MCP-437 Do not log from stderr

src/main/java/org/sonarsource/sonarqube/mcp/SonarQubeMcpServer.java

@@ -198,6 +198,12 @@ public void start() {
         .capabilities(McpSchema.ServerCapabilities.builder().tools(true).logging().build())
         .tools(filterForEnabledTools(supportedTools).stream().map(this::toStdioSpec).toArray(McpServerFeatures.SyncToolSpecification[]::new))
         .build();
+      LOG.setNotifier(notification -> transportProvider.notifyClients(McpSchema.METHOD_NOTIFICATION_MESSAGE, notification)
+        .subscribe(v -> {
+          // success: nothing to do, already written to file
+        }, e -> {
+          // swallow: notification the SDK already logs dispatch failure
+        }));
     }
 
     Runtime.getRuntime().addShutdownHook(new Thread(this::shutdown));
@@ -629,6 +635,7 @@ public void shutdown() {
     shutdownMcpServer();
     shutdownAnalytics();
     shutdownBackend();
+    LOG.clearNotifier();
   }
 
   private void shutdownAnalytics() {

src/main/java/org/sonarsource/sonarqube/mcp/log/McpLogger.java

@@ -16,19 +16,32 @@
  */
 package org.sonarsource.sonarqube.mcp.log;
 
+import io.modelcontextprotocol.spec.McpSchema;
+import java.io.PrintWriter;
+import java.io.StringWriter;
+import java.util.Objects;
+import java.util.function.Consumer;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 /**
- * MCP-specific logger that outputs to both:
- * - STDERR: for MCP clients (the MCP protocol uses STDERR for diagnostic logs, STDOUT is reserved for JSON-RPC messages)
+ * MCP-specific logger that outputs to:
  * - Log file: via SLF4J/Logback for persistence and debugging
+ * - MCP client: as {@code notifications/message} (per the MCP logging spec) when a notifier is configured.
+ *   Outside of a session (e.g. during startup, before the MCP server is built) the notifier is a no-op
+ *   and only SLF4J file logging happens.
  */
 public class McpLogger {
 
   private static final Logger LOG = LoggerFactory.getLogger(McpLogger.class);
   private static final McpLogger INSTANCE = new McpLogger();
   private static final String SONARQUBE_DEBUG_ENABLED = "SONARQUBE_DEBUG_ENABLED";
+  private static final String LOGGER_NAME = "sonarqube-mcp-server";
+  private static final Consumer<McpSchema.LoggingMessageNotification> NO_OP = notification -> {
+    // no-op
+  };
+
+  private Consumer<McpSchema.LoggingMessageNotification> notifier = NO_OP;
 
   public static McpLogger getInstance() {
     return INSTANCE;
@@ -46,36 +59,53 @@ private static boolean resolveDebugEnabled() {
     return "true".equalsIgnoreCase(System.getProperty(SONARQUBE_DEBUG_ENABLED));
   }
 
+  public void setNotifier(Consumer<McpSchema.LoggingMessageNotification> notifier) {
+    this.notifier = Objects.requireNonNull(notifier);
+  }
+
+  public void clearNotifier() {
+    this.notifier = NO_OP;
+  }
+
   public void info(String message) {
     LOG.info(message);
-    logToStderr("INFO", message);
+    notify(McpSchema.LoggingLevel.INFO, message);
   }
 
   public void debug(String message) {
     if (isDebugEnabled()) {
       LOG.debug(message);
-      logToStderr("DEBUG", message);
+      notify(McpSchema.LoggingLevel.DEBUG, message);
     }
   }
 
   public void warn(String message) {
     LOG.warn(message);
-    logToStderr("WARN", message);
+    notify(McpSchema.LoggingLevel.WARNING, message);
   }
 
   public void error(String message, Throwable throwable) {
     LOG.error(message, throwable);
-    logToStderr("ERROR", message);
-    throwable.printStackTrace(System.err);
+    notify(McpSchema.LoggingLevel.ERROR, message + System.lineSeparator() + stackTraceOf(throwable));
   }
 
   public void error(String message) {
     LOG.error(message);
-    logToStderr("ERROR", message);
+    notify(McpSchema.LoggingLevel.ERROR, message);
+  }
+
+  private void notify(McpSchema.LoggingLevel level, String message) {
+    try {
+      notifier.accept(new McpSchema.LoggingMessageNotification(level, LOGGER_NAME, message, null));
+    } catch (Exception e) {
+      LOG.warn("Failed to dispatch MCP log notification", e);
+    }
   }
 
