Send string response bodies as-is instead of JSON-serializing them

DevProxy.Abstractions.Tests/Utils/ProxyUtilsTests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Text.Json;
 using DevProxy.Abstractions.Utils;
 using Xunit;
 
@@ -76,4 +77,25 @@ public void GetFullPath_ResolvesTildeSlashToHomeDirectoryInsteadOfConfigFolder()
             System.IO.Path.GetFullPath(System.IO.Path.Combine(home, "mocks", "mocks.json")),
             ProxyUtils.GetFullPath("~/mocks/mocks.json", configFile));
     }
+
+    [Fact]
+    public void SerializeBody_NullReturnsEmptyString() =>
+        Assert.Equal(string.Empty, ProxyUtils.SerializeBody(null));
+
+    [Fact]
+    public void SerializeBody_StringIsReturnedAsIs() =>
+        Assert.Equal("hello", ProxyUtils.SerializeBody("hello"));
+
+    [Fact]
+    public void SerializeBody_JsonStringElementIsReturnedAsIs() =>
+        Assert.Equal("hello", ProxyUtils.SerializeBody(JsonSerializer.Deserialize<object>("\"hello\"")));
+
+    [Fact]
+    public void SerializeBody_ObjectIsSerializedAsJson()
+    {
+        var body = ProxyUtils.SerializeBody(JsonSerializer.Deserialize<object>("""{ "message": "hello" }"""));
+
+        using var doc = JsonDocument.Parse(body);
+        Assert.Equal("hello", doc.RootElement.GetProperty("message").GetString());
+    }
 }

DevProxy.Abstractions/Utils/ProxyUtils.cs

@@ -104,6 +104,19 @@ static ProxyUtils()
         JsonSerializerOptions.Converters.Add(new JsonStringEnumConverter(JsonNamingPolicy.CamelCase));
     }
 
+    /// <summary>
+    /// Converts a configured response body to the string sent to the client.
+    /// String bodies are returned as-is; other values are serialized as JSON.
+    /// </summary>
+    public static string SerializeBody(object? body) => body switch
+    {
+        null => string.Empty,
+        string s => s,
+        JsonElement { ValueKind: JsonValueKind.String } el => el.GetString() ?? string.Empty,
+        JValue { Type: JTokenType.String } jv => jv.Value<string>() ?? string.Empty,
+        _ => JsonSerializer.Serialize(body, JsonSerializerOptions)
+    };
+
     public static bool IsGraphRequest(IHttpRequest request)
     {
         ArgumentNullException.ThrowIfNull(request);

DevProxy.Integration.Tests/BehaviorPluginsIntegrationTests.cs

@@ -66,6 +66,42 @@ public async Task GenericRandomError_Rate100_OverridesOriginWithConfiguredError(
         Assert.Equal(HttpStatusCode.ServiceUnavailable, response.StatusCode);
     }
 
+    [Fact]
+    public async Task GenericRandomError_StringBody_IsSentAsIs()
+    {
+        await using var origin = await FakeOrigin.StartAsync();
+        var urls = KestrelProxyHarness.BuildUrlsToWatch(origin.Host);
+
+        var config = PluginConfig.FromJson($$"""
+            {
+              "rate": 100,
+              "errors": [
+                {
+                  "request": { "url": "http://{{origin.Host}}/*" },
+                  "responses": [ { "statusCode": 503, "body": "service down" } ]
+                }
+              ]
+            }
+            """);
+        var plugin = new GenericRandomErrorPlugin(
+            SharedHttpClient,
+            NullLogger<GenericRandomErrorPlugin>.Instance,
+            urls,
+            ProxyConfig,
+            config);
+
+        await using var proxy = await KestrelProxyHarness.StartAsync(
+            origin.Host, [plugin]);
+        using var client = proxy.CreateHttpClient();
+
+        using var response = await client.GetAsync(new Uri($"http://{origin.Host}/get"));
+        var body = await response.Content.ReadAsStringAsync();
+
+        Assert.Equal(HttpStatusCode.ServiceUnavailable, response.StatusCode);
+        // string bodies must not be JSON-serialized (no surrounding quotes)
+        Assert.Equal("service down", body);
+    }
+
     [Theory]
     [InlineData("/Get")]
     [InlineData("/Ge*")]
@@ -268,6 +304,60 @@ public async Task RateLimiting_ThrottlesOnceResourcesAreExhausted()
         Assert.Equal(HttpStatusCode.TooManyRequests, second.StatusCode);
     }
 
