ogen

OpenAPI v3 Code Generator for Go.

Install

console
go install -v github.com/ogen-go/ogen/cmd/ogen@latest

Usage

go
//go:generate go run github.com/ogen-go/ogen/cmd/ogen --target target/dir -package api --clean schema.json

or using container:

shell
docker run --rm \
  --volume ".:/workspace" \
  ghcr.io/ogen-go/ogen:latest --target workspace/petstore --clean workspace/petstore.yml

Features

No reflection or interface{}
- The json encoding is code-generated, optimized and uses go-faster/jx for speed and overcoming encoding/json limitations
- Validation is code-generated according to spec
Code-generated static radix router
No more boilerplate
- Structures are generated from OpenAPI v3 specification
- Arguments, headers, url queries are parsed according to specification into structures
- String formats like uuid, date, date-time, uri are represented by go types directly
Statically typed client and server
Convenient support for optional, nullable and optional nullable fields
- No more pointers
- Generated Optional[T], Nullable[T] or OptionalNullable[T] wrappers with helpers
- Special case for array handling with nil semantics relevant to specification
  - When array is optional, nil denotes absence of value
  - When nullable, nil denotes that value is nil
  - When required, nil currently the same as [], but is actually invalid
  - If both nullable and required, wrapper will be generated (TODO)
Support for untyped parameters (any)
- Parameters with no type specified in schema are represented as Go any
- Decoded as strings from URI (path, query, header, cookie)
- Client encoding uses fmt.Sprint for flexible value conversion
- Useful for legacy APIs or dynamic parameter types
Generated sum types for oneOf
- Primitive types (string, number) are detected by type
- Discriminator field is used if defined in schema
- Type is inferred by unique fields if possible
  - Field name discrimination: variants with different field names
  - Field type discrimination: variants with same field names but different types (e.g., {id: string} vs {id: integer})
  - Field value discrimination: variants with same field names and types but different enum values
Extra Go struct field tags in the generated types
OpenTelemetry tracing and metrics
Server-Sent Events (SSE) support

Example generated structure from schema:

go
// Pet describes #/components/schemas/Pet.
type Pet struct {
	Birthday     time.Time     `json:"birthday"`
	Friends      []Pet         `json:"friends"`
	ID           int64         `json:"id"`
	IP           net.IP        `json:"ip"`
	IPV4         net.IP        `json:"ip_v4"`
	IPV6         net.IP        `json:"ip_v6"`
	Kind         PetKind       `json:"kind"`
	Name         string        `json:"name"`
	Next         OptData       `json:"next"`
	Nickname     NilString     `json:"nickname"`
	NullStr      OptNilString  `json:"nullStr"`
	Rate         time.Duration `json:"rate"`
	Tag          OptUUID       `json:"tag"`
	TestArray1   [][]string    `json:"testArray1"`
	TestDate     OptTime       `json:"testDate"`
	TestDateTime OptTime       `json:"testDateTime"`
	TestDuration OptDuration   `json:"testDuration"`
	TestFloat1   OptFloat64    `json:"testFloat1"`
	TestInteger1 OptInt        `json:"testInteger1"`
	TestTime     OptTime       `json:"testTime"`
	Type         OptPetType    `json:"type"`
	URI          url.URL       `json:"uri"`
	UniqueID     uuid.UUID     `json:"unique_id"`
}

Example generated server interface:

go
// Server handles operations described by OpenAPI v3 specification.
type Server interface {
	PetGetByName(ctx context.Context, params PetGetByNameParams) (Pet, error)
	// ...
}

Example generated client method signature:

go
type PetGetByNameParams struct {
    Name string
}

// GET /pet/{name}
func (c *Client) PetGetByName(ctx context.Context, params PetGetByNameParams) (res Pet, err error)

Generics

Instead of using pointers, ogen generates generic wrappers.

For example, OptNilString is string that is optional (no value) and can be null.

go
// OptNilString is optional nullable string.
type OptNilString struct {
	Value string
	Set   bool
	Null  bool
}

Multiple convenience helper methods and functions are generated, some of them:

go
func (OptNilString) Get() (v string, ok bool)
func (OptNilString) IsNull() bool
func (OptNilString) IsSet() bool
func (OptNilString) IsEmpty() bool

func NewOptNilString(v string) OptNilString

Recursive types

If ogen encounters recursive types that can't be expressed in go, pointers are used as fallback.

