...

.github/copilot-instructions.md

@@ -0,0 +1,5 @@
+# Copilot Instructions
+
+## Project Guidelines
+- When running tests in this workspace, do not run tests for the net48 target framework.
+- When changing System.Text.Json code in this workspace, verify API availability for netstandard2.0 and netstandard2.1 instead of assuming newer APIs exist.

src/WireMock.Net.Minimal/Settings/WireMockServerSettingsParser.cs

@@ -10,6 +10,7 @@ using WireMock.Constants;
 using WireMock.Logging;
 using WireMock.Models;
 using WireMock.Types;
+using WireMock.Transformers;
 using WireMock.Util;
 
 namespace WireMock.Settings;
@@ -74,6 +75,8 @@ public static class WireMockServerSettingsParser
             WatchStaticMappingsInSubdirectories = parser.GetBoolValue(nameof(WireMockServerSettings.WatchStaticMappingsInSubdirectories)),
         };
 
+        settings.DefaultJsonBodyTransformer = new NewtonsoftJsonBodyTransformer(settings);
+
 #if USE_ASPNETCORE
         settings.CorsPolicyOptions = parser.GetEnumValue(nameof(WireMockServerSettings.CorsPolicyOptions), CorsPolicyOptions.None);
         settings.ClientCertificateMode = parser.GetEnumValue(nameof(WireMockServerSettings.ClientCertificateMode), ClientCertificateMode.NoCertificate);

src/WireMock.Net.Minimal/Transformers/Handlebars/IHandlebarsContext.cs

@@ -1,6 +1,7 @@
 // Copyright © WireMock.Net
 
 using HandlebarsDotNet;
+using WireMock.Transformers;
 
 namespace WireMock.Transformers.Handlebars;
 

src/WireMock.Net.Minimal/Transformers/ITransformerContext.cs

@@ -1,14 +0,0 @@
-// Copyright © WireMock.Net
-
-using WireMock.Handlers;
-
-namespace WireMock.Transformers;
-
-internal interface ITransformerContext
-{
-    IFileSystemHandler FileSystemHandler { get; }
-
-    string ParseAndRender(string text, object model);
-
-    object? ParseAndEvaluate(string text, object model);
-}

src/WireMock.Net.Minimal/Transformers/Transformer.cs

@@ -1,10 +1,5 @@
 // Copyright © WireMock.Net
 
-using System.Collections;
-using System.Linq;
-using HandlebarsDotNet.Helpers.Models;
-using Newtonsoft.Json;
-using Newtonsoft.Json.Linq;
 using Stef.Validation;
 using WireMock.Settings;
 using WireMock.Types;
@@ -14,17 +9,13 @@ namespace WireMock.Transformers;
 
 internal class Transformer : ITransformer
 {
-    private readonly JsonSerializer _jsonSerializer;
+    private readonly IJsonBodyTransformer _jsonBodyTransformer;
     private readonly ITransformerContextFactory _factory;
 
     public Transformer(WireMockServerSettings settings, ITransformerContextFactory factory)
     {
         _factory = Guard.NotNull(factory);
-
+        _jsonBodyTransformer = Guard.NotNull(settings).DefaultJsonBodyTransformer;
-        _jsonSerializer = new JsonSerializer
-        {
-            Culture = Guard.NotNull(settings).Culture
-        };
     }
 
     public IBodyData? TransformBody(
@@ -121,13 +112,17 @@ internal class Transformer : ITransformer
         });
     }
 
-    private IBodyData? TransformBodyData(ITransformerContext transformerContext, ReplaceNodeOptions options, TransformModel model, IBodyData original, bool useTransformerForBodyAsFile)
+    private BodyData? TransformBodyData(ITransformerContext transformerContext, ReplaceNodeOptions options, TransformModel model, IBodyData original, bool useTransformerForBodyAsFile)
     {
         switch (original.DetectedBodyType)
         {
             case BodyType.Json:
             case BodyType.ProtoBuf:
-                return TransformBodyAsJson(transformerContext, options, model, original);
+                return _jsonBodyTransformer.TransformBodyAsJson(
+                    transformerContext,
+                    options,
+                    model,
+                    original);
 
             case BodyType.File:
                 return TransformBodyAsFile(transformerContext, model, original, useTransformerForBodyAsFile);
@@ -159,185 +154,7 @@ internal class Transformer : ITransformer
         return newHeaders;
     }
 
