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 d4ce73065c..ade733e3df 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
@@ -47,21 +47,23 @@ 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).
+  it('repairs a label mermaid returned as HTML, so the image stays loadable', async () => {
+    // `A["a<br/>b&nbsp;c"]` comes back from mermaid with an open `<br>` and an
+    // `&nbsp;` entity — 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>'
+      svg: '<svg xmlns="http://www.w3.org/2000/svg" width="100%" viewBox="0 0 260.34375 70"><foreignObject width="19.109375" height="48"><div xmlns="http://www.w3.org/1999/xhtml"><span class="nodeLabel"><p>a<br>b&nbsp;c</p></span></div></foreignObject></svg>'
     })
 
-    const { container } = render(<MermaidRenderer code="graph TD; A[x<br/>y] --> B" />)
+    const { container } = render(<MermaidRenderer code='graph TD; A["a<br/>b&nbsp;c"]' />)
 
     await waitFor(() => expect(container.querySelectorAll('img')).toHaveLength(1))
 
     const src = decodeURIComponent([...container.querySelectorAll('img')][0]?.src.split(',')[1] ?? '')
+    const doc = new DOMParser().parseFromString(src, 'image/svg+xml')
 
-    expect(src).toContain('<br/>')
-    expect(new DOMParser().parseFromString(src, 'image/svg+xml').documentElement.tagName).toBe('svg')
+    expect(doc.documentElement.tagName).toBe('svg')
+    expect(doc.querySelector('p')?.textContent).toBe('ab c')
   })
 })
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 2dc7a93491..8b5857638a 100644
--- a/apps/desktop/src/components/assistant-ui/embeds/mermaid-embed.tsx
+++ b/apps/desktop/src/components/assistant-ui/embeds/mermaid-embed.tsx
@@ -36,9 +36,10 @@ const renderCache = createMermaidRenderCache({
     const id = `mmd-${Math.random().toString(36).slice(2)}`
     const result = await mermaid.render(id, code)
 
-    // 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).
+    // Mermaid's markup targets inline HTML, so a label's `<br/>` reaches us as
+    // an open `<br>` and a non-breaking space as `&nbsp;`; make it XML before
+    // the size pass and 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
diff --git a/apps/desktop/src/lib/svg-image.test.ts b/apps/desktop/src/lib/svg-image.test.ts
index 4197f39290..87be49ad1d 100644
--- a/apps/desktop/src/lib/svg-image.test.ts
+++ b/apps/desktop/src/lib/svg-image.test.ts
@@ -65,40 +65,27 @@ describe('normalizeSvgSize', () => {
   })
 })
 
-// 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).
+// Real mermaid 11.16 label island for `A["a<br/>b&nbsp;c"]` (verified against
+// the installed package): the HTML serialisation leaves `<br>` open and writes
+// the non-breaking space as `&nbsp;`, and neither is well-formed XML. Every
+// strict `image/svg+xml` parse degrades on it — 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).
+const MERMAID_LABEL_SVG = `<svg xmlns="http://www.w3.org/2000/svg" width="100%" viewBox="0 0 260.34375 70"><g class="label"><foreignObject width="19.109375" height="48"><div xmlns="http://www.w3.org/1999/xhtml" style="display: table-cell; white-space: nowrap; line-height: 1.5; max-width: 200px; text-align: center;"><span class="nodeLabel"><p>a<br>b&nbsp;c</p></span></div></foreignObject></g></svg>`
+
 describe('xmlWellFormedSvg', () => {
-  const rootOf = (svg: string) => new DOMParser().parseFromString(svg, 'image/svg+xml').documentElement.tagName
+  const parseXml = (svg: string) => new DOMParser().parseFromString(svg, 'image/svg+xml')
 
-  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('makes a mermaid label with <br> and &nbsp; parse as an <svg> instead of a parsererror page', () => {
+    expect(parseXml(MERMAID_LABEL_SVG).documentElement.tagName).not.toBe('svg')
+    expect(parseXml(xmlWellFormedSvg(MERMAID_LABEL_SVG)).documentElement.tagName).toBe('svg')
+    expect(normalizeSvgSize(xmlWellFormedSvg(MERMAID_LABEL_SVG))).toContain('width="260.34375"')
   })
 
-  it('closes an open void tag that carries attributes', () => {
-    expect(xmlWellFormedSvg('<br class="label">')).toBe('<br class="label"/>')
-  })
+  it('keeps the label content: the line break and the non-breaking space', () => {
+    const label = parseXml(xmlWellFormedSvg(MERMAID_LABEL_SVG)).querySelector('p')
 
-  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"')
+    expect(label?.querySelectorAll('br')).toHaveLength(1)
+    expect(label?.textContent).toBe('ab c')
   })
 })
diff --git a/apps/desktop/src/lib/svg-image.ts b/apps/desktop/src/lib/svg-image.ts
index 22cb71c617..d5e8d759b1 100644
--- a/apps/desktop/src/lib/svg-image.ts
+++ b/apps/desktop/src/lib/svg-image.ts
@@ -16,22 +16,31 @@ 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(`<(${VOID_TAGS})((?:\\s[^<>]*?)?)(/?)>`, 'gi')
-
+// `mermaid.render()` returns the HTML serialisation of the diagram, aimed at
+// inline HTML. Its label islands (XHTML inside <foreignObject>) therefore carry
+// HTML-only syntax: a `<br/>` in the source comes back as an open `<br>`, and a
+// non-breaking space (`&nbsp;`, mermaid's `#nbsp;`, or a literal U+00A0) comes
+// back as the `&nbsp;` entity, which XML does not define. Everything below
+// parses that string as strict XML (`image/svg+xml`) — and so does Blink when
+// the same string reaches an <img> as a data: URI — so either one turns the
+// document into a parsererror page and the diagram silently stops rendering.
+// Parse it with the HTML parser it was serialised for and re-serialise as XML,
+// which fixes every such construct at once rather than one tag at a time.
 export function xmlWellFormedSvg(svg: string): string {
-  return svg.replace(VOID_TAG_RE, (match, tag: string, attrs: string, slash: string) =>
-    slash ? match : `<${tag}${attrs}/>`
-  )
+  const el = new DOMParser().parseFromString(svg, 'text/html').querySelector('svg')
+
+  if (!el) {
+    return svg
+  }
+
+  // The HTML parser keeps a label's `xmlns="…/xhtml"` as a plain attribute. The
+  // serializer declares that namespace itself, and a spec-conformant one (jsdom)
+  // also writes the plain attribute — a duplicate `xmlns`, malformed again.
+  for (const node of el.querySelectorAll('[xmlns]')) {
+    node.removeAttribute('xmlns')
+  }
+
+  return new XMLSerializer().serializeToString(el)
 }
 
 export function normalizeSvgSize(svg: string): string {