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reaper-mcp

reaper-mcp

by T-Rzeznik
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Python
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reaper_set_fx_preset

Idempotent

Set an FX on a track to a specific preset using its name or index. Use with reaper_list_fx_presets to discover valid presets.

Instructions

Switch an FX to a preset by name or by index.

Provide exactly one of preset_name or preset_index. Use reaper_list_fx_presets to discover valid values.

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": {
      "track_index": "<track_index>",
      "fx_index": "<fx_index>"
    },
    "name": "reaper_set_fx_preset"
  }
}

Input Schema

NameRequiredDescriptionDefault
track_indexYes0-based track index
fx_indexYes0-based FX index within the track chain
preset_nameNoPreset name to select (use this OR preset_index)
preset_indexNoPreset index to select; -1 means 'use preset_name instead'

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.1.0

TDQS

A4/5.0
Behavior3/5

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

Warns about providing exactly one of preset_name or preset_index, which is a behavioral constraint. Annotations already indicate idempotent and open-world behavior; description adds no additional behavioral traits beyond that.

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 concise sentences: first states purpose, second adds necessary usage instruction. No redundant information.

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?

Covers the main action and parameter constraint well. Lacks mention of return value or error handling, but adequate for an agent to select and invoke correctly.

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?

Adds value by clarifying the mutual exclusivity of preset_name and preset_index, and by referencing a sibling tool for discovering valid values. Schema coverage is 100%, but the description enhances understanding.

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 states a specific verb ('switch'), resource ('FX'), and target ('to a preset by name or by index'). It clearly distinguishes from sibling tools that set parameters or enable/disable FX.

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

Usage Guidelines3/5

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

Provides a prerequisite ('Use reaper_list_fx_presets to discover valid values'), but does not explicitly state when to use this tool versus alternatives like reaper_set_fx_param or reaper_set_fx_enabled.

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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