Sum types

For oneOf sum-types are generated. ID that is one of [string, integer] will be represented like that:

go
type ID struct {
	Type   IDType
	String string
	Int    int
}

// Also, some helpers:
func NewStringID(v string) ID
func NewIntID(v int) ID

Discriminator Inference

ogen automatically infers how to discriminate between oneOf variants using several strategies:

1. Type-based discrimination (for primitive types)

Variants with different JSON types are discriminated by checking the JSON type at runtime:

json
{
  "oneOf": [
    {"type": "string"},
    {"type": "integer"}
  ]
}

2. Explicit discriminator (when discriminator field is specified)

When a discriminator field is defined in the schema, ogen uses it directly:

json
{
  "oneOf": [...],
  "discriminator": {
    "propertyName": "type",
    "mapping": {"user": "#/components/schemas/User", ...}
  }
}

3. Field-based discrimination (automatic inference from unique fields)

ogen analyzes the fields in each variant to find discriminating characteristics:

Field name discrimination: Variants have different field names

json
{
  "oneOf": [
    {"type": "object", "required": ["userId"], "properties": {"userId": {"type": "string"}}},
    {"type": "object", "required": ["orderId"], "properties": {"orderId": {"type": "string"}}}
  ]
}

Field type discrimination: Variants have fields with the same name but different types

json
{
  "oneOf": [
    {
      "type": "object",
      "required": ["id", "value"],
      "properties": {
        "id": {"type": "string"},
        "value": {"type": "string"}
      }
    },
    {
      "type": "object",
      "required": ["id", "value"],
      "properties": {
        "id": {"type": "integer"},
        "value": {"type": "number"}
      }
    }
  ]
}

In this case, ogen checks the JSON type of the id field at runtime to determine which variant to decode.

Field value discrimination: Variants have fields with the same name and type but different enum values

json
{
  "oneOf": [
    {
      "type": "object",
      "required": ["status"],
      "properties": {
        "status": {"type": "string", "enum": ["active", "pending"]}
      }
    },
    {
      "type": "object",
      "required": ["status"],
      "properties": {
        "status": {"type": "string", "enum": ["inactive", "deleted"]}
      }
    }
  ]
}

In this case, ogen checks the actual string value of the status field at runtime and matches it against each variant's enum values. The enum values must be disjoint (non-overlapping) for this to work. If enum values overlap, ogen will report an error and suggest using an explicit discriminator.

Const values

ogen supports the JSON Schema const keyword, which specifies that a field must have a fixed value (introduced in JSON Schema draft 6 and supported in OpenAPI 3.0+). When a field has a const value, it is encoded directly in the generated JSON encoder without requiring the struct field to be set.

Example schema with const values

yaml
components:
  schemas:
    ErrorResponse:
      type: object
      properties:
        code:
          type: integer
          const: 400
        status:
          type: string
          const: "error"
        message:
          type: string

Generated code

The generated struct includes the field, but the encoder hardcodes the const value:

go
type ErrorResponse struct {
    Code    int64  `json:"code"`    // const: 400
    Status  string `json:"status"`  // const: "error"
    Message string `json:"message"`
}

func (s *ErrorResponse) encodeFields(e *jx.Encoder) {
    {
        e.FieldStart("code")
        e.Int64(400)  // Const value encoded directly
    }
    {
        e.FieldStart("status")
        e.Str("error")  // Const value encoded directly
    }
    {
        e.FieldStart("message")
        e.Str(s.Message)  // Regular field
    }
}

Benefits

Simplified initialization: You don't need to set const fields when creating struct instances
Type safety: Const values are validated at code generation time
Performance: Const values are encoded directly without runtime lookups
Works with allOf: Const values are preserved when merging schemas with allOf

Supported const value types

Primitives: integer, number, string, boolean
Special values: null, empty strings (""), zero values (0, false)
Complex types: object, array (when specified in schema)

Extension properties

OpenAPI enables Specification Extensions,
which are implemented as patterned fields that are always prefixed by x-.

