diff --git a/changelog.d/fixes/1037-vercel-relay-sso-protection-check.md b/changelog.d/fixes/1037-vercel-relay-sso-protection-check.md
new file mode 100644
index 00000000000..e03d00dd02d
--- /dev/null
+++ b/changelog.d/fixes/1037-vercel-relay-sso-protection-check.md
@@ -0,0 +1 @@
+- **fix(api):** Vercel Relay deploy now checks the Deployment Protection (SSO) PATCH response and surfaces `ssoProtectionWarning` when Vercel rejects it, instead of silently activating a relay that later returns an undiagnosed `403 Access denied`. (thanks @ricatix)
diff --git a/src/app/api/settings/proxy/vercel-deploy/route.ts b/src/app/api/settings/proxy/vercel-deploy/route.ts
index 42f432f2c6b..6d6b859396e 100644
--- a/src/app/api/settings/proxy/vercel-deploy/route.ts
+++ b/src/app/api/settings/proxy/vercel-deploy/route.ts
@@ -98,6 +98,92 @@ export default async function handler(req) {
  */
 export const __buildRelayFunctionForTest = buildRelayFunction;
 
+/**
+ * Disable Vercel project SSO/Deployment Protection so the relay is publicly
+ * reachable. The PATCH response was previously fired-and-forgotten
+ * (`.catch(() => {})`, no `res.ok` check) — if Vercel rejects or no-ops the
+ * request (plan does not allow disabling protection, an under-scoped token,
+ * etc.), the relay still got saved and activated as a healthy proxy pool,
+ * and later requests through it failed with an undiagnosed
+ * `403 Access denied` from Vercel's own deployment protection. Callers must
+ * now check `.ok` and surface the failure instead of assuming success.
+ */
+async function disableSsoProtection(
+  vercelApiBase: string,
+  projectId: string,
+  token: string
+): Promise<{ ok: boolean; status?: number }> {
+  try {
+    const res = await fetch(`${vercelApiBase}/v9/projects/${projectId}`, {
+      method: "PATCH",
+      headers: {
+        Authorization: `Bearer ${token}`,
+        "Content-Type": "application/json",
+      },
+      body: JSON.stringify({ ssoProtection: null }),
+    });
+    return { ok: res.ok, status: res.status };
+  } catch {
+    return { ok: false };
+  }
+}
+
+/**
+ * Test-only hook exposing `disableSsoProtection` so the regression test can
+ * assert the PATCH response is checked instead of silently swallowed. Not
+ * part of the route contract.
+ */
+export const __disableSsoProtectionForTest = disableSsoProtection;
+
+/**
+ * Builds the sanitized error response for a rejected Vercel deployment
+ * request. Extracted from POST to keep the handler's cognitive complexity
+ * within the ratchet — parses the canonical `{ error: { message } } }` shape
+ * and never forwards raw upstream error text (may contain project IDs, team
+ * slugs, deployment hashes or internal Vercel error strings).
+ */
+async function buildDeployErrorResponse(deployRes: Response) {
+  let upstreamMessage = "Vercel API rejected the deployment";
+  try {
+    const parsed = (await deployRes.json().catch(() => null)) as {
+      error?: { message?: string };
+    } | null;
+    const candidate = parsed?.error?.message;
+    if (typeof candidate === "string" && candidate.trim()) {
+      upstreamMessage = candidate.trim().slice(0, 200);
+    }
+  } catch {
+    /* fall through to generic message */
+  }
+  return createErrorResponse({
+    status: deployRes.status,
+    message: `Vercel deployment failed: ${upstreamMessage}`,
+    type: "upstream_error",
+  });
+}
+
+/**
+ * Disables Vercel SSO/Deployment Protection for the deployed project and
+ * returns a caller-facing warning when it could not be disabled. Extracted
+ * from POST to keep the handler's cognitive complexity within the ratchet.
+ * See `disableSsoProtection` doc comment for the bug this guards against.
+ */
+async function resolveSsoProtectionWarning(
+  projectId: string | undefined,
+  vercelApiBase: string,
+  token: string
+): Promise<string | undefined> {
+  if (!projectId) return undefined;
+  const ssoResult = await disableSsoProtection(vercelApiBase, projectId, token);
+  if (ssoResult.ok) return undefined;
+  return (
+    "Could not disable Vercel Deployment Protection (SSO) for this project" +
+    (ssoResult.status ? ` (status ${ssoResult.status})` : "") +
+    ". Requests through this relay may fail with a 403 Access denied from " +
+    "Vercel until protection is disabled manually in the Vercel dashboard."
+  );
+}
+
 async function pollDeployment(deploymentApiUrl: string, token: string): Promise<"READY" | "ERROR"> {
   for (let i = 0; i < POLL_MAX_ATTEMPTS; i++) {
     await new Promise((r) => setTimeout(r, POLL_INTERVAL_MS));
@@ -171,27 +257,9 @@ export async function POST(request: Request) {
     });
 
