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obs-mcp-server

by LarsCanGit
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set_volume

Adjust audio input volume in OBS Studio by specifying the input name and volume level from 0.0 (silent) to 1.0 (full).

Instructions

Set the volume of an audio input (0.0 – 1.0, linear mul).

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
input_nameYesName of the audio input.
volumeYesVolume multiplier: 0.0 (silent) to 1.0 (full).

Implementation Reference

  • obs-mcp-server.js:507-513 (handler)
    Handler implementation for the set_volume tool. Calls obs.call("SetInputVolume") with the provided input name and volume multiplier.
    case "set_volume": {
  await obs.call("SetInputVolume", {
    inputName: args.input_name,
    inputVolumeMul: args.volume,
  });
  return ok({ input: args.input_name, volume: args.volume });
}
  • obs-mcp-server.js:176-193 (schema)
    Tool registration for set_volume, including the input schema defining input_name (string) and volume (number).
    {
  name: "set_volume",
  description: "Set the volume of an audio input (0.0 – 1.0, linear mul).",
  inputSchema: {
    type: "object",
    properties: {
      input_name: {
        type: "string",
        description: "Name of the audio input.",
      },
      volume: {
        type: "number",
        description: "Volume multiplier: 0.0 (silent) to 1.0 (full).",
      },
    },
    required: ["input_name", "volume"],
  },
},

Tool Definition Quality

A3.5/5.0
Behavior3/5

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

With no annotations provided, the description carries full disclosure burden. It adds valuable mathematical context ('linear mul') and valid range, but lacks critical behavioral details: whether changes persist across sessions, if they apply immediately to active recordings/streams, or failure modes when 'input_name' is invalid.

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?

Every element earns its place in a single dense sentence: the verb establishes the operation, the parenthetical supplies the precise range and scaling semantics without redundant explanation, and there is no extraneous text.

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

Completeness4/5

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

Given the tool's simplicity (two primitive parameters with complete schema coverage) and lack of output schema, the description is reasonably complete for invocation. It appropriately delegates specific input requirements to the schema while providing the essential domain context (linear 0.0–1.0 scale).

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?

With 100% schema coverage, the baseline is 3. The description adds meaningful semantic value beyond the schema by specifying the 'linear mul' scaling behavior, which clarifies how the numeric value maps to audio gain (as opposed to logarithmic/decibel scaling).

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 clearly states the specific action (set) and resource (volume of an audio input), using precise terminology. However, it does not explicitly distinguish this from siblings like 'set_mute' or 'toggle_mute', which control similar but distinct aspects of audio inputs.

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?

While the parenthetical provides valid parameter constraints (0.0–1.0 range), it offers no guidance on when to select this tool versus alternatives like 'set_mute' (silencing) or 'toggle_mute', nor does it mention prerequisites such as verifying the input exists via 'list_audio_inputs'.

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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