feat: add model routing UI settings, fix api handler restoration bug

- Add translation keys for MODEL_ROUTING in locales/en/settings.json
- Create ModelRoutingSettings component with toggle and light model ID input
- Wire up MODEL_ROUTING special-case in ExperimentalSettings.tsx
- Add modelRoutingLightModelId state binding in SettingsView.tsx
- Fix bug: restore this.api to primary handler on all paths (stream
  failure, empty-response retry, catch) not just the happy path
- Fix nit: remove unnecessary as any cast in ModelRouter.isEnabled()

src/core/task/ModelRouter.ts

@@ -187,7 +187,7 @@ export class ModelRouter {
 		if (!experimentsConfig || !lightModelId || lightModelId.trim() === "") {
 			return false
 		}
-		return experiments.isEnabled(experimentsConfig, EXPERIMENT_IDS.MODEL_ROUTING as any)
+		return experiments.isEnabled(experimentsConfig, EXPERIMENT_IDS.MODEL_ROUTING)
 	}
 
 	/**

src/core/task/Task.ts

@@ -2795,6 +2795,9 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 			await this.saveClineMessages()
 			await this.providerRef.deref()?.postStateToWebviewWithoutTaskHistory()
 
+			// Model routing: declare outside try block so catch can restore it
+			let primaryApiHandler: typeof this.api | undefined
+
 			try {
 				let cacheWriteTokens = 0
 				let cacheReadTokens = 0
@@ -2898,7 +2901,6 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 				await this.diffViewProvider.reset()
 
 				// Model routing: temporarily swap to light model if heuristics say so
-				let primaryApiHandler: typeof this.api | undefined
 				{
 					const routingState = await this.providerRef.deref()?.getState()
 					if (
@@ -3604,6 +3606,14 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 					presentAssistantMessage(this)
 				}
 
+				// Model routing: always restore primary API handler after streaming completes,
+				// regardless of whether the turn had content or not. This prevents permanently
+				// losing the primary model on empty-response retries or error paths.
+				if (primaryApiHandler) {
+					this.api = primaryApiHandler
+					primaryApiHandler = undefined
+				}
+
 				if (hasTextContent || hasToolUses) {
 					// NOTE: This comment is here for future reference - this was a
 					// workaround for `userMessageContent` not getting set to true.
@@ -3623,12 +3633,8 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 
 					await pWaitFor(() => this.userMessageContentReady)
 
-					// Model routing: end current turn and restore primary handler
+					// Model routing: end current turn classification
 					this.modelRouter.endTurn()
-					if (primaryApiHandler) {
-						this.api = primaryApiHandler
-						primaryApiHandler = undefined
-					}
 
 					// If the model did not tool use, then we need to tell it to
 					// either use a tool or attempt_completion.
@@ -3770,6 +3776,11 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 				// If we reach here without continuing, return false (will always be false for now)
 				return false
 			} catch (error) {
+				// Model routing: restore primary API handler on error paths
+				if (primaryApiHandler) {
+					this.api = primaryApiHandler
+					primaryApiHandler = undefined
+				}
 				// This should never happen since the only thing that can throw an
 				// error is the attemptApiRequest, which is wrapped in a try catch
 				// that sends an ask where if noButtonClicked, will clear current

webview-ui/src/components/settings/ExperimentalSettings.tsx

@@ -14,6 +14,7 @@ import { SearchableSetting } from "./SearchableSetting"
 import { ExperimentalFeature } from "./ExperimentalFeature"
 import { ImageGenerationSettings } from "./ImageGenerationSettings"
 import { CustomToolsSettings } from "./CustomToolsSettings"
+import { ModelRoutingSettings } from "./ModelRoutingSettings"
 
 type ExperimentalSettingsProps = HTMLAttributes<HTMLDivElement> & {
 	experiments: Experiments
@@ -26,6 +27,8 @@ type ExperimentalSettingsProps = HTMLAttributes<HTMLDivElement> & {
 	setImageGenerationProvider?: (provider: ImageGenerationProvider) => void
 	setOpenRouterImageApiKey?: (apiKey: string) => void
 	setImageGenerationSelectedModel?: (model: string) => void
+	modelRoutingLightModelId?: string
+	setModelRoutingLightModelId?: (modelId: string) => void
 }
 
 export const ExperimentalSettings = ({
@@ -39,6 +42,8 @@ export const ExperimentalSettings = ({
 	setImageGenerationProvider,
 	setOpenRouterImageApiKey,
 	setImageGenerationSelectedModel,
+	modelRoutingLightModelId,
+	setModelRoutingLightModelId,
 	className,
 	...props
 }: ExperimentalSettingsProps) => {
@@ -83,6 +88,24 @@ export const ExperimentalSettings = ({
 								</SearchableSetting>
 							)
 						}
+						if (config[0] === "MODEL_ROUTING" && setModelRoutingLightModelId) {
+							return (
+								<SearchableSetting
+									key={config[0]}
+									settingId={`experimental-${config[0].toLowerCase()}`}
+									section="experimental"
+									label={label}>
+									<ModelRoutingSettings
+										enabled={experiments[EXPERIMENT_IDS.MODEL_ROUTING] ?? false}
+										onChange={(enabled) =>
+											setExperimentEnabled(EXPERIMENT_IDS.MODEL_ROUTING, enabled)
+										}
+										modelRoutingLightModelId={modelRoutingLightModelId}
+										setModelRoutingLightModelId={setModelRoutingLightModelId}
+									/>
+								</SearchableSetting>
+							)
+						}
 						if (config[0] === "CUSTOM_TOOLS") {
 							return (
 								<SearchableSetting

