diff --git a/harness/udpstream/frame.py b/harness/udpstream/frame.py
index e6bdefa..6de19ae 100644
--- a/harness/udpstream/frame.py
+++ b/harness/udpstream/frame.py
@@ -103,39 +103,55 @@ def _checked(name: str, value: int, limit: int) -> int:
 
 
 def pack_header(kind: int, frame_no: int, payload_len: int, rows: int,
-                stretch: int, payload: bytes = b"") -> bytes:
+                stretch: int, payload: bytes | None = None) -> bytes:
     """Build the 16-byte header for a frame.
 
     The CRC is computed over header bytes 0..11 plus ``payload``.  Pass the
-    payload here to seal a complete frame in one call; with the default empty
-    payload the header carries the CRC of a header-only frame (use
-    :func:`seal_header` to add the CRC later -- that is what the chunked sender
-    does, since it builds the header before splitting the payload).
+    payload here to seal a complete frame in one call -- and then it must be
+    exactly ``payload_len`` bytes, or a frame the CRC covers but the length
+    field denies would go on the wire.  Omit ``payload`` to build the header
+    before the body exists and seal it later with :func:`seal_header`.
     """
+    kind = _checked("kind", kind, 0xFF)
+    frame_no = _checked("frame_no", frame_no, 0xFFFF)
+    payload_len = _checked("payload_len", payload_len, 0xFFFF)
+    rows = _checked("rows", rows, 0xFF)
+    stretch = _checked("stretch", stretch, 0xFF)
+    if payload is None:
+        body = b""
+    else:
+        body = bytes(payload)
+        if len(body) != payload_len:
+            raise ValueError("payload is %d bytes but payload_len says %d"
+                             % (len(body), payload_len))
     prefix = _PREFIX.pack(
         MAGIC,
         VERSION,
-        _checked("kind", kind, 0xFF),
-        _checked("frame_no", frame_no, 0xFFFF),
-        _checked("payload_len", payload_len, 0xFFFF),
-        _checked("rows", rows, 0xFF),
-        _checked("stretch", stretch, 0xFF),
+        kind,
+        frame_no,
+        payload_len,
+        rows,
+        stretch,
         0,                                  # reserved
     )
     assert len(prefix) == CRC_OFFSET
-    crc = crc16(prefix + bytes(payload))
+    crc = crc16(prefix + body)
     return prefix + _TAIL.pack(crc, 0)
 
 
 def seal_header(header: bytes, payload: bytes = b"") -> bytes:
     """Return ``header`` with its CRC field recomputed over ``payload``.
 
+    ``header`` must be exactly :data:`HEADER_SIZE` bytes: a longer buffer is
+    rejected rather than silently truncated, so a caller cannot pass
+    ``header + payload`` here and lose the payload from the CRC.
+
     Byte-for-byte identical to ``pack_header(..., payload=payload)`` for a
     header that came from :func:`pack_header`; kept separate so a caller that
     assembles a frame in pieces (header, then payload) can still seal it.
     """
-    if len(header) < HEADER_SIZE:
-        raise ValueError("header shorter than %d bytes: %d"
+    if len(header) != HEADER_SIZE:
+        raise ValueError("header must be exactly %d bytes, got %d"
                          % (HEADER_SIZE, len(header)))
     crc = crc16(bytes(header[:CRC_OFFSET]) + bytes(payload))
     return bytes(header[:CRC_OFFSET]) + _TAIL.pack(crc, 0)
diff --git a/harness/udpstream/tests/test_frame.py b/harness/udpstream/tests/test_frame.py
index 1406de0..837e70b 100644
--- a/harness/udpstream/tests/test_frame.py
+++ b/harness/udpstream/tests/test_frame.py
@@ -79,6 +79,34 @@ def test_seal_header_after_the_payload_is_known():
                                  payload=payload)
 
 
+def test_seal_header_rejects_over_length_input():
+    """A buffer longer than a header must not be silently truncated."""
+    h = pack_header(KIND_STRIP, 3, 4, 8, 128)
+    with pytest.raises(ValueError):
+        seal_header(h + b"\x01\x02\x03\x04")      # header + payload, not header
+    with pytest.raises(ValueError):
+        seal_header(h + b"\x00" * HEADER_SIZE)
+    with pytest.raises(ValueError):
+        seal_header(h[:HEADER_SIZE - 1])
+    # Exactly a header is still accepted, and matches the one-shot call.
+    assert seal_header(h, b"\x01\x02\x03\x04") == pack_header(
+        KIND_STRIP, 3, 4, 8, 128, payload=b"\x01\x02\x03\x04")
+
+
+def test_pack_header_rejects_payload_length_mismatch():
+    """A CRC sealed over N bytes must not be sent under a different payload_len."""
+    payload = b"\xAA" * 10
+    pack_header(KIND_STRIP, 1, 10, 8, 128, payload=payload)     # exact: fine
+    with pytest.raises(ValueError):
+        pack_header(KIND_STRIP, 1, 9, 8, 128, payload=payload)
+    with pytest.raises(ValueError):
+        pack_header(KIND_STRIP, 1, 11, 8, 128, payload=payload)
+    with pytest.raises(ValueError):
+        pack_header(KIND_STRIP, 1, 10, 8, 128, payload=b"")     # explicit empty
+    # Omitting the payload is the header-only path: no length to disagree with.
+    assert unpack_header(pack_header(KIND_STRIP, 1, 10, 8, 128))["crc_ok"] is True
+
+
 # --- the byte layout the C guest will mirror -----------------------------
 
 def test_byte_offsets_and_endianness():