feat: add grok-4-fast model to xAI provider

- Added grok-4-fast model configuration to xaiModels in packages/types/src/providers/xai.ts
- Added test case to verify grok-4-fast model selection works correctly
- Model includes 2M context window and 30K max tokens support

Fixes #8474

packages/types/src/providers/xai.ts

@@ -28,6 +28,15 @@ export const xaiModels = {
 		cacheReadsPrice: 0.75,
 		description: "xAI's Grok-4 model with 256K context window",
 	},
+	"grok-4-fast": {
+		maxTokens: 30_000,
+		contextWindow: 2_000_000,
+		supportsImages: false,
+		supportsPromptCache: false,
+		inputPrice: 1.0,
+		outputPrice: 5.0,
+		description: "xAI's Grok-4 Fast model with 2M context window and SOTA cost-efficiency",
+	},
 	"grok-3": {
 		maxTokens: 8192,
 		contextWindow: 131072,

src/api/providers/__tests__/xai.spec.ts

@@ -69,6 +69,17 @@ describe("XAIHandler", () => {
 		expect(model.info).toEqual(xaiModels[testModelId])
 	})
 
+	test("should return grok-4-fast model when specified", () => {
+		const testModelId = "grok-4-fast"
+		const handlerWithModel = new XAIHandler({ apiModelId: testModelId })
+		const model = handlerWithModel.getModel()
+
+		expect(model.id).toBe(testModelId)
+		expect(model.info).toEqual(xaiModels[testModelId])
+		expect(model.info.contextWindow).toBe(2_000_000)
+		expect(model.info.maxTokens).toBe(30_000)
+	})
+
 	it("should include reasoning_effort parameter for mini models", async () => {
 		const miniModelHandler = new XAIHandler({
 			apiModelId: "grok-3-mini",