Drawpenapi — design HTTP APIs visually

Drawpenapi is a free, browser-based designer for HTTP APIs. Build endpoints with parameters, request bodies and responses in a Swagger-style surface, define reusable schemas, and the OpenAPI YAML is generated as you work — 3.0, 3.1 or 3.2. Paste an existing spec and the rendered API appears instantly, with nothing lost when you edit it — the sync runs both ways. No account, no backend: specs are saved in your own browser.

• Swagger-style operation rows with method badges, and a guided “new endpoint” composer with CRUD templates.
• Tag groups, expandable parameter / request / response detail.
• A JSON Schema editor: properties, arrays, oneOf / allOf / anyOf, enums, constraints and $ref links.
• Security schemes (API key, HTTP, OAuth 2 flows, OpenID Connect, mutual TLS), servers with variables, webhooks, callbacks, links, examples and every component type.
• A problems list that checks the spec against its declared OpenAPI version.
• Live OpenAPI 3.0 / 3.1 / 3.2 YAML with syntax highlighting, and one-click copy.
• Paste an existing OpenAPI 3.x document and edit it visually.
• Export your spec as a .yaml file.
• Light and dark themes; works offline.