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QGIS MCP

QGIS MCP

by lefcgis
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
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Set Project Variable

set_project_variable
Idempotent

Set project-level variables accessible in QGIS expressions as @key to store and reuse values across your project.

Instructions

Set a project-level variable. Variables are accessible in expressions as @key.

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": {
      "key": "<key>",
      "value": "<value>"
    },
    "name": "set_project_variable"
  }
}

Input Schema

NameRequiredDescriptionDefault
keyYes
valueYes

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.5.0

TDQS

B3.1/5.0
Behavior3/5

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

Annotations already indicate idempotency. The description adds context about expression usage (@key) but does not disclose other behavioral traits like overwrite behavior, persistence scope, or required permissions. It adds some value beyond annotations.

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?

Two sentences, no filler, directly communicates the core functionality and usage hint. Every word is purposeful.

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

Completeness2/5

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

The description lacks details on idempotent behavior (e.g., overwriting existing keys), scope (project-level), and retrieval. Given no output schema and minimal schema coverage, the description is insufficient for complete understanding.

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

Parameters1/5

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

Schema coverage is 0%, so the description must compensate for lack of parameter documentation. It provides no details about key format, value constraints, or allowed characters, leaving the agent without guidance on valid inputs.

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 sets a project-level variable and explains the syntax for access in expressions. It unambiguously identifies the action and resource, distinguishing it from sibling tools like get_project_variables.

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?

No guidance on when to use this tool versus alternatives, such as other variable-related tools. There is no mention of prerequisites, when to avoid use, or comparison with get_project_variables.

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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