# oaspec
[](https://hex.pm/packages/oaspec)
[](https://github.com/nao1215/oaspec/actions/workflows/ci.yml)
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 357 unit tests, ShellSpec CLI tests, 40 integration compile tests, and 94 OSS-derived fixtures (130 total)
## 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:
```text
gen/my_api/
types.gleam
decode.gleam
encode.gleam
request_types.gleam
response_types.gleam
middleware.gleam
guards.gleam
handlers.gleam
router.gleam
gen_client/my_api/
types.gleam
decode.gleam
encode.gleam
request_types.gleam
response_types.gleam
middleware.gleam
guards.gleam
client.gleam
```
Example generated code:
```gleam
/// 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
todo
}
```
## Quickstart
### Install from GitHub release
Requires Erlang/OTP 27+.
```sh
curl -fSL -o oaspec https://github.com/nao1215/oaspec/releases/latest/download/oaspec
chmod +x oaspec
sudo mv oaspec /usr/local/bin/
```
### Build from source
Requires Gleam 1.15+, Erlang/OTP 27+, and `rebar3`.
```sh
git clone https://github.com/nao1215/oaspec.git
cd oaspec
gleam deps download
gleam run -m gleescript
sudo mv oaspec /usr/local/bin/
```
### Generate code
1. Create a config file.
```sh
oaspec init
```
2. Edit `oaspec.yaml`.
```yaml
input: openapi.yaml
package: my_api
output:
dir: ./gen
```
3. Run the generator.
```sh
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.
| Field | Required | Default | Description |
|-------|----------|---------|-------------|
| `input` | yes | - | Path to an OpenAPI 3.x spec in YAML or JSON |
| `package` | no | `api` | Gleam module namespace prefix |
| `mode` | no | `both` | `server`, `client`, or `both` |
| `output.dir` | no | `./gen` | Base output directory |
| `output.server` | no | `<dir>/<package>` | Server output path |
| `output.client` | no | `<dir>_client/<package>` | Client output path |
CLI options:
```text
--config=<path> Path to config file (default: ./oaspec.yaml)
--mode=<mode> server, client, or both (default: both)
--output=<path> Override output base directory
```
## 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, `$ref` responses, `default` responses, and text or binary passthrough cases
- Security: `apiKey`, HTTP auth schemes, OAuth2, and OpenID Connect
- Generation safety: name collision handling, keyword escaping, validation guards, and capability errors with clear failure modes
## Current Boundaries
These are the most important limitations today:
- The following JSON Schema 2020-12 keywords are detected and rejected at parse time: `const`, `$defs`, `prefixItems`, `if`/`then`/`else`, `dependentSchemas`, `not`, `unevaluatedProperties`, `unevaluatedItems`, `contentEncoding`, `contentMediaType`, `contentSchema`
- OpenAPI 3.1 multi-type unions such as `type: [string, integer]` should be modeled with `oneOf` instead
- `xml` annotations are not used for code generation
- Some fields are parsed and preserved but not yet used by codegen, including `webhooks`, `externalDocs`, top-level `tags`, parameter examples, media examples and encoding, response headers and links, operation-level servers, and component headers/examples/links
## Development
This project uses [mise](https://mise.jdx.dev/) for tool versions and [just](https://just.systems/) as a task runner.
```sh
mise install
just check
just shellspec
just integration
```
Test structure:
| Command | Tool | What it tests |
|---------|------|---------------|
| `just test` | gleeunit | Parser, validator, naming, config, collision detection |
| `just shellspec` | ShellSpec | CLI behaviour, file generation, content, unsupported feature detection |
| `just integration` | gleeunit | Generated code compiles and the generated modules work together |
## License
[MIT](LICENSE)
