chore: deprecate the SSE transport

README.md

@@ -865,6 +865,11 @@ install(CORS) {
 
 ### SSE Transport
 
+> [!WARNING]
+> The SSE transport is [deprecated](https://modelcontextprotocol.io/specification/2026-07-28/deprecated) in the MCP
+> specification and will be removed in a future release.
+> Use the [Streamable HTTP transport](#streamable-http-transport) instead.
+
 Server-Sent Events remain available for backwards compatibility with older MCP clients. Two Ktor helpers are provided:
 
 - **`Application.mcp { }`** — installs SSE and `ContentNegotiation` with `McpJson` automatically, then registers MCP
@@ -913,8 +918,6 @@ embeddedServer(CIO, port = 3000) {
 -->
 <!--- KNIT example-server-routes-03.kt -->
 
-Prefer Streamable HTTP for new projects.
-
 ### WebSocket Transport
 
 `WebSocketClientTransport` plus the matching server utilities provide full-duplex, low-latency connections—useful when

docs/moduledoc.md

@@ -41,8 +41,8 @@ High-level client for connecting to MCP servers and invoking their features:
   `serverVersion`, and `serverInstructions`.
 - Typed operations for tools, prompts, resources, completion, logging, roots, sampling, and elicitation with capability
   enforcement.
-- Transports: `StdioClientTransport`, `SseClientTransport`, `WebSocketClientTransport`, and `StreamableHttpClientTransport`,
-  plus Ktor client extensions for quick wiring.
+- Transports: `StdioClientTransport`, `StreamableHttpClientTransport`, `WebSocketClientTransport`, and the deprecated
+  `SseClientTransport`, plus Ktor client extensions for quick wiring.
 
 Minimal WebSocket client:
 
@@ -66,15 +66,15 @@ Server toolkit for exposing MCP tools, prompts, and resources:
 
 - `Server` runtime coordinates sessions, initialization flow, and capability enforcement with registries for tools,
   prompts, resources, and templates.
-- Transports: `StdioServerTransport` for CLI/editor bridges; Ktor extensions (`mcp` for SSE + POST back-channel and
-  `mcpWebSocket` for WebSocket) for HTTP hosting.
+- Transports: `StdioServerTransport` for CLI/editor bridges; Ktor extensions (`mcpStreamableHttp` for Streamable HTTP,
+  `mcpWebSocket` for WebSocket, and the deprecated `mcp` for SSE) for HTTP hosting.
 - Built-in notifications for list changes and resource subscriptions when capabilities enable them.
 
