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mcp-mgba

by dmang-dev
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mgba_load_state

Restore a saved game state from a specific slot or file path, ensuring the state matches the current ROM and mGBA version.

Instructions

Load a previously-saved emulator state. Pass either slot (0-9) or path (absolute file path). The state must come from the same ROM and a compatible mGBA version.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
slotNoSave state slot (0-9)
pathNoAbsolute file path (alternative to slot)

Implementation Reference

  • src/tools.ts:232-242 (schema)
    Tool definition (name, description, and inputSchema) for mgba_load_state. Accepts optional 'slot' (0-9) or 'path' (absolute file path).
    {
  name: "mgba_load_state",
  description: "Load a previously-saved emulator state. Pass either `slot` (0-9) or `path` (absolute file path). The state must come from the same ROM and a compatible mGBA version.",
  inputSchema: {
    type: "object",
    properties: {
      slot: { type: "integer", minimum: 0, maximum: 9, description: "Save state slot (0-9)" },
      path: { type: "string", description: "Absolute file path (alternative to slot)" },
    },
  },
},
  • src/tools.ts:396-405 (handler)
    Handler for mgba_load_state. Validates that either slot or path is provided, then calls the mGBA bridge 'load_state' RPC method and returns a success message with the slot or path used.
    case "mgba_load_state": {
  if (p.slot === undefined && p.path === undefined) {
    throw new Error("provide either `slot` (0-9) or `path`");
  }
  const r = await mgba.call<{ slot?: number; path?: string }>("load_state", {
    ...(p.slot !== undefined ? { slot: p.slot } : {}),
    ...(p.path !== undefined ? { path: p.path } : {}),
  });
  return ok(r.path ? `Loaded state from ${r.path}` : `Loaded state from slot ${r.slot}`);
}
  • src/tools.ts:258-411 (registration)
    The registerTools function registers all tools (including mgba_load_state) with the MCP server via ListToolsRequestSchema and CallToolRequestSchema handlers.
    export function registerTools(server: Server, mgba: MgbaClient): void {
  server.setRequestHandler(ListToolsRequestSchema, async () => ({ tools: TOOLS }));

  server.setRequestHandler(CallToolRequestSchema, async (req) => {
    const { name, arguments: args = {} } = req.params;
    const p = args as Record<string, unknown>;

    switch (name) {
      case "mgba_ping": {
        const r = await mgba.call<string>("ping");
        return ok(r);
      }

      case "mgba_get_info": {
        const r = await mgba.call<{
          title?: string;
          code?: string;
          frame?: number;
          platform?: number | string;
          capabilities?: Record<string, boolean>;
        }>("get_info");
        const lines = [
          `Title:    ${r.title ?? "(unavailable)"}`,
          `Code:     ${r.code ?? "(unavailable)"}`,
          `Platform: ${r.platform ?? "(unavailable)"}`,
          `Frame:    ${r.frame ?? "(unavailable)"}`,
        ];
        if (r.capabilities) {
          const present = Object.entries(r.capabilities).filter(([, v]) => v).map(([k]) => k);
          const missing = Object.entries(r.capabilities).filter(([, v]) => !v).map(([k]) => k);
          lines.push("");
          lines.push(`Capabilities present: ${present.length ? present.join(", ") : "(none)"}`);
          if (missing.length) lines.push(`Missing on this build: ${missing.join(", ")}`);
        }
        return ok(lines.join("\n"));
      }

      case "mgba_read8": {
        const v = await mgba.call<number>("read8", { address: p.address });
        return ok(`0x${(p.address as number).toString(16).toUpperCase()}: ${formatHex(v)}`);
      }

      case "mgba_read16": {
        const v = await mgba.call<number>("read16", { address: p.address });
        return ok(`0x${(p.address as number).toString(16).toUpperCase()}: ${formatHex(v)}`);
      }

      case "mgba_read32": {
        const v = await mgba.call<number>("read32", { address: p.address });
        return ok(`0x${(p.address as number).toString(16).toUpperCase()}: ${formatHex(v)}`);
      }

      case "mgba_write8": {
        await mgba.call("write8", { address: p.address, value: p.value });
        return ok(`Wrote ${formatHex(p.value)} → 0x${(p.address as number).toString(16).toUpperCase()}`);
      }

      case "mgba_write16": {
        await mgba.call("write16", { address: p.address, value: p.value });
        return ok(`Wrote ${formatHex(p.value)} → 0x${(p.address as number).toString(16).toUpperCase()}`);
      }

      case "mgba_write32": {
        await mgba.call("write32", { address: p.address, value: p.value });
        return ok(`Wrote ${formatHex(p.value)} → 0x${(p.address as number).toString(16).toUpperCase()}`);
      }

      case "mgba_read_range": {
        const bytes = await mgba.call<number[]>("read_range", {
          address: p.address,
          length:  p.length,
        });
        const hex = bytes
          .map((b) => b.toString(16).padStart(2, "0").toUpperCase())
          .join(" ");
        const addr = (p.address as number).toString(16).toUpperCase();
        return ok(`0x${addr} [${bytes.length} bytes]:\n${hex}`);
      }

