Add support for YAML-based config by Copilot · Pull Request #1491 · dotnet/dev-proxy

Copilot · 2026-01-13T13:14:44Z

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

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

Co-authored-by: waldekmastykarz <11164679+waldekmastykarz@users.noreply.github.com>

waldekmastykarz

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

Copilot · 2026-01-14T11:46:44Z

@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

DevProxy.Abstractions/Plugins/BaseLoader.cs

Copilot

Pull request overview

Adds first-class YAML support for Dev Proxy configuration files so users can author configs in either JSON/JSONC or YAML (including anchors and merge keys), while keeping existing JSON configs working.

Changes:

Added a YAML-to-JSON conversion utility (ProxyYaml) and a YAML IConfigurationProvider for loading root config from .yaml/.yml.
Updated config file discovery/loading to recognize devproxyrc.yaml/.yml and route through a new AddConfigFile helper.
Updated sample config/mock detection to include YAML files when scanning downloaded config folders.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
DevProxy/Extensions/ConfigurationManagerExtensions.cs	Adds `.yaml/.yml` to devproxyrc discovery and uses `AddConfigFile` for JSON/YAML loading.
DevProxy/Commands/ConfigCommand.cs	Expands sample config/mock detection to include YAML files.
DevProxy.Abstractions/Utils/ProxyYaml.cs	Introduces YAML-to-JSON conversion with merge-key handling and scalar typing.
DevProxy.Abstractions/Plugins/BaseLoader.cs	Converts YAML plugin/mock files to JSON before schema validation and parsing.
DevProxy.Abstractions/Extensions/YamlConfigurationExtensions.cs	Adds YAML configuration source/provider and `AddConfigFile` extension method.

Copilot · 2026-02-22T10:25:34Z

DevProxy.Abstractions/Extensions/YamlConfigurationExtensions.cs

+            // Skip nested merge keys in merged content
+            if (key == "<<")
+            {


CollectMergedValues skips nested YAML merge keys (<<) inside merged content. This means an anchored mapping that itself uses merges will not be fully expanded, which breaks expected YAML merge semantics and the PR’s stated merge-key support. Consider handling nested merge keys recursively (similar to FlattenMappingNode’s merge handling) rather than skipping them.

Suggested change

// Skip nested merge keys in merged content

if (key == "<<")

{

if (key == "<<")

{

// Handle nested merge keys recursively so that anchored mappings that

// themselves use merges are fully expanded.

YamlNode mergeNode = entry.Value;

if (mergeNode is YamlAliasNode aliasNode && aliasNode.Target is not null)

{

mergeNode = aliasNode.Target;

}

switch (mergeNode)

{

case YamlMappingNode mergeMapping:

// Merge the mapping into the current prefix

CollectMergedValues(mergeMapping, prefix, values);

break;

case YamlSequenceNode mergeSequence:

// Each mapping in the sequence is merged in order

foreach (var child in mergeSequence.Children)

{

YamlNode childNode = child;

if (childNode is YamlAliasNode childAlias && childAlias.Target is not null)

{

childNode = childAlias.Target;

}

if (childNode is YamlMappingNode childMapping)

{

CollectMergedValues(childMapping, prefix, values);

}

}

break;

}

// Skip normal processing for the merge key itself

Copilot · 2026-02-22T10:25:34Z

DevProxy.Abstractions/Utils/ProxyYaml.cs

+    // YAML 1.1 and 1.2 null values
+    private static bool IsNullValue(string value) =>
+        value.Length == 0 ||
+        value.Equals("~", StringComparison.Ordinal) ||
+        value.Equals("null", StringComparison.OrdinalIgnoreCase);


IsNullValue treats any empty string (value.Length == 0) as null. This will convert an explicitly quoted empty string (e.g., "" or '') into JSON null, which changes configuration semantics. Consider distinguishing an empty plain scalar (null) from a quoted empty string (empty) using YamlScalarNode.Style/tag info, or avoid treating length==0 as null here.

Copilot · 2026-02-22T10:25:34Z

DevProxy.Abstractions/Extensions/YamlConfigurationExtensions.cs

+                Data[prefix] = scalarNode.Value;
+                break;
+        }
+    }
+


YamlConfigurationProvider currently stores scalar values verbatim (e.g., "yes"/"no"/"on"/"off"). ConfigurationBinder only parses booleans reliably from "true"/"false", so YAML 1.1 boolean values will not bind to bool properties like ValidateSchemas/AsSystemProxy as described in the PR. Normalize these YAML boolean tokens (and optionally null tokens like "~"/"null") to the canonical strings expected by the binder when flattening scalars.

