diff --git a/src/defib/agent/client.py b/src/defib/agent/client.py
index fd5b0a9..67116ec 100644
--- a/src/defib/agent/client.py
+++ b/src/defib/agent/client.py
@@ -318,12 +318,28 @@ async def read_memory(
         await send_packet(self._transport, CMD_READ, payload)
 
         received = bytearray()
+        # Track the 16-bit seq prefix the agent stamps on every RSP_DATA
+        # (`pkt[0..1] = seq` in agent/main.c handle_read). Without this,
+        # a COBS-CRC-rejected packet was silently dropped by recv_packet
+        # and read_memory returned short data with no error — see kaeru
+        # `agent-read-silent-packet-loss-on-long-uart-streams-2026-05-15`.
+        expected_seq = 0
         while True:
             cmd, data = await recv_packet(self._transport, timeout=60.0)
             if cmd == RSP_READY:
                 continue
             elif cmd == RSP_DATA and len(data) > 2:
+                seq = data[0] | (data[1] << 8)
+                if seq != expected_seq:
+                    raise RuntimeError(
+                        f"Read packet seq gap at offset {len(received)} "
+                        f"(addr={addr + len(received):#010x}): expected "
+                        f"seq={expected_seq}, got seq={seq}; likely UART "
+                        f"corruption on a long stream. Try a smaller read "
+                        f"or a lower baud."
+                    )
                 received.extend(data[2:])
+                expected_seq = (expected_seq + 1) & 0xFFFF
                 if on_progress:
                     on_progress(len(received), size)
             elif cmd == RSP_ACK:
@@ -334,6 +350,14 @@ async def read_memory(
                         f"Read rejected by agent: status=0x{status:02x} "
                         f"({detail}); addr={addr:#010x} size={size}"
                     )
+                # Final size check — catches the case where the agent
+                # ACKs early but we never saw a seq gap (shouldn't happen
+                # with the seq-check above, but cheap insurance).
+                if len(received) != size:
+                    raise RuntimeError(
+                        f"Read returned {len(received)} bytes but agent "
+                        f"ACKed for {size}; addr={addr:#010x}"
+                    )
                 break
             else:
                 raise RuntimeError(f"Unexpected response: cmd=0x{cmd:02x}")
diff --git a/tests/test_agent_protocol.py b/tests/test_agent_protocol.py
index 66ef909..c9bdb1f 100644
--- a/tests/test_agent_protocol.py
+++ b/tests/test_agent_protocol.py
@@ -871,3 +871,79 @@ async def test_crc32_raises_on_flash_error_ack(self):
         t.enqueue_rx(make_device_packet(RSP_ACK, bytes([ACK_FLASH_ERROR])))
         with pytest.raises(RuntimeError, match="CRC32 rejected"):
             await client.crc32(0x10100000, 64)
+
+
+class TestReadMemorySeqValidation:
+    """When recv_packet silently drops a CRC-invalid RSP_DATA packet
+    mid-stream during a long read (e.g. UART corruption on a multi-MB
+    eMMC dump), read_memory must NOT return short data and pretend
+    success. It must spot the gap via the seq prefix the agent stamps
+    on every RSP_DATA and raise loudly so the caller can chunk + retry."""
+
+    @pytest.mark.asyncio
+    async def test_normal_sequential_seq_passes(self):
+        # seq=0,1,2 followed by ACK_OK — must succeed cleanly.
+        from defib.transport.mock import MockTransport
+        from defib.agent.client import FlashAgentClient
+
+        t = MockTransport(flush_clears_buffer=False)
+        t.enqueue_rx(make_device_packet(RSP_READY, b"DEFIB"))
+        client = FlashAgentClient(t)
+        assert await client.connect(timeout=1.0)
+
+        # 3 packets of 4 bytes each => 12 bytes total
+        for seq in range(3):
+            seq_bytes = struct.pack("<H", seq)
+            payload = bytes([seq * 4, seq * 4 + 1, seq * 4 + 2, seq * 4 + 3])
+            t.enqueue_rx(make_device_packet(RSP_DATA, seq_bytes + payload))
+        t.enqueue_rx(make_device_packet(RSP_ACK, bytes([ACK_OK])))
+
+        data = await client.read_memory(0x80000000, 12, fast=False)
+        assert data == bytes(range(12))
+
+    @pytest.mark.asyncio
+    async def test_seq_gap_raises(self):
+        # seq=0 then seq=2 — packet 1 was dropped by recv_packet (CRC fail).
+        # Without the seq check the client would silently return 8 bytes;
+        # with it, we raise.
+        from defib.transport.mock import MockTransport
+        from defib.agent.client import FlashAgentClient
+
+        t = MockTransport(flush_clears_buffer=False)
+        t.enqueue_rx(make_device_packet(RSP_READY, b"DEFIB"))
+        client = FlashAgentClient(t)
+        assert await client.connect(timeout=1.0)
+
+        t.enqueue_rx(make_device_packet(
+            RSP_DATA, struct.pack("<H", 0) + b"AAAA"
+        ))
+        t.enqueue_rx(make_device_packet(
+            RSP_DATA, struct.pack("<H", 2) + b"CCCC"
+        ))
+        t.enqueue_rx(make_device_packet(RSP_ACK, bytes([ACK_OK])))
+
+        with pytest.raises(RuntimeError, match="seq gap"):
+            await client.read_memory(0x80000000, 12, fast=False)
+
+    @pytest.mark.asyncio
+    async def test_size_mismatch_at_ack_raises(self):
+        # No seq gap, but agent ACKs after fewer bytes than the host asked
+        # for. Real-hardware bug surfaced when the agent's handle_read
+        # short-circuited; client now fails loudly instead of returning
+        # truncated data.
+        from defib.transport.mock import MockTransport
+        from defib.agent.client import FlashAgentClient
+
+        t = MockTransport(flush_clears_buffer=False)
+        t.enqueue_rx(make_device_packet(RSP_READY, b"DEFIB"))
+        client = FlashAgentClient(t)
+        assert await client.connect(timeout=1.0)
+
+        # Only 8 bytes delivered, but client asked for 16
+        t.enqueue_rx(make_device_packet(
+            RSP_DATA, struct.pack("<H", 0) + b"AAAABBBB"
+        ))
+        t.enqueue_rx(make_device_packet(RSP_ACK, bytes([ACK_OK])))
+
+        with pytest.raises(RuntimeError, match="Read returned 8 bytes but agent ACKed for 16"):
+            await client.read_memory(0x80000000, 16, fast=False)