diff --git a/packages/ai/README.md b/packages/ai/README.md
index 16b3be096..e010fdf25 100644
--- a/packages/ai/README.md
+++ b/packages/ai/README.md
@@ -794,7 +794,7 @@ Many models support thinking/reasoning capabilities where they can show their in
 const model = models.getModel('anthropic', 'claude-sonnet-4-5')!;
 // or models.getModel('openai', 'gpt-5-mini');
 // or models.getModel('google', 'gemini-2.5-flash');
-// or models.getModel('xai', 'grok-4.5');
+// or models.getModel('xai', 'grok-4.6');
 
 // Check if model supports reasoning
 if (model.reasoning) {
diff --git a/packages/ai/scripts/generate-models.ts b/packages/ai/scripts/generate-models.ts
index e36d5551b..1f3a49bcb 100644
--- a/packages/ai/scripts/generate-models.ts
+++ b/packages/ai/scripts/generate-models.ts
@@ -410,7 +410,6 @@ const OPENAI_RESPONSES_NONE_REASONING_MODELS = new Set([
 	"gpt-5.6-terra",
 	"gpt-5.6-luna",
 ]);
-const XAI_RESPONSES_MODEL_ID = "grok-4.5";
 const XAI_BUILTIN_EXCLUDED_MODEL_IDS = new Set([
 	"grok-3",
 	"grok-3-fast",
@@ -418,10 +417,6 @@ const XAI_BUILTIN_EXCLUDED_MODEL_IDS = new Set([
 	"grok-4.20-0309-reasoning",
 	"grok-code-fast-1",
 ]);
-const XAI_RESPONSES_EFFORT_LEVEL_MAP = {
-	off: null,
-	minimal: null,
-} as const;
 const XAI_RESPONSES_COMPAT: OpenAIResponsesCompat = {
 	supportsLongCacheRetention: false,
 };
@@ -823,8 +818,10 @@ function applyThinkingLevelMetadata(model: Model<any>): void {
 	) {
 		mergeThinkingLevelMap(model, { off: "none" });
 	}
-	if (model.provider === "xai" && model.api === "openai-responses" && model.id === XAI_RESPONSES_MODEL_ID) {
-		mergeThinkingLevelMap(model, XAI_RESPONSES_EFFORT_LEVEL_MAP);
+	// xAI models without verified effort options (e.g. grok-build-0.1) must not
+	// send the undocumented "none"/"minimal" efforts.
+	if (model.provider === "xai" && model.api === "openai-responses" && model.thinkingLevelMap === undefined) {
+		mergeThinkingLevelMap(model, { off: null, minimal: null });
 	}
 	if (supportsOpenAiXhigh(model.id)) {
 		mergeThinkingLevelMap(model, { xhigh: "xhigh" });
@@ -1632,15 +1629,14 @@ async function loadModelsDevData(): Promise<Model<any>[]> {
 			for (const [modelId, model] of Object.entries(data.xai.models)) {
 				const m = model as ModelsDevModel;
 				if (m.tool_call !== true) continue;
-				const useResponsesApi = modelId === XAI_RESPONSES_MODEL_ID;
 
 				models.push({
 					id: modelId,
 					name: m.name || modelId,
-					api: useResponsesApi ? "openai-responses" : "openai-completions",
+					api: "openai-responses",
 					provider: "xai",
 					baseUrl: "https://api.x.ai/v1",
-					...(useResponsesApi ? { compat: { ...XAI_RESPONSES_COMPAT } } : {}),
+					compat: { ...XAI_RESPONSES_COMPAT },
 					reasoning: m.reasoning === true,
 					input: m.modalities?.input?.includes("image") ? ["text", "image"] : ["text"],
 					cost: {
@@ -1946,10 +1942,10 @@ async function loadModelsDevData(): Promise<Model<any>[]> {
 
