(OAS3-GEN) OpenAPI 3.1+ Rust Generator

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oas3-gen is a command-line interface (CLI) for generating idiomatic Rust type definitions from an OpenAPI v3.1.x specification. The tool produces clean, production-ready code designed for seamless integration into any Rust project. Its primary function is to provide a robust and reliable method for type generation, ensuring the resulting code is correct, efficient, and well-documented.

Generated output preserves the declaration order written in the OpenAPI document for schemas, fields, enum variants, union variants, operations, and header constants. Map-like generated types use indexmap::IndexMap, and arrays marked with uniqueItems: true use indexmap::IndexSet so runtime insertion order is preserved too. Pass --no-ordered-collections to opt out of the indexmap runtime types and emit std::collections::HashMap<String, T> plus Vec<T> instead.

Quick Start

1. Installation

Install the tool directly from crates.io using cargo.

cargo install oas3-gen

Alternative build with Nix

• Make sure you have nix installed or install it with:

  sh <(curl --proto '=https' --tlsv1.2 -L https://nixos.org/nix/install) --daemon
  

• Now simply run it with:
  • nix run github:eklipse2k8/oas3-gen will run the binary, fetching the git repo automatically.
  • Or locally in the repo with nix run
  • To globally install use nix profile install

This takes cares of build dependencies (openssl, pkg-config) and they are packaged reproducibly and defined in flake.nix. A development shell is included and can be accessed by running nix develop or use direnv allow if available.

2. Generation

Provide a path to an OpenAPI specification (JSON or YAML) and specify an output file for the generated Rust code. The format is auto-detected based on file extension.

# generate types (structs and enums)
oas3-gen generate -i path/to/openapi.json -o path/to/types.rs
oas3-gen generate -i path/to/openapi.yaml -o path/to/types.rs

# generate client operations
oas3-gen generate client -i path/to/openapi.json -o path/to/client.rs

# generate server module (types.rs, server.rs, mod.rs)
oas3-gen generate server-mod -i path/to/openapi.json -o path/to/output/

Example

Consider the following Pet OpenAPI schema definition in fixtures/petstore.json:

{
  "Pet": {
    "type": "object",
    "required": ["id", "name"],
    "properties": {
     "id": {
      "type": "integer",
      "format": "int64"
     },
     "name": {
      "type": "string"
     },
     "tag": {
      "type": "string"
     }
    }
  }
}

Executing oas3-gen produces the corresponding Rust types.

// src/types.rs

use serde::Deserialize;

#[derive(Debug, Clone, PartialEq, Deserialize, oas3_gen_support::Default)]
pub struct Pet {
  pub id: i64,
  pub name: String,
  pub tag: Option<String>,
}

Server Generation

The server-mod command generates an axum-based server trait with handler functions and router configuration.

oas3-gen generate server-mod -i path/to/openapi.json -o path/to/output/

This generates three files:

• types.rs - All request/response types
• server.rs - Server trait, handlers, and router
• mod.rs - Module exports

Generated Server Trait

// server.rs

pub trait ApiServer: Send + Sync {
    fn list_pets(&self, request: ListPetsRequest)
        -> impl std::future::Future<Output = anyhow::Result<ListPetsResponse>> + Send;

    fn create_pet(&self, request: CreatePetRequest)
        -> impl std::future::Future<Output = anyhow::Result<CreatePetResponse>> + Send;

    fn get_pet_by_id(&self, request: GetPetByIdRequest)
        -> impl std::future::Future<Output = anyhow::Result<GetPetByIdResponse>> + Send;
}

Implementing the Server

use generated::{ApiServer, ListPetsRequest, ListPetsResponse, /* ... */};

#[derive(Clone)]
struct MyServer {
    db: DatabasePool,
}

impl ApiServer for MyServer {
    fn list_pets(&self, request: ListPetsRequest)
        -> impl std::future::Future<Output = anyhow::Result<ListPetsResponse>> + Send
    {
        async move {
            let pets = self.db.query_pets(request.query.limit).await?;
            Ok(ListPetsResponse::Ok(pets))
        }
    }
    // ... implement other methods
}

#[tokio::main]
async fn main() {
    let server = MyServer { db: create_pool().await };
    let app = generated::router(server);
    let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await.unwrap();
    axum::serve(listener, app).await.unwrap();
}

The generated router automatically:

• Extracts path, query, and header parameters
• Deserializes request bodies (JSON, form, multipart)
• Routes requests to the correct handler based on path and HTTP method
• Converts response enums to proper HTTP responses with status codes

Key Features

Missing features

• OAS 3.1 Links and $dynamic-ref (oas3 doesn't support this yet)
• OAS 3.2 Event-stream support
• External schema references
• HTTP schema references and fetching

Command-Line Options

OpenAPI to Rust code generator

Usage: oas3-gen [OPTIONS] <COMMAND>

Commands:
  list      List information from OpenAPI specification
  generate  Generates idiomatic, type-safe Rust code from an OpenAPI v3.1 (OAS31) specification
  help      Print this message or the help of the given subcommand(s)