     if (!deployRes.ok) {
-      // Avoid forwarding 200 bytes of raw Vercel error text — it may contain
-      // project IDs, team slugs, deployment hashes or internal Vercel error
-      // strings. Parse the canonical { error: { message } } shape and surface
-      // only the human-readable message (or a generic fallback).
-      let upstreamMessage = "Vercel API rejected the deployment";
-      try {
-        const parsed = (await deployRes.json().catch(() => null)) as {
-          error?: { message?: string };
-        } | null;
-        const candidate = parsed?.error?.message;
-        if (typeof candidate === "string" && candidate.trim()) {
-          upstreamMessage = candidate.trim().slice(0, 200);
-        }
-      } catch {
-        /* fall through to generic message */
-      }
-      return createErrorResponse({
-        status: deployRes.status,
-        message: `Vercel deployment failed: ${upstreamMessage}`,
-        type: "upstream_error",
-      });
+      // Avoid forwarding raw Vercel error text — it may contain project IDs,
+      // team slugs, deployment hashes or internal Vercel error strings.
+      return buildDeployErrorResponse(deployRes);
     }
 
     const deployment = (await deployRes.json()) as {
@@ -208,17 +276,17 @@ export async function POST(request: Request) {
       });
     }
 
-    // Disable Vercel SSO protection so the relay is publicly accessible
-    if (deployment.projectId) {
-      await fetch(`${VERCEL_API_BASE}/v9/projects/${deployment.projectId}`, {
-        method: "PATCH",
-        headers: {
-          Authorization: `Bearer ${token}`,
-          "Content-Type": "application/json",
-        },
-        body: JSON.stringify({ ssoProtection: null }),
-      }).catch(() => {});
-    }
+    // Disable Vercel SSO protection so the relay is publicly accessible.
+    // The PATCH response is checked — if Vercel rejects/no-ops it (plan
+    // doesn't allow disabling protection, under-scoped token, etc.) the
+    // relay is still deployed and saved, but the caller is warned so a
+    // later `403 Access denied` can be diagnosed as Vercel-side deployment
+    // protection rather than an upstream provider rejection.
+    const ssoProtectionWarning = await resolveSsoProtectionWarning(
+      deployment.projectId,
+      VERCEL_API_BASE,
+      token
+    );
 