 				// Claude 4.x and 5.x models route to Anthropic Messages API
 				const isCopilotClaude = /^claude-(haiku|sonnet|opus)-[45]([.\-]|$)/.test(modelId);
-				// Grok 4.5, gpt-5, oswe, and MAI-Code models are only served through
+				// Grok, gpt-5, oswe, and MAI-Code models are only served through
 				// the Copilot /responses endpoint.
 				const needsResponsesApi =
-					modelId === "grok-4.5" ||
+					modelId.startsWith("grok-") ||
 					modelId.startsWith("gpt-5") ||
 					modelId.startsWith("oswe") ||
 					modelId.startsWith("mai-");
diff --git a/packages/ai/src/api/openai-completions.ts b/packages/ai/src/api/openai-completions.ts
index c62ef9f87..268e4f4f8 100644
--- a/packages/ai/src/api/openai-completions.ts
+++ b/packages/ai/src/api/openai-completions.ts
@@ -39,6 +39,7 @@ import { AssistantMessageEventStream } from "../utils/event-stream.ts";
 import { shortHash } from "../utils/hash.ts";
 import { headersToRecord } from "../utils/headers.ts";
 import { parseStreamingJson } from "../utils/json-parse.ts";
+import { forcePiUserAgent } from "../utils/pi-user-agent.ts";
 import { getProviderEnvValue } from "../utils/provider-env.ts";
 import { retryProviderRequest } from "../utils/provider-retry.ts";
 import { sanitizeSurrogates } from "../utils/sanitize-unicode.ts";
@@ -670,6 +671,10 @@ function createClient(
 		Object.assign(headers, optionsHeaders);
 	}
 
+	if (model.provider === "xai") {
+		forcePiUserAgent(headers);
+	}
+
 	return new OpenAI({
 		apiKey,
 		baseURL: model.baseUrl,
diff --git a/packages/ai/src/api/openai-responses.ts b/packages/ai/src/api/openai-responses.ts
index 578398e8e..58d179c7c 100644
--- a/packages/ai/src/api/openai-responses.ts
+++ b/packages/ai/src/api/openai-responses.ts
@@ -19,6 +19,7 @@ import { splitDeferredTools } from "../utils/deferred-tools.ts";
 import { formatProviderError, normalizeProviderError } from "../utils/error-body.ts";
 import { AssistantMessageEventStream } from "../utils/event-stream.ts";
 import { headersToRecord } from "../utils/headers.ts";
+import { forcePiUserAgent } from "../utils/pi-user-agent.ts";
 import { getProviderEnvValue } from "../utils/provider-env.ts";
 import { retryProviderRequest } from "../utils/provider-retry.ts";
 import { createGrammarToolInputProperties } from "./constrained-sampling.ts";
@@ -246,6 +247,10 @@ function createClient(
 		Object.assign(headers, optionsHeaders);
 	}
 
+	if (model.provider === "xai") {
+		forcePiUserAgent(headers);
+	}
+
 	return new OpenAI({
 		apiKey,
 		baseURL: model.baseUrl,
diff --git a/packages/ai/src/providers/xai.ts b/packages/ai/src/providers/xai.ts
index c9fe6c349..8e2b179c2 100644
--- a/packages/ai/src/providers/xai.ts
+++ b/packages/ai/src/providers/xai.ts
@@ -1,11 +1,10 @@
-import { openAICompletionsApi } from "../api/openai-completions.lazy.ts";
 import { openAIResponsesApi } from "../api/openai-responses.lazy.ts";
 import { envApiKeyAuth, lazyOAuth } from "../auth/helpers.ts";
 import { loadXaiOAuth } from "../auth/oauth/load.ts";
 import { createProvider, type Provider } from "../models.ts";
 import { XAI_MODELS } from "./xai.models.ts";
 
-export function xaiProvider(): Provider<"openai-completions" | "openai-responses"> {
+export function xaiProvider(): Provider<"openai-responses"> {
 	return createProvider({
 		id: "xai",
 		name: "xAI",
@@ -20,9 +19,6 @@ export function xaiProvider(): Provider<"openai-completions" | "openai-responses
 			}),
 		},
 		models: Object.values(XAI_MODELS),
-		api: {
-			"openai-completions": openAICompletionsApi(),
-			"openai-responses": openAIResponsesApi(),
-		},
+		api: openAIResponsesApi(),
 	});
 }
diff --git a/packages/ai/src/utils/pi-user-agent.ts b/packages/ai/src/utils/pi-user-agent.ts
index 93b23dd35..c36d5ffbc 100644
--- a/packages/ai/src/utils/pi-user-agent.ts
+++ b/packages/ai/src/utils/pi-user-agent.ts
@@ -1,4 +1,5 @@
 import type * as NodeOs from "node:os";
+import type { ProviderHeaders } from "../types.ts";
 
