byok: default statefulResponses to true for custom Responses endpoints

The Open Responses spec requires servers to honor previous_response_id or fail with previous_response_not_found, which the existing full-history retry already handles. Chain by default and let stateless servers that silently ignore the ID (e.g. DeepSeek) opt out with statefulResponses: false.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

extensions/copilot/package.json

@@ -2158,8 +2158,8 @@
                   },
                   "statefulResponses": {
                     "type": "boolean",
-                    "default": false,
-                    "markdownDescription": "Whether the Responses API server retains prior responses. When `true`, requests chain via `previous_response_id` and send only new history; leave `false` for stateless servers (for example DeepSeek), which require the full history on every request. Ignored when `zeroDataRetentionEnabled` is `true`."
+                    "default": true,
+                    "markdownDescription": "Whether the Responses API server retains prior responses. When `true`, requests chain via `previous_response_id` and send only new history. Set to `false` for stateless servers that ignore `previous_response_id` (for example DeepSeek), so the full history is sent on every request. Ignored when `zeroDataRetentionEnabled` is `true`."
                   },
                   "supportsReasoningEffort": {
                     "type": "array",

extensions/copilot/src/extension/byok/vscode-node/customEndpointProvider.ts

@@ -208,8 +208,8 @@ export class CustomEndpointBYOKModelProvider extends AbstractOpenAICompatibleLMP
  *    conflicting credentials.
  * 4. Omits the Responses API `store` property when Zero Data Retention was not
  *    explicitly configured, allowing custom implementations to use their own default.
- * 5. Sends full Responses history instead of chaining via `previous_response_id`
- *    unless the user declares the server stateful with `statefulResponses: true`.
+ * 5. Chains Responses requests via `previous_response_id` per the Open Responses spec,
+ *    unless the user marks the server stateless with `statefulResponses: false`.
  */
 export class CustomEndpointOAIEndpoint extends OpenAIEndpoint {
 	/**
@@ -261,11 +261,11 @@ export class CustomEndpointOAIEndpoint extends OpenAIEndpoint {
 	}
 
 	/**
-	 * Many OpenAI-compatible Responses implementations (e.g. DeepSeek) are stateless and
-	 * ignore `previous_response_id`, so only chain when the user opts in via `statefulResponses`.
+	 * Spec-compliant Responses servers honor `previous_response_id`, but some (e.g. DeepSeek)
+	 * silently ignore it, so users can opt out with `statefulResponses: false`.
 	 */
 	protected override get supportsStatefulResponses(): boolean {
-		return this.modelMetadata.statefulResponses === true;
+		return this.modelMetadata.statefulResponses !== false;
 	}
 
 	override createRequestBody(options: ICreateEndpointBodyOptions): IEndpointBody {

extensions/copilot/src/extension/byok/vscode-node/test/customEndpointProvider.spec.ts

@@ -236,7 +236,7 @@ describe('CustomEndpointBYOKModelProvider', () => {
 			};
 		}
 
-		it('omits store and previous_response_id after cloning a Custom Endpoint Responses endpoint when zeroDataRetentionEnabled is omitted', async () => {
+		it('omits store but keeps previous_response_id after cloning a Custom Endpoint Responses endpoint when both flags are omitted', async () => {
 			const endpoint = (await createConfiguredResponsesEndpoint()).cloneWithTokenOverride(64000);
 			const body = createResponsesBody(endpoint);
 
@@ -247,12 +247,12 @@ describe('CustomEndpointBYOKModelProvider', () => {
 			}).toEqual({
 				storePresent: false,
 				store: undefined,
-				previousResponseId: undefined,
+				previousResponseId: customResponsesMarker,
 			});
 		});
 
-		it('issue #331636: sends the full tool-call history to stateless Custom Endpoint Responses servers', async () => {
-			const endpoint = await createConfiguredResponsesEndpoint();
+		it('issue #331636: sends the full tool-call history when statefulResponses is false', async () => {
+			const endpoint = await createConfiguredResponsesEndpoint(undefined, false);
 			const body = endpoint.createRequestBody({
 				debugName: 'test',
 				messages: [
@@ -280,8 +280,8 @@ describe('CustomEndpointBYOKModelProvider', () => {
 			});
 		});
 
-		it('enables store but not previous_response_id when zeroDataRetentionEnabled is false without statefulResponses', async () => {
-			const endpoint = await createConfiguredResponsesEndpoint(false);
+		it('enables store but not previous_response_id when zeroDataRetentionEnabled and statefulResponses are false', async () => {
+			const endpoint = await createConfiguredResponsesEndpoint(false, false);
 			const body = createResponsesBody(endpoint);
 
 			expect({
@@ -295,8 +295,8 @@ describe('CustomEndpointBYOKModelProvider', () => {
 			});
 		});
 
-		it('chains via previous_response_id when statefulResponses is true', async () => {
-			const endpoint = await createConfiguredResponsesEndpoint(false, true);
+		it('enables store and previous_response_id when zeroDataRetentionEnabled is false and statefulResponses is omitted', async () => {
+			const endpoint = await createConfiguredResponsesEndpoint(false);
 			const body = createResponsesBody(endpoint);
 
 			expect({

extensions/copilot/src/platform/endpoint/common/endpointProvider.ts

@@ -156,8 +156,9 @@ export type IChatModelInformation = IModelAPIResponse & {
 	modelOptions?: Readonly<IChatModelRequestOptions>;
 	zeroDataRetentionEnabled?: boolean;
 	/**
-	 * BYOK Custom Endpoint opt-in declaring that the Responses API server retains prior
-	 * responses, so requests may chain via `previous_response_id` and send only post-marker history.
+	 * BYOK Custom Endpoint setting declaring whether the Responses API server retains prior
+	 * responses, so requests may chain via `previous_response_id` and send only post-marker
+	 * history. Unset is treated as `true`; set `false` for stateless servers.
 	 */
 	statefulResponses?: boolean;
 	/**