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Reaper Daemon

by wretcher207
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Python
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set_fx_param

Set a single FX parameter in REAPER using normalized, formatted, or relative values. Supports dry-run preview and wraps changes in an undo block for safe automation.

Instructions

Set one FX parameter. Give normalized_value (0-1), formatted_value (e.g. '-16.00 dB', '80 Hz' — the bridge binary-searches the normalized value whose display matches), or relative ('+0.1'). Scan with get_fx_parameters first and prefer param_index. Undo-block wrapped; supports dry_run.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
trackNoExact track name (case-insensitive); "master" targets the master track. Used when no GUID is given; errors if two tracks share the name.
dry_runNoPreview: return what would run without changing the project.
fx_indexNo
fx_scopeNo
relativeNo
param_indexNo
track_containsNoCase-insensitive substring; used only when no GUID or exact name is given. Errors if it matches more than one track.
formatted_valueNo
fx_name_containsNo
normalized_valueNo
target_track_guidNoStable REAPER track GUID; preferred when available. Wins over every other track selector.
use_selected_trackNoTarget the first selected track; ignored when any other track selector is given. Errors if nothing is selected.
param_name_containsNo

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed4 schema fields changedv3.21.0
    • changedInput schema / properties / target_track_guid / description
      Previous value: -"Stable REAPER track GUID; preferred when available."New value: +"Stable REAPER track GUID; preferred when available. Wins over every other track selector."
    • changedInput schema / properties / track / description
      Previous value: -"Exact track name (case-insensitive)."New value: +"Exact track name (case-insensitive); \"master\" targets the master track. Used when no GUID is given; errors if two tracks share the name."
    • changedInput schema / properties / track_contains / description
      Previous value: -"Case-insensitive substring; errors if it matches more than one track."New value: +"Case-insensitive substring; used only when no GUID or exact name is given. Errors if it matches more than one track."
    • changedInput schema / properties / use_selected_track / description
      Previous value: -"Target the currently selected track instead of naming one."New value: +"Target the first selected track; ignored when any other track selector is given. Errors if nothing is selected."
  2. First observedv0.1.0

TDQS

A4.1/5.0
Behavior4/5

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

With no annotations, the description carries the full behavioral disclosure burden. It usefully discloses undo-block wrapping, dry_run support, and the bridge's binary-search behavior for formatted_value matching. It does not describe conflict/error behavior when multiple value modes are provided, but the transparency is still strong.

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?

The description is compact and front-loaded: it states the action, then the value modes with concrete examples, then the workflow and safety features. Every sentence carries useful information with no repetition or 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 13-parameter polymorphic tool with no output schema and no annotations, the description gives the core semantics and workflow but leaves important gaps. It does not explain how to select the target FX via fx_index/fx_scope/fx_name_contains, nor what happens if multiple value or selector modes are provided, so an agent may still need to infer or experiment.

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?

Schema description coverage is only 38%, so the description must compensate for sparse parameter documentation. It adds meaningful semantics for normalized_value, formatted_value, relative, and param_index. However, key FX-targeting parameters such as fx_index, fx_scope, fx_name_contains, and param_name_contains remain undocumented in both schema and description.

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 opens with 'Set one FX parameter,' a specific verb and resource. It also names the companion workflow tool, get_fx_parameters, and clearly distinguishes the action from scanning or preset-level operations.

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

Usage Guidelines4/5

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

It gives explicit operational guidance: 'Scan with get_fx_parameters first and prefer param_index' and explains the three value-supply modes. It does not enumerate when-not-to-use alternatives like tune_param or set_fx_preset, but the intended workflow is clear.

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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