fix: preserve custom model IDs instead of falling back to defaults

When users configure a custom model ID (e.g. for third-party gateways),
several providers silently replaced it with the default model ID if it
was not found in the predefined models map. This fix ensures the
user-provided model ID is always used as-is, with only the model info
(capabilities, pricing, etc.) falling back to the default when the
custom model is not in the known models map.

Affected providers: Anthropic, Gemini, OpenAI Native, OpenAI Codex,
Vertex, Anthropic-Vertex, Minimax, xAI, and all providers extending
BaseOpenAiCompatibleProvider (SambaNova, Fireworks, Baseten, etc.).

Fixes #11936

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

@@ -165,13 +165,14 @@ describe("GeminiHandler", () => {
 			expect(modelInfo.info).toBeDefined()
 		})
 
-		it("should return default model if invalid model specified", () => {
+		it("should preserve custom model id when not in known models", () => {
 			const invalidHandler = new GeminiHandler({
-				apiModelId: "invalid-model",
+				apiModelId: "custom-model",
 				geminiApiKey: "test-key",
 			})
 			const modelInfo = invalidHandler.getModel()
-			expect(modelInfo.id).toBe(geminiDefaultModelId) // Default model
+			expect(modelInfo.id).toBe("custom-model") // Custom model ID is preserved
+			expect(modelInfo.info).toBeDefined() // Falls back to default model info
 		})
 
 		it("should exclude apply_diff and include edit in tool preferences", () => {

src/api/providers/__tests__/openai-codex.spec.ts

@@ -16,12 +16,12 @@ describe("OpenAiCodexHandler.getModel", () => {
 		},
 	)
 
-	it("should fall back to default model when an invalid model id is provided", () => {
+	it("should preserve custom model id when not in known models", () => {
 		const handler = new OpenAiCodexHandler({ apiModelId: "not-a-real-model" })
 		const model = handler.getModel()
 
-		expect(model.id).toBe("gpt-5.3-codex")
-		expect(model.info).toBeDefined()
+		expect(model.id).toBe("not-a-real-model") // Custom model ID is preserved
+		expect(model.info).toBeDefined() // Falls back to default model info
 	})
 
 	it("should use Spark-specific limits and capabilities", () => {

src/api/providers/anthropic-vertex.ts

@@ -207,8 +207,8 @@ 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 = modelId ?? vertexDefaultModelId
+		let info: ModelInfo = vertexModels[id as VertexModelId] ?? vertexModels[vertexDefaultModelId]
 
 		// Check if 1M context beta should be enabled for supported models
 		const supports1MContext = VERTEX_1M_CONTEXT_MODEL_IDS.includes(

src/api/providers/anthropic.ts

@@ -334,8 +334,8 @@ export class AnthropicHandler extends BaseProvider implements SingleCompletionHa
 
 	getModel() {
 		const modelId = this.options.apiModelId
-		let id = modelId && modelId in anthropicModels ? (modelId as AnthropicModelId) : anthropicDefaultModelId
-		let info: ModelInfo = anthropicModels[id]
+		let id = modelId ?? anthropicDefaultModelId
+		let info: ModelInfo = anthropicModels[id as AnthropicModelId] ?? anthropicModels[anthropicDefaultModelId]
 
 		// If 1M context beta is enabled for supported models, update the model info
 		if (

src/api/providers/base-openai-compatible-provider.ts

@@ -250,11 +250,9 @@ export abstract class BaseOpenAiCompatibleProvider<ModelName extends string>
 	}
 
 	override getModel() {
-		const id =
-			this.options.apiModelId && this.options.apiModelId in this.providerModels
-				? (this.options.apiModelId as ModelName)
-				: this.defaultProviderModelId
+		const id = this.options.apiModelId ?? this.defaultProviderModelId
+		const info = this.providerModels[id as ModelName] ?? this.providerModels[this.defaultProviderModelId]
 
-		return { id, info: this.providerModels[id] }
+		return { id, info }
 	}
 }

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]
+		let id = modelId ?? geminiDefaultModelId
+		let info: ModelInfo = geminiModels[id as GeminiModelId] ?? geminiModels[geminiDefaultModelId]
 
 		const params = getModelParams({
 			format: "gemini",

src/api/providers/minimax.ts

@@ -271,8 +271,8 @@ export class MiniMaxHandler extends BaseProvider implements SingleCompletionHand
 
 	getModel() {
 		const modelId = this.options.apiModelId
-		const id = modelId && modelId in minimaxModels ? (modelId as MinimaxModelId) : minimaxDefaultModelId
-		const info = minimaxModels[id]
+		const id = modelId ?? minimaxDefaultModelId
+		const info = minimaxModels[id as MinimaxModelId] ?? minimaxModels[minimaxDefaultModelId]
 
 		const params = getModelParams({
 			format: "anthropic",

src/api/providers/openai-codex.ts

@@ -1117,9 +1117,10 @@ export class OpenAiCodexHandler extends BaseProvider implements SingleCompletion
 	override getModel() {
 		const modelId = this.options.apiModelId
 
-		let id = modelId && modelId in openAiCodexModels ? (modelId as OpenAiCodexModelId) : openAiCodexDefaultModelId
+		let id = modelId ?? openAiCodexDefaultModelId
 
-		const info: ModelInfo = openAiCodexModels[id]
+		const info: ModelInfo =
+			openAiCodexModels[id as OpenAiCodexModelId] ?? openAiCodexModels[openAiCodexDefaultModelId]
 
 		const params = getModelParams({
 			format: "openai",

src/api/providers/openai-native.ts

@@ -1435,10 +1435,10 @@ export class OpenAiNativeHandler extends BaseProvider implements SingleCompletio
 	override getModel() {
 		const modelId = this.options.apiModelId
 
-		let id =
-			modelId && modelId in openAiNativeModels ? (modelId as OpenAiNativeModelId) : openAiNativeDefaultModelId
+		let id = modelId ?? openAiNativeDefaultModelId
 
-		const info: ModelInfo = openAiNativeModels[id]
+		const info: ModelInfo =
+			openAiNativeModels[id as OpenAiNativeModelId] ?? openAiNativeModels[openAiNativeDefaultModelId]
 
 		const params = getModelParams({
 			format: "openai",

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]
+		let id = modelId ?? vertexDefaultModelId
+		let info: ModelInfo = vertexModels[id as VertexModelId] ?? vertexModels[vertexDefaultModelId]
 		const params = getModelParams({
 			format: "gemini",
 			modelId: id,

src/api/providers/xai.ts

@@ -37,12 +37,9 @@ export class XAIHandler extends BaseProvider implements SingleCompletionHandler
 	}
 
 	override getModel() {
-		const id =
-			this.options.apiModelId && this.options.apiModelId in xaiModels
-				? (this.options.apiModelId as XAIModelId)
-				: xaiDefaultModelId
+		const id = this.options.apiModelId ?? xaiDefaultModelId
 
-		const info = xaiModels[id]
+		const info = xaiModels[id as XAIModelId] ?? xaiModels[xaiDefaultModelId]
 		const params = getModelParams({
 			format: "openai",
 			modelId: id,