-    private IBodyData TransformBodyAsJson(ITransformerContext transformerContext, ReplaceNodeOptions options, object model, IBodyData original)
+    private static BodyData TransformBodyAsString(ITransformerContext transformerContext, object model, IBodyData original)
-    {
-        JToken? jToken = null;
-        switch (original.BodyAsJson)
-        {
-            case JObject bodyAsJObject:
-                jToken = bodyAsJObject.DeepClone();
-                WalkNode(transformerContext, options, jToken, model);
-                break;
-
-            case JArray bodyAsJArray:
-                jToken = bodyAsJArray.DeepClone();
-                WalkNode(transformerContext, options, jToken, model);
-                break;
-
-            case var bodyAsEnumerable when bodyAsEnumerable is IEnumerable and not string:
-                jToken = JArray.FromObject(bodyAsEnumerable, _jsonSerializer);
-                WalkNode(transformerContext, options, jToken, model);
-                break;
-
-            case string bodyAsString:
-                jToken = ReplaceSingleNode(transformerContext, options, bodyAsString, model);
-                break;
-
-            case not null:
-                jToken = JObject.FromObject(original.BodyAsJson, _jsonSerializer);
-                WalkNode(transformerContext, options, jToken, model);
-                break;
-        }
-
-        return new BodyData
-        {
-            Encoding = original.Encoding,
-            DetectedBodyType = original.DetectedBodyType,
-            DetectedBodyTypeFromContentType = original.DetectedBodyTypeFromContentType,
-            ProtoDefinition = original.ProtoDefinition,
-            ProtoBufMessageType = original.ProtoBufMessageType,
-            BodyAsJson = jToken
-        };
-    }
-
-    private JToken ReplaceSingleNode(ITransformerContext transformerContext, ReplaceNodeOptions options, string stringValue, object model)
-    {
-        var transformedString = transformerContext.ParseAndRender(stringValue, model);
-
-        if (!string.Equals(stringValue, transformedString))
-        {
-            const string property = "_";
-            JObject dummy = JObject.Parse($"{{ \"{property}\": null }}");
-            if (dummy[property] == null)
-            {
-                // TODO: check if just returning null is fine
-                return string.Empty;
-            }
-
-            JToken node = dummy[property]!;
-
-            ReplaceNodeValue(options, node, transformedString);
-
-            return dummy[property]!;
-        }
-
-        return stringValue;
-    }
-
-    private void WalkNode(ITransformerContext transformerContext, ReplaceNodeOptions options, JToken node, object model)
-    {
-        switch (node.Type)
-        {
-            case JTokenType.Object:
-                // In case of Object, loop all children. Do a ToArray() to avoid `Collection was modified` exceptions.
-                foreach (var child in node.Children<JProperty>().ToArray())
-                {
-                    WalkNode(transformerContext, options, child.Value, model);
-                }
-                break;
-
-            case JTokenType.Array:
-                // In case of Array, loop all items. Do a ToArray() to avoid `Collection was modified` exceptions.
-                foreach (var child in node.Children().ToArray())
-                {
-                    WalkNode(transformerContext, options, child, model);
-                }
-                break;
-
-            case JTokenType.String:
-                // In case of string, try to transform the value.
-                var stringValue = node.Value<string>();
-                if (string.IsNullOrEmpty(stringValue))
-                {
-                    return;
-                }
-
-                var transformed = transformerContext.ParseAndEvaluate(stringValue!, model);
-                if (!Equals(stringValue, transformed))
-                {
-                    ReplaceNodeValue(options, node, transformed);
-                }
-                break;
-        }
-    }
-
-    // ReSharper disable once UnusedParameter.Local
-    private void ReplaceNodeValue(ReplaceNodeOptions options, JToken node, object? transformedValue)
-    {
-        switch (transformedValue)
-        {
-            case JValue jValue:
-                node.Replace(jValue);
-                return;
-
-            case string transformedString:
-                var (isConvertedFromString, convertedValueFromString) = TryConvert(options, transformedString);
-                if (isConvertedFromString)
-                {
-                    node.Replace(JToken.FromObject(convertedValueFromString, _jsonSerializer));
-                }
-                else
-                {
-                    node.Replace(ParseAsJObject(transformedString));
-                }
-                break;
-
-            case WireMockList<string> strings:
-                switch (strings.Count)
-                {
-                    case 1:
-                        node.Replace(ParseAsJObject(strings[0]));
-                        return;
-
-                    case > 1:
-                        node.Replace(JToken.FromObject(strings.ToArray(), _jsonSerializer));
-                        return;
-                }
-                break;
-
-            case { }:
-                var (isConverted, convertedValue) = TryConvert(options, transformedValue);
-                if (isConverted)
-                {
-                    node.Replace(JToken.FromObject(convertedValue, _jsonSerializer));
-                }
-                return;
-
-            default: // It's null, skip it. Maybe remove it ?
-                return;
-        }
-    }
-
-    private static JToken ParseAsJObject(string stringValue)
-    {
-        return JsonUtils.TryParseAsJObject(stringValue, out var parsedAsjObject) ? parsedAsjObject : stringValue;
-    }
-
-    private static (bool IsConverted, object ConvertedValue) TryConvert(ReplaceNodeOptions options, object value)
-    {
-        var valueAsString = value as string;
-
-        if (options == ReplaceNodeOptions.Evaluate)
-        {
-            if (valueAsString != null && WrappedString.TryDecode(valueAsString, out var decoded))
-            {
-                return (true, decoded);
-            }
-
-            return (false, value);
-        }
-
-        if (valueAsString != null)
-        {
-            return WrappedString.TryDecode(valueAsString, out var decoded) ?
-                (true, decoded) :
-                StringUtils.TryConvertToKnownType(valueAsString);
-        }
-
-        return (false, value);
-    }
-
-    private static IBodyData TransformBodyAsString(ITransformerContext transformerContext, object model, IBodyData original)
     {
         return new BodyData
         {
@@ -348,7 +165,7 @@ internal class Transformer : ITransformer
         };
     }
 
