n8n Code Tool Skill

Expert guidance for writing code inside the n8n Custom Code Tool (@n8n/n8n-nodes-langchain.toolCode) — the AI-agent-callable tool, not the regular Code node.

⚠️ This is NOT the Code node

Same editor UI, completely different contract:

If you carry over Code-node habits, it fails with cryptic errors. This skill teaches the Code Tool's actual contract.

What This Skill Teaches

Core Concepts

1. Return a string — JSON.stringify() for structured output
2. Input lives in query (JS) or _query (Python)
3. No $fromAI() — doesn't exist in this sandbox
4. Unstructured vs structured input — when to add a JSON Schema
5. Tool name and description are the LLM-facing contract, not docs

Top 5 Errors This Skill Prevents

1. "Cannot assign to read only property 'name'..." — $fromAI() misuse
2. "Wrong output type returned" — returning [{json:{...}}]
3. "The response property should be a string, but it is an object" — unstringified object
4. AI never calls the tool — generic name or vague description
5. LLM sends malformed query — no schema, no example

Skill Activation

Activates when you:

• Build a Code Tool attached to an AI Agent
• Get "Wrong output type returned" or "No execution data available" errors
• Decide between unstructured query parsing and specifyInputSchema
• Wonder why $fromAI() or $helpers.httpRequest() don't work
• Choose between Code Tool, HTTP Request Tool, and toolWorkflow

Example queries:

• "Why is my Code Tool throwing 'Wrong output type returned'?"
• "How do I pass multiple parameters to a Code Tool?"
• "Does $fromAI work in @n8n/n8n-nodes-langchain.toolCode?"
• "What's the difference between Code Tool and the Code node?"
• "How do I use specifyInputSchema for structured tool input?"

File Structure

SKILL.md

Main skill content — loaded when the skill activates.

• Why Code Tool ≠ Code node (the cheat-sheet table)
• Quick-start JS and Python examples
• The two input modes: unstructured query vs structured schema
• Return-format rules
• Tool name and description as prompt engineering
• What's NOT in the sandbox ($input, $helpers, $fromAI, state)
• When to choose Code Tool vs toolWorkflow vs HTTP Request Tool
• Complete working example
• Quick-reference checklist

INPUT_SCHEMA.md

Structured-input deep dive — specifyInputSchema: true.

• Why schemas help (DynamicStructuredTool vs DynamicTool)
• Style A: fromJson (infer schema from an example, v≥1.3)
• Style B: manual (write the JSON Schema yourself)
• How query behaves with vs without schema
• Version compatibility
• Decision tree: when to stay unstructured, go structured, or jump to toolWorkflow

ERROR_PATTERNS.md

Full error catalog with exact strings, causes, and fixes.

• The three signature runtime errors
• AI-never-calls-tool diagnostic
• LLM-sends-malformed-query fixes
• Sandbox-missing-helper error
• Python-specific query vs _query
• Debugging tips

Quick Reference

Minimal JavaScript Code Tool

return `You asked: ${query}`;

Minimal Python Code Tool

return f"You asked: {_query}"

Return a structured result

return JSON.stringify({
  result: 42,
  currency: "SEK"
});

Parse a JSON-string input (unstructured mode)

const params = typeof query === 'string' ? JSON.parse(query) : query;
const price = Number(params.price);

Use a typed input (structured mode, specifyInputSchema: true)

const { price, months, residual_percent } = query;

Tool name rules

• [A-Za-z0-9_]+ — snake_case, no spaces/hyphens/emoji
• Verb-y and domain-specific: calculate_car_loan, not Code Tool

Integration with Other Skills

n8n-code-javascript (Code node): most JS patterns transfer, but I/O is different — don't copy $input.all() or [{json:{...}}] return.

n8n-node-configuration: specifyInputSchema is a typical conditional-field pattern — use get_node({detail: "standard"}) on toolCode to explore.

n8n-workflow-patterns: Code Tool sits inside the AI-Agent-with-tools pattern. Usually alongside HTTP Request Tool, toolWorkflow, and memory.

n8n-validation-expert: the three signature errors have exact strings that map cleanly to fixes — if you see them in validation output, the fix is mechanical.

When to Use Code Tool vs Alternatives

Rule of thumb: if you catch yourself reaching for $fromAI(), you want toolWorkflow instead.

Success Metrics

After using this skill, you should be able to:

• Distinguish Code Tool from Code node by node type and contract
• Return a string (or JSON.stringify() result) — never a bare object or items array
• Read input from query/_query without reaching for $fromAI
• Decide between unstructured (JSON-in-string) and structured (specifyInputSchema) patterns
• Write tool names/descriptions that the LLM will actually invoke
• Diagnose the three signature errors by message alone
• Pick the right tool type (Code Tool vs toolWorkflow vs HTTP Request Tool)

Sources

Authoritative facts in this skill come from:

• ToolCode source — sandbox contract, query binding, return handling
• n8n Custom Code Tool docs
• LangChain tool docs — DynamicTool / DynamicStructuredTool semantics

Version

Version: 1.0.0 Compatibility: n8n with @n8n/n8n-nodes-langchain.toolCode v1.1+; structured fromJson requires v≥1.3.

Credits

Part of the n8n-skills project.

Remember: Code Tool is a LangChain tool wearing a Code-node UI. Contract is string in, string out. Everything else follows from that.