diff --git a/apps/desktop/src/components/assistant-ui/embeds/mermaid-embed.test.tsx b/apps/desktop/src/components/assistant-ui/embeds/mermaid-embed.test.tsx
index d333bf3fa0..d4ce73065c 100644
--- a/apps/desktop/src/components/assistant-ui/embeds/mermaid-embed.test.tsx
+++ b/apps/desktop/src/components/assistant-ui/embeds/mermaid-embed.test.tsx
@@ -46,4 +46,22 @@ describe('MermaidRenderer', () => {
     ])
     expect(decodeURIComponent(images[0]?.src.split(',')[1] ?? '')).toContain('marker-end="url(#arrow)"')
   })
+
+  it('repairs a label mermaid returned with an open <br>, so the image stays loadable', async () => {
+    // `A[x<br/>y]` comes back from mermaid as `<br>` — malformed XML, so the
+    // data: URI fails to decode and the message shows a broken image with the
+    // alt text instead of the diagram (#133089).
+    renderMermaid.mockResolvedValueOnce({
+      svg: '<svg xmlns="http://www.w3.org/2000/svg" width="100%" viewBox="0 0 260.34375 70"><text><tspan><br>x</tspan></text></svg>'
+    })
+
+    const { container } = render(<MermaidRenderer code="graph TD; A[x<br/>y] --> B" />)
+
+    await waitFor(() => expect(container.querySelectorAll('img')).toHaveLength(1))
+
+    const src = decodeURIComponent([...container.querySelectorAll('img')][0]?.src.split(',')[1] ?? '')
+
+    expect(src).toContain('<br/>')
+    expect(new DOMParser().parseFromString(src, 'image/svg+xml').documentElement.tagName).toBe('svg')
+  })
 })
diff --git a/apps/desktop/src/components/assistant-ui/embeds/mermaid-embed.tsx b/apps/desktop/src/components/assistant-ui/embeds/mermaid-embed.tsx
index a7514c5426..2dc7a93491 100644
--- a/apps/desktop/src/components/assistant-ui/embeds/mermaid-embed.tsx
+++ b/apps/desktop/src/components/assistant-ui/embeds/mermaid-embed.tsx
@@ -4,7 +4,7 @@ import type { Mermaid as MermaidApi } from 'mermaid'
 import { useEffect, useState } from 'react'
 
 import { Zoomable } from '@/components/ui/zoomable'
-import { copySvgAsPng, normalizeSvgSize } from '@/lib/svg-image'
+import { copySvgAsPng, normalizeSvgSize, xmlWellFormedSvg } from '@/lib/svg-image'
 import { cn } from '@/lib/utils'
 
 import { createMermaidRenderCache, createRetryableLoader, nextPaint } from './mermaid-render-cache'
@@ -36,7 +36,10 @@ const renderCache = createMermaidRenderCache({
     const id = `mmd-${Math.random().toString(36).slice(2)}`
     const result = await mermaid.render(id, code)
 
-    return normalizeSvgSize(result.svg)
+    // Mermaid's markup targets inline HTML, so a `<br/>` in a label reaches us
+    // as an open `<br>`; close void tags before the size pass and before the
+    // data: URI below, or the diagram decodes to a broken image (#133089).
+    return normalizeSvgSize(xmlWellFormedSvg(result.svg))
   },
   // Defer until the source fallback has had a frame to paint, so the mermaid
   // runtime import and parse/layout cannot contend with first paint of the
diff --git a/apps/desktop/src/lib/svg-image.test.ts b/apps/desktop/src/lib/svg-image.test.ts
index 2e81a5958c..4197f39290 100644
--- a/apps/desktop/src/lib/svg-image.test.ts
+++ b/apps/desktop/src/lib/svg-image.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest'
 
-import { normalizeSvgSize, svgSize } from './svg-image'
+import { normalizeSvgSize, svgSize, xmlWellFormedSvg } from './svg-image'
 
