diff --git a/packages/plugin-protocol/src/__tests__/manifest.test.ts b/packages/plugin-protocol/src/__tests__/manifest.test.ts
index d8c824d..bc9141c 100644
--- a/packages/plugin-protocol/src/__tests__/manifest.test.ts
+++ b/packages/plugin-protocol/src/__tests__/manifest.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it } from 'vitest';
 import {
+  GHOST_MANIFEST_SUMMARY_MAX_CHARS,
   GHOST_MANIFEST_SCHEMA_VERSION,
   GHOST_OAUTH_SCOPES_MAX,
   compareCindyVersions,
@@ -64,6 +65,27 @@ describe('Ghost manifest contract', () => {
     expect(validateGhostManifest({ ...validManifest, schemaVersion: 1 }).ok).toBe(false);
   });
 
+  it('shares the 300-character limit across description and whenToUse', () => {
+    expect(GHOST_MANIFEST_SUMMARY_MAX_CHARS).toBe(300);
+    for (const field of ['description', 'whenToUse'] as const) {
+      expect(
+        validateGhostManifest({
+          ...validManifest,
+          [field]: 'x'.repeat(GHOST_MANIFEST_SUMMARY_MAX_CHARS),
+        }).ok,
+      ).toBe(true);
+      expect(
+        validateGhostManifest({
+          ...validManifest,
+          [field]: 'x'.repeat(GHOST_MANIFEST_SUMMARY_MAX_CHARS + 1),
+        }),
+      ).toEqual({
+        ok: false,
+        reason: `${field} 必须是 1–${GHOST_MANIFEST_SUMMARY_MAX_CHARS} 字符的非空字符串`,
+      });
+    }
+  });
+
   it('accepts an oidc-token secret without a settings page and normalizes its exact host', () => {
     const result = validateGhostManifest({
       ...validManifest,
diff --git a/packages/plugin-protocol/src/manifest.ts b/packages/plugin-protocol/src/manifest.ts
index 97251eb..0c7511c 100644
--- a/packages/plugin-protocol/src/manifest.ts
+++ b/packages/plugin-protocol/src/manifest.ts
@@ -7,6 +7,9 @@ export const CINDY_FILE_EXT = '.cindy';
 /** ghost.json 格式版本；与 Plugin HTTP API envelope 版本独立演进。 */
 export const GHOST_MANIFEST_SCHEMA_VERSION = 2 as const;
 
+/** ghost.json 的 description / whenToUse 字符上限。 */
+export const GHOST_MANIFEST_SUMMARY_MAX_CHARS = 300;
+
 /** Cindy host locales supported by Plugin manifest resources. */
 export const GHOST_LOCALES = ['zh-CN', 'en', 'ja', 'ko'] as const;
 export type GhostLocale = (typeof GHOST_LOCALES)[number];
@@ -1152,17 +1155,23 @@ export function validateGhostManifest(raw: unknown): ManifestValidation {
     raw.description !== undefined &&
     (typeof raw.description !== 'string' ||
       raw.description.trim().length === 0 ||
-      raw.description.length > 300)
+      raw.description.length > GHOST_MANIFEST_SUMMARY_MAX_CHARS)
   ) {
-    return { ok: false, reason: 'description 必须是 1–300 字符的非空字符串' };
+    return {
+      ok: false,
+      reason: `description 必须是 1–${GHOST_MANIFEST_SUMMARY_MAX_CHARS} 字符的非空字符串`,
+    };
   }
   if (
     raw.whenToUse !== undefined &&
     (typeof raw.whenToUse !== 'string' ||
       raw.whenToUse.trim().length === 0 ||
-      raw.whenToUse.length > 300)
+      raw.whenToUse.length > GHOST_MANIFEST_SUMMARY_MAX_CHARS)
   ) {
-    return { ok: false, reason: 'whenToUse 必须是 1–300 字符的非空字符串' };
+    return {
+      ok: false,
+      reason: `whenToUse 必须是 1–${GHOST_MANIFEST_SUMMARY_MAX_CHARS} 字符的非空字符串`,
+    };
   }
   if (raw.icon !== undefined) {
     if (!isSafeGhostRelativePath(raw.icon)) {