Marshal Options Example

This example demonstrates the WithMarshalOptions ServerOption, which threads protojson.MarshalOptions through every JSON response a generated sebuf HTTP server produces.

The problem#

Proto3 silently drops zero values from JSON. A bool accepting_orders = false is indistinguishable from "field never set" — both produce the same {} on the wire. For fields where false (or 0, or "") carries semantic meaning, that's lossy.

message Status {
  bool accepting_orders = 1;   // false ≠ "unset" in app logic
  int32 subscriber_count = 2;  // 0 subscribers is a real, visible state
  string note = 3;             // empty note is a deliberate choice
}

The fix#

import "google.golang.org/protobuf/encoding/protojson"
 
err := api.RegisterOfferingServiceServer(handler,
    api.WithMux(mux),
    api.WithMarshalOptions(protojson.MarshalOptions{
        EmitUnpopulated: true,
    }),
)

WithMarshalOptions is process-scoped (set once at server registration) and threads through every marshal site: standard responses, error bodies, and SSE event streams. Zero-value MarshalOptions{} preserves the previous wire output byte-for-byte — opt-in only.

Run it#

make demo

This boots two servers backed by the same handler, both returning a Status{} with proto3 zero values for every field:

• :8080 — default opts
• :8081 — WithMarshalOptions(protojson.MarshalOptions{EmitUnpopulated: true})

In another terminal:

make test

Expected output:

=== Default opts (port 8080) — zero values omitted ===
{}

=== EmitUnpopulated: true (port 8081) — zero values surfaced ===
{"acceptingOrders":false, "subscriberCount":0, "note":""}

Same handler. Same proto. Different wire output, controlled by one registration-time option.

Other useful options#

protojson.MarshalOptions exposes more knobs that flow through the same plumbing:

See protojson docs for the full list.