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QGIS MCP

QGIS MCP

by lefcgis
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
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Field Calculator

field_calculator

Add or populate a field in a QGIS layer by evaluating an expression for each feature in-place. Supports multiple field types including string, int, double, bool, date, and datetime.

Instructions

Add (if missing) + populate a field from a QGIS expression, per feature, in-place. field_type: string|int|double|bool|date|datetime (default double). Example: expression='$area', field_name='area_m2'. Returns updated feature count.

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": {
      "layer_id": "<layer_id>",
      "field_name": "<field_name>",
      "expression": "<expression>"
    },
    "name": "field_calculator"
  }
}

Input Schema

NameRequiredDescriptionDefault
lengthNo
layer_idYes
precisionNo
expressionYes
field_nameYes
field_typeNodouble

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.5.0

TDQS

B3.4/5.0
Behavior3/5

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

With no annotations, the description carries the full burden. It discloses that the operation is in-place, adds the field only if missing, processes per feature, and returns the updated count. However, it omits details about access rights, whether existing fields are overwritten (only 'if missing' is mentioned), and potential side effects (e.g., irreversible modification).

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

Conciseness4/5

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

The description is concise: two sentences and an example. The main action is in the first sentence, front-loading the purpose. Every part is useful, though the example could be integrated more naturally. No unnecessary words.

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

Completeness3/5

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

For a tool with 6 parameters, no output schema, and no annotations, the description covers the core functionality (adding and populating a field) but leaves gaps: parameters 'length' and 'precision' are undocumented, and the return value 'updated feature count' lacks type/format. Sibling tools are numerous, but the description doesn't explicitly differentiate from close ones like 'add_field' or 'evaluate_expression'.

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?

The description adds meaning for 'field_type' by listing allowed values (string, int, double, etc.) and default (double). It uses an example to clarify 'expression' and 'field_name'. However, 'length' and 'precision' (integer with default 0) are not explained at all, and schema coverage is 0%, so the description only partially compensates.

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 adds a field if missing and populates it using a QGIS expression per feature in-place. It lists valid field types and provides a concrete example, making the purpose unambiguous. It distinguishes from siblings like 'add_field' (which only adds) and 'evaluate_expression' (which only evaluates).

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 gives an example but offers no explicit guidance on when to use this tool versus alternatives like 'add_field', 'update_features', or 'evaluate_expression'. It does not state prerequisites, contraindications, or post-conditions.

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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