-    private static IBodyData TransformBodyAsFile(ITransformerContext transformerContext, object model, IBodyData original, bool useTransformerForBodyAsFile)
+    private static BodyData TransformBodyAsFile(ITransformerContext transformerContext, object model, IBodyData original, bool useTransformerForBodyAsFile)
     {
         var transformedBodyAsFilename = transformerContext.ParseAndRender(original.BodyAsFile!, model);
 

src/WireMock.Net.Shared/Settings/WireMockServerSettings.cs

@@ -14,6 +14,7 @@ using WireMock.Logging;
 using WireMock.Matchers;
 using WireMock.Models;
 using WireMock.RegularExpressions;
+using WireMock.Transformers;
 using WireMock.Types;
 
 namespace WireMock.Settings;
@@ -362,11 +363,31 @@ public class WireMockServerSettings
     /// Default is <see cref="NewtonsoftJsonConverter"/>.
     /// </remarks>
     [PublicAPI]
-    public IJsonConverter DefaultJsonSerializer { get; set; } = new NewtonsoftJsonConverter();
+    public IJsonConverter DefaultJsonSerializer { get; set; }
+
+    /// <summary>
+    /// Gets or sets the default JSON body transformer used for template-based JSON body transformations.
+    /// </summary>
+    /// <remarks>
+    /// Set this property to provide a custom implementation for transforming JSON and ProtoBuf body content.
+    /// Default is <see cref="NewtonsoftJsonBodyTransformer"/>.
+    /// </remarks>
+    [PublicAPI]
+    [JsonIgnore]
+    public IJsonBodyTransformer DefaultJsonBodyTransformer { get; set; }
 
     /// <summary>
     /// WebSocket settings.
     /// </summary>
     [PublicAPI]
     public WebSocketSettings? WebSocketSettings { get; set; }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="WireMockServerSettings"/> class.
+    /// </summary>
+    public WireMockServerSettings()
+    {
+        DefaultJsonSerializer = new NewtonsoftJsonConverter();
+        DefaultJsonBodyTransformer = new NewtonsoftJsonBodyTransformer(this);
+    }
 }

src/WireMock.Net.Shared/Transformers/IJsonBodyTransformer.cs

@@ -0,0 +1,28 @@
+// Copyright © WireMock.Net
+
+using JetBrains.Annotations;
+using WireMock.Types;
+using WireMock.Util;
+
+namespace WireMock.Transformers;
+
+/// <summary>
+/// Defines the contract for transforming JSON-like body data using a transformer context.
+/// </summary>
+[PublicAPI]
+public interface IJsonBodyTransformer
+{
+    /// <summary>
+    /// Transforms the JSON body using the provided transformer context and model.
+    /// </summary>
+    /// <param name="transformerContext">The transformer context used to render and evaluate template values.</param>
+    /// <param name="options">The JSON node replacement behavior to apply during transformation.</param>
+    /// <param name="model">The model used when rendering or evaluating template values.</param>
+    /// <param name="original">The original body data to transform.</param>
+    /// <returns>The transformed JSON body data.</returns>
+    BodyData TransformBodyAsJson(
+        ITransformerContext transformerContext,
+        ReplaceNodeOptions options,
+        object model,
+        IBodyData original);
+}

