diff --git a/protocol.py b/protocol.py
index 5c9f15a..55ee78f 100644
--- a/protocol.py
+++ b/protocol.py
@@ -86,9 +86,21 @@ class ProtocolReader:
 
     def __init__(self, reader: asyncio.StreamReader):
         self._reader = reader
+        self._leftover = b""  # bytes after the delimiter salvaged from a drain
 
     async def read_message(self) -> dict | None:
         """Read and parse one message. Returns None on EOF."""
+        prefix = b""
+        if self._leftover:
+            nl = self._leftover.find(b"\n")
+            if nl != -1:
+                line = self._leftover[:nl + 1]
+                self._leftover = self._leftover[nl + 1:]
+                if len(line) > MAX_MESSAGE_BYTES:
+                    raise ValueError("Message exceeds maximum size")
+                return parse_message(line.strip())
+            prefix = self._leftover
+            self._leftover = b""
         try:
             line = await self._reader.readuntil(b"\n")
         except (asyncio.IncompleteReadError, ConnectionError):
@@ -102,11 +114,19 @@ class ProtocolReader:
                 chunk = await self._reader.read(max(remaining, 4096))
                 if not chunk:
                     return None  # EOF while draining
-                if b"\n" in chunk:
-                    break  # found delimiter, oversized message fully drained
+                nl = chunk.find(b"\n")
+                if nl != -1:
+                    # Bytes after the delimiter belong to the NEXT message —
+                    # discarding them would corrupt framing for the rest of
+                    # the connection.
+                    self._leftover = chunk[nl + 1:]
+                    break
             raise ValueError("Message exceeds maximum size")
         if not line:
             return None
+        line = prefix + line
+        if len(line) > MAX_MESSAGE_BYTES:
+            raise ValueError("Message exceeds maximum size")
         return parse_message(line.strip())