Anonymous embedded struct produces invalid JSON: "go build -tags=go_json "

[zhuzaiye]

Warning: go build -tags=go_json

Code example (minimal reproducible)

type skillUpStudyLabel struct {
    HasPartList   bool               `json:"has_part_list"`
    PartItems     []*skillUpResLabel `json:"part_items"`
    ResourceItems []*skillUpResource `json:"resource_items"`
}

type skillUpResLabel struct {
    ClassId     int    `json:"class_id"`
    ClassName   string `json:"class_name"`
    PercentRate int    `json:"percent_rate"`
    IsCompleted bool   `json:"is_completed"`

    // anonymous embedded struct
    skillUpStudyLabel
}

Expected JSON output

{
  "class_id": 150301,
  "class_name": "PreA1",
  "percent_rate": 0,
  "is_completed": false,
  "has_part_list": false,
  "part_items": [],
  "resource_items": [...]
}

Actual JSON output

macOS build produces invalid JSON, where the anonymous embedded struct is emitted as an object without a key:

{
    "class_id": 150301,
    "class_name": "PreA1",
    "percent_rate": 0,
    "is_completed": false,
    {
        "has_part_list": false,
        "part_items": [],
        "resource_items": [ ... ]
    }
}

Environment

macOS environment
GOOS=darwin
GOARCH=amd64
GOVERSION=go1.25.4
GOROOT=/usr/local/go
CGO_ENABLED=1

Build script:

export GOARCH=amd64
export GOOS=linux

go build -v -tags=go_json -ldflags="-s -w" -o run-pkg-api2 ../main.go

[ismaildasci]

This is a known issue with go_json (goccy/go-json) and anonymous embedded structs on macOS builds.

The problem:

go_json handles anonymous structs differently than standard library in some edge cases. This is specifically triggered with the -tags=go_json build flag.

Solutions:

1. Remove the anonymous embedding (recommended):

type skillUpResLabel struct {
    ClassId     int    `json:"class_id"`
    ClassName   string `json:"class_name"`
    // ... other fields
    
    // Instead of anonymous embed, use named field:
    StudyLabel skillUpStudyLabel `json:"study_label,omitempty"`
}

Or flatten manually:

type skillUpResLabel struct {
    ClassId       int    `json:"class_id"`
    HasPartList   bool   `json:"has_part_list"`
    PartItems     []*skillUpResLabel `json:"part_items"`
    ResourceItems []*skillUpResource `json:"resource_items"`
}

2. Use standard library JSON:

Build without the go_json tag:

go build ./...  # no tags

3. Use sonic instead:

go build -tags=sonic ./...

This is a goccy/go-json bug. You might want to report it to their repo if not already filed. Anonymous struct embedding with circular references is tricky for all JSON libs.