diff --git a/integration-test/src/jvmTest/typescript/package-lock.json b/integration-test/src/jvmTest/typescript/package-lock.json
index 0ad4874..496bfa0 100644
--- a/integration-test/src/jvmTest/typescript/package-lock.json
+++ b/integration-test/src/jvmTest/typescript/package-lock.json
@@ -989,7 +989,6 @@
       "resolved": "https://registry.npmjs.org/express/-/express-5.2.1.tgz",
       "integrity": "sha512-hIS4idWWai69NezIdRt2xFVofaF4j+6INOpJlVOLDO8zXGpUVEVzIYk12UUi2JzjEzWL3IOAxcTubgz9Po0yXw==",
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "accepts": "^2.0.0",
         "body-parser": "^2.2.1",
@@ -1824,7 +1823,6 @@
       "resolved": "https://registry.npmjs.org/zod/-/zod-4.3.5.tgz",
       "integrity": "sha512-k7Nwx6vuWx1IJ9Bjuf4Zt1PEllcwe7cls3VNzm4CQ1/hgtFUK2bRNG3rvnpPUhFjmqJKAKtjV576KnUkHocg/g==",
       "license": "MIT",
-      "peer": true,
       "funding": {
         "url": "https://github.com/sponsors/colinhacks"
       }
diff --git a/kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StdioClientTransport.kt b/kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StdioClientTransport.kt
index 66d43f7..0b774a6 100644
--- a/kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StdioClientTransport.kt
+++ b/kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StdioClientTransport.kt
@@ -230,8 +230,18 @@ public class StdioClientTransport @JvmOverloads public constructor(
     }
 
     override suspend fun send(message: JSONRPCMessage, options: TransportSendOptions?) {
-        check(initialized.load()) { "Transport is not started" }
-        check(!onCloseCalled.load()) { "Transport is closed" }
+        if (!initialized.load()) {
+            throw McpException(
+                code = CONNECTION_CLOSED,
+                message = "Transport is not started",
+            )
+        }
+        if (onCloseCalled.load()) {
+            throw McpException(
+                code = CONNECTION_CLOSED,
+                message = "Transport is closed",
+            )
+        }
         @Suppress("TooGenericExceptionCaught", "SwallowedException")
         try {
             sendChannel.send(message)
diff --git a/kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StreamableHttpClientTransport.kt b/kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StreamableHttpClientTransport.kt
index a171217..c8f250a 100644
--- a/kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StreamableHttpClientTransport.kt
+++ b/kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StreamableHttpClientTransport.kt
@@ -27,7 +27,9 @@ import io.modelcontextprotocol.kotlin.sdk.types.JSONRPCMessage
 import io.modelcontextprotocol.kotlin.sdk.types.JSONRPCNotification
 import io.modelcontextprotocol.kotlin.sdk.types.JSONRPCRequest
 import io.modelcontextprotocol.kotlin.sdk.types.JSONRPCResponse
+import io.modelcontextprotocol.kotlin.sdk.types.McpException
 import io.modelcontextprotocol.kotlin.sdk.types.McpJson
+import io.modelcontextprotocol.kotlin.sdk.types.RPCError.ErrorCode.CONNECTION_CLOSED
 import io.modelcontextprotocol.kotlin.sdk.types.RequestId
 import kotlinx.coroutines.CancellationException
 import kotlinx.coroutines.CoroutineName
@@ -105,7 +107,12 @@ public class StreamableHttpClientTransport(
         resumptionToken: String?,
         onResumptionToken: ((String) -> Unit)? = null,
     ) {
-        check(initialized.load()) { "Transport is not started" }
+        if (!initialized.load()) {
+            throw McpException(
+                code = CONNECTION_CLOSED,
+                message = "Transport is not started",
+            )
+        }
         logger.debug { "Client sending message via POST to $url: ${McpJson.encodeToString(message)}" }
 
         // If we have a resumption token, reconnect the SSE stream with it
diff --git a/kotlin-sdk-client/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/AbstractClientTransportLifecycleTest.kt b/kotlin-sdk-client/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/AbstractClientTransportLifecycleTest.kt
index 8afc81f..02b6d25 100644
--- a/kotlin-sdk-client/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/AbstractClientTransportLifecycleTest.kt
+++ b/kotlin-sdk-client/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/AbstractClientTransportLifecycleTest.kt
@@ -4,7 +4,9 @@ import io.kotest.assertions.throwables.shouldThrow
 import io.kotest.matchers.shouldBe
 import io.kotest.matchers.string.shouldContain
 import io.modelcontextprotocol.kotlin.sdk.shared.AbstractTransport
+import io.modelcontextprotocol.kotlin.sdk.types.McpException
 import io.modelcontextprotocol.kotlin.sdk.types.PingRequest
+import io.modelcontextprotocol.kotlin.sdk.types.RPCError.ErrorCode.CONNECTION_CLOSED
 import io.modelcontextprotocol.kotlin.sdk.types.toJSON
 import kotlinx.coroutines.delay
 import kotlinx.coroutines.test.runTest
@@ -46,10 +48,11 @@ abstract class AbstractClientTransportLifecycleTest<T : AbstractTransport> {
     fun `should throw when sending before start`() = runTest {
         val transport = createTransport()
 
-        val exception = shouldThrow<IllegalStateException> {
+        val exception = shouldThrow<McpException> {
             transport.send(PingRequest().toJSON())
         }
         exception.message shouldContain "not started"
+        exception.code shouldBe CONNECTION_CLOSED
     }
 
     @Test
diff --git a/kotlin-sdk-client/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/stdio/StdioClientTransportErrorHandlingTest.kt b/kotlin-sdk-client/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/stdio/StdioClientTransportErrorHandlingTest.kt
index da18741..b227d5c 100644
--- a/kotlin-sdk-client/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/stdio/StdioClientTransportErrorHandlingTest.kt
+++ b/kotlin-sdk-client/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/stdio/StdioClientTransportErrorHandlingTest.kt
@@ -6,6 +6,7 @@ import io.kotest.matchers.shouldBe
 import io.kotest.matchers.types.shouldBeInstanceOf
 import io.kotest.matchers.types.shouldBeSameInstanceAs
 import io.mockk.coEvery
+import io.mockk.every
 import io.mockk.mockk
 import io.modelcontextprotocol.kotlin.sdk.client.StdioClientTransport
 import io.modelcontextprotocol.kotlin.sdk.types.JSONRPCMessage
@@ -151,7 +152,16 @@ class StdioClientTransportErrorHandlingTest {
     fun `Send should handle exceptions`(throwable: Throwable, shouldWrap: Boolean, expectedCode: Int?) = runTest {
         val sendChannel: Channel<JSONRPCMessage> = mockk(relaxed = true)
 
-        val stdin = createNonEmptyBuffer { "id: 1\ndata:\n" }
+        // Create a stdin Source that never returns EOF to prevent transport from closing
+        // Use mockk since Source is a sealed interface
+        val stdin: kotlinx.io.Source = mockk(relaxed = true)
+        every { stdin.readAtMostTo(any<Buffer>(), any()) } coAnswers {
+            // Return 0 to indicate no data available (but not EOF)
+            // This keeps the transport alive without blocking
+            delay(10) // Small delay to prevent busy-waiting
+            0L
+        }
+
         transport = StdioClientTransport(
             input = stdin,
             output = Buffer(),