Fix default parameters missing from RestResponse.Request.Parameters (#2282)

Merge default parameters into RestRequest.Parameters early in
ExecuteRequestAsync so they are visible via response.Request.Parameters.
Remove the now-redundant separate merges in RequestContent,
BuildUriExtensions, RequestHeaders, and OAuth1Authenticator.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: alexeyzimarev

src/RestSharp/Authenticators/OAuth/OAuth1Authenticator.cs

@@ -258,7 +258,6 @@ internal static void AddOAuthData(
                 ? x => BaseQuery(x) && x.Name != null && x.Name.StartsWith("oauth_")
                 : (Func<Parameter, bool>)BaseQuery;
 
-        parameters.AddRange(client.DefaultParameters.Where(query).ToWebParameters());
         parameters.AddRange(request.Parameters.Where(query).ToWebParameters());
 
         workflow.RequestUrl = url;

src/RestSharp/BuildUriExtensions.cs

@@ -53,8 +53,7 @@ public Uri BuildUriWithoutQueryParameters(RestRequest request) {
             var (uri, resource) = client.Options.BaseUrl.GetUrlSegmentParamsValues(
                 request.Resource,
                 client.Options.Encode,
-                request.Parameters,
-                client.DefaultParameters
+                request.Parameters
             );
             return uri.MergeBaseUrlAndResource(resource);
         }
@@ -66,9 +65,7 @@ public Uri BuildUriWithoutQueryParameters(RestRequest request) {
         /// <returns></returns>
         [PublicAPI]
         public string? GetRequestQuery(RestRequest request) {
-            var parametersCollections = new ParametersCollection[] { request.Parameters, client.DefaultParameters };
-
-            var parameters = parametersCollections.SelectMany(x => x.GetQueryParameters(request.Method)).ToList();
+            var parameters = request.Parameters.GetQueryParameters(request.Method).ToList();
 
             return parameters.Count == 0 ? null : string.Join("&", parameters.Select(EncodeParameter).ToArray());
 

src/RestSharp/Request/RequestContent.cs

@@ -24,7 +24,7 @@ namespace RestSharp;
 
 class RequestContent(IRestClient client, RestRequest request) : IDisposable {
     readonly List<Stream>         _streams    = [];
-    readonly ParametersCollection _parameters = new RequestParameters(request.Parameters.Union(client.DefaultParameters));
+    readonly ParametersCollection _parameters = request.Parameters;
 
     HttpContent? Content { get; set; }
 

src/RestSharp/RestClient.Async.cs

@@ -100,6 +100,14 @@ async Task<HttpResponse> ExecuteRequestAsync(RestRequest request, CancellationTo
         }
 #endif
         CombineInterceptors(request);
+
+        // Merge default parameters into the request so they are visible in response.Request.Parameters
+        foreach (var defaultParam in DefaultParameters) {
+            if (!request.Parameters.Any(p => p.Name == defaultParam.Name && p.Type == defaultParam.Type)) {
+                request.Parameters.AddParameter(defaultParam);
+            }
+        }
+
         await OnBeforeRequest(request, cancellationToken).ConfigureAwait(false);
         request.ValidateParameters();
         var authenticator = request.Authenticator ?? Options.Authenticator;
@@ -131,7 +139,6 @@ async Task<HttpResponse> ExecuteRequestAsync(RestRequest request, CancellationTo
 
         var headers = new RequestHeaders()
             .AddHeaders(request.Parameters)
-            .AddHeaders(DefaultParameters)
             .AddAcceptHeader(AcceptedContentTypes)
             .AddCookieHeaders(url, cookieContainer)
             .AddCookieHeaders(url, Options.CookieContainer);

test/RestSharp.Tests.Integrated/HttpHeadersTests.cs

@@ -68,6 +68,25 @@ public async Task Should_sent_custom_UserAgent() {
         response.GetHeaderValue("Server").Should().Be("Kestrel");
     }
 
+    [Fact]
+    public async Task Default_headers_should_appear_in_response_request_parameters() {
+        const string headerName  = "X-Custom-Default";
+        const string headerValue = "DefaultValue123";
+
+        _client.AddDefaultHeader(headerName, headerValue);
+
+        var request  = new RestRequest("/headers");
+        var response = await _client.ExecuteAsync(request);
+
+        response.StatusCode.Should().Be(HttpStatusCode.OK);
+
+        var param = response.Request.Parameters
+            .FirstOrDefault(p => p.Name == headerName && p.Type == ParameterType.HttpHeader);
+
+        param.Should().NotBeNull();
+        param!.Value.Should().Be(headerValue);
+    }
+
     static void CheckHeader(RestResponse<TestServerResponse[]> response, Header header) {
         var h = FindHeader(response, header.Name);
         h.Should().NotBeNull();