diff --git a/packages/proxy/__tests__/proxyRequests.test.ts b/packages/proxy/__tests__/proxyRequests.test.ts
index 23bc8f94e..9fb4ac179 100644
--- a/packages/proxy/__tests__/proxyRequests.test.ts
+++ b/packages/proxy/__tests__/proxyRequests.test.ts
@@ -24,6 +24,7 @@ beforeEach(() => {
 	setGlobalDispatcher(mockAgent); // enabled the mock client to intercept requests
 
 	mockPool = mockAgent.get('https://discord.com');
+	api.setAgent(mockAgent);
 });
 
 afterEach(async () => {
diff --git a/packages/rest/__tests__/REST.test.ts b/packages/rest/__tests__/REST.test.ts
index 2f451699d..f733b8d28 100644
--- a/packages/rest/__tests__/REST.test.ts
+++ b/packages/rest/__tests__/REST.test.ts
@@ -37,6 +37,8 @@ beforeEach(() => {
 	setGlobalDispatcher(mockAgent); // enabled the mock client to intercept requests
 
 	mockPool = mockAgent.get('https://discord.com');
+	api.setAgent(mockAgent);
+	fetchApi.setAgent(mockAgent);
 });
 
 afterEach(async () => {
diff --git a/packages/rest/__tests__/RequestHandler.test.ts b/packages/rest/__tests__/RequestHandler.test.ts
index 6471a1add..16f02769c 100644
--- a/packages/rest/__tests__/RequestHandler.test.ts
+++ b/packages/rest/__tests__/RequestHandler.test.ts
@@ -481,6 +481,8 @@ test('perm server outage', async () => {
 test('server responding too slow', async () => {
 	const api2 = new REST({ timeout: 1 }).setToken('A-Very-Really-Real-Token');
 
+	api2.setAgent(mockAgent);
+
 	mockPool
 		.intercept({
 			path: genPath('/slow'),
diff --git a/packages/rest/__tests__/RequestManager.test.ts b/packages/rest/__tests__/RequestManager.test.ts
index e0a6b6ced..c158663de 100644
--- a/packages/rest/__tests__/RequestManager.test.ts
+++ b/packages/rest/__tests__/RequestManager.test.ts
@@ -15,6 +15,7 @@ beforeEach(() => {
 	setGlobalDispatcher(mockAgent);
 
 	mockPool = mockAgent.get('https://discord.com');
+	api.setAgent(mockAgent);
 });
 
 afterEach(async () => {
diff --git a/packages/rest/__tests__/UndiciRequest.test.ts b/packages/rest/__tests__/UndiciRequest.test.ts
index 95e0f4b8d..90bcc057f 100644
--- a/packages/rest/__tests__/UndiciRequest.test.ts
+++ b/packages/rest/__tests__/UndiciRequest.test.ts
@@ -28,6 +28,7 @@ beforeEach(() => {
 	setGlobalDispatcher(mockAgent); // enabled the mock client to intercept requests
 
 	mockPool = mockAgent.get('https://discord.com');
+	api.setAgent(mockAgent);
 });
 
 afterEach(async () => {
diff --git a/packages/rest/src/strategies/undiciRequest.ts b/packages/rest/src/strategies/undiciRequest.ts
index 5eee90c5e..bafd05028 100644
--- a/packages/rest/src/strategies/undiciRequest.ts
+++ b/packages/rest/src/strategies/undiciRequest.ts
@@ -1,12 +1,14 @@
 import { STATUS_CODES } from 'node:http';
 import { URLSearchParams } from 'node:url';
 import { types } from 'node:util';
-import { type RequestInit, request, Headers, FormData as UndiciFormData } from 'undici';
+import { type RequestInit, request, Headers, FormData as UndiciFormData, Agent } from 'undici';
 import type { HeaderRecord } from 'undici/types/header.js';
 import type { ResponseLike } from '../shared.js';
 
 export type RequestOptions = Exclude<Parameters<typeof request>[1], undefined>;
 
+let localAgent: Agent | null = null;
+
 export async function makeRequest(url: string, init: RequestInit): Promise<ResponseLike> {
 	// The cast is necessary because `headers` and `method` are narrower types in `undici.request`
 	// our request path guarantees they are of acceptable type for `undici.request`
@@ -14,6 +16,15 @@ export async function makeRequest(url: string, init: RequestInit): Promise<Respo
 		...init,
 		body: await resolveBody(init.body),
 	} as RequestOptions;
+
+	// Mismatched dispatchers from the Node.js-bundled undici and package-installed undici breaks file uploads.
+	// So we ensure that we always pass an Agent to request()
+	// https://github.com/nodejs/node/issues/59012
+	if (!options.dispatcher) {
+		localAgent ??= new Agent();
+		options.dispatcher = localAgent;
+	}
+
 	const res = await request(url, options);
 	return {
 		body: res.body,