Add MCP Tool

Add a new tool to an existing FastMCP server with interactive configuration.

Usage

/add-mcp-tool [tool-name]

Arguments

• [tool-name]: Optional – Name for the new tool (will prompt if not provided)

Execution Instructions for Claude Code

When this command is run:

1. Locate the FastMCP server in the current project
2. Gather tool requirements through interactive questions
3. Generate tool code based on configuration
4. Add tool to server by modifying existing files or creating new ones

Interactive Session Flow

1. Locate Server

First, find the FastMCP server in the project:

Looking for FastMCP server...

Found: src/server.ts

Is this the correct server file? (yes/no):

If not found:

I couldn't find a FastMCP server. Please provide the path to your server file:

2. Tool Basics

Let's add a new tool to your MCP server.

Tool name (lowercase, use hyphens):
Example: "fetch-data", "create-file", "query-database"

Tool name:

Tool description (shown to LLM clients):
Example: "Fetches data from the specified API endpoint"

Description:

3. Parameters

Does this tool require parameters?

1. No parameters
2. Simple parameters (strings, numbers, booleans)
3. Complex parameters (objects, arrays, enums)

Select (1-3):

If parameters needed:

Let's define the parameters one by one.

Parameter 1:
  Name:
  Type (string/number/boolean/array/object):
  Required? (yes/no):
  Description:

Add another parameter? (yes/no):

For enum types:

Enum values (comma-separated):
Example: "json,xml,csv"

Values:

4. Return Type

What does this tool return?

1. Simple text response
2. Multiple content items
3. Image content
4. Audio content
5. Resource reference
6. Custom structure

Select (1-6):

5. Tool Features

Which features do you need?

[ ] Logging (log.info, log.error, etc.)
[ ] Progress reporting (for long operations)
[ ] Streaming output (for text generation)
[ ] Authorization check (canAccess)
[ ] Error handling with UserError

Select features (comma-separated numbers, or 'none'):

6. Tool Annotations

Optional annotations for LLM clients:

Read-only hint (tool doesn't modify state)? (yes/no) [default: no]:

Open-world hint (tool accesses external systems)? (yes/no) [default: no]:

Streaming hint (tool produces streaming output)? (yes/no) [default: no]:

Generated Code Examples

Simple Tool (no parameters)

server.addTool({
  name: "{{tool-name}}",
  description: "{{description}}",
  execute: async () => {
    return "Result here";
  },
});

Tool with Parameters

server.addTool({
  name: "{{tool-name}}",
  description: "{{description}}",
  parameters: z.object({
    {{#each parameters}}
    {{name}}: z.{{type}}(){{#if description}}.describe("{{description}}"){{/if}}{{#unless required}}.optional(){{/unless}},
    {{/each}}
  }),
  execute: async (args) => {
    const { {{parameterNames}} } = args;

    // TODO: Implement tool logic
    return "Processed: " + JSON.stringify(args);
  },
});

Tool with Logging

server.addTool({
  name: "{{tool-name}}",
  description: "{{description}}",
  parameters: z.object({
    url: z.string().describe("URL to fetch"),
  }),
  execute: async (args, { log }) => {
    log.info("Starting operation", { url: args.url });

    try {
      // TODO: Implement logic
      const result = await fetchData(args.url);
      log.info("Operation completed successfully");
      return result;
    } catch (error) {
      log.error("Operation failed", { error: String(error) });
      throw error;
    }
  },
});

Tool with Progress Reporting

server.addTool({
  name: "{{tool-name}}",
  description: "{{description}}",
  parameters: z.object({
    items: z.array(z.string()).describe("Items to process"),
  }),
  execute: async (args, { reportProgress }) => {
    const total = args.items.length;
    const results: string[] = [];

    for (let i = 0; i < total; i++) {
      await reportProgress({ progress: i, total });

      // TODO: Process each item
      results.push("Processed: " + args.items[i]);
    }

    await reportProgress({ progress: total, total });
    return results.join("\n");
  },
});

Tool with Streaming Output

server.addTool({
  name: "{{tool-name}}",
  description: "{{description}}",
  parameters: z.object({
    prompt: z.string().describe("Input prompt"),
  }),
  annotations: {
    streamingHint: true,
  },
  execute: async (args, { streamContent }) => {
    // Stream content progressively
    await streamContent({ type: "text", text: "Processing...\n" });

    // TODO: Generate content
    const chunks = ["First ", "part, ", "second ", "part."];
    for (const chunk of chunks) {
      await streamContent({ type: "text", text: chunk });
      await new Promise(r => setTimeout(r, 100));
    }

    return; // Return undefined when using streaming
  },
});

Tool with Authorization

server.addTool({
  name: "{{tool-name}}",
  description: "{{description}}",
  canAccess: (auth) => {
    // Return true if user is authorized
    return auth?.role === "admin";
  },
  execute: async (args, { session }) => {
    // Tool is only executed if canAccess returns true
    return "Welcome, " + session.id;
  },
});

Tool with UserError

import { UserError } from "fastmcp";

server.addTool({
  name: "{{tool-name}}",
  description: "{{description}}",
  parameters: z.object({
    id: z.string().describe("Resource ID"),
  }),
  execute: async (args) => {
    // Validate input
    if (!args.id.match(/^[a-z0-9-]+$/)) {
      throw new UserError("Invalid ID format. Use lowercase letters, numbers, and hyphens only.");
    }

    // Check resource exists
    const resource = await findResource(args.id);
    if (!resource) {
      throw new UserError(`Resource not found: ${args.id}`);
    }

    return JSON.stringify(resource);
  },
});

Tool with Image Content

import { imageContent } from "fastmcp";

server.addTool({
  name: "{{tool-name}}",
  description: "{{description}}",
  parameters: z.object({
    imageId: z.string().describe("Image identifier"),
  }),
  execute: async (args) => {
    // Return image from URL
    return imageContent({
      url: `https://example.com/images/${args.imageId}.png`,
    });

    // Or from file path:
    // return imageContent({ path: `/path/to/${args.imageId}.png` });

    // Or from buffer:
    // return imageContent({ buffer: imageBuffer });
  },
});

Tool with Multiple Content Types

server.addTool({
  name: "{{tool-name}}",
  description: "{{description}}",
  execute: async () => {
    return {
      content: [
        { type: "text", text: "Here's the analysis:" },
        { type: "text", text: "Line 1: Found 5 issues" },
        { type: "text", text: "Line 2: 3 warnings" },
      ],
    };
  },
});

File Placement Options

Where should the tool code be added?

1. Inline in server.ts (simple projects)
2. New file in tools/ directory (recommended for organization)
3. Existing tools file (specify which)

Select (1-3):

Implementation Notes

1. Find existing server: Use Glob/Grep to locate FastMCP server files
2. Parse existing structure: Understand current tool organization
3. Generate imports: Add necessary imports (z, UserError, imageContent, etc.)
4. Insert code: Add tool at appropriate location
5. Preserve formatting: Match existing code style
6. Summary: Show the generated tool and file location