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jimmcq

Lemonade Stand MCP Server

by jimmcq

set_price

Adjust the cost per cup of lemonade in the Lemonade Stand business simulation to optimize revenue based on weather conditions and customer demand.

Instructions

Set the price per cup of lemonade

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
gameIdYesThe game ID
priceYesPrice per cup in dollars

Implementation Reference

  • Handler for the 'set_price' tool. Retrieves the game state, validates existence, updates pricePerCup from arguments, sets status to 'selling', persists the state, and returns success with updated game state.
    case 'set_price': {
      const priceGame = games.get(request.params.arguments?.gameId);
      if (!priceGame) {
        throw new McpError(ErrorCode.InvalidRequest, "Game not found");
      }
      
      priceGame.pricePerCup = parseFloat(request.params.arguments.price);
      priceGame.status = 'selling';
      games.set(request.params.arguments.gameId, priceGame);
      
      return {
        content: [{
          type: "text",
          text: JSON.stringify({ success: true, gameState: priceGame })
        }]
      };
    }
  • server.js:260-271 (registration)
    Tool registration in ListTools response, including name, description, and input schema for 'set_price'.
    {
      name: "set_price",
      description: "Set the price per cup of lemonade",
      inputSchema: {
        type: "object",
        properties: {
          gameId: { type: "string", description: "The game ID" },
          price: { type: "number", description: "Price per cup in dollars" }
        },
        required: ["gameId", "price"]
      }
    },
  • Input schema definition for the 'set_price' tool, specifying gameId (string, required) and price (number, required).
    inputSchema: {
      type: "object",
      properties: {
        gameId: { type: "string", description: "The game ID" },
        price: { type: "number", description: "Price per cup in dollars" }
      },
      required: ["gameId", "price"]
    }
Behavior2/5

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

With no annotations provided, the description carries the full burden of behavioral disclosure. It states 'Set the price per cup of lemonade,' which implies a mutation operation, but doesn't clarify permissions needed, whether the change is reversible, side effects (e.g., impacting sales in 'sell_lemonade'), or expected response format. This is inadequate for a mutation tool with zero annotation coverage.

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 a single, direct sentence with zero wasted words. It's front-loaded with the core action and resource, making it highly efficient and easy to parse. Every word earns its place without redundancy.

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

Completeness2/5

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

Given the tool's mutation nature, lack of annotations, and no output schema, the description is incomplete. It doesn't address behavioral aspects like side effects, error conditions, or how it integrates with sibling tools (e.g., 'sell_lemonade'). For a tool that likely affects game state, more context is needed for effective agent use.

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?

The schema description coverage is 100%, with both parameters ('gameId' and 'price') clearly documented in the schema. The description adds no additional parameter semantics beyond what the schema provides (e.g., units, constraints, or examples). This meets the baseline for high schema coverage but doesn't enhance understanding.

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 action ('Set') and the target resource ('price per cup of lemonade'), making the purpose immediately understandable. However, it doesn't explicitly differentiate from sibling tools like 'sell_lemonade' or 'next_day', which might also involve price-related operations in the lemonade stand context.

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?

The description provides no guidance on when to use this tool versus alternatives. It doesn't mention prerequisites (e.g., needing an active game), exclusions, or how it relates to sibling tools like 'sell_lemonade' (which might depend on the price set). This leaves the agent with insufficient context for optimal tool selection.

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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