diff --git a/src/components/MermaidDiagram.test.tsx b/src/components/MermaidDiagram.test.tsx
index cb861de0..fd85d6f6 100644
--- a/src/components/MermaidDiagram.test.tsx
+++ b/src/components/MermaidDiagram.test.tsx
@@ -1,5 +1,6 @@
 import { fireEvent, render, screen, waitFor } from '@testing-library/react'
 import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { RUNTIME_STYLE_NONCE } from '../lib/runtimeStyleNonce'
 import { MermaidDiagram } from './MermaidDiagram'
 
 const mermaidMock = vi.hoisted(() => ({
@@ -50,6 +51,26 @@ describe('MermaidDiagram', () => {
     expect(screen.getByTestId('mermaid-diagram-dialog-viewport').querySelector('svg')).not.toBeNull()
   })
 
+  it('tags Mermaid SVG style elements with the runtime CSP nonce', async () => {
+    mermaidMock.render.mockResolvedValueOnce({
+      svg: '<svg aria-label="Rendered Mermaid"><style>.node{fill:#000}</style><g><text>A to B</text></g></svg>',
+    })
+
+    render(
+      <MermaidDiagram
+        diagram={'flowchart LR\nA --> B'}
+        source={'```mermaid\nflowchart LR\nA --> B\n```'}
+      />,
+    )
+
+    await waitFor(() => {
+      expect(screen.getByTestId('mermaid-diagram-viewport').querySelector('style')).not.toBeNull()
+    })
+
+    const style = screen.getByTestId('mermaid-diagram-viewport').querySelector('style')
+    expect(style?.getAttribute('nonce')).toBe(RUNTIME_STYLE_NONCE)
+  })
+
   it('falls back to the original source when Mermaid cannot render', async () => {
     mermaidMock.render.mockRejectedValueOnce(new Error('parse error'))
 
diff --git a/src/components/MermaidDiagram.tsx b/src/components/MermaidDiagram.tsx
index 6ca39809..0f76bc22 100644
--- a/src/components/MermaidDiagram.tsx
+++ b/src/components/MermaidDiagram.tsx
@@ -8,6 +8,7 @@ import {
   DialogTitle,
   DialogTrigger,
 } from '@/components/ui/dialog'
+import { RUNTIME_STYLE_NONCE } from '@/lib/runtimeStyleNonce'
 
 type MermaidApi = typeof import('mermaid')['default']
 
@@ -51,6 +52,18 @@ function initializeMermaid(mermaid: MermaidApi) {
   initialized = true
 }
 
+function withRuntimeStyleNonce(svg: string): string {
+  if (!svg.includes('<style')) return svg
+  if (typeof document === 'undefined') return svg
+
+  const container = document.createElement('div')
+  container.innerHTML = svg
+  container.querySelectorAll('style').forEach((style) => {
+    style.setAttribute('nonce', RUNTIME_STYLE_NONCE)
+  })
+  return container.innerHTML
+}
+
 async function renderMermaidDiagram({
   diagram,
   renderId,
@@ -62,7 +75,7 @@ async function renderMermaidDiagram({
     const mermaid = (await import('mermaid')).default
     initializeMermaid(mermaid)
     const result = await mermaid.render(renderId, diagram)
-    return result.svg
+    return withRuntimeStyleNonce(result.svg)
   }
   const nextRender = renderQueue.then(render, render)
   renderQueue = nextRender.then(() => undefined, () => undefined)
diff --git a/tests/smoke/mermaid-diagram-notes.spec.ts b/tests/smoke/mermaid-diagram-notes.spec.ts
index 3a42eab4..78df2c39 100644
--- a/tests/smoke/mermaid-diagram-notes.spec.ts
+++ b/tests/smoke/mermaid-diagram-notes.spec.ts
@@ -3,6 +3,7 @@ import fs from 'fs'
 import path from 'path'
 import { createFixtureVaultCopy, openFixtureVault, removeFixtureVaultCopy } from '../helpers/fixtureVault'
 import { executeCommand, openCommandPalette } from './helpers'
+import { RUNTIME_STYLE_NONCE } from '../../src/lib/runtimeStyleNonce'
 
 let tempVaultDir: string
 
@@ -200,6 +201,13 @@ async function computedCss(locator: Locator, property: 'color' | 'fill' | 'strok
   ), property)
 }
 
+async function readStyleNonce(svg: Locator): Promise<string | null> {
+  return svg.locator('style').first().evaluate((style) => {
+    const nonce = (style as HTMLElement).nonce
+    return nonce.length > 0 ? nonce : style.getAttribute('nonce')
+  })
+}
+
 async function labelFitsNode(node: Locator): Promise<boolean> {
   return node.evaluate((element) => {
     const shape = element.querySelector<SVGGraphicsElement>('rect, polygon, circle, ellipse, path')!
@@ -230,6 +238,7 @@ async function readReportedDiagramMetrics(page: Page, diagramIndex: number) {
       await labelFitsNode(mermaidNode(page, diagramIndex, 'Contract path')),
       await labelFitsNode(mermaidNode(page, diagramIndex, 'unscheduled clocking')),
     ].every(Boolean),
+    styleNonce: await readStyleNonce(svg),
     text: await svg.evaluate((element) => element.textContent ?? ''),
   }
 }
@@ -248,6 +257,7 @@ test('Mermaid diagrams render when opening saved notes directly', async ({ page
     scheduledStroke: 'rgb(122, 91, 0)',
     scheduledTextColor: 'rgb(58, 44, 0)',
     labelsFit: true,
+    styleNonce: RUNTIME_STYLE_NONCE,
     text: expect.stringContaining('Linked to a planned shift?'),
   })
 })
@@ -283,6 +293,7 @@ ${SYSTEM_OVERVIEW_DIAGRAM}
     scheduledStroke: 'rgb(122, 91, 0)',
     scheduledTextColor: 'rgb(58, 44, 0)',
     labelsFit: true,
+    styleNonce: RUNTIME_STYLE_NONCE,
     text: expect.stringContaining('Linked to a planned shift?'),
   })
   await expect(page.locator('[data-testid="mermaid-diagram-error"]')).toHaveCount(1)