Add validation for user-defined JsonSerializerOptions.

Author: eiriktsarpalis

src/ModelContextProtocol/Client/McpClientExtensions.cs

@@ -188,6 +188,7 @@ public static Task<GetPromptResult> GetPromptAsync(
         Throw.IfNull(client);
         Throw.IfNullOrWhiteSpace(name);
         serializerOptions ??= McpJsonUtilities.DefaultOptions;
+        McpJsonUtilities.ValidateSerializerOptions(serializerOptions);
         var parametersTypeInfo = serializerOptions.GetTypeInfo<IReadOnlyDictionary<string, object?>>();
 
         return client.SendRequestAsync(
@@ -454,6 +455,7 @@ public static Task<CallToolResponse> CallToolAsync(
         Throw.IfNull(client);
         Throw.IfNull(toolName);
         serializerOptions ??= McpJsonUtilities.DefaultOptions;
+        McpJsonUtilities.ValidateSerializerOptions(serializerOptions);
         var parametersTypeInfo = serializerOptions.GetTypeInfo<IReadOnlyDictionary<string, object?>>();
 
         return client.SendRequestAsync(

src/ModelContextProtocol/McpEndpointExtensions.cs

@@ -75,6 +75,7 @@ public static Task<TResult> SendRequestAsync<TParameters, TResult>(
         where TResult : notnull
     {
         serializerOptions ??= McpJsonUtilities.DefaultOptions;
+        McpJsonUtilities.ValidateSerializerOptions(serializerOptions);
         JsonTypeInfo<TParameters> paramsTypeInfo = serializerOptions.GetTypeInfo<TParameters>();
         JsonTypeInfo<TResult> resultTypeInfo = serializerOptions.GetTypeInfo<TResult>();
         return SendRequestAsync(endpoint, method, parameters, paramsTypeInfo, resultTypeInfo, requestId, cancellationToken);
@@ -132,6 +133,7 @@ public static Task SendNotificationAsync<TParameters>(
         CancellationToken cancellationToken = default)
     {
         serializerOptions ??= McpJsonUtilities.DefaultOptions;
+        McpJsonUtilities.ValidateSerializerOptions(serializerOptions);
         JsonTypeInfo<TParameters> parametersTypeInfo = serializerOptions.GetTypeInfo<TParameters>();
         return SendNotificationAsync(endpoint, method, parameters, parametersTypeInfo, cancellationToken);
     }

src/ModelContextProtocol/Utils/Json/McpJsonUtilities.cs

@@ -74,6 +74,16 @@ private static JsonSerializerOptions CreateDefaultOptions()
     internal static JsonTypeInfo<T> GetTypeInfo<T>(this JsonSerializerOptions options) =>
         (JsonTypeInfo<T>)options.GetTypeInfo(typeof(T));
 
+    internal static void ValidateSerializerOptions(JsonSerializerOptions options)
+    {
+        if (options.WriteIndented)
+        {
+            throw new InvalidOperationException("JsonSerializerOptions.WriteIndented is not supported for JSON-RPC workloads.");
+        }
+
+        options.MakeReadOnly();
+    }
+
     internal static JsonElement DefaultMcpToolSchema { get; } = ParseJsonElement("""{"type":"object"}"""u8);
     internal static object? AsObject(this JsonElement element) => element.ValueKind is JsonValueKind.Null ? null : element;