feat: add custom model support for Vertex AI provider

- Add custom model input field to Vertex provider settings UI
- Update provider settings schema to include vertexCustomModelId field
- Modify useSelectedModel hook to prioritize custom models for Vertex
- Update AnthropicVertexHandler to handle custom model names with thinking suffix support
- Add comprehensive tests for custom model functionality
- Add i18n translations for custom model UI elements

Fixes #5751: Users can now input custom Vertex AI model IDs in the correct @ format (e.g., claude-sonnet-4@20250514) to avoid 404 errors

Author: roomote

packages/types/src/provider-settings.ts

@@ -116,6 +116,7 @@ const vertexSchema = apiModelIdProviderModelSchema.extend({
 	vertexJsonCredentials: z.string().optional(),
 	vertexProjectId: z.string().optional(),
 	vertexRegion: z.string().optional(),
+	vertexCustomModelId: z.string().optional(),
 })
 
 const openAiSchema = baseProviderSettingsSchema.extend({

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

@@ -809,4 +809,124 @@ describe("VertexHandler", () => {
 			)
 		})
 	})
+
+	describe("custom model handling", () => {
+		it("should use custom model when vertexCustomModelId is provided", () => {
+			handler = new AnthropicVertexHandler({
+				apiModelId: "claude-3-5-sonnet-v2@20241022",
+				vertexCustomModelId: "claude-sonnet-4@20250514",
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const modelInfo = handler.getModel()
+			expect(modelInfo.id).toBe("claude-sonnet-4@20250514")
+			// Should use default model info as fallback
+			expect(modelInfo.info).toBeDefined()
+			expect(modelInfo.info.maxTokens).toBe(8192)
+			expect(modelInfo.info.contextWindow).toBe(200_000)
+		})
+
+		it("should trim whitespace from custom model ID", () => {
+			handler = new AnthropicVertexHandler({
+				apiModelId: "claude-3-5-sonnet-v2@20241022",
+				vertexCustomModelId: "  claude-sonnet-4@20250514  ",
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const modelInfo = handler.getModel()
+			expect(modelInfo.id).toBe("claude-sonnet-4@20250514")
+		})
+
+		it("should handle custom model with thinking suffix", () => {
+			handler = new AnthropicVertexHandler({
+				apiModelId: "claude-3-5-sonnet-v2@20241022",
+				vertexCustomModelId: "claude-sonnet-4@20250514:thinking",
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+				modelMaxTokens: 16384,
+				modelMaxThinkingTokens: 4096,
+			})
+
+			const modelInfo = handler.getModel()
+			expect(modelInfo.id).toBe("claude-sonnet-4@20250514")
+			// For custom models with thinking suffix, reasoning parameters should be set
+			expect(modelInfo.reasoningBudget).toBe(4096)
+			expect(modelInfo.temperature).toBe(1.0)
+		})
+
+		it("should fall back to predefined model when custom model is empty", () => {
+			handler = new AnthropicVertexHandler({
+				apiModelId: "claude-3-5-sonnet-v2@20241022",
+				vertexCustomModelId: "",
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const modelInfo = handler.getModel()
+			expect(modelInfo.id).toBe("claude-3-5-sonnet-v2@20241022")
+		})
+
+		it("should fall back to predefined model when custom model is only whitespace", () => {
+			handler = new AnthropicVertexHandler({
+				apiModelId: "claude-3-5-sonnet-v2@20241022",
+				vertexCustomModelId: "   ",
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const modelInfo = handler.getModel()
+			expect(modelInfo.id).toBe("claude-3-5-sonnet-v2@20241022")
+		})
+
+		it("should fall back to default model when custom model is not provided", () => {
+			handler = new AnthropicVertexHandler({
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const modelInfo = handler.getModel()
+			expect(modelInfo.id).toBe("claude-sonnet-4@20250514") // default model
+		})
+
+		it("should use custom model in API calls", async () => {
+			handler = new AnthropicVertexHandler({
+				apiModelId: "claude-3-5-sonnet-v2@20241022",
+				vertexCustomModelId: "claude-sonnet-4@20250514",
+				vertexProjectId: "test-project",
+				vertexRegion: "us-central1",
+			})
+
+			const mockCreate = vitest.fn().mockImplementation(async (options) => {
+				return {
+					async *[Symbol.asyncIterator]() {
+						yield {
+							type: "message_start",
+							message: {
+								usage: {
+									input_tokens: 10,
+									output_tokens: 5,
+								},
+							},
+						}
+					},
+				}
+			})
+			;(handler["client"].messages as any).create = mockCreate
+
+			const stream = handler.createMessage("You are a helpful assistant", [{ role: "user", content: "Hello" }])
+
+			// Consume the stream
+			for await (const _chunk of stream) {
+				// Just consume the stream
+			}
+
+			expect(mockCreate).toHaveBeenCalledWith(
+				expect.objectContaining({
+					model: "claude-sonnet-4@20250514",
+				}),
+			)
+		})
+	})
 })

src/api/providers/anthropic-vertex.ts

@@ -163,6 +163,45 @@ export class AnthropicVertexHandler extends BaseProvider implements SingleComple
 	}
 
 	getModel() {
+		// Check if a custom model is specified
+		const customModelId = this.options.vertexCustomModelId
+		if (customModelId && customModelId.trim()) {
+			// For custom models, use default model info as fallback
+			const defaultInfo: ModelInfo = vertexModels[vertexDefaultModelId]
+			const trimmedId = customModelId.trim()
+
+			// Check if custom model has thinking suffix
+			const hasThinkingSuffix = trimmedId.endsWith(":thinking")
+			const actualModelId = hasThinkingSuffix ? trimmedId.replace(":thinking", "") : trimmedId
+
+			// For thinking models, create a model info that supports reasoning
+			let modelInfo: ModelInfo = defaultInfo
+			if (hasThinkingSuffix) {
+				modelInfo = {
+					...defaultInfo,
+					supportsReasoningBudget: true,
+					requiredReasoningBudget: true,
+					maxThinkingTokens: defaultInfo.maxThinkingTokens || 8192,
+				}
+			}
+
+			// Use the full model ID (with :thinking suffix) for getModelParams to get proper reasoning parameters
+			const modelIdForParams = hasThinkingSuffix ? trimmedId : actualModelId
+			const params = getModelParams({
+				format: "anthropic",
+				modelId: modelIdForParams,
+				model: modelInfo,
+				settings: this.options,
+			})
+
+			return {
+				id: actualModelId,
+				info: modelInfo,
+				...params,
+			}
+		}
+
+		// Use predefined models
 		const modelId = this.options.apiModelId
 		let id = modelId && modelId in vertexModels ? (modelId as VertexModelId) : vertexDefaultModelId
 		const info: ModelInfo = vertexModels[id]

webview-ui/src/components/settings/providers/Vertex.tsx

@@ -91,6 +91,16 @@ export const Vertex = ({ apiConfiguration, setApiConfigurationField }: VertexPro
 					</SelectContent>
 				</Select>
 			</div>
+			<VSCodeTextField
+				value={apiConfiguration?.vertexCustomModelId || ""}
+				onInput={handleInputChange("vertexCustomModelId")}
+				placeholder="claude-sonnet-4@20250514"
+				className="w-full">
+				<label className="block font-medium mb-1">{t("settings:providers.customModel")}</label>
+			</VSCodeTextField>
+			<div className="text-sm text-vscode-descriptionForeground">
+				{t("settings:providers.vertex.customModelDescription")}
+			</div>
 		</>
 	)
 }