diff --git a/src/api/routes.ts b/src/api/routes.ts
index feb5c6861..5815af128 100644
--- a/src/api/routes.ts
+++ b/src/api/routes.ts
@@ -128,7 +128,7 @@ import { handleOrbIngest, readOrbIngestBody } from "../orb/ingest";
 import { handleOrbWebhook } from "../orb/webhook";
 import { handleOrbOAuthCallback } from "../orb/oauth";
 import { brokerOrbToken, isOrbBrokerEnabled, issueOrbEnrollment } from "../orb/broker";
-import { registerOrbRelay } from "../orb/relay";
+import { readOrbRelayRegisterBody, registerValidatedOrbRelay, validateOrbRelayEnrollment } from "../orb/relay";
 import { computeFleetAnalytics } from "../orb/analytics";
 import { handleMcpRequest } from "../mcp/server";
 import { buildOpenApiSpec } from "../openapi/spec";
@@ -2916,10 +2916,19 @@ export function createApp() {
     const auth = c.req.header("authorization") ?? "";
     const secret = auth.startsWith("Bearer ") ? auth.slice(7).trim() : "";
     if (!secret) return c.json({ error: "missing_enrollment_secret" }, 401);
-    const body = (await c.req.json().catch(() => null)) as { relayUrl?: unknown } | null;
+    const enrollment = await validateOrbRelayEnrollment(c.env, secret);
+    if ("error" in enrollment) return c.json(enrollment, enrollment.error === "invalid_enrollment" ? 401 : 403);
+    const rawBody = await readOrbRelayRegisterBody(c.req.raw, c.req.header("content-length"));
+    if (rawBody === null) return c.json({ error: "payload_too_large" }, 413);
+    let body: { relayUrl?: unknown } | null;
+    try {
+      body = JSON.parse(rawBody) as { relayUrl?: unknown };
+    } catch {
+      body = null;
+    }
     const relayUrl = typeof body?.relayUrl === "string" ? body.relayUrl.trim() : "";
     if (!relayUrl) return c.json({ error: "missing_relay_url" }, 400);
-    const result = await registerOrbRelay(c.env, secret, relayUrl);
+    const result = await registerValidatedOrbRelay(c.env, enrollment, secret, relayUrl);
     if ("error" in result) {
       const status = result.error === "invalid_enrollment" ? 401 : result.error === "installation_not_eligible" ? 403 : result.error === "encryption_unavailable" ? 500 : 400;
       return c.json(result, status);
diff --git a/src/orb/relay.ts b/src/orb/relay.ts
index ef9fe1f2c..829f07927 100644
--- a/src/orb/relay.ts
+++ b/src/orb/relay.ts
@@ -55,11 +55,41 @@ export type RegisterResult =
   | { ok: true; installationId: number }
   | { error: "invalid_enrollment" | "installation_not_eligible" | "invalid_relay_url" | "encryption_unavailable" };
 
