diff --git a/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs b/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
index ec545c9..6077efa 100644
--- a/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
+++ b/src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
@@ -4,6 +4,7 @@ using Microsoft.AspNetCore.Http.Features;
 using Microsoft.AspNetCore.WebUtilities;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
+using Microsoft.Extensions.Primitives;
 using Microsoft.Net.Http.Headers;
 using ModelContextProtocol.AspNetCore.Stateless;
 using ModelContextProtocol.Protocol;
@@ -28,9 +29,6 @@ internal sealed class StreamableHttpHandler(
 {
     private static readonly JsonTypeInfo<JsonRpcError> s_errorTypeInfo = GetRequiredJsonTypeInfo<JsonRpcError>();
 
-    private static readonly MediaTypeHeaderValue s_applicationJsonMediaType = new("application/json");
-    private static readonly MediaTypeHeaderValue s_textEventStreamMediaType = new("text/event-stream");
-
     public ConcurrentDictionary<string, HttpMcpSession<StreamableHttpServerTransport>> Sessions { get; } = new(StringComparer.Ordinal);
 
     public HttpServerTransportOptions HttpServerTransportOptions => httpServerTransportOptions.Value;
@@ -43,8 +41,8 @@ internal sealed class StreamableHttpHandler(
         // ASP.NET Core Minimal APIs mostly try to stay out of the business of response content negotiation,
         // so we have to do this manually. The spec doesn't mandate that servers MUST reject these requests,
         // but it's probably good to at least start out trying to be strict.
-        var acceptHeaders = context.Request.GetTypedHeaders().Accept;
-        if (!acceptHeaders.Contains(s_applicationJsonMediaType) || !acceptHeaders.Contains(s_textEventStreamMediaType))
+        var typedHeaders = context.Request.GetTypedHeaders();
+        if (!typedHeaders.Accept.Any(MatchesApplicationJsonMediaType) || !typedHeaders.Accept.Any(MatchesTextEventStreamMediaType))
         {
             await WriteJsonRpcErrorAsync(context,
                 "Not Acceptable: Client must accept both application/json and text/event-stream",
@@ -85,8 +83,7 @@ internal sealed class StreamableHttpHandler(
 
     public async Task HandleGetRequestAsync(HttpContext context)
     {
-        var acceptHeaders = context.Request.GetTypedHeaders().Accept;
-        if (!acceptHeaders.Contains(s_textEventStreamMediaType))
+        if (!context.Request.GetTypedHeaders().Accept.Any(MatchesTextEventStreamMediaType))
         {
             await WriteJsonRpcErrorAsync(context,
                 "Not Acceptable: Client must accept text/event-stream",
@@ -331,6 +328,12 @@ internal sealed class StreamableHttpHandler(
 
     private static JsonTypeInfo<T> GetRequiredJsonTypeInfo<T>() => (JsonTypeInfo<T>)McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(T));
 
+    private static bool MatchesApplicationJsonMediaType(MediaTypeHeaderValue acceptHeaderValue)
+        => acceptHeaderValue.MatchesMediaType("application/json");
+
+    private static bool MatchesTextEventStreamMediaType(MediaTypeHeaderValue acceptHeaderValue)
+        => acceptHeaderValue.MatchesMediaType("text/event-stream");
+
     private sealed class HttpDuplexPipe(HttpContext context) : IDuplexPipe
     {
         public PipeReader Input => context.Request.BodyReader;
diff --git a/src/ModelContextProtocol/Client/StreamableHttpClientSessionTransport.cs b/src/ModelContextProtocol/Client/StreamableHttpClientSessionTransport.cs
index f903490..55ecb96 100644
--- a/src/ModelContextProtocol/Client/StreamableHttpClientSessionTransport.cs
+++ b/src/ModelContextProtocol/Client/StreamableHttpClientSessionTransport.cs
@@ -59,7 +59,7 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
         using var content = new StringContent(
             JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage),
             Encoding.UTF8,
-            "application/json"
+            "application/json; charset=utf-8"
         );
 #endif
 
diff --git a/tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs b/tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs
index 17b1234..3efc104 100644
--- a/tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs
+++ b/tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs
@@ -105,7 +105,7 @@ public class StreamableHttpServerConformanceTests(ITestOutputHelper outputHelper
     [InlineData("text/event-stream")]
     [InlineData("application/json")]
     [InlineData("application/json-text/event-stream")]
-    public async Task PostRequest_IsNotAcceptable_WithSingleAcceptHeader(string singleAcceptValue)
+    public async Task PostRequest_IsNotAcceptable_WithSingleSpecificAcceptHeader(string singleAcceptValue)
     {
         await StartAsync();
 
@@ -116,6 +116,20 @@ public class StreamableHttpServerConformanceTests(ITestOutputHelper outputHelper
         Assert.Equal(HttpStatusCode.NotAcceptable, response.StatusCode);
     }
 
+    [Theory]
+    [InlineData("*/*")]
+    [InlineData("text/event-stream, application/json;q=0.9")]
+    public async Task PostRequest_IsAcceptable_WithWildcardOrAddedQualityInAcceptHeader(string acceptHeaderValue)
+    {
+        await StartAsync();
+
+        HttpClient.DefaultRequestHeaders.Accept.Clear();
+        HttpClient.DefaultRequestHeaders.TryAddWithoutValidation(HeaderNames.Accept, acceptHeaderValue);
+
+        using var response = await HttpClient.PostAsync("", JsonContent(InitializeRequest), TestContext.Current.CancellationToken);
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+    }
+
     [Fact]
     public async Task GetRequest_IsNotAcceptable_WithoutTextEventStreamAcceptHeader()
     {
@@ -128,6 +142,22 @@ public class StreamableHttpServerConformanceTests(ITestOutputHelper outputHelper
         Assert.Equal(HttpStatusCode.NotAcceptable, response.StatusCode);
     }
 
+    [Theory]
+    [InlineData("*/*")]
+    [InlineData("application/json, text/event-stream;q=0.9")]
+    public async Task GetRequest_IsAcceptable_WithWildcardOrAddedQualityInAcceptHeader(string acceptHeaderValue)
+    {
+        await StartAsync();
+
+        HttpClient.DefaultRequestHeaders.Accept.Clear();
+        HttpClient.DefaultRequestHeaders.TryAddWithoutValidation(HeaderNames.Accept, acceptHeaderValue);
+
+        await CallInitializeAndValidateAsync();
+
+        using var response = await HttpClient.GetAsync("", HttpCompletionOption.ResponseHeadersRead, TestContext.Current.CancellationToken);
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+    }
+
     [Fact]
     public async Task PostRequest_IsNotFound_WithUnrecognizedSessionId()
     {