PostFader V10 — FL Studio MCP Server

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fl_set_mixer_send_level

Destructive

Set how much signal from one mixer track reaches another by changing its send level. Use 0.8 for unity gain; the send route must already exist.

Instructions

Set how much of one track reaches another. 0.8 is unity, as on the fader.

The send must already exist; create it with `fl_set_mixer_send` first. FL
raises rather than reporting a level for a route that is not active, so
this is refused outright rather than written and reported unverified.

Input Schema

TableJSON Schema

Name	Required	Description
`track_index`	Yes	Zero-based index of the sending track.
`allow_master`	No	Required only to send FROM track 0.
`expected_before`	No	Optional expected current send amount; refuse if it changed.
`level_normalized`	Yes	Send amount, 0..1. 0.8 is unity.
`session_fingerprint`	No	Optional bridge/project-session fingerprint from a recent read. The write refuses after bridge reload or a reported project load. This is a concurrency guard, not authentication or a durable project identity.
`destination_track_index`	Yes	Zero-based index of the receiving track.

Output Schema

TableJSON Schema

Name	Required	Default
`verified`	Yes
`warnings`	No
`applied_at`	Yes
`send_active`	No
`track_index`	Yes
`project_saved`	No
`bridge_command`	No	mixer.set_send_level
`schema_version`	No	1.0
`targeted_master`	No
`undo_point_created`	No
`verification_basis`	No	readback_on_a_later_fl_idle_tick
`session_fingerprint`	No
`verification_summary`	Yes
`after_level_normalized`	No
`before_level_normalized`	No
`destination_track_index`	Yes
`expected_before_applied`	No
`requested_level_normalized`	Yes
`session_precondition_applied`	No

Schema Changelog

Changes observed during successful MCP inspections.

First observedv10.0.0Sep 14, 2026

TDQS

A4.5/5.0

Behavior4/5

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

Annotations already mark this as a destructive, non-idempotent write, and the description adds a non-obvious behavioral detail: FL raises rather than reporting a level for an inactive route, so the call is refused outright. This goes beyond the structured hints by explaining the validation/refusal model. It does not enumerate all side effects, but the destructive annotation carries that safety signal.

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 two short, focused paragraphs: the first defines the operation, the second gives the prerequisite and failure behavior. Every sentence earns its place, and there is no redundant restatement of annotations or schema.

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 six-parameter write tool with a full input schema and an output schema, the description covers the core concept, the scale convention, the key prerequisite, and the failure mode. Nothing an agent needs in order to select and invoke the tool correctly is missing.

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 coverage is 100%, so the baseline applies. The description's '0.8 is unity, as on the fader' adds a small calibration cue, but the schema already documents the same unity fact and describes each parameter's role. The description does not materially compensate for anything missing because nothing is missing from the schema.

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 a specific action and object: 'Set how much of one track reaches another,' which clearly identifies this as a mixer send-level write. It also adds the key domain fact that 0.8 is unity, which distinguishes it from volume, pan, and send-creation operations. This is clearly distinct from sibling tools like fl_set_mixer_send and fl_set_mixer_volume.

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?

The description explicitly states the prerequisite: the send must already exist and must be created with `fl_set_mixer_send` first. It also explains when the operation should not proceed — for an inactive route — and that the tool refuses rather than writing an unverified value. This gives clear when-to-use and when-not-to-use guidance.

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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