src/WireMock.Net.Shared/Transformers/ITransformerContext.cs

@@ -0,0 +1,34 @@
+// Copyright © WireMock.Net
+
+using JetBrains.Annotations;
+using WireMock.Handlers;
+
+namespace WireMock.Transformers;
+
+/// <summary>
+/// Defines the transformer context used to render and evaluate templates during response transformation.
+/// </summary>
+[PublicAPI]
+public interface ITransformerContext
+{
+    /// <summary>
+    /// Gets the file system handler used by the current transformer context.
+    /// </summary>
+    IFileSystemHandler FileSystemHandler { get; }
+
+    /// <summary>
+    /// Renders the specified template text using the supplied model.
+    /// </summary>
+    /// <param name="text">The template text to render.</param>
+    /// <param name="model">The model used during rendering.</param>
+    /// <returns>The rendered text.</returns>
+    string ParseAndRender(string text, object model);
+
+    /// <summary>
+    /// Evaluates the specified template text using the supplied model.
+    /// </summary>
+    /// <param name="text">The template text to evaluate.</param>
+    /// <param name="model">The model used during evaluation.</param>
+    /// <returns>The evaluated value.</returns>
+    object? ParseAndEvaluate(string text, object model);
+}

src/WireMock.Net.Shared/Transformers/NewtonsoftJsonBodyTransformer.cs

@@ -0,0 +1,271 @@
+// Copyright © WireMock.Net
+
+using System.Collections;
+using HandlebarsDotNet.Helpers.Models;
+using JetBrains.Annotations;
+using JsonConverter.Abstractions;
+using JsonConverter.Newtonsoft.Json;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+using WireMock.Settings;
+using WireMock.Types;
+using WireMock.Util;
+
+namespace WireMock.Transformers;
+
+/// <summary>
+/// Default JSON body transformer implementation based on Newtonsoft.Json.
+/// </summary>
+/// <remarks>
+/// Initializes a new instance of the <see cref="NewtonsoftJsonBodyTransformer"/> class.
+/// </remarks>
+/// <param name="settings">The server settings used to configure JSON transformation behavior.</param>
+[PublicAPI]
+public class NewtonsoftJsonBodyTransformer(WireMockServerSettings settings) : IJsonBodyTransformer
+{
+    private readonly IJsonConverter _jsonConverter = new NewtonsoftJsonConverter();
+
+    /// <inheritdoc />
+    public BodyData TransformBodyAsJson(
+        ITransformerContext transformerContext,
+        ReplaceNodeOptions options,
+        object model,
+        IBodyData original)
+    {
+        var jsonSerializer = new JsonSerializer
+        {
+            Culture = settings.Culture
+        };
+
+        JToken? jToken = null;
+        switch (original.BodyAsJson)
+        {
+            case JObject bodyAsJObject:
+                jToken = bodyAsJObject.DeepClone();
+                WalkNode(transformerContext, jsonSerializer, options, jToken, model);
+                break;
+
+            case JArray bodyAsJArray:
+                jToken = bodyAsJArray.DeepClone();
+                WalkNode(transformerContext, jsonSerializer, options, jToken, model);
+                break;
+
+            case var bodyAsEnumerable when bodyAsEnumerable is IEnumerable and not string:
+                jToken = JArray.FromObject(bodyAsEnumerable, jsonSerializer);
+                WalkNode(transformerContext, jsonSerializer, options, jToken, model);
+                break;
+
+            case string bodyAsString:
+                jToken = ReplaceSingleNode(transformerContext, jsonSerializer, options, bodyAsString, model);
+                break;
+
+            case not null:
+                jToken = JObject.FromObject(original.BodyAsJson, jsonSerializer);
+                WalkNode(transformerContext, jsonSerializer, options, jToken, model);
+                break;
+        }
+
+        return new BodyData
+        {
+            Encoding = original.Encoding,
+            DetectedBodyType = original.DetectedBodyType,
+            DetectedBodyTypeFromContentType = original.DetectedBodyTypeFromContentType,
+            ProtoDefinition = original.ProtoDefinition,
+            ProtoBufMessageType = original.ProtoBufMessageType,
+            BodyAsJson = jToken
+        };
+    }
+
+    private JToken ParseAsJObject(string stringValue)
+    {
+        if (_jsonConverter.IsValidJson(stringValue))
+        {
+            try
+            {
+                // Try to convert this string into a JObject
+                return JObject.Parse(stringValue!);
+            }
+            catch
+            {
+            }
+        }
+
+        return stringValue;
+    }
+
+    private JToken ReplaceSingleNode(ITransformerContext transformerContext, JsonSerializer jsonSerializer, ReplaceNodeOptions options, string stringValue, object model)
+    {
+        var transformedString = transformerContext.ParseAndRender(stringValue, model);
+
+        if (!string.Equals(stringValue, transformedString))
+        {
+            const string property = "_";
+            JObject dummy = JObject.Parse($"{{ \"{property}\": null }}");
+            if (dummy[property] == null)
+            {
+                return string.Empty;
+            }
+
+            JToken node = dummy[property]!;
+
