diff --git a/apps/sim/lib/copilot/vfs/document-style.test.ts b/apps/sim/lib/copilot/vfs/document-style.test.ts
new file mode 100644
index 00000000000..372718b4703
--- /dev/null
+++ b/apps/sim/lib/copilot/vfs/document-style.test.ts
@@ -0,0 +1,168 @@
+/**
+ * @vitest-environment node
+ */
+import { deflateRawSync } from 'zlib'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+const { mockLoadAsync } = vi.hoisted(() => ({ mockLoadAsync: vi.fn() }))
+
+vi.mock('jszip', () => ({ default: { loadAsync: mockLoadAsync } }))
+
+import { extractDocumentStyle } from '@/lib/copilot/vfs/document-style'
+
+const THEME_XML = `<?xml version="1.0"?>
+<a:theme xmlns:a="http://schemas.openxmlformats.org/drawingml/2006/main">
+  <a:themeElements>
+    <a:clrScheme name="Office">
+      <a:dk1><a:sysClr val="windowText" lastClr="000000"/></a:dk1>
+      <a:lt1><a:sysClr val="window" lastClr="FFFFFF"/></a:lt1>
+      <a:accent1><a:srgbClr val="4472C4"/></a:accent1>
+    </a:clrScheme>
+    <a:fontScheme name="Office">
+      <a:majorFont><a:latin typeface="Calibri Light"/></a:majorFont>
+      <a:minorFont><a:latin typeface="Calibri"/></a:minorFont>
+    </a:fontScheme>
+  </a:themeElements>
+</a:theme>`
+
+interface ZipEntryInput {
+  name: string
+  content: string
+  /** Overrides the uncompressed size written to both headers, to model a lying archive. */
+  declaredUncompressedSize?: number
+}
+
+/**
+ * Emit a ZIP archive byte by byte. Generating one with a library and patching it
+ * afterwards cannot express a declared size that never matched the payload, and
+ * that divergence is the whole subject of these tests.
+ */
+function buildZip(entries: ZipEntryInput[]): Buffer {
+  const locals: Buffer[] = []
+  const centrals: Buffer[] = []
+  let offset = 0
+
+  for (const entry of entries) {
+    const name = Buffer.from(entry.name, 'utf8')
+    const raw = Buffer.from(entry.content, 'utf8')
+    const deflated = deflateRawSync(raw)
+    const declared = entry.declaredUncompressedSize ?? raw.length
+
+    const local = Buffer.alloc(30 + name.length)
+    local.writeUInt32LE(0x04034b50, 0)
+    local.writeUInt16LE(20, 4)
+    local.writeUInt16LE(8, 8) // deflate
+    local.writeUInt32LE(0, 14) // crc32 — nothing under test validates it
+    local.writeUInt32LE(deflated.length, 18)
+    local.writeUInt32LE(declared, 22)
+    local.writeUInt16LE(name.length, 26)
+    name.copy(local, 30)
+    locals.push(local, deflated)
+
+    const central = Buffer.alloc(46 + name.length)
+    central.writeUInt32LE(0x02014b50, 0)
+    central.writeUInt16LE(20, 4)
+    central.writeUInt16LE(20, 6)
+    central.writeUInt16LE(8, 10) // deflate
+    central.writeUInt32LE(0, 16)
+    central.writeUInt32LE(deflated.length, 20)
+    central.writeUInt32LE(declared, 24)
+    central.writeUInt16LE(name.length, 28)
+    central.writeUInt32LE(offset, 42)
+    name.copy(central, 46)
+    centrals.push(central)
+
+    offset += local.length + deflated.length
+  }
+
+  const centralDirectory = Buffer.concat(centrals)
+  const eocd = Buffer.alloc(22)
+  eocd.writeUInt32LE(0x06054b50, 0)
+  eocd.writeUInt16LE(entries.length, 8)
+  eocd.writeUInt16LE(entries.length, 10)
+  eocd.writeUInt32LE(centralDirectory.length, 12)
+  eocd.writeUInt32LE(offset, 16)
+
+  return Buffer.concat([...locals, centralDirectory, eocd])
+}
+
+/** Stand-in for a loaded JSZip archive, serving the parts the extractor asks for. */
+function fakeArchive(files: Record<string, string>) {
+  return {
+    file: (path: string) => (files[path] ? { async: async () => files[path] } : null),
+  }
+}
+
+describe('extractDocumentStyle', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+  })
+
+  it('extracts theme colors and fonts from a well-formed docx', async () => {
+    mockLoadAsync.mockResolvedValue(fakeArchive({ 'word/theme/theme1.xml': THEME_XML }))
