PhoenixSpectral

PhoenixSpectral integrates Spectral with Phoenix, making controller typespecs the single source of truth for OpenAPI 3.1 spec generation and request/response validation. Define your types once — PhoenixSpectral derives the API docs and enforces them at runtime.

Installation

Add phoenix_spectral to your dependencies in mix.exs:

def deps do
  [
    {:phoenix_spectral, "~> 0.4.0"}
  ]
end

Usage

Step 1: Define typed structs with Spectral

Spectral is an Elixir library that validates, decodes, and encodes data according to your @type definitions. Add use Spectral to a module and your types become the schema — PhoenixSpectral reads them to validate requests, decode inputs, encode responses, and generate the OpenAPI spec.

defmodule MyApp.User do
  use Spectral

  defstruct [:id, :name, :email]

  spectral(title: "User", description: "A user resource")
  @type t :: %__MODULE__{
    id: integer(),
    name: String.t(),
    email: String.t()
  }
end

defmodule MyApp.Error do
  use Spectral

  defstruct [:message]

  spectral(title: "Error")
  @type t :: %__MODULE__{message: String.t()}
end

Step 2: Create a typed controller

use PhoenixSpectral.Controller replaces the standard Phoenix action(conn, params) convention with five typed arguments. The four request inputs are kept separate rather than merged into one params map: the body can be a typed struct, which cannot be merged into a flat map alongside path args and query params without losing its type, and the OpenAPI generator needs to know where each field comes from — path, query, header, or body — to produce a correct spec.

@spec update(Plug.Conn.t(), %{id: integer()}, %{}, %{}, MyApp.User.t()) ::
        {200, %{}, MyApp.User.t()}
        | {404, %{}, MyApp.Error.t()}
def update(_conn, %{id: id}, _query_params, _headers, body), do: ...

• conn (Plug.Conn.t()) — the Plug connection, for out-of-band context (conn.assigns, conn.remote_ip, etc.)
• path_args (map, e.g. %{id: integer()}) — path parameters declared in the router, decoded from strings to the types declared in the spec
• query_params (map) — query string parameters, decoded to typed values; required keys use atom syntax (key: type), optional keys use arrow syntax (optional(key) => type)
• headers (map) — request headers, decoded from binary strings to typed values; required keys use atom syntax (key: type), optional keys use arrow syntax (optional(key) => type)
• body (any Elixir type (e.g., a struct), or nil) — decoded and validated request body, or nil for requests without a body

Note: Use conn only for context that isn't already captured in the typed arguments — primarily conn.assigns (auth data from upstream plugs), conn.remote_ip, conn.host, or conn.method. Do not read conn.path_params, conn.query_params, conn.req_headers, or conn.body_params directly; use the decoded and validated arguments instead.

Actions return {status_code, response_headers, response_body}. Union return types produce multiple OpenAPI response entries.

Use the spectral/1 macro to annotate actions with OpenAPI metadata such as summary and description:

defmodule MyAppWeb.UserController do
  use PhoenixSpectral.Controller, formats: [:json]  # opts forwarded to use Phoenix.Controller

  spectral(summary: "Get user", description: "Returns a user by ID")
  @spec show(Plug.Conn.t(), %{id: integer()}, %{}, %{}, nil) ::
          {200, %{}, MyApp.User.t()}
          | {404, %{}, MyApp.Error.t()}
  def show(_conn, %{id: id}, _query, _headers, _body) do
    case MyApp.Users.get(id) do
      {:ok, user} -> {200, %{}, user}
      :not_found -> {404, %{}, %MyApp.Error{message: "User not found"}}
    end
  end

  spectral(summary: "Create user")
  @spec create(Plug.Conn.t(), %{}, %{}, %{}, MyApp.User.t()) :: {201, %{}, MyApp.User.t()}
  def create(_conn, _path_args, _query, _headers, body) do
    {201, %{}, MyApp.Users.insert!(body)}
  end
end

Parameter descriptions

To add a description to a path or header parameter in the OpenAPI output, define a named type alias and annotate it with spectral:

spectral(description: "The user's unique identifier")
@type user_id :: integer()

@spec show(Plug.Conn.t(), %{id: user_id()}, %{}, %{}, nil) ::
        {200, %{}, MyApp.User.t()}
        | {404, %{}, MyApp.Error.t()}
def show(_conn, %{id: id}, _query, _headers, _body), do: ...

Typed response headers

Response headers are declared in the return type map:

@spec show(Plug.Conn.t(), %{id: integer()}, %{}, %{}, nil) ::
        {200, %{"x-request-id": String.t()}, MyApp.User.t()}
def show(_conn, %{id: id}, _query, _headers, _body) do
  {200, %{"x-request-id": "abc123"}, MyApp.Users.get!(id)}
end

Step 3: Serve the OpenAPI spec

defmodule MyAppWeb.OpenAPIController do
  use PhoenixSpectral.OpenAPIController,
    router: MyAppWeb.Router,
    title: "My API",
    version: "1.0.0"
end

Add routes in your router:

scope "/api" do
  get "/users/:id", MyAppWeb.UserController, :show
  post "/users", MyAppWeb.UserController, :create
  get "/openapi", MyAppWeb.OpenAPIController, :show
  get "/swagger", MyAppWeb.OpenAPIController, :swagger
end

GET /openapi returns the OpenAPI JSON spec. GET /swagger serves a Swagger UI page.

