RPC v2 JSON

smithy.protocols#rpcv2Json is the JSON variant of Smithy’s RPC v2 protocol. It shares routing, headers, errors, and event streams with RPC v2 CBOR and carries JSON bodies instead of CBOR. NSmithy generates typed clients and ASP.NET Core servers for unary and event stream operations.

See Protocol Status for maturity and current conformance numbers.

Protocol behavior

Area	rpcv2Json
Route	`POST /service/{Service}/operation/{Operation}`
Body	JSON
Content type	`application/json`
Operation header	`Smithy-Protocol: rpc-v2-json`
Errors	Modeled type in the JSON `__type` member
Streaming	Input, output, and duplex event streams
HTTP bindings	Not used

JSON serialization differs from restJson1 in three places:

Shape or trait	rpcv2Json
`@jsonName`	Ignored. Properties use member names.
`timestamp`	Always epoch seconds. `@timestampFormat` is ignored.
`bigInteger`, `bigDecimal`	JSON strings, such as `"9223372036854775808"`

Modeling

Apply @rpcv2Json to the service. Operations do not use @http:

$version: "2"

namespace example.weather

use smithy.protocols#rpcv2Json

@rpcv2Json
service Weather {
    version: "2026-01-01"
    operations: [GetCity]
}

A service can declare both @rpcv2Cbor and @rpcv2Json. Both protocols use the same routes, so a server maps each on its own endpoint route builder.

On the wire

POST /service/Weather/operation/GetCity HTTP/1.1
Host: api.example.com
Smithy-Protocol: rpc-v2-json
Content-Type: application/json
Accept: application/json

{"cityId":"123"}

HTTP/1.1 400 Bad Request
Smithy-Protocol: rpc-v2-json
Content-Type: application/json

{"__type":"example.weather#NoSuchResource","resourceType":"city"}

Streaming

Event streams use the same framing as rpcv2Cbor. Event payloads and the initial-request and initial-response messages are JSON.

Packages

Surface	Packages
Client	`NSmithy.Client`, `NSmithy.Codecs.Json`, `NSmithy.Protocols.RpcV2Json`
Server	`NSmithy.Server.AspNetCore`, `NSmithy.Codecs.Json`, `NSmithy.Protocols.RpcV2Json`