diff --git a/scripts/test-layout/layout.json b/scripts/test-layout/layout.json
index 53a8e65898..5aa8568c24 100644
--- a/scripts/test-layout/layout.json
+++ b/scripts/test-layout/layout.json
@@ -1298,6 +1298,7 @@
     "ws-upstream-reuse.test.ts": "responses",
     "ws-upstream.test.ts": "responses",
     "xai-client.test.ts": "images",
+    "xai-endpoint-validation.test.ts": "providers/xai",
     "xai-oauth-retry.test.ts": "providers/xai",
     "xai-refresh-lock.test.ts": "providers/xai",
     "xai-tool-schema.test.ts": "providers/xai",
diff --git a/src/oauth/xai.ts b/src/oauth/xai.ts
index f876b18b38..600e4a24e4 100644
--- a/src/oauth/xai.ts
+++ b/src/oauth/xai.ts
@@ -44,11 +44,19 @@ function requestSignal(signal: AbortSignal | undefined): AbortSignal {
   return signal ? AbortSignal.any([signal, timeoutSignal]) : timeoutSignal;
 }
 
-function validateXaiEndpoint(rawUrl: string): string {
+const TRUSTED_XAI_AUTH_HOSTS = new Set(["auth.x.ai", "accounts.x.ai"]);
+
+export function validateXaiEndpoint(rawUrl: string): string {
   const parsed = new URL(rawUrl);
   const host = parsed.hostname.toLowerCase();
-  if (parsed.protocol !== "https:" || (host !== "x.ai" && !host.endsWith(".x.ai"))) {
-    throw new Error(`xAI OAuth discovery returned an unexpected endpoint: ${rawUrl}`);
+  if (
+    parsed.protocol !== "https:" ||
+    parsed.username ||
+    parsed.password ||
+    !TRUSTED_XAI_AUTH_HOSTS.has(host)
+  ) {
+    const sanitized = `${parsed.protocol}//${parsed.host}${parsed.pathname}${parsed.search}${parsed.hash}`;
+    throw new Error(`xAI OAuth discovery returned an unexpected endpoint: ${sanitized}`);
   }
   return parsed.toString();
 }
diff --git a/tests/fixtures/test-layout-expected.json b/tests/fixtures/test-layout-expected.json
index da6be012bc..5bc3e80ebb 100644
--- a/tests/fixtures/test-layout-expected.json
+++ b/tests/fixtures/test-layout-expected.json
@@ -1133,6 +1133,7 @@
   "ws-upstream-reuse.test.ts": "responses",
   "ws-upstream.test.ts": "responses",
   "xai-client.test.ts": "images",
+  "xai-endpoint-validation.test.ts": "providers/xai",
   "xai-oauth-retry.test.ts": "providers/xai",
   "xai-refresh-lock.test.ts": "providers/xai",
   "xai-tool-schema.test.ts": "providers/xai",
diff --git a/tests/providers/xai/xai-endpoint-validation.test.ts b/tests/providers/xai/xai-endpoint-validation.test.ts
new file mode 100644
index 0000000000..b48458ef51
--- /dev/null
+++ b/tests/providers/xai/xai-endpoint-validation.test.ts
@@ -0,0 +1,78 @@
+import { afterEach, describe, expect, test } from "bun:test";
+import {
+  discoverXaiOAuthEndpoints,
+  validateXaiEndpoint,
+} from "../../../src/oauth/xai";
+
+const originalFetch = globalThis.fetch;
+afterEach(() => {
+  globalThis.fetch = originalFetch;
+});
+
+describe("xAI endpoint validation (#4048)", () => {
+  test("allows trusted auth.x.ai and accounts.x.ai endpoints", () => {
+    expect(validateXaiEndpoint("https://auth.x.ai/oauth2/token")).toBe(
+      "https://auth.x.ai/oauth2/token"
+    );
+    expect(validateXaiEndpoint("https://accounts.x.ai/oauth2/token")).toBe(
+      "https://accounts.x.ai/oauth2/token"
+    );
+    expect(validateXaiEndpoint("https://AUTH.X.AI/token")).toBe(
+      "https://auth.x.ai/token"
+    );
+  });
+
+  test("rejects arbitrary x.ai subdomains and root domain", () => {
+    expect(() => validateXaiEndpoint("https://evil.x.ai/token")).toThrow(
+      /unexpected endpoint/
+    );
+    expect(() => validateXaiEndpoint("https://anything.x.ai/token")).toThrow(
+      /unexpected endpoint/
+    );
+    expect(() => validateXaiEndpoint("https://x.ai/token")).toThrow(
+      /unexpected endpoint/
+    );
+    expect(() => validateXaiEndpoint("https://api.x.ai/token")).toThrow(
+      /unexpected endpoint/
+    );
+  });
+
+  test("rejects endpoints with embedded userinfo", () => {
+    expect(() =>
+      validateXaiEndpoint(["https://user:pass", "@", "auth.x.ai/token"].join(""))
+    ).toThrow(/unexpected endpoint/);
+    expect(() =>
+      validateXaiEndpoint(["https://user", "@", "auth.x.ai/token"].join(""))
+    ).toThrow(/unexpected endpoint/);
+    expect(() =>
+      validateXaiEndpoint(["https://:pass", "@", "auth.x.ai/token"].join(""))
+    ).toThrow(/unexpected endpoint/);
+  });
+
+  test("rejects non-https schemes and non-x.ai domains", () => {
+    expect(() => validateXaiEndpoint("http://auth.x.ai/token")).toThrow(
+      /unexpected endpoint/
+    );
+    expect(() => validateXaiEndpoint("https://attacker.com/token")).toThrow(
+      /unexpected endpoint/
+    );
+    expect(() =>
+      validateXaiEndpoint("https://auth.x.ai.attacker.com/token")
+    ).toThrow(/unexpected endpoint/);
+  });
+
+  test("discoverXaiOAuthEndpoints rejects untrusted discovery endpoints", async () => {
+    globalThis.fetch = (async () =>
+      new Response(
+        JSON.stringify({
+          authorization_endpoint: "https://auth.x.ai/oauth2/auth",
+          token_endpoint: "https://evil.x.ai/oauth2/token",
+        })
+      )) as typeof fetch;
+
+    await expect(discoverXaiOAuthEndpoints()).rejects.toThrow(
+      /unexpected endpoint/
+    );
+  });
+});
+