Fix DefaultParameters not set up using a cached client instance

docs/docs/usage/client.md

@@ -48,11 +48,12 @@ Another way to create the client instance is to use a simple client factory. The
 * `ClientCertificates`
 * `MaxRedirects`
 * `Timeout`
-* `UserAgent`
 * `Expect100Continue`
 
 Constructor parameters to configure the `HttpMessageHandler` and default `HttpClient` headers configuration are also ignored for the cached instance as the factory only configures the handler once.
 
+The `UserAgent` option is not affected by caching: it is added to each `RestClient` instance's default parameters and applied per request, so every client gets its own `User-Agent` header even when the underlying `HttpClient` is reused.
+
 You need to set the `useClientFactory` parameter to `true` in the `RestClient` constructor to enable the factory.
 
 ```csharp

src/RestSharp/RestClient.cs

@@ -76,6 +76,8 @@ public RestClient(
         ConfigureSerializers(configureSerialization);
         Options           = new(options);
         DefaultParameters = new(Options);
+        // Must run per RestClient instance, not inside GetClient: the factory may return a cached HttpClient without invoking GetClient
+        ConfigureDefaultParameters(options);
 
         if (useClientFactory) {
             _disposeHttpClient = false;
@@ -97,7 +99,6 @@ HttpClient GetClient() {
             // We will use Options.Timeout in ExecuteAsInternalAsync method
             httpClient.Timeout = Timeout.InfiniteTimeSpan;
 
-            ConfigureDefaultParameters(options);
             configureDefaultHeaders?.Invoke(httpClient.DefaultRequestHeaders);
             return httpClient;
         }
@@ -185,10 +186,7 @@ public RestClient(
         var opt = options ?? new RestClientOptions();
         Options           = new(opt);
         DefaultParameters = new(Options);
-
-        if (options != null) {
-            ConfigureDefaultParameters(options);
-        }
+        ConfigureDefaultParameters(opt);
     }
 
     /// <summary>

test/RestSharp.Tests/RestClientTests.cs

@@ -124,6 +124,47 @@ public void ConfigureDefaultParameters_sets_user_agent_given_httpClient_instance
         Assert.Empty(httpClient.DefaultRequestHeaders.UserAgent);
     }
 
+    [Fact]
+    public void ConfigureDefaultParameters_sets_user_agent_using_factory_twice() {
+        // arrange
+        const string expectedAgentString = "Agent/1.0";
+
+        // The base URL is unique to this test to keep the process-wide factory cache isolated from other tests
+        var clientOptions = new RestClientOptions { BaseUrl = new Uri("https://localhost:8888"), UserAgent = expectedAgentString };
+
+        // act
+        using var firstRestClient  = new RestClient(clientOptions, useClientFactory: true);
+        using var secondRestClient = new RestClient(clientOptions, useClientFactory: true);
+
+        //assert
+        secondRestClient.HttpClient.Should().BeSameAs(firstRestClient.HttpClient, "the regression only manifests on a factory cache hit");
+        AssertHasUserAgent(firstRestClient);
+        AssertHasUserAgent(secondRestClient);
+        return;
+
+        static void AssertHasUserAgent(RestClient restClient)
+            => Assert.Single(
+                restClient.DefaultParameters,
+                parameter => parameter is { Type: ParameterType.HttpHeader, Name: KnownHeaders.UserAgent, Value: expectedAgentString }
+            );
+    }
+
+    [Fact]
+    public void ConfigureDefaultParameters_sets_user_agent_given_httpClient_and_null_options() {
+        // arrange
+        var httpClient = new HttpClient();
+
+        // act
+        using var restClient = new RestClient(httpClient, options: null);
+
+        //assert
+        Assert.Single(
+            restClient.DefaultParameters,
+            parameter => parameter is { Type: ParameterType.HttpHeader, Name: KnownHeaders.UserAgent, Value: string valueAsString } &&
+                valueAsString == new RestClientOptions().UserAgent
+        );
+    }
+
     [Fact]
     public void Should_not_set_expect_continue_on_shared_http_client_default_headers() {
         // arrange