update Anthropic thinking budget to default to 4000 (#2588)

* update Anthropic thinking budget to default to 4000

* test: update thinking_budget test to use default value of 4000

Author: bhavyaus

package.json

@@ -3258,13 +3258,11 @@
 						]
 					},
 					"github.copilot.chat.anthropic.thinking.budgetTokens": {
-						"type": [
-							"number",
-							"null"
-						],
+						"type": "number",
 						"markdownDescription": "%github.copilot.config.anthropic.thinking.budgetTokens%",
 						"minimum": 0,
 						"maximum": 32000,
+						"default": 4000,
 						"tags": [
 							"experimental",
 							"onExp"
@@ -5130,4 +5128,4 @@
 		"string_decoder": "npm:[email protected]",
 		"node-gyp": "npm:[email protected]"
 	}
-}
+}

src/platform/configuration/common/configurationService.ts

@@ -815,7 +815,7 @@ export namespace ConfigKey {
 	export const ResponsesApiReasoningSummary = defineSetting<'off' | 'detailed'>('chat.responsesApiReasoningSummary', ConfigType.ExperimentBased, 'detailed');
 	export const EnableChatImageUpload = defineSetting<boolean>('chat.imageUpload.enabled', ConfigType.ExperimentBased, true);
 	/** Thinking token budget for Anthropic extended thinking. If set, enables extended thinking. */
-	export const AnthropicThinkingBudget = defineSetting<number | undefined>('chat.anthropic.thinking.budgetTokens', ConfigType.ExperimentBased, undefined);
+	export const AnthropicThinkingBudget = defineSetting<number>('chat.anthropic.thinking.budgetTokens', ConfigType.ExperimentBased, 4000);
 	/** Enable Anthropic web search tool for BYOK Claude models */
 	export const AnthropicWebSearchToolEnabled = defineSetting<boolean>('chat.anthropic.tools.websearch.enabled', ConfigType.ExperimentBased, false);
 	/** Maximum number of web searches allowed per request */

src/platform/endpoint/node/test/copilotChatEndpoint.spec.ts

@@ -374,7 +374,7 @@ describe('ChatEndpoint - Anthropic Thinking Budget', () => {
 			expect(body.thinking_budget).toBe(maxOutputTokens - 1);
 		});
 
-		it('should not set thinking_budget when configuredBudget is undefined', () => {
+		it('should use default thinking_budget of 4000', () => {
 			mockServices.configurationService.setConfig(ConfigKey.AnthropicThinkingBudget, undefined);
 			const modelMetadata = createAnthropicModelMetadata('claude-sonnet-4-5');
 
@@ -396,7 +396,7 @@ describe('ChatEndpoint - Anthropic Thinking Budget', () => {
 			const options = createTestOptions([createUserMessage('Hello')]);
 			const body = endpoint.createRequestBody(options);
 
-			expect(body.thinking_budget).toBeUndefined();
+			expect(body.thinking_budget).toBe(4000);
 		});
 
 		it('should not set thinking_budget for non-Anthropic models (gpt)', () => {