diff --git a/src/org/labkey/test/tests/ClientAPITest.java b/src/org/labkey/test/tests/ClientAPITest.java
index a9b4549d91..3a6ceea1d1 100644
--- a/src/org/labkey/test/tests/ClientAPITest.java
+++ b/src/org/labkey/test/tests/ClientAPITest.java
@@ -57,6 +57,7 @@
 import org.labkey.test.util.PermissionsHelper;
 import org.labkey.test.util.PortalHelper;
 import org.labkey.test.util.StudyHelper;
+import org.labkey.test.util.TestDataGenerator;
 import org.labkey.test.util.data.TestDataUtils;
 import org.labkey.test.util.UIUserHelper;
 import org.labkey.test.util.WikiHelper;
@@ -77,9 +78,12 @@
 
 import static org.hamcrest.CoreMatchers.containsString;
 import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.endsWith;
+import static org.hamcrest.Matchers.startsWith;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 import static org.labkey.test.WebTestHelper.getHttpResponse;
 import static org.labkey.test.params.FieldDefinition.DOMAIN_TRICKY_CHARACTERS;
@@ -93,8 +97,10 @@ public class ClientAPITest extends BaseWebDriverTest
 
     private static final String PROJECT_NAME = "ClientAPITestProject";
     private static final String OTHER_PROJECT = "OtherClientAPITestProject"; // for cross-project query test
-    protected static final String FOLDER_NAME = "api folder";
+    protected static final String API_FOLDER_NAME = "api folder";
+    private static final String API_FOLDER_PATH = PROJECT_NAME + "/" + API_FOLDER_NAME;
     private static final String SUBFOLDER_NAME = "subfolder";
+    private static final String SUBFOLDER_PATH = API_FOLDER_PATH + "/" + SUBFOLDER_NAME;
     private static final String TIME_STUDY_FOLDER = "timeStudyFolder";
     private static final String TIME_STUDY_NAME = "timeStudyName";
     private static final String VISIT_STUDY_FOLDER = "visitStudyFolder";
@@ -129,10 +135,6 @@ public class ClientAPITest extends BaseWebDriverTest
 
     public static final String TEST_DIV_NAME = "testDiv";
 
-    private static final String GRIDTEST_GRIDTITLE = "ClientAPITest Grid Title";
-
-    private static final int PAGE_SIZE = 4;
-
     public static final String SRC_PREFIX = "<script type=\"text/javascript\">\n" +
             "    LABKEY.requiresExt3ClientAPI(true /* left on purpose to test old signature */, function() {\n" +
             "    Ext.namespace('demoNamespace'); //define namespace with some 'name'\n" +
@@ -204,16 +206,16 @@ public static void setupProject() throws Exception
         init._containerHelper.createProject(OTHER_PROJECT, null);
         init._containerHelper.createProject(PROJECT_NAME, null);
 
-        init._containerHelper.createSubfolder(PROJECT_NAME, FOLDER_NAME);
+        init._containerHelper.createSubfolder(PROJECT_NAME, API_FOLDER_NAME);
         init._containerHelper.createSubfolder(PROJECT_NAME, TIME_STUDY_FOLDER);
         init._containerHelper.createSubfolder(PROJECT_NAME, VISIT_STUDY_FOLDER);
         init._containerHelper.createSubfolder(PROJECT_NAME, ASSAY_STUDY_FOLDER);
 
-        init._containerHelper.createSubfolder(PROJECT_NAME, FOLDER_NAME, SUBFOLDER_NAME, "None", null);
+        init._containerHelper.createSubfolder(PROJECT_NAME, API_FOLDER_NAME, SUBFOLDER_NAME, "None", null);
 
         init.createStudies();
 
-        init.clickFolder(FOLDER_NAME);
+        init.clickFolder(API_FOLDER_NAME);
 
         init.createWiki();
 
@@ -229,10 +231,10 @@ public void preTest() throws Exception
     {
         if (dirtyList)
         {
-            refreshPeopleList(LIST_NAME, createDefaultConnection(), getProjectName() + "/" + FOLDER_NAME);
+            refreshPeopleList(LIST_NAME, createDefaultConnection(), API_FOLDER_PATH);
             dirtyList = false;
         }
-        navigateToFolder(getProjectName(), FOLDER_NAME);
+        navigateToFolder(getProjectName(), API_FOLDER_NAME);
     }
 
     @NotNull
@@ -247,7 +249,7 @@ public static ListDefinition getPeopleListDefinition(String listName)
     protected void checkLinks()
     {
         //delete the test page so the crawler doesn't refetch a test and cause errors
-        getHttpResponse(WebTestHelper.buildURL("wiki", getProjectName() + "/" + FOLDER_NAME, "delete", Maps.of("name", WIKIPAGE_NAME)), "POST");
+        getHttpResponse(WebTestHelper.buildURL("wiki", API_FOLDER_PATH, "delete", Maps.of("name", WIKIPAGE_NAME)), "POST");
         super.checkLinks();
     }
 
@@ -255,11 +257,11 @@ protected void checkLinks()
     private void createLists() throws Exception
     {
         Connection connection = createDefaultConnection();
-        createPeopleList(LIST_NAME, connection, PROJECT_NAME + "/" + FOLDER_NAME);
+        createPeopleList(LIST_NAME, connection, API_FOLDER_PATH);
         createLargePeopleList();
 
         // Create lists for cross-folder query test.
-        createPeopleList(SUBFOLDER_LIST, connection, PROJECT_NAME + "/" + FOLDER_NAME + "/" + SUBFOLDER_NAME);
+        createPeopleList(SUBFOLDER_LIST, connection, SUBFOLDER_PATH);
         createPeopleList(OTHER_PROJECT_LIST, connection, OTHER_PROJECT);
     }
 