Suggested change

Data[prefix] = scalarNode.Value;

break;

}

}

Data[prefix] = NormalizeScalar(scalarNode);

break;

}

}

private static string? NormalizeScalar(YamlScalarNode scalarNode)

{

var value = scalarNode.Value;

if (value is null)

{

return null;

}

// Only normalize plain (unquoted) scalars; quoted values should remain verbatim.

if (scalarNode.Style != YamlDotNet.Core.ScalarStyle.Plain)

{

return value;

}

var trimmed = value.Trim();

if (trimmed.Length == 0)

{

return value;

}

var lower = trimmed.ToLowerInvariant();

// YAML 1.1 boolean true values

switch (lower)

{

case "y":

case "yes":

case "true":

case "on":

return "true";

case "n":

case "no":

case "false":

case "off":

return "false";

case "~":

case "null":

return null;

default:

return value;

}

}

Initial plan

c1b6598

Copilot AI assigned Copilot and waldekmastykarz Jan 13, 2026

Copilot started work on behalf of waldekmastykarz January 13, 2026 13:15 View session

Copilot AI and others added 4 commits January 13, 2026 13:25

Add YAML configuration support with anchors and merge keys

766dc40

Co-authored-by: waldekmastykarz <11164679+waldekmastykarz@users.noreply.github.com>

Fix YAML merge key precedence logic

77ccd75

Co-authored-by: waldekmastykarz <11164679+waldekmastykarz@users.noreply.github.com>

Fix unsafe casts in YAML processing

31f1cc9

Co-authored-by: waldekmastykarz <11164679+waldekmastykarz@users.noreply.github.com>

Improve YAML boolean and null value detection per YAML spec

be07f52

Co-authored-by: waldekmastykarz <11164679+waldekmastykarz@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Add support for YAML-based configuration~~ Add support for YAML-based config Jan 13, 2026

Copilot AI requested a review from waldekmastykarz January 13, 2026 13:40

Copilot finished work on behalf of waldekmastykarz January 13, 2026 13:40

waldekmastykarz requested changes Jan 14, 2026

View reviewed changes

Copilot started work on behalf of waldekmastykarz January 14, 2026 11:45 View session

waldekmastykarz reviewed Jan 14, 2026

View reviewed changes

DevProxy.Abstractions/Plugins/BaseLoader.cs Show resolved Hide resolved

Copilot AI requested a review from waldekmastykarz January 14, 2026 11:47

Copilot finished work on behalf of waldekmastykarz January 14, 2026 11:47

Copilot started work on behalf of waldekmastykarz January 14, 2026 11:47 View session

Copilot finished work on behalf of waldekmastykarz January 14, 2026 11:49

waldekmastykarz added 3 commits January 14, 2026 13:23

Merge branch 'main' into copilot/add-yaml-config-support

0188110

Merge branch 'main' into copilot/add-yaml-config-support

a5db9ea

Merge branch 'main' into copilot/add-yaml-config-support

82b9f58

waldekmastykarz marked this pull request as ready for review February 22, 2026 10:22

waldekmastykarz requested a review from a team as a code owner February 22, 2026 10:22

Copilot AI review requested due to automatic review settings February 22, 2026 10:22

Copilot started reviewing on behalf of waldekmastykarz February 22, 2026 10:23 View session

Copilot AI reviewed Feb 22, 2026

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Comments

Add support for YAML-based config#1491

Add support for YAML-based config#1491
Copilot wants to merge 8 commits intomainfrom
copilot/add-yaml-config-support

Copilot AI commented Jan 13, 2026 •

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-            // Skip nested merge keys in merged content
-            if (key == "<<")
-            {
+            if (key == "<<")
+            {
+                // Handle nested merge keys recursively so that anchored mappings that
+                // themselves use merges are fully expanded.
+                YamlNode mergeNode = entry.Value;
+                if (mergeNode is YamlAliasNode aliasNode && aliasNode.Target is not null)
+                {
+                    mergeNode = aliasNode.Target;
+                }
+                switch (mergeNode)
+                {
+                    case YamlMappingNode mergeMapping:
+                        // Merge the mapping into the current prefix
+                        CollectMergedValues(mergeMapping, prefix, values);
+                        break;
+                    case YamlSequenceNode mergeSequence:
+                        // Each mapping in the sequence is merged in order
+                        foreach (var child in mergeSequence.Children)
+                        {
+                            YamlNode childNode = child;
+                            if (childNode is YamlAliasNode childAlias && childAlias.Target is not null)
+                            {
+                                childNode = childAlias.Target;
+                            }
+                            if (childNode is YamlMappingNode childMapping)
+                            {
+                                CollectMergedValues(childMapping, prefix, values);
+                            }
+                        }
+                        break;
+                }
+                // Skip normal processing for the merge key itself

-                Data[prefix] = scalarNode.Value;
-                break;
-        }
-    }
+                Data[prefix] = NormalizeScalar(scalarNode);
+                break;
+        }
+    }
+    private static string? NormalizeScalar(YamlScalarNode scalarNode)
+    {
+        var value = scalarNode.Value;
+        if (value is null)
+        {
+            return null;
+        }
+        // Only normalize plain (unquoted) scalars; quoted values should remain verbatim.
+        if (scalarNode.Style != YamlDotNet.Core.ScalarStyle.Plain)
+        {
+            return value;
+        }
+        var trimmed = value.Trim();
+        if (trimmed.Length == 0)
+        {
+            return value;
+        }
+        var lower = trimmed.ToLowerInvariant();
+        // YAML 1.1 boolean true values
+        switch (lower)
+        {
+            case "y":
+            case "yes":
+            case "true":
+            case "on":
+                return "true";
+            case "n":
+            case "no":
+            case "false":
+            case "off":
+                return "false";
+            case "~":
+            case "null":
+                return null;
+            default:
+                return value;
+        }
+    }

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Copilot AI commented Jan 13, 2026 •

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