PoC: Simulating Code Execution via MCP Meta-Tools

Author: yannbonzom

README.md

@@ -784,6 +784,39 @@ const transport = new StdioServerTransport();
 await server.connect(transport);
 ```
 
+### Code Mode Wrapper (experimental)
+
+You can also run a lightweight “code-mode” wrapper that proxies multiple MCP servers through a single stdio endpoint and only loads tool definitions on demand. Create a config file (for example `code-config.mcp-servers.json`) inside your local checkout of this SDK:
+
+```json
+{
+    "downstreams": [
+        {
+            "id": "playwright",
+            "description": "Browser automation via Playwright",
+            "command": "node",
+            "args": ["/Users/you/Desktop/playwright-mcp/cli.js", "--headless", "--browser=chromium"]
+        }
+    ]
+}
+```
+
+Then launch the wrapper:
+
+```bash
+pnpm code-mode -- --config ./code-config.mcp-servers.json
+```
+
+Point your MCP client (Cursor, VS Code, etc.) at the wrapper command instead of individual servers. The wrapper publishes four meta-tools:
+
+1. `list_mcp_servers` — enumerate the configured downstream servers (IDs + descriptions).
+2. `list_tool_names` — requires a `serverId` and returns just the tool names/descriptions for that server.
+3. `get_tool_implementation` — loads the full schema and a generated TypeScript stub for a specific tool.
+4. `call_tool` — proxies the downstream tool call unchanged.
+
+This mirrors the progressive disclosure workflow described in Anthropic’s [Code execution with MCP: Building more efficient agents](https://www.anthropic.com/engineering/code-execution-with-mcp): models explore servers first, then drill into tools only when needed. You can keep
+many MCP servers configured without loading all of their schemas into the prompt, and the LLM pays the context cost only for the server/tool it decides to use.
+
 ### Testing and Debugging
 
 To test your server, you can use the [MCP Inspector](https://github.com/modelcontextprotocol/inspector). See its README for more information.

package.json

@@ -75,7 +75,8 @@
         "test:watch": "vitest",
         "start": "npm run server",
         "server": "tsx watch --clear-screen=false scripts/cli.ts server",
-        "client": "tsx scripts/cli.ts client"
+        "client": "tsx scripts/cli.ts client",
+        "code-mode": "tsx scripts/code-mode.ts"
     },
     "dependencies": {
         "ajv": "^8.17.1",

scripts/code-mode.ts

@@ -0,0 +1,119 @@
+import { readFile } from 'node:fs/promises';
+import path from 'node:path';
+import process from 'node:process';
+import { CodeModeWrapper } from '../src/code-mode/index.js';
+import type { DownstreamConfig } from '../src/code-mode/downstream.js';
+import { StdioServerTransport } from '../src/server/stdio.js';
+import type { Implementation } from '../src/types.js';
+
+type CodeModeConfig = {
+    server?: Implementation;
+    downstreams: DownstreamConfig[];
+};
+
+function parseArgs(argv: string[]): string | undefined {
+    for (let i = 0; i < argv.length; i += 1) {
+        const current = argv[i];
+        if (current === '--config' || current === '-c') {
+            return argv[i + 1];
+        }
+
+        if (current?.startsWith('--config=')) {
+            return current.split('=')[1];
+        }
+    }
+
+    return undefined;
+}
+
+function assertDownstreamConfig(value: unknown): asserts value is DownstreamConfig[] {
+    if (!Array.isArray(value) || value.length === 0) {
+        throw new Error('Config must include a non-empty "downstreams" array.');
+    }
+
+    for (const entry of value) {
+        if (!entry || typeof entry !== 'object') {
+            throw new Error('Invalid downstream entry.');
+        }
+
+        const { id, command, args, env, cwd } = entry as DownstreamConfig;
+        if (!id || typeof id !== 'string') {
+            throw new Error('Each downstream requires a string "id".');
