refactor(test): migrate to logback ListAppender for log testing

- Replaced captureStderr with a Logback ListAppender-based LogCapture utility
- Swapped slf4j-simple for logback-classic in kotlin-sdk-server's test dependencies
- Added logback-test.xml (matching the previous simplelogger.properties levels)
- Tests now assert directly on captured log events instead of relying on stderr output

Author: kpavlov

gradle/libs.versions.toml

@@ -24,6 +24,7 @@ mockk = "1.14.9"
 mokksy = "0.8.1"
 serialization = "1.10.0"
 slf4j = "2.0.17"
+logback = "1.5.32"
 junit = "5.14.3"
 
 [libraries]
@@ -67,6 +68,7 @@ mockk = { module = "io.mockk:mockk", version.ref = "mockk" }
 mokksy = { group = "dev.mokksy", name = "mokksy", version.ref = "mokksy" }
 netty-bom = { group = "io.netty", name = "netty-bom", version.ref = "netty" }
 slf4j-simple = { group = "org.slf4j", name = "slf4j-simple", version.ref = "slf4j" }
+logback-classic = { group = "ch.qos.logback", name = "logback-classic", version.ref = "logback" }
 junit-jupiter-params = { module = "org.junit.jupiter:junit-jupiter-params", version.ref = "junit" }
 
 # Samples

kotlin-sdk-server/build.gradle.kts

@@ -39,7 +39,7 @@ kotlin {
                 implementation(libs.ktor.server.content.negotiation)
                 implementation(libs.ktor.server.test.host)
 
-                runtimeOnly(libs.slf4j.simple)
+                implementation(libs.logback.classic)
             }
         }
     }

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

@@ -0,0 +1,35 @@
+package io.modelcontextprotocol.kotlin.sdk.server
+
+import ch.qos.logback.classic.Logger
+import ch.qos.logback.classic.spi.ILoggingEvent
+import ch.qos.logback.core.read.ListAppender
+import org.slf4j.LoggerFactory
+
+/**
+ * Captures log events from the given logger (and its children) using a Logback [ListAppender].
+ *
+ * Usage:
+ * ```
+ * LogCapture("io.modelcontextprotocol.kotlin.sdk.server").use { logs ->
+ *     // … code that emits log messages …
+ *     logs.messages.shouldExist { "expected text" in it }
+ * }
+ * ```
+ */
+internal class LogCapture(loggerName: String) : AutoCloseable {
+    private val logger: Logger = LoggerFactory.getLogger(loggerName) as Logger
+    private val appender: ListAppender<ILoggingEvent> = ListAppender()
+
+    init {
+        appender.start()
+        logger.addAppender(appender)
+    }
+
+    val messages: List<String>
+        get() = appender.list.map { it.formattedMessage }
+
+    override fun close() {
+        logger.detachAppender(appender)
+        appender.stop()
+    }
+}

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

@@ -1,7 +1,6 @@
 package io.modelcontextprotocol.kotlin.sdk.server
 
-import io.kotest.matchers.string.shouldContain
-import io.kotest.matchers.string.shouldNotBeEmpty
+import io.kotest.matchers.collections.shouldExist
 import io.ktor.client.request.get
 import io.ktor.serialization.kotlinx.json.json
 import io.ktor.server.application.install
@@ -11,16 +10,18 @@ import io.modelcontextprotocol.kotlin.sdk.types.ServerCapabilities
 import kotlin.test.Test
 import io.ktor.server.plugins.contentnegotiation.ContentNegotiation as ServerContentNegotiation
 
+private const val MCP_SERVER_PACKAGE = "io.modelcontextprotocol.kotlin.sdk.server"
+
 class StreamableHttpServerConfigurationTest {
 
-    private fun testServer() = Server(
+    private fun testServer(): Server = Server(
         Implementation("test-server", "1.0"),
         ServerOptions(capabilities = ServerCapabilities()),
     )
 
     @Test
     fun `mcpStreamableHttp warns when ContentNegotiation is pre-installed`() {
-        val logOutput = captureStderr {
+        LogCapture(MCP_SERVER_PACKAGE).use { logs ->
             testApplication {
                 application {
                     install(ServerContentNegotiation) {
@@ -30,14 +31,13 @@ class StreamableHttpServerConfigurationTest {
                 }
                 client.get("/mcp")
             }
+            logs.messages.shouldExist { "already installed with an unknown JSON configuration" in it }
         }
-
-        logOutput shouldContain "already installed with an unknown JSON configuration"
     }
 
     @Test
     fun `mcpStatelessStreamableHttp warns when ContentNegotiation is pre-installed`() {
-        val logOutput = captureStderr {
+        LogCapture(MCP_SERVER_PACKAGE).use { logs ->
             testApplication {
                 application {
                     install(ServerContentNegotiation) {
@@ -47,14 +47,13 @@ class StreamableHttpServerConfigurationTest {
                 }
                 client.get("/mcp")
             }
+            logs.messages.shouldExist { "already installed with an unknown JSON configuration" in it }
         }
-
-        logOutput shouldContain "already installed with an unknown JSON configuration"
     }
 
     @Test
     fun `mcp warns when ContentNegotiation is pre-installed`() {
-        val logOutput = captureStderr {
+        LogCapture(MCP_SERVER_PACKAGE).use { logs ->
             testApplication {
                 application {
                     install(ServerContentNegotiation) {
@@ -64,8 +63,7 @@ class StreamableHttpServerConfigurationTest {
                 }
                 client.get("/sse")
             }
+            logs.messages.shouldExist { "already installed with an unknown JSON configuration" in it }
         }
-
-        logOutput shouldContain "already installed with an unknown JSON configuration"
     }
 }

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

@@ -0,0 +1,15 @@
+<configuration>
+    <appender name="STDERR" class="ch.qos.logback.core.ConsoleAppender">
+        <target>System.err</target>
+        <encoder>
+            <pattern>[%thread] %level %logger - %msg%n</pattern>
+        </encoder>
+    </appender>
+
+    <logger name="io.ktor.server" level="DEBUG"/>
+    <logger name="io.modelcontextprotocol" level="DEBUG"/>
+
+    <root level="INFO">
+        <appender-ref ref="STDERR"/>
+    </root>
+</configuration>