+
+    const summary = await extractDocumentStyle(
+      buildZip([{ name: 'word/theme/theme1.xml', content: THEME_XML }]),
+      'docx'
+    )
+
+    expect(mockLoadAsync).toHaveBeenCalledOnce()
+    expect(summary?.theme?.colors).toMatchObject({
+      dk1: '000000',
+      lt1: 'FFFFFF',
+      accent1: '4472C4',
+    })
+    expect(summary?.theme?.fonts).toEqual({ major: 'Calibri Light', minor: 'Calibri' })
+  })
+
+  it('extracts theme data from a well-formed pptx', async () => {
+    mockLoadAsync.mockResolvedValue(fakeArchive({ 'ppt/theme/theme1.xml': THEME_XML }))
+
+    const summary = await extractDocumentStyle(
+      buildZip([{ name: 'ppt/theme/theme1.xml', content: THEME_XML }]),
+      'pptx'
+    )
+
+    expect(summary?.theme?.fonts.minor).toBe('Calibri')
+  })
+
+  it('never hands JSZip an archive declaring more expansion than the guard allows', async () => {
+    // JSZip would also fail this archive — but only after inflating the entry,
+    // which is the memory cost the guard exists to avoid. Asserting on the
+    // return value alone cannot tell the two apart, so assert JSZip is never
+    // reached.
+    const bomb = buildZip([
+      {
+        name: 'word/theme/theme1.xml',
+        content: THEME_XML,
+        declaredUncompressedSize: 2 * 1024 * 1024 * 1024,
+      },
+    ])
+
+    expect(await extractDocumentStyle(bomb, 'docx')).toBeNull()
+    expect(mockLoadAsync).not.toHaveBeenCalled()
+  })
+
+  it('never hands JSZip an archive with an implausible compression ratio', async () => {
+    // 400 MiB sits under the 1 GiB absolute cap, so this exercises the ratio
+    // check rather than the size check. Declaring the expansion rather than
+    // carrying it keeps the fixture a few hundred bytes — the guard reads the
+    // declared total, so a real payload would only cost the suite memory.
+    const bomb = buildZip([
+      {
+        name: 'word/theme/theme1.xml',
+        content: THEME_XML,
+        declaredUncompressedSize: 400 * 1024 * 1024,
+      },
+    ])
+
+    expect(await extractDocumentStyle(bomb, 'docx')).toBeNull()
+    expect(mockLoadAsync).not.toHaveBeenCalled()
+  })
+
+  it('returns null for a buffer that is not a ZIP archive', async () => {
+    expect(await extractDocumentStyle(Buffer.from('not an archive at all'), 'docx')).toBeNull()
+    expect(await extractDocumentStyle(Buffer.alloc(2), 'docx')).toBeNull()
+    expect(mockLoadAsync).not.toHaveBeenCalled()
+  })
+})
diff --git a/apps/sim/lib/copilot/vfs/document-style.ts b/apps/sim/lib/copilot/vfs/document-style.ts
index 7edbe202fc2..29d2b120a83 100644
--- a/apps/sim/lib/copilot/vfs/document-style.ts
+++ b/apps/sim/lib/copilot/vfs/document-style.ts
@@ -1,11 +1,9 @@
 import { createLogger } from '@sim/logger'
 import { toError } from '@sim/utils/errors'
+import { assertOoxmlArchiveWithinLimits, isZipShaped } from '@/lib/file-parsers/zip-guard'
 
 const logger = createLogger('DocumentStyle')
 
-// ZIP magic bytes: PK\x03\x04
-const ZIP_MAGIC = [0x50, 0x4b, 0x03, 0x04]
-
 interface ThemeColors {
   dk1: string
   lt1: string
@@ -389,12 +387,15 @@ export async function extractDocumentStyle(
     return extractPdfStyle(buffer)
   }
 
-  if (buffer.length < 4) return null
-  for (let i = 0; i < 4; i++) {
-    if (buffer[i] !== ZIP_MAGIC[i]) return null
-  }
+  if (!isZipShaped(buffer)) return null
 
   try {
+    // Reading a handful of named parts still means handing an attacker-controlled
+    // archive to JSZip, which inflates whatever those entries hold. Bound it with
+    // the same guard the document parsers use rather than trusting the entry
+    // sizes JSZip reports, which come from the archive itself.
+    assertOoxmlArchiveWithinLimits(buffer)
+
     const JSZip = (await import('jszip')).default
     const zip = await JSZip.loadAsync(buffer)