mcp: order extension host delegates to avoid unnecessary CORS errors (#245165)

Cherry-picks #245163

src/vs/workbench/api/browser/mainThreadMcp.ts

@@ -40,6 +40,8 @@ export class MainThreadMcp extends Disposable implements MainThreadMcpShape {
 		const proxy = _extHostContext.getProxy(ExtHostContext.ExtHostMcp);
 		this._mcpEnabled = observableConfigValue(mcpEnabledSection, true, configurationService);
 		this._register(this._mcpRegistry.registerDelegate({
+			// Prefer Node.js extension hosts when they're available. No CORS issues etc.
+			priority: _extHostContext.extensionHostKind === ExtensionHostKind.LocalWebWorker ? 0 : 1,
 			waitForInitialProviderPromises() {
 				return proxy.$waitForInitialCollectionProviders();
 			},

src/vs/workbench/contrib/mcp/common/mcpRegistry.ts

@@ -112,6 +112,8 @@ export class McpRegistry extends Disposable implements IMcpRegistry {
 
 	public registerDelegate(delegate: IMcpHostDelegate): IDisposable {
 		this._delegates.push(delegate);
+		this._delegates.sort((a, b) => b.priority - a.priority);
+
 		return {
 			dispose: () => {
 				const index = this._delegates.indexOf(delegate);

src/vs/workbench/contrib/mcp/common/mcpRegistryTypes.ts

@@ -27,6 +27,8 @@ export interface IMcpMessageTransport extends IDisposable {
 }
 
 export interface IMcpHostDelegate {
+	/** Priority for this delegate, delegates are tested in descending priority order */
+	readonly priority: number;
 	waitForInitialProviderPromises(): Promise<void>;
 	canStart(collectionDefinition: McpCollectionDefinition, serverDefinition: McpServerDefinition): boolean;
 	start(collectionDefinition: McpCollectionDefinition, serverDefinition: McpServerDefinition, resolvedLaunch: McpServerLaunch): IMcpMessageTransport;

src/vs/workbench/contrib/mcp/test/common/mcpRegistry.test.ts

@@ -72,6 +72,8 @@ class TestConfigurationResolverService implements Partial<IConfigurationResolver
 }
 
 class TestMcpHostDelegate implements IMcpHostDelegate {
+	priority = 0;
+
 	canStart(): boolean {
 		return true;
 	}

src/vs/workbench/contrib/mcp/test/common/mcpServerConnection.test.ts

@@ -26,6 +26,8 @@ class TestMcpHostDelegate extends Disposable implements IMcpHostDelegate {
 	private readonly _transport: TestMcpMessageTransport;
 	private _canStartValue = true;
 
+	priority = 0;
+
 	constructor() {
 		super();
 		this._transport = this._register(new TestMcpMessageTransport());

src/vs/workbench/contrib/mcp/test/common/mcpServerRequestHandler.test.ts

@@ -24,6 +24,8 @@ import { CancellationTokenSource } from '../../../../../base/common/cancellation
 class TestMcpHostDelegate extends Disposable implements IMcpHostDelegate {
 	private readonly _transport: TestMcpMessageTransport;
 
+	priority = 0;
+
 	constructor() {
 		super();
 		this._transport = this._register(new TestMcpMessageTransport());