+    [Fact]
+    public async Task RateLimiting_CustomResponseStringBody_IsSentAsIs()
+    {
+        await using var origin = await FakeOrigin.StartAsync();
+        var urls = KestrelProxyHarness.BuildUrlsToWatch(origin.Host);
+
+        var dir = Directory.CreateTempSubdirectory("devproxy-ratelimit-");
+        try
+        {
+            var responseFile = Path.Combine(dir.FullName, "rate-limit-response.json");
+            await File.WriteAllTextAsync(responseFile, """
+                { "statusCode": 403, "body": "slow down" }
+                """);
+            var proxyConfig = new TestProxyConfiguration
+            {
+                ConfigFile = Path.Combine(dir.FullName, "devproxyrc.json"),
+            };
+            var plugin = new RateLimitingPlugin(
+                SharedHttpClient,
+                NullLogger<RateLimitingPlugin>.Instance,
+                urls,
+                proxyConfig,
+                PluginConfig.FromJson("""
+                    {
+                      "rateLimit": 2,
+                      "costPerRequest": 2,
+                      "resetTimeWindowSeconds": 300,
+                      "whenLimitExceeded": "custom",
+                      "customResponseFile": "rate-limit-response.json"
+                    }
+                    """));
+
+            using var host = new PluginTestHost(proxyConfig);
+            await plugin.InitializeAsync(host.CreateInitArgs(), CancellationToken.None);
+
+            await using var proxy = await KestrelProxyHarness.StartAsync(
+                origin.Host, [plugin]);
+            using var client = proxy.CreateHttpClient();
+
+            using var first = await client.GetAsync(new Uri($"http://{origin.Host}/get"));
+            using var throttled = await client.GetAsync(new Uri($"http://{origin.Host}/get"));
+            var body = await throttled.Content.ReadAsStringAsync();
+
+            Assert.Equal(HttpStatusCode.OK, first.StatusCode);
+            Assert.Equal(HttpStatusCode.Forbidden, throttled.StatusCode);
+            // string bodies must not be JSON-serialized (no surrounding quotes)
+            Assert.Equal("slow down", body);
+        }
+        finally
+        {
+            dir.Delete(recursive: true);
+        }
+    }
+
     [Fact]
     public async Task RateLimitingPlusRetryAfter_ThrottledResponseCarriesRetryAfter()
     {

DevProxy.Integration.Tests/ManipulationAndMockingIntegrationTests.cs

@@ -317,7 +317,8 @@ public async Task MockResponse_ShortCircuitsWithConfiguredMock()
 
         // /get normally returns 200 "hello get"; the mock overrides it.
         Assert.Equal(HttpStatusCode.Created, response.StatusCode);
-        Assert.Contains("mocked-by-test", body, StringComparison.Ordinal);
+        // string bodies must not be JSON-serialized (no surrounding quotes)
+        Assert.Equal("mocked-by-test", body);
     }
 
     [Theory]

DevProxy.Plugins/Behavior/GenericRandomErrorPlugin.cs

@@ -265,16 +265,14 @@ private void UpdateProxyResponse(ProxyRequestArgs e, GenericErrorResponseRespons
         }
 
         var statusCode = (HttpStatusCode)(error.StatusCode ?? 400);
-        var body = error.Body is null ? string.Empty : JsonSerializer.Serialize(error.Body, ProxyUtils.JsonSerializerOptions);
-        // we get a JSON string so need to start with the opening quote
-        if (body.StartsWith("\"@"))
+        string body = ProxyUtils.SerializeBody(error.Body);
+        if (body.StartsWith('@'))
         {
             // we've got a mock body starting with @-token which means we're sending
             // a response from a file on disk
             // if we can read the file, we can immediately send the response and
             // skip the rest of the logic in this method
-            // remove the surrounding quotes and the @-token
-            var filePath = Path.Combine(Path.GetDirectoryName(Configuration.ErrorsFile) ?? "", ProxyUtils.ReplacePathTokens(body.Trim('"').Substring(1)));
+            var filePath = Path.Combine(Path.GetDirectoryName(Configuration.ErrorsFile) ?? "", ProxyUtils.ReplacePathTokens(body[1..]));
             if (!File.Exists(filePath))
             {
                 Logger.LogError("File {FilePath} not found. Serving file path in the mock response", (string?)filePath);

DevProxy.Plugins/Behavior/RateLimitingPlugin.cs

@@ -177,9 +177,7 @@ public override Task BeforeRequestAsync(ProxyRequestArgs e, CancellationToken ca
                         ));
                     }
 
-                    string body = Configuration.CustomResponse.Body is not null ?
-                        JsonSerializer.Serialize(Configuration.CustomResponse.Body, ProxyUtils.JsonSerializerOptions) :
-                        "";
+                    string body = ProxyUtils.SerializeBody(Configuration.CustomResponse.Body);
                     e.ProxySession.Respond(body, responseCode, headers);
                     state.HasBeenSet = true;
                 }

DevProxy.Plugins/Mocking/MockResponsePlugin.cs

@@ -380,16 +380,14 @@ private void ProcessMockResponseInternal(ProxyRequestArgs e, MockResponse matchi
 
         if (matchingResponse.Response?.Body is not null)
         {
-            var bodyString = JsonSerializer.Serialize(matchingResponse.Response.Body, ProxyUtils.JsonSerializerOptions) as string;
-            // we get a JSON string so need to start with the opening quote
-            if (bodyString?.StartsWith("\"@", StringComparison.OrdinalIgnoreCase) ?? false)
+            string bodyString = ProxyUtils.SerializeBody(matchingResponse.Response.Body);
+            if (bodyString.StartsWith('@'))
             {
                 // we've got a mock body starting with @-token which means we're sending
                 // a response from a file on disk
                 // if we can read the file, we can immediately send the response and
                 // skip the rest of the logic in this method
-                // remove the surrounding quotes and the @-token
-                var filePath = Path.Combine(Path.GetDirectoryName(Configuration.MocksFile) ?? "", ProxyUtils.ReplacePathTokens(bodyString.Trim('"')[1..]));
+                var filePath = Path.Combine(Path.GetDirectoryName(Configuration.MocksFile) ?? "", ProxyUtils.ReplacePathTokens(bodyString[1..]));
                 if (!File.Exists(filePath))
                 {