diff --git a/.changeset/fuzzy-dodos-work.md b/.changeset/fuzzy-dodos-work.md
new file mode 100644
index 000000000..aeac90bf9
--- /dev/null
+++ b/.changeset/fuzzy-dodos-work.md
@@ -0,0 +1,5 @@
+---
+'@electric-sql/pglite': patch
+---
+
+Report a clear error when OPFS-AHP is used outside a Dedicated Web Worker.
diff --git a/docs/docs/filesystems.md b/docs/docs/filesystems.md
index d3ef16ad3..c96319eec 100644
--- a/docs/docs/filesystems.md
+++ b/docs/docs/filesystems.md
@@ -86,7 +86,7 @@ The IndexedDB filesystem works at the file level, storing whole files (Postgres
 
 ## OPFS AHP FS
 
-The OPFS AHP filesystem is built on top of the [Origin Private Filesystem](https://developer.mozilla.org/en-US/docs/Web/API/File_System_API/Origin_private_file_system) in the browser and uses an "access handle pool". It is only available when PGlite is run in a Web Worker, this could be any worker you configure. We provide a [Multi Tab Worker](./multi-tab-worker.md) to aid in using PGlite from multiple tabs in the browser.
+The OPFS AHP filesystem is built on top of the [Origin Private Filesystem](https://developer.mozilla.org/en-US/docs/Web/API/File_System_API/Origin_private_file_system) in the browser and uses an "access handle pool". It is only available when PGlite is run in a Dedicated Web Worker. We provide a [Multi Tab Worker](./multi-tab-worker.md) to aid in using PGlite from multiple tabs in the browser.
 
 To use the OPFS AHP FS you can use one of these methods:
 
diff --git a/packages/pglite/src/fs/opfs-ahp-support.ts b/packages/pglite/src/fs/opfs-ahp-support.ts
new file mode 100644
index 000000000..965d29d28
--- /dev/null
+++ b/packages/pglite/src/fs/opfs-ahp-support.ts
@@ -0,0 +1,19 @@
+interface FileSystemFileHandleConstructor {
+  prototype: object
+}
+
+export function isOpfsAhpSupported(
+  fileSystemFileHandle:
+    | FileSystemFileHandleConstructor
+    | undefined = typeof FileSystemFileHandle === 'undefined'
+    ? undefined
+    : FileSystemFileHandle,
+): boolean {
+  return (
+    typeof (
+      fileSystemFileHandle?.prototype as {
+        createSyncAccessHandle?: unknown
+      }
+    )?.createSyncAccessHandle === 'function'
+  )
+}
diff --git a/packages/pglite/src/fs/opfs-ahp.ts b/packages/pglite/src/fs/opfs-ahp.ts
index 433af52bc..c3b00b613 100644
--- a/packages/pglite/src/fs/opfs-ahp.ts
+++ b/packages/pglite/src/fs/opfs-ahp.ts
@@ -1,6 +1,7 @@
 import { BaseFilesystem, ERRNO_CODES, type FsStats } from './base.js'
 import type { PostgresMod } from '../postgresMod.js'
 import { PGlite } from '../pglite.js'
+import { isOpfsAhpSupported } from './opfs-ahp-support.js'
 
 export interface OpfsAhpOptions {
   initialPoolSize?: number
@@ -129,6 +130,12 @@ export class OpfsAhpFS extends BaseFilesystem {
   }
 
   async #init() {
+    if (!isOpfsAhpSupported()) {
+      throw new Error(
+        'OPFS-AHP is only supported in a Dedicated Web Worker because FileSystemFileHandle.createSyncAccessHandle() is not available in this context.',
+      )
+    }
+
     this.#opfsRootAh = await navigator.storage.getDirectory()
     this.#rootAh = await this.#resolveOpfsDirectory(this.dataDir!, {
       create: true,
diff --git a/packages/pglite/tests/opfs-ahp.test.ts b/packages/pglite/tests/opfs-ahp.test.ts
new file mode 100644
index 000000000..9080904e6
--- /dev/null
+++ b/packages/pglite/tests/opfs-ahp.test.ts
@@ -0,0 +1,42 @@
+import { describe, expect, it, vi } from 'vitest'
+
+vi.mock('../src/fs/base.js', () => ({
+  BaseFilesystem: class {
+    protected dataDir?: string
+
+    constructor(dataDir?: string) {
+      this.dataDir = dataDir
+    }
+  },
+  ERRNO_CODES: {},
+}))
+
+vi.mock('../src/pglite.js', () => ({ PGlite: class {} }))
+
+import { OpfsAhpFS } from '../src/fs/opfs-ahp.js'
+import { isOpfsAhpSupported } from '../src/fs/opfs-ahp-support.js'
+
+describe('isOpfsAhpSupported', () => {
+  it('rejects environments without the sync access handle API', () => {
+    expect(isOpfsAhpSupported(undefined)).toBe(false)
+    expect(isOpfsAhpSupported({ prototype: {} })).toBe(false)
+  })
+
+  it('accepts the API shape exposed in Dedicated Web Workers', () => {
+    expect(
+      isOpfsAhpSupported({
+        prototype: { createSyncAccessHandle: () => undefined },
+      }),
+    ).toBe(true)
+  })
+})
+
+describe('OpfsAhpFS', () => {
+  it('explains that OPFS-AHP requires a Dedicated Web Worker', async () => {
+    const fs = new OpfsAhpFS('test')
+
+    await expect(fs.init(undefined as never, {})).rejects.toThrow(
+      'OPFS-AHP is only supported in a Dedicated Web Worker because FileSystemFileHandle.createSyncAccessHandle() is not available in this context.',
+    )
+  })
+})