diff --git a/extensions/discord/src/client.proxy.test.ts b/extensions/discord/src/client.proxy.test.ts
index 3d5e583547d..bf6dbf2a977 100644
--- a/extensions/discord/src/client.proxy.test.ts
+++ b/extensions/discord/src/client.proxy.test.ts
@@ -1,6 +1,9 @@
+import http from "node:http";
+import { fetch as undiciFetch } from "undici";
 import { beforeEach, describe, expect, it, vi } from "vitest";
 import type { OpenClawConfig } from "../../../src/config/config.js";
 import { createDiscordRestClient } from "./client.js";
+import { createDiscordRequestClient } from "./proxy-request-client.js";
 
 const makeProxyFetchMock = vi.hoisted(() => vi.fn());
 
@@ -118,4 +121,56 @@ describe("createDiscordRestClient proxy support", () => {
     expect(makeProxyFetchMock).toHaveBeenCalledWith("http://[::1]:8080");
     expect(requestClient.options?.fetch).toEqual(expect.any(Function));
   });
+
+  it("serializes multipart media with undici-compatible FormData for proxy fetches", async () => {
+    const received = await new Promise<{
+      contentType: string | undefined;
+      body: string;
+    }>((resolve, reject) => {
+      const server = http.createServer((req, res) => {
+        const chunks: Buffer[] = [];
+        req.on("data", (chunk: Buffer) => chunks.push(chunk));
+        req.on("error", reject);
+        req.on("end", () => {
+          resolve({
+            contentType: req.headers["content-type"],
+            body: Buffer.concat(chunks).toString("utf8"),
+          });
+          res.setHeader("Content-Type", "application/json");
+          res.end(JSON.stringify({ id: "message-id", channel_id: "channel-id" }));
+          server.close();
+        });
+      });
+      server.on("error", reject);
+      server.listen(0, "127.0.0.1", () => {
+        const address = server.address();
+        if (!address || typeof address === "string") {
+          reject(new Error("failed to bind test server"));
+          server.close();
+          return;
+        }
+        const rest = createDiscordRequestClient("test-token", {
+          baseUrl: `http://127.0.0.1:${address.port}`,
+          fetch: undiciFetch as typeof fetch,
+          queueRequests: false,
+        });
+        void rest
+          .post("/channels/123/messages", {
+            body: {
+              content: "with image",
+              files: [{ data: Buffer.from("png-data"), name: "image.png" }],
+            },
+          })
+          .catch((err: unknown) => {
+            reject(err);
+            server.close();
+          });
+      });
+    });
+
+    expect(received.contentType).toMatch(/^multipart\/form-data; boundary=/);
+    expect(received.body).toContain('name="files[0]"; filename="image.png"');
+    expect(received.body).toContain('name="payload_json"');
+    expect(received.body).toContain('"attachments":[{"id":0,"filename":"image.png"}]');
+  });
 });
diff --git a/extensions/discord/src/proxy-request-client.ts b/extensions/discord/src/proxy-request-client.ts
index bd68846dd3f..a78bc25aae2 100644
--- a/extensions/discord/src/proxy-request-client.ts
+++ b/extensions/discord/src/proxy-request-client.ts
@@ -7,6 +7,7 @@ import {
   type RequestClientOptions,
 } from "@buape/carbon";
 import { isRecord } from "openclaw/plugin-sdk/text-runtime";
+import { FormData as UndiciFormData } from "undici";
 
 export type ProxyRequestClientOptions = RequestClientOptions & {
   fetch?: typeof fetch;
@@ -281,7 +282,7 @@ class ProxyRequestClientCompat {
         typeof payload === "string"
           ? { content: payload, attachments: [] }
           : { ...payload, attachments: [] };
-      const formData = new FormData();
+      const formData = new UndiciFormData();
       const files = getMultipartFiles(payload);
 
       for (const [index, file] of files.entries()) {