# Switching to AgentBack AgentBack isn't a faster router or another MCP library. It's one Zod schema turned into your REST routes, your OpenAPI 3.1 document, your MCP tools, your typed client, and your runtime validation — served from a single process with a real dependency-injection container. If you're arriving from one of the frameworks below, here's what carries over and what you gain. | Coming from | What carries over | What you gain | | -------------------------------------- | -------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------- | | [**LoopBack 4**](https://loopback.io/) | DI instincts — `@inject`, `@injectable`, `Context`, binding scopes, extension points map 1:1 | Zod-first schemas, MCP tools, OpenAPI 3.1 emitted from the same Zod, ESM/Node 22, no `@loopback/repository` baggage | | [**NestJS**](https://nestjs.com/) | Decorated classes and a DI container | One Zod-on-decorator source instead of `nestjs-zod` + `@nestjs/swagger` + a community MCP bridge across two metadata systems | | [**tRPC**](https://trpc.io/) | A no-codegen, end-to-end-typed client | Your API is _also_ a public OpenAPI 3.1 document and MCP tools — first-class outputs, not a bolt-on — plus a DI container | | [**ts-rest**](https://ts-rest.com/) | Contract-first discipline | The same contract becomes MCP tools an agent can call, under one `@authorize` policy, plus a DI container | | [**Hono**](https://hono.dev/) | — | `@hono/zod-openapi` + `@hono/mcp` + hand-written SDK tools collapse to one schema, one process | Each section below shows the same forecast endpoint — a REST route _and_ an agent-callable tool — first in that framework, then in AgentBack. The AgentBack class is identical every time; it's spelled out in full under **From LoopBack 4** and abbreviated after that. ## From LoopBack 4 The DI core is identical; what changes is the schema layer. LB4's `@model` / `@property` + `getModelSchemaRef` become one Zod object on the decorator — which also defines an MCP tool, something LB4 has no answer for.

LoopBack 4

// model + JSON-schema decorators
@model()
class Forecast {
  @property() city: string;
  @property() tempC: number;
}
class WeatherController {
  @get('/forecast/{city}')
  @response(200, {content: {'application/json':
    {schema: getModelSchemaRef(Forecast)}}})
  forecast(@param.path.string('city') city: string) {
    return lookup(city);
  }
}
// MCP is not built in.

AgentBack

const City = z.object({city: z.string()});
const Out = z.object({tempC: z.number()});
@api() @mcpServer()
class Weather {
  @get('/forecast/{city}', {path: City, response: Out})
  route(i: {path: {city: string}}) {
    return lookup(i.path.city);
  }
  @tool('forecast', {input: City, output: Out})
  tool(i: {city: string}) {
    return lookup(i.city);
  }
}

## From NestJS Decorated classes and constructor injection carry over. The `class-validator` DTO, the `@nestjs/swagger` decorator, and a community MCP-Nest tool — three declarations across two metadata systems — collapse into one Zod object.

NestJS

// DTO: class-validator + swagger
class ForecastDto {
  @ApiProperty() @IsString() city: string;
  @ApiProperty() @IsNumber() tempC: number;
}
@Controller()
class WeatherController {
  @Get('forecast/:city')
  @ApiOkResponse({type: ForecastDto})
  forecast(@Param('city') city: string) {
    return lookup(city);
  }
}
// + a separate MCP-Nest @Tool method

AgentBack — same class

// the Weather class from the LoopBack section:
// one Zod schema → a @get route and a @tool
@api() @mcpServer()
class Weather {
  @get('/forecast/{city}', {path: City, response: Out})
  route(i) { ... }
  @tool('forecast', {input: City, output: Out})
  tool(i) { ... }
}

## From tRPC The typed client is what carries over. In tRPC the contract is the exported router _type_ (TypeScript-only); in AgentBack it's the Zod schema — so the same procedure is also real REST, an OpenAPI 3.1 document, and an MCP tool.

tRPC

// RPC router + zod input
const appRouter = router({
  forecast: publicProcedure
    .input(z.object({city: z.string()}))
    .query(({input}) => lookup(input.city)),
});
export type AppRouter = typeof appRouter;
// typed client call
trpc.forecast.query({city: 'sf'});
// REST / OpenAPI / MCP: add-ons only

AgentBack — same class

// the Weather class from the LoopBack section:
// one Zod schema → a @get route and a @tool
@api() @mcpServer()
class Weather {
  @get('/forecast/{city}', {path: City, response: Out})
  route(i) { ... }
  @tool('forecast', {input: City, output: Out})
  tool(i) { ... }
}

## From ts-rest Contract-first carries over. ts-rest keeps the contract as a standalone object you implement separately — two artifacts to align. In AgentBack the decorator's Zod schema _is_ the contract, and it also yields MCP tools and OpenAPI.

ts-rest

// 1. contract — a separate artifact
const contract = c.router({
  forecast: {
    method: 'GET',
    path: '/forecast/:city',
    responses: {200: z.object({tempC: z.number()})},
  },
});
// 2. implement it separately
const router = s.router(contract, {
  forecast: async ({params}) =>
    ({status: 200, body: await lookup(params.city)}),
});

AgentBack — same class

// the Weather class from the LoopBack section:
// one Zod schema → a @get route and a @tool
@api() @mcpServer()
class Weather {
  @get('/forecast/{city}', {path: City, response: Out})
  route(i) { ... }
  @tool('forecast', {input: City, output: Out})
  tool(i) { ... }
}

## From Hono Hono reaches this with `@hono/zod-openapi` + `@hono/mcp` + hand-written SDK tools, each with its own schema to keep aligned. AgentBack does routing, validation, OpenAPI, and MCP from one class. (Hono is an excellent edge router; if raw routing speed is your priority it belongs on your list — the pitch here is consolidation, not benchmarks.)

Hono

// @hono/zod-openapi route
const route = createRoute({
  method: 'get', path: '/forecast/{city}',
  responses: {200: {content: {'application/json':
    {schema: Out}}}},
});
app.openapi(route, c =>
  c.json(lookup(c.req.param('city'))));
// separate MCP SDK tool, mounted via @hono/mcp
mcp.registerTool('forecast',
  {inputSchema: z.object({city: z.string()})}, handler);

AgentBack — same class

// the Weather class from the LoopBack section:
// one Zod schema → a @get route and a @tool
@api() @mcpServer()
class Weather {
  @get('/forecast/{city}', {path: City, response: Out})
  route(i) { ... }
  @tool('forecast', {input: City, output: Out})
  tool(i) { ... }
}

## Get started Whatever you're coming from, the first step is the same: `npm create agentback`