Fix for #56 - missing method attribute from client RPC message

[IlyaGulya]

Motivation and Context

This PR fixes issue #56 where some MCP clients (including Cursor) don't include the "method" attribute in the params map of their notification messages. The absence of this attribute caused NPEs during initialization. The solution adds the method parameter to the params object when missing, ensuring compatibility with various clients.

How Has This Been Tested?

I've implemented a test suite that covers all potential scenarios:

• When method is missing from JsonObject params
• When method already exists in JsonObject params
• When params is a JsonArray (should remain unmodified)
• When params is JsonNull (should remain JsonNull)
• Testing the fallback notification handler behavior
• Error handling when a handler throws an exception

Breaking Changes

None

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the [MCP Documentation](https://modelcontextprotocol.io)
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

This PR builds upon the initial work by @tomakehurst to fix issue #56.
It retains the fix while adding test coverage and removing unintended version changes as requested by the reviewer.

[Copilot]

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/shared/TestTransport.kt:1

• The package name appears to be duplicated; consider changing it to 'io.modelcontextprotocol.kotlin.sdk.shared' to align with the directory structure and avoid confusion.

package io.modelcontextprotocol.kotlin.sdk.io.modelcontextprotocol.kotlin.sdk.shared

[IlyaGulya]

Fixed package for TestTransport, thanks for noticing 🙂

[tomakehurst]

The odd thing is that the latest release seems to have somehow fixed this particular problem despite not including my PR or something like it. I haven't had a chance to debug it and find out what's happening, but it might be worth examining before merging this.

[IlyaGulya]

Hmm, I will check, because in my case this issue still was present

[IlyaGulya]

Yeah, seems to be fixed for me as well. There's another bug left but I will make a separate PR for it, I suppose.

