fix(server): GET SSE stream crash on Netty in Streamable HTTP (#681)

`appendSseHeaders()` was called inside Ktor's `sse {}` block where
response headers are already committed. On Netty this throws
`UnsupportedOperationException`, killing the SSE stream and causing the
client to retry in a loop

## How Has This Been Tested?
unit, simple-streamable-server sample

## Breaking Changes
none

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

Author: devcrocod

kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/KtorServer.kt

@@ -3,13 +3,17 @@
 package io.modelcontextprotocol.kotlin.sdk.server
 
 import io.github.oshai.kotlinlogging.KotlinLogging
+import io.ktor.http.HttpMethod
 import io.ktor.http.HttpStatusCode
 import io.ktor.server.application.Application
 import io.ktor.server.application.ApplicationCall
+import io.ktor.server.application.ApplicationCallPipeline
 import io.ktor.server.application.MissingApplicationPluginException
 import io.ktor.server.application.install
 import io.ktor.server.request.ApplicationRequest
 import io.ktor.server.request.header
+import io.ktor.server.request.httpMethod
+import io.ktor.server.response.header
 import io.ktor.server.response.respond
 import io.ktor.server.routing.Route
 import io.ktor.server.routing.RoutingContext
@@ -114,6 +118,16 @@ private fun Application.mcpStreamableHttp(
 
     routing {
         route(path) {
+            // Set Mcp-Session-Id on GET responses before Ktor's sse {} commits headers.
+            intercept(ApplicationCallPipeline.Plugins) {
+                if (context.request.httpMethod == HttpMethod.Get) {
+                    val sessionId = context.request.header(MCP_SESSION_ID_HEADER)
+                    if (sessionId != null && transportManager.getTransport(sessionId) != null) {
+                        context.response.header(MCP_SESSION_ID_HEADER, sessionId)
+                    }
+                }
+            }
+
             sse {
                 val transport = existingStreamableTransport(call, transportManager) ?: return@sse
                 transport.handleRequest(this, call)

kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/StreamableHttpServerTransport.kt

@@ -453,8 +453,8 @@ public class StreamableHttpServerTransport(private val configuration: Configurat
             return
         }
 
-        call.appendSseHeaders()
-        flushSse(sseSession) // flush headers immediately
+        // SSE headers (Content-Type, Cache-Control, Connection) are already set by the framework's SSE handler
+        flushSse(sseSession)
         streamsMapping[STANDALONE_SSE_STREAM_ID] = SessionContext(sseSession, call)
         maybeSendPrimingEvent(STANDALONE_SSE_STREAM_ID, sseSession, call.request.header(MCP_PROTOCOL_VERSION_HEADER))
         sseSession.coroutineContext.job.invokeOnCompletion {
@@ -529,8 +529,8 @@ public class StreamableHttpServerTransport(private val configuration: Configurat
                 }
             }
 
-            call.appendSseHeaders()
-            flushSse(session) // flush headers immediately
+            // SSE headers are already set by the framework's SSE handler.
+            flushSse(session)
 
             val streamId = store.replayEventsAfter(lastEventId) { eventId, message ->
                 try {

kotlin-sdk-server/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/server/StreamableHttpServerTransportTest.kt

@@ -2,6 +2,7 @@ package io.modelcontextprotocol.kotlin.sdk.server
 
 import io.kotest.matchers.collections.shouldContainAll
 import io.kotest.matchers.equals.shouldBeEqual
+import io.kotest.matchers.nulls.shouldNotBeNull
 import io.kotest.matchers.shouldBe
 import io.ktor.client.HttpClient
 import io.ktor.client.call.body
@@ -10,7 +11,9 @@ import io.ktor.client.plugins.logging.Logging
 import io.ktor.client.request.HttpRequestBuilder
 import io.ktor.client.request.header
 import io.ktor.client.request.post
+import io.ktor.client.request.prepareGet
 import io.ktor.client.request.setBody
+import io.ktor.client.statement.bodyAsChannel
 import io.ktor.http.ContentType
 import io.ktor.http.HttpHeaders
 import io.ktor.http.HttpStatusCode
@@ -21,6 +24,7 @@ import io.ktor.server.routing.post
 import io.ktor.server.routing.routing
 import io.ktor.server.testing.ApplicationTestBuilder
 import io.ktor.server.testing.testApplication
+import io.ktor.utils.io.readUTF8Line
 import io.modelcontextprotocol.kotlin.sdk.types.ClientCapabilities
 import io.modelcontextprotocol.kotlin.sdk.types.EmptyResult
 import io.modelcontextprotocol.kotlin.sdk.types.Implementation
@@ -36,6 +40,7 @@ import io.modelcontextprotocol.kotlin.sdk.types.ListToolsResult
 import io.modelcontextprotocol.kotlin.sdk.types.McpJson
 import io.modelcontextprotocol.kotlin.sdk.types.Method
 import io.modelcontextprotocol.kotlin.sdk.types.RequestId
+import io.modelcontextprotocol.kotlin.sdk.types.ServerCapabilities
 import io.modelcontextprotocol.kotlin.sdk.types.Tool
 import io.modelcontextprotocol.kotlin.sdk.types.ToolSchema
 import io.modelcontextprotocol.kotlin.sdk.types.toJSON
@@ -433,6 +438,47 @@ class StreamableHttpServerTransportTest {
         }
     }
 
+    @Test
+    fun `GET SSE stream includes Mcp-Session-Id header and stays open`() = testApplication {
+        val mcpPath = "/mcp"
+
+        application {
+            mcpStreamableHttp(mcpPath) {
+                Server(
+                    Implementation("test-server", "1.0.0"),
+                    ServerOptions(capabilities = ServerCapabilities()),
+                )
+            }
+        }
+
+        val client = createTestClient()
+
+        // Step 1: Initialize session via POST
+        val initResponse = client.post(mcpPath) {
+            addStreamableHeaders()
+            setBody(buildInitializeRequestPayload())
+        }
+        initResponse.status shouldBe HttpStatusCode.OK
+        val sessionId = assertNotNull(initResponse.headers[MCP_SESSION_ID_HEADER])
+
+        // Step 2: Open GET SSE stream with session ID
+        client.prepareGet(mcpPath) {
+            header(HttpHeaders.Accept, ContentType.Text.EventStream.toString())
+            header(MCP_SESSION_ID_HEADER, sessionId)
+            header("mcp-protocol-version", LATEST_PROTOCOL_VERSION)
+        }.execute { response ->
+            // Verify Mcp-Session-Id is present on the SSE response
+            response.status shouldBe HttpStatusCode.OK
+            response.headers[MCP_SESSION_ID_HEADER] shouldBe sessionId
+
+            // Verify the stream is alive by reading at least one line (flush event)
+            val channel = response.bodyAsChannel()
+            val firstLine = channel.readUTF8Line()
+            firstLine.shouldNotBeNull()
+            channel.isClosedForRead shouldBe false
+        }
+    }
+
     private fun ApplicationTestBuilder.configureTransportEndpoint(transport: StreamableHttpServerTransport) {
         application {
             routing {