 // Real mermaid 11.16 render output shape (verified against the installed
 // package): width="100%" + inline style="max-width: Npx" + viewBox.
@@ -64,3 +64,41 @@ describe('normalizeSvgSize', () => {
     expect(normalizeSvgSize(svg)).toBe(svg)
   })
 })
+
+// A mermaid label keeps HTML void tags open: `<br/>` in the source comes back
+// from `mermaid.render()` as `<br>`. That makes the string malformed XML, so
+// every strict `image/svg+xml` parse below degrades — and Blink fails the same
+// way when the string is handed to an <img> as a data: URI, which is how a
+// diagram silently stops rendering (broken image, no error anywhere).
+describe('xmlWellFormedSvg', () => {
+  const rootOf = (svg: string) => new DOMParser().parseFromString(svg, 'image/svg+xml').documentElement.tagName
+
+  it('closes a void tag mermaid left open inside a label', () => {
+    expect(xmlWellFormedSvg('<text><tspan><br>x</tspan></text>')).toBe('<text><tspan><br/>x</tspan></text>')
+  })
+
+  it('closes an open void tag that carries attributes', () => {
+    expect(xmlWellFormedSvg('<br class="label">')).toBe('<br class="label"/>')
+  })
+
+  it('leaves already-closed void tags untouched', () => {
+    const svg = '<text><br/><hr /></text>'
+
+    expect(xmlWellFormedSvg(svg)).toBe(svg)
+  })
+
+  it('leaves non-void tags and void-tag lookalikes untouched', () => {
+    const svg = '<blockquote><break>x</break><bring>y</bring><image href="a"/></blockquote>'
+
+    expect(xmlWellFormedSvg(svg)).toBe(svg)
+  })
+
+  it('makes a label with <br> parse as an <svg> instead of a parsererror page', () => {
+    const svg =
+      '<svg xmlns="http://www.w3.org/2000/svg" width="100%" viewBox="0 0 260.34375 70"><text><tspan><br>x</tspan></text></svg>'
+
+    expect(rootOf(svg)).not.toBe('svg')
+    expect(rootOf(xmlWellFormedSvg(svg))).toBe('svg')
+    expect(normalizeSvgSize(xmlWellFormedSvg(svg))).toContain('width="260.34375"')
+  })
+})
diff --git a/apps/desktop/src/lib/svg-image.ts b/apps/desktop/src/lib/svg-image.ts
index a9397dc969..22cb71c617 100644
--- a/apps/desktop/src/lib/svg-image.ts
+++ b/apps/desktop/src/lib/svg-image.ts
@@ -16,6 +16,24 @@ function viewBoxSize(el: Element): { height: number; width: number } | null {
   return vbW > 0 && vbH > 0 ? { height: vbH, width: vbW } : null
 }
 
+// Mermaid emits SVG aimed at inline HTML: its labels keep HTML void tags
+// unclosed, so a `<br/>` in the source comes back from `mermaid.render()` as
+// `<br>`. Everything below parses that string as strict XML
+// (`image/svg+xml`) — and so does Blink when the same string reaches an <img>
+// as a data: URI — where an open void tag is a malformed document: the parse
+// yields a parsererror page rather than an <svg> and the diagram silently stops
+// rendering. Close void tags before anything XML-parses the markup. Already
+// closed tags, and names that merely start with a void tag (`<break>`), pass
+// through untouched.
+const VOID_TAGS = 'area|base|br|col|embed|hr|img|input|link|meta|param|source|track|wbr'
+const VOID_TAG_RE = new RegExp(`<(${VOID_TAGS})((?:\\s[^<>]*?)?)(/?)>`, 'gi')
+
+export function xmlWellFormedSvg(svg: string): string {
+  return svg.replace(VOID_TAG_RE, (match, tag: string, attrs: string, slash: string) =>
+    slash ? match : `<${tag}${attrs}/>`
+  )
+}
+
 export function normalizeSvgSize(svg: string): string {
   const el = new DOMParser().parseFromString(svg, 'image/svg+xml').documentElement