21:05:43.651 [DefaultDispatcher-worker-2] ERROR io.modelcontextprotocol.kotlin.sdk.shared.Protocol -- Error handling notification: notifications/initialized
java.util.NoSuchElementException: Key method is missing in the map.
	at kotlin.collections.MapsKt__MapWithDefaultKt.getOrImplicitDefaultNullable(MapWithDefault.kt:24)
	at kotlin.collections.MapsKt__MapsKt.getValue(Maps.kt:372)
	at io.modelcontextprotocol.kotlin.sdk.Types_utilKt.selectClientNotificationDeserializer(types.util.kt:137)
	at io.modelcontextprotocol.kotlin.sdk.Types_utilKt.access$selectClientNotificationDeserializer(types.util.kt:1)
	at io.modelcontextprotocol.kotlin.sdk.NotificationPolymorphicSerializer.selectDeserializer(types.util.kt:187)
	at kotlinx.serialization.json.JsonContentPolymorphicSerializer.deserialize(JsonContentPolymorphicSerializer.kt:93)
	at kotlinx.serialization.json.internal.AbstractJsonTreeDecoder.decodeSerializableValue(TreeJsonDecoder.kt:337)
	at kotlinx.serialization.json.internal.TreeJsonDecoderKt.readJson(TreeJsonDecoder.kt:26)
	at kotlinx.serialization.json.Json.decodeFromJsonElement(Json.kt:186)
	at io.modelcontextprotocol.kotlin.sdk.TypesKt.fromJSON(types.kt:1461)
	at io.modelcontextprotocol.kotlin.sdk.shared.Protocol$setNotificationHandler$1.invokeSuspend(Protocol.kt:476)
	at io.modelcontextprotocol.kotlin.sdk.shared.Protocol$setNotificationHandler$1.invoke(Protocol.kt)
	at io.modelcontextprotocol.kotlin.sdk.shared.Protocol$setNotificationHandler$1.invoke(Protocol.kt)
	at io.modelcontextprotocol.kotlin.sdk.shared.Protocol.onNotification(Protocol.kt:198)
	at io.modelcontextprotocol.kotlin.sdk.shared.Protocol.access$onNotification(Protocol.kt:93)
	at io.modelcontextprotocol.kotlin.sdk.shared.Protocol$connect$4.invokeSuspend(Protocol.kt:167)
	at io.modelcontextprotocol.kotlin.sdk.shared.Protocol$connect$4.invoke(Protocol.kt)
	at io.modelcontextprotocol.kotlin.sdk.shared.Protocol$connect$4.invoke(Protocol.kt)
	at io.modelcontextprotocol.kotlin.sdk.shared.AbstractTransport$onMessage$1.invokeSuspend(Transport.kt:94)
	at io.modelcontextprotocol.kotlin.sdk.shared.AbstractTransport$onMessage$1.invoke(Transport.kt)
	at io.modelcontextprotocol.kotlin.sdk.shared.AbstractTransport$onMessage$1.invoke(Transport.kt)
	at io.modelcontextprotocol.kotlin.sdk.server.SseServerTransport.handleMessage(SSEServerTransport.kt:102)
	at io.modelcontextprotocol.kotlin.sdk.server.SseServerTransport.handlePostMessage(SSEServerTransport.kt:86)
	at io.modelcontextprotocol.kotlin.sdk.server.KtorServerKt.mcpPostEndpoint(KtorServer.kt:108)
	at io.modelcontextprotocol.kotlin.sdk.server.KtorServerKt$mcp$4$2.invokeSuspend(KtorServer.kt:55)
	at io.modelcontextprotocol.kotlin.sdk.server.KtorServerKt$mcp$4$2.invoke(KtorServer.kt)
	at io.modelcontextprotocol.kotlin.sdk.server.KtorServerKt$mcp$4$2.invoke(KtorServer.kt)
	at io.ktor.server.routing.RoutingNode$buildPipeline$1$1.invokeSuspend(RoutingNode.kt:116)
	at io.ktor.server.routing.RoutingNode$buildPipeline$1$1.invoke(RoutingNode.kt)
	at io.ktor.server.routing.RoutingNode$buildPipeline$1$1.invoke(RoutingNode.kt)
	at io.ktor.util.pipeline.PipelineJvmKt.pipelineStartCoroutineUninterceptedOrReturn(PipelineJvm.kt:15)
	at io.ktor.util.pipeline.SuspendFunctionGun.loop(SuspendFunctionGun.kt:131)
	at io.ktor.util.pipeline.SuspendFunctionGun.proceed(SuspendFunctionGun.kt:89)
	at io.ktor.util.pipeline.SuspendFunctionGun.execute$ktor_utils(SuspendFunctionGun.kt:109)
	at io.ktor.util.pipeline.Pipeline.execute(Pipeline.kt:86)
	at io.ktor.server.routing.RoutingRoot$executeResult$$inlined$execute$1.invokeSuspend(Pipeline.kt:488)
	at io.ktor.server.routing.RoutingRoot$executeResult$$inlined$execute$1.invoke(Pipeline.kt)
	at io.ktor.server.routing.RoutingRoot$executeResult$$inlined$execute$1.invoke(Pipeline.kt)
	at io.ktor.util.debug.ContextUtilsKt.initContextInDebugMode(ContextUtils.kt:17)
	at io.ktor.server.routing.RoutingRoot.executeResult(RoutingRoot.kt:193)
	at io.ktor.server.routing.RoutingRoot.interceptor(RoutingRoot.kt:66)
