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Fix missing resultType on complete result responses #1684

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2 changes: 1 addition & 1 deletion src/ModelContextProtocol.Core/Protocol/EmptyResult.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,5 +9,5 @@ namespace ModelContextProtocol.Protocol;
public sealed class EmptyResult : Result
{
[JsonIgnore]
internal static EmptyResult Instance { get; } = new();
internal static EmptyResult Instance { get; } = new() { ResultType = "complete" };
}
2 changes: 1 addition & 1 deletion src/ModelContextProtocol.Core/Protocol/Result.cs
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Expand Up @@ -27,7 +27,7 @@ private protected Result()
/// </summary>
/// <remarks>
/// <para>
/// When absent or set to <c>"complete"</c>, the result is a normal completed response.
/// When set to <c>"complete"</c>, the result is a normal completed response.

@PranavSenthilnathan PranavSenthilnathan Jul 1, 2026

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I don't think we want to lock into these semantics. The spec says that client should treat null as complete but that is a behavior that shouldn't leak into the model IMO.

/// When set to <c>"input_required"</c>, the result is an <see cref="InputRequiredResult"/> indicating
/// that additional input is needed before the request can be completed.
/// When set to <c>"task"</c>, the result is a <see cref="CreateTaskResult"/> indicating that the server
Expand Down
40 changes: 38 additions & 2 deletions src/ModelContextProtocol.Core/Server/McpServerImpl.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -384,6 +384,7 @@ private void ConfigureInitialize(McpServerOptions options)
Instructions = options.ServerInstructions,
ServerInfo = options.ServerInfo ?? DefaultImplementation,
Capabilities = ServerCapabilities ?? new(),
ResultType = "complete",
};
},
McpJsonUtilities.JsonContext.Default.InitializeRequestParams,
Expand Down Expand Up @@ -414,6 +415,7 @@ private void ConfigureDiscover(McpServerOptions options)
// their "do not cache" behavior while satisfying the wire requirement.
TimeToLive = TimeSpan.Zero,
CacheScope = CacheScope.Private,
ResultType = "complete",
});
},
McpJsonUtilities.JsonContext.Default.DiscoverRequestParams,
Expand Down Expand Up @@ -458,7 +460,7 @@ private void ConfigureSubscriptions(McpServerOptions options)

await SendSubscriptionAckAsync(statelessSubscription, cancellationToken).ConfigureAwait(false);

return new EmptyResult();
return EmptyResult.Instance;
}

// Filter the requested notifications against what the server actually supports.
Expand Down Expand Up @@ -498,7 +500,7 @@ private void ConfigureSubscriptions(McpServerOptions options)
_activeSubscriptions.TryRemove(jsonRpcRequest.Id, out _);
}

return new EmptyResult();
return EmptyResult.Instance;
},
McpJsonUtilities.JsonContext.Default.SubscriptionsListenRequestParams,
McpJsonUtilities.JsonContext.Default.EmptyResult);
Expand Down Expand Up @@ -1664,6 +1666,21 @@ private void SetHandler<TParams, TResult>(
};
}

if (typeof(Result).IsAssignableFrom(typeof(TResult)))
{
var innerHandler = handler;
handler = async (request, cancellationToken) =>
{
var result = await innerHandler(request, cancellationToken).ConfigureAwait(false);
if (result is Result protocolResult && protocolResult.ResultType is null)
{
protocolResult.ResultType = "complete";
}

return result;
};
}

_requestHandlers.Set(method,
(request, jsonRpcRequest, cancellationToken) =>
InvokeHandlerAsync(handler, request, jsonRpcRequest, cancellationToken),
Expand All @@ -1678,6 +1695,25 @@ private void SetTaskAugmentedHandler<TParams, TResult>(
JsonTypeInfo<CreateTaskResult> taskResultTypeInfo)
where TResult : Result
{
var innerHandler = handler;
handler = async (request, cancellationToken) =>
{
var result = await innerHandler(request, cancellationToken).ConfigureAwait(false);
if (result.IsTask)
{
if (result.TaskCreated is { ResultType: null } taskCreated)
{
taskCreated.ResultType = "task";
}
}
else if (result.Result is { ResultType: null } immediateResult)
{
immediateResult.ResultType = "complete";
}

return result;
};

