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mattlemmone

Expo MCP Server

by mattlemmone

readFile

Retrieve file contents by specifying the file path using this tool, designed for managing Expo-based React Native applications in the Expo MCP Server environment.

Instructions

Read the contents of a file

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
filePathYesThe path to the file to read

Implementation Reference

  • The readFile tool handler function that reads and returns the content of the specified file.
    export async function readFile(args: { filePath: string }, { log }: LogContext) {
      try {
        log.info(`Reading file at path: ${args.filePath}`);
    
        const normalizedPath = path.normalize(args.filePath);
        const fileContent = await fs.promises.readFile(normalizedPath, "utf8");
    
        log.info(`Successfully read file: ${normalizedPath}`);
    
        return {
          content: [
            {
              type: "text",
              text: fileContent,
            },
          ],
        };
      } catch (error: any) {
        log.error(`Error reading file: ${error.message}`);
        throw new Error(`Failed to read file: ${error.message}`);
      }
    }
  • src/index.ts:18-25 (registration)
    Registration of the readFile tool on the MCP server, including schema definition and handler reference.
    addTool({
      name: "readFile",
      description: "Read the contents of a file",
      parameters: z.object({
        filePath: z.string().describe("The path to the file to read"),
      }),
      execute: readFile,
    });
Behavior2/5

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

With no annotations provided, the description carries full burden for behavioral disclosure. While 'Read' implies a read-only operation, it doesn't specify whether this requires file permissions, how it handles large files, encoding issues, or what happens on non-existent paths. For a file I/O tool with zero annotation coverage, this leaves significant behavioral gaps.

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, efficient sentence that directly states the tool's function without unnecessary words. It's appropriately sized for a simple tool and front-loads the core purpose immediately.

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 complexity of file operations (permissions, encoding, size limits) and the absence of both annotations and output schema, the description is insufficiently complete. It doesn't explain what the tool returns (text content, binary data, error formats) or address common file system considerations, leaving the agent with inadequate context for reliable 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?

Schema description coverage is 100%, so the schema already documents the 'filePath' parameter adequately. The description adds no additional parameter semantics beyond what the schema provides, such as path format examples or supported file types. This meets the baseline for high schema coverage.

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 verb ('Read') and resource ('contents of a file'), making the purpose immediately understandable. However, it doesn't differentiate from sibling tools like 'tailFile' (which also reads files but with a streaming approach) or 'listFiles' (which lists metadata rather than contents), so it doesn't achieve full sibling differentiation.

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 like 'tailFile' for streaming or 'listFiles' for metadata. There's no mention of prerequisites, file size limitations, or error conditions, leaving 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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