Server name

Optionally, server name can be specified by x-ogen-server-name, for example:

json
{
  "openapi": "3.0.3",
  "servers": [
    {
      "x-ogen-server-name": "production",
      "url": "https://{region}.example.com/{val}/v1",
    },
    {
      "x-ogen-server-name": "prefix",
      "url": "/{val}/v1",
    },
    {
      "x-ogen-server-name": "const",
      "url": "https://cdn.example.com/v1"
    }
  ],
(...)

Custom type name

Optionally, type name can be specified by x-ogen-name, for example:

json
{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "type": "object",
  "x-ogen-name": "Name",
  "properties": {
    "foobar": {
      "$ref": "#/$defs/FooBar"
    }
  },
  "$defs": {
    "FooBar": {
      "x-ogen-name": "FooBar",
      "type": "object",
      "properties": {
        "foo": {
          "type": "string"
        }
      }
    }
  }
}

Custom field name

Optionally, type name can be specified by x-ogen-properties, for example:

yaml
components:
  schemas:
    Node:
      type: object
      properties:
        parent:
          $ref: "#/components/schemas/Node"
        child:
          $ref: "#/components/schemas/Node"
      x-ogen-properties:
        parent:
          name: "Prev"
        child:
          name: "Next"

The generated source code looks like:

go
// Ref: #/components/schemas/Node
type Node struct {
    Prev *Node `json:"parent"`
    Next *Node `json:"child"`
}

Extra struct field tags

Optionally, additional Go struct field tags can be specified by x-oapi-codegen-extra-tags, for example:

yaml
components:
  schemas:
    Pet:
      type: object
      required:
        - id
      properties:
        id:
          type: integer
          format: int64
          x-oapi-codegen-extra-tags:
            gorm: primaryKey
            valid: customIdValidator

The generated source code looks like:

go
// Ref: #/components/schemas/Pet
type Pet struct {
    ID   int64     `gorm:"primaryKey" valid:"customNameValidator" json:"id"`
}

Streaming JSON encoding

By default, ogen loads the entire JSON body into memory before decoding it.
Optionally, streaming JSON encoding can be enabled by x-ogen-json-streaming, for example:

yaml
requestBody:
  required: true
  content:
    application/json:
      x-ogen-json-streaming: true
      schema:
        type: array
        items:
          type: number

Custom validation

Optionally, custom validation can be specified by x-ogen-validate, for example:

yaml
components:
  schemas:
    Product:
      type: object
      properties:
        name:
          type: string
          x-ogen-validate:
            minWords: 2
        tags:
          type: array
          items:
            type: string
          x-ogen-validate:
            uniqueItems: true
        metadata:
          type: object
          additionalProperties: true
          x-ogen-validate:
            fieldCount:
              min: 1
              max: 10

Custom validators must be registered before validation is performed:

go
import "github.com/ogen-go/ogen/validate"

// Register validators
validate.RegisterValidator("minWords", func(value any, params any) error {
    // ... validate minimum word count
})

validate.RegisterValidator("uniqueItems", func(value any, params any) error {
    // ... validate array has no duplicate items
})

validate.RegisterValidator("fieldCount", func(value any, params any) error {
    // ... validate object field count within min/max range
})

Operation groups

Optionally, operations can be grouped so a handler interface will be generated for each group of operations.
This is useful for organizing operations for large APIs.

The group for operations on a path or individual operations can be specified by x-ogen-operation-group, for example:

yaml
paths:
  /images:
    x-ogen-operation-group: Images
    get:
      operationId: listImages
      ...
  /images/{imageID}:
    x-ogen-operation-group: Images
    get:
      operationId: getImageByID
      ...
  /users:
    x-ogen-operation-group: Users
    get:
      operationId: listUsers
      ...

