diff --git a/powertools-common/src/main/java/software/amazon/lambda/powertools/common/internal/ClassPreLoader.java b/powertools-common/src/main/java/software/amazon/lambda/powertools/common/internal/ClassPreLoader.java
index ca599429e..77318722c 100644
--- a/powertools-common/src/main/java/software/amazon/lambda/powertools/common/internal/ClassPreLoader.java
+++ b/powertools-common/src/main/java/software/amazon/lambda/powertools/common/internal/ClassPreLoader.java
@@ -22,12 +22,17 @@
 import java.net.URLConnection;
 import java.util.Enumeration;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 /**
  * Used to preload classes to support automatic priming for SnapStart
  */
 public final class ClassPreLoader {
     public static final String CLASSES_FILE = "classesloaded.txt";
 
+    private static final Logger LOG = LoggerFactory.getLogger(ClassPreLoader.class);
+
     private ClassPreLoader() {
         // Hide default constructor
     }
@@ -57,6 +62,7 @@ public static void preloadClasses() {
      * @param is
      */
     private static void preloadClassesFromStream(InputStream is) {
+        int loaded = 0;
         try (is;
              InputStreamReader isr = new InputStreamReader(is, StandardCharsets.UTF_8);
              BufferedReader reader = new BufferedReader(isr)) {
@@ -66,26 +72,30 @@ private static void preloadClassesFromStream(InputStream is) {
                 if (idx != -1) {
                     line = line.substring(0, idx);
                 }
-                final String className = line.stripTrailing();
-                if (!className.isBlank()) {
-                    loadClassIfFound(className);
+                final String className = line.strip();
+                if (!className.isBlank() && loadClassIfFound(className)) {
+                    loaded++;
                 }
             }
         } catch (Exception ignored) {
             // No action is required if preloading fails for any reason
         }
+        LOG.debug("SnapStart priming: preloaded {} class(es) from {}", loaded, CLASSES_FILE);
     }
 
     /**
      * Initializes the class with given name if found, ignores otherwise
      *
-     * @param className
+     * @param className the binary name of the class to load
+     * @return true if the class was found and loaded, false otherwise
      */
-    private static void loadClassIfFound(String className) {
+    private static boolean loadClassIfFound(String className) {
         try {
             Class.forName(className, true, ClassPreLoader.class.getClassLoader());
+            return true;
         } catch (ClassNotFoundException e) {
             // No action is required if the class with given name cannot be found
+            return false;
         }
     }
 }
\ No newline at end of file
diff --git a/powertools-common/src/test/java/software/amazon/lambda/powertools/common/internal/ClassPreLoaderTest.java b/powertools-common/src/test/java/software/amazon/lambda/powertools/common/internal/ClassPreLoaderTest.java
index 03991688e..fb1359434 100644
--- a/powertools-common/src/test/java/software/amazon/lambda/powertools/common/internal/ClassPreLoaderTest.java
+++ b/powertools-common/src/test/java/software/amazon/lambda/powertools/common/internal/ClassPreLoaderTest.java
@@ -8,6 +8,7 @@ class ClassPreLoaderTest {
 
     // Making this volatile so the Thread Context doesn't need any special handling
     static volatile boolean dummyClassLoaded = false;
+    static volatile boolean leadingSpaceDummyClassLoaded = false;
 
     /**
      * Dummy class to be loaded by ClassPreLoader in test.
@@ -20,15 +21,35 @@ static class DummyClass {
             dummyClassLoaded = true;
         }
     }
+
+    /**
+     * Dummy class whose name is referenced with a leading space in
+     * <i>powertools-common/src/test/resources/classesloaded.txt</i>.
+     * Some modules ship a classesloaded.txt with a leading space on every line, so the loader must
+     * strip leading whitespace before calling Class.forName.
+     */
+    static class LeadingSpaceDummyClass {
+        static {
+            leadingSpaceDummyClassLoaded = true;
+        }
+    }
+
     @Test
     void preloadClasses_shouldIgnoreInvalidClassesAndLoadValidClasses() {
 
         dummyClassLoaded = false;
-        // powertools-common/src/test/resources/classesloaded.txt has a class that does not exist
+        leadingSpaceDummyClassLoaded = false;
+        // A class only runs its static initializer the first time it is loaded, so this test calls
+        // preloadClasses once and asserts on all classes listed in classesloaded.txt together.
+        // powertools-common/src/test/resources/classesloaded.txt has a class that does not exist.
         // Verify that the missing class did not throw any exception
         assertDoesNotThrow(ClassPreLoader::preloadClasses);
 