+        }
+
+        if (!command || typeof command !== 'string') {
+            throw new Error(`Downstream "${id}" is missing a "command".`);
+        }
+
+        if (args && !Array.isArray(args)) {
+            throw new Error(`Downstream "${id}" has invalid "args"; expected an array.`);
+        }
+
+        if (env && typeof env !== 'object') {
+            throw new Error(`Downstream "${id}" has invalid "env"; expected an object.`);
+        }
+
+        if (cwd && typeof cwd !== 'string') {
+            throw new Error(`Downstream "${id}" has invalid "cwd"; expected a string.`);
+        }
+    }
+}
+
+async function readConfig(configPath: string): Promise<CodeModeConfig> {
+    const resolved = path.resolve(process.cwd(), configPath);
+    const raw = await readFile(resolved, 'utf8');
+    const parsed = JSON.parse(raw);
+
+    assertDownstreamConfig(parsed.downstreams);
+
+    return {
+        server: parsed.server,
+        downstreams: parsed.downstreams
+    };
+}
+
+function printUsage(): void {
+    console.log('Usage: npm run code-mode -- --config ./code-mode.config.json');
+}
+
+async function main(): Promise<void> {
+    const configPath = parseArgs(process.argv.slice(2));
+    if (!configPath) {
+        printUsage();
+        process.exitCode = 1;
+        return;
+    }
+
+    const config = await readConfig(configPath);
+    const wrapper = new CodeModeWrapper({
+        serverInfo: config.server,
+        downstreams: config.downstreams
+    });
+
+    const transport = new StdioServerTransport();
+    await wrapper.connect(transport);
+    console.log('Code Mode wrapper is running on stdio.');
+
+    let shuttingDown = false;
+    const shutdown = async () => {
+        if (shuttingDown) {
+            return;
+        }
+
+        shuttingDown = true;
+        await wrapper.close();
+    };
+
+    process.on('SIGINT', () => {
+        void shutdown().finally(() => process.exit(0));
+    });
+
+    process.on('SIGTERM', () => {
+        void shutdown().finally(() => process.exit(0));
+    });
+}
+
+main().catch(error => {
+    console.error(error);
+    process.exit(1);
+});

src/code-mode/downstream.ts

@@ -0,0 +1,112 @@
+import { Client } from '../client/index.js';
+import { StdioClientTransport, getDefaultEnvironment } from '../client/stdio.js';
+import { CallToolResultSchema, ToolListChangedNotificationSchema } from '../types.js';
+import type { CallToolResult, Implementation, Tool } from '../types.js';
+
+export type DownstreamConfig = {
+    id: string;
+    command: string;
+    args?: string[];
+    env?: Record<string, string>;
+    cwd?: string;
+    description?: string;
+};
+
+export interface DownstreamHandle {
+    readonly config: DownstreamConfig;
+    listTools(): Promise<Tool[]>;
+    getTool(toolName: string): Promise<Tool | undefined>;
+    callTool(toolName: string, args?: Record<string, unknown>): Promise<CallToolResult>;
+    close(): Promise<void>;
+}
+
+export class DefaultDownstreamHandle implements DownstreamHandle {
+    private readonly _clientInfo: Implementation;
+    private _client?: Client;
+    private _toolsCache?: Tool[];
+    private _listPromise?: Promise<Tool[]>;
+
+    constructor(
+        private readonly _config: DownstreamConfig,
+        clientInfo: Implementation
+    ) {
+        this._clientInfo = clientInfo;
+    }
+
+    get config(): DownstreamConfig {
+        return this._config;
+    }
+
+    async listTools(): Promise<Tool[]> {
+        if (this._toolsCache) {
+            return this._toolsCache;
+        }
+
+        if (this._listPromise) {
+            return this._listPromise;
+        }
+
+        this._listPromise = this._ensureClient()
+            .then(async client => {
+                const result = await client.listTools();
+                this._toolsCache = result.tools;
