diff --git a/docs/web-transport.md b/docs/web-transport.md
index 922ac8fe..ce77eba4 100644
--- a/docs/web-transport.md
+++ b/docs/web-transport.md
@@ -10,7 +10,8 @@ The SDK can run ACP over two remote connectivity profiles in addition to stdio:
   session), and `DELETE` to terminate. `initialize` returns `200 OK` with a JSON
   body; all other POSTs return `202 Accepted`. **Requires HTTP/2.**
 - **WebSocket** — a `GET` upgrade on the same endpoint carrying full-duplex
-  JSON-RPC text frames.
+  JSON-RPC text frames, one message per frame. Binary frames are ignored, as
+  the transport RFD specifies; the server logs a warning on the first one.
 
 Both reuse the existing JSON-RPC message format and ACP lifecycle
 (`initialize` → session methods → close).
diff --git a/src/acp/ws/server.py b/src/acp/ws/server.py
index f6179eab..cb63ab96 100644
--- a/src/acp/ws/server.py
+++ b/src/acp/ws/server.py
@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import json
+import logging
 import uuid
 from typing import TYPE_CHECKING, Any
 
@@ -22,6 +23,7 @@ class _WebSocketTransport:
 
     def __init__(self, websocket: WebSocket) -> None:
         self._ws = websocket
+        self._warned_binary = False
 
     async def send(self, message: dict[str, Any]) -> None:
         if self._ws.client_state == WebSocketState.DISCONNECTED:
@@ -34,6 +36,9 @@ async def receive(self) -> dict[str, Any] | None:
             if event["type"] == "websocket.disconnect":
                 return None
             if event.get("text") is None:
+                if event.get("bytes") is not None and not self._warned_binary:
+                    self._warned_binary = True
+                    logging.warning("Ignoring binary WebSocket frame; ACP messages must be sent as text frames")
                 continue
             try:
                 message = json.loads(event["text"])
diff --git a/tests/http/test_websocket.py b/tests/http/test_websocket.py
index bb88c071..eca6cbd5 100644
--- a/tests/http/test_websocket.py
+++ b/tests/http/test_websocket.py
@@ -4,6 +4,7 @@
 
 import asyncio
 import json
+import logging
 from typing import Any
 
 import pytest
@@ -202,6 +203,31 @@ async def test_asgi_websocket_ignores_non_message_frames() -> None:
         await asyncio.wait_for(handler, timeout=1)
 
 
+@pytest.mark.asyncio
+async def test_asgi_websocket_warns_once_on_binary_frames(caplog: pytest.LogCaptureFixture) -> None:
+    socket = _FakeAsgiSocket()
+    socket.client_connect()
+    for _ in range(2):
+        socket._incoming.put_nowait({"type": "websocket.receive", "bytes": b'{"jsonrpc":"2.0"}\n'})
+    handler = asyncio.create_task(
+        handle_websocket(lambda conn: _Agent(), WebSocket({"type": "websocket"}, socket.receive, socket.send))
+    )
+    try:
+        with caplog.at_level(logging.WARNING):
+            socket.client_send_text({
+                "jsonrpc": "2.0",
+                "id": 0,
+                "method": "initialize",
+                "params": {"protocolVersion": 1},
+            })
+            await socket.wait_for_send(lambda m: m.get("id") == 0)
+        warnings = [r for r in caplog.records if "binary WebSocket frame" in r.getMessage()]
+        assert len(warnings) == 1
+    finally:
+        socket.client_disconnect()
+        await asyncio.wait_for(handler, timeout=1)
+
+
 @pytest.mark.asyncio
 async def test_asgi_websocket_full_flow() -> None:
     socket = _FakeAsgiSocket()