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n8n MCP Server

n8n MCP Server

by verzth
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TypeScript
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Update n8n Variable

n8n_update_variable
Idempotent

Update a workflow variable's value in n8n by specifying its ID, key, and new value, enabling programmatic variable changes.

Instructions

Update a workflow variable's value.

Args:

  • id (string): Variable ID

  • key (string): Variable key/name

  • value (string): New variable value

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "id": "<id>",
      "key": "<key>",
      "value": "<value>"
    },
    "name": "n8n_update_variable"
  }
}

Input Schema

NameRequiredDescriptionDefault
idYesVariable ID
keyYesVariable key/name
valueYesNew variable value

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv1.0.0

TDQS

A3.6/5.0
Behavior3/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint=false, destructiveHint=false, and idempotentHint=true, which outline the safety profile. The description adds no behavioral details beyond the action 'update'—for example, whether it overwrites the entire value, requires an existing variable, or returns the updated object. It neither contradicts the annotations nor enriches them, so a mid-range score is appropriate.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is extremely concise: a one-sentence purpose followed by a clean, bullet-style Args list. Every word earns its place, and the structure front-loads the core action before parameter details. No redundant fluff exists.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a simple update tool with only three required parameters, no output schema, and annotations covering safety semantics, the description is largely sufficient. It states the operation and lists all arguments, though it lacks any mention of return values or when a variable must already exist. This minor gap keeps it from a perfect score.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, and the description repeats the same parameter descriptions as the schema (id, key, value). No additional meaning is added regarding value format, key semantics, or relationship between id and key. Since the schema already fully documents parameters, the baseline of 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool's action: 'Update a workflow variable's value.' This specifies a distinct verb (update) and resource (workflow variable), and the sibling set includes create and delete variants, so it is clearly differentiated from alternatives.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines2/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides no guidance on when to use this tool versus creating or deleting a variable. It does not mention any prerequisites, such as needing an existing variable, nor does it reference sibling tools for alternative actions. The 'Args' section only restates parameter names, not usage context.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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