+                return this._toolsCache;
+            })
+            .finally(() => {
+                this._listPromise = undefined;
+            });
+
+        return this._listPromise;
+    }
+
+    async getTool(toolName: string): Promise<Tool | undefined> {
+        const tools = await this.listTools();
+        return tools.find(tool => tool.name === toolName);
+    }
+
+    async callTool(toolName: string, args?: Record<string, unknown>): Promise<CallToolResult> {
+        const client = await this._ensureClient();
+        return client.callTool(
+            {
+                name: toolName,
+                arguments: args
+            },
+            CallToolResultSchema
+        ) as Promise<CallToolResult>;
+    }
+
+    async close(): Promise<void> {
+        await this._client?.close();
+        this._client = undefined;
+        this._toolsCache = undefined;
+    }
+
+    private async _ensureClient(): Promise<Client> {
+        if (this._client) {
+            return this._client;
+        }
+
+        const transport = new StdioClientTransport({
+            command: this._config.command,
+            args: this._config.args,
+            env: {
+                ...getDefaultEnvironment(),
+                ...this._config.env
+            },
+            cwd: this._config.cwd
+        });
+
+        const client = new Client({
+            name: `code-mode:${this._config.id}`,
+            version: '0.1.0'
+        });
+
+        await client.connect(transport);
+        client.setNotificationHandler(ToolListChangedNotificationSchema, () => {
+            this._toolsCache = undefined;
+        });
+
+        this._client = client;
+        return client;
+    }
+}

src/code-mode/index.ts

@@ -0,0 +1,13 @@
+export { CodeModeWrapper, type CodeModeWrapperOptions } from './wrapper.js';
+export type { DownstreamConfig, DownstreamHandle } from './downstream.js';
+export {
+    ListToolNamesInputSchema,
+    ListToolNamesOutputSchema,
+    type ListToolNamesResult,
+    type ToolSummary,
+    GetToolImplementationInputSchema,
+    GetToolImplementationOutputSchema,
+    type GetToolImplementationResult,
+    CallToolInputSchema,
+    type CallToolInput
+} from './metaTools.js';

src/code-mode/metaTools.ts

@@ -0,0 +1,54 @@
+import { z } from 'zod';
+
+export const ServerSummarySchema = z.object({
+    serverId: z.string(),
+    description: z.string().optional()
+});
+
+export const ListMcpServersOutputSchema = z.object({
+    servers: z.array(ServerSummarySchema)
+});
+
+export type ListMcpServersResult = z.infer<typeof ListMcpServersOutputSchema>;
+
+export const ToolSummarySchema = z.object({
+    serverId: z.string(),
+    toolName: z.string(),
+    description: z.string().optional()
+});
+
+export const ListToolNamesInputSchema = z.object({
+    serverId: z.string()
+});
+
+export const ListToolNamesOutputSchema = z.object({
+    tools: z.array(ToolSummarySchema)
+});
+
+export type ToolSummary = z.infer<typeof ToolSummarySchema>;
+export type ListToolNamesResult = z.infer<typeof ListToolNamesOutputSchema>;
+
+export const GetToolImplementationInputSchema = z.object({
+    serverId: z.string(),
+    toolName: z.string()
+});
+
+export const GetToolImplementationOutputSchema = z.object({
+    serverId: z.string(),
+    toolName: z.string(),
+    signature: z.string(),
+    description: z.string().optional(),
+    annotations: z.record(z.unknown()).optional(),
+    inputSchema: z.record(z.unknown()).optional(),
+    outputSchema: z.record(z.unknown()).optional()
+});
+
+export type GetToolImplementationResult = z.infer<typeof GetToolImplementationOutputSchema>;
+
+export const CallToolInputSchema = z.object({
+    serverId: z.string(),
+    toolName: z.string(),
+    arguments: z.record(z.unknown()).optional()
+});
+
+export type CallToolInput = z.infer<typeof CallToolInputSchema>;