-  private static void logToStderr(String level, String message) {
-    System.err.println(level + " SonarQube MCP Server - " + message);
+  private static String stackTraceOf(Throwable throwable) {
+    var writer = new StringWriter();
+    throwable.printStackTrace(new PrintWriter(writer));
+    return writer.toString();
   }
 
 }

src/test/java/org/sonarsource/sonarqube/mcp/SonarQubeMcpServerGenericTest.java

@@ -17,15 +17,17 @@
 package org.sonarsource.sonarqube.mcp;
 
 import io.modelcontextprotocol.common.McpTransportContext;
-import java.io.ByteArrayOutputStream;
+import io.modelcontextprotocol.spec.McpSchema;
 import java.io.IOException;
-import java.io.PrintStream;
 import java.net.ServerSocket;
-import java.nio.charset.StandardCharsets;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.stream.Collectors;
 import org.sonarsource.sonarqube.mcp.harness.SonarQubeMcpServerTest;
 import org.sonarsource.sonarqube.mcp.harness.SonarQubeMcpServerTestHarness;
+import org.sonarsource.sonarqube.mcp.log.McpLogger;
 import org.sonarsource.sonarqube.mcp.tools.proxied.ProxiedMcpTool;
 import org.sonarsource.sonarqube.mcp.transport.HttpServerTransportProvider;
 import org.sonarsource.sonarqube.mcp.transport.StdioServerTransportProvider;
@@ -58,9 +60,7 @@ void get_should_return_server_api_in_stdio_mode(SonarQubeMcpServerTestHarness ha
   @SonarQubeMcpServerTest
   void should_log_sanitized_config_when_elevated_debug_enabled(SonarQubeMcpServerTestHarness harness) {
     System.setProperty("SONARQUBE_DEBUG_ENABLED", "true");
-    var originalErr = System.err;
-    var errBuffer = new ByteArrayOutputStream();
-    System.setErr(new PrintStream(errBuffer, true, StandardCharsets.UTF_8));
+    List<McpSchema.LoggingMessageNotification> capturedNotifications = new CopyOnWriteArrayList<>();
 
     try {
       var environment = createStdioEnvironment(harness.getMockSonarQubeServer().baseUrl());
@@ -71,6 +71,9 @@ void should_log_sanitized_config_when_elevated_debug_enabled(SonarQubeMcpServerT
         null,
         environment);
       server.start();
+      // Override the server-installed notifier with a capturing one so we can assert on the
+      // notifications/message payloads emitted during the asynchronous startup logging.
+      McpLogger.getInstance().setNotifier(capturedNotifications::add);
       var configuration = server.getMcpConfiguration();
 
       assertThat(System.getProperty("os.name")).isNotNull().isNotEmpty();
@@ -81,10 +84,12 @@ void should_log_sanitized_config_when_elevated_debug_enabled(SonarQubeMcpServerT
       assertThat(configuration.getLogFilePath().toAbsolutePath().toString()).isNotNull().isNotEmpty();
 
       await().atMost(2, SECONDS).untilAsserted(() -> {
-        var stderrOutput = errBuffer.toString(StandardCharsets.UTF_8);
+        var joined = capturedNotifications.stream()
+          .map(McpSchema.LoggingMessageNotification::data)
+          .collect(Collectors.joining("\n"));
         var proxySelector = java.net.ProxySelector.getDefault();
         var proxySelectorName = proxySelector != null ? proxySelector.getClass().getName() : "none";
-        assertThat(stderrOutput)
+        assertThat(joined)
           .contains("SSL/TLS - OS: " + System.getProperty("os.name"))
           .contains("SSL/TLS configured - protocol: TLS")
           .contains("Proxy selector: " + proxySelectorName)
@@ -102,7 +107,7 @@ void should_log_sanitized_config_when_elevated_debug_enabled(SonarQubeMcpServerT
       server.shutdown();
     } finally {
       System.clearProperty("SONARQUBE_DEBUG_ENABLED");
-      System.setErr(originalErr);
+      McpLogger.getInstance().clearNotifier();
     }
   }
 

