feat(core, server): improve error handling and add synthetic constructors

- Enhanced `McpException` default message logic to ensure consistency and eliminate redundant prefixes.
- Simplified test assertions in integration tests using `shouldBe` for better readability.
- Added synthetic constructors in `ServerOptions` and `McpException` for better Kotlin compatibility.
- Updated `Protocol` to rethrow `CancellationException` explicitly, ensuring proper cancellation behavior.

Author: kpavlov

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

@@ -1,5 +1,6 @@
 package io.modelcontextprotocol.kotlin.sdk.client
 
+import io.kotest.matchers.shouldBe
 import io.modelcontextprotocol.kotlin.sdk.server.Server
 import io.modelcontextprotocol.kotlin.sdk.server.ServerOptions
 import io.modelcontextprotocol.kotlin.sdk.server.ServerSession
@@ -266,8 +267,8 @@ class ClientTest {
             client.connect(failingTransport)
         }
 
-        assertEquals(-32600, exception.code)
-        assertEquals("MCP error -32600: Invalid Request", exception.message)
+        exception.code shouldBe -32600
+        exception.message shouldBe "Invalid Request"
 
         assertTrue(closed)
     }

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

@@ -1,6 +1,7 @@
 package io.modelcontextprotocol.kotlin.sdk.integration.kotlin
 
 import io.kotest.assertions.withClue
+import io.kotest.matchers.shouldBe
 import io.kotest.matchers.string.shouldContain
 import io.modelcontextprotocol.kotlin.sdk.types.GetPromptRequest
 import io.modelcontextprotocol.kotlin.sdk.types.GetPromptRequestParams
@@ -688,13 +689,13 @@ abstract class AbstractPromptIntegrationTest : KotlinTestBase() {
             }
         }
 
-        val expectedMessage = "MCP error -32603: Prompt not found: non-existent-prompt"
+        val expectedMessage = "Prompt not found: non-existent-prompt"
 
-        assertEquals(
-            RPCError.ErrorCode.INTERNAL_ERROR,
-            exception.code,
-            "Exception code should be INTERNAL_ERROR: ${RPCError.ErrorCode.INTERNAL_ERROR}",
-        )
-        assertEquals(expectedMessage, exception.message, "Unexpected error message for non-existent prompt")
+        withClue("Exception code should be INTERNAL_ERROR: -32603") {
+            exception.code shouldBe -32603
+        }
+        withClue("Unexpected error message for non-existent prompt") {
+            exception.message shouldBe expectedMessage
+        }
     }
 }

kotlin-sdk-core/api/kotlin-sdk-core.api

@@ -2713,6 +2713,7 @@ public abstract interface annotation class io/modelcontextprotocol/kotlin/sdk/ty
 }
 
 public final class io/modelcontextprotocol/kotlin/sdk/types/McpException : java/lang/Exception {
+	public fun <init> (I)V
 	public fun <init> (ILjava/lang/String;)V
 	public fun <init> (ILjava/lang/String;Lkotlinx/serialization/json/JsonElement;)V
 	public fun <init> (ILjava/lang/String;Lkotlinx/serialization/json/JsonElement;Ljava/lang/Throwable;)V

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/shared/Protocol.kt

@@ -336,6 +336,8 @@ public abstract class Protocol(@PublishedApi internal val options: ProtocolOptio
                     RPCError(code = RPCError.ErrorCode.INTERNAL_ERROR, message = cause.message ?: "Internal error")
                 }
                 transport?.send(JSONRPCError(id = request.id, error = rpcError))
+            } catch (e: CancellationException) {
+                throw e
             } catch (sendError: Throwable) {
                 logger.error(sendError) {
                     "Failed to send error response for request: ${request.method} (id: ${request.id})"

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

@@ -8,12 +8,13 @@ import kotlin.jvm.JvmOverloads
  *
  * @property code The MCP/JSON‑RPC error code.
  * @property data Optional additional error payload as a JSON element; `null` when not provided.
- * @param message The error message.
+ * @param message The error message. Used verbatim as [Exception.message] — no error code prefix is prepended.
+ *   Defaults to `"MCP error $code"` when not provided.
  * @param cause The original cause.
  */
 public class McpException @JvmOverloads public constructor(
     public val code: Int,
-    message: String?,
+    message: String = "MCP error $code",
     public val data: JsonElement? = null,
     cause: Throwable? = null,
-) : Exception("MCP error $code: $message", cause)
+) : Exception(message, cause)

kotlin-sdk-server/api/kotlin-sdk-server.api

@@ -148,6 +148,7 @@ public class io/modelcontextprotocol/kotlin/sdk/server/Server {
 public final class io/modelcontextprotocol/kotlin/sdk/server/ServerOptions : io/modelcontextprotocol/kotlin/sdk/shared/ProtocolOptions {
 	public fun <init> (Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities;)V
 	public fun <init> (Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities;Z)V
+	public synthetic fun <init> (Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities;ZILkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public fun <init> (Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities;ZLio/modelcontextprotocol/kotlin/sdk/utils/ResourceTemplateMatcherFactory;)V
 	public synthetic fun <init> (Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities;ZLio/modelcontextprotocol/kotlin/sdk/utils/ResourceTemplateMatcherFactory;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public final fun getCapabilities ()Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities;

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

@@ -66,12 +66,17 @@ private val logger = KotlinLogging.logger {}
  * @property resourceTemplateMatcherFactory The factory used to create [ResourceTemplateMatcher] instances
  *   for matching resource URIs against registered templates. Defaults to [PathSegmentTemplateMatcher.factory].
  */
-public class ServerOptions @JvmOverloads public constructor(
+public class ServerOptions(
     public val capabilities: ServerCapabilities,
     enforceStrictCapabilities: Boolean = true,
-    public val resourceTemplateMatcherFactory: ResourceTemplateMatcherFactory =
-        PathSegmentTemplateMatcher.factory,
-) : ProtocolOptions(enforceStrictCapabilities = enforceStrictCapabilities)
+    public val resourceTemplateMatcherFactory: ResourceTemplateMatcherFactory = PathSegmentTemplateMatcher.factory,
+) : ProtocolOptions(enforceStrictCapabilities = enforceStrictCapabilities) {
+    @JvmOverloads
+    public constructor(
+        capabilities: ServerCapabilities,
+        enforceStrictCapabilities: Boolean = true,
+    ) : this(capabilities, enforceStrictCapabilities, PathSegmentTemplateMatcher.factory)
+}
 
 /**
  * An MCP server is responsible for storing features and handling new connections.