Tell the agent what a tool returns

load_tool publishes an outputSchema when the endpoint declares a success body. With it, an agent can plan "fetch the order, take its customerId, fetch the customer" before it makes the first call.

Where the declaration comes from differs between the SDKs, and the difference is not a design choice.

Declare the response type

On ASP.NET Core the response type is read from ApiExplorer, so an endpoint that documents itself for Swagger is already done. On NestJS, TypeScript erases generics before the type reaches runtime, so the response type has to be declared.

[HttpGet("/orders/{id:int}")]
[ProducesResponseType(typeof(OrderResponse), StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status404NotFound)]
[Authorize(Policy = "OrdersRead")]
public IActionResult GetOrder(int id) => ...;

Produces<T>() on a minimal API works the same way, and nothing on [McpTool] controls this.

On NestJS, four forms are accepted per status code:

A DTO class needs class-validator decorators, as a request DTO does. If you already annotate with @nestjs/swagger, @ApiOkResponse({ type: OrderResponse }) is read as a fallback, and a sync handler with a plain class return type is the last resort. The responses declaration wins.

What the agent gets

{
  "name": "get_order",
  "inputSchema": {
    "type": "object",
    "properties": { "id": { "type": "integer" } },
    "...": "..."
  },
  "outputSchema": {
    "type": "object",
    "properties": {
      "id": { "type": "integer" },
      "item": { "type": "string" },
      "quantity": { "type": "integer" },
      "owner": { "type": "string" }
    },
    "required": ["id", "item", "quantity", "owner"]
  }
}

• One status wins. 200, 201, 202 and 204 are tried in that order, then the lowest remaining 2xx. Error statuses do not change outputSchema.
• A non-object root is wrapped. MCP requires an object root, so a list is published under a result property.
• Read-only members survive. A property your request schema drops because the caller cannot set it is exactly what a response is made of.

An endpoint that returns 204, or declares no 2xx, publishes no outputSchema. The key is absent, not empty.