+            ReplaceNodeValue(jsonSerializer, options, node, transformedString);
+
+            return dummy[property]!;
+        }
+
+        return stringValue;
+    }
+
+    private void WalkNode(ITransformerContext transformerContext, JsonSerializer jsonSerializer, ReplaceNodeOptions options, JToken node, object model)
+    {
+        switch (node.Type)
+        {
+            case JTokenType.Object:
+                foreach (var child in node.Children<JProperty>().ToArray())
+                {
+                    WalkNode(transformerContext, jsonSerializer, options, child.Value, model);
+                }
+                break;
+
+            case JTokenType.Array:
+                foreach (var child in node.Children().ToArray())
+                {
+                    WalkNode(transformerContext, jsonSerializer, options, child, model);
+                }
+                break;
+
+            case JTokenType.String:
+                var stringValue = node.Value<string>();
+                if (string.IsNullOrEmpty(stringValue))
+                {
+                    return;
+                }
+
+                var transformed = transformerContext.ParseAndEvaluate(stringValue!, model);
+                if (!Equals(stringValue, transformed))
+                {
+                    ReplaceNodeValue(jsonSerializer, options, node, transformed);
+                }
+                break;
+        }
+    }
+
+    private void ReplaceNodeValue(JsonSerializer jsonSerializer, ReplaceNodeOptions options, JToken node, object? transformedValue)
+    {
+        switch (transformedValue)
+        {
+            case JValue jValue:
+                node.Replace(jValue);
+                return;
+
+            case string transformedString:
+                var (isConvertedFromString, convertedValueFromString) = TryConvert(options, transformedString);
+                if (isConvertedFromString)
+                {
+                    node.Replace(JToken.FromObject(convertedValueFromString, jsonSerializer));
+                }
+                else
+                {
+                    node.Replace(ParseAsJObject(transformedString));
+                }
+                break;
+
+            case WireMockList<string> strings:
+                switch (strings.Count)
+                {
+                    case 1:
+                        node.Replace(ParseAsJObject(strings[0]));
+                        return;
+
+                    case > 1:
+                        node.Replace(JToken.FromObject(strings.ToArray(), jsonSerializer));
+                        return;
+                }
+                break;
+
+            case { }:
+                var (isConverted, convertedValue) = TryConvert(options, transformedValue);
+                if (isConverted)
+                {
+                    node.Replace(JToken.FromObject(convertedValue, jsonSerializer));
+                }
+                return;
+
+            default:
+                return;
+        }
+    }
+
+    private static (bool IsConverted, object ConvertedValue) TryConvert(ReplaceNodeOptions options, object value)
+    {
+        var valueAsString = value as string;
+
+        if (options == ReplaceNodeOptions.Evaluate)
+        {
+            if (valueAsString != null && WrappedString.TryDecode(valueAsString, out var decoded))
+            {
+                return (true, decoded);
+            }
+
+            return (false, value);
+        }
+
+        if (valueAsString != null)
+        {
+            return WrappedString.TryDecode(valueAsString, out var decoded)
+                ? (true, decoded)
+                : TryConvertToKnownType(valueAsString);
+        }
+
+        return (false, value);
+    }
+
+    internal static (bool IsConverted, object ConvertedValue) TryConvertToKnownType(string value)
+    {
+        if (bool.TryParse(value, out var boolResult))
+        {
+            return (true, boolResult);
+        }
+
+        if (int.TryParse(value, out var intResult))
+        {
+            return (true, intResult);
+        }
+
+        if (long.TryParse(value, out var longResult))
+        {
+            return (true, longResult);
+        }
+
+        if (double.TryParse(value, out var doubleResult))
+        {
+            return (true, doubleResult);
+        }
+
+        if (Guid.TryParseExact(value, "D", out var guidResult))
+        {
+            return (true, guidResult);
+        }
+
+        if (TimeSpan.TryParse(value, out var timeSpanResult))
+        {
+            return (true, timeSpanResult);
+        }
+
+        if (DateTime.TryParse(value, out var dateTimeResult))
+        {
+            return (true, dateTimeResult);
+        }
+
+        if ((value.StartsWith("ftp://", StringComparison.OrdinalIgnoreCase) ||
+            value.StartsWith("ftps://", StringComparison.OrdinalIgnoreCase) ||
+            value.StartsWith("http://", StringComparison.OrdinalIgnoreCase) ||
+            value.StartsWith("https://", StringComparison.OrdinalIgnoreCase)) &&
+            Uri.TryCreate(value, UriKind.RelativeOrAbsolute, out var uriResult))
+        {
+            return (true, uriResult);
+        }
+
+        return (false, value);
+    }
+}

