Code Mode MCP

Use JavaScript to discover and compose MCP tools through one agent-agnostic stdio server.

Code Mode exposes one model-facing tool, exec. A program can search configured MCP servers, inspect exact schemas, call tools and reduce intermediate results. Upstream schemas stay out of the model's initial context.

Choose Code Mode by task shape

Use Code Mode for stages that involve:

• large tool catalogs

• repeated calls followed by filtering or aggregation

• long deterministic chains

• large intermediate results that the model does not need to inspect

Keep direct tools available for short tasks, semantic decisions, approvals, errors and rich results. A workflow can switch between direct tools and Code Mode at each stage.

The direct tools and Code Mode benchmark explains this recommendation.

Related MCP server: programmatic-mcp

How it works

MCP client
  └─ exec({ code })
      └─ code-mode-mcp
          ├─ search and describe upstream tools
          ├─ call tools from JavaScript
          └─ return selected results

Code Mode supports:

• stdio, Streamable HTTP and legacy SSE upstream servers

• bearer authentication and OAuth

• cancellation, progress, elicitation, sampling, roots and logging

• text, image, audio, resource and structured results

• explicit JSON-only session state held in memory

Code Mode does not store tool results, screenshots, console output or intermediate values automatically.

Requirements

You need:

• Node.js 22 or newer

• an MCP client that can launch a stdio server

Install

Install version 0.4.0 from npm:

npm install --global @tmustier/code-mode-mcp@0.4.0
code-mode-mcp --help

Configure upstream servers

Create ~/.config/code-mode-mcp/mcp.json:

{
  "mcpServers": {
    "local": {
      "command": "node",
      "args": ["/absolute/path/to/server.js"]
    },
    "remote": {
      "url": "https://example.com/mcp",
      "auth": "bearer",
      "bearerTokenEnv": "EXAMPLE_MCP_TOKEN"
    }
  }
}

Check the file without starting MCP:

code-mode-mcp --check-config \
  --config ~/.config/code-mode-mcp/mcp.json

The check excludes commands, arguments, headers, tokens and environment values from its summary.

See configuration for transports, environment expansion, OAuth and config lookup.

Add Code Mode to an MCP client

Add the outer Code Mode server to your client's MCP configuration:

{
  "mcpServers": {
    "code-mode": {
      "command": "npx",
      "args": [
        "-y",
        "@tmustier/code-mode-mcp@0.4.0",
        "--config",
        "/Users/you/.config/code-mode-mcp/mcp.json"
      ]
    }
  }
}

Keep the upstream config separate. Do not configure Code Mode as its own upstream server.

Use the exec tool

The code field contains a JavaScript async function body:

{
  "code": "return search('app screenshot accessibility', { limit: 5 });",
  "session_id": "optional-session",
  "timeout_ms": 120000,
  "max_output_chars": 51200
}

The execution context provides:

• search() for ranked tool discovery

• describe() for exact schemas

• tools.<name>(args) and call(name, args) for tool calls

• ALL_TOOLS and ALL_SERVERS for bounded custom discovery

• text(), image() and emit() for output selection

• store(), load() and clearStore() for in-memory JSON state

• signal for cancellation

For example:

const apps = await tools.mcp__computer_use__list_apps({});
const selected = ["Calculator", "TextEdit"];
const states = await Promise.all(
  selected.map(app => tools.mcp__computer_use__get_app_state({ app }))
);

return states.map((state, index) => ({
  app: selected[index],
  text: state.content.find(block => block.type === "text")?.text.slice(0, 500)
}));

Code Mode preserves a complete MCP CallToolResult when you return it. Filter or aggregate large results inside the program to keep model context small.

See the exec API for discovery, canonical tool names, output limits and session state.

Understand host authority

Code passed to exec has the same authority as the Node.js process. It can access the filesystem, network, environment, processes and child processes.

node:vm controls execution and interrupts synchronous loops. It is not a security sandbox. Run Code Mode inside the operating system, container, account and credential boundary you want the agent to have.

See the security model before using Code Mode with sensitive systems.

Documentation

• configuration

• exec API

• architecture

• security model

• architecture decision record

• direct tools and Code Mode benchmark

Develop

npm ci
npm run check
npm test
npm run prepublishOnly
npm pack --dry-run