Node Code Sandbox MCP

Instructions

Implementation Reference

• src/tools/runJs.ts:52-149 (handler)

  The main handler function that installs npm dependencies (if provided) and executes the given JavaScript code inside the specified Docker container. Handles both one-shot execution and long-running servers (with listenOnPort). Detects and reports file changes in the workspace.
  export default async function runJs({ container_id, code, dependencies = [], listenOnPort, }: { container_id: string; code: string; dependencies?: DependenciesArray; listenOnPort?: number; }): Promise<McpResponse> { if (!isDockerRunning()) { return { content: [textContent(DOCKER_NOT_RUNNING_ERROR)] }; } const telemetry: Record<string, unknown> = {}; const dependenciesRecord: Record<string, string> = Object.fromEntries( dependencies.map(({ name, version }) => [name, version]) ); // Create workspace in container const localWorkspace = await prepareWorkspace({ code, dependenciesRecord }); execSync(`docker cp ${localWorkspace.name}/. ${container_id}:/workspace`); let rawOutput: string = ''; // Generate snapshot of the workspace const snapshotStartTime = Date.now(); const snapshot = await getSnapshot(getMountPointDir()); if (listenOnPort) { if (dependencies.length > 0) { const installStart = Date.now(); const installOutput = execSync( `docker exec ${container_id} /bin/sh -c ${JSON.stringify( `npm install --omit=dev --prefer-offline --no-audit --loglevel=error` )}`, { encoding: 'utf8' } ); telemetry.installTimeMs = Date.now() - installStart; telemetry.installOutput = installOutput; } else { telemetry.installTimeMs = 0; telemetry.installOutput = 'Skipped npm install (no dependencies)'; } const { error, duration } = safeExecNodeInContainer({ containerId: container_id, command: `nohup node index.js > output.log 2>&1 &`, }); telemetry.runTimeMs = duration; if (error) return getContentFromError(error, telemetry); await waitForPortHttp(listenOnPort); rawOutput = `Server started in background; logs at /output.log`; } else { if (dependencies.length > 0) { const installStart = Date.now(); const fullCmd = `npm install --omit=dev --prefer-offline --no-audit --loglevel=error`; const installOutput = execSync( `docker exec ${container_id} /bin/sh -c ${JSON.stringify(fullCmd)}`, { encoding: 'utf8' } ); telemetry.installTimeMs = Date.now() - installStart; telemetry.installOutput = installOutput; } else { telemetry.installTimeMs = 0; telemetry.installOutput = 'Skipped npm install (no dependencies)'; } const { output, error, duration } = safeExecNodeInContainer({ containerId: container_id, }); if (output) rawOutput = output; telemetry.runTimeMs = duration; if (error) return getContentFromError(error, telemetry); } // Detect the file changed during the execution of the tool in the mounted workspace // and report the changes to the user const changes = await detectChanges( snapshot, getMountPointDir(), snapshotStartTime ); const extractedContents = await changesToMcpContent(changes); localWorkspace.removeCallback(); return { content: [ textContent(`Node.js process output:\n${rawOutput}`), ...extractedContents, textContent(`Telemetry:\n${JSON.stringify(telemetry, null, 2)}`), ], }; }
• src/tools/runJs.ts:26-48 (schema)

  Zod schema defining the input parameters for the run_js tool: container_id, dependencies (array of {name,version}), code, and optional listenOnPort.
  export const argSchema = { container_id: z.string().describe('Docker container identifier'), // We use an array of { name, version } items instead of a record // because the OpenAI function-calling schema doesn’t reliably support arbitrary // object keys. An explicit array ensures each dependency has a clear, uniform // structure the model can populate. // Schema for a single dependency item dependencies: z .array(NodeDependency) .default([]) .describe( 'A list of npm dependencies to install before running the code. ' + 'Each item must have a `name` (package) and `version` (range). ' + 'If none, returns an empty array.' ), code: z.string().describe('JavaScript code to run inside the container.'), listenOnPort: z .number() .optional() .describe( 'If set, leaves the process running and exposes this port to the host.' ), };
• src/server.ts:66-73 (registration)

  Registration of the 'run_js' tool on the MCP server using server.tool(), providing name, long description, input schema, and handler function.
  'run_js', `Install npm dependencies and run JavaScript code inside a running sandbox container. After running, you must manually stop the sandbox to free resources. The code must be valid ESModules (import/export syntax). Best for complex workflows where you want to reuse the environment across multiple executions. When reading and writing from the Node.js processes, you always need to read from and write to the "./files" directory to ensure persistence on the mounted volume.`, runJsSchema, runJs );