-Minimal Ktor SSE server:
+Minimal Ktor Streamable HTTP server:
 
 ```kotlin
 fun Application.module() {
-    mcp {
+    mcpStreamableHttp {
         Server(
             serverInfo = Implementation("sample-server", "1.0.0"),
             options = ServerOptions(ServerCapabilities(

integration-test/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/SseTransportTest.kt

@@ -18,6 +18,7 @@ import kotlin.test.Test
 import io.ktor.client.plugins.sse.SSE as ClientSSE
 import io.ktor.server.sse.SSE as ServerSSE
 
+@Suppress("DEPRECATION")
 class SseTransportTest {
 
     @Test

integration-test/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/integration/ClientDisconnectIntegrationTest.kt

@@ -54,6 +54,7 @@ class ClientDisconnectIntegrationTest {
 
     private fun testServer() = Server(Implementation("test-server", "1.0"), ServerOptions(ServerCapabilities()))
 
+    @Suppress("DEPRECATION")
     @Test
     fun `an SSE session ends when its client disconnects`(): Unit = runBlocking {
         val server = testServer()
@@ -75,6 +76,7 @@ class ClientDisconnectIntegrationTest {
         }
     }
 
+    @Suppress("DEPRECATION")
     @Test
     fun `an SSE handler ends when the server closes its session`(): Unit = runBlocking {
         val server = testServer()
@@ -101,6 +103,7 @@ class ClientDisconnectIntegrationTest {
         val handlerStarted = CompletableDeferred<Unit>()
         val handlerCompleted = CompletableDeferred<Boolean>()
         withCioServer({
+            @Suppress("DEPRECATION")
             mcp { testServer() }
             routing {
                 get("/slow") {

integration-test/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/integration/TransportIntegrationTest.kt

@@ -10,9 +10,9 @@ import io.ktor.client.plugins.websocket.WebSockets
 import io.ktor.server.engine.EmbeddedServer
 import io.ktor.server.engine.embeddedServer
 import io.modelcontextprotocol.kotlin.sdk.client.Client
-import io.modelcontextprotocol.kotlin.sdk.client.SseClientTransport
 import io.modelcontextprotocol.kotlin.sdk.client.StdioClientTransport
 import io.modelcontextprotocol.kotlin.sdk.client.StreamableHttpClientTransport
+import io.modelcontextprotocol.kotlin.sdk.client.mcpSseTransport
 import io.modelcontextprotocol.kotlin.sdk.client.mcpWebSocketTransport
 import io.modelcontextprotocol.kotlin.sdk.server.Server
 import io.modelcontextprotocol.kotlin.sdk.server.ServerOptions
@@ -139,7 +139,10 @@ class TransportIntegrationTest {
             engine = when (kind) {
                 TransportKind.STDIO -> null
 
-                TransportKind.SSE -> embeddedServer(ServerCIO, host = HOST, port = 0) { mcp { server } }
+                TransportKind.SSE -> embeddedServer(ServerCIO, host = HOST, port = 0) {
+                    @Suppress("DEPRECATION")
+                    mcp { server }
+                }
 
                 TransportKind.STREAMABLE_HTTP -> embeddedServer(ServerCIO, host = HOST, port = 0) {
                     mcpStreamableHttp { server }
@@ -170,7 +173,9 @@ class TransportIntegrationTest {
         private suspend fun clientTransport(): Transport = when (kind) {
             TransportKind.STDIO -> stdioClientTransport()
 
-            TransportKind.SSE -> SseClientTransport(httpClient { install(SSE) }, "http://$HOST:${port()}")
+            TransportKind.SSE ->
+                @Suppress("DEPRECATION")
+                httpClient { install(SSE) }.mcpSseTransport("http://$HOST:${port()}")
 
             TransportKind.STREAMABLE_HTTP ->
                 StreamableHttpClientTransport(httpClient { install(SSE) }, "http://$HOST:${port()}/mcp")

integration-test/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/integration/sse/SseAuthenticationTest.kt

@@ -1,3 +1,5 @@
+@file:Suppress("DEPRECATION")
+
 package io.modelcontextprotocol.kotlin.sdk.integration.sse
 
 import io.kotest.matchers.shouldBe

integration-test/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/integration/sse/SseSubpathTest.kt

@@ -1,3 +1,5 @@
+@file:Suppress("DEPRECATION")
+
 package io.modelcontextprotocol.kotlin.sdk.integration.sse
 
 import io.kotest.matchers.shouldBe

kotlin-sdk-client/Module.md

@@ -14,7 +14,8 @@ protocol types from `kotlin-sdk-core` and ships with transports for CLI and Ktor
 - **Capability enforcement**: Methods fail fast if the server does not advertise the needed capability; optional
   strictness mirrors the protocol options used by the server SDK.
 - **Transports**: Ready-to-use implementations for STDIO (CLI interoperability), Ktor SSE (`SseClientTransport` with
-  POST back-channel), Ktor WebSocket, and Streamable HTTP for environments that prefer request/response streaming.
+  POST back-channel, deprecated), Ktor WebSocket, and Streamable HTTP for environments that prefer request/response
+  streaming.
 - **Ktor client integration**: Extension helpers wire MCP over Ktor client engines with minimal setup for SSE,
   Streamable HTTP, or WebSocket.
 
@@ -50,7 +51,8 @@ println(result.content)
 ### Ktor-based transports
 
 - **SSE** (streaming GET + POST back-channel): create `SseClientTransport(httpClient, url)` (or
-  `HttpClient.mcpSseTransport(url)`); sessions are keyed by `sessionId` managed by the SDK.
+  `HttpClient.mcpSseTransport(url)`); sessions are keyed by `sessionId` managed by the SDK. Deprecated; use Streamable
+  HTTP instead.
 - **WebSocket**: use `WebSocketClientTransport(httpClient, url)` (or `HttpClient.mcpWebSocketTransport(url)`) to get
   bidirectional messaging in one connection.
 - **Streamable HTTP**: `StreamableHttpClientTransport(httpClient, url)` enables MCP over streaming HTTP

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

@@ -15,6 +15,12 @@ import kotlin.time.Duration
  * @param requestBuilder Optional lambda to configure the HTTP request.
  * @return A [SseClientTransport] configured for MCP communication.
  */
+@Deprecated(
+    "The SSE transport is deprecated in the MCP specification (SEP-2596) " +
+        "and will be removed in a future release. Use mcpStreamableHttpTransport instead.",
+    level = DeprecationLevel.WARNING,
+)
+@Suppress("DEPRECATION")
 public fun HttpClient.mcpSseTransport(
     urlString: String? = null,
     reconnectionTime: Duration? = null,
@@ -29,6 +35,12 @@ public fun HttpClient.mcpSseTransport(
  * @param requestBuilder Optional lambda to configure the HTTP request.
  * @return A connected [Client] ready for MCP communication.
  */
+@Deprecated(
+    "The SSE transport is deprecated in the MCP specification (SEP-2596) " +
+        "and will be removed in a future release. Use mcpStreamableHttp instead.",
+    level = DeprecationLevel.WARNING,
+)
+@Suppress("DEPRECATION")
 public suspend fun HttpClient.mcpSse(
     urlString: String? = null,
     reconnectionTime: Duration? = null,

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

@@ -41,6 +41,11 @@ import kotlin.time.Duration
  * Client transport for SSE: this will connect to a server using Server-Sent Events for receiving
  * messages and make separate POST requests for sending messages.
  */
+@Deprecated(
+    "The SSE transport is deprecated in the MCP specification (SEP-2596) " +
+        "and will be removed in a future release. Use StreamableHttpClientTransport instead.",
+    level = DeprecationLevel.WARNING,
+)
 @OptIn(ExperimentalAtomicApi::class)
 public class SseClientTransport(
     private val client: HttpClient,

kotlin-sdk-client/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/sse/SseClientTransportTest.kt

@@ -1,3 +1,5 @@
+@file:Suppress("DEPRECATION")
+
 package io.modelcontextprotocol.kotlin.sdk.client.sse
 
 import io.kotest.assertions.throwables.shouldThrow

kotlin-sdk-server/Module.md

@@ -10,8 +10,8 @@ that make it straightforward to expose tools, prompts, and resources to MCP clie
   tracks active sessions and cleans them up when transports close.
 - **Feature registries**: Helpers to register and remove tools, prompts, resources, and resource templates. Registry
   listeners emit list-changed notifications when the capability is enabled.
-- **Transports**: Ready-to-use transports for STDIO (CLI-style hosting), Ktor Server-Sent Events (SSE) with POST
-  back-channel, and Ktor WebSocket. All share the common `Transport` abstraction from the SDK.
+- **Transports**: Ready-to-use transports for STDIO (CLI-style hosting), Ktor Server-Sent Events (SSE, deprecated) with
+  POST back-channel, and Ktor WebSocket. All share the common `Transport` abstraction from the SDK.
 - **Ktor integration**: Extension functions (`Route.mcp`, `Route.mcpWebSocket`, and `Application` variants) that
   plug MCP into an existing Ktor server with minimal boilerplate.
 - **Notifications**: Built-in support for resource subscription, resource-updated events, and feature list change
@@ -58,6 +58,9 @@ server.createSession(transport)
 
 ### Hosting with Ktor (SSE)
 
+The SSE transport is deprecated in the MCP specification and will be removed in a future release. Use
+`mcpStreamableHttp` instead.
+
 ```kotlin
 fun Application.module() {
     mcp {