 type ProcessWithOsBuiltinModule = typeof process & {
 	getBuiltinModule?: (id: "node:os") => typeof NodeOs;
@@ -17,3 +18,10 @@ const nodeOs = loadNodeOs();
 export function getPiUserAgent(): string {
 	return nodeOs ? `pi (${nodeOs.platform()} ${nodeOs.release()}; ${nodeOs.arch()})` : "pi (browser)";
 }
+
+export function forcePiUserAgent(headers: ProviderHeaders): void {
+	for (const name of Object.keys(headers)) {
+		if (name.toLowerCase() === "user-agent") delete headers[name];
+	}
+	headers["User-Agent"] = getPiUserAgent();
+}
diff --git a/packages/ai/test/model-catalog-types.test.ts b/packages/ai/test/model-catalog-types.test.ts
index 0facda3db..0918228fe 100644
--- a/packages/ai/test/model-catalog-types.test.ts
+++ b/packages/ai/test/model-catalog-types.test.ts
@@ -6,7 +6,9 @@ it("derives model API, ID, and provider literals from grouped model data", () =>
 	expectTypeOf(XAI_MODELS["grok-4.5"].api).toEqualTypeOf<"openai-responses">();
 	expectTypeOf(XAI_MODELS["grok-4.5"].id).toEqualTypeOf<"grok-4.5">();
 	expectTypeOf(XAI_MODELS["grok-4.5"].provider).toEqualTypeOf<"xai">();
-	expectTypeOf(XAI_MODELS["grok-4.3"].api).toEqualTypeOf<"openai-completions">();
+	expectTypeOf(XAI_MODELS["grok-4.6"].api).toEqualTypeOf<"openai-responses">();
+	expectTypeOf(XAI_MODELS["grok-4.6"].id).toEqualTypeOf<"grok-4.6">();
+	expectTypeOf(XAI_MODELS["grok-4.3"].api).toEqualTypeOf<"openai-responses">();
 });
 
 it("routes GitHub Copilot Grok 4.5 through the Responses API", () => {
diff --git a/packages/ai/test/stream.test.ts b/packages/ai/test/stream.test.ts
index d3e7a0211..bf68823d2 100644
--- a/packages/ai/test/stream.test.ts
+++ b/packages/ai/test/stream.test.ts
@@ -542,7 +542,7 @@ describe("Generate E2E Tests", () => {
 		});
 	});
 
-	describe.skipIf(!process.env.XAI_API_KEY)("xAI Provider (grok-4.3 via OpenAI Completions)", () => {
+	describe.skipIf(!process.env.XAI_API_KEY)("xAI Provider (grok-4.3 via OpenAI Responses)", () => {
 		const llm = getModel("xai", "grok-4.3");
 
 		it("should complete basic text generation", { retry: 3 }, async () => {
diff --git a/packages/ai/test/supports-xhigh.test.ts b/packages/ai/test/supports-xhigh.test.ts
index a1fd7ff8f..8d97b7607 100644
--- a/packages/ai/test/supports-xhigh.test.ts
+++ b/packages/ai/test/supports-xhigh.test.ts
@@ -147,6 +147,12 @@ describe("getSupportedThinkingLevels", () => {
 		expect(getSupportedThinkingLevels(model!)).toContain("max");
 	});
 
+	it("includes xhigh but not off or max for xAI Grok 4.6", () => {
+		const model = getModel("xai", "grok-4.6");
+		expect(model).toBeDefined();
+		expect(getSupportedThinkingLevels(model!)).toEqual(["low", "medium", "high", "xhigh"]);
+	});
+
 	it("includes xhigh and max but not off for Bedrock Claude Fable 5", () => {
 		const model = getModel("amazon-bedrock", "global.anthropic.claude-fable-5");
 		expect(model).toBeDefined();
diff --git a/packages/ai/test/xai-responses.test.ts b/packages/ai/test/xai-responses.test.ts
index 92e464203..00e890815 100644
--- a/packages/ai/test/xai-responses.test.ts
+++ b/packages/ai/test/xai-responses.test.ts
@@ -1,10 +1,15 @@
+import { arch, platform, release } from "node:os";
 import { afterEach, describe, expect, it, vi } from "vitest";
+import { stream as streamOpenAICompletions } from "../src/api/openai-completions.ts";
 import type { OpenAIResponsesOptions } from "../src/api/openai-responses.ts";
+import { stream as streamOpenAIResponses } from "../src/api/openai-responses.ts";
 import { getSupportedThinkingLevels } from "../src/models.ts";
 import { XAI_MODELS } from "../src/providers/xai.models.ts";
 import { xaiProvider } from "../src/providers/xai.ts";
 import type { Context, Model } from "../src/types.ts";
 