src/test/java/org/sonarsource/sonarqube/mcp/http/HttpClientProxyTests.java

@@ -17,22 +17,23 @@
 package org.sonarsource.sonarqube.mcp.http;
 
 import com.github.tomakehurst.wiremock.junit5.WireMockExtension;
+import io.modelcontextprotocol.spec.McpSchema;
 import java.io.IOException;
-import java.io.ByteArrayOutputStream;
-import java.io.PrintStream;
 import java.net.InetSocketAddress;
 import java.net.Proxy;
 import java.net.ProxySelector;
 import java.net.SocketAddress;
 import java.net.URI;
-import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.CompletionException;
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.stream.Collectors;
 import org.apache.hc.core5.http.HttpStatus;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.RegisterExtension;
+import org.sonarsource.sonarqube.mcp.log.McpLogger;
 
 import static com.github.tomakehurst.wiremock.client.WireMock.aResponse;
 import static com.github.tomakehurst.wiremock.client.WireMock.get;
@@ -50,20 +51,14 @@ class HttpClientProxyTests {
     .build();
 
   private ProxySelector originalProxySelector;
-  private PrintStream originalErr;
-  private ByteArrayOutputStream errBuffer;
 
   @AfterEach
   void tearDown() {
     // Restore original proxy selector
     if (originalProxySelector != null) {
       ProxySelector.setDefault(originalProxySelector);
     }
-    if (originalErr != null) {
-      System.setErr(originalErr);
-      originalErr = null;
-      errBuffer = null;
-    }
+    McpLogger.getInstance().clearNotifier();
     // Clear proxy system properties
     System.clearProperty("http.proxyHost");
     System.clearProperty("http.proxyPort");
@@ -77,6 +72,18 @@ void tearDown() {
     System.clearProperty("SONARQUBE_DEBUG_ENABLED");
   }
 
+  private static List<McpSchema.LoggingMessageNotification> captureLogNotifications() {
+    var captured = new CopyOnWriteArrayList<McpSchema.LoggingMessageNotification>();
+    McpLogger.getInstance().setNotifier(captured::add);
+    return captured;
+  }
+
+  private static String joinedLogData(List<McpSchema.LoggingMessageNotification> notifications) {
+    return notifications.stream()
+      .map(McpSchema.LoggingMessageNotification::data)
+      .collect(Collectors.joining("\n"));
+  }
+
   @Test
   void should_invoke_proxy_selector_when_making_requests() {
     var trackingProxySelector = new TrackingProxySelector();
@@ -163,15 +170,13 @@ void should_log_proxy_settings_when_elevated_debug_enabled() {
     System.setProperty("https.proxyHost", "proxy.local");
     System.setProperty("https.proxyPort", "3129");
 
-    originalErr = System.err;
-    errBuffer = new ByteArrayOutputStream();
-    System.setErr(new PrintStream(errBuffer, true, StandardCharsets.UTF_8));
+    var captured = captureLogNotifications();
 
     var provider = new HttpClientProvider(USER_AGENT);
     provider.logConnectionSettings();
 
-    var stderrOutput = errBuffer.toString(StandardCharsets.UTF_8);
-    assertThat(stderrOutput)
+    var output = joinedLogData(captured);
+    assertThat(output)
       .contains("SSL/TLS - OS: ")
       .contains("SSL/TLS configured - protocol: ")
       .contains("Proxy selector: ")
@@ -187,15 +192,13 @@ void should_log_socks_proxy_settings_when_elevated_debug_enabled() {
     System.setProperty("socksProxyHost", "socks.local");
     System.setProperty("socksProxyPort", "1080");
 
-    originalErr = System.err;
-    errBuffer = new ByteArrayOutputStream();
-    System.setErr(new PrintStream(errBuffer, true, StandardCharsets.UTF_8));
+    var captured = captureLogNotifications();
 
     var provider = new HttpClientProvider(USER_AGENT);
     provider.logConnectionSettings();
 
