diff --git a/docs/DEPLOY.en.md b/docs/DEPLOY.en.md
index a7716a3..b2ed75e 100644
--- a/docs/DEPLOY.en.md
+++ b/docs/DEPLOY.en.md
@@ -271,6 +271,7 @@ VERCEL_TEAM_ID=team_xxxxxxxxxxxx   # optional for personal accounts
 | `VERCEL_TOKEN` | Vercel sync token | — |
 | `VERCEL_PROJECT_ID` | Vercel project ID | — |
 | `VERCEL_TEAM_ID` | Vercel team ID | — |
+| `DS2API_CHAT_HISTORY_PATH` | Chat history storage path (must be set to `/tmp/chat_history.json` on Vercel, otherwise unavailable due to read-only filesystem) | `data/chat_history.json` |
 | `DS2API_VERCEL_PROTECTION_BYPASS` | Deployment protection bypass for internal Node→Go calls | — |
 
 ### 3.4 Vercel Architecture
@@ -360,6 +361,22 @@ If API responses return Vercel HTML `Authentication Required`:
 - **Option B**: Add `x-vercel-protection-bypass` header to requests
 - **Option C**: Set `VERCEL_AUTOMATION_BYPASS_SECRET` (or `DS2API_VERCEL_PROTECTION_BYPASS`) for internal Node→Go calls
 
+#### Chat History Unavailable (read-only file system)
+
+```text
+create chat history dir: mkdir /var/task/data: read-only file system
+```
+
+**Cause**: Vercel Serverless functions have a read-only filesystem (`/var/task`). Chat history fails because it cannot create directories there.
+
+**Fix**: Add the following in Vercel Project Settings → Environment Variables:
+
+```text
+DS2API_CHAT_HISTORY_PATH=/tmp/chat_history.json
+```
+
+`/tmp` is the only writable directory in Vercel Serverless. Data is ephemeral (not persisted across cold starts), but the feature works within a single instance lifetime.
+
 ### 3.6 Build Artifacts Not Committed
 
 - `static/admin` directory is not in Git
diff --git a/docs/DEPLOY.md b/docs/DEPLOY.md
index 3ff20ed..4a1b75f 100644
--- a/docs/DEPLOY.md
+++ b/docs/DEPLOY.md
@@ -271,6 +271,7 @@ VERCEL_TEAM_ID=team_xxxxxxxxxxxx   # 个人账号可留空
 | `VERCEL_TOKEN` | Vercel 同步 token | — |
 | `VERCEL_PROJECT_ID` | Vercel 项目 ID | — |
 | `VERCEL_TEAM_ID` | Vercel 团队 ID | — |
+| `DS2API_CHAT_HISTORY_PATH` | Chat history 存储路径（Vercel 上必须设为 `/tmp/chat_history.json`，否则因文件系统只读而不可用） | `data/chat_history.json` |
 | `DS2API_VERCEL_PROTECTION_BYPASS` | 部署保护绕过密钥（内部 Node→Go 调用） | — |
 
 ### 3.3 运行时行为配置（通过 Admin API 设置）
@@ -370,6 +371,22 @@ No Output Directory named "public" found after the Build completed.
 - **方案 B**：请求中添加 `x-vercel-protection-bypass` 头
 - **方案 C**：设置 `VERCEL_AUTOMATION_BYPASS_SECRET`（或 `DS2API_VERCEL_PROTECTION_BYPASS`），仅影响内部 Node→Go 调用
 
+#### Chat History 不可用（read-only file system）
+
+```text
+create chat history dir: mkdir /var/task/data: read-only file system
+```
+
+**原因**：Vercel Serverless 函数的文件系统（`/var/task`）为只读，chat history 尝试在该路径下创建目录失败。
+
+**解决**：在 Vercel Project Settings → Environment Variables 中添加：
+
+```text
+DS2API_CHAT_HISTORY_PATH=/tmp/chat_history.json
+```
+
+`/tmp` 是 Vercel Serverless 环境中唯一可写的目录。数据在函数冷启动之间不会持久化（ephemeral），但在单个实例生命周期内功能正常。
+
 ### 3.6 仓库不提交构建产物
 
 - `static/admin` 目录不在 Git 中