diff --git a/src/jdk.jshell/share/classes/jdk/internal/jshell/tool/JShellTool.java b/src/jdk.jshell/share/classes/jdk/internal/jshell/tool/JShellTool.java
index 1adcb472b77..5d95bc7c77d 100644
--- a/src/jdk.jshell/share/classes/jdk/internal/jshell/tool/JShellTool.java
+++ b/src/jdk.jshell/share/classes/jdk/internal/jshell/tool/JShellTool.java
@@ -72,6 +72,7 @@ import java.util.Scanner;
 import java.util.Set;
 import java.util.function.Consumer;
 import java.util.function.Predicate;
+import java.util.function.UnaryOperator;
 import java.util.prefs.Preferences;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -805,46 +806,46 @@ public class JShellTool implements MessageHandler {
     }
 
     /**
-     * Add normal prefixing/postfixing to embedded newlines in a string,
+     * Add normal prefixing/postfixing to embedded newlines in a format string,
      * bracketing with normal prefix/postfix
      *
-     * @param s the string to prefix
-     * @return the pre/post-fixed and bracketed string
+     * @param format the format string to prefix/postfix
+     * @return the pre/post-fixed and bracketed format string
      */
-    String prefix(String s) {
-         return prefix(s, feedback.getPre(), feedback.getPost());
+    String prefix(String format) {
+         return prefix(format, feedback.getPre(), feedback.getPost());
     }
 
     /**
-     * Add error prefixing/postfixing to embedded newlines in a string,
+     * Add error prefixing/postfixing to embedded newlines in a format string,
      * bracketing with error prefix/postfix
      *
-     * @param s the string to prefix
-     * @return the pre/post-fixed and bracketed string
+     * @param format the format string to prefix/postfix
+     * @return the pre/post-fixed and bracketed format string
      */
-    String prefixError(String s) {
-         return prefix(s, feedback.getErrorPre(), feedback.getErrorPost());
+    String prefixError(String format) {
+         return prefix(format, feedback.getErrorPre(), feedback.getErrorPost());
     }
 
     /**
-     * Add prefixing/postfixing to embedded newlines in a string,
+     * Add prefixing/postfixing to embedded newlines in a format string,
      * bracketing with prefix/postfix.  No prefixing when non-interactive.
-     * Result is expected to be the format for a printf.
+     * Both input and result strings are expected to be the format for a printf.
      *
-     * @param s the string to prefix
-     * @param pre the string to prepend to each line
-     * @param post the string to append to each line (replacing newline)
-     * @return the pre/post-fixed and bracketed string
+     * @param format the format string to prefix
+     * @param pre the string to prepend to each line (printf safe)
+     * @param post the string to append to each line (replacing newline; printf safe)
+     * @return the pre/post-fixed and bracketed format string
      */
-    String prefix(String s, String pre, String post) {
-        if (s == null) {
+    String prefix(String format, String pre, String post) {
+        if (format == null) {
             return "";
         }
         if (!interactiveModeBegun) {
             // messages expect to be new-line terminated (even when not prefixed)
-            return s + "%n";
+            return format + "%n";
         }
-        String pp = s.replaceAll("\\R", post + pre);
+        String pp = format.replaceAll("\\R", post + pre);
         if (pp.endsWith(post + pre)) {
             // prevent an extra prefix char and blank line when the string
             // already terminates with newline
@@ -859,7 +860,7 @@ public class JShellTool implements MessageHandler {
      * @param key the resource key
      */
     void hardrb(String key) {
-        hard(getResourceString(key));
+        hard(escape(getResourceString(key)));
     }
 
     /**
@@ -882,7 +883,7 @@ public class JShellTool implements MessageHandler {
      */
     @Override
     public void hardmsg(String key, Object... args) {
-        hard(messageFormat(key, args));
+        hard(escape(messageFormat(key, args)));
     }
 
     /**
@@ -912,7 +913,7 @@ public class JShellTool implements MessageHandler {
     }
 
