Code Tool Error Patterns

The most common failure modes for @n8n/n8n-nodes-langchain.toolCode, with exact error strings, root causes, and fixes.

Error 1: "Cannot assign to read only property 'name' of object: Error: No execution data available"

Full message (wrapped by n8n):

There was an error: "Cannot assign to read only property 'name' of object 'Error: No execution data available'"

Cause: Calling $fromAI() inside the Code Tool sandbox. $fromAI() is a helper intended for other tool-enabled nodes (HTTP Request Tool, SendGrid Tool, toolWorkflow) where AI-supplied values flow through workflow execution data. The Code Tool sandbox has no execution data — it receives input directly via query. The helper throws, n8n tries to annotate the error's name property, and that assignment fails because the error object is frozen.

Fix: remove $fromAI(). Read from query (or define an input schema, see INPUT_SCHEMA.md).

// ❌ Broken
const price = $fromAI('price', 'Car price in SEK', 'number');

// ✅ Unstructured — parse a JSON string
const params = JSON.parse(query);
const price = Number(params.price);

// ✅ Structured — with specifyInputSchema: true
const { price } = query;

Error 2: "Wrong output type returned"

Cause: You returned the workflow item format ([{json: {...}}]) from the Code Tool. That format is for regular Code nodes; tools follow the LangChain contract and must return a string.

Fix: return a string. For structured output, stringify:

// ❌ Broken
return [{ json: { monthly_payment: 5405 } }];

// ✅ Fixed
return JSON.stringify({ monthly_payment: 5405 });

Error 3: "The response property should be a string, but it is an <type>"

Where <type> is object, undefined, function, etc.

Cause: You returned a bare object, array, or nothing at all.

Numbers are fine — n8n auto-converts them to strings:

return 42;  // ✅ becomes "42"

Booleans are NOT auto-converted — stringify explicitly:

return String(someBoolean);  // ✅
return JSON.stringify(someBoolean);  // ✅

Error 4: AI never calls the tool

Symptom: the agent answers from its own reasoning and ignores the tool. No tool invocation shows up in the execution trace.

Common causes and fixes:

1. Generic name. Default names like Code Tool or My Tool give the LLM no signal.

   • Fix: rename to verb-y, domain-specific snake_case: calculate_car_loan, search_orders, lookup_customer.

2. Description doesn't state the trigger. "Calculates things" is too vague.

   • Fix: explicitly list the user intents that should invoke the tool. "Use this whenever the user asks about monthly cost, loan breakdown, or total interest."

3. Tool isn't wired. The node sits in the canvas but isn't connected to the AI Agent's ai_tool input.

   • Fix: connect it. Check the workflow JSON connections block has "<tool_name>": { "ai_tool": [[{ "node": "AI Agent", "type": "ai_tool", "index": 0 }]] }.

4. Name violates [A-Za-z0-9_]+. Spaces, hyphens, and emoji in the tool name cause silent skip on v1.1+.

   • Fix: rename to snake_case_only.

Error 5: LLM sends malformed query

Symptom: your JSON.parse(query) throws, or fields come through as wrong types.

Causes:

• You're in unstructured mode and the description is ambiguous, so the LLM invents a format.
• You asked for a JSON string but the LLM sent a natural-language sentence.
• Numeric fields arrive as strings because the LLM serialized them that way.

Fixes, in order of preference:

1. Switch to structured mode. Set specifyInputSchema: true and define fields. The LLM now gets a typed schema and n8n validates before your code runs.

2. Give a concrete example in the description. LLMs imitate examples well:

   Copy

   Call with a single JSON string. Example:
   {"price":439900,"down_payment":87980,"interest_rate":6.95}
   

3. Coerce defensively:

   javascriptCopy

   const params = JSON.parse(query);
   const price = Number(params.price);
   if (!isFinite(price)) throw new Error('price must be numeric');
   

Error 6: "$helpers is not defined" / "$input is not defined"

Cause: you assumed the Code Tool sandbox exposes the same helpers as the Code node. It doesn't.

Unavailable in Code Tool:

• $input, $json, $binary
• $node["OtherNode"]
• $helpers.httpRequest()
• $jmespath()
• this.getContext(...), $getWorkflowStaticData(...)
• $fromAI()

Fix:

• Pure computation? Stay in Code Tool, use plain JS.
• Need HTTP? Move to HTTP Request Tool (with $fromAI() in URL/body).
• Need other-node data or credentials? Move to Call Sub-workflow Tool (toolWorkflow) — its sub-workflow has a full Code node sandbox.
• Need state across calls? Not possible in Code Tool. Use a sub-workflow that reads/writes a Data Table, Redis, etc.

Error 7: Python-specific — "name 'query' is not defined"

Cause: in Python, the input variable is _query (underscore prefix), not query.

# ❌ Broken
result = process(query)

# ✅ Fixed
result = process(_query)

Error Prevention Checklist

Before saving a Code Tool:

• Tool name is snake_case, descriptive, and unique
• Description tells the LLM when to call it, with an example if unstructured
• No $fromAI() in the code body
• No $input, $json, $helpers — not in this sandbox
• Input read from query (JS) or _query (Python)
• All code paths return a string (or a number that auto-converts)
• If returning structured data, wrapped in JSON.stringify(...)
• Wired to an AI Agent via ai_tool connection
• For multi-field input: either example JSON in description, or specifyInputSchema: true

Debugging tips

• Use the Execution view, not just the test output. The agent's tool invocation and raw input/output are visible there — you can see exactly what query the LLM sent.
• Log inside the tool by including fields in the returned JSON:
  javascriptCopy

  return JSON.stringify({ received_query: query, result: /* ... */ });
  

  The LLM sees the echo, and you can spot malformed input.
• Test the tool without the LLM by temporarily turning the tool node into a standalone Code node with hard-coded query, running it manually, then swapping back.