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QGIS MCP

QGIS MCP

by lefcgis
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
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Evaluate Expression

evaluate_expression
Read-only

Evaluate QGIS expressions to compute scalar values like aggregates, project variables, or dates. Optionally include a layer for context.

Instructions

Evaluate a standalone QGIS expression to a scalar value (e.g. aggregate('layer','sum','field'), @project_var, now()). Optional layer_id adds layer scope. Distinct from validate_expression (validate only) and field_calculator (per-feature).

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": {
      "expression": "<expression>"
    },
    "name": "evaluate_expression"
  }
}

Input Schema

NameRequiredDescriptionDefault
layer_idNo
expressionYes

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.5.0

TDQS

A4.6/5.0
Behavior4/5

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

Annotations indicate readOnlyHint=true. Description adds behavioral details: evaluates to scalar value, standalone, with optional layer scope. No contradictions. Lacks error handling details but sufficient.

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, well-structured, no redundant information. Front-loaded with core purpose and examples.

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?

Given no output schema, the description sufficiently explains tool behavior and usage. It covers purpose, parameters, and distinguishes from siblings. Minor gap: no mention of return value format, but implied as scalar.

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

Parameters4/5

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

Schema has no property descriptions (0% coverage). Description adds meaning: layer_id adds layer scope. Expression is required and standalone. Provides value beyond schema.

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?

Description clearly states the tool evaluates a standalone QGIS expression to a scalar value, with specific examples. It distinguishes itself from sibling tools validate_expression and field_calculator.

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

Usage Guidelines5/5

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

Explicitly mentions optional layer_id for layer scope and contrasts with alternatives for validation and per-feature calculation, guiding when to use.

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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