feat(mcp): implement sampling/createMessage capability

packages/core/src/mcp/client/connection.ts

@@ -33,6 +33,10 @@ import {
   registerMcpClientRequestHandlers,
   unregisterMcpClientRequestHandlers,
 } from './roots'
+import {
+  registerMcpSamplingHandler,
+  unregisterMcpSamplingHandler,
+} from './sampling'
 import type { WrappedClient } from './types'
 
 type GlobalWithWebSocket = { WebSocket?: unknown }
@@ -101,6 +105,7 @@ export function createMcpClient(
     createMcpClientSdkOptions(name),
   )
   registerMcpClientRequestHandlers(client)
+  registerMcpSamplingHandler(client)
   client.setNotificationHandler(
     LoggingMessageNotificationSchema,
     notification => {
@@ -121,6 +126,7 @@ export function createMcpClient(
 
 export async function closeMcpClient(client: Client): Promise<void> {
   unregisterMcpClientRequestHandlers(client)
+  unregisterMcpSamplingHandler(client)
   try {
     await client.close()
   } catch {}

packages/core/src/mcp/client/index.ts

@@ -76,3 +76,9 @@ export {
   type McpLogMessageEvent,
   type McpLoggingLevel,
 } from './logging'
+export {
+  __resetMcpSamplingForTests,
+  __setMcpSamplingEnabledForTests,
+  isMcpSamplingEnabled,
+  setMcpSamplingEnabled,
+} from './sampling'

packages/core/src/mcp/client/roots.ts

@@ -11,6 +11,7 @@ import {
 import { checkHasTrustDialogAccepted } from '#core/utils/config'
 import { logMCPError } from '#core/utils/log'
 import { getCwd, subscribeCwdChanged } from '#core/utils/state'
+import { isMcpSamplingEnabled } from './sampling'
 
 let exposeRootsOverrideForTests: boolean | null = null
 const rootsClients = new Set<Client>()
@@ -43,10 +44,17 @@ export function shouldExposeMcpRoots(): boolean {
 }
 
 export function getMcpClientCapabilities(): ClientCapabilities {
-  if (!shouldExposeMcpRoots()) return {}
-  return {
-    roots: { listChanged: true },
+  const capabilities: ClientCapabilities = {}
+
+  if (shouldExposeMcpRoots()) {
+    capabilities.roots = { listChanged: true }
+  }
+
+  if (isMcpSamplingEnabled()) {
+    capabilities.sampling = {}
   }
+
+  return capabilities
 }
 
 function ensureCwdChangedSubscription(): void {