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

@@ -64,8 +64,13 @@ private val logger = KotlinLogging.logger {}
  *      that creates and returns the [Server] to handle the connection.
  * @throws IllegalStateException if the [SSE] plugin is not installed.
  */
+@Deprecated(
+    "The SSE transport is deprecated in the MCP specification (SEP-2596) " +
+        "and will be removed in a future release. Use StreamableHttpServerTransport instead.",
+    level = DeprecationLevel.WARNING,
+)
 @KtorDsl
-@Suppress("LongParameterList")
+@Suppress("DEPRECATION", "LongParameterList")
 public fun Route.mcp(
     path: String,
     enableDnsRebindingProtection: Boolean = true,
@@ -103,8 +108,13 @@ public fun Route.mcp(
  *      that creates and returns the [Server] to handle the connection.
  * @throws IllegalStateException if the [SSE] plugin is not installed.
  */
+@Deprecated(
+    "The SSE transport is deprecated in the MCP specification (SEP-2596) " +
+        "and will be removed in a future release. Use StreamableHttpServerTransport instead.",
+    level = DeprecationLevel.WARNING,
+)
 @KtorDsl
-@Suppress("LongParameterList")
+@Suppress("DEPRECATION", "LongParameterList")
 public fun Route.mcp(
     enableDnsRebindingProtection: Boolean = true,
     allowedHosts: List<String>? = null,
@@ -162,8 +172,13 @@ public fun Route.mcp(
  * @param block factory block with access to the [ServerSSESession]
  *      that creates and returns the [Server] to handle the connection.
  */
+@Deprecated(
+    "The SSE transport is deprecated in the MCP specification (SEP-2596) " +
+        "and will be removed in a future release. Use mcpStreamableHttp instead.",
+    level = DeprecationLevel.WARNING,
+)
 @KtorDsl
-@Suppress("LongParameterList")
+@Suppress("DEPRECATION", "LongParameterList")
 public fun Application.mcp(
     enableDnsRebindingProtection: Boolean = true,
     allowedHosts: List<String>? = null,
@@ -397,6 +412,7 @@ public fun Application.mcpStatelessStreamableHttp(
     )
 }
 
