feat: add support for Grok-4 Fast model via OpenRouter and xAI

- Added grok-4-fast to xAI models configuration with free pricing
- Added special handling in OpenRouter fetcher to override pricing for free promotion
- Added tests for the new model in both xAI and OpenRouter test suites
- Model supports 256K context window and image inputs

Fixes #8217

packages/types/src/providers/xai.ts

@@ -17,6 +17,17 @@ export const xaiModels = {
 		cacheReadsPrice: 0.02,
 		description: "xAI's Grok Code Fast model with 256K context window",
 	},
+	"grok-4-fast": {
+		maxTokens: 8192,
+		contextWindow: 256000,
+		supportsImages: true,
+		supportsPromptCache: true,
+		inputPrice: 0.0, // Currently free on OpenRouter
+		outputPrice: 0.0, // Currently free on OpenRouter
+		cacheWritesPrice: 0.0,
+		cacheReadsPrice: 0.0,
+		description: "xAI's Grok-4 Fast model with 256K context window - currently free on OpenRouter",
+	},
 	"grok-4": {
 		maxTokens: 8192,
 		contextWindow: 256000,

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

@@ -69,6 +69,18 @@ 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])
+		// Verify it's currently free
+		expect(model.info.inputPrice).toBe(0.0)
+		expect(model.info.outputPrice).toBe(0.0)
+	})
+
 	it("should include reasoning_effort parameter for mini models", async () => {
 		const miniModelHandler = new XAIHandler({
 			apiModelId: "grok-3-mini",

src/api/providers/fetchers/__tests__/openrouter.spec.ts

@@ -382,5 +382,34 @@ describe("OpenRouter API", () => {
 			expect(textResult.maxTokens).toBe(64000)
 			expect(imageResult.maxTokens).toBe(64000)
 		})
+
+		it("sets xai/grok-4-fast model to free pricing", () => {
+			const mockModel = {
+				name: "Grok 4 Fast",
+				description: "xAI's Grok-4 Fast model",
+				context_length: 256000,
+				max_completion_tokens: 8192,
+				pricing: {
+					prompt: "0.003",
+					completion: "0.015",
+				},
+			}
+
+			const result = parseOpenRouterModel({
+				id: "xai/grok-4-fast",
+				model: mockModel,
+				inputModality: ["text"],
+				outputModality: ["text"],
+				maxTokens: 8192,
+			})
+
+			// Should override pricing to free
+			expect(result.inputPrice).toBe(0.0)
+			expect(result.outputPrice).toBe(0.0)
+			expect(result.cacheWritesPrice).toBe(0.0)
+			expect(result.cacheReadsPrice).toBe(0.0)
+			expect(result.contextWindow).toBe(256000)
+			expect(result.maxTokens).toBe(8192)
+		})
 	})
 })

src/api/providers/fetchers/openrouter.ts

@@ -263,5 +263,14 @@ export const parseOpenRouterModel = ({
 		modelInfo.maxTokens = 32768
 	}
 
+	// Set xai/grok-4-fast model configuration (currently free on OpenRouter)
+	if (id === "xai/grok-4-fast") {
+		// Override pricing to reflect current free promotion
+		modelInfo.inputPrice = 0.0
+		modelInfo.outputPrice = 0.0
+		modelInfo.cacheWritesPrice = 0.0
+		modelInfo.cacheReadsPrice = 0.0
+	}
+
 	return modelInfo
 }