fix: add Gemma 4 models to geminiModels map and stop silent model ID fallback

- Add gemma-4-31b-it, gemma-4-12b-it, gemma-4-6b-it, gemma-4-4b-it,
  and gemma-4-1b-it to the geminiModels map with correct context windows
  and capabilities
- Fix silent fallback in GeminiHandler.getModel() that replaced unknown
  model IDs with geminiDefaultModelId (gemini-3.1-pro-preview)
- Apply the same fix to VertexHandler.getModel()
- Now preserves user-provided model ID and only falls back on model info
- Update tests to verify unknown models are passed through and Gemma 4
  models are natively recognized

Closes #12166

packages/types/src/providers/gemini.ts

@@ -283,4 +283,46 @@ export const geminiModels = {
 		supportsReasoningBudget: true,
 		maxThinkingTokens: 24_576,
 	},
+	// Gemma 4 models
+	// https://ai.google.dev/gemma/docs/core
+	"gemma-4-31b-it": {
+		maxTokens: 8_192,
+		contextWindow: 131_072,
+		supportsImages: true,
+		supportsPromptCache: false,
+		inputPrice: 0,
+		outputPrice: 0,
+	},
+	"gemma-4-12b-it": {
+		maxTokens: 8_192,
+		contextWindow: 131_072,
+		supportsImages: true,
+		supportsPromptCache: false,
+		inputPrice: 0,
+		outputPrice: 0,
+	},
+	"gemma-4-6b-it": {
+		maxTokens: 8_192,
+		contextWindow: 32_768,
+		supportsImages: true,
+		supportsPromptCache: false,
+		inputPrice: 0,
+		outputPrice: 0,
+	},
+	"gemma-4-4b-it": {
+		maxTokens: 8_192,
+		contextWindow: 32_768,
+		supportsImages: true,
+		supportsPromptCache: false,
+		inputPrice: 0,
+		outputPrice: 0,
+	},
+	"gemma-4-1b-it": {
+		maxTokens: 8_192,
+		contextWindow: 32_768,
+		supportsImages: false,
+		supportsPromptCache: false,
+		inputPrice: 0,
+		outputPrice: 0,
+	},
 } as const satisfies Record<string, ModelInfo>

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

@@ -165,13 +165,33 @@ describe("GeminiHandler", () => {
 			expect(modelInfo.info).toBeDefined()
 		})
 
-		it("should return default model if invalid model specified", () => {
+		it("should preserve unknown model ID instead of silently falling back to default", () => {
 			const invalidHandler = new GeminiHandler({
 				apiModelId: "invalid-model",
 				geminiApiKey: "test-key",
 			})
 			const modelInfo = invalidHandler.getModel()
-			expect(modelInfo.id).toBe(geminiDefaultModelId) // Default model
+			expect(modelInfo.id).toBe("invalid-model") // Preserves user-provided ID
+			expect(modelInfo.info).toBeDefined() // Falls back to default model info
+		})
+
+		it("should use default model when no model ID is provided", () => {
+			const noModelHandler = new GeminiHandler({
+				geminiApiKey: "test-key",
+			})
+			const modelInfo = noModelHandler.getModel()
+			expect(modelInfo.id).toBe(geminiDefaultModelId)
+		})
+
+		it("should recognize Gemma 4 models natively", () => {
+			const gemmaHandler = new GeminiHandler({
+				apiModelId: "gemma-4-31b-it",
+				geminiApiKey: "test-key",
+			})
+			const modelInfo = gemmaHandler.getModel()
+			expect(modelInfo.id).toBe("gemma-4-31b-it")
+			expect(modelInfo.info.contextWindow).toBe(131_072)
+			expect(modelInfo.info.supportsImages).toBe(true)
 		})
 
 		it("should exclude apply_diff and include edit in tool preferences", () => {

src/api/providers/gemini.ts

@@ -348,8 +348,8 @@ export class GeminiHandler extends BaseProvider implements SingleCompletionHandl
 
 	override getModel() {
 		const modelId = this.options.apiModelId
-		let id = modelId && modelId in geminiModels ? (modelId as GeminiModelId) : geminiDefaultModelId
-		let info: ModelInfo = geminiModels[id]
+		const id: string = modelId ?? geminiDefaultModelId
+		let info: ModelInfo = geminiModels[id as GeminiModelId] ?? geminiModels[geminiDefaultModelId]
 
 		const params = getModelParams({
 			format: "gemini",

src/api/providers/vertex.ts

@@ -14,8 +14,8 @@ export class VertexHandler extends GeminiHandler implements SingleCompletionHand
 
 	override getModel() {
 		const modelId = this.options.apiModelId
-		let id = modelId && modelId in vertexModels ? (modelId as VertexModelId) : vertexDefaultModelId
-		let info: ModelInfo = vertexModels[id]
+		const id: string = modelId ?? vertexDefaultModelId
+		let info: ModelInfo = vertexModels[id as VertexModelId] ?? vertexModels[vertexDefaultModelId]
 		const params = getModelParams({
 			format: "gemini",
 			modelId: id,