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thevalmarch

Opal Emu MCP

by thevalmarch
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TypeScript
Local

Load ROM

load_rom
Destructive

Load a ROM by library name or Base64, start the game paused, and return the first PNG screenshot, replacing the current game.

Instructions

Loads a ROM into the emulator and starts the game, paused, ready for the next tool call. Provide either "name" (from list_roms) or "romBase64" + "fileName" for a ROM not in the library. Returns the first PNG screenshot once the game has booted. Replaces the current game and its unsaved progress. Local ROM files are limited to 128 MiB; direct Base64 ROMs are limited to 6 MiB decoded. Some disc images need multiple files or manual system selection and cannot load through this tool. First use of a system may download a core and can take up to 100 seconds; MCP clients with shorter call deadlines may time out.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
nameNoA ROM name or fileName from list_roms().
fileNameNoRequired with romBase64; use a plain filename with an extension, such as game.nes.
romBase64NoBase64-encoded ROM bytes, up to 6 MiB decoded over MCP stdio.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed7 schema fields changedv1.1.0
    • changedInput schema / properties / fileName / description
      Previous value: -"Required with romBase64 — needs a real extension (e.g. \"game.nes\") for system detection."New value: +"Required with romBase64; use a plain filename with an extension, such as game.nes."
    • addedInput schema / properties / fileName / maxLength
      Added value: +255
    • addedInput schema / properties / fileName / minLength
      Added value: +1
    • addedInput schema / properties / name / maxLength
      Added value: +255
    • addedInput schema / properties / name / minLength
      Added value: +1
    • changedInput schema / properties / romBase64 / description
      Previous value: -"Base64-encoded ROM bytes, for a ROM not in the library."New value: +"Base64-encoded ROM bytes, up to 6 MiB decoded over MCP stdio."
    • addedInput schema / properties / romBase64 / maxLength
      Added value: +8388608
  2. First observedv0.1.0

TDQS

A4.9/5.0
Behavior5/5

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

Annotations already flag destructiveHint=true and non-idempotent, and the description reinforces and extends them: it names exactly what is destroyed ('the current game and its unsaved progress'), gives size ceilings (128 MiB local, 6 MiB decoded Base64), and warns that first use of a system may download a core taking up to 100 seconds and time out MCP clients. This is behavioral context well beyond the annotations.

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?

Front-loaded with the core action and post-condition, then alternatives, then constraints and caveats. Dense but every sentence carries a distinct, actionable fact; nothing is padding.

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?

With no output schema, the description still tells the agent what comes back ('the first PNG screenshot once the game has booted'), and covers destructive effect, input modes, size limits, unsupported disc images, and latency/timeout risk. Nothing an agent needs to call this correctly is missing.

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 coverage is 100%, so the three parameters are already documented; the description nonetheless adds the mutual-exclusion rule (either 'name' or 'romBase64'+'fileName') and the decoded-size limit that the schema's maxLength does not express in human terms. It stops short of clarifying conflict handling if both 'name' and 'romBase64' are supplied.

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+resource ('Loads a ROM into the emulator') plus the resulting state ('starts the game, paused, ready for the next tool call'). It also names the sibling it draws its input from (list_roms), so an agent can distinguish it from list_roms/reset_emulator without opening schemas.

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?

Explicitly says when to use 'name' versus 'romBase64' + 'fileName', and adds an exclusion: some disc images needing multiple files or manual system selection 'cannot load through this tool.' That is when/when-not guidance rather than mere context.

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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