feat: restart MCP servers on /mcp refresh (#5479)

Co-authored-by: Brian Ray <62354532+emeryray2002@users.noreply.github.com>
Co-authored-by: N. Taylor Mullen <ntaylormullen@google.com>

packages/core/src/tools/mcp-client.ts

@@ -69,6 +69,134 @@ export enum MCPDiscoveryState {
   COMPLETED = 'completed',
 }
 
+/**
+ * A client for a single MCP server.
+ *
+ * This class is responsible for connecting to, discovering tools from, and
+ * managing the state of a single MCP server.
+ */
+export class McpClient {
+  private client: Client;
+  private transport: Transport | undefined;
+  private status: MCPServerStatus = MCPServerStatus.DISCONNECTED;
+  private isDisconnecting = false;
+
+  constructor(
+    private readonly serverName: string,
+    private readonly serverConfig: MCPServerConfig,
+    private readonly toolRegistry: ToolRegistry,
+    private readonly promptRegistry: PromptRegistry,
+    private readonly workspaceContext: WorkspaceContext,
+    private readonly debugMode: boolean,
+  ) {
+    this.client = new Client({
+      name: `gemini-cli-mcp-client-${this.serverName}`,
+      version: '0.0.1',
+    });
+  }
+
+  /**
+   * Connects to the MCP server.
+   */
+  async connect(): Promise<void> {
+    this.isDisconnecting = false;
+    this.updateStatus(MCPServerStatus.CONNECTING);
+    try {
+      this.transport = await this.createTransport();
+
+      this.client.onerror = (error) => {
+        if (this.isDisconnecting) {
+          return;
+        }
+        console.error(`MCP ERROR (${this.serverName}):`, error.toString());
+        this.updateStatus(MCPServerStatus.DISCONNECTED);
+      };
+
+      this.client.registerCapabilities({
+        roots: {},
+      });
+
+      this.client.setRequestHandler(ListRootsRequestSchema, async () => {
+        const roots = [];
+        for (const dir of this.workspaceContext.getDirectories()) {
+          roots.push({
+            uri: pathToFileURL(dir).toString(),
+            name: basename(dir),
+          });
+        }
+        return {
+          roots,
+        };
+      });
+
+      await this.client.connect(this.transport, {
+        timeout: this.serverConfig.timeout,
+      });
+
+      this.updateStatus(MCPServerStatus.CONNECTED);
+    } catch (error) {
+      this.updateStatus(MCPServerStatus.DISCONNECTED);
+      throw error;
+    }
+  }
+
+  /**
+   * Discovers tools and prompts from the MCP server.
+   */
+  async discover(): Promise<void> {
+    if (this.status !== MCPServerStatus.CONNECTED) {
+      throw new Error('Client is not connected.');
+    }
+
+    const prompts = await this.discoverPrompts();
+    const tools = await this.discoverTools();
+
+    if (prompts.length === 0 && tools.length === 0) {
+      throw new Error('No prompts or tools found on the server.');
+    }
+
+    for (const tool of tools) {
+      this.toolRegistry.registerTool(tool);
+    }
+  }
+
+  /**
+   * Disconnects from the MCP server.
+   */
+  async disconnect(): Promise<void> {
+    this.isDisconnecting = true;
+    if (this.transport) {
+      await this.transport.close();
+    }
+    this.client.close();
+    this.updateStatus(MCPServerStatus.DISCONNECTED);
+  }
+
+  /**
+   * Returns the current status of the client.
+   */
+  getStatus(): MCPServerStatus {
+    return this.status;
+  }
+
+  private updateStatus(status: MCPServerStatus): void {
+    this.status = status;
+    updateMCPServerStatus(this.serverName, status);
+  }
+
+  private async createTransport(): Promise<Transport> {
+    return createTransport(this.serverName, this.serverConfig, this.debugMode);
+  }
+
+  private async discoverTools(): Promise<DiscoveredMCPTool[]> {
+    return discoverTools(this.serverName, this.serverConfig, this.client);
+  }
+
+  private async discoverPrompts(): Promise<Prompt[]> {
+    return discoverPrompts(this.serverName, this.client, this.promptRegistry);
+  }
+}
+
 /**
  * Map to track the status of each MCP server within the core package
  */
@@ -117,7 +245,7 @@ export function removeMCPStatusChangeListener(
 /**
  * Update the status of an MCP server
  */
-function updateMCPServerStatus(
+export function updateMCPServerStatus(
   serverName: string,
   status: MCPServerStatus,
 ): void {