oaspec

Generate usable Gleam code from OpenAPI 3.x specifications.

oaspec is aimed at practical, typed code generation rather than a feature checklist. It handles the OpenAPI cases that tend to break real projects, such as $ref resolution, allOf, oneOf and anyOf, deepObject query parameters, form bodies, multipart bodies, and multiple security schemes, while failing fast when a spec goes outside the supported subset.

• Generate client and server-side modules from a single spec
• Produce readable Gleam types, encoders, decoders, request types, and response types
• Handle real-world OpenAPI patterns: unions, nullable fields, additionalProperties, form bodies, multipart, and security
• Backed by 615 unit tests, ShellSpec CLI tests, 40 integration compile tests, and 179 test fixtures (including 94 OSS-derived edge-case specs)

Why oaspec

• Built for Gleam: the generated code is shaped like normal Gleam modules, not generic templates awkwardly translated from another ecosystem.
• Focused on practical OpenAPI: coverage is strongest around the features teams actually ship with, not just toy Petstore specs.
• Strict by default: unsupported features are reported explicitly instead of being silently dropped into broken output.

What you get

Given one OpenAPI spec, oaspec generates modules you can keep in your repository:

gen/my_api/
  types.gleam
  decode.gleam
  encode.gleam
  request_types.gleam
  response_types.gleam
  middleware.gleam
  guards.gleam          (only if schemas have validation constraints)
  handlers.gleam
  router.gleam

gen_client/my_api/
  types.gleam
  decode.gleam
  encode.gleam
  request_types.gleam
  response_types.gleam
  middleware.gleam
  guards.gleam          (only if schemas have validation constraints)
  client.gleam

Example generated code:

/// A pet in the store
pub type Pet {
  Pet(
    id: Int,
    name: String,
    status: PetStatus,
    tag: Option(String),
  )
}

pub type PetStatus {
  PetStatusAvailable
  PetStatusPending
  PetStatusSold
}

pub fn create_pet(config: ClientConfig, body: types.CreatePetRequest)
  -> Result(response_types.CreatePetResponse, ClientError) {
  // ...
}

pub fn list_pets(req: request_types.ListPetsRequest)
  -> response_types.ListPetsResponse {
  let _ = req
  panic as "unimplemented: list_pets"
}

Quickstart

Install from GitHub release (Linux / macOS)

Requires Erlang/OTP 27+. The release binary is an Erlang escript that runs on any platform with Erlang installed.

curl -fSL -o oaspec https://github.com/nao1215/oaspec/releases/latest/download/oaspec
chmod +x oaspec
sudo mv oaspec /usr/local/bin/

On Windows, download oaspec from the latest release and run it with escript oaspec <command>. Erlang/OTP 27+ must be on your PATH.

Build from source (all platforms)

Requires Gleam 1.15+, Erlang/OTP 27+, and rebar3. Works on Linux, macOS, and Windows.

git clone https://github.com/nao1215/oaspec.git
cd oaspec
gleam deps download
gleam run -m gleescript

On Linux/macOS, move the binary into your PATH:

sudo mv oaspec /usr/local/bin/

On Windows, move oaspec to a directory on your PATH and run it with escript oaspec <command>.

Generate code

1. Create a config file.

oaspec init

1. Edit oaspec.yaml.

input: openapi.yaml
package: my_api
output:
  dir: ./gen

1. Run the generator.

oaspec generate --config=oaspec.yaml

You can also run gleam run -- generate --config=oaspec.yaml.

Configuration

Generated server code is written to <dir>/<package>. Generated client code is written to <dir>_client/<package>. The basename of each output directory must match package so imports such as import my_api/types resolve correctly.

