PostFader V12 — FL Studio MCP Server

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plugin_set_parameter

Destructive

Set a loaded FL Studio plug-in parameter by display value, option text, or raw 0..1 value, with readback verification and no auto-save.

Instructions

Set one plug-in parameter by display value, option text, or raw 0..1 value.

Supply exactly one value form. display_value searches the control until its
own display reads the number asked for, so no unit curve is guessed; use it
for anything with units (Hz, dB, ms, %). option sets a word-valued control
(Key, Scale, Mode) by sweeping it, which moves the control while it looks;
the result lists every option found, and an unknown option restores the
original value before the error. normalized_value writes 0..1 directly.
Requires write mode. Read controls with plugin_list_parameters first.
Inspect verified and the readback; does not save the project.

Input Schema

TableJSON Schema

Name	Required	Description
`unit`	No	Optional unit for display_value: Hz, kHz, ms, seconds, dB, percent, or ratio; converts across prefixes.
`option`	No	Exact option text for a control that shows words, such as 'Major' or 'Low Male'.
`target`	Yes	Loaded plug-in: {"kind": "mixer_effect", "track_index", "slot_index"} (Master also needs "allow_master": true) or {"kind": "channel_generator", "channel_index"}. Read targets with plugin_list_loaded.
`parameter`	Yes	Parameter index from plugin_list_parameters, or text matched against parameter names and display strings (many controls have no name). normalized_value needs an index.
`tolerance`	No	How close the displayed number must land; defaults to 2% of the target, at least 0.01.
`sweep_steps`	No	Resolution of the option sweep; defaults to 64. Raise it only if an option is missed.
`display_value`	No	The number the plug-in should display: 20 for '20 ms', -18 for '-18.0 dB', 4000 with unit='Hz' for '4.0kHz'.
`expected_before`	No	Optional current normalized value and/or exact display text; refuse if either changed.
`normalized_value`	No	Raw 0..1 value. Only when you know the control's mapping; prefer display_value or option.
`session_fingerprint`	No	Optional session_fingerprint from a recent read. The call refuses after a bridge reload or a reported project load. This is a concurrency guard, not authentication or a durable project identity.

Output Schema

TableJSON Schema

Name	Required	Description	Default
`result`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Addedv11.0.2Oct 5, 2026

TDQS

A4.8/5.0

Behavior5/5

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

Annotations already declare destructive/non-idempotent/write semantics, and the description adds substantial behavior beyond them: display_value sweeps until the control's own display matches (no unit curve guessed), option sweeps the control while it looks, an unknown option restores the original value before erroring, and the project is not saved. It also tells the agent to inspect 'verified' and the readback, which is exactly the post-condition context a mutating tool needs.

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

Conciseness4/5

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

Front-loads the core action and the three value forms, then proceeds through behavior, prerequisites, and post-conditions. Dense but nearly every sentence carries distinct information; the value-form paragraph is slightly long but not padded.

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?

For a 10-parameter mutation tool with an output schema, the description covers what an agent still needs: write-mode requirement, target/parameter lookup path, the three value forms and their trade-offs, error/rollback behavior for unknown options, and the fact that the project is not saved. Return-value details are rightly left to the output schema.

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 100%, so the baseline is 3, but the description adds real meaning the schema cannot express: the mutual-exclusion rule that exactly one value form must be supplied (the schema's nullable anyOf does not enforce this), and guidance on preferring display_value/option over normalized_value. Some overlap with schema text (e.g. normalized_value needing an index) keeps it from a 5.

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 a specific verb (set) and resource (one plug-in parameter) plus the three accepted value forms, and clearly separates itself from the read-side siblings plugin_list_parameters and plugin_list_loaded, which it names. An agent can identify the operation without opening the schema.

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

Usage Guidelines5/5

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

Gives explicit selection rules among the three value forms ('use it for anything with units (Hz, dB, ms, %)', 'option sets a word-valued control', 'normalized_value ... only when you know the control's mapping; prefer display_value or option'), plus prerequisites ('Requires write mode', 'Read controls with plugin_list_parameters first'). Both when-to-use and when-not-to-use are covered.

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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