diff --git a/src/extensions/SafeClipboard.hostileThrow.test.ts b/src/extensions/SafeClipboard.hostileThrow.test.ts
new file mode 100644
index 00000000..6c9ea33f
--- /dev/null
+++ b/src/extensions/SafeClipboard.hostileThrow.test.ts
@@ -0,0 +1,50 @@
+import { describe, expect, it, vi } from 'vitest';
+
+import {
+  isClipboardSanitizationError,
+  sanitizeRichClipboardHtml,
+  type ClipboardConfig,
+} from './SafeClipboard.js';
+
+/**
+ * Exercise the direct sanitizer boundary with a hostile configuration failure.
+ * Unknown thrown values must be normalized without prototype inspection.
+ */
+describe('SafeClipboard sanitizer hostile thrown-value containment', () => {
+  it('rejects primitive values without consulting the WeakSet', () => {
+    expect(isClipboardSanitizationError('private primitive sentinel')).toBe(false);
+    expect(isClipboardSanitizationError(1)).toBe(false);
+    expect(isClipboardSanitizationError(null)).toBe(false);
+  });
+
+  it('normalizes hostile configuration failures without prototype inspection', () => {
+    const privateSentinel = new Error('private sanitizer prototype sentinel');
+    const getPrototypeOf = vi.fn(() => {
+      throw privateSentinel;
+    });
+    const hostileThrownValue = new Proxy(Object.create(null) as object, {
+      getPrototypeOf,
+    });
+    const hostileConfig = new Proxy(Object.create(null) as ClipboardConfig, {
+      ownKeys() {
+        throw hostileThrownValue;
+      },
+    });
+
+    let observed: unknown;
+    try {
+      sanitizeRichClipboardHtml('<p>private source</p>', hostileConfig, document);
+    } catch (error) {
+      observed = error;
+    }
+
+    expect(getPrototypeOf).not.toHaveBeenCalled();
+    expect(observed).toEqual(
+      expect.objectContaining({
+        name: 'ClipboardSanitizationError',
+        code: 'invalid_configuration',
+        message: 'Rich clipboard configuration is invalid.',
+      }),
+    );
+  });
+});
diff --git a/src/extensions/SafeClipboard.ts b/src/extensions/SafeClipboard.ts
index a3eaef4c..5e4aef8c 100644
--- a/src/extensions/SafeClipboard.ts
+++ b/src/extensions/SafeClipboard.ts
@@ -58,6 +58,8 @@ const ERROR_MESSAGES: Readonly<Record<ClipboardSanitizationErrorCode, string>> =
     invalid_html: 'Rich clipboard HTML could not be sanitized.',
   });
 
+const CLIPBOARD_SANITIZATION_ERRORS = new WeakSet<object>();
+
 /** Error whose stable code and message never disclose clipboard content. */
 export class ClipboardSanitizationError extends Error {
   /** Machine-readable rejection category safe for host telemetry. */
@@ -68,9 +70,19 @@ export class ClipboardSanitizationError extends Error {
     super(ERROR_MESSAGES[code]);
     this.name = 'ClipboardSanitizationError';
     this.code = code;
+    CLIPBOARD_SANITIZATION_ERRORS.add(this);
   }
 }
 
+/** Return whether an unknown value is a genuine module-created sanitizer error. */
+export function isClipboardSanitizationError(
+  value: unknown,
+): value is ClipboardSanitizationError {
+  return (
+    (typeof value === 'object' && value !== null) || typeof value === 'function'
+  ) && CLIPBOARD_SANITIZATION_ERRORS.has(value as object);
+}
+
 interface ResolvedClipboardConfig {
   readonly maxHtmlBytes: number;
   readonly maxNodes: number;
@@ -241,7 +253,7 @@ function resolveClipboardConfig(
     });
   } catch (error) {
     if (
-      error instanceof ClipboardSanitizationError &&
+      isClipboardSanitizationError(error) &&
       error.code === 'invalid_configuration'
     ) {
       throw error;
@@ -577,7 +589,7 @@ export function sanitizeRichClipboardHtml(
     }
     return outputContainer.innerHTML;
   } catch (error) {
-    if (error instanceof ClipboardSanitizationError) throw error;
+    if (isClipboardSanitizationError(error)) throw error;
     throw new ClipboardSanitizationError('invalid_html');
   }
 }
@@ -620,10 +632,9 @@ export const SafeClipboard = Extension.create<SafeClipboardOptions>({
           : this.options.config;
       return sanitizeRichClipboardHtml(html, config, this.options.document);
     } catch (error) {
-      const clipboardError =
-        error instanceof ClipboardSanitizationError
-          ? error
-          : new ClipboardSanitizationError('invalid_html');
+      const clipboardError = isClipboardSanitizationError(error)
+        ? error
+        : new ClipboardSanitizationError('invalid_html');
       try {
         this.options.onError?.(clipboardError);
       } catch {
diff --git a/src/extensions/SafeClipboardExtension.hostileThrow.test.ts b/src/extensions/SafeClipboardExtension.hostileThrow.test.ts
new file mode 100644
index 00000000..6624c620
--- /dev/null
+++ b/src/extensions/SafeClipboardExtension.hostileThrow.test.ts
@@ -0,0 +1,97 @@
+import { describe, expect, it, vi } from 'vitest';
+import {
+  DEFAULT_CLIPBOARD_HTML_BYTES,
+  DEFAULT_CLIPBOARD_MAX_DEPTH,
+  DEFAULT_CLIPBOARD_MAX_NODES,
+  type ClipboardSanitizationError,
+} from './SafeClipboard.js';
+import {
+  SafeClipboard,
+  type SafeClipboardOptions,
+} from './SafeClipboardExtension.js';
+
+/**
+ * Exercise the real ProseMirror paste transform with hostile values thrown by
+ * host option access. Unknown thrown values must never escape Inkspan.