+const PI_USER_AGENT = `pi (${platform()} ${release()}; ${arch()})`;
+
 type CapturedRequest = {
 	url: string;
 	headers: Headers;
@@ -72,10 +77,14 @@ describe("xAI Responses provider", () => {
 		}
 	});
 
-	it("uses Responses with low/medium/high efforts only for Grok 4.5", () => {
-		expect(XAI_MODELS["grok-4.5"].api).toBe("openai-responses");
+	it("routes every built-in xAI model through Responses", () => {
+		for (const model of Object.values(XAI_MODELS)) {
+			expect(model.api, model.id).toBe("openai-responses");
+		}
 		expect(getSupportedThinkingLevels(XAI_MODELS["grok-4.5"])).toEqual(["low", "medium", "high"]);
-		expect(XAI_MODELS["grok-4.3"].api).toBe("openai-completions");
+		expect(getSupportedThinkingLevels(XAI_MODELS["grok-4.6"])).toEqual(["low", "medium", "high", "xhigh"]);
+		expect(getSupportedThinkingLevels(XAI_MODELS["grok-4.3"])).toEqual(["off", "low", "medium", "high"]);
+		expect(getSupportedThinkingLevels(XAI_MODELS["grok-build-0.1"])).toEqual(["low", "medium", "high"]);
 	});
 
 	it("uses /responses with bearer auth and xAI-compatible request fields", async () => {
@@ -95,6 +104,7 @@ describe("xAI Responses provider", () => {
 
