markdown-cache.tsx b/packages/session-ui/src/components/markdown-cache.tsx
index 80cb3fa9e04..9da44b82eee 100644
--- a/packages/session-ui/src/components/markdown-cache.tsx
+++ b/packages/session-ui/src/components/markdown-cache.tsx
@@ -10,6 +10,8 @@ export type MarkdownCacheEntry = {
 
 const max = 200
 const cache = new Map<string, MarkdownCacheEntry>()
+// Mermaid registers hooks on the shared instance that overwrite link attributes.
+const purifier = typeof window !== "undefined" ? DOMPurify(window) : DOMPurify
 const config = {
   USE_PROFILES: { html: true, mathMl: true },
   SANITIZE_NAMED_PROPS: true,
@@ -19,8 +21,8 @@ const config = {
   ADD_ATTR: ["d", "viewBox", "preserveAspectRatio", "xmlns", "target"],
 }
 
-if (typeof window !== "undefined" && DOMPurify.isSupported) {
-  DOMPurify.addHook("afterSanitizeAttributes", (node: Element) => {
+if (typeof window !== "undefined" && purifier.isSupported) {
+  purifier.addHook("afterSanitizeAttributes", (node: Element) => {
     if (!(node instanceof HTMLAnchorElement)) return
     if (node.target !== "_blank") return
 
@@ -33,8 +35,8 @@ if (typeof window !== "undefined" && DOMPurify.isSupported) {
 }
 
 export function sanitizeMarkdown(html: string) {
-  if (!DOMPurify.isSupported) return ""
-  return DOMPurify.sanitize(html, config)
+  if (!purifier.isSupported) return ""
+  return purifier.sanitize(html, config)
 }
 
 export function getCachedMarkdown(key: string) {
diff --git a/packages/session-ui/src/components/markdown.tsx b/packages/session-ui/src/components/markdown.tsx
index 19a6031dafc..bdc71ff442e 100644
--- a/packages/session-ui/src/components/markdown.tsx
+++ b/packages/session-ui/src/components/markdown.tsx
@@ -631,6 +631,7 @@ function updateBlock(container: HTMLDivElement, index: number, block: RenderedBl
   const rendered = renderedMarkdown.get(next)
   // Keep live renderers in control of their DOM, including after completion.
   const source = rendered || block.mode === "live" ? document.createElement("div") : next
+  if (source === next) disposeCopyButtons(next)
   source.innerHTML = block.html
   markInlineCode(source)
   markCodeLinks(source)
@@ -647,6 +648,23 @@ function updateBlock(container: HTMLDivElement, index: number, block: RenderedBl
       raw: block.raw,
     })
   }
+  if (block.mode !== "live") {
+    next.querySelectorAll<HTMLElement>("pre > code").forEach((code) => {
+      const pre = code.parentElement!
+      const wrapper = document.createElement("div")
+      wrapper.dataset.component = "markdown-code"
+      applyCodeMetadata(
+        wrapper,
+        Array.from(code.classList)
+          .find((name) => name.startsWith("language-"))
+          ?.slice(9),
+      )
+      pre.replaceWith(wrapper)
+      wrapper.appendChild(pre)
+      wrapper.appendChild(createCopyButton(labels))
+      decorateMermaid(wrapper, code, true)
+    })
+  }
 
   if (existing) return
   if (!current) {
diff --git a/packages/session-ui/src/timeline/mermaid.stories.tsx b/packages/session-ui/src/timeline/mermaid.stories.tsx
new file mode 100644
index 00000000000..5cd028ba9a4
--- /dev/null
+++ b/packages/session-ui/src/timeline/mermaid.stories.tsx
@@ -0,0 +1,67 @@
+import { createMemo, createSignal } from "solid-js"
+import type { SessionDocument } from "../document"
+import { CURRENT_SESSION_ID, STORY_MODEL, STORY_TIME } from "../storybook/current-session-fixtures"
+import { CurrentSessionProviders } from "../storybook/current-session-story"
+import { SessionTimeline } from "./session-timeline"
+
+export default {
+  title: "OpenCode/Conversation/Mermaid diagrams",
+  id: "current-session-mermaid",
+  component: SessionTimeline,
+  parameters: { layout: "fullscreen" },
+}
+
+export const Diagrams = {
+  args: { streaming: false },
+  render: (args: { streaming: boolean }) => <MermaidTimeline streaming={args.streaming} />,
+}
+
+function MermaidTimeline(props: { streaming: boolean }) {
+  const [completed, setCompleted] = createSignal(!props.streaming)
+  const document = createMemo(
+    (): SessionDocument => ({
+      sessionID: CURRENT_SESSION_ID,
+      status: { type: completed() ? "idle" : "busy" },
+      diffs: [],
+      messages: [
+        {
+          id: "msg_mermaid_user",
+          type: "user",
+          text: "Show the request flow and sequence as Mermaid diagrams.",
+          time: { created: STORY_TIME },
+          metadata: { agent: "build", model: STORY_MODEL },
+        },
+        {
+          id: "msg_mermaid_assistant",
+          type: "assistant",
+          agent: "build",
+          model: STORY_MODEL,
+          time: { created: STORY_TIME + 100, ...(completed() ? { completed: STORY_TIME + 1000 } : {}) },
+          content: [
+            {
+              type: "text",
+              text: [
+                "## Request flow",
+                "```mermaid\nflowchart LR\n Client[Client] --> Server[Server]\n Server --> Model[Model]\n```",
+                "## Request sequence",
+                "```mermaid\nsequenceDiagram\n Client->>Server: Send prompt\n Server->>Model: Generate response\n Model-->>Client: Response\n" +
+                  (completed() ? "```" : ""),
+              ].join("\n\n"),
+              ...(completed() ? {} : { state: { phase: "streaming" } }),
+            },
+          ],
+        },
+      ],
+    }),
+  )
+  return (
+    <section class="mx-auto flex w-full max-w-[840px] flex-col gap-4 p-6">
+      <button type="button" onClick={() => setCompleted((value) => !value)}>
+        {completed() ? "Stream response" : "Complete response"}
+      </button>
+      <CurrentSessionProviders document={document()}>
+        <SessionTimeline document={document()} />
+      </CurrentSessionProviders>
+    </section>
+  )
+}
diff --git a/packages/ui/package.json b/packages/ui/package.json
index 773b63fd7ad..6962e81228a 100644
--- a/packages/ui/package.json
+++ b/packages/ui/package.json
@@ -119,7 +119,7 @@
     "@solid-primitives/resize-observer": "2.1.3",
     "@shikijs/stream": "catalog:",
     "diff": "catalog:",
-    "dompurify": "3.3.1",
+    "dompurify": "3.4.14",
     "fuzzysort": "catalog:",
     "katex": "0.16.47",
     "luxon": "catalog:",