diff --git a/tests/test_coap_wire.py b/tests/test_coap_wire.py
new file mode 100644
index 0000000..27ed460
--- /dev/null
+++ b/tests/test_coap_wire.py
@@ -0,0 +1,45 @@
+from samsung_appliance.coap_dtls import (
+    build_coap, parse_coap, encode_options, block_value, fmt_code,
+    TYPE_CON, METHOD_GET, URI_PATH, ACCEPT, CF_CBOR, BLOCK2,
+)
+
+
+def test_build_then_parse_roundtrip_no_payload():
+    opts = [(URI_PATH, b'device'), (URI_PATH, b'0'), (ACCEPT, CF_CBOR)]
+    datagram = build_coap(TYPE_CON, METHOD_GET, 0xABCD, b'\x01\x02', opts)
+    mtype, code, mid, tok, parsed_opts, payload = parse_coap(datagram)
+    assert mtype == TYPE_CON
+    assert code == METHOD_GET
+    assert mid == 0xABCD
+    assert tok == b'\x01\x02'
+    assert payload == b''
+    assert sorted(parsed_opts) == sorted(opts)
+
+
+def test_build_then_parse_roundtrip_with_payload():
+    datagram = build_coap(TYPE_CON, 0x45, 1, b'\xff', [], payload=b'\xa1\x01\x02')
+    _, code, _, _, _, payload = parse_coap(datagram)
+    assert code == 0x45
+    assert payload == b'\xa1\x01\x02'
+
+
+def test_encode_options_orders_by_option_number():
+    # ACCEPT (17) must be encoded after URI_PATH (11) regardless of input order
+    encoded_in_order = encode_options([(URI_PATH, b'x'), (ACCEPT, CF_CBOR)])
+    encoded_reversed = encode_options([(ACCEPT, CF_CBOR), (URI_PATH, b'x')])
+    assert encoded_in_order == encoded_reversed
+
+
+def test_block_value_encodes_num_more_szx():
+    # num=2, more=1, szx=6 -> (2<<4)|(1<<3)|6 = 0x2E
+    assert block_value(2, 1, 6) == bytes([0x2E])
+
+
+def test_block_value_promotes_to_two_bytes_when_num_is_large():
+    v = block_value(num=0xFFF, more=0, szx=0)
+    assert len(v) == 2
+
+
+def test_fmt_code_formats_class_dot_detail():
+    assert fmt_code(0x45) == '2.05'
+    assert fmt_code(0x84) == '4.04'