OpenAPIController options

Streaming and raw responses

An action can return a Plug.Conn directly instead of {status, headers, body}. This enables send_file/3, send_chunked/2, and any other conn-based response mechanism:

@spec download(Plug.Conn.t(), %{id: String.t()}, %{}, %{}, nil) :: {200, %{}, nil}
def download(conn, %{id: id}, _query, _headers, _body) do
  path = MyApp.Files.path_for(id)
  conn
  |> put_resp_content_type("application/octet-stream")
  |> send_file(200, path)
end

When a conn is returned, PhoenixSpectral passes it through without schema validation. The typespec still documents the endpoint for the OpenAPI spec, but the actual response is your responsibility.

Ecto schemas

PhoenixSpectral can work directly with Ecto schema structs. Two features in Spectral make this practical: struct defaults and field filtering with only.

Struct defaults

When decoding a JSON request body into a struct, fields absent from the JSON are filled from the struct's defstruct defaults — the same values you get from %MyStruct{}. Whether a field is required or optional in the JSON depends on its default and type:

This means Ecto fields declared with timestamps() (which default to nil) are handled automatically: they are optional on input (the client omits them) and omitted from output when nil. No special handling required.

Field filtering with only

The only option restricts which struct fields appear in encode, decode, and schema generation. Fields not in the list are silently dropped on encode and filled from struct defaults on decode. This is the Ecto equivalent of @derive {Jason.Encoder, only: [...]}.

defmodule MyApp.User do
  use Ecto.Schema
  use Spectral

  schema "users" do
    field :name, :string
    field :email, :string
    field :password_hash, :string
    has_many :posts, MyApp.Post
    timestamps()
  end

  # Expose only name and email. password_hash, posts association, and
  # timestamps are excluded — Spectral never tries to encode or decode them.
  spectral only: [:name, :email]
  @type public_t :: %__MODULE__{
          name: String.t() | nil,
          email: String.t() | nil,
          password_hash: String.t() | nil,
          posts: term(),
          inserted_at: DateTime.t() | nil,
          updated_at: DateTime.t() | nil
        }
end

You can define multiple types on the same module for different API views — for example a create_t for write input and a response_t for read output, each with a different only list.

only without Ecto

only is also useful on plain Elixir structs to expose different views of the same struct or to keep a single struct for both internal and external use:

defmodule User do
  use Spectral

  defstruct [:id, :name, :email, :password_hash]

  @type t :: %User{id: pos_integer(), name: String.t(), email: String.t(), password_hash: binary() | nil}

  spectral only: [:id, :name, :email]
  @type public_t :: %User{id: pos_integer(), name: String.t(), email: String.t(), password_hash: binary() | nil}
end

The example app in example/ demonstrates both features: User uses only to hide password_hash, and UserInput uses a nil struct default to make email optional in the request body.

Request/Response Behavior

• Invalid requests (type mismatch, missing required fields) return 400 Bad Request with a JSON error body listing the validation errors
• Response encoding failures return 500 Internal Server Error and log the error
• Missing or malformed typespecs raise at runtime — actions without @spec crash on dispatch; malformed specs crash on spec generation
• Only routes whose controllers use PhoenixSpectral.Controller appear in the generated OpenAPI spec; standard Phoenix controllers are ignored

Example

The example/ directory contains a complete runnable Phoenix app demonstrating a CRUD user API with path parameters, typed request headers, union return types, and an OpenAPI/Swagger UI endpoint. To run it:

cd example
mix deps.get
make integration-test   # starts server, runs curl checks, stops server

Configuration

PhoenixSpectral delegates encoding, decoding, and schema generation to Spectral / spectra. Configure them directly in config/config.exs (or config/runtime.exs).

Custom codecs

Spectral ships codecs for DateTime, Date, and MapSet that are not active by default. Register them — and any application-level custom codecs — under the :spectra application:

# config/config.exs
config :spectra, :codecs, %{
  {DateTime, {:type, :t, 0}} => Spectral.Codec.DateTime,
  {Date,     {:type, :t, 0}} => Spectral.Codec.Date,
  {MapSet,   {:type, :t, 1}} => Spectral.Codec.MapSet
}

The key is {ModuleOwningType, {:type, type_name, arity}}. See the Spectral codec guide for writing your own codecs with use Spectral.Codec.

Production: enable the module types cache

By default, spectra extracts type info from BEAM metadata on every decode/encode call. In production, enable persistent-term caching to avoid that overhead:

# config/prod.exs
config :spectra, :use_module_types_cache, true

This stores __spectra_type_info__/0 results in :persistent_term after the first call. Safe whenever modules are not hot-reloaded (i.e., in Mix releases). Clear manually with spectra_module_types:clear(Module) if needed.

Unicode validation

spectra skips Unicode validation of list-based strings by default. Enable it when strict validation matters:

config :spectra, :check_unicode, true

Design

• Typespecs are the single source of truth — no separate schema definitions; @spec drives both docs and validation
• Action convention — (conn, path_args, query_params, headers, body) → {status, headers, body}; union return types produce multiple OpenAPI response entries
• Crash on bad code, error on bad user input — malformed typespecs raise; invalid requests return 400, encoding failures return 500
• Automatic encoding/decoding — Spectral handles struct serialization
• Optional caching — via persistent_term for production performance