Add support for YAML-based config

[Copilot]

Adds YAML as an alternative to JSON for all configuration files. Supports YAML anchors and merge keys for reusable configuration blocks.

New Files

• DevProxy.Abstractions/Utils/ProxyYaml.cs - YAML-to-JSON conversion with anchor/alias/merge key support
• DevProxy.Abstractions/Extensions/YamlConfigurationExtensions.cs - ASP.NET Core IConfigurationProvider for YAML files

Modified Files

• ConfigurationManagerExtensions.cs - Added devproxyrc.yaml/.yml to config file discovery
• BaseLoader.cs - Convert YAML plugin configs to JSON before processing
• ConfigCommand.cs - Detect YAML config/mock files in downloaded samples

Features

• .yaml and .yml extensions supported everywhere JSON is used
• YAML 1.1 boolean values (yes/no, on/off, true/false)
• Full anchor/alias support with merge key precedence (explicit values override merged)

Example

definitions:
  throttled: &throttled
    statusCode: 429
    body: '{"error": "Too many requests"}'

mocks:
  - request:
      url: https://graph.microsoft.com/v1.0/users
    response:
      <<: *throttled
  - request:
      url: https://graph.microsoft.com/v1.0/groups
    response:
      <<: *throttled

Backward compatible—existing JSON configuration continues to work unchanged.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• aka.ms
  • Triggering command: /home/REDACTED/work/dev-proxy/dev-proxy/DevProxy/bin/Debug/net10.0/devproxy /home/REDACTED/work/dev-proxy/dev-proxy/DevProxy/bin/Debug/net10.0/devproxy --help (dns block)
  • Triggering command: /home/REDACTED/work/dev-proxy/dev-proxy/DevProxy/bin/Debug/net10.0/devproxy /home/REDACTED/work/dev-proxy/dev-proxy/DevProxy/bin/Debug/net10.0/devproxy --config-file devproxyrc.yaml (dns block)
  • Triggering command: /home/REDACTED/work/dev-proxy/dev-proxy/DevProxy/bin/Debug/net10.0/devproxy /home/REDACTED/work/dev-proxy/dev-proxy/DevProxy/bin/Debug/net10.0/devproxy --config-file devproxyrc-mocks.yaml (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add support for YAML-based config</issue_title>
<issue_description>Support both JSON- and YAML-based config.

YAML would be particularly beneficial for including comments (although they are supported in JSONC, but that's not always obvious) and multiline strings (ie. mocks).

One more thing that would be beneficial are anchors for repeated patterns:

definitions:
  throttled: &throttled
    statusCode: 429
    body: '{"error": "Too many requests"}'

responses:
  - url: https://graph.microsoft.com/v1.0/users
    <<: *throttled
  - url: https://graph.microsoft.com/v1.0/groups
    <<: *throttled

Notice how for both responses we reuse the same definition, significantly simplifying the config.

YAML config should be supported everywhere where we allow JSON-based config (root, plugins, etc)</issue_description>

Comments on the Issue (you are @copilot in this section)

@garrytrinder Nice idea! Let's do it 💪

• Fixes Add support for YAML-based config #1489

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[waldekmastykarz]

@copilot have you also considered supporting YAML in config files for plugins, like mocks, CRUD API, etc?

[Copilot]

@copilot have you also considered supporting YAML in config files for plugins, like mocks, CRUD API, etc?

Yes, YAML is already supported for all plugin config files. The BaseLoader class (which all loaders like MockResponsesLoader, CrudApiDefinitionLoader, etc. inherit from) automatically detects .yaml/.yml files and converts them to JSON before processing.

Users can specify YAML files for mocks, CRUD API definitions, error responses, rate limiting configs, and all other plugin-specific configuration files - they just need to use the .yaml or .yml extension.

[waldekmastykarz]

@copilot