Fix Streamable handling to support JSON-only responses

kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StreamableHttpClientTransport.kt

@@ -235,10 +235,22 @@ public class StreamableHttpClientTransport(
             }
             logger.debug { "Client SSE session started successfully." }
         } catch (e: SSEClientException) {
-            if (e.response?.status == HttpStatusCode.MethodNotAllowed) {
+            val responseStatus = e.response?.status
+            val responseContentType = e.response?.contentType()
+
+            // 405 means server doesn't support SSE at GET endpoint - this is expected and valid
+            if (responseStatus == HttpStatusCode.MethodNotAllowed) {
                 logger.info { "Server returned 405 for GET/SSE, stream disabled." }
                 return
             }
+
+            // If server returns application/json, it means it doesn't support SSE for this session
+            // This is valid per spec - server can choose to only use JSON responses
+            if (responseContentType?.match(ContentType.Application.Json) == true) {
+                logger.info { "Server returned application/json for GET/SSE, using JSON-only mode." }
+                return
+            }
+
             _onError(e)
             throw e
         }

kotlin-sdk-client/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StreamableHttpClientTest.kt

@@ -1,6 +1,7 @@
 package io.modelcontextprotocol.kotlin.sdk.client
 
 import io.kotest.matchers.collections.shouldContain
+import io.ktor.http.HttpMethod
 import io.ktor.http.HttpStatusCode
 import io.ktor.sse.ServerSentEvent
 import io.modelcontextprotocol.kotlin.sdk.ClientCapabilities
@@ -148,4 +149,40 @@ internal class StreamableHttpClientTest : AbstractStreamableHttpClientTest() {
 
         client.close()
     }
+
+    @Test
+    fun `handle streaming not supported`() = runBlocking {
+        val client = Client(
+            clientInfo = Implementation(name = "client2", version = "1.0.0"),
+            options = ClientOptions(
+                capabilities = ClientCapabilities(),
+            ),
+        )
+
+        val sessionId = UUID.randomUUID().toString()
+
+        mockMcp.onInitialize(clientName = "client2", sessionId = sessionId)
+
+        mockMcp.handleJSONRPCRequest(
+            jsonRpcMethod = "notifications/initialized",
+            expectedSessionId = sessionId,
+            sessionId = sessionId,
+            statusCode = HttpStatusCode.Accepted,
+        )
+
+        mockMcp.onSubscribe(
+            httpMethod = HttpMethod.Get,
+            sessionId = sessionId,
+        ) respondsWith {
+            headers += MCP_SESSION_ID_HEADER to sessionId
+            body = null
+            httpStatus = HttpStatusCode.UnsupportedMediaType
+        }
+
+        mockMcp.mockUnsubscribeRequest(sessionId = sessionId)
+
+        connect(client)
+
+        client.close()
+    }
 }