     // Poll until READY
     const deploymentApiUrl = `${VERCEL_API_BASE}/v13/deployments/${deployment.id}`;
@@ -254,6 +322,7 @@ export async function POST(request: Request) {
       success: true,
       relayUrl: `https://${deployment.url}`,
       poolProxyId: poolProxy?.id,
+      ...(ssoProtectionWarning ? { ssoProtectionWarning } : {}),
     });
   } catch (error) {
     return createErrorResponseFromUnknown(error, "Vercel deploy failed");
diff --git a/tests/unit/vercel-deploy-sso-protection-check.test.ts b/tests/unit/vercel-deploy-sso-protection-check.test.ts
new file mode 100644
index 00000000000..d3fb945e965
--- /dev/null
+++ b/tests/unit/vercel-deploy-sso-protection-check.test.ts
@@ -0,0 +1,95 @@
+// Regression guard for upstream report: "Vercel Relay with Codex returns 403
+// Access denied and lacks source diagnostics".
+//
+// Root cause: the Vercel deploy route disables project SSO/Deployment
+// Protection via a PATCH request, but fired it with `.catch(() => {})` and
+// never inspected `res.ok`. If Vercel rejects or no-ops the PATCH (plan
+// doesn't allow disabling protection, stale/under-scoped token, etc.), the
+// relay is still saved and activated as a healthy proxy pool — later
+// requests routed through it fail with an undiagnosed `403 Access denied`
+// from Vercel's own deployment protection, indistinguishable from an
+// upstream-provider 403.
+//
+// Fix: check the PATCH response and surface the failure back to the caller
+// (`ssoProtectionWarning` in the JSON response) instead of silently
+// swallowing it, so the UI/API consumer can diagnose the 403 source.
+import { describe, it, beforeEach, afterEach } from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { join, dirname } from "node:path";
+import { fileURLToPath } from "node:url";
+import { __disableSsoProtectionForTest } from "../../src/app/api/settings/proxy/vercel-deploy/route";
+
+const ROOT = join(dirname(fileURLToPath(import.meta.url)), "..", "..");
+const ROUTE_PATH = join(
+  ROOT,
+  "src/app/api/settings/proxy/vercel-deploy/route.ts"
+);
+
+describe("disableSsoProtection — checks the Vercel PATCH response instead of swallowing it", () => {
+  const originalFetch = global.fetch;
+
+  afterEach(() => {
+    global.fetch = originalFetch;
+  });
+
+  it("reports failure when Vercel rejects the PATCH (e.g. plan does not allow disabling protection)", async () => {
+    global.fetch = (async () =>
+      new Response(JSON.stringify({ error: { message: "Forbidden" } }), {
+        status: 403,
+      })) as typeof fetch;
+
+    const result = await __disableSsoProtectionForTest(
+      "https://api.vercel.com",
+      "proj_123",
+      "test-token"
+    );
+
+    assert.equal(result.ok, false, "must report ok:false on a non-2xx PATCH response");
+    assert.equal(result.status, 403);
+  });
+
+  it("reports success when Vercel accepts the PATCH", async () => {
+    global.fetch = (async () => new Response(null, { status: 200 })) as typeof fetch;
+
+    const result = await __disableSsoProtectionForTest(
+      "https://api.vercel.com",
+      "proj_123",
+      "test-token"
+    );
+
+    assert.equal(result.ok, true);
+  });
+
+  it("reports failure (not a thrown exception) when the PATCH request itself fails", async () => {
+    global.fetch = (async () => {
+      throw new Error("network down");
+    }) as typeof fetch;
+
+    const result = await __disableSsoProtectionForTest(
+      "https://api.vercel.com",
+      "proj_123",
+      "test-token"
+    );
+
+    assert.equal(result.ok, false);
+  });
+});
+
+describe("vercel-deploy route — wires the SSO-protection check into the response", () => {
+  const src = readFileSync(ROUTE_PATH, "utf8");
+
+  it("no longer fires the PATCH with a silent `.catch(() => {})`", () => {
+    assert.ok(
+      !/ssoProtection:\s*null[\s\S]*?\.catch\(\s*\(\)\s*=>\s*\{\s*\}\s*\)/.test(src),
+      "the ssoProtection PATCH must not be silently swallowed with .catch(() => {})"
+    );
+  });
+
+  it("surfaces a warning in the JSON response when disabling SSO protection failed", () => {
+    assert.ok(
+      src.includes("ssoProtectionWarning"),
+      "POST handler must surface ssoProtectionWarning in the response payload when the PATCH failed"
+    );
+  });
+});