-/** Register (or update) the container's relay target for a valid enrollment. Validates the secret (→ the bound,
- *  registered, non-suspended install — same gate as the token broker), SSRF-validates the relay URL, then stores
- *  the URL + the enrollment secret encrypted at rest (for the forward-time HMAC). The container presents its OWN
- *  plaintext enrollment secret as the Bearer, so this is self-service + bound to that install. */
-export async function registerOrbRelay(env: Env, secret: string, relayUrl: string): Promise<RegisterResult> {
+export const MAX_ORB_RELAY_REGISTER_BODY_BYTES = 4096;
+
+function parseContentLength(header: string | null | undefined): number | null {
+  if (typeof header !== "string") return null;
+  const n = Number(header);
+  return Number.isInteger(n) && n >= 0 ? n : null;
+}
+
+/** Read the relay-registration JSON with a small hard ceiling; returns null when the sender exceeds it. */
+export async function readOrbRelayRegisterBody(request: Request, contentLengthHeader: string | null | undefined): Promise<string | null> {
+  const declared = parseContentLength(contentLengthHeader);
+  if (declared !== null && declared > MAX_ORB_RELAY_REGISTER_BODY_BYTES) return null;
+
+  const stream = request.body;
+  if (!stream) return "";
+  const reader = stream.getReader();
+  const decoder = new TextDecoder();
+  let total = 0;
+  let out = "";
+  for (;;) {
+    const { done, value } = await reader.read();
+    if (done) break;
+    total += value.byteLength;
+    if (total > MAX_ORB_RELAY_REGISTER_BODY_BYTES) {
+      await reader.cancel();
+      return null;
+    }
+    out += decoder.decode(value, { stream: true });
+  }
+  return out + decoder.decode();
+}
+
+export type RelayEnrollment = { enrollId: string; installationId: number };
+
+export async function validateOrbRelayEnrollment(env: Env, secret: string): Promise<RelayEnrollment | { error: "invalid_enrollment" | "installation_not_eligible" }> {
   const row = await env.DB
     .prepare("SELECT enroll_id, installation_id, state, revoked_at FROM orb_enrollments WHERE secret_hash = ?")
     .bind(await hashToken(secret))
@@ -70,6 +100,10 @@ export async function registerOrbRelay(env: Env, secret: string, relayUrl: strin
     .bind(row.installation_id)
     .first<{ registered: number; suspended_at: string | null; removed_at: string | null }>();
   if (!install || install.registered !== 1 || install.suspended_at !== null || install.removed_at !== null) return { error: "installation_not_eligible" };
+  return { enrollId: row.enroll_id, installationId: row.installation_id };
+}
+
+export async function registerValidatedOrbRelay(env: Env, enrollment: RelayEnrollment, secret: string, relayUrl: string): Promise<RegisterResult> {
   // SSRF guard: the Orb will POST events to this URL — it must be a public https endpoint (no loopback / private /
   // link-local host), so a registered relay URL can never coerce the Orb into hitting an internal service.
   if (!isSafeHttpUrl(relayUrl)) return { error: "invalid_relay_url" };
@@ -77,9 +111,19 @@ export async function registerOrbRelay(env: Env, secret: string, relayUrl: strin
   const enc = await encryptSecret(secret, env.TOKEN_ENCRYPTION_SECRET);
   await env.DB
     .prepare("UPDATE orb_enrollments SET relay_url = ?, relay_secret_enc = ?, relay_secret_iv = ?, relay_secret_salt = ?, relay_registered_at = CURRENT_TIMESTAMP WHERE enroll_id = ?")
-    .bind(relayUrl, enc.ciphertext, enc.iv, enc.salt, row.enroll_id)
+    .bind(relayUrl, enc.ciphertext, enc.iv, enc.salt, enrollment.enrollId)
     .run();
-  return { ok: true, installationId: row.installation_id };
+  return { ok: true, installationId: enrollment.installationId };
+}
+
+/** Register (or update) the container's relay target for a valid enrollment. Validates the secret (→ the bound,
+ *  registered, non-suspended install — same gate as the token broker), SSRF-validates the relay URL, then stores
+ *  the URL + the enrollment secret encrypted at rest (for the forward-time HMAC). The container presents its OWN
+ *  plaintext enrollment secret as the Bearer, so this is self-service + bound to that install. */
+export async function registerOrbRelay(env: Env, secret: string, relayUrl: string): Promise<RegisterResult> {
+  const enrollment = await validateOrbRelayEnrollment(env, secret);
+  if ("error" in enrollment) return enrollment;
+  return registerValidatedOrbRelay(env, enrollment, secret, relayUrl);
 }
 
 const RELAY_RETRY_MAX_ATTEMPTS = 5;
diff --git a/test/integration/orb-relay.test.ts b/test/integration/orb-relay.test.ts
index 997c4ab04..e51099353 100644
--- a/test/integration/orb-relay.test.ts
+++ b/test/integration/orb-relay.test.ts
@@ -1,7 +1,7 @@
 import { describe, expect, it } from "vitest";
 import { createApp } from "../../src/api/routes";
 import { issueOrbEnrollment } from "../../src/orb/broker";
-import { forwardOrbEvent, registerOrbRelay, relaySignature, relayVerify, retryFailedRelays, storeRelayFailure } from "../../src/orb/relay";
+import { forwardOrbEvent, MAX_ORB_RELAY_REGISTER_BODY_BYTES, readOrbRelayRegisterBody, registerOrbRelay, relaySignature, relayVerify, retryFailedRelays, storeRelayFailure } from "../../src/orb/relay";
 import { createTestEnv, type TestD1Database } from "../helpers/d1";
 
 const db = (e: Env) => e.DB as unknown as TestD1Database;
@@ -79,12 +79,13 @@ describe("POST /v1/orb/relay/register", () => {
     expect(await ok.json()).toMatchObject({ ok: true, installationId: 710 });
   });
 
