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by danielkinahan
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set_fx_preset

Load a preset for an FX plugin on a track in REAPER. Specify track index, FX index, and preset name or file path to apply the desired effect configuration.

Instructions

Load a named preset for an FX plugin on a track. preset_name can be:

  • A plain preset name or index-based name (for plugins with internal preset banks)

  • A full absolute file path to a .ffp/.fxp/.fxb file (FabFilter and others). Use the 'path' field from list_fx_presets() for file-based plugins. On failure, returns loaded=false with a failure_reason: 'preset_name_not_found' - name not in the plugin's preset list 'plugin_rejected_state' - plugin returned false despite the name existing 'plugin_has_no_presets' - plugin exposes no preset bank at all 'file_unreadable' - path given but file could not be opened

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
track_indexYes
fx_indexYes
preset_nameYes

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault

No arguments

Implementation Reference

  • src/reaper_mcp/server.py:561-577 (handler)
    The MCP tool handler for set_fx_preset in server.py, which wraps the adapter call.
    def set_fx_preset(track_index: int, fx_index: int, preset_name: str) -> dict[str, Any]:
    """
    Load a named preset for an FX plugin on a track.
    preset_name can be:
    - A plain preset name or index-based name (for plugins with internal preset banks)
    - A full absolute file path to a .ffp/.fxp/.fxb file (FabFilter and others).
      Use the 'path' field from list_fx_presets() for file-based plugins.
    On failure, returns loaded=false with a failure_reason:
      'preset_name_not_found'   - name not in the plugin's preset list
      'plugin_rejected_state'   - plugin returned false despite the name existing
      'plugin_has_no_presets'   - plugin exposes no preset bank at all
      'file_unreadable'         - path given but file could not be opened
    """
    try:
        return _wrap(
            adapter.set_fx_preset(
                track_index=track_index, fx_index=fx_index, preset_name=preset_name
  • src/reaper_mcp/reaper_adapter.py:320-328 (handler)
    The adapter method that forwards the set_fx_preset request to the underlying Reaper client.
    def set_fx_preset(
    self, track_index: int, fx_index: int, preset_name: str
) -> dict[str, Any]:
    return self._client.call(
        "set_fx_preset",
        track_index=track_index,
        fx_index=fx_index,
        preset_name=preset_name,
    )

Tool Definition Quality

A4.7/5.0
Behavior5/5

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

No annotations provided, so description carries full burden. Excellent disclosure: documents specific file extensions (.ffp/.fxp/.fxb), pinpoints four distinct failure modes with semantic error codes, and describes exact failure return structure ('loaded=false with a failure_reason').

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?

Efficient three-section structure: purpose statement, input format specification with clear bullet hierarchy, and error return documentation. No redundant words; every sentence provides actionable information front-loaded for quick agent parsing.

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

Completeness5/5

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

Comprehensive for a 3-parameter state-changing tool. Despite output schema existing (per context signals), description proactively documents failure responses. Preset loading complexity and file path requirements are thoroughly covered, leaving no ambiguous gaps.

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 has 0% description coverage (titles only). Description extensively compensates for complex preset_name parameter, detailing three distinct input formats (plain name, index-based, absolute file path) and cross-referencing list_fx_presets(). track_index and fx_index rely on schema titles, but these are relatively self-evident in the DAW context.

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?

States specific action ('Load') with clear resource ('named preset') and context ('for an FX plugin on a track'). Clearly distinguishes from sibling set_fx_param (individual parameter control) and list_fx_presets (discovery).

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?

Explicitly references sibling tool list_fx_presets() as the source for path values ('Use the path field from list_fx_presets()'). Documents three input formats for preset_name, guiding correct invocation. Could slightly improve by explicitly stating when NOT to use (e.g., vs set_fx_param).

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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