webview-ui/src/components/settings/ModelRoutingSettings.tsx

@@ -0,0 +1,50 @@
+import { VSCodeCheckbox, VSCodeTextField } from "@vscode/webview-ui-toolkit/react"
+
+import { useAppTranslation } from "@/i18n/TranslationContext"
+
+interface ModelRoutingSettingsProps {
+	enabled: boolean
+	onChange: (enabled: boolean) => void
+	modelRoutingLightModelId: string | undefined
+	setModelRoutingLightModelId: (modelId: string) => void
+}
+
+export const ModelRoutingSettings = ({
+	enabled,
+	onChange,
+	modelRoutingLightModelId,
+	setModelRoutingLightModelId,
+}: ModelRoutingSettingsProps) => {
+	const { t } = useAppTranslation()
+
+	return (
+		<div className="space-y-4">
+			<div>
+				<div className="flex items-center gap-2">
+					<VSCodeCheckbox checked={enabled} onChange={(e: any) => onChange(e.target.checked)}>
+						<span className="font-medium">{t("settings:experimental.MODEL_ROUTING.name")}</span>
+					</VSCodeCheckbox>
+				</div>
+				<p className="text-vscode-descriptionForeground text-sm mt-0">
+					{t("settings:experimental.MODEL_ROUTING.description")}
+				</p>
+			</div>
+
+			{enabled && (
+				<div className="ml-2 space-y-3">
+					<div>
+						<label className="block font-medium mb-1">
+							{t("settings:experimental.MODEL_ROUTING.lightModelIdLabel")}
+						</label>
+						<VSCodeTextField
+							value={modelRoutingLightModelId ?? ""}
+							placeholder={t("settings:experimental.MODEL_ROUTING.lightModelIdPlaceholder")}
+							onInput={(e: any) => setModelRoutingLightModelId(e.target.value)}
+							className="w-full"
+						/>
+					</div>
+				</div>
+			)}
+		</div>
+	)
+}

webview-ui/src/components/settings/SettingsView.tsx

@@ -207,6 +207,7 @@ const SettingsView = forwardRef<SettingsViewRef, SettingsViewProps>(({ onDone, t
 		imageGenerationProvider,
 		openRouterImageApiKey,
 		openRouterImageGenerationSelectedModel,
+		modelRoutingLightModelId,
 		reasoningBlockCollapsed,
 		enterBehavior,
 		includeCurrentTime,
@@ -338,6 +339,16 @@ const SettingsView = forwardRef<SettingsViewRef, SettingsViewProps>(({ onDone, t
 		})
 	}, [])
 
+	const setModelRoutingLightModelId = useCallback((modelId: string) => {
+		setCachedState((prevState) => {
+			if (prevState.modelRoutingLightModelId !== modelId) {
+				setChangeDetected(true)
+			}
+
+			return { ...prevState, modelRoutingLightModelId: modelId }
+		})
+	}, [])
+
 	const setCustomSupportPromptsField = useCallback((prompts: Record<string, string | undefined>) => {
 		setCachedState((prevState) => {
 			const previousStr = JSON.stringify(prevState.customSupportPrompts)
@@ -422,6 +433,7 @@ const SettingsView = forwardRef<SettingsViewRef, SettingsViewProps>(({ onDone, t
 					imageGenerationProvider,
 					openRouterImageApiKey,
 					openRouterImageGenerationSelectedModel,
+					modelRoutingLightModelId,
 					experiments,
 					customSupportPrompts,
 				},
@@ -927,6 +939,8 @@ const SettingsView = forwardRef<SettingsViewRef, SettingsViewProps>(({ onDone, t
 								setImageGenerationProvider={setImageGenerationProvider}
 								setOpenRouterImageApiKey={setOpenRouterImageApiKey}
 								setImageGenerationSelectedModel={setImageGenerationSelectedModel}
+								modelRoutingLightModelId={modelRoutingLightModelId}
+								setModelRoutingLightModelId={setModelRoutingLightModelId}
 							/>
 						)}
 

webview-ui/src/i18n/locales/en/settings.json

@@ -941,6 +941,12 @@
 			"refreshSuccess": "Tools refreshed successfully",
 			"refreshError": "Failed to refresh tools",
 			"toolParameters": "Parameters"
+		},
+		"MODEL_ROUTING": {
+			"name": "Enable model routing",
+			"description": "When enabled, Roo dynamically routes API calls to a lighter (cheaper) model during information-gathering phases. The light model must be from the same provider as your primary model.",
+			"lightModelIdLabel": "Light Model ID",
+			"lightModelIdPlaceholder": "e.g. claude-3-haiku-20241022"
 		}
 	},
 	"promptCaching": {