N8n code mode

Code-Mode — Code-First n8n Runtime

Code-mode collapses sequential LLM tool calls into a single sandboxed TypeScript execution. Instead of an AI agent making 5 serial HTTP-request tool calls (5 round trips, 5x context), it writes one TypeScript block that calls all 5 — executed in a V8 isolate sandbox.

Benchmark result: 96% token savings on a 5-tool pipeline (4,847 → 202 tokens).

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Architecture

@utcp/code-mode (upstream)        @code-mode/core (engine)         Wrappers
──────────────────────          ─────────────────────          ──────────
CodeModeUtcpClient              CodeModeEngine                 n8n community node
  - V8 sandbox (isolated-vm)     - Setup caching (FIFO/16)     - supplyData() thin wrapper
  - UTCP tool registration        - Execution tracing           - Sibling auto-register
  - TypeScript compilation         - External tool composition   - MCP presets UI
                                   - Collision-safe bridges
                                   - Security hardening        MCP server (code-mode-tools)
                                                                - Standalone stdio server
                                                                - tools.json config

Package relationships

@utcp/[email protected]         <- upstream (raw sandbox + UTCP client)
    wraps
@code-mode/core                 <- production engine (caching, tracing, composition)
    consumed by
[email protected]  <- n8n community node
[email protected]          <- CLI + MCP server (any MCP client)

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Sandbox Rules (CRITICAL)

n8n Code nodes and code-mode both run in sandboxed environments with strict restrictions.

n8n toolCode sandbox

• No fetch — not available in sandbox
• No require('http') — blocked
• No require('fs') — blocked
• Only this.helpers.httpRequest works for HTTP calls
• query variable with specifyInputSchema: true — query is an object {query: "..."}, not a string. Access via query.query
• Sibling tool args — use args ?? {} not args || {} (falsy primitives like 0 or "" are valid)

code-mode V8 isolate sandbox

• Runs in isolated-vm — separate V8 context, no Node.js APIs
• Tool calls go through registered bridges (MCP, HTTP, sibling)
• TypeScript compiled at setup time, cached via FIFO (16 entries)
• Timeout and memory limits configurable per execution

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Lazy Imports (CRITICAL for Windows n8n)

All heavy imports in the n8n node MUST be lazy — await import() inside supplyData(). Eager top-level imports crash n8n on Windows because isolated-vm native addon gets loaded at n8n startup.

// WRONG — crashes n8n on Windows
import { CodeModeEngine } from '@code-mode/core';
import { Client } from '@utcp/mcp';
import { z } from 'zod';

// CORRECT — lazy import inside supplyData()
async supplyData() {
  const { CodeModeEngine } = await import('@code-mode/core');
  const { Client } = await import('@utcp/mcp');
  const { z } = await import('zod');
  // ...
}

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Sibling Auto-Register (v2.1+)

The n8n code-mode node automatically discovers sibling tool nodes connected to the same AI Agent and registers them in the sandbox. The AI agent can call both:

• MCP/manual tools registered via toolSources config
• Sibling n8n tool nodes connected via the n8n canvas

[HTTP Request Tool] ─┐
[Calculator Tool]  ──┤── AI Agent ── Code-Mode Tool
[Custom Tool]     ───┘       ↑
                     siblingAdapter.ts converts
                     LangChain DynamicTool → ToolLike[] + CallToolFn

Engine API for external tools:

const result = await engine.execute(code, {
  externalTools: siblingToolDescriptions,
  externalCallToolFn: callSiblingTool,
});

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LLM Gotchas for n8n AI Agents

Model selection

• Never Sonnet for n8n agents — $10 burned in 8 tool calls once. Use Haiku max ($0.80/$4 per 1M tokens)
• Gemini 2.0/2.5 Flash + n8n tools = broken — sends null tool arguments in toolCode. Use Claude Haiku instead
• OpenRouter model IDs: anthropic/claude-haiku-4-5 works. anthropic/claude-3.5-sonnet is dead

n8n LLM node config

• lmChatOpenAi typeVersion 1 — accepts plain string model IDs
• lmChatOpenAi typeVersion 1.3 — requires { mode: 'list', value: 'model-id' } object
• Context overflow — Haiku's 200k context overflows if agent lists all workflows. System prompt must skip listing

Credential references

• OpenRouter uses openAiApi credential type with custom base URL
• Google Gemini uses googlePalmApi credential type
• Always get credential ID from npx --yes n8nac credential list --json

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MCP Server Setup

code-mode-tools exposes execute_code_chain and list_available_tools as MCP tools:

{
  "mcpServers": {
    "code-mode": {
      "command": "node",
      "args": ["/path/to/code-mode-tools/dist/index.js"],
      "env": { "TOOLS_CONFIG": "/path/to/tools.json" }
    }
  }
}

Works with: Claude Desktop, Claude Code, Cursor, Windsurf, any MCP client.

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MCP Tool Sources (Known limitation)

MCP tool sources work in the source repo but spawn Linux child processes. When n8n runs on Windows and code-mode spawns an MCP server, it fails because the child process is Linux.

Workaround: Use n8n-native toolCode with this.helpers.httpRequest instead of MCP tool sources when n8n runs on Windows.

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Engine API Quick Reference

const engine = await CodeModeEngine.create();

// Register tool sources
await engine.registerToolSource({
  name: "fs",
  call_template_type: "mcp",
  config: { mcpServers: { filesystem: { transport: "stdio", command: "node", args: [...] } } }
});

// Execute code in sandbox
const result = await engine.execute(code, {
  timeout: 30000,
  memoryLimit: 128,
  enableTrace: true,           // ToolCallRecord[] with timing
  externalTools: [...],        // v2.1: merge additional tools
  externalCallToolFn: fn       // v2.1: route external tool calls
});

// Get LLM-ready tool description
const description = engine.getToolDescription();

engine.close();

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Build Commands

# n8n community node (monorepo)
cd n8n-nodes-utcp-codemode && npm run build && npm test  # 78 tests

# code-mode-tools (CLI + MCP server)
cd code-mode-tools && npm run build && npm test  # 44 tests

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npm Packages

Package	Description
n8n-nodes-utcp-codemode	n8n community node
code-mode-tools	CLI + MCP server
@code-mode/core	Platform-agnostic engine (internal)