-    var stderrOutput = errBuffer.toString(StandardCharsets.UTF_8);
-    assertThat(stderrOutput)
+    var output = joinedLogData(captured);
+    assertThat(output)
       .contains("SOCKS proxy: socks.local:1080")
       .doesNotContain("No proxy system properties configured");
   }
@@ -204,15 +207,13 @@ void should_log_socks_proxy_settings_when_elevated_debug_enabled() {
   void should_log_no_proxy_when_no_proxy_properties_set() {
     System.setProperty("SONARQUBE_DEBUG_ENABLED", "true");
 
-    originalErr = System.err;
-    errBuffer = new ByteArrayOutputStream();
-    System.setErr(new PrintStream(errBuffer, true, StandardCharsets.UTF_8));
+    var captured = captureLogNotifications();
 
     var provider = new HttpClientProvider(USER_AGENT);
     provider.logConnectionSettings();
 
-    var stderrOutput = errBuffer.toString(StandardCharsets.UTF_8);
-    assertThat(stderrOutput).contains("No proxy system properties configured");
+    var output = joinedLogData(captured);
+    assertThat(output).contains("No proxy system properties configured");
   }
 
   @Test

src/test/java/org/sonarsource/sonarqube/mcp/serverapi/ServerApiTests.java

@@ -17,9 +17,10 @@
 package org.sonarsource.sonarqube.mcp.serverapi;
 
 import com.github.tomakehurst.wiremock.junit5.WireMockExtension;
-import java.io.ByteArrayOutputStream;
-import java.io.PrintStream;
-import java.nio.charset.StandardCharsets;
+import io.modelcontextprotocol.spec.McpSchema;
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.stream.Collectors;
 import java.util.stream.Stream;
 import org.apache.hc.core5.http.HttpStatus;
 import org.junit.jupiter.api.AfterEach;
@@ -31,6 +32,7 @@
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
 import org.sonarsource.sonarqube.mcp.http.HttpClientProvider;
+import org.sonarsource.sonarqube.mcp.log.McpLogger;
 import org.sonarsource.sonarqube.mcp.serverapi.exception.ForbiddenException;
 import org.sonarsource.sonarqube.mcp.serverapi.exception.NotFoundException;
 import org.sonarsource.sonarqube.mcp.serverapi.exception.ServerInternalErrorException;
@@ -50,8 +52,6 @@ class ServerApiTests {
 
   private static final String USER_AGENT = "SonarQube MCP tests";
   private ServerApiHelper serverApiHelper;
-  private PrintStream originalErr;
-  private ByteArrayOutputStream errBuffer;
 
   @RegisterExtension
   static WireMockExtension sonarqubeMock = WireMockExtension.newInstance()
@@ -69,11 +69,7 @@ void init() {
   @AfterEach
   void tearDown() {
     System.clearProperty("SONARQUBE_DEBUG_ENABLED");
-    if (originalErr != null) {
-      System.setErr(originalErr);
-      originalErr = null;
-      errBuffer = null;
-    }
+    McpLogger.getInstance().clearNotifier();
   }
 
   static Stream<Arguments> getErrorResponses() {
@@ -121,16 +117,17 @@ void it_should_parse_error_message_from_body(String responseBody, String message
   @Test
   void it_should_log_error_response_code() {
     System.setProperty("SONARQUBE_DEBUG_ENABLED", "true");
-    originalErr = System.err;
-    errBuffer = new ByteArrayOutputStream();
-    System.setErr(new PrintStream(errBuffer, true, StandardCharsets.UTF_8));
+    List<McpSchema.LoggingMessageNotification> capturedNotifications = new CopyOnWriteArrayList<>();
+    McpLogger.getInstance().setNotifier(capturedNotifications::add);
 
     sonarqubeMock.stubFor(get("/test").willReturn(jsonResponse("{\"errors\": [{\"msg\": \"Missing permission\",\"code\":\"insufficient_privileges\"}]}", HttpStatus.SC_FORBIDDEN)));
 
     assertThrows(ForbiddenException.class, () -> serverApiHelper.get("/test"));
 
-    var stderrOutput = errBuffer.toString(StandardCharsets.UTF_8);
-    assertThat(stderrOutput)
+    var joined = capturedNotifications.stream()
+      .map(McpSchema.LoggingMessageNotification::data)
+      .collect(Collectors.joining("\n"));
+    assertThat(joined)
       .contains("HTTP error - URL: " + sonarqubeMock.baseUrl() + "/test")
       .contains("status: 403");
   }