src/WireMock.Net.Shared/Transformers/SystemTextJsonBodyTransformer.cs

@@ -0,0 +1,210 @@
+// Copyright © WireMock.Net
+
+using System.Collections;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+using HandlebarsDotNet.Helpers.Models;
+using JetBrains.Annotations;
+using JsonConverter.Abstractions;
+using JsonConverter.System.Text.Json;
+using WireMock.Settings;
+using WireMock.Types;
+using WireMock.Util;
+
+namespace WireMock.Transformers;
+
+/// <summary>
+/// JSON body transformer implementation based on System.Text.Json.
+/// </summary>
+/// <param name="settings">The server settings used to configure JSON transformation behavior.</param>
+[PublicAPI]
+public class SystemTextJsonBodyTransformer(WireMockServerSettings settings) : IJsonBodyTransformer
+{
+    private readonly IJsonConverter _jsonConverter = new SystemTextJsonConverter();
+
+    /// <inheritdoc />
+    public BodyData TransformBodyAsJson(
+        ITransformerContext transformerContext,
+        ReplaceNodeOptions options,
+        object model,
+        IBodyData original)
+    {
+        JsonNode? jsonNode = null;
+        switch (original.BodyAsJson)
+        {
+            case JsonObject bodyAsJsonObject:
+                jsonNode = CloneNode(bodyAsJsonObject);
+                jsonNode = WalkNode(transformerContext, options, jsonNode, model);
+                break;
+
+            case JsonArray bodyAsJsonArray:
+                jsonNode = CloneNode(bodyAsJsonArray);
+                jsonNode = WalkNode(transformerContext, options, jsonNode, model);
+                break;
+
+            case var bodyAsEnumerable when bodyAsEnumerable is IEnumerable and not string:
+                jsonNode = JsonSerializer.SerializeToNode(bodyAsEnumerable);
+                if (jsonNode != null)
+                {
+                    jsonNode = WalkNode(transformerContext, options, jsonNode, model);
+                }
+                break;
+
+            case string bodyAsString:
+                jsonNode = ReplaceSingleNode(transformerContext, options, bodyAsString, model);
+                break;
+
+            case not null:
+                jsonNode = JsonSerializer.SerializeToNode(original.BodyAsJson);
+                if (jsonNode != null)
+                {
+                    jsonNode = WalkNode(transformerContext, options, jsonNode, model);
+                }
+                break;
+        }
+
+        return new BodyData
+        {
+            Encoding = original.Encoding,
+            DetectedBodyType = original.DetectedBodyType,
+            DetectedBodyTypeFromContentType = original.DetectedBodyTypeFromContentType,
+            ProtoDefinition = original.ProtoDefinition,
+            ProtoBufMessageType = original.ProtoBufMessageType,
+            BodyAsJson = jsonNode
+        };
+    }
+
+    private JsonNode ParseAsJsonObject(string stringValue)
+    {
+        if (_jsonConverter.IsValidJson(stringValue))
+        {
+            try
+            {
+                var parsed = JsonNode.Parse(stringValue);
+                if (parsed is JsonObject)
+                {
+                    return parsed;
+                }
+            }
+            catch
+            {
+                // Ignore and return as string.
+            }
+        }
+
+        return JsonValue.Create(stringValue)!;
+    }
+
+    private JsonNode? ReplaceSingleNode(ITransformerContext transformerContext, ReplaceNodeOptions options, string stringValue, object model)
+    {
+        var transformedString = transformerContext.ParseAndRender(stringValue, model);
+
+        if (!string.Equals(stringValue, transformedString))
+        {
+            return ReplaceNodeValue(options, transformedString);
+        }
+
+        return JsonValue.Create(stringValue);
+    }
+
+    private JsonNode? WalkNode(ITransformerContext transformerContext, ReplaceNodeOptions options, JsonNode? node, object model)
+    {
+        switch (node)
+        {
+            case JsonObject jsonObject:
+                foreach (var property in jsonObject.ToArray())
+                {
+                    jsonObject[property.Key] = WalkNode(transformerContext, options, property.Value, model);
+                }
+                return jsonObject;
+
+            case JsonArray jsonArray:
+                for (var i = 0; i < jsonArray.Count; i++)
+                {
+                    jsonArray[i] = WalkNode(transformerContext, options, jsonArray[i], model);
+                }
+                return jsonArray;
+
+            case JsonValue jsonValue when jsonValue.TryGetValue<string>(out var stringValue):
+                if (string.IsNullOrEmpty(stringValue))
+                {
+                    return jsonValue;
+                }
+
+                var transformed = transformerContext.ParseAndEvaluate(stringValue!, model);
+                return !Equals(stringValue, transformed) ? ReplaceNodeValue(options, transformed) ?? jsonValue : jsonValue;
+
+            default:
+                return node;
+        }
+    }
+
+    private JsonNode? ReplaceNodeValue(ReplaceNodeOptions options, object? transformedValue)
+    {
+        switch (transformedValue)
+        {
+            case JsonNode jsonNode:
+                return CloneNode(jsonNode);
+
+            case string transformedString:
+                var (isConvertedFromString, convertedValueFromString) = TryConvert(options, transformedString);
+                return isConvertedFromString
+                    ? JsonSerializer.SerializeToNode(convertedValueFromString)
+                    : ParseAsJsonObject(transformedString);
+
+            case WireMockList<string> strings:
+                switch (strings.Count)
+                {
+                    case 1:
+                        return ParseAsJsonObject(strings[0]);
+
+                    case > 1:
+                        return JsonSerializer.SerializeToNode(strings.ToArray());
+                }
+                break;
+
+            case { }:
+                var (isConverted, convertedValue) = TryConvert(options, transformedValue);
+                if (isConverted)
+                {
+                    return JsonSerializer.SerializeToNode(convertedValue);
+                }
+                break;
+        }
+
+        return null;
+    }
+
+    private static JsonNode? CloneNode(JsonNode? node)
+    {
+#if NET8_0_OR_GREATER
+        return node?.DeepClone();
+#else
+        return node == null ? null : JsonNode.Parse(node.ToJsonString());
+#endif
+    }
+
+    private static (bool IsConverted, object ConvertedValue) TryConvert(ReplaceNodeOptions options, object value)
+    {
+        var valueAsString = value as string;
+
+        if (options == ReplaceNodeOptions.Evaluate)
+        {
+            if (valueAsString != null && WrappedString.TryDecode(valueAsString, out var decoded))
+            {
+                return (true, decoded);
+            }
+
+            return (false, value);
+        }
+
+        if (valueAsString != null)
+        {
+            return WrappedString.TryDecode(valueAsString, out var decoded)
+                ? (true, decoded)
+                : NewtonsoftJsonBodyTransformer.TryConvertToKnownType(valueAsString);
+        }
+
+        return (false, value);
+    }
+}