@@ -283,7 +285,7 @@ private static void refreshPeopleList(String listName, Connection connection, St
     private void createLargePeopleList()
     {
         // Create Larger list for query test.
-        _listHelper.createListFromFile(getProjectName() + "/" + FOLDER_NAME, QUERY_LIST_NAME, TEST_XLS_DATA_FILE);
+        _listHelper.createListFromFile(API_FOLDER_PATH, QUERY_LIST_NAME, TEST_XLS_DATA_FILE);
         _listHelper.goToList(QUERY_LIST_NAME);
         waitForElement(Locator.linkWithText("Norbert"));
     }
@@ -324,7 +326,7 @@ protected String waitForDivPopulation()
     @LogMethod
     private void createWiki()
     {
-        beginAt(WebTestHelper.buildURL("wiki", getProjectName() + "/" + FOLDER_NAME, "edit"));
+        beginAt(WebTestHelper.buildURL("wiki", API_FOLDER_PATH, "edit"));
         setFormElement(Locator.name("name"), WIKIPAGE_NAME);
         setFormElement(Locator.name("title"), WIKIPAGE_NAME);
         _wikiHelper.setWikiBody("placeholder text");
@@ -1042,13 +1044,12 @@ protected String setSource(String srcFragment)
     @LogMethod
     protected String setSource(String srcFragment, boolean excludeTags)
     {
-        beginAt(WebTestHelper.buildURL("wiki", getProjectName() + "/" + FOLDER_NAME, "edit", Maps.of("name", WIKIPAGE_NAME)));
+        beginAt(WebTestHelper.buildURL("wiki", API_FOLDER_PATH, "edit", Maps.of("name", WIKIPAGE_NAME)));
 
         String fullSource = srcFragment;
         if (!excludeTags)
             fullSource = getFullSource(srcFragment);
         log("Setting wiki page source:");
-//        log(fullSource);
         _wikiHelper.setWikiBody(fullSource);
         _wikiHelper.saveWikiPage();
         return waitForDivPopulation();
@@ -1530,19 +1531,18 @@ public void suggestedColumnsInQueryDetailsTest() throws Exception
     {
         assumeTestModules();
 
-        final String PATH = getProjectName() + "/" + FOLDER_NAME;
         final String SCHEMA = "vehicle";
         final String QUERY = "UserDefinedEmissions";
-        beginAt(PATH);
+        beginAt(API_FOLDER_PATH);
         _containerHelper.enableModules(Collections.singletonList("simpletest"));
         Connection cn = getConnection(true);
 
         log("Verify default behavior to include suggested columns in details of user defined queries.");
-        List<GetQueryDetailsResponse.Column> columns = new GetQueryDetailsCommand(SCHEMA, QUERY).execute(cn, PATH).getColumns();
+        List<GetQueryDetailsResponse.Column> columns = new GetQueryDetailsCommand(SCHEMA, QUERY).execute(cn, API_FOLDER_PATH).getColumns();
         assertTrue("Suggested column 'Container' for user defined query was missing in response.",
                 columns.stream().anyMatch(col -> col.getName().equalsIgnoreCase("Container")));
         log("Verify optional behavior to exclude suggested columns from details of user defined queries.");
-        columns = new GetQueryDetailsCommand(SCHEMA, QUERY, false).execute(cn, PATH).getColumns();
+        columns = new GetQueryDetailsCommand(SCHEMA, QUERY, false).execute(cn, API_FOLDER_PATH).getColumns();
         assertTrue("Suggested column 'Container' for user defined query was included in response.",
                 columns.stream().noneMatch(col -> col.getName().equalsIgnoreCase("Container")));
     }
@@ -1562,6 +1562,35 @@ public void testBootstrapButtonEncoding()
         waitForElement(Locator.buttonContainingText("<script>alert('XSS Input Success')</script><span>Loading XSS</span>"));
     }
 
+    @Test // Issue 53699
+    public void testLargeMultibytePayload()
+    {
+        for (int i = 0; i < 10; i++)
+        {
+            // Arrange
+            int characterCount = 5_000;
+            // We had problems fetching the JSON content server-side due to Tomcat's InputBuffer not dealing with
+            // characters that span multiple "pages" of the byte buffer. Test with different numbers of single-byte
+            // UTF-8 characters before the 4-byte characters to ensure we get different boundary conditions
+            String tooManyMultibyteCharacters = "a".repeat(i) + "\uD83D\uDC7E".repeat(characterCount);
+
+            TestDataGenerator dataGenerator = new TestDataGenerator("lists", LIST_NAME, API_FOLDER_PATH)
+                    .addCustomRow(Map.of("FirstName", tooManyMultibyteCharacters, "LastName", "Chaplin", "Age", 42));
+
+            // Act
+            // Prior to fix for Issue 53699 this would throw a BufferUnderflowException when processing the JSON payload
+            Exception exception = assertThrows(Exception.class, dataGenerator::insertRows);
+
+            // Assert
+            // Prior to fix length validation incorrectly checked for string length rather than character code point length
+            String expectedPrefix = "Value is too long for column 'FirstName', a maximum length of 4000 is allowed.";
+            String expectedSuffix = String.format("was %d characters long.", characterCount + i);
+
+            assertThat(exception.getMessage(), startsWith(expectedPrefix));
+            assertThat(exception.getMessage(), endsWith(expectedSuffix));
+        }
+    }
+
     @Override
     public BrowserType bestBrowser()
     {