implementing-mcp-tools

name	implementing-mcp-tools
description	Implement new MCP tools in the deno-mcp-template project. Provides the exact file structure, type signatures, registration steps, and patterns for standard tools, sampling tools, form and URL elicitation, resource-backed tools, and notification tools. Use when adding a new tool, creating MCP tools, or asking how tools work in this project.

Implementing MCP Tools

Workflow

Task Progress:
- [ ] Step 1: Create tool file in src/mcp/tools/
- [ ] Step 2: Define Zod schema, name, config, and callback
- [ ] Step 3: Export as default ToolModule
- [ ] Step 4: Register in src/mcp/tools/mod.ts
- [ ] Step 5: Run `deno task ci` to verify

Tool File Template

Every tool follows this structure. Create a new file in src/mcp/tools/.

import type { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import type { CallToolResult } from "@modelcontextprotocol/sdk/types.js";
import { z } from "zod/v3";

import type { ToolConfig, ToolModule } from "$/shared/types.ts";
import {
  createCallToolErrorResponse,
  createCallToolTextResponse,
} from "$/shared/utils.ts";

// 1. Input schema
const schema = z.object({
  myField: z.string().min(1, "Required").describe("What this field does"),
});

// 2. Tool name (kebab-case)
const name = "my-tool-name";

// 3. Config
// deno-lint-ignore no-explicit-any
const config: ToolConfig<typeof schema.shape, any> = {
  title: "Human-readable title",
  description: "What this tool does — shown to the LLM",
  inputSchema: schema.shape,
};

// 4. Callback factory: receives McpServer, returns the handler
// deno-lint-ignore no-explicit-any
const callback = (_mcp: McpServer) => async (args: any): Promise<CallToolResult> => {
  const parsed = schema.safeParse(args);
  if (!parsed.success) {
    return createCallToolErrorResponse({
      error: "Invalid arguments",
      details: parsed.error.flatten(),
      received: args,
    });
  }

  const { myField } = parsed.data;

  // ... tool logic ...

  return createCallToolTextResponse({ result: myField });
};

// 5. Export as ToolModule tuple
// deno-lint-ignore no-explicit-any
const module: ToolModule<typeof schema.shape, any> = [name, config, callback];

export default module;

Registration

In src/mcp/tools/mod.ts:

1. Import the tool: import myTool from "./myTool.ts";
2. Add it to the tools array:

export const tools: ToolModule<any>[] = [
  // ... existing tools
  myTool,
];

The ToolManager in the same file handles binding and registration automatically.

Key Types

From src/shared/types.ts:

• ToolModule<InputArgs, OutputArgs> — Export format: [name, config, callbackFactory]
• ToolConfig<InputArgs, OutputArgs> — { title, description, inputSchema, outputSchema?, annotations? }
• ToolPlugin — Bound format: [name, config, callback] (created by ToolManager)

Response Helpers

From src/shared/utils.ts:

• createCallToolTextResponse(obj, structuredContent?) — Wraps obj as JSON text content
• createCallToolErrorResponse(obj, structuredContent?) — Same but sets isError: true

Validation Pattern

Always validate with Zod safeParse. For optional args, use args ?? {}:

const parsed = schema.safeParse(args ?? {});
if (!parsed.success) {
  return createCallToolErrorResponse({
    error: "Invalid arguments",
    details: parsed.error.flatten(),
    received: args,
  });
}

Error Handling Pattern

Wrap operational logic in try/catch:

try {
  const result = await doSomething(parsed.data);
  return createCallToolTextResponse({ result });
} catch (error) {
  return createCallToolErrorResponse({
    error: error instanceof Error ? error.message : "Unknown error",
    operation: "my-operation",
  });
}

Tool Categories

Standard Tool (no MCP server interaction)

Use _mcp (unused) in the callback factory. See the template above.