      case "mgba_write_range": {
        const r = await mgba.call<{ written: number }>("write_range", {
          address: p.address,
          bytes:   p.bytes,
        });
        const addr = (p.address as number).toString(16).toUpperCase();
        return ok(`Wrote ${r.written} bytes → 0x${addr}`);
      }

      case "mgba_press_buttons": {
        const r = await mgba.call<{ queued: boolean; queue_size: number }>("press_buttons", {
          buttons:        p.buttons,
          frames:         p.frames         ?? 1,
          release_frames: p.release_frames ?? 1,
        });
        const keys = (p.buttons as string[]).join("+");
        return ok(
          `Queued press: ${keys} ` +
          `(hold ${p.frames ?? 1}f, release ${p.release_frames ?? 1}f). ` +
          `Queue size: ${r.queue_size}`,
        );
      }

      case "mgba_advance_frames": {
        const frame = await mgba.call<number>("advance_frames", { count: p.count ?? 1 });
        return ok(`Advanced ${p.count ?? 1} frame(s). Current frame: ${frame}`);
      }

      case "mgba_pause": {
        await mgba.call("pause");
        return ok("Emulation paused");
      }

      case "mgba_unpause": {
        await mgba.call("unpause");
        return ok("Emulation resumed");
      }

      case "mgba_reset": {
        await mgba.call("reset");
        return ok("ROM reset");
      }

      case "mgba_screenshot": {
        const path = await mgba.call<string>("screenshot", p.path ? { path: p.path } : {});
        return ok(`Screenshot saved: ${path}`);
      }

      case "mgba_save_state": {
        if (p.slot === undefined && p.path === undefined) {
          throw new Error("provide either `slot` (0-9) or `path`");
        }
        const r = await mgba.call<{ slot?: number; path?: string }>("save_state", {
          ...(p.slot !== undefined ? { slot: p.slot } : {}),
          ...(p.path !== undefined ? { path: p.path } : {}),
        });
        return ok(r.path ? `Saved state to ${r.path}` : `Saved state to slot ${r.slot}`);
      }

      case "mgba_load_state": {
        if (p.slot === undefined && p.path === undefined) {
          throw new Error("provide either `slot` (0-9) or `path`");
        }
        const r = await mgba.call<{ slot?: number; path?: string }>("load_state", {
          ...(p.slot !== undefined ? { slot: p.slot } : {}),
          ...(p.path !== undefined ? { path: p.path } : {}),
        });
        return ok(r.path ? `Loaded state from ${r.path}` : `Loaded state from slot ${r.slot}`);
      }

      default:
        throw new Error(`Unknown tool: ${name}`);
    }
  });
}
  • src/mgba.ts:89-131 (helper)
    MgbaClient.call() method — the generic RPC helper used by the handler to send the 'load_state' request to the mGBA Lua bridge over a TCP socket.
    async call<T = unknown>(
  method: string,
  params?: Record<string, unknown>,
): Promise<T> {
  // Lazy (re)connect — bridge.lua reloads kill the socket, and the user
  // shouldn't have to restart the MCP host every time they edit the script.
  if (!this.socket || this.socket.destroyed) {
    try {
      await this.connect();
    } catch (err) {
      throw new Error(
        `Cannot reach mGBA bridge at ${this.host}:${this.port}. ` +
        `Make sure mGBA is running with bridge.lua loaded (Tools > Scripting). ` +
        `Underlying error: ${(err as Error).message}`,
      );
    }
  }

  return new Promise<T>((resolve, reject) => {
    const sock = this.socket;
    if (!sock) {
      reject(new Error("socket vanished after connect"));
      return;
    }

    const id = this.nextId++;
    this.pending.set(id, (resp) => {
      if (resp.error) {
        reject(new Error(`mGBA RPC error [${resp.error.code}]: ${resp.error.message}`));
      } else {
        resolve(resp.result as T);
      }
    });

    const msg = JSON.stringify({ id, method, params: params ?? {} }) + "\n";
    sock.write(msg, (err) => {
      if (err) {
        this.pending.delete(id);
        reject(err);
      }
    });
  });
}

Tool Definition Quality

A4.2/5.0
Behavior3/5

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

With no annotations, the description carries the burden of behavioral disclosure. It mentions the ROM/version compatibility requirement but omits details about side effects (e.g., overwriting current state, emulation pause) or error handling.

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?

Two sentences, no redundancy, front-loads the purpose and immediately provides parameter guidance. Every word earns its place.

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 no output schema and no annotations, the description is fairly complete for a straightforward load operation. It explains usage and a key constraint, though more behavioral context could be added.

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?

The schema covers both parameters (slot and path) with descriptions, but the description adds value by clarifying that exactly one of them is needed and that path must be absolute, which is not in 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 clearly states it loads a saved emulator state and distinguishes it from siblings like mgba_save_state. It specifies the two methods (slot or path) and includes a constraint about ROM and version compatibility.

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?

The description indicates when to use slot vs path and notes a prerequisite (same ROM and mGBA version). However, it lacks explicit guidance on when not to use this tool or alternatives among siblings.

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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