src/WireMock.Net.Shared/WireMock.Net.Shared.csproj

@@ -31,6 +31,7 @@
         <PackageReference Include="AnyOf" Version="0.5.0.1" />
         <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" Version="8.0.2" />
         <PackageReference Include="JsonConverter.Newtonsoft.Json" Version="0.10.0" />
+        <PackageReference Include="JsonConverter.System.Text.Json" Version="0.10.0" />
     </ItemGroup>
 
     <ItemGroup>

test/WireMock.Net.Tests/Transformers/JsonBodyTransformerTests.cs

@@ -0,0 +1,176 @@
+// Copyright © WireMock.Net
+
+using System.Text;
+using System.Text.Json.Nodes;
+using Moq;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+using WireMock.Handlers;
+using WireMock.Settings;
+using WireMock.Transformers;
+using WireMock.Types;
+using WireMock.Util;
+
+namespace WireMock.Net.Tests.Transformers;
+
+public class JsonBodyTransformerTests
+{
+    public static TheoryData<JsonBodyTransformerTestContext> Transformers
+    {
+        get
+        {
+            return new TheoryData<JsonBodyTransformerTestContext>
+            {
+                new JsonBodyTransformerTestContext(
+                    () => new NewtonsoftJsonBodyTransformer(new WireMockServerSettings()),
+                    JObject.Parse,
+                    body => ((JToken)body).ToString(Formatting.None)),
+
+                new JsonBodyTransformerTestContext(
+                    () => new SystemTextJsonBodyTransformer(new WireMockServerSettings()),
+                    json => JsonNode.Parse(json)!,
+                    body => ((JsonNode)body).ToJsonString())
+            };
+        }
+    }
+
+    [Theory]
+    [MemberData(nameof(Transformers))]
+    public void TransformBodyAsJson_Replaces_String_Value_And_Preserves_Original(JsonBodyTransformerTestContext testContext)
+    {
+        // Arrange
+        var transformer = testContext.CreateTransformer();
+        var originalJson = testContext.ParseJson("{\"value\":\"{{number}}\"}");
+        var bodyData = new BodyData
+        {
+            Encoding = Encoding.UTF8,
+            DetectedBodyType = BodyType.Json,
+            DetectedBodyTypeFromContentType = BodyType.Json,
+            ProtoBufMessageType = "My.Message",
+            BodyAsJson = originalJson
+        };
+
+        var transformerContext = new FakeTransformerContext(
+            text => text,
+            text => text == "{{number}}" ? "123" : text);
+
+        // Act
+        var result = transformer.TransformBodyAsJson(transformerContext, ReplaceNodeOptions.EvaluateAndTryToConvert, new { }, bodyData);
+
+        // Assert
+        result.Encoding.Should().Be(Encoding.UTF8);
+        result.DetectedBodyType.Should().Be(BodyType.Json);
+        result.DetectedBodyTypeFromContentType.Should().Be(BodyType.Json);
+        result.ProtoBufMessageType.Should().Be("My.Message");
+        result.BodyAsJson.Should().NotBeNull();
+        testContext.SerializeJson(result.BodyAsJson).Should().Be("{\"value\":123}");
+        testContext.SerializeJson(originalJson).Should().Be("{\"value\":\"{{number}}\"}");
+    }
+
+    [Theory]
+    [MemberData(nameof(Transformers))]
+    public void TransformBodyAsJson_With_String_Body_Replaces_Single_Node_With_Object(JsonBodyTransformerTestContext testContext)
+    {
+        // Arrange
+        var transformer = testContext.CreateTransformer();
+        var bodyData = new BodyData
+        {
+            DetectedBodyType = BodyType.Json,
+            BodyAsJson = "{{json}}"
+        };
+
+        var transformerContext = new FakeTransformerContext(
+            text => text == "{{json}}" ? "{\"name\":\"test\"}" : text,
+            text => text);
+
+        // Act