_requestHandlers.SetTaskAugmented(method,
(request, jsonRpcRequest, cancellationToken) =>
InvokeHandlerAsync(handler, request, jsonRpcRequest, cancellationToken),
Expand Down
2 changes: 2 additions & 0 deletions tests/ModelContextProtocol.Tests/Client/July2026ProtocolConnectionTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -67,6 +67,7 @@ public async Task LegacyClient_CanCallServerDiscover()

var discoverResult = JsonSerializer.Deserialize<DiscoverResult>(response.Result, McpJsonUtilities.DefaultOptions);
Assert.NotNull(discoverResult);
Assert.Equal("complete", discoverResult.ResultType);
Assert.NotEmpty(discoverResult.SupportedVersions);
Assert.Contains(LatestStableVersion, discoverResult.SupportedVersions);
Assert.Equal(nameof(July2026ProtocolConnectionTests), discoverResult.ServerInfo.Name);
Expand All @@ -85,6 +86,7 @@ public async Task ServerDiscover_IncludesJuly2026ProtocolVersion()

var discoverResult = JsonSerializer.Deserialize<DiscoverResult>(response.Result, McpJsonUtilities.DefaultOptions);
Assert.NotNull(discoverResult);
Assert.Equal("complete", discoverResult.ResultType);
Assert.Contains(McpHttpHeaders.July2026ProtocolVersion, discoverResult.SupportedVersions);
}
}
21 changes: 21 additions & 0 deletions tests/ModelContextProtocol.Tests/Server/McpServerTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -283,6 +283,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<InitializeResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result);
Assert.Equal("complete", result.ResultType);
Assert.Equal(expectedAssemblyName.Name, result.ServerInfo.Name);
Assert.Equal(expectedAssemblyName.Version?.ToString() ?? "1.0.0", result.ServerInfo.Version);
Assert.Equal("2024", result.ProtocolVersion);
Expand All @@ -304,6 +305,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<InitializeResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result);
Assert.Equal("complete", result.ResultType);
Assert.NotNull(result.Capabilities.Extensions);
Assert.True(result.Capabilities.Extensions.ContainsKey("io.myext"));
});
Expand All @@ -323,6 +325,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<InitializeResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result);
Assert.Equal("complete", result.ResultType);
Assert.NotNull(result.Capabilities.Experimental);
Assert.True(result.Capabilities.Experimental.ContainsKey("customFeature"));
});
Expand Down Expand Up @@ -354,6 +357,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<InitializeResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result);
Assert.Equal("complete", result.ResultType);

// Use reflection to verify every public property on ServerCapabilities is non-null.
// This catches cases where new capability properties are added but not copied
Expand Down Expand Up @@ -400,6 +404,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<CompleteResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result?.Completion);
Assert.Equal("complete", result.ResultType);
Assert.Equal(["test"], result.Completion.Values);
Assert.Equal(2, result.Completion.Total);
Assert.True(result.Completion.HasMore);
Expand Down Expand Up @@ -443,6 +448,7 @@ await transport.SendMessageAsync(new JsonRpcRequest
Assert.NotNull(response);
var result = JsonSerializer.Deserialize<CompleteResult>(response.Result, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result?.Completion);
Assert.Equal("complete", result.ResultType);
Assert.Equal(["cat"], result.Completion.Values);
Assert.Equal(1, result.Completion.Total);

Expand Down Expand Up @@ -486,6 +492,7 @@ await transport.SendMessageAsync(new JsonRpcRequest
Assert.NotNull(response);
var result = JsonSerializer.Deserialize<CompleteResult>(response.Result, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result?.Completion);
Assert.Equal("complete", result.ResultType);
Assert.Empty(result.Completion.Values);

await transport.DisposeAsync();
Expand Down Expand Up @@ -535,6 +542,7 @@ await transport.SendMessageAsync(new JsonRpcRequest
Assert.NotNull(response);
var result = JsonSerializer.Deserialize<CompleteResult>(response.Result, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result?.Completion);
Assert.Equal("complete", result.ResultType);
Assert.Equal(["us-east-1", "us-west-2"], result.Completion.Values);
Assert.Equal(2, result.Completion.Total);

