require-tool-input-schema

Require an input schema when an MCP tool handler reads arguments.

• CWE: CWE-20 — Improper Input Validation
• OWASP: A03:2021 — Injection
• CVSS: 5.3 (Medium)
• Presets: minimal, recommended, strict

WhyCopy Anchor Link

When a tool declares no inputSchema, the SDK calls its handler as handler(extra) — v1's executeToolHandler, v2's createToolExecutor. No client argument reaches it. A schema-less zero-argument tool (get_time, list_projects, whoami) is therefore correct, and this rule is silent on it.

What is wrong is a schema-less handler that expects arguments. The keys it reads were never declared, so nothing validated them, and the SDK never passes them: the handler is reading the request context. Either way, the fix is to declare the schema.

Rule detailsCopy Anchor Link

The rule fires only in files that import an MCP server package (@modelcontextprotocol/sdk, the v2 @modelcontextprotocol/* packages, mcp-handler). It reports a registerTool(...) / legacy tool(...) call with no input schema whose handler — inline, or a function declared in the same file — does one of:

• destructures a key from its first parameter that is not a request-context key (signal, authInfo, sessionId, _meta, requestInfo, mcpReq, http, …);
• reads such a named property off its first parameter (args.path);
• declares a second parameter, which without a schema is always undefined.

It stays silent when it cannot see the configuration or the handler: a config passed by reference or containing a spread, a legacy object it cannot classify as a params shape or annotations, and a handler imported from another module.

IncorrectCopy Anchor Link

import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';

// `path` is read off the request context — it is never the caller's argument.
server.registerTool('read_file', { description: 'Read a file' }, async ({ path }) =>
  readFileSync(path, 'utf8'),
);

server.tool('delete_record', async (args) => db.delete(args.id));

CorrectCopy Anchor Link

import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';
import { z } from 'zod';

server.registerTool('read_file', {
  description: 'Read a file',
  inputSchema: { path: z.string() },
}, async ({ path }) => readFileSync(path, 'utf8'));

server.tool('delete_record', { id: z.string().uuid() }, async (args) => db.delete(args.id));

// A zero-argument tool needs no schema.
server.registerTool('get_time', { description: 'Server time' }, async () => ({
  content: [{ type: 'text', text: new Date().toISOString() }],
}));

When not to use itCopy Anchor Link

If the server registers tools through a wrapper that injects the schema, the rule cannot see it. Disable it at that call site rather than repeating the schema.

Further readingCopy Anchor Link

• MCP — Tools
• CWE-20 — Improper Input Validation