-        // When the classloaded.txt is a mixed bag of valid and invalid classes, Valid class must load
+        // When the classloaded.txt is a mixed bag of valid and invalid classes, valid classes must load
         assertTrue(dummyClassLoaded, "DummyClass should be loaded");
+        // classesloaded.txt references LeadingSpaceDummyClass with a leading space. The loader must
+        // strip it before Class.forName, otherwise the class is never loaded.
+        assertTrue(leadingSpaceDummyClassLoaded,
+                "LeadingSpaceDummyClass should be loaded despite the leading space");
     }
 }
\ No newline at end of file
diff --git a/powertools-common/src/test/resources/classesloaded.txt b/powertools-common/src/test/resources/classesloaded.txt
index 498ffdf69..a6cda309e 100644
--- a/powertools-common/src/test/resources/classesloaded.txt
+++ b/powertools-common/src/test/resources/classesloaded.txt
@@ -1,2 +1,3 @@
 software.amazon.lambda.powertools.common.internal.NonExistingClass
-software.amazon.lambda.powertools.common.internal.ClassPreLoaderTest$DummyClass
\ No newline at end of file
+software.amazon.lambda.powertools.common.internal.ClassPreLoaderTest$DummyClass
+ software.amazon.lambda.powertools.common.internal.ClassPreLoaderTest$LeadingSpaceDummyClass
diff --git a/powertools-tracing/src/test/java/software/amazon/lambda/powertools/tracing/handlers/PowerTracerToolEnabledForResponseWithCustomMapper.java b/powertools-tracing/src/test/java/software/amazon/lambda/powertools/tracing/handlers/PowerTracerToolEnabledForResponseWithCustomMapper.java
index a18b1580d..3627ab355 100644
--- a/powertools-tracing/src/test/java/software/amazon/lambda/powertools/tracing/handlers/PowerTracerToolEnabledForResponseWithCustomMapper.java
+++ b/powertools-tracing/src/test/java/software/amazon/lambda/powertools/tracing/handlers/PowerTracerToolEnabledForResponseWithCustomMapper.java
@@ -28,7 +28,11 @@
 import software.amazon.lambda.powertools.tracing.TracingUtils;
 
 public class PowerTracerToolEnabledForResponseWithCustomMapper implements RequestHandler<Object, Object> {
-    static {
+    public PowerTracerToolEnabledForResponseWithCustomMapper() {
+        // Set the custom mapper in the constructor rather than a static initializer. A static
+        // initializer only runs the first time this class is loaded, which makes the resulting
+        // global state depend on when the class happens to be loaded (for example by SnapStart
+        // class priming). Setting it in the constructor makes each instantiation deterministic.
         ObjectMapper objectMapper = new ObjectMapper();
         SimpleModule simpleModule = new SimpleModule();
         simpleModule.addSerializer(ChildClass.class, new ChildSerializer());
diff --git a/powertools-tracing/src/test/java/software/amazon/lambda/powertools/tracing/internal/LambdaTracingAspectTest.java b/powertools-tracing/src/test/java/software/amazon/lambda/powertools/tracing/internal/LambdaTracingAspectTest.java
index b22f7a9af..bf0e9aa90 100644
--- a/powertools-tracing/src/test/java/software/amazon/lambda/powertools/tracing/internal/LambdaTracingAspectTest.java
+++ b/powertools-tracing/src/test/java/software/amazon/lambda/powertools/tracing/internal/LambdaTracingAspectTest.java
@@ -41,6 +41,7 @@
 import software.amazon.lambda.powertools.tracing.handlers.PowerTracerToolEnabledExplicitlyForResponseAndError;
 import software.amazon.lambda.powertools.tracing.handlers.PowerTracerToolEnabledForError;
 import software.amazon.lambda.powertools.tracing.handlers.PowerTracerToolEnabledForResponse;
+import software.amazon.lambda.powertools.tracing.TracingUtils;
 import software.amazon.lambda.powertools.tracing.handlers.PowerTracerToolEnabledForResponseWithCustomMapper;
 import software.amazon.lambda.powertools.tracing.handlers.PowerTracerToolEnabledForStream;
 import software.amazon.lambda.powertools.tracing.handlers.PowerTracerToolEnabledForStreamWithNoMetaData;
@@ -70,6 +71,10 @@ void setUp() throws IllegalAccessException {
     @AfterEach
     void tearDown() {
         AWSXRay.endSegment();
+        // Some tests (e.g. the custom mapper handler) set the static TracingUtils.objectMapper.
+        // Reset it so it does not leak into later tests running in the same JVM fork and change how
+        // responses and errors are serialized.
+        TracingUtils.defaultObjectMapper(null);
     }
 
     @Test