diff --git a/src/StreamJsonRpc/Reflection/MessageFormatterRpcMarshaledContextTracker.cs b/src/StreamJsonRpc/Reflection/MessageFormatterRpcMarshaledContextTracker.cs
index 5e62b2da..221fac6c 100644
--- a/src/StreamJsonRpc/Reflection/MessageFormatterRpcMarshaledContextTracker.cs
+++ b/src/StreamJsonRpc/Reflection/MessageFormatterRpcMarshaledContextTracker.cs
@@ -57,7 +57,8 @@ internal partial class MessageFormatterRpcMarshaledContextTracker
     private static readonly ConcurrentDictionary<Type, (JsonRpcProxyOptions? ProxyOptions, JsonRpcTargetOptions TargetOptions, RpcMarshalableAttribute Attribute)> MarshaledTypes = new();
     private static readonly JsonRpcTargetOptions RpcMarshalableInterfaceDefaultTargetOptions = new() { NotifyClientOfEvents = false, DisposeOnDisconnect = true };
     private static readonly MethodInfo ReleaseMarshaledObjectMethodInfo = typeof(MessageFormatterRpcMarshaledContextTracker).GetMethod(nameof(ReleaseMarshaledObject), BindingFlags.NonPublic | BindingFlags.Instance)!;
-    private static readonly ConcurrentDictionary<Type, RpcMarshalableOptionalInterfaceAttribute[]> MarshalableOptionalInterfaces = new ConcurrentDictionary<Type, RpcMarshalableOptionalInterfaceAttribute[]>();
+    private static readonly ConcurrentDictionary<Type, RpcMarshalableOptionalInterfaceAttribute[]> MarshalableOptionalInterfaces = new();
+    private static readonly ConcurrentDictionary<Type, RpcMarshalableAttribute?> RpcMarshalableAttributeCache = new();
 
     private readonly Dictionary<long, (RpcMarshaledContext Context, IDisposable Revert)> marshaledObjects = new Dictionary<long, (RpcMarshaledContext Context, IDisposable Revert)>();
     private readonly JsonRpc jsonRpc;
@@ -106,12 +107,13 @@ internal static bool TryGetMarshalOptionsForType<T>(
         [NotNullWhen(true)] out JsonRpcTargetOptions? targetOptions,
         [NotNullWhen(true)] out RpcMarshalableAttribute? rpcMarshalableAttribute)
     {
-        if (TryGetMarshalOptionsForTypeHelper(typeShape.Type, defaultProxyOptions, out proxyOptions, out targetOptions, out rpcMarshalableAttribute))
+        bool? helperResult = TryGetMarshalOptionsForTypeHelper(typeShape.Type, defaultProxyOptions, out proxyOptions, out targetOptions, out rpcMarshalableAttribute);
+        if (helperResult is not null)
         {
-            return true;
+            return helperResult.Value;
         }
 
-        if (typeShape.Type.GetCustomAttribute<RpcMarshalableAttribute>() is RpcMarshalableAttribute marshalableAttribute)
+        if (TryGetRpcMarshalableAttribute(typeShape.Type, out RpcMarshalableAttribute? marshalableAttribute))
         {
             // Validation requires more trim annotations than our NativeAOT callers can provide.
             // And besides, analyzers should have called out any issues at compile-time.
@@ -137,19 +139,23 @@ internal static bool TryGetMarshalOptionsForType(
         [NotNullWhen(true)] out JsonRpcTargetOptions? targetOptions,
         [NotNullWhen(true)] out RpcMarshalableAttribute? rpcMarshalableAttribute)
     {
-        if (TryGetMarshalOptionsForTypeHelper(type, defaultProxyOptions, out proxyOptions, out targetOptions, out rpcMarshalableAttribute))
+        bool? helperResult = TryGetMarshalOptionsForTypeHelper(type, defaultProxyOptions, out proxyOptions, out targetOptions, out rpcMarshalableAttribute);
+        if (helperResult is not null)
         {
-            // Because events are not checked by the Nerdbank.MessagePack formatter,
-            // Remove this check when issues related to https://github.com/eiriktsarpalis/PolyType/issues/226 are resolved in this file.
-            if (type.GetEvents().Length > 0)
+            if (helperResult is true)
             {
-                throw new NotSupportedException(string.Format(CultureInfo.CurrentCulture, Resources.MarshalableInterfaceHasEvents, type.FullName));
+                // Because events are not checked by the Nerdbank.MessagePack formatter,
+                // Remove this check when issues related to https://github.com/eiriktsarpalis/PolyType/issues/226 are resolved in this file.
+                if (type.GetEvents().Length > 0)
+                {
+                    throw new NotSupportedException(string.Format(CultureInfo.CurrentCulture, Resources.MarshalableInterfaceHasEvents, type.FullName));
+                }
             }
 
-            return true;
+            return helperResult.Value;
         }
 
-        if (type.GetCustomAttribute<RpcMarshalableAttribute>() is RpcMarshalableAttribute marshalableAttribute)
+        if (TryGetRpcMarshalableAttribute(type, out RpcMarshalableAttribute? marshalableAttribute))
         {
             // Validation requires more trim annotations than our NativeAOT callers can provide.
             // And besides, analyzers should have called out any issues at compile-time.
@@ -199,7 +205,7 @@ internal static RpcMarshalableOptionalInterfaceAttribute[] GetMarshalableOptiona
 
             foreach (RpcMarshalableOptionalInterfaceAttribute attribute in attributes)
             {
-                if (attribute.OptionalInterface.GetCustomAttribute<RpcMarshalableAttribute>() is null)
+                if (!TryGetRpcMarshalableAttribute(attribute.OptionalInterface, out _))
                 {
                     throw new NotSupportedException(string.Format(CultureInfo.CurrentCulture, Resources.RpcMarshalableOptionalInterfaceMustBeMarshalable, attribute.OptionalInterface.FullName));
                 }
@@ -441,7 +447,7 @@ internal MarshalToken GetToken(
         }
     }
 
-    private static bool TryGetMarshalOptionsForTypeHelper(
+    private static bool? TryGetMarshalOptionsForTypeHelper(
         Type type,
         JsonRpcProxyOptions defaultProxyOptions,
         [NotNullWhen(true)] out JsonRpcProxyOptions? proxyOptions,
@@ -453,6 +459,7 @@ private static bool TryGetMarshalOptionsForTypeHelper(
         rpcMarshalableAttribute = null;
         if (type.IsInterface is false)
         {
+            // Definitely not.
             return false;
         }
 
@@ -479,7 +486,8 @@ private static bool TryGetMarshalOptionsForTypeHelper(
             }
         }
 
-        return false;
+        // Unknown. Caller may want to do more work.
+        return null;
     }
 
     /// <summary>
@@ -545,6 +553,12 @@ private static void ValidateMarshalableInterface(
     }
 #endif
 
+    private static bool TryGetRpcMarshalableAttribute(Type type, [NotNullWhen(true)] out RpcMarshalableAttribute? attribute)
+    {
+        attribute = RpcMarshalableAttributeCache.GetOrAdd(type, static type => type.GetCustomAttribute<RpcMarshalableAttribute>());
+        return attribute is not null;
+    }
+
     /// <summary>
     /// Releases memory associated with marshaled objects.
     /// </summary>