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SapperMCP

by MasterX1582
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Python
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Capture Screen Region

capture_screen_region

Captures a specified screen area using x/y coordinates and width/height, returning a PNG image for visual inspection or debugging.

Instructions

Capture a screen region (x,y top-left, width/height px); returns a PNG image.

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": {
      "x": "<x>",
      "y": "<y>",
      "width": "<width>",
      "height": "<height>"
    },
    "name": "capture_screen_region"
  }
}

Input Schema

NameRequiredDescriptionDefault
xYes
yYes
widthYes
heightYes

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.1.0

TDQS

B3.4/5.0
Behavior3/5

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

With no annotations and no output schema, the description carries the full burden, and it does disclose the one thing an agent most needs: the return is a PNG image. It does not disclose how the image is delivered (inline content vs base64/text), permission requirements, or behavior for out-of-bounds or multi-monitor coordinates.

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 short clauses, front-loaded with the action and followed by the parameter meaning and return type. Nothing is redundant and no sentence is filler.

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 four-required-parameter capture tool with no annotations and no output schema, the description covers the essentials (what it captures, the geometry parameters, and the PNG return). It leaves gaps around delivery format, screen-capture permissions, and error conditions that matter for correct invocation.

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 description coverage is 0%, so the description must compensate, and it does: x/y are identified as the top-left origin and width/height as pixels, covering all four required parameters. It stops short of specifying the coordinate origin (whole desktop vs active monitor) or accepted ranges.

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

Purpose4/5

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

States a specific verb and resource ('Capture a screen region') and immediately qualifies the scope with the coordinate/width/height parameters, which implicitly distinguishes it from a full-screen capture. It does not name capture_screen or capture_screen_to_file, so the differentiation is left to inference rather than stated.

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?

There is no when-to-use guidance and no mention of alternatives such as capturing the whole screen or writing to a file. The agent must infer from the tool name alone that this is the right choice when only a sub-rectangle is needed.

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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