diff --git a/src/components/editorAccessibility.ts b/src/components/editorAccessibility.ts
index f2cc1779..376a9a0b 100644
--- a/src/components/editorAccessibility.ts
+++ b/src/components/editorAccessibility.ts
@@ -37,6 +37,20 @@ function normalizedAccessibilityValue(
   return normalized ? normalized : undefined;
 }
 
+/** Reject runtime direction values outside Inkspan's public finite contract. */
+function validateEditorTextDirection(
+  value: EditorTextDirection | undefined,
+): void {
+  if (
+    value !== undefined &&
+    value !== 'ltr' &&
+    value !== 'rtl' &&
+    value !== 'auto'
+  ) {
+    throw new RangeError('Editor text direction must be ltr, rtl, or auto.');
+  }
+}
+
 /**
  * Normalize the shared visual and semantic empty-editor guidance.
  *
@@ -61,6 +75,7 @@ export function normalizeEditorPlaceholder(
 export function buildEditorAccessibilityAttributes(
   options: EditorAccessibilityOptions,
 ): Record<string, string> {
+  validateEditorTextDirection(options.textDirection);
   const placeholder = normalizeEditorPlaceholder(options.placeholder);
   const languageTag = normalizedAccessibilityValue(options.languageTag);
   const labelledBy = normalizedAccessibilityValue(options.ariaLabelledBy);
diff --git a/src/components/editorAccessibilityRuntime.test.ts b/src/components/editorAccessibilityRuntime.test.ts
new file mode 100644
index 00000000..04ce1ea4
--- /dev/null
+++ b/src/components/editorAccessibilityRuntime.test.ts
@@ -0,0 +1,29 @@
+import { describe, expect, it } from 'vitest';
+import { buildEditorAccessibilityAttributes } from './editorAccessibility.js';
+
+describe('editor accessibility runtime contracts', () => {
+  it.each(['ltr', 'rtl', 'auto'] as const)(
+    'preserves the valid %s text direction',
+    (textDirection) => {
+      expect(
+        buildEditorAccessibilityAttributes({
+          defaultLabel: 'Editor',
+          editable: true,
+          textDirection,
+        }).dir,
+      ).toBe(textDirection);
+    },
+  );
+
+  it('rejects a runtime text direction outside the public enumerated states', () => {
+    expect(() =>
+      buildEditorAccessibilityAttributes({
+        defaultLabel: 'Editor',
+        editable: true,
+        textDirection: 'sideways' as never,
+      }),
+    ).toThrowError(
+      new RangeError('Editor text direction must be ltr, rtl, or auto.'),
+    );
+  });
+});