fix: validate protocol version header on initialization requests (#697)

Validate `mcp-protocol-version` HTTP header during `InitializeRequest`

Fix `DEFAULT_NEGOTIATED_PROTOCOL_VERSION` to match the spec

fixes #547 

## How Has This Been Tested?
Updated existing test (removed TODO referencing #547) to verify init
request

## 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

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/common.kt

@@ -12,7 +12,7 @@ import kotlinx.serialization.json.JsonObject
 public const val LATEST_PROTOCOL_VERSION: String = "2025-11-25"
 
 /** The default protocol version used when negotiation is not performed. */
-public const val DEFAULT_NEGOTIATED_PROTOCOL_VERSION: String = "2025-06-18"
+public const val DEFAULT_NEGOTIATED_PROTOCOL_VERSION: String = "2025-03-26"
 
 /** All MCP protocol versions supported by this SDK. */
 public val SUPPORTED_PROTOCOL_VERSIONS: List<String> = listOf(

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

@@ -367,6 +367,7 @@ public class StreamableHttpServerTransport(private val configuration: Configurat
                     )
                     return
                 }
+                if (!validateProtocolVersion(call)) return
                 if (messages.size > 1) {
                     call.reject(
                         HttpStatusCode.BadRequest,
@@ -393,11 +394,23 @@ public class StreamableHttpServerTransport(private val configuration: Configurat
                 return
             }
 
+            // Extract protocol version for priming event decision.
+            // For initialize requests, get from request params.
+            // For other requests, get from header (already validated).
+            val clientProtocolVersion = if (isInitializationRequest) {
+                val initRequest = messages.first() as JSONRPCRequest
+                (initRequest.params as? JsonObject)?.get("protocolVersion")
+                    ?.let { McpJson.decodeFromJsonElement<String>(it) }
+                    ?: DEFAULT_NEGOTIATED_PROTOCOL_VERSION
+            } else {
+                call.request.header(MCP_PROTOCOL_VERSION_HEADER) ?: DEFAULT_NEGOTIATED_PROTOCOL_VERSION
+            }
+
             val streamId = Uuid.random().toString()
             if (!configuration.enableJsonResponse) {
                 call.appendSseHeaders()
                 flushSse(session) // flush headers immediately
-                maybeSendPrimingEvent(streamId, session, call.request.header(MCP_PROTOCOL_VERSION_HEADER))
+                maybeSendPrimingEvent(streamId, session, clientProtocolVersion)
             }
 
             streamMutex.withLock {
@@ -456,7 +469,9 @@ public class StreamableHttpServerTransport(private val configuration: Configurat
         // 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))
+        val clientProtocolVersion =
+            call.request.header(MCP_PROTOCOL_VERSION_HEADER) ?: DEFAULT_NEGOTIATED_PROTOCOL_VERSION
+        maybeSendPrimingEvent(STANDALONE_SSE_STREAM_ID, sseSession, clientProtocolVersion)
         sseSession.coroutineContext.job.invokeOnCompletion {
             streamsMapping.remove(STANDALONE_SSE_STREAM_ID)
         }
@@ -568,9 +583,9 @@ public class StreamableHttpServerTransport(private val configuration: Configurat
             return false
         }
 
-        val sessionHeaderValues = call.request.headers.getAll(MCP_SESSION_ID_HEADER)
+        val headerId = call.request.header(MCP_SESSION_ID_HEADER)
 
-        if (sessionHeaderValues.isNullOrEmpty()) {
+        if (headerId == null) {
             call.reject(
                 HttpStatusCode.BadRequest,
                 RPCError.ErrorCode.CONNECTION_CLOSED,
@@ -579,17 +594,6 @@ public class StreamableHttpServerTransport(private val configuration: Configurat
             return false
         }
 
-        if (sessionHeaderValues.size > 1) {
-            call.reject(
-                HttpStatusCode.BadRequest,
-                RPCError.ErrorCode.CONNECTION_CLOSED,
-                "Bad Request: Mcp-Session-Id header must be a single value",
-            )
-            return false
-        }
-
-        val headerId = sessionHeaderValues.single()
-
         return when (headerId) {
             sessionId -> true
 
@@ -605,8 +609,7 @@ public class StreamableHttpServerTransport(private val configuration: Configurat
     }
 
     private suspend fun validateProtocolVersion(call: ApplicationCall): Boolean {
-        val protocolVersions = call.request.headers.getAll(MCP_PROTOCOL_VERSION_HEADER)
+        val version = call.request.headers[MCP_PROTOCOL_VERSION_HEADER] ?: return true
-        val version = protocolVersions?.lastOrNull() ?: DEFAULT_NEGOTIATED_PROTOCOL_VERSION
 
         return when (version) {
             !in SUPPORTED_PROTOCOL_VERSIONS -> {
@@ -715,14 +718,14 @@ public class StreamableHttpServerTransport(private val configuration: Configurat
     private suspend fun maybeSendPrimingEvent(
         streamId: String,
         session: ServerSSESession?,
-        clientProtocolVersion: String? = null,
+        clientProtocolVersion: String,
     ) {
         val store = configuration.eventStore
         if (store == null || session == null) return
         // Priming events have empty data which older clients cannot handle.
         // Only send priming events to clients with protocol version >= 2025-11-25
         // which includes the fix for handling empty SSE data.
-        if (clientProtocolVersion != null && clientProtocolVersion < MIN_PRIMING_EVENT_PROTOCOL_VERSION) return
+        if (clientProtocolVersion < MIN_PRIMING_EVENT_PROTOCOL_VERSION) return
         try {
             val primingEventId = store.storeEvent(streamId, JSONRPCEmptyMessage)
             session.send(

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

@@ -176,6 +176,31 @@ class StreamableHttpServerTransportTest {
         secondResponse.status shouldBe HttpStatusCode.BadRequest
     }
 
+    @Test
+    fun `init request with unsupported protocol version returns an HTTP error`() = testApplication {
+        configTestServer()
+
+        val client = createTestClient()
+
+        val transport = StreamableHttpServerTransport(enableJsonResponse = true)
+        transport.onMessage { message ->
+            if (message is JSONRPCRequest) {
+                transport.send(JSONRPCResponse(message.id, EmptyResult()))
+            }
+        }
+
+        configureTransportEndpoint(transport)
+
+        val initResponse = client.post(path) {
+            addStreamableHeaders()
+            header("mcp-protocol-version", "1900-01-01")
+            setBody(buildInitializeRequestPayload())
+        }
+
+        initResponse.status shouldBe HttpStatusCode.BadRequest
+        initResponse.headers[MCP_SESSION_ID_HEADER] shouldBe null
+    }
+
     @Test
     fun `request with unsupported protocol version returns an HTTP error`() = testApplication {
         configTestServer()
@@ -191,18 +216,15 @@ class StreamableHttpServerTransportTest {
 
         configureTransportEndpoint(transport)
 
-        val initPayload = buildInitializeRequestPayload()
         val initResponse = client.post(path) {
             addStreamableHeaders()
-            setBody(initPayload)
+            setBody(buildInitializeRequestPayload())
         }
 
         initResponse.status shouldBe HttpStatusCode.OK
         val sessionId = initResponse.headers[MCP_SESSION_ID_HEADER]
         assertNotNull(sessionId)
 
-        // TODO When https://github.com/modelcontextprotocol/kotlin-sdk/issues/547 is fixed,
-        //  check the incompatible mcp-protocol-version in the InitializeRequest and delete the part below
         val response = client.post(path) {
             addStreamableHeaders()
             header("mcp-session-id", sessionId)