+ */
+describe('SafeClipboard hostile thrown-value containment', () => {
+  it('fails closed without prototype inspection when a config getter throws a proxy', () => {
+    const privateSentinel = new Error('private prototype sentinel');
+    const hostileThrownValue = new Proxy(Object.create(null) as object, {
+      getPrototypeOf() {
+        throw privateSentinel;
+      },
+    });
+    const onError = vi.fn((_error: ClipboardSanitizationError) => undefined);
+    const hostileOptions = {
+      get config(): never {
+        throw hostileThrownValue;
+      },
+      maxHtmlBytes: DEFAULT_CLIPBOARD_HTML_BYTES,
+      maxNodes: DEFAULT_CLIPBOARD_MAX_NODES,
+      maxDepth: DEFAULT_CLIPBOARD_MAX_DEPTH,
+      onError,
+      document,
+    } as SafeClipboardOptions;
+
+    const addPlugins = SafeClipboard.config.addProseMirrorPlugins;
+    if (!addPlugins) throw new Error('SafeClipboard plugin factory is unavailable');
+    const plugins = addPlugins.call({ options: hostileOptions } as never);
+    const plugin = plugins[0];
+    const transform = plugin?.props.transformPastedHTML;
+    if (!plugin || !transform) {
+      throw new Error('SafeClipboard paste transform is unavailable');
+    }
+
+    let transformed: string | undefined;
+    expect(() => {
+      transformed = transform.call(plugin, '<p>private source</p>', {} as never);
+    }).not.toThrow();
+
+    expect(transformed).toBe('');
+    expect(onError).toHaveBeenCalledTimes(1);
+    expect(onError).toHaveBeenCalledWith(
+      expect.objectContaining({
+        code: 'invalid_html',
+        message: 'Rich clipboard HTML could not be sanitized.',
+      }),
+    );
+  });
+
+  it('fails closed when a config getter throws a primitive value', () => {
+    const onError = vi.fn((_error: ClipboardSanitizationError) => undefined);
+    const hostileOptions = {
+      get config(): never {
+        throw 'private primitive sentinel';
+      },
+      maxHtmlBytes: DEFAULT_CLIPBOARD_HTML_BYTES,
+      maxNodes: DEFAULT_CLIPBOARD_MAX_NODES,
+      maxDepth: DEFAULT_CLIPBOARD_MAX_DEPTH,
+      onError,
+      document,
+    } as SafeClipboardOptions;
+
+    const addPlugins = SafeClipboard.config.addProseMirrorPlugins;
+    if (!addPlugins) throw new Error('SafeClipboard plugin factory is unavailable');
+    const plugins = addPlugins.call({ options: hostileOptions } as never);
+    const plugin = plugins[0];
+    const transform = plugin?.props.transformPastedHTML;
+    if (!plugin || !transform) {
+      throw new Error('SafeClipboard paste transform is unavailable');
+    }
+
+    let transformed: string | undefined;
+    expect(() => {
+      transformed = transform.call(plugin, '<p>private source</p>', {} as never);
+    }).not.toThrow();
+
+    expect(transformed).toBe('');
+    expect(onError).toHaveBeenCalledTimes(1);
+    expect(onError).toHaveBeenCalledWith(
+      expect.objectContaining({
+        code: 'invalid_html',
+        message: 'Rich clipboard HTML could not be sanitized.',
+      }),
+    );
+  });
+});
diff --git a/src/extensions/SafeClipboardExtension.ts b/src/extensions/SafeClipboardExtension.ts
index 4778313a..71d8302a 100644
--- a/src/extensions/SafeClipboardExtension.ts
+++ b/src/extensions/SafeClipboardExtension.ts
@@ -9,6 +9,7 @@ import {
   DEFAULT_CLIPBOARD_HTML_BYTES,
   DEFAULT_CLIPBOARD_MAX_DEPTH,
   DEFAULT_CLIPBOARD_MAX_NODES,
+  isClipboardSanitizationError,
   sanitizeRichClipboardHtml,
   type ClipboardConfig,
 } from './SafeClipboard.js';
@@ -55,10 +56,9 @@ function transformPastedClipboardHtml(
         : options.config;
     return sanitizeRichClipboardHtml(html, config, options.document);
   } catch (error) {
-    const clipboardError =
-      error instanceof ClipboardSanitizationError
-        ? error
-        : new ClipboardSanitizationError('invalid_html');
+    const clipboardError = isClipboardSanitizationError(error)
+      ? error
+      : new ClipboardSanitizationError('invalid_html');
     try {
       options.onError?.(clipboardError);
     } catch {