diff --git a/release-notes/CREDITS-2.x b/release-notes/CREDITS-2.x
index 1e895809d5..22f1acceed 100644
--- a/release-notes/CREDITS-2.x
+++ b/release-notes/CREDITS-2.x
@@ -257,3 +257,7 @@ Jendrik Johannes (jjohannes@github)
 Jonathan Haber (jhaber@github)
   * Contributed #573: More customizable TokenFilter inclusion (using `Tokenfilter.Inclusion`)
   (2.12.0)
+
+Jeffrey Ye (jeffreye@github)
+  * Reported #702: `ArrayOutOfBoundException` at `WriterBasedJsonGenerator.writeString(Reader, int)`
+  (2.12.4)
diff --git a/release-notes/VERSION-2.x b/release-notes/VERSION-2.x
index dbd2bef52a..3064fbd319 100644
--- a/release-notes/VERSION-2.x
+++ b/release-notes/VERSION-2.x
@@ -14,6 +14,11 @@ JSON library.
 === Releases ===
 ------------------------------------------------------------------------
 
+2.12.4 (not yet released)
+
+#702: `ArrayOutOfBoundException` at `WriterBasedJsonGenerator.writeString(Reader, int)`
+ (reported by Jeffrey Y)
+
 2.12.3 (12-Apr-2021)
 2.12.2 (03-Mar-2021)
 2.12.1 (08-Jan-2021)
diff --git a/src/main/java/com/fasterxml/jackson/core/json/WriterBasedJsonGenerator.java b/src/main/java/com/fasterxml/jackson/core/json/WriterBasedJsonGenerator.java
index d5db1de742..70058a813f 100644
--- a/src/main/java/com/fasterxml/jackson/core/json/WriterBasedJsonGenerator.java
+++ b/src/main/java/com/fasterxml/jackson/core/json/WriterBasedJsonGenerator.java
@@ -430,37 +430,32 @@ public void writeString(String text) throws IOException
     }
 
     @Override
-    public void writeString(Reader reader, int len) throws IOException {
+    public void writeString(Reader reader, final int len) throws IOException
+    {
         _verifyValueWrite(WRITE_STRING);
         if (reader == null) {
             _reportError("null reader");
             return; // just to block warnings by lgtm.com
         }
         int toRead = (len >= 0) ? len : Integer.MAX_VALUE;
-        //Add leading quote
-        if ((_outputTail + len) >= _outputEnd) {
+        // Add leading quote
+        if (_outputTail >= _outputEnd) {
             _flushBuffer();
         }
         _outputBuffer[_outputTail++] = _quoteChar;
 
         final char[] buf = _allocateCopyBuffer();
-        //read
         while (toRead > 0) {
             int toReadNow = Math.min(toRead, buf.length);
             int numRead = reader.read(buf, 0, toReadNow);
             if (numRead <= 0) {
                 break;
             }
-            if ((_outputTail + len) >= _outputEnd) {
-                _flushBuffer();
-            }
             _writeString(buf, 0, numRead);
-
-            //decrease tracker
             toRead -= numRead;
         }
-        //Add trailing quote
-        if ((_outputTail + len) >= _outputEnd) {
+        // Add trailing quote
+        if (_outputTail >= _outputEnd) {
             _flushBuffer();
         }
         _outputBuffer[_outputTail++] = _quoteChar;
diff --git a/src/test/java/com/fasterxml/jackson/core/json/StringGenerationFromReaderTest.java b/src/test/java/com/fasterxml/jackson/core/json/StringGenerationFromReaderTest.java
index 5f8bc9c03d..f21bda04c6 100644
--- a/src/test/java/com/fasterxml/jackson/core/json/StringGenerationFromReaderTest.java
+++ b/src/test/java/com/fasterxml/jackson/core/json/StringGenerationFromReaderTest.java
@@ -30,9 +30,10 @@ public void testBasicEscaping() throws Exception
     // for [core#194]
     public void testMediumStringsBytes() throws Exception
     {
+        final JsonFactory jsonF = new JsonFactory();
         for (int mode : ALL_BINARY_MODES) {
-            for (int size : new int[] { 1100, 2300, 3800, 7500, 19000 }) {
-                _testMediumStrings(mode, size);
+            for (int size : new int[] { 1100, 2300, 3800, 7500, 19000, 33333 }) {
+                _testMediumStrings(jsonF, mode, size);
             }
         }
     }
@@ -40,18 +41,18 @@ public void testMediumStringsBytes() throws Exception
     // for [core#194]
     public void testMediumStringsChars() throws Exception
     {
+        final JsonFactory jsonF = new JsonFactory();
         for (int mode : ALL_TEXT_MODES) {
-            for (int size : new int[] { 1100, 2300, 3800, 7500, 19000 }) {
-                _testMediumStrings(mode, size);
+            for (int size : new int[] { 1100, 2300, 3800, 7500, 19000, 33333 }) {
+                _testMediumStrings(jsonF, mode, size);
             }
         }
     }
 
     public void testLongerRandomSingleChunk() throws Exception
     {
-        /* Let's first generate 100k of pseudo-random characters, favoring
-         * 7-bit ascii range
-         */
+        // Let's first generate 100k of pseudo-random characters, favoring
+        // 7-bit ascii range
         for (int mode : ALL_TEXT_MODES) {
             for (int round = 0; round < 80; ++round) {
                 String content = generateRandom(75000+round);
@@ -62,9 +63,8 @@ public void testLongerRandomSingleChunk() throws Exception
 
     public void testLongerRandomMultiChunk() throws Exception
     {
-        /* Let's first generate 100k of pseudo-random characters, favoring
-         * 7-bit ascii range
-         */
+        // Let's first generate 100k of pseudo-random characters, favoring
+        // 7-bit ascii range
         for (int mode : ALL_TEXT_MODES) {
             for (int round = 0; round < 70; ++round) {
                 String content = generateRandom(73000+round);
@@ -126,14 +126,15 @@ private String generateRandom(int len)
         return sb.toString();
     }
 
-    private void _testMediumStrings(int readMode, int length) throws Exception
+    private void _testMediumStrings(JsonFactory jsonF,
+            int readMode, int length) throws Exception
     {
         String text = _generareMediumText(length);
         StringWriter sw = new StringWriter();
         ByteArrayOutputStream bytes = new ByteArrayOutputStream();
 
-        JsonGenerator gen = (readMode != MODE_READER) ? FACTORY.createGenerator(bytes)
-                : FACTORY.createGenerator(sw);
+        JsonGenerator gen = (readMode != MODE_READER) ? jsonF.createGenerator(bytes)
+                : jsonF.createGenerator(sw);
         gen.writeStartArray();
 
         StringReader reader = new StringReader(text);
@@ -143,9 +144,9 @@ private void _testMediumStrings(int readMode, int length) throws Exception
 
         JsonParser p;
         if (readMode == MODE_READER) {
-            p = FACTORY.createParser(sw.toString());
+            p = jsonF.createParser(sw.toString());
         } else {
-            p = createParser(FACTORY, readMode, bytes.toByteArray());
+            p = createParser(jsonF, readMode, bytes.toByteArray());
         }
         assertToken(JsonToken.START_ARRAY, p.nextToken());
         assertToken(JsonToken.VALUE_STRING, p.nextToken());
@@ -153,7 +154,7 @@ private void _testMediumStrings(int readMode, int length) throws Exception
         assertToken(JsonToken.END_ARRAY, p.nextToken());
         p.close();
     }
-    
+
     private void doTestBasicEscaping() throws Exception
     {
         for (int i = 0; i < SAMPLES.length; ++i) {