feat: allow custom model ID passthrough for Vertex AI providers

- Add gemini-3.1-flash-lite-preview to vertex models list
- Update VertexHandler.getModel() to pass through unknown model IDs
  with sensible default ModelInfo instead of falling back to default model
- Update AnthropicVertexHandler.getModel() with same custom model ID
  passthrough behavior
- Add tests for custom model ID passthrough in both handlers

Closes #12232

packages/types/src/providers/vertex.ts

@@ -35,6 +35,17 @@ export const vertexModels = {
 			},
 		],
 	},
+	"gemini-3.1-flash-lite-preview": {
+		maxTokens: 65_536,
+		contextWindow: 1_048_576,
+		supportsImages: true,
+		supportsPromptCache: true,
+
+		supportsTemperature: true,
+		defaultTemperature: 1,
+		inputPrice: 0.0,
+		outputPrice: 0.0,
+	},
 	"gemini-3.1-pro-preview-customtools": {
 		maxTokens: 65_536,
 		contextWindow: 1_048_576,

src/api/providers/__tests__/anthropic-vertex.spec.ts

@@ -869,6 +869,34 @@ describe("VertexHandler", () => {
 			expect(result.temperature).toBe(0)
 		})
 
+		it("should pass through custom/unknown model IDs with sensible defaults", () => {
+			const handler = new AnthropicVertexHandler({
+				apiModelId: "claude-5-opus@20260101",
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const modelInfo = handler.getModel()
+			expect(modelInfo.id).toBe("claude-5-opus@20260101")
+			expect(modelInfo.info).toBeDefined()
+			expect(modelInfo.info.maxTokens).toBe(8192)
+			expect(modelInfo.info.contextWindow).toBe(200_000)
+			expect(modelInfo.info.supportsImages).toBe(true)
+			expect(modelInfo.info.supportsPromptCache).toBe(false)
+		})
+
+		it("should fall back to default model when no model ID is provided", () => {
+			const handler = new AnthropicVertexHandler({
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const modelInfo = handler.getModel()
+			expect(modelInfo.id).toBeDefined()
+			expect(modelInfo.info).toBeDefined()
+			expect(modelInfo.info.maxTokens).toBeGreaterThan(0)
+		})
+
 		it("should enable 1M context for Claude Sonnet 4 when beta flag is set", () => {
 			const handler = new AnthropicVertexHandler({
 				apiModelId: VERTEX_1M_CONTEXT_MODEL_IDS[0],

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

@@ -163,5 +163,35 @@ describe("VertexHandler", () => {
 			expect(excludedCount).toBe(1)
 			expect(includedCount).toBe(1)
 		})
+
+		it("should pass through custom/unknown model IDs with sensible defaults", () => {
+			const testHandler = new VertexHandler({
+				apiModelId: "gemini-4.0-ultra-preview",
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const modelInfo = testHandler.getModel()
+			expect(modelInfo.id).toBe("gemini-4.0-ultra-preview")
+			expect(modelInfo.info).toBeDefined()
+			expect(modelInfo.info.maxTokens).toBe(8192)
+			expect(modelInfo.info.contextWindow).toBe(1_048_576)
+			expect(modelInfo.info.supportsImages).toBe(true)
+			expect(modelInfo.info.supportsPromptCache).toBe(false)
+			expect(modelInfo.info.excludedTools).toContain("apply_diff")
+			expect(modelInfo.info.includedTools).toContain("edit")
+		})
+
+		it("should fall back to default model when no model ID is provided", () => {
+			const testHandler = new VertexHandler({
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const modelInfo = testHandler.getModel()
+			expect(modelInfo.id).toBeDefined()
+			expect(modelInfo.info).toBeDefined()
+			expect(modelInfo.info.maxTokens).toBeGreaterThan(0)
+		})
 	})
 })

src/api/providers/anthropic-vertex.ts

@@ -207,8 +207,27 @@ export class AnthropicVertexHandler extends BaseProvider implements SingleComple
 
 	getModel() {
 		const modelId = this.options.apiModelId
-		let id = modelId && modelId in vertexModels ? (modelId as VertexModelId) : vertexDefaultModelId
-		let info: ModelInfo = vertexModels[id]
+		let id: string
+		let info: ModelInfo
+
+		if (modelId && modelId in vertexModels) {
+			id = modelId as VertexModelId
+			info = vertexModels[id as VertexModelId]
+		} else if (modelId) {
+			// Pass through custom/unknown model IDs with sensible defaults
+			id = modelId
+			info = {
+				maxTokens: 8192,
+				contextWindow: 200_000,
+				supportsImages: true,
+				supportsPromptCache: false,
+				inputPrice: 0,
+				outputPrice: 0,
+			}
+		} else {
+			id = vertexDefaultModelId
+			info = vertexModels[vertexDefaultModelId]
+		}
 
 		// Check if 1M context beta should be enabled for supported models
 		const supports1MContext = VERTEX_1M_CONTEXT_MODEL_IDS.includes(

src/api/providers/vertex.ts

@@ -14,8 +14,27 @@ 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]
+		let id: string
+		let info: ModelInfo
+
+		if (modelId && modelId in vertexModels) {
+			id = modelId as VertexModelId
+			info = vertexModels[id as VertexModelId]
+		} else if (modelId) {
+			// Pass through custom/unknown model IDs with sensible defaults
+			id = modelId
+			info = {
+				maxTokens: 8192,
+				contextWindow: 1_048_576,
+				supportsImages: true,
+				supportsPromptCache: false,
+				inputPrice: 0,
+				outputPrice: 0,
+			}
+		} else {
+			id = vertexDefaultModelId
+			info = vertexModels[vertexDefaultModelId]
+		}
 		const params = getModelParams({
 			format: "gemini",
 			modelId: id,