CLI commands

CLI options for generate

CLI options for validate

Validate

Check a spec for unsupported patterns without generating code:

oaspec validate --config=oaspec.yaml

CI integration

Use --check and --fail-on-warnings to verify generated code stays in sync:

# Fail if generated code would differ from what's committed
oaspec generate --config=oaspec.yaml --check --fail-on-warnings

Best For

• Generating typed Gleam clients from an OpenAPI contract
• Keeping request and response types in sync with an external API spec
• Bootstrapping server-side types, handlers, and router support from the same source spec
• Catching unsupported spec features early in CI instead of after code generation

OpenAPI Support

oaspec supports OpenAPI 3.0.x and a practical subset of OpenAPI 3.1 in YAML or JSON.

Coverage is strongest in these areas:

• Schemas: component schemas, primitive aliases, enums, nullable fields, arrays, objects, allOf, oneOf, anyOf, and typed additionalProperties
• References: local $ref resolution for schemas, parameters, request bodies, responses, and path items, including circular-reference detection
• Parameters: path, query, header, and cookie parameters, including array serialization and style: deepObject
• Request bodies: application/json, application/x-www-form-urlencoded, and multipart/form-data
• Responses: typed status-code variants, $ref responses, default responses, and text or binary passthrough cases
• Security: apiKey (header, query, cookie), HTTP auth (bearer, basic, digest), OAuth2, and OpenID Connect. For OAuth2 and OpenID Connect, the generated client attaches a bearer token to requests; token acquisition, refresh, and flow execution are outside the generated code.
• Generation safety: name collision handling, keyword escaping, validation guards, and capability errors with clear failure modes

Current Boundaries

These boundaries are generated from the capability registry in src/oaspec/capability.gleam.

These are the most important limitations today:

• The following keywords are detected and rejected: $defs, prefixItems, if/then/else, dependentSchemas, not, unevaluatedProperties, unevaluatedItems, contentEncoding, contentMediaType, contentSchema, mutualTLS
• xml annotations are not handled by the parser
• Some fields are parsed and preserved but not yet used by codegen: webhooks, externalDocs, tags, examples, links, response headers, encoding
• Operation-level and path-level server overrides are supported in generated clients (precedence: operation > path > top-level)
• The following are normalized to supported equivalents:
• const: String const normalized to single-value enum
• type: [T, null]: Normalized to nullable
• type: [T1, T2]: Normalized to oneOf <!-- END GENERATED:BOUNDARIES -->

Mode-Specific Support

oaspec generates different files depending on the --mode flag. Some features have mode-specific restrictions enforced at validation time.

Generated files

Feature restrictions by mode

Library API

oaspec can be used as a Gleam library, not just a CLI tool. The generation pipeline is pure (no IO) and split into composable steps.

Pipeline overview

parse → normalize → resolve → capability check → hoist → dedup → validate → codegen

The oaspec/generate module wraps this pipeline into two entry points:

• generate.generate(spec, config) — run the full pipeline and return generated files
• generate.validate_only(spec, config) — run validation without code generation

Example: generate files from a parsed spec

import oaspec/config
import oaspec/generate
import oaspec/openapi/parser

let assert Ok(spec) = parser.parse_file("openapi.yaml")
let cfg = config.Config(
  input: "openapi.yaml",
  output_server: "./gen/my_api",
  output_client: "./gen_client/my_api",
  package: "my_api",
  mode: config.Both,
)

case generate.generate(spec, cfg) {
  Ok(summary) -> {
    // summary.files: List(GeneratedFile) — path and content for each file
    // summary.warnings: List(Diagnostic) — non-blocking warnings
    // summary.spec_title: String
  }
  Error(generate.ValidationErrors(errors:)) -> {
    // errors: List(Diagnostic) — blocking validation errors
  }
}

Example: validate without generating

case generate.validate_only(spec, cfg) {
  Ok(summary) -> // spec is valid; summary.warnings may be non-empty
  Error(generate.ValidationErrors(errors:)) -> // spec has errors
}

Key modules

Development

This project uses mise for tool versions and just as a task runner.

mise install
just check
just shellspec
just integration

Test structure:

License

MIT