Sampling Tool (LLM-powered)

Use mcp.server.createMessage() to request LLM completions:

const callback = (mcp: McpServer) => async (args: any): Promise<CallToolResult> => {
  // ... validate args ...

  const response = await mcp.server.createMessage(
    {
      messages: [{ role: "user", content: { type: "text", text: prompt } }],
      maxTokens: 1024,
      temperature: 0.7,
    },
    { timeout: 30000 },
  );

  const content = Array.isArray(response.content)
    ? response.content[0]
    : response.content;

  if (!content || content.type !== "text") {
    return createCallToolErrorResponse({ error: "No text response from sampling" });
  }

  return createCallToolTextResponse({ result: content.text });
};

Elicitation Tool (user input)

Experimental elicitation is enabled in src/mcp/serverDefinition.ts (experimentalElicitation: true). Two modes matter in practice:

Mode	Use case	Mechanism
form	Structured, non-sensitive fields; validated in the MCP client UI	await mcp.server.elicitInput({ mode: "form", message, requestedSchema })
url	Sensitive or browser-only flows (confirmations, OAuth, secrets typed in your page)	Throw UrlElicitationRequiredError with a URL; complete out-of-band, then createElicitationCompletionNotifier fires notifications/elicitation/complete

Form mode

Use mcp.server.elicitInput() with a JSON Schema (requestedSchema):

const callback = (mcp: McpServer) => async (args: any): Promise<CallToolResult> => {
  // ... validate args ...

  const result = await mcp.server.elicitInput({
    mode: "form",
    message: "Please provide details",
    requestedSchema: {
      type: "object",
      properties: {
        name: { type: "string", title: "Name", description: "Your name" },
      },
      required: ["name"],
    },
  });

  return createCallToolTextResponse({ elicitationResult: result });
};

Branch on result.action (accept with content, decline, or cancelled) before assuming data exists. For multiple steps, call elicitInput more than once in one tool (see src/mcp/tools/elicitFormWizard.ts).

URL mode

The client opens a browser URL you control. The tool does not return a normal success result; it throws UrlElicitationRequiredError from @modelcontextprotocol/sdk/types.js with an array of { mode: "url", message, url, elicitationId }.

In this template:

1. McpServerFactoryContext includes urlElicitation: { baseUrl, registry } (src/mcp/context.ts). Resolve baseUrl with MCP_PUBLIC_BASE_URL or the derived bind URL (src/shared/publicBaseUrl.ts).
2. Register pending state and the SDK notifier: mcp.server.createElicitationCompletionNotifier(elicitationId), then ctx.urlElicitation.registry.registerPending({ elicitationId, sessionId, label, completionNotifier }).
3. Require extra.sessionId on the tool handler (streamable HTTP). STDIO has no session; return createCallToolErrorResponse instead of throwing.
4. Browser routes live in src/app/http/urlElicitationRoutes.ts (GET/POST /mcp-elicitation/confirm). They validate session + elicitation id against the registry and active transport, then call registry.complete(elicitationId). Bearer auth is skipped for /mcp-elicitation so users are not prompted for the MCP token in a normal browser tab (src/app/http/httpBearerAuthMiddleware.ts).
5. Reference implementation: registerUrlElicitationDemoTool in src/mcp/tools/urlElicitationDemo.ts, wired from src/mcp/mod.ts when mcpServerDefinition.urlElicitationDemo is true.

Do not log secrets submitted on elicitation HTML forms.

Resource-Backed Tool

Import resource helpers and mutate state. Resource subscriptions handle notifications automatically via KV watchers:

import { COUNTER_URI, incrementCounterValue } from "../resources/counter.ts";

// In callback:
const value = await incrementCounterValue(delta);
return createCallToolTextResponse({ value, uri: COUNTER_URI });

Notification Tool (logging)

Use mcp.server.sendLoggingMessage():

await mcp.server.sendLoggingMessage({
  level: "info",    // debug | info | notice | warning | error | critical | alert | emergency
  logger: "my-logger",
  data: { message: "Something happened" },
});

Notification Tool (list changed)

Use mcp.sendToolListChanged(), mcp.sendPromptListChanged(), or mcp.sendResourceListChanged() to notify clients that available items have changed.

Annotations

Add annotations to config for client hints:

const config: ToolConfig<typeof schema.shape, any> = {
  title: "My Tool",
  description: "...",
  inputSchema: schema.shape,
  annotations: {
    title: "My Tool",
    readOnlyHint: true,
    openWorldHint: false,
  },
};

Additional Resources

• For complete tool examples by category, see examples.md
• MCP Tool spec: https://modelcontextprotocol.io/specification/2025-06-18/server/tools
• MCP Sampling: https://modelcontextprotocol.io/docs/concepts/sampling
• MCP Elicitation: https://modelcontextprotocol.io/docs/concepts/elicitation