feat: add "Enable Reasoning" checkbox for Bedrock custom ARNs

- Add awsCustomArnEnableReasoning field to ProviderSettings type
- Add UI checkbox in Bedrock settings when custom ARN is used
- Update bedrock.ts logic to check awsCustomArnEnableReasoning for custom ARNs
- Only enable reasoning for custom ARNs when explicitly enabled by user
- Fixes issue where custom ARNs like Nova Lite received unsupported reasoning flag

Addresses #10040

packages/types/src/provider-settings.ts

@@ -226,6 +226,7 @@ const bedrockSchema = apiModelIdProviderModelSchema.extend({
 	awsApiKey: z.string().optional(),
 	awsUseApiKey: z.boolean().optional(),
 	awsCustomArn: z.string().optional(),
+	awsCustomArnEnableReasoning: z.boolean().optional(), // Enable reasoning for custom ARNs
 	awsModelContextWindow: z.number().optional(),
 	awsBedrockEndpointEnabled: z.boolean().optional(),
 	awsBedrockEndpoint: z.string().optional(),

src/api/providers/bedrock.ts

@@ -384,7 +384,13 @@ export class AwsBedrockHandler extends BaseProvider implements SingleCompletionH
 			modelConfig.reasoning &&
 			modelConfig.reasoningBudget
 
-		if ((isThinkingExplicitlyEnabled || isThinkingEnabledBySettings) && modelConfig.info.supportsReasoningBudget) {
+		// For custom ARNs, only enable reasoning if explicitly enabled via the awsCustomArnEnableReasoning setting
+		// For regular models, use the standard logic
+		const shouldEnableThinking = this.options.awsCustomArn
+			? this.options.awsCustomArnEnableReasoning && modelConfig.info.supportsReasoningBudget
+			: (isThinkingExplicitlyEnabled || isThinkingEnabledBySettings) && modelConfig.info.supportsReasoningBudget
+
+		if (shouldEnableThinking) {
 			thinkingEnabled = true
 			additionalModelRequestFields = {
 				thinking: {
@@ -717,11 +723,16 @@ export class AwsBedrockHandler extends BaseProvider implements SingleCompletionH
 
 			// For completePrompt, thinking is typically not used, but we should still check
 			// if thinking was somehow enabled in the model config
-			const thinkingEnabled =
+			// For custom ARNs, only enable if explicitly set via awsCustomArnEnableReasoning
+			const thinkingEnabledBySettings =
 				shouldUseReasoningBudget({ model: modelConfig.info, settings: this.options }) &&
 				modelConfig.reasoning &&
 				modelConfig.reasoningBudget
 
+			const thinkingEnabled = this.options.awsCustomArn
+				? this.options.awsCustomArnEnableReasoning && modelConfig.reasoning && modelConfig.reasoningBudget
+				: thinkingEnabledBySettings
+
 			const inferenceConfig: BedrockInferenceConfig = {
 				maxTokens: modelConfig.maxTokens || (modelConfig.info.maxTokens as number),
 				temperature: modelConfig.temperature ?? (this.options.modelTemperature as number),

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

@@ -226,6 +226,20 @@ export const Bedrock = ({ apiConfiguration, setApiConfigurationField, selectedMo
 					</div>
 				</>
 			)}
+			{apiConfiguration?.awsCustomArn && (
+				<div>
+					<Checkbox
+						checked={apiConfiguration?.awsCustomArnEnableReasoning ?? false}
+						onChange={(checked: boolean) => {
+							setApiConfigurationField("awsCustomArnEnableReasoning", checked)
+						}}>
+						Enable Reasoning
+					</Checkbox>
+					<div className="text-sm text-vscode-descriptionForeground mt-1 ml-6">
+						Enable extended thinking for this custom ARN. Only enable if the model supports reasoning.
+					</div>
+				</div>
+			)}
 		</>
 	)
 }