The generated handler interfaces look like this:

go
// x-ogen-operation-group: Images
type ImagesHandler interface {
    ListImages(ctx context.Context, req *ListImagesRequest) (*ListImagesResponse, error)
    GetImageByID(ctx context.Context, req *GetImagesByIDRequest) (*GetImagesByIDResponse, error)
}

// x-ogen-operation-group: Users
type UsersHandler interface {
    ListUsers(ctx context.Context, req *ListUsersRequest) (*ListUsersResponse, error)
}

type Handler interface {
    ImagesHandler
    UsersHandler
    // All un-grouped operations will be on this interface
}

JSON

Code generation provides very efficient and flexible encoding and decoding of json:

go
// Decode decodes Error from json.
func (s *Error) Decode(d *jx.Decoder) error {
	if s == nil {
		return errors.New("invalid: unable to decode Error to nil")
	}
	return d.ObjBytes(func(d *jx.Decoder, k []byte) error {
		switch string(k) {
		case "code":
			if err := func() error {
				v, err := d.Int64()
				s.Code = int64(v)
				if err != nil {
					return err
				}
				return nil
			}(); err != nil {
				return errors.Wrap(err, "decode field \"code\"")
			}
		case "message":
			if err := func() error {
				v, err := d.Str()
				s.Message = string(v)
				if err != nil {
					return err
				}
				return nil
			}(); err != nil {
				return errors.Wrap(err, "decode field \"message\"")
			}
		default:
			return d.Skip()
		}
		return nil
	})
}

SSE

Server-Sent Events (SSE) code generation is supported in ogen for text/event-stream
responses, following the
HTML Server-Sent Events specification
with some Go-specific behavior.

[!NOTE]
Only SSE client generation is supported in ogen for now.

Event shapes

There is no official standard for representing text/event-stream in OpenAPI
before OAS 3.2. Because of this, ogen supports multiple ways of representing SSE
in schema. In ogen, this is called an SSE event shape.

An SSE event may contain the standard id, event, data, and retry fields.
The generated client dispatches an event only when at least one data: line is
present. If the event field is omitted, it defaults to "message" as defined
by the SSE specification.

You can represent SSE events in OpenAPI with multiple shapes. The shape is
selected with x-ogen-sse-event-shape.

data-only

By default, text/event-stream uses the data-only shape:

yaml
text/event-stream:
  schema:
    type: object
    properties:
      message:
        type: string
      createdAt:
        type: string
        format: date-time

This shape describes only the SSE data field. Standard SSE fields are still
parsed by the client and exposed on the generated event type.

full

full shape describes the full SSE event envelope. This is useful when you need
discriminators on the event field or schema validation for the full envelope.

yaml
text/event-stream:
  x-ogen-sse-event-shape: full
  schema:
    oneOf:
      - $ref: "#/components/schemas/EventA"
      - $ref: "#/components/schemas/EventB"
    discriminator:
      propertyName: event
      mapping:
        event_a: "#/components/schemas/EventA"
        event_b: "#/components/schemas/EventB"

# ...

EventA:
  type: object
  required: [ event, data ]
  properties:
    event:
      type: string
      enum: [ event_a ]
    data:
      $ref: "#/components/schemas/EventAData"
EventB:
  type: object
  required: [ event, data ]
  properties:
    event:
      type: string
      enum: [ event_b ]
    data:
      $ref: "#/components/schemas/EventBData"

In full mode the schema describes the full SSE event envelope.

It is not required to specify every standard field in the schema. Omitted
standard fields continue to follow default SSE event semantics.

full-array

full-array is the array form of full shape. The schema must be an array of
full SSE event envelopes.

Generated client

Generated SSE clients handle reconnection automatically. Reconnect behavior can
be configured with generated SSE client options, including the initial
Last-Event-ID, retry delay, maximum reconnect attempts, decoder buffer size,
maximum event size, and a retry error handler.

The stream-level last event ID and retry interval are updated automatically as
valid id: and retry: fields are parsed, even if the event is not
dispatched.

If the configured maximum event size is set and reached while parsing an event,
the client returns ErrEventTooLarge and drains the remaining part of that event
before continuing with the next one, without closing the stream.

Generated SSE responses comply with this interface:

go
type Client[E any] interface {
    Next(ctx context.Context) (E, error)         // Returns the next event.
    All(ctx context.Context) iter.Seq2[E, error] // Exposes iterator over events.
    State() (state sse.State, latestErr error)   // Reports current stream state with the latest or terminal error.
    Close() error                                // Closes the stream.
}