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codecov Maintainability Rating .NET license

OpenCaptcha is an open source, free, public API that generates CAPTCHA images from text.

OpenCaptcha provides:

• A free, public API at api.opencaptcha.io.
• Documentation at opencaptcha.io.
• Self-hosting support (see the Installation section).

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Try it out

curl -X 'POST' \
 'https://api.opencaptcha.io/captcha' \
 -H 'Content-Type: application/json' \
 -d '{ "text": "captcha" }'

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Try it out

curl -X 'POST' \
 'https://api.opencaptcha.io/captcha' \
 -H 'Content-Type: application/json' \
 -d '{ "text": "普通" }'

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Try it out

curl -X 'POST' \
 'https://api.opencaptcha.io/captcha' \
 -H 'Content-Type: application/json' \
 -d '{
 "text": "hello world",
 "theme": {
 "primaryColor": "#ADD8E6",
 "secondaryColor": "#FFFFFF"
 }
 }'

• Docker

1. From the root folder of the project, run the following commands:

   docker build -t opencaptcha -f ./src/Captcha.WebApi/Dockerfile .
   docker run -it -p 5280:8080 opencaptcha

2. The API can be accessed at http://localhost:5280.

• Visual Studio 2026
  • With ASP.NET and web development installed from the Visual Studio Installer
• .NET 10 SDK

1. Open the solution in Visual Studio 2026.
2. Build and launch the Captcha.WebApi project.
3. The API can be accessed at https://localhost:5280.

1. Open the solution in Visual Studio 2026.
2. Run the tests in Test Explorer.

A significant portion of this project, especially the CAPTCHA generation code, is inspired by work originally published on February 9, 2004, by BrainJar.

Pull requests are accepted.

MIT