diff --git a/src/core/net/chat-websocket.ts b/src/core/net/chat-websocket.ts
index 80231ef4..8eabf692 100644
--- a/src/core/net/chat-websocket.ts
+++ b/src/core/net/chat-websocket.ts
@@ -18,6 +18,7 @@ import {
   type ChatAction,
   type CommercialAction,
 } from "@/data/schemas/chat";
+import { createClientMessageId } from "@/lib/chat/client_message_id";
 
 const log = new Logger("ChatWebSocket");
 const RECONNECT_DELAY_MS = 3000;
@@ -101,8 +102,11 @@ export class ChatWebSocket {
     }
   }
 
-  sendMessage(content: string): boolean {
-    const payload = { type: "message", content };
+  sendMessage(
+    content: string,
+    clientMessageId: string = createClientMessageId(),
+  ): boolean {
+    const payload = { type: "message", content, clientMessageId };
     if (!this.isConnected) {
       logWebSocketWarn("→ WS SEND SKIPPED: socket is not open", payload);
       return false;
diff --git a/src/data/repositories/__tests__/chat_send_idempotency.test.ts b/src/data/repositories/__tests__/chat_send_idempotency.test.ts
new file mode 100644
index 00000000..d45b495f
--- /dev/null
+++ b/src/data/repositories/__tests__/chat_send_idempotency.test.ts
@@ -0,0 +1,57 @@
+import { describe, expect, it, vi } from "vitest";
+
+import { ChatRemoteDataSource } from "../chat_remote_data_source";
+import { SendMessageRequestSchema } from "@/data/schemas/chat";
+
+
+describe("chat send clientMessageId", () => {
+  it("keeps legacy requests compatible but validates an explicit UUID", () => {
+    expect(
+      SendMessageRequestSchema.parse({ characterId: "elio", message: "hello" })
+        .clientMessageId,
+    ).toBeUndefined();
+    expect(() =>
+      SendMessageRequestSchema.parse({
+        characterId: "elio",
+        message: "hello",
+        clientMessageId: "not-a-uuid",
+      }),
+    ).toThrow();
+  });
+
+  it("adds one UUID before the HTTP client owns any retry", async () => {
+    const sendMessage = vi.fn().mockResolvedValue({
+      reply: "hi",
+      messageId: "server-message-1",
+      image: { type: null, url: null },
+      lockDetail: { locked: false, reason: null },
+    });
+    const source = new ChatRemoteDataSource({ sendMessage } as never);
+
+    await source.sendMessage("elio", "hello");
+
+    const request = sendMessage.mock.calls[0][0];
+    expect(request.clientMessageId).toMatch(
+      /^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/,
+    );
+  });
+
+  it("reuses an explicitly supplied key for a protocol fallback", async () => {
+    const sendMessage = vi.fn().mockResolvedValue({
+      reply: "hi",
+      messageId: "server-message-1",
+      image: { type: null, url: null },
+      lockDetail: { locked: false, reason: null },
+    });
+    const source = new ChatRemoteDataSource({ sendMessage } as never);
+    const clientMessageId = "11111111-1111-4111-8111-111111111111";
+
+    await source.sendMessage("elio", "hello", { clientMessageId, useWebSocket: false });
+    await source.sendMessage("elio", "hello", { clientMessageId, useWebSocket: true });
+
+    expect(sendMessage.mock.calls.map(([request]) => request.clientMessageId)).toEqual([
+      clientMessageId,
+      clientMessageId,
+    ]);
+  });
+});
diff --git a/src/data/repositories/chat_remote_data_source.ts b/src/data/repositories/chat_remote_data_source.ts
index f3021712..30cc056b 100644
--- a/src/data/repositories/chat_remote_data_source.ts
+++ b/src/data/repositories/chat_remote_data_source.ts
@@ -17,6 +17,7 @@ import {
 } from "@/data/schemas/chat";
 import type { ChatApi } from "@/data/services/api";
 import { Result } from "@/utils/result";
+import { createClientMessageId } from "@/lib/chat/client_message_id";
 
 export class ChatRemoteDataSource {
   constructor(private readonly api: ChatApi) {}
@@ -29,6 +30,7 @@ export class ChatRemoteDataSource {
     return Result.wrap(async () => {
       const request = SendMessageRequestSchema.parse({
         characterId,
+        clientMessageId: options?.clientMessageId ?? createClientMessageId(),
         message,
         ...(options?.imageId ? { imageId: options.imageId } : {}),
         ...(options?.imageThumbUrl
diff --git a/src/data/repositories/interfaces/ichat_repository.ts b/src/data/repositories/interfaces/ichat_repository.ts
index fcbc7d24..2342b369 100644
--- a/src/data/repositories/interfaces/ichat_repository.ts
+++ b/src/data/repositories/interfaces/ichat_repository.ts
@@ -49,6 +49,8 @@ export interface ChatRequestOptions {
 }
 
 export interface ChatSendOptions extends ChatRequestOptions {
+  /** 同一次逻辑发送在重试或 HTTP/WS 切换时必须复用。 */
+  clientMessageId?: string;
   imageId?: string;
   imageThumbUrl?: string;
   imageMediumUrl?: string;
diff --git a/src/data/schemas/chat/request/send_message_request.ts b/src/data/schemas/chat/request/send_message_request.ts
index 1695596b..c25fbfdc 100644
--- a/src/data/schemas/chat/request/send_message_request.ts
+++ b/src/data/schemas/chat/request/send_message_request.ts
@@ -9,6 +9,7 @@ import { booleanOrFalse } from "../../nullable-defaults";
 export const SendMessageRequestSchema = z
   .object({
     characterId: z.string().min(1),
+    clientMessageId: z.uuid().optional(),
     message: z.string().max(4000).optional(),
     imageId: z.string().min(1).optional(),
     imageThumbUrl: z.string().min(1).optional(),
diff --git a/src/lib/chat/client_message_id.ts b/src/lib/chat/client_message_id.ts
new file mode 100644
index 00000000..44d64bba
--- /dev/null
+++ b/src/lib/chat/client_message_id.ts
@@ -0,0 +1,24 @@
+/** 为一次逻辑发送生成稳定 UUID；网络层重放同一请求体时会复用该值。 */
+export function createClientMessageId(): string {
+  if (typeof globalThis.crypto?.randomUUID === "function") {
+    return globalThis.crypto.randomUUID();
+  }
+  const bytes = new Uint8Array(16);
+  if (typeof globalThis.crypto?.getRandomValues === "function") {
+    globalThis.crypto.getRandomValues(bytes);
+  } else {
+    for (let index = 0; index < bytes.length; index += 1) {
+      bytes[index] = Math.floor(Math.random() * 256);
+    }
+  }
+  bytes[6] = (bytes[6] & 0x0f) | 0x40;
+  bytes[8] = (bytes[8] & 0x3f) | 0x80;
+  const hex = Array.from(bytes, (value) => value.toString(16).padStart(2, "0"));
+  return [
+    hex.slice(0, 4).join(""),
+    hex.slice(4, 6).join(""),
+    hex.slice(6, 8).join(""),
+    hex.slice(8, 10).join(""),
+    hex.slice(10, 16).join(""),
+  ].join("-");
+}