     <T> void hardPairs(Stream<T> stream, Function<T, String> a, Function<T, String> b) {
-        Map<String, String> a2b = stream.collect(toMap(a, b,
+        Map<String, String> a2b = stream.collect(toMap(a, b.andThen(this::escape),
                 (m1, m2) -> m1,
                 LinkedHashMap::new));
         for (Entry<String, String> e : a2b.entrySet()) {
@@ -921,6 +922,13 @@ public class JShellTool implements MessageHandler {
         }
     }
 
+    /**
+     * Escape '%' signs in a plain string to make it a valid format string.
+     */
+    String escape(Object s) {
+        return s.toString().replace("%", "%%");
+    }
+
     /**
      * Trim whitespace off end of string
      *
@@ -2332,7 +2340,7 @@ public class JShellTool implements MessageHandler {
             sb.append(startup.show(false));
             sb.append(startup.showDetail());
         }
-        hard(sb.toString());
+        hard(escape(sb));
     }
 
     private void showIndent() {
@@ -3262,7 +3270,7 @@ public class JShellTool implements MessageHandler {
                 sb.append(a);
             }
             if (sb.length() > 0) {
-                hard(sb.toString());
+                hard(escape(sb));
             }
             return false;
         }
@@ -3777,7 +3785,7 @@ public class JShellTool implements MessageHandler {
             case NONEXISTENT:
             default:
                 // Should not occur
-                error("Unexpected status: " + previousStatus.toString() + "=>" + status.toString());
+                error("Unexpected status: %s=>%s", previousStatus, status);
                 act = FormatAction.DROPPED;
         }
         return act;
diff --git a/src/jdk.jshell/share/classes/jdk/internal/jshell/tool/resources/l10n.properties b/src/jdk.jshell/share/classes/jdk/internal/jshell/tool/resources/l10n.properties
index 4cf667a3787..44ad74611e5 100644
--- a/src/jdk.jshell/share/classes/jdk/internal/jshell/tool/resources/l10n.properties
+++ b/src/jdk.jshell/share/classes/jdk/internal/jshell/tool/resources/l10n.properties
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2016, 2023, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 2016, 2024, Oracle and/or its affiliates. All rights reserved.
 # DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 #
 # This code is free software; you can redistribute it and/or modify it
@@ -1052,7 +1052,7 @@ Show the normal prompt and the continuation-prompts:\n\
 \n\
 Where <mode> is the name of a previously defined feedback mode.\n\
 Where <prompt> and <continuation-prompt> are quoted strings to be printed as input prompts.\n\
-Both may optionally contain '%%s' which will be substituted with the next snippet ID --\n\
+Both may optionally contain '%s' which will be substituted with the next snippet ID --\n\
 note that what is entered may not be assigned that ID, for example it may be an error or command.\n\
 The continuation-prompt is used on the second and subsequent lines of a multi-line snippet.\n\
 \n\
diff --git a/test/langtools/jdk/jshell/StartupWithFormatSpecifierTest.java b/test/langtools/jdk/jshell/StartupWithFormatSpecifierTest.java
new file mode 100644
index 00000000000..b0695ae8636
--- /dev/null
+++ b/test/langtools/jdk/jshell/StartupWithFormatSpecifierTest.java
@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2024, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8341495
+ * @summary Test for crash caused by format specifier in startup script
+ * @modules jdk.compiler/com.sun.tools.javac.api
+ *          jdk.compiler/com.sun.tools.javac.main
+ *          jdk.jshell/jdk.internal.jshell.tool
+ * @library /tools/lib
+ * @build toolbox.ToolBox
+ * @build KullaTesting Compiler
+ * @run testng StartupWithFormatSpecifierTest
+ */
+
+import java.nio.file.Path;
+
+import org.testng.annotations.Test;
+
+@Test
+public class StartupWithFormatSpecifierTest extends ReplToolTesting {
+
+    public void testStartupWithFormatSpecifier() {
+        Compiler compiler = new Compiler();
+        String startupScript = "String.format(\"This is a %s.\", \"test\");";
+        Path startupFile = compiler.getPath("StartupFileOption/startup.txt");
+        compiler.writeToFile(startupFile, startupScript);
+        test(new String[] { "--startup", startupFile.toString() },
+                (a) -> assertCommandOutputContains(a, "/set start", startupScript)
+        );
+    }
+}