Create a custom plugin

2025-06-26

In this article, you learn how to create a custom plugin for the Dev Proxy. By creating plugins for Dev Proxy, you can extend its functionality and add custom features to fit your needs.

Prerequisites

Before you start creating a custom plugin, make sure you have the following prerequisites:

.NET v9 Core SDK
The latest version of the Dev Proxy Abstractions DLL, which you can find on the Dev Proxy GitHub releases page

Create a new plugin

Follow the next steps to create a new project:

Create a new class library project using the dotnet new classlib command.
```
dotnet new classlib -n MyCustomPlugin
```
Open the newly created project in Visual Studio Code.
```
code MyCustomPlugin
```
Add the Dev Proxy Abstractions DLL (DevProxy.Abstractions.dll) to the project folder.

Add the DevProxy.Abstractions.dll as a reference to your project DevProxyCustomPlugin.csproj file.

<ItemGroup>
  <Reference Include="DevProxy.Abstractions">
    <HintPath>.\DevProxy.Abstractions.dll</HintPath>
    <Private>false</Private>
    <ExcludeAssets>runtime</ExcludeAssets>
  </Reference>
</ItemGroup>

Add the NuGet packages required for your project.

dotnet add package Microsoft.Extensions.Configuration
dotnet add package Microsoft.Extensions.Configuration.Binder
dotnet add package Microsoft.Extensions.Logging.Abstractions
dotnet add package Unobtanium.Web.Proxy

Exclude the dependency DLLs from the build output by adding a ExcludeAssets tag per PackageReference in the DevProxyCustomPlugin.csproj file.
```
<ExcludeAssets>runtime</ExcludeAssets>
```

Create a new class that inherits from the BaseProxy class.

using DevProxy.Abstractions.Plugins;
using DevProxy.Abstractions.Proxy;
using Microsoft.Extensions.Logging;

namespace MyCustomPlugin;

public sealed class CatchApiCallsPlugin(
    ILogger<CatchApiCallsPlugin> logger,
    ISet<UrlToWatch> urlsToWatch) : BasePlugin(logger, urlsToWatch)
{
    public override string Name => nameof(CatchApiCallsPlugin);

    public override Task BeforeRequestAsync(ProxyRequestArgs e)
    {
        Logger.LogTrace("{Method} called", nameof(BeforeRequestAsync));

        ArgumentNullException.ThrowIfNull(e);

        if (!e.HasRequestUrlMatch(UrlsToWatch))
        {
            Logger.LogRequest("URL not matched", MessageType.Skipped, new(e.Session));
            return Task.CompletedTask;
        }

        var headers = e.Session.HttpClient.Request.Headers;
        var header = headers.Where(h => h.Name == "Authorization").FirstOrDefault();
        if (header is null)
        {
            Logger.LogRequest($"Does not contain the Authorization header", MessageType.Warning, new LoggingContext(e.Session));
            return Task.CompletedTask;
        }

        Logger.LogTrace("Left {Name}", nameof(BeforeRequestAsync));
        return Task.CompletedTask;
    }
}

Build your project.
```
dotnet build
```

Use your custom plugin

To use your custom plugin, you need to add it to the Dev Proxy configuration file:

Add the new plugin configuration in the devproxyrc.json file.

{
  "plugins": [{
    "name": "CatchApiCallsPlugin",
    "enabled": true,
    "pluginPath": "./bin/Debug/net9.0/MyCustomPlugin.dll",
  }]
}

Run the Dev Proxy.
```
devproxy
```

The example plugin checks all matching URLs for the required Authorization header. If the header isn't present, it shows a warning message.

Adding custom configuration to your plugin (optional)

You can extend your plugin's logic by adding custom configuration:

Inherit from the BasePlugin<TConfiguration> class. Dev Proxy exposes at runtime parsed plugin configuration through the Configuration property.

using DevProxy.Abstractions.Plugins;
using DevProxy.Abstractions.Proxy;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Logging;

namespace MyCustomPlugin;

public sealed class CatchApiCallsConfiguration
{
    public string? RequiredHeader { get; set; }
}

public sealed class CatchApiCallsPlugin(
    ILogger<CatchApiCallsPlugin> logger,
    ISet<UrlToWatch> urlsToWatch,
    IProxyConfiguration proxyConfiguration,
    IConfigurationSection pluginConfigurationSection) :
    BasePlugin<CatchApiCallsConfiguration>(
        logger,
        urlsToWatch,
        proxyConfiguration,
        pluginConfigurationSection)
{
    public override string Name => nameof(CatchApiCallsPlugin);

    public override Task BeforeRequestAsync(ProxyRequestArgs e)
    {
        Logger.LogTrace("{Method} called", nameof(BeforeRequestAsync));

        ArgumentNullException.ThrowIfNull(e);

        if (!e.HasRequestUrlMatch(UrlsToWatch))
        {
            Logger.LogRequest("URL not matched", MessageType.Skipped, new(e.Session));
            return Task.CompletedTask;
        }

        // Start using your custom configuration
        var requiredHeader = Configuration.RequiredHeader ?? string.Empty;
        if (string.IsNullOrEmpty(requiredHeader))
        {
            // Required header is not set, so we don't need to do anything
            Logger.LogRequest("Required header not set", MessageType.Skipped, new LoggingContext(e.Session));
            return Task.CompletedTask;
        }

        var headers = e.Session.HttpClient.Request.Headers;
        var header = headers.Where(h => h.Name == requiredHeader).FirstOrDefault();
        if (header is null)
        {
            Logger.LogRequest($"Does not contain the {requiredHeader} header", MessageType.Warning, new LoggingContext(e.Session));
            return Task.CompletedTask;
        }

        Logger.LogTrace("Left {Name}", nameof(BeforeRequestAsync));
        return Task.CompletedTask;
    }
}

Build your project.
```
dotnet build
```

Update your devproxyrc.json file to include the new configuration.

{
  "plugins": [{
    "name": "CatchApiCallsPlugin",
    "enabled": true,
    "pluginPath": "./bin/Debug/net9.0/MyCustomPlugin.dll",
    "configSection": "catchApiCalls"
  }],
  "catchApiCalls": {
    "requiredHeader": "Authorization" // Example configuration
  }
}

Run the Dev Proxy.
```
devproxy
```

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