diff --git a/pcapkit/toolkit/pypcapfile.py b/pcapkit/toolkit/pypcapfile.py
index dd73f5360..bdcce8430 100644
--- a/pcapkit/toolkit/pypcapfile.py
+++ b/pcapkit/toolkit/pypcapfile.py
@@ -568,8 +568,8 @@ def tcp_reassembly(packet: 'Packet', *, count: 'int' = -1) -> 'TCP_Packet | None
         fin=bool(tcp.fin),                    # finish flag
         header=segment[:hdr_len],             # raw bytes type header
         payload=bytearray(payload),           # raw bytearray type payload
-        first=tcp.seqnum,                     # this sequence number
-        last=tcp.seqnum + len(payload),       # next (wanted) sequence number
+        first=tcp.seqnum,                     # first sequence number of payload
+        last=tcp.seqnum + len(payload) - 1,   # last sequence number of payload
         len=len(payload),                     # payload length, header excludes
         timestamp=packet2timestamp(packet),   # capture timestamp
     )
diff --git a/tests/toolkit/test_pypcapfile_tcp_last_unit.py b/tests/toolkit/test_pypcapfile_tcp_last_unit.py
new file mode 100644
index 000000000..24e0d1075
--- /dev/null
+++ b/tests/toolkit/test_pypcapfile_tcp_last_unit.py
@@ -0,0 +1,107 @@
+"""Regression tests for the TCP sequence range of
+:func:`pcapkit.toolkit.pypcapfile.tcp_reassembly` (GH#1100).
+
+:attr:`TCP_Packet.last <pcapkit.foundation.reassembly.data.tcp.Packet.last>` is
+the *inclusive* sequence number of the segment's last payload octet, as every
+other toolkit adapter computes it. `pypcapfile`_ is not needed: its TCP decoder
+is replaced by a stand-in carrying only the attributes the adapter reads.
+
+.. _pypcapfile: https://github.com/kisom/pypcapfile
+
+"""
+
+from __future__ import annotations
+
+import importlib.util
+import struct
+import sys
+import types
+import unittest
+import unittest.mock
+
+RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
+HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
+
+
+class FakeTCP:
+    """Stand-in for :class:`pcapfile.protocols.transport.tcp.TCP`."""
+
+    def __init__(self, segment: bytes) -> None:
+        (self.src_port, self.dst_port, self.seqnum, self.acknum,
+         offset, flags) = struct.unpack('!HHIIBB', segment[:14])
+        self.data_offset = (offset >> 4) * 4
+        self.fin = flags & 0x01
+        self.syn = (flags >> 1) & 0x01
+        self.rst = (flags >> 2) & 0x01
+
+
+class FakeIP:
+    """Stand-in for :class:`pcapfile.protocols.network.ip.IP`."""
+
+    def __init__(self, payload: bytes) -> None:
+        self.p = 6
+        self.src = b'10.1.1.2'
+        self.dst = b'10.1.1.3'
+        self.payload = payload
+
+
+# NOTE: the toolkit recognises a decoded network layer by its class name.
+FakeIP.__name__ = FakeIP.__qualname__ = 'IP'
+
+
+def make_tcp(payload: bytes, *, seq: int = 1000) -> bytes:
+    """Build a raw TCP segment with no options."""
+    return struct.pack('!HHIIBBHHH', 51000, 22, seq, 2000, 5 << 4, 0x18,
+                       8192, 0xCAFE, 0) + payload
+
+
+def make_packet(segment: bytes) -> types.SimpleNamespace:
+    """Build a stand-in for :class:`pcapfile.structs.pcap_packet`."""
+    return types.SimpleNamespace(
+        header=[types.SimpleNamespace(ns_resolution=False)],
+        timestamp=1511106545,
+        timestamp_us=471719,
+        packet=types.SimpleNamespace(payload=FakeIP(segment)),
+    )
+
+
+def fake_pcapfile() -> dict[str, types.ModuleType]:
+    """Build the :mod:`pcapfile` module tree the adapter imports from."""
+    modules = {name: types.ModuleType(name) for name in (
+        'pcapfile', 'pcapfile.protocols', 'pcapfile.protocols.transport',
+        'pcapfile.protocols.transport.tcp',
+    )}
+    modules['pcapfile.protocols.transport.tcp'].TCP = FakeTCP  # type: ignore[attr-defined]
+    return modules
+
+
+@unittest.skipUnless(HAS_RUNTIME, 'runtime dependencies not installed')
+class PyPCAPFileTCPLastTests(unittest.TestCase):
+    def setUp(self) -> None:
+        patcher = unittest.mock.patch.dict(sys.modules, fake_pcapfile())
+        patcher.start()
+        self.addCleanup(patcher.stop)
+
+    def test_last_is_the_inclusive_sequence_number_of_the_last_octet(self) -> None:
+        from pcapkit.toolkit.pypcapfile import tcp_reassembly
+
+        payload = b'SSH-2.0-OpenSSH_9.3\r\n'
+        data = tcp_reassembly(make_packet(make_tcp(payload, seq=1000)), count=1)
+        self.assertIsNotNone(data)
+        self.assertEqual(data.first, 1000)
+        self.assertEqual(data.len, len(payload))
+        self.assertEqual(data.last, 1000 + len(payload) - 1)
+        self.assertEqual(data.last - data.first + 1, data.len)
+
+    def test_an_empty_segment_has_last_one_below_first(self) -> None:
+        from pcapkit.toolkit.pypcapfile import tcp_reassembly
+
+        data = tcp_reassembly(make_packet(make_tcp(b'', seq=5000)), count=1)
+        self.assertIsNotNone(data)
+        self.assertEqual(data.len, 0)
+        self.assertEqual(data.first, 5000)
+        self.assertEqual(data.last, 4999)
+
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/tests/toolkit/test_pypcapfile_unit.py b/tests/toolkit/test_pypcapfile_unit.py
index 1c70fe7f4..dc3630a37 100644
--- a/tests/toolkit/test_pypcapfile_unit.py
+++ b/tests/toolkit/test_pypcapfile_unit.py
@@ -429,7 +429,7 @@ def test_tcp_reassembly_splits_the_real_segment_exactly(self) -> None:
         self.assertEqual(data.dsn, 1000)
         self.assertEqual(data.ack, 2000)
         self.assertEqual(data.first, 1000)
-        self.assertEqual(data.last, 1021)
+        self.assertEqual(data.last, 1020)
         self.assertTrue(data.syn)
         self.assertTrue(data.fin)
         self.assertFalse(data.rst)