-  it("maps each failure to its status: 401 bad secret, 403 ineligible, 400 SSRF, 500 no-encryption", async () => {
+  it("maps each failure to its status: 401 bad secret, 403 ineligible, 400 SSRF, 413 too-large body, 500 no-encryption", async () => {
     const e = brokeredEnv();
     const sBad = "Bearer orbsec_bad";
     expect((await app.request("/v1/orb/relay/register", { method: "POST", headers: { authorization: sBad }, body: JSON.stringify({ relayUrl: "https://x.example" }) }, e)).status).toBe(401);
     const s1 = await enroll(e, 711);
     expect((await app.request("/v1/orb/relay/register", { method: "POST", headers: { authorization: `Bearer ${s1}` }, body: JSON.stringify({ relayUrl: "http://127.0.0.1" }) }, e)).status).toBe(400);
+    expect((await app.request("/v1/orb/relay/register", { method: "POST", headers: { authorization: `Bearer ${s1}` }, body: JSON.stringify({ relayUrl: `${"https://x.example/"}${"a".repeat(4096)}` }) }, e)).status).toBe(413);
     const s2 = await enroll(e, 712);
     await db(e).prepare("UPDATE orb_github_installations SET registered=0 WHERE installation_id=712").run();
     expect((await app.request("/v1/orb/relay/register", { method: "POST", headers: { authorization: `Bearer ${s2}` }, body: JSON.stringify({ relayUrl: "https://x.example" }) }, e)).status).toBe(403);
@@ -92,6 +93,49 @@ describe("POST /v1/orb/relay/register", () => {
     const s3 = await enroll(noEnc, 713);
     expect((await app.request("/v1/orb/relay/register", { method: "POST", headers: { authorization: `Bearer ${s3}` }, body: JSON.stringify({ relayUrl: "https://x.example/relay" }) }, noEnc)).status).toBe(500);
   });
+
+  it("rejects an invalid enrollment before reading the registration body", async () => {
+    const e = brokeredEnv();
+    let bodyAccesses = 0;
+    const req = new Request("http://localhost/v1/orb/relay/register", { method: "POST", headers: { authorization: "Bearer orbsec_bad" } });
+    Object.defineProperty(req, "body", {
+      get() {
+        bodyAccesses += 1;
+        throw new Error("body should not be read before enrollment validation");
+      },
+    });
+    const res = await app.fetch(req, e);
+    expect(res.status).toBe(401);
+    expect(bodyAccesses).toBe(0);
+  });
+});
+
+describe("readOrbRelayRegisterBody", () => {
+  it("returns an empty string when the request has no body stream", async () => {
+    // A request without a body (e.g. a bodyless POST) has request.body === null — the empty-body path.
+    const req = new Request("http://localhost/v1/orb/relay/register", { method: "POST" });
+    expect(req.body).toBeNull();
+    expect(await readOrbRelayRegisterBody(req, null)).toBe(""); // null content-length → declared null → empty stream
+    expect(await readOrbRelayRegisterBody(req, undefined)).toBe(""); // undefined header → typeof !== "string" arm
+  });
+
+  it("rejects an oversized DECLARED content-length up front (before touching the stream)", async () => {
+    // Past the ceiling: parseContentLength returns a valid integer that exceeds MAX → short-circuit to null.
+    const req = new Request("http://localhost/v1/orb/relay/register", { method: "POST", body: "{}" });
+    expect(await readOrbRelayRegisterBody(req, String(MAX_ORB_RELAY_REGISTER_BODY_BYTES + 1))).toBeNull();
+  });
+
+  it("ignores a malformed or negative content-length and reads the actual body", async () => {
+    // Number("abc")=NaN and "-1"<0 → parseContentLength returns null → the declared-too-large guard is skipped.
+    expect(await readOrbRelayRegisterBody(new Request("http://localhost/r", { method: "POST", body: "{}" }), "abc")).toBe("{}"); // non-integer
+    expect(await readOrbRelayRegisterBody(new Request("http://localhost/r", { method: "POST", body: "{}" }), "-1")).toBe("{}"); // negative
+  });
+
+  it("returns null when the STREAMED body exceeds the ceiling regardless of the declared length", async () => {
+    // No content-length declared, but the stream itself runs past MAX → reader is cancelled, null returned.
+    const req = new Request("http://localhost/r", { method: "POST", body: "x".repeat(MAX_ORB_RELAY_REGISTER_BODY_BYTES + 1) });
+    expect(await readOrbRelayRegisterBody(req, null)).toBeNull();
+  });
 });
 
 describe("relaySignature", () => {