Select Reflection

Prerequisites

Install MCP Fusion before following this recipe: npm install @vinkius-core/mcp-fusion @modelcontextprotocol/sdk zod — or scaffold a project with npx fusion create.

• Introduction
• The Problem — Over-Fetching
• Using Zod Shape Inspection
• Database Integration

Introduction

When the Presenter schema declares 5 fields but the handler's database query fetches 25 columns, you waste bandwidth and CPU processing data that will be stripped anyway. Select Reflection lets you introspect the Presenter's Zod schema at query time, so your handler only fetches the columns the AI will actually see.

The Problem — Over-Fetching

// Handler fetches ALL columns
const user = await ctx.db.users.findUnique({
  where: { id: input.id },
});
// Returns: id, name, email, role, password_hash, stripe_id, internal_flags, ...

// Presenter keeps only 4 fields
// UserPresenter.schema: { id, name, email, role }
// Everything else is stripped by Zod .strict()

The Presenter already strips the extra fields — but you still paid for the database I/O and memory to fetch them.

Using Zod Shape Inspection

Since Presenter schemas are standard Zod objects, you can extract the field names using Zod's .shape property:

import { createPresenter, t } from '@vinkius-core/mcp-fusion';

const UserPresenter = createPresenter('User')
  .schema({
    id:    t.string,
    name:  t.string,
    email: t.string,
    role:  t.enum('admin', 'member', 'guest'),
  });

// Extract declared field names from the Zod shape
const schema = UserPresenter.getSchema();
const selectKeys = Object.keys(schema.shape);
// ['id', 'name', 'email', 'role']

// Convert to Prisma select format
const select = Object.fromEntries(selectKeys.map(k => [k, true]));
// { id: true, name: true, email: true, role: true }

Database Integration

Wire it into your handlers:

Prisma

export const getUser = f.query('users.get')
  .describe('Get a user by ID')
  .withString('id', 'User ID')
  .returns(UserPresenter)
  .handle(async (input, ctx) => {
    const shape = UserPresenter.getSchema().shape;
    const select = Object.fromEntries(
      Object.keys(shape).map(k => [k, true]),
    );

    return ctx.db.users.findUnique({
      where: { id: input.id },
      select,   // ← only fetch declared fields
    });
  });

Raw SQL

export const listUsers = f.query('users.list')
  .describe('List all users')
  .returns(UserPresenter)
  .handle(async (input, ctx) => {
    const columns = Object.keys(UserPresenter.getSchema().shape).join(', ');
    return ctx.db.query(`SELECT ${columns} FROM users`);
  });

TIP

Select Reflection is optional. If your query needs extra fields for business logic (e.g., checking role before deciding what to return), fetch what you need — the Presenter strips the rest automatically.