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Fix invalid initialize request handling #1709

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Fix invalid initialize request handling #1709

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16 changes: 16 additions & 0 deletions src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -45,6 +45,7 @@ internal sealed class StreamableHttpHandler(

private static readonly JsonTypeInfo<JsonRpcMessage> s_messageTypeInfo = GetRequiredJsonTypeInfo<JsonRpcMessage>();
private static readonly JsonTypeInfo<JsonRpcError> s_errorTypeInfo = GetRequiredJsonTypeInfo<JsonRpcError>();
private static readonly JsonTypeInfo<InitializeRequestParams> s_initializeRequestParamsTypeInfo = GetRequiredJsonTypeInfo<InitializeRequestParams>();

private static bool AllowNewSessionForNonInitializeRequests { get; } =
AppContext.TryGetSwitch("ModelContextProtocol.AspNetCore.AllowNewSessionForNonInitializeRequests", out var enabled) && enabled;
Expand Down Expand Up @@ -116,6 +117,21 @@ await WriteJsonRpcErrorAsync(context,
return;
}

if (message is JsonRpcRequest { Method: RequestMethods.Initialize } initializeRequest)
{
try
{
JsonSerializer.Deserialize(initializeRequest.Params, s_initializeRequestParamsTypeInfo);

@PranavSenthilnathan PranavSenthilnathan Jul 23, 2026

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Missing params or "params": null can deserialize to null without throwing, so this still proceeds to GetOrCreateSessionAsync, creates a session, and leaves the original observable side effect in place. Could we treat a null result as InvalidParams too and add test coverage for both missing and explicit-null params?

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}
catch (JsonException ex)
{
await WriteJsonRpcErrorAsync(context,
$"Bad Request: The initialize request parameters were invalid. {ex.Message}",
StatusCodes.Status400BadRequest, (int)McpErrorCode.InvalidParams, requestId);
return;
}
}

var session = await GetOrCreateSessionAsync(context, message, requestId);
if (session is null)
{
Expand Down
19 changes: 19 additions & 0 deletions tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerConformanceTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -293,6 +293,25 @@ public async Task PostMalformedJson_Returns400_InvalidRequest_WithNullId()
Assert.Equal((int)McpErrorCode.InvalidRequest, doc.RootElement.GetProperty("error").GetProperty("code").GetInt32());
}

[Fact]
public async Task InitializeWithMissingClientVersion_Returns400_InvalidParams_WithRequestId()
{
await StartAsync();

const string request = """
{"jsonrpc":"2.0","id":7,"method":"initialize","params":{"protocolVersion":"2025-03-26","capabilities":{},"clientInfo":{"name":"IntegrationTestClient"}}}
""";

using var response = await HttpClient.PostAsync("", JsonContent(request), TestContext.Current.CancellationToken);
Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
Assert.False(response.Headers.Contains("mcp-session-id"));

using var doc = JsonDocument.Parse(await response.Content.ReadAsStringAsync(TestContext.Current.CancellationToken));
Assert.Equal(7, doc.RootElement.GetProperty("id").GetInt64());
Assert.Equal((int)McpErrorCode.InvalidParams, doc.RootElement.GetProperty("error").GetProperty("code").GetInt32());
Assert.Contains("version", doc.RootElement.GetProperty("error").GetProperty("message").GetString(), StringComparison.OrdinalIgnoreCase);
}

[Fact]
public async Task PostRequestWithExplicitNullId_Returns400_InvalidRequest_WithNullId()
{
Expand Down