 		expect(captured.url).toBe("https://api.x.ai/v1/responses");
 		expect(captured.headers.get("authorization")).toBe("Bearer xai-test-token");
+		expect(captured.headers.get("user-agent")).toBe(PI_USER_AGENT);
 		expect(captured.headers.get("session_id")).toBe("pi-session-123");
 		expect(captured.body).toMatchObject({
 			model: "grok-4.5",
@@ -114,4 +124,132 @@ describe("xAI Responses provider", () => {
 			]),
 		);
 	});
+
+	it("requests encrypted reasoning without an effort override", async () => {
+		const captured = await captureRequest(
+			XAI_MODELS["grok-4.5"],
+			{ messages: [{ role: "user", content: "hello", timestamp: 1 }] },
+			{ apiKey: "xai-test-token" },
+		);
+
+		expect(captured.body).toMatchObject({
+			model: "grok-4.5",
+			store: false,
+			include: ["reasoning.encrypted_content"],
+		});
+		expect(captured.body).not.toHaveProperty("reasoning");
+	});
+
+	it("uses /responses for Grok 4.6 with xhigh effort and encrypted reasoning", async () => {
+		const captured = await captureRequest(
+			XAI_MODELS["grok-4.6"],
+			{
+				systemPrompt: "You are a careful coding assistant.",
+				messages: [{ role: "user", content: "hello", timestamp: 1 }],
+			},
+			{
+				apiKey: "xai-test-token",
+				reasoningEffort: "xhigh",
+			},
+		);
+
+		expect(captured.url).toBe("https://api.x.ai/v1/responses");
+		expect(captured.body).toMatchObject({
+			model: "grok-4.6",
+			store: false,
+			stream: true,
+			reasoning: { effort: "xhigh" },
+			include: ["reasoning.encrypted_content"],
+		});
+	});
+
+	it("uses /responses for Grok 4.3", async () => {
+		const captured = await captureRequest(
+			XAI_MODELS["grok-4.3"],
+			{
+				messages: [{ role: "user", content: "hello", timestamp: 1 }],
+			},
+			{
+				apiKey: "xai-test-token",
+				reasoningEffort: "low",
+			},
+		);
+
+		expect(captured.url).toBe("https://api.x.ai/v1/responses");
+		expect(captured.body).toMatchObject({
+			model: "grok-4.3",
+			store: false,
+			include: ["reasoning.encrypted_content"],
+			reasoning: { effort: "low" },
+		});
+	});
+
+	it("keeps the SDK User-Agent for non-xAI Responses requests", async () => {
+		let userAgent: string | null = null;
+		vi.spyOn(globalThis, "fetch").mockImplementation(async (input, init) => {
+			userAgent = new Request(input, init).headers.get("user-agent");
+			return completedResponse();
+		});
+
+		const openaiModel: Model<"openai-responses"> = {
+			...XAI_MODELS["grok-4.5"],
+			provider: "openai",
+			baseUrl: "https://api.openai.com/v1",
+		};
+		const result = await streamOpenAIResponses(
+			openaiModel,
+			{ messages: [{ role: "user", content: "hello", timestamp: 1 }] },
+			{ apiKey: "test-token" },
+		).result();
+
+		expect(result.stopReason, result.errorMessage).toBe("stop");
+		expect(userAgent).not.toBeNull();
+		expect(userAgent).not.toBe(PI_USER_AGENT);
+	});
+
+	it("forces pi's User-Agent on custom xAI Completions models over caller headers", async () => {
+		let userAgent: string | null = null;
+		vi.spyOn(globalThis, "fetch").mockImplementation(async (input, init) => {
+			userAgent = new Request(input, init).headers.get("user-agent");
+			const chunks = [
+				{ id: "chatcmpl-ua", choices: [{ delta: { content: "ok" }, finish_reason: null, index: 0 }] },
+				{
+					id: "chatcmpl-ua",
+					choices: [{ delta: {}, finish_reason: "stop", index: 0 }],
+					usage: {
+						prompt_tokens: 1,
+						completion_tokens: 1,
+						prompt_tokens_details: { cached_tokens: 0 },
+						completion_tokens_details: { reasoning_tokens: 0 },
+					},
+				},
+			];
+			const body = `${chunks.map((chunk) => `data: ${JSON.stringify(chunk)}`).join("\n\n")}\n\ndata: [DONE]\n\n`;
+			return new Response(body, {
+				status: 200,
+				headers: { "content-type": "text/event-stream" },
+			});
+		});
+
+		const customModel: Model<"openai-completions"> = {
+			id: "grok-custom",
+			name: "Grok Custom",
+			api: "openai-completions",
+			provider: "xai",
+			baseUrl: "https://api.x.ai/v1",
+			reasoning: false,
+			input: ["text"],
+			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+			contextWindow: 128000,
+			maxTokens: 16384,
+		};
+		const result = await streamOpenAICompletions(
+			customModel,
+			{ messages: [{ role: "user", content: "hello", timestamp: 1 }] },
+			{ apiKey: "xai-test-token", headers: { "User-Agent": "custom-agent" } },
+		).result();
+
+		expect(result.stopReason, result.errorMessage).toBe("stop");
+		expect(userAgent).toBe(PI_USER_AGENT);
+	});
 });
diff --git a/packages/coding-agent/src/core/model-resolver.ts b/packages/coding-agent/src/core/model-resolver.ts
index cfc36ec16..aea6717e1 100644
--- a/packages/coding-agent/src/core/model-resolver.ts
+++ b/packages/coding-agent/src/core/model-resolver.ts
@@ -32,7 +32,7 @@ export const defaultModelPerProvider: Record<KnownProvider, string> = {
 	"github-copilot": "gpt-5.4",
 	openrouter: "moonshotai/kimi-k2.6",
 	"vercel-ai-gateway": "zai/glm-5.1",
-	xai: "grok-4.5",
+	xai: "grok-4.6",
 	groq: "openai/gpt-oss-120b",
 	cerebras: "zai-glm-4.7",
 	zai: "glm-5.3",
diff --git a/packages/coding-agent/test/model-resolver.test.ts b/packages/coding-agent/test/model-resolver.test.ts
index ddf83c569..c38b59f04 100644
--- a/packages/coding-agent/test/model-resolver.test.ts
+++ b/packages/coding-agent/test/model-resolver.test.ts
@@ -723,6 +723,10 @@ describe("default model selection", () => {
 		expect(defaultModelPerProvider["vercel-ai-gateway"]).toBe("zai/glm-5.1");
 	});
 
+	test("xai default tracks current model", () => {
+		expect(defaultModelPerProvider.xai).toBe("grok-4.6");
+	});
+
 	test("qwen token plan individual default tracks current model", () => {
 		expect(defaultModelPerProvider["qwen-token-plan-individual"]).toBe("qwen3.8-max");
 	});