Options:
  -h, --help     Print help
  -V, --version  Print version

Terminal Output:
      --color <WHEN>   Coloring [default: auto] [possible values: always, auto, never]
      --theme <THEME>  Theme [default: auto] [possible values: dark, light, auto]

Generate Command

Generates idiomatic, type-safe Rust code from an OpenAPI v3.1 (OAS31) specification

Usage: oas3-gen generate [OPTIONS] --input <FILE> [MODE]

Arguments:
  [MODE]  Sets the generation mode [default: types] [possible values: types, client, client-mod, server-mod]

Required:
  -i, --input <FILE>   Path to the OpenAPI specification file
  -o, --output <PATH>  Path for generated output (file for types/client, directory for client-mod/server-mod)

Code Generation:
  -C, --visibility <PUB>       Module visibility for generated items [default: public] [possible values: public, crate, file]
      --odata-support          Enable OData-specific field optionality rules (makes @odata.* fields optional on concrete types)
      --enum-mode <ENUM_MODE>  Specifies how to handle enum case sensitivity and duplicates [default: merge] [possible values: merge, preserve, relaxed]
      --no-helpers             Disable generation of ergonomic helper methods for enum variants
  -c, --customize <TYPE=PATH>  Custom serde_as type overrides (format: type_name=custom::Path)
      --all-headers            Emit header constants for all parameters defined in components, not just those used in operations
      --enable-builders        Enable bon builder derives on schema structs and builder methods on request structs
      --doc-format             Format documentation comments using mdformat (requires mdformat installed)
      --no-ordered-collections Emit std::collections::HashMap and Vec instead of indexmap::IndexMap/IndexSet for map fields and uniqueItems arrays

Operation Filtering:
      --only <id_1,id_2,...>     Include only the specified comma-separated operation IDs
      --exclude <id_1,id_2,...>  Exclude the specified comma-separated operation IDs
      --all-schemas              Generate all schemas, even those unreferenced by selected operations

List Command

List information from OpenAPI specification

Usage: oas3-gen list [OPTIONS] <COMMAND>

Commands:
  operations  List all operations defined in the OpenAPI specification
  help        Print this message or the help of the given subcommand(s)

Options:
  -h, --help     Print help
  -V, --version  Print version

Terminal Output:
      --color <WHEN>   Coloring [default: auto] [possible values: always, auto, never]
      --theme <THEME>  Theme [default: auto] [possible values: dark, light, auto]

Examples

# Basic usage to generate a client module from a json schema file
oas3-gen generate client-mod -i openapi.json -o generated

# Generate a server module with axum trait scaffolding
oas3-gen generate server-mod -i openapi.json -o generated

# Or generate types and client code individually ...

# Generate types with crate-level visibility
oas3-gen generate types -i openapi.json -o types.rs -C crate
oas3-gen generate client -i openapi.json -o client.rs -C crate

# Generate all schemas types including unused ones
oas3-gen generate -i openapi.json -o types.rs --all-schemas

# Generate code for specific operation types only
oas3-gen generate -i openapi.json -o types.rs --only create_user,get_user,update_user

# Generate code excluding certain operation types
oas3-gen generate -i openapi.json -o types.rs --exclude delete_user,list_users

# Generate all schemas but only specific operation types (includes unreferenced schemas)
oas3-gen generate -i openapi.json -o types.rs --all-schemas --only create_user

# Enable OData support for Microsoft Graph
oas3-gen generate -i graph-api.json -o types.rs --odata-support

# Enable relaxed (case-insensitive) enum deserialization
oas3-gen generate -i openapi.json -o types.rs --enum-mode relaxed

# Enable custom parsing through serde_as traits
oas3-gen generate client-mod -i openapi.json -o generated --customize datetime=MyCustomDateTime

# Enable bon builder derives for ergonomic struct construction
oas3-gen generate client-mod -i openapi.json -o generated --enable-builders

# Opt out of indexmap runtime types (emit std::collections::HashMap and Vec)
oas3-gen generate client-mod -i openapi.json -o generated --no-ordered-collections

# Format documentation comments with mdformat
oas3-gen generate client-mod -i openapi.json -o generated --doc-format

# List all operations in the specification
oas3-gen list operations -i openapi.json

Documentation Formatting with mdformat

The --doc-format flag pipes generated documentation comments through mdformat, a CommonMark-compliant Markdown formatter. This reformats long descriptions from OpenAPI summary and description fields into consistently wrapped, readable rustdoc comments. Lines are wrapped at 100 characters and markdown structure (headings, blockquotes, lists) is normalized.

The mdformat binary must be available on your PATH when using --doc-format. If it is not installed, the command will fail with an error. The flag is entirely optional and has no effect on the structure or correctness of generated code.

License

This project is licensed under the MIT License. See the LICENSE.md file for details.

Copyright (c) 2026 Individual contributors

Contribution

Contributions are welcome! Please see CONTRIBUTING.md for more details on how to get started.

All contributions submitted for inclusion in the work must be code-signed by an active HUMAN GitHub account and shall be licensed as MIT, without any additional terms or conditions. No bot or agent generated contributions will be accepted and will be immediately rejected.

See [OpenAPI v3.1.2]: https://spec.openapis.org/oas/v3.1.2