	at io.ktor.server.routing.RoutingRoot$Plugin$install$1.invokeSuspend(RoutingRoot.kt:143)
	at io.ktor.server.routing.RoutingRoot$Plugin$install$1.invoke(RoutingRoot.kt)
	at io.ktor.server.routing.RoutingRoot$Plugin$install$1.invoke(RoutingRoot.kt)
	at io.ktor.util.pipeline.PipelineJvmKt.pipelineStartCoroutineUninterceptedOrReturn(PipelineJvm.kt:15)
	at io.ktor.util.pipeline.SuspendFunctionGun.loop(SuspendFunctionGun.kt:131)
	at io.ktor.util.pipeline.SuspendFunctionGun.proceed(SuspendFunctionGun.kt:89)
	at io.ktor.server.engine.BaseApplicationEngineKt$installDefaultTransformationChecker$1.invokeSuspend(BaseApplicationEngine.kt:112)
	at io.ktor.server.engine.BaseApplicationEngineKt$installDefaultTransformationChecker$1.invoke(BaseApplicationEngine.kt)
	at io.ktor.server.engine.BaseApplicationEngineKt$installDefaultTransformationChecker$1.invoke(BaseApplicationEngine.kt)
	at io.ktor.util.pipeline.PipelineJvmKt.pipelineStartCoroutineUninterceptedOrReturn(PipelineJvm.kt:15)
	at io.ktor.util.pipeline.SuspendFunctionGun.loop(SuspendFunctionGun.kt:131)
	at io.ktor.util.pipeline.SuspendFunctionGun.proceed(SuspendFunctionGun.kt:89)
	at io.ktor.util.pipeline.SuspendFunctionGun.execute$ktor_utils(SuspendFunctionGun.kt:109)
	at io.ktor.util.pipeline.Pipeline.execute(Pipeline.kt:86)
	at io.ktor.server.engine.DefaultEnginePipelineKt$defaultEnginePipeline$1$invokeSuspend$$inlined$execute$1.invokeSuspend(Pipeline.kt:488)
	at io.ktor.server.engine.DefaultEnginePipelineKt$defaultEnginePipeline$1$invokeSuspend$$inlined$execute$1.invoke(Pipeline.kt)
	at io.ktor.server.engine.DefaultEnginePipelineKt$defaultEnginePipeline$1$invokeSuspend$$inlined$execute$1.invoke(Pipeline.kt)
	at io.ktor.util.debug.ContextUtilsKt.initContextInDebugMode(ContextUtils.kt:17)
	at io.ktor.server.engine.DefaultEnginePipelineKt$defaultEnginePipeline$1.invokeSuspend(DefaultEnginePipeline.kt:123)
	at io.ktor.server.engine.DefaultEnginePipelineKt$defaultEnginePipeline$1.invoke(DefaultEnginePipeline.kt)
	at io.ktor.server.engine.DefaultEnginePipelineKt$defaultEnginePipeline$1.invoke(DefaultEnginePipeline.kt)
	at io.ktor.util.pipeline.PipelineJvmKt.pipelineStartCoroutineUninterceptedOrReturn(PipelineJvm.kt:15)
	at io.ktor.util.pipeline.SuspendFunctionGun.loop(SuspendFunctionGun.kt:131)
	at io.ktor.util.pipeline.SuspendFunctionGun.proceed(SuspendFunctionGun.kt:89)
	at io.ktor.util.pipeline.SuspendFunctionGun.execute$ktor_utils(SuspendFunctionGun.kt:109)
	at io.ktor.util.pipeline.Pipeline.execute(Pipeline.kt:86)
	at io.ktor.server.cio.CIOApplicationEngine$handleRequest$2$invokeSuspend$$inlined$execute$1.invokeSuspend(Pipeline.kt:488)
	at io.ktor.server.cio.CIOApplicationEngine$handleRequest$2$invokeSuspend$$inlined$execute$1.invoke(Pipeline.kt)
	at io.ktor.server.cio.CIOApplicationEngine$handleRequest$2$invokeSuspend$$inlined$execute$1.invoke(Pipeline.kt)
	at io.ktor.util.debug.ContextUtilsKt.initContextInDebugMode(ContextUtils.kt:17)
	at io.ktor.server.cio.CIOApplicationEngine$handleRequest$2.invokeSuspend(CIOApplicationEngine.kt:229)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
	at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:101)
	at kotlinx.coroutines.internal.LimitedDispatcher$Worker.run(LimitedDispatcher.kt:113)
	at kotlinx.coroutines.scheduling.TaskImpl.run(Tasks.kt:89)
	at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:589)
	at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:823)
	at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:720)
	at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:707)

[apikas]

Well done, @tomakehurst and @IlyaGulya !
I have built both of your commits locally (06597f3 and 7006ce9), and they both solve the problem I saw in 0.3.0 and 0.4.0 (MCP clients (including Cursor) don't include the "method" attribute in the params map of their notification messages).
I have tried my own MCP server build on the kotlin-sdk with both Claude Code 0.2.66 and MCP Inspector v0.8.1.
My opinion is that this is a showstopper, and that it would be very nice if a fix is included in the next release.