+        var result = transformer.TransformBodyAsJson(transformerContext, ReplaceNodeOptions.EvaluateAndTryToConvert, new { }, bodyData);
+
+        // Assert
+        result.BodyAsJson.Should().NotBeNull();
+        testContext.SerializeJson(result.BodyAsJson).Should().Be("{\"name\":\"test\"}");
+    }
+
+    [Theory]
+    [MemberData(nameof(Transformers))]
+    public void TransformBodyAsJson_Replaces_String_Value_With_WireMockList_As_Array(JsonBodyTransformerTestContext testContext)
+    {
+        // Arrange
+        var transformer = testContext.CreateTransformer();
+        var bodyData = new BodyData
+        {
+            DetectedBodyType = BodyType.Json,
+            BodyAsJson = testContext.ParseJson("{\"values\":\"{{list}}\"}")
+        };
+
+        var transformerContext = new FakeTransformerContext(
+            text => text,
+            text => text == "{{list}}" ? new WireMockList<string>(new[] { "a", "b" }) : text);
+
+        // Act
+        var result = transformer.TransformBodyAsJson(transformerContext, ReplaceNodeOptions.EvaluateAndTryToConvert, new { }, bodyData);
+
+        // Assert
+        result.BodyAsJson.Should().NotBeNull();
+        testContext.SerializeJson(result.BodyAsJson).Should().Be("{\"values\":[\"a\",\"b\"]}");
+    }
+
+    public sealed class JsonBodyTransformerTestContext
+    {
+        private readonly Func<IJsonBodyTransformer> _createTransformer;
+        private readonly Func<string, object> _parseJson;
+        private readonly Func<object, string> _serializeJson;
+
+        public JsonBodyTransformerTestContext(
+            Func<IJsonBodyTransformer> createTransformer,
+            Func<string, object> parseJson,
+            Func<object, string> serializeJson)
+        {
+            _createTransformer = createTransformer;
+            _parseJson = parseJson;
+            _serializeJson = serializeJson;
+        }
+
+        public IJsonBodyTransformer CreateTransformer()
+        {
+            return _createTransformer();
+        }
+
+        public object ParseJson(string json)
+        {
+            return _parseJson(json);
+        }
+
+        public string SerializeJson(object body)
+        {
+            return _serializeJson(body);
+        }
+    }
+
+    private sealed class FakeTransformerContext : ITransformerContext
+    {
+        private readonly Func<string, string> _parseAndRender;
+        private readonly Func<string, object> _parseAndEvaluate;
+
+        public FakeTransformerContext(
+            Func<string, string> parseAndRender,
+            Func<string, object> parseAndEvaluate)
+        {
+            _parseAndRender = parseAndRender;
+            _parseAndEvaluate = parseAndEvaluate;
+            FileSystemHandler = Mock.Of<IFileSystemHandler>();
+        }
+
+        public IFileSystemHandler FileSystemHandler { get; private set; }
+
+        public string ParseAndRender(string text, object model)
+        {
+            return _parseAndRender(text);
+        }
+
+        public object ParseAndEvaluate(string text, object model)
+        {
+            return _parseAndEvaluate(text);
+        }
+    }
+}

test/WireMock.Net.Tests/WireMock.Net.Tests.csproj

@@ -76,7 +76,6 @@
         </PackageReference>
         <PackageReference Include="Moq" Version="4.20.72" />
         <PackageReference Include="SimMetrics.Net" Version="1.0.5" />
-        <PackageReference Include="JsonConverter.System.Text.Json" Version="0.10.0" />
         <PackageReference Include="Google.Protobuf" Version="3.33.5" />
         <PackageReference Include="Grpc.Net.Client" Version="2.76.0" />
         <PackageReference Include="Grpc.Tools" Version="2.78.0">