reaper-reapy-mcp

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set_fx_param

Set an FX parameter value in REAPER by specifying track, FX index, parameter name, and value. Adjust plugin settings programmatically.

Instructions

Set an FX parameter value.

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>",
      "param_name": "<param_name>",
      "value": "<value>"
    },
    "name": "set_fx_param"
  }
}

Input Schema

Name	Required	Description	Default
`value`	Yes
`fx_index`	Yes
`param_name`	Yes
`track_index`	Yes

Output Schema

Name	Required	Description	Default
`result`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

First observedv0.1.4Aug 7, 2026

TDQS

C2.3/5.0

Behavior2/5

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

With no annotations provided, the description itself must convey behavioral traits. It implies a mutating operation but does not disclose side effects, requirements (e.g., valid fx_index), or response format. This is a significant gap for a setter tool.

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

Conciseness2/5

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

The one-sentence description is concise with no fluff, but it is severely under-specified for a tool with four parameters. It restates the tool's name almost verbatim and does not earn its place as a useful explanation.

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

Completeness1/5

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

Given the tool's complexity (four required parameters, no annotations, and an output schema whose semantics are not described), the one-line description is completely inadequate. It fails to explain indexing conventions, value formats, or even that the FX must already exist, leaving the agent to guess.

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

Parameters1/5

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

The input schema has four required parameters with zero descriptions (0% coverage), and the description adds no parameter explanations. The agent is left without any guidance on what values are valid or how parameters relate, making this dimension very weak.

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?

The description 'Set an FX parameter value' clearly states the action (set) and the resource (FX parameter), distinguishing it from related tools like get_fx_param or toggle_fx. However, it does not explicitly mention scope or differentiate from siblings by name, so it falls short of a perfect score.

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 guidance on when to use this tool versus alternatives, nor any prerequisites such as needing an FX to exist on the track. The description provides no contextual usage information beyond the basic action.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.