fix(xai): pass through reasoning effort (#42160)

Co-authored-by: Aiden Cline <rekram1-node@users.noreply.github.com>

packages/core/test/provider-xai-responses.test.ts

@@ -30,3 +30,56 @@ test("xAI Responses sends promptCacheKey as prompt_cache_key", async () => {
 
   expect(body?.prompt_cache_key).toBe("session-123")
 })
+
+test("xAI Responses passes through xhigh reasoning effort", async () => {
+  let body: Record<string, unknown> | undefined
+  const mockFetch = Object.assign(
+    async (_input: Parameters<typeof fetch>[0], init?: RequestInit) => {
+      body = JSON.parse(String(init?.body))
+      return Response.json({
+        id: "response-1",
+        created_at: 0,
+        model: "grok-4",
+        object: "response",
+        output: [],
+        usage: { input_tokens: 1, output_tokens: 0 },
+        status: "completed",
+      })
+    },
+    { preconnect: fetch.preconnect },
+  )
+  const model = createXai({ apiKey: "test", fetch: mockFetch }).responses("grok-4")
+
+  await model.doGenerate({
+    prompt: [{ role: "user", content: [{ type: "text", text: "Hello" }] }],
+    providerOptions: { xai: { reasoningEffort: "xhigh" } },
+  })
+
+  expect(body?.reasoning).toEqual({ effort: "xhigh" })
+})
+
+test("xAI Chat passes through xhigh reasoning effort", async () => {
+  let body: Record<string, unknown> | undefined
+  const mockFetch = Object.assign(
+    async (_input: Parameters<typeof fetch>[0], init?: RequestInit) => {
+      body = JSON.parse(String(init?.body))
+      return Response.json({
+        id: "chat-1",
+        created: 0,
+        model: "grok-4",
+        object: "chat.completion",
+        choices: [{ index: 0, message: { role: "assistant", content: "Hello" }, finish_reason: "stop" }],
+        usage: { prompt_tokens: 1, completion_tokens: 1, total_tokens: 2 },
+      })
+    },
+    { preconnect: fetch.preconnect },
+  )
+  const model = createXai({ apiKey: "test", fetch: mockFetch }).chat("grok-4")
+
+  await model.doGenerate({
+    prompt: [{ role: "user", content: [{ type: "text", text: "Hello" }] }],
+    providerOptions: { xai: { reasoningEffort: "xhigh" } },
+  })
+
+  expect(body?.reasoning_effort).toBe("xhigh")
+})