diff --git a/src/main/runtime/character-dictionary-availability.test.ts b/src/main/runtime/character-dictionary-availability.test.ts
new file mode 100644
index 0000000..77a3aa4
--- /dev/null
+++ b/src/main/runtime/character-dictionary-availability.test.ts
@@ -0,0 +1,20 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import {
+  getCharacterDictionaryDisabledReason,
+  isCharacterDictionaryRuntimeEnabled,
+} from './character-dictionary-availability';
+
+test('character dictionary runtime is enabled when external Yomitan profile is not configured', () => {
+  assert.equal(isCharacterDictionaryRuntimeEnabled(''), true);
+  assert.equal(isCharacterDictionaryRuntimeEnabled('   '), true);
+  assert.equal(getCharacterDictionaryDisabledReason(''), null);
+});
+
+test('character dictionary runtime is disabled when external Yomitan profile is configured', () => {
+  assert.equal(isCharacterDictionaryRuntimeEnabled('/tmp/gsm-profile'), false);
+  assert.equal(
+    getCharacterDictionaryDisabledReason('/tmp/gsm-profile'),
+    'Character dictionary is disabled while yomitan.externalProfilePath is configured.',
+  );
+});
diff --git a/src/main/runtime/character-dictionary-availability.ts b/src/main/runtime/character-dictionary-availability.ts
new file mode 100644
index 0000000..4d74f6b
--- /dev/null
+++ b/src/main/runtime/character-dictionary-availability.ts
@@ -0,0 +1,10 @@
+export function isCharacterDictionaryRuntimeEnabled(externalProfilePath: string): boolean {
+  return externalProfilePath.trim().length === 0;
+}
+
+export function getCharacterDictionaryDisabledReason(externalProfilePath: string): string | null {
+  if (isCharacterDictionaryRuntimeEnabled(externalProfilePath)) {
+    return null;
+  }
+  return 'Character dictionary is disabled while yomitan.externalProfilePath is configured.';
+}