+@Suppress("DEPRECATION")
 private suspend fun ServerSSESession.mcpSseEndpoint(
     postEndpoint: String,
     transportManager: TransportManager<SseServerTransport>,
@@ -424,6 +440,7 @@ private suspend fun ServerSSESession.mcpSseEndpoint(
     }
 }
 
+@Suppress("DEPRECATION")
 private fun ServerSSESession.mcpSseTransport(
     postEndpoint: String,
     transportManager: TransportManager<SseServerTransport>,
@@ -460,6 +477,7 @@ private suspend fun RoutingContext.mcpStatelessStreamableHttpEndpoint(
     }
 }
 
+@Suppress("DEPRECATION")
 private suspend fun RoutingContext.mcpPostEndpoint(transportManager: TransportManager<SseServerTransport>) {
     val sessionId: String = call.request.queryParameters["sessionId"] ?: run {
         call.respond(HttpStatusCode.BadRequest, "sessionId query parameter is not provided")

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

@@ -31,6 +31,11 @@ internal const val SESSION_ID_PARAM = "sessionId"
  * @param maxRequestBodySize maximum allowed size, in bytes, of an incoming POST body; larger requests are
  *      rejected with `413 Payload Too Large` without being buffered in full. Defaults to 4 MiB.
  */
+@Deprecated(
+    "The SSE transport is deprecated in the MCP specification (SEP-2596) " +
+        "and will be removed in a future release. Use StreamableHttpServerTransport instead.",
+    level = DeprecationLevel.WARNING,
+)
 @OptIn(ExperimentalAtomicApi::class)
 public class SseServerTransport(
     private val endpoint: String,

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

@@ -202,6 +202,7 @@ class DnsRebindingProtectionTest {
         ex.message shouldContain "not-a-url"
     }
 
+    @Suppress("DEPRECATION")
     @Test
     fun `Route mcp with DNS protection enabled rejects non-localhost`() = testApplication {
         application {
@@ -218,6 +219,7 @@ class DnsRebindingProtectionTest {
         response.shouldHaveStatus(HttpStatusCode.Forbidden)
     }
 
+    @Suppress("DEPRECATION")
     @Test
     fun `Route mcp with DNS protection disabled allows any host`() = testApplication {
         application {

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

@@ -31,6 +31,7 @@ import kotlin.time.Duration.Companion.seconds
 
 class KtorExtensionsTest {
 
+    @Suppress("DEPRECATION")
     @Test
     fun `Route mcp should throw at registration time if SSE plugin is not installed`() {
         val exception = shouldThrow<IllegalStateException> {
@@ -46,6 +47,7 @@ class KtorExtensionsTest {
         exception.message shouldContain "install(SSE)"
     }
 
+    @Suppress("DEPRECATION")
     @Test
     fun `Route mcp should register SSE and POST endpoints at the given subpath`() = testApplication {
         application {
@@ -60,6 +62,7 @@ class KtorExtensionsTest {
         client.assertMcpEndpointsAt("/api/mcp")
     }
 
+    @Suppress("DEPRECATION")
     @Test
     fun `Route mcp with path parameter should register endpoints at the resolved path`() = testApplication {
         application {
@@ -79,6 +82,7 @@ class KtorExtensionsTest {
      * Verifies that a closed session stops receiving POSTs and is not retained until the
      * [Server] itself closes, even while its SSE connection is still open.
      */
+    @Suppress("DEPRECATION")
     @Test
     fun `Route mcp stops routing POSTs to a session once it closes`() = testApplication {
         val server = testServer()
@@ -101,6 +105,7 @@ class KtorExtensionsTest {
         }
     }
 
+    @Suppress("DEPRECATION")
     @Test
     fun `Application mcp should install SSE and register endpoints at the root`() = testApplication {
         application {
@@ -110,6 +115,7 @@ class KtorExtensionsTest {
         client.assertMcpEndpointsAt("/")
     }
 
+    @Suppress("DEPRECATION")
     @Test
     fun `Application mcp should reuse an installed SSE plugin`() = testApplication {
         application {

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

@@ -16,6 +16,7 @@ import io.ktor.server.testing.testApplication
 import java.util.concurrent.atomic.AtomicBoolean
 import kotlin.test.Test
 
+@Suppress("DEPRECATION")
 class SseServerTransportTest {
 
     @Test