diff --git a/src/selfhost/setup-wizard.ts b/src/selfhost/setup-wizard.ts
index 2ee0c6ad03..4e67961b89 100644
--- a/src/selfhost/setup-wizard.ts
+++ b/src/selfhost/setup-wizard.ts
@@ -5,6 +5,26 @@
 // disabled once an App is configured (server.ts gates on GITHUB_APP_ID), so this can't rebind a live install.
 import { createHmac, timingSafeEqual } from "node:crypto";
 
+export const SETUP_TOKEN_FORM_MAX_BYTES = 4096;
+
+export function setupTokenFormRejection(headers: Headers): Response | undefined {
+  const contentLength = headers.get("content-length");
+  if (!contentLength) return new Response("setup token form requires Content-Length", { status: 411 });
+  const byteLength = Number(contentLength);
+  if (!Number.isSafeInteger(byteLength) || byteLength < 0) {
+    return new Response("invalid setup token form length", { status: 400 });
+  }
+  if (byteLength > SETUP_TOKEN_FORM_MAX_BYTES) {
+    return new Response("setup token form is too large", { status: 413 });
+  }
+
+  const mediaType = headers.get("content-type")?.split(";", 1)[0]?.trim().toLowerCase();
+  if (mediaType !== "application/x-www-form-urlencoded" && mediaType !== "multipart/form-data") {
+    return new Response("unsupported setup token form content type", { status: 415 });
+  }
+  return undefined;
+}
+
 export interface AppCredentials {
   id: number;
   slug: string;
diff --git a/src/server.ts b/src/server.ts
index fdfcb0faae..4b6bf2b19f 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -22,6 +22,7 @@ import {
   renderSetupPage,
   renderTokenEntryPage,
   setupAuthCookieValue,
+  setupTokenFormRejection,
   timingSafeStrEqual,
 } from "./selfhost/setup-wizard";
 import { isOrbBrokerMode, registerOrbRelayTarget } from "./orb/broker-client";
@@ -290,6 +291,8 @@ async function main(): Promise<void> {
               request.headers.get("authorization")?.replace(/^Bearer\s+/i, "") ??
               "";
             if (!suppliedToken && request.method === "POST") {
+              const rejection = setupTokenFormRejection(request.headers);
+              if (rejection) return rejection;
               const form = await request.formData().catch(() => null);
               const field = form?.get("token");
               suppliedToken = typeof field === "string" ? field : "";
diff --git a/test/unit/selfhost-setup-wizard.test.ts b/test/unit/selfhost-setup-wizard.test.ts
index 4b627ecc04..3e33470de8 100644
--- a/test/unit/selfhost-setup-wizard.test.ts
+++ b/test/unit/selfhost-setup-wizard.test.ts
@@ -8,7 +8,9 @@ import {
   renderBrokeredSetupPage,
   renderSetupPage,
   renderTokenEntryPage,
+  SETUP_TOKEN_FORM_MAX_BYTES,
   setupAuthCookieValue,
+  setupTokenFormRejection,
   timingSafeStrEqual,
 } from "../../src/selfhost/setup-wizard";
 
@@ -91,6 +93,30 @@ describe("setup-wizard (#981 GitHub App Manifest)", () => {
     expect(timingSafeStrEqual("", "")).toBe(true);
   });
 
+  it("rejects unsafe setup token form uploads before parsing the body", async () => {
+    const oversized = setupTokenFormRejection(
+      new Headers({ "content-length": String(SETUP_TOKEN_FORM_MAX_BYTES + 1), "content-type": "application/x-www-form-urlencoded" }),
+    );
+    expect(oversized?.status).toBe(413);
+    await expect(oversized?.text()).resolves.toContain("too large");
+
+    expect(setupTokenFormRejection(new Headers({ "content-type": "application/x-www-form-urlencoded" }))?.status).toBe(411);
+    expect(
+      setupTokenFormRejection(new Headers({ "content-length": "nope", "content-type": "application/x-www-form-urlencoded" }))?.status,
+    ).toBe(400);
+    expect(
+      setupTokenFormRejection(new Headers({ "content-length": "-1", "content-type": "application/x-www-form-urlencoded" }))?.status,
+    ).toBe(400);
+    expect(setupTokenFormRejection(new Headers({ "content-length": "5", "content-type": "text/plain" }))?.status).toBe(415);
+
+    expect(
+      setupTokenFormRejection(new Headers({ "content-length": "12", "content-type": "application/x-www-form-urlencoded; charset=UTF-8" })),
+    ).toBeUndefined();
+    expect(
+      setupTokenFormRejection(new Headers({ "content-length": "12", "content-type": "multipart/form-data; boundary=x" })),
+    ).toBeUndefined();
+  });
+
   it("renderTokenEntryPage renders a POST form (token in the body, not the URL) + an error variant", () => {
     const page = renderTokenEntryPage();
     expect(page).toContain(`<form action="/setup" method="post">`);