Expand Down Expand Up @@ -590,6 +598,7 @@ await transport.SendMessageAsync(new JsonRpcRequest
Assert.NotNull(response);
var result = JsonSerializer.Deserialize<CompleteResult>(response.Result, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result?.Completion);
Assert.Equal("complete", result.ResultType);
// Custom handler values + auto-populated values should be combined
Assert.Equal(["custom-value", "dog", "cat"], result.Completion.Values);
Assert.Equal(3, result.Completion.Total);
Expand Down Expand Up @@ -637,6 +646,7 @@ await transport.SendMessageAsync(new JsonRpcRequest
Assert.NotNull(response);
var result = JsonSerializer.Deserialize<CompleteResult>(response.Result, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result?.Completion);
Assert.Equal("complete", result.ResultType);
Assert.Equal(["a", "b"], result.Completion.Values);

await transport.DisposeAsync();
Expand Down Expand Up @@ -675,6 +685,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<ListResourceTemplatesResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result?.ResourceTemplates);
Assert.Equal("complete", result.ResultType);
Assert.NotEmpty(result.ResourceTemplates);
Assert.Equal("test", result.ResourceTemplates[0].UriTemplate);
});
Expand Down Expand Up @@ -704,6 +715,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<ListResourcesResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result?.Resources);
Assert.Equal("complete", result.ResultType);
Assert.NotEmpty(result.Resources);
Assert.Equal("test", result.Resources[0].Uri);
});
Expand Down Expand Up @@ -739,6 +751,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<ReadResourceResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result?.Contents);
Assert.Equal("complete", result.ResultType);
Assert.NotEmpty(result.Contents);

TextResourceContents textResource = Assert.IsType<TextResourceContents>(result.Contents[0]);
Expand Down Expand Up @@ -776,6 +789,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<ListPromptsResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result?.Prompts);
Assert.Equal("complete", result.ResultType);
Assert.NotEmpty(result.Prompts);
Assert.Equal("test", result.Prompts[0].Name);
});
Expand Down Expand Up @@ -805,6 +819,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<GetPromptResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result);
Assert.Equal("complete", result.ResultType);
Assert.Equal("test", result.Description);
});
}
Expand Down Expand Up @@ -839,6 +854,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<ListToolsResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result);
Assert.Equal("complete", result.ResultType);
Assert.NotEmpty(result.Tools);
Assert.Equal("test", result.Tools[0].Name);
});
Expand Down Expand Up @@ -874,6 +890,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<CallToolResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result);
Assert.Equal("complete", result.ResultType);
Assert.NotEmpty(result.Content);
Assert.Equal("test", Assert.IsType<TextContentBlock>(result.Content[0]).Text);
});
Expand Down Expand Up @@ -907,6 +924,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<CallToolResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result);
Assert.Equal("complete", result.ResultType);
Assert.True(result.IsError);
Assert.NotEmpty(result.Content);
var textContent = Assert.IsType<TextContentBlock>(result.Content[0]);
Expand Down Expand Up @@ -935,6 +953,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<CallToolResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result);
Assert.Equal("complete", result.ResultType);
Assert.True(result.IsError);
Assert.NotEmpty(result.Content);
var textContent = Assert.IsType<TextContentBlock>(result.Content[0]);
Expand Down Expand Up @@ -970,6 +989,7 @@ await Can_Handle_Requests(
{
var result = JsonSerializer.Deserialize<CallToolResult>(response, McpJsonUtilities.DefaultOptions);
Assert.NotNull(result);
Assert.Equal("complete", result.ResultType);
Assert.True(result.IsError, "Input validation errors should be returned as tool execution errors (IsError=true), not protocol errors");
Assert.NotEmpty(result.Content);
var textContent = Assert.IsType<TextContentBlock>(result.Content[0]);
Expand Down Expand Up @@ -1230,6 +1250,7 @@ await transport.SendClientMessageAsync(new JsonRpcNotification
Assert.NotNull(response.Result);
var initResult = JsonSerializer.Deserialize<InitializeResult>(response.Result, McpJsonUtilities.DefaultOptions);
Assert.NotNull(initResult);
Assert.Equal("complete", initResult.ResultType);
Assert.NotNull(initResult.ServerInfo);

await transport.DisposeAsync();
Expand Down
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