diff --git a/msgpack/_packer.pyx b/msgpack/_packer.pyx
index 94d1462c..75b3f7f5 100644
--- a/msgpack/_packer.pyx
+++ b/msgpack/_packer.pyx
@@ -291,6 +291,10 @@ cdef class Packer:
             raise ValueError("ext data too large")
         msgpack_pack_ext(&self.pk, typecode, len(data))
         msgpack_pack_raw_body(&self.pk, data, len(data))
+        if self.autoreset:
+            buf = PyBytes_FromStringAndSize(self.pk.buf, self.pk.length)
+            self.pk.length = 0
+            return buf
 
     @cython.critical_section
     def pack_array_header(self, long long size):
diff --git a/msgpack/fallback.py b/msgpack/fallback.py
index b02e47cf..6b54ffbf 100644
--- a/msgpack/fallback.py
+++ b/msgpack/fallback.py
@@ -861,6 +861,10 @@ def pack_ext_type(self, typecode, data):
             self._buffer.write(b"\xc9" + struct.pack(">I", L))
         self._buffer.write(struct.pack("B", typecode))
         self._buffer.write(data)
+        if self._autoreset:
+            ret = self._buffer.getvalue()
+            self._buffer = BytesIO()
+            return ret
 
     def _pack_array_header(self, n):
         if n <= 0x0F:
diff --git a/pyproject.toml b/pyproject.toml
index c69d5a7c..4e4510b4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -19,6 +19,7 @@ classifiers = [
     "Programming Language :: Python :: Implementation :: CPython",
     "Programming Language :: Python :: Implementation :: PyPy",
 ]
+dependencies = []
 
 [project.urls]
 Homepage = "https://msgpack.org/"
@@ -43,3 +44,9 @@ lint.select = [
     "I", # isort
     #"UP", pyupgrade
 ]
+
+[dependency-groups]
+dev = [
+    "cython>=3.2.5",
+    "pytest>=9.0.3",
+]
diff --git a/test/test_extension.py b/test/test_extension.py
index aaf0fd92..61852f15 100644
--- a/test/test_extension.py
+++ b/test/test_extension.py
@@ -6,7 +6,7 @@
 
 def test_pack_ext_type():
     def p(s):
-        packer = msgpack.Packer()
+        packer = msgpack.Packer(autoreset=False)
         packer.pack_ext_type(0x42, s)
         return packer.bytes()
 
@@ -20,6 +20,15 @@ def p(s):
     assert p(b"A" * 0x00012345) == b"\xc9\x00\x01\x23\x45\x42" + b"A" * 0x00012345  # ext 32
 
 
+def test_pack_ext_type_autoreset():
+    packer = msgpack.Packer()
+
+    assert packer.pack_ext_type(0x42, b"A") == b"\xd4\x42A"
+    assert packer.bytes() == b""
+    assert packer.pack_ext_type(0x42, b"ABC") == b"\xc7\x03\x42ABC"
+    assert packer.bytes() == b""
+
+
 def test_unpack_ext_type():
     def check(b, expected):
         assert msgpack.unpackb(b) == expected