diff --git a/apps/desktop/src/app/chat/right-rail/preview-file.tsx b/apps/desktop/src/app/chat/right-rail/preview-file.tsx
index 408e9d86b88..48e0649647a 100644
--- a/apps/desktop/src/app/chat/right-rail/preview-file.tsx
+++ b/apps/desktop/src/app/chat/right-rail/preview-file.tsx
@@ -6,7 +6,7 @@ import type {
   MouseEvent as ReactMouseEvent,
   ReactNode
 } from 'react'
-import { Fragment, useCallback, useEffect, useMemo, useRef, useState } from 'react'
+import { Fragment, lazy, Suspense, useCallback, useEffect, useMemo, useRef, useState } from 'react'
 import ShikiHighlighter from 'react-shiki'
 import { Streamdown } from 'streamdown'
 
@@ -34,6 +34,10 @@ import { setPreviewDirty } from '@/store/preview-edit'
 import { $currentCwd } from '@/store/session'
 import { notifyWorkspaceChanged } from '@/store/workspace-events'
 
+const MermaidDiagram = lazy(() =>
+  import('@/components/chat/mermaid-block').then(m => ({ default: m.MermaidBlock }))
+)
+
 const SHIKI_THEME = { dark: 'github-dark-default', light: 'github-light-default' } as const
 const TEXT_PREVIEW_MAX_BYTES = 512 * 1024
 const SOURCE_CHUNK_LINES = 200
@@ -291,6 +295,16 @@ function MarkdownCode({ className, children, ...props }: ComponentProps<'code'>)
     )
   }
 
+  if (language === 'mermaid') {
+    return (
+      <Suspense
+        fallback={<div className="my-3 text-xs text-muted-foreground">Loading diagram…</div>}
+      >
+        <MermaidDiagram chart={String(children).replace(/\n$/, '')} />
+      </Suspense>
+    )
+  }
+
   return (
     <ShikiHighlighter
       addDefaultStyles={false}
diff --git a/apps/desktop/src/components/chat/mermaid-block.tsx b/apps/desktop/src/components/chat/mermaid-block.tsx
new file mode 100644
index 00000000000..157dc0d2e03
--- /dev/null
+++ b/apps/desktop/src/components/chat/mermaid-block.tsx
@@ -0,0 +1,84 @@
+import { useEffect, useRef, useState } from 'react'
+
+let mermaidPromise: Promise<typeof import('mermaid')> | null = null
+
+function loadMermaid() {
+  if (!mermaidPromise) {
+    mermaidPromise = import('mermaid').then(async mod => {
+      const m = mod.default
+
+      m.initialize({
+        securityLevel: 'strict',
+        startOnLoad: false,
+        theme: document.documentElement.classList.contains('dark') ? 'dark' : 'default'
+      })
+
+      return mod
+    })
+  }
+
+  return mermaidPromise
+}
+
+interface MermaidBlockProps {
+  chart: string
+}
+
+export function MermaidBlock({ chart }: MermaidBlockProps) {
+  const containerRef = useRef<HTMLDivElement>(null)
+  const [error, setError] = useState<string | null>(null)
+  const [svg, setSvg] = useState<string | null>(null)
+
+  useEffect(() => {
+    let cancelled = false
+
+    async function render() {
+      try {
+        const { default: mermaid } = await loadMermaid()
+        const id = `mermaid-${Math.random().toString(36).slice(2, 10)}`
+        const { svg: rendered } = await mermaid.render(id, chart)
+
+        if (!cancelled) {
+          setSvg(rendered)
+          setError(null)
+        }
+      } catch (err) {
+        if (!cancelled) {
+          setError(err instanceof Error ? err.message : String(err))
+          setSvg(null)
+        }
+      }
+    }
+
+    void render()
+
+    return () => {
+      cancelled = true
+    }
+  }, [chart])
+
+  if (error) {
+    return (
+      <div className="my-3 rounded-md border border-border bg-muted/30 px-3 py-2 text-xs text-muted-foreground">
+        <div className="mb-1 font-medium text-foreground">Mermaid diagram error</div>
+        <pre className="whitespace-pre-wrap break-words text-[0.75rem]">{error}</pre>
+        <details className="mt-2">
+          <summary className="cursor-pointer text-muted-foreground/70 hover:text-foreground">View source</summary>
+          <pre className="mt-1 whitespace-pre-wrap break-words text-[0.75rem]">{chart}</pre>
+        </details>
+      </div>
+    )
+  }
+
+  if (!svg) {
+    return <div className="my-3 text-xs text-muted-foreground">Rendering diagram…</div>
+  }
+
+  return (
+    <div
+      ref={containerRef}
+      className="my-3 overflow-auto rounded-md border border-border bg-background p-3 [&_svg]:max-w-full"
+      dangerouslySetInnerHTML={{ __html: svg }}
+    />
+  )
+}