diff --git a/CHANGES/11761.bugfix.rst b/CHANGES/11761.bugfix.rst
new file mode 100644
index 00000000000..d4661c6d4a1
--- /dev/null
+++ b/CHANGES/11761.bugfix.rst
@@ -0,0 +1,4 @@
+Fixed ``AssertionError`` when the transport is ``None`` during WebSocket
+preparation or file response sending (e.g. when a client disconnects
+immediately after connecting). A ``ConnectionResetError`` is now raised
+instead -- by :user:`agners`.
diff --git a/aiohttp/web_fileresponse.py b/aiohttp/web_fileresponse.py
index eeaa2010f98..f339bec9662 100644
--- a/aiohttp/web_fileresponse.py
+++ b/aiohttp/web_fileresponse.py
@@ -128,7 +128,8 @@ async def _sendfile(
 
         loop = request._loop
         transport = request.transport
-        assert transport is not None
+        if transport is None:
+            raise ConnectionResetError("Connection lost")
 
         try:
             await loop.sendfile(transport, fobj, offset, count)
diff --git a/aiohttp/web_ws.py b/aiohttp/web_ws.py
index ca129bb0f30..2aeeb6dec1f 100644
--- a/aiohttp/web_ws.py
+++ b/aiohttp/web_ws.py
@@ -361,7 +361,8 @@ def _pre_start(self, request: BaseRequest) -> tuple[str | None, WebSocketWriter]
         self.force_close()
         self._compress = compress
         transport = request._protocol.transport
-        assert transport is not None
+        if transport is None:
+            raise ConnectionResetError("Connection lost")
         writer = WebSocketWriter(
             request._protocol,
             transport,
diff --git a/tests/test_web_sendfile_functional.py b/tests/test_web_sendfile_functional.py
index 87be2db182b..1d695d332c4 100644
--- a/tests/test_web_sendfile_functional.py
+++ b/tests/test_web_sendfile_functional.py
@@ -1,5 +1,6 @@
 import asyncio
 import bz2
+import contextlib
 import gzip
 import pathlib
 import socket
@@ -15,6 +16,7 @@
 from aiohttp.compression_utils import ZLibBackend
 from aiohttp.pytest_plugin import AiohttpClient, AiohttpServer
 from aiohttp.typedefs import PathLike
+from aiohttp.web_fileresponse import NOSENDFILE
 
 try:
     import brotlicffi as brotli
@@ -1156,3 +1158,64 @@ async def handler(request: web.Request) -> web.FileResponse:
 
     resp.release()
     await client.close()
+
+
+@pytest.mark.skipif(NOSENDFILE, reason="OS sendfile not available")
+async def test_sendfile_after_client_disconnect(
+    aiohttp_client: AiohttpClient, tmp_path: pathlib.Path
+) -> None:
+    """Test ConnectionResetError when client disconnects before sendfile.
+
+    Reproduces the race condition where:
+    - Client sends a GET request for a file
+    - Handler does async work (e.g. auth check) before returning a FileResponse
+    - Client disconnects while the handler is busy
+    - Server then calls sendfile() → ConnectionResetError (not AssertionError)
+
+    _send_headers_immediately is set to False so that super().prepare()
+    only buffers the headers without writing to the transport. Otherwise
+    _write() raises ClientConnectionResetError first and _sendfile()'s own
+    transport check is never reached.
+    """
+    filepath = tmp_path / "test.txt"
+    filepath.write_bytes(b"x" * 1024)
+
+    handler_started = asyncio.Event()
+    prepare_done = asyncio.Event()
+    captured_protocol = None
+
+    async def handler(request: web.Request) -> web.Response:
+        nonlocal captured_protocol
+        resp = web.FileResponse(filepath)
+        resp._send_headers_immediately = False
+        captured_protocol = request._protocol
+        handler_started.set()
+        # Simulate async work (e.g., auth check) during which client disconnects.
+        await asyncio.sleep(0)
+        with pytest.raises(ConnectionResetError, match="Connection lost"):
+            await resp.prepare(request)
+        prepare_done.set()
+        return web.Response(status=503)
+
+    app = web.Application()
+    app.router.add_get("/", handler)
+    client = await aiohttp_client(app)
+
+    request_task = asyncio.create_task(client.get("/"))
+
+    # Wait until the handler is running but has not yet returned the response.
+    await handler_started.wait()
+    assert captured_protocol is not None
+
+    # Simulate the client disconnecting by setting transport to None directly.
+    # We cannot use force_close() because closing the TCP transport triggers
+    # connection_lost() which cancels the handler task before it can call
+    # prepare() and hit the ConnectionResetError in _sendfile().
+    captured_protocol.transport = None
+
+    # Wait for the handler to resume, call prepare(), and hit ConnectionResetError.
+    await asyncio.wait_for(prepare_done.wait(), timeout=1)
+
+    request_task.cancel()
+    with contextlib.suppress(asyncio.CancelledError):
+        await request_task
diff --git a/tests/test_web_websocket_functional.py b/tests/test_web_websocket_functional.py
index 1e202649c6a..7257c47ba73 100644
--- a/tests/test_web_websocket_functional.py
+++ b/tests/test_web_websocket_functional.py
@@ -11,7 +11,7 @@
 import pytest
 
 import aiohttp
-from aiohttp import WSServerHandshakeError, web
+from aiohttp import WSServerHandshakeError, hdrs, web
 from aiohttp.http import WSCloseCode, WSMsgType
 from aiohttp.pytest_plugin import AiohttpClient, AiohttpServer
 
@@ -1661,3 +1661,58 @@ async def websocket_handler(
     assert msg.type is aiohttp.WSMsgType.TEXT
     assert msg.data == "success"
     await ws.close()
+
+
+async def test_prepare_after_client_disconnect(aiohttp_client: AiohttpClient) -> None:
+    """Test ConnectionResetError when client disconnects before ws.prepare().
+
+    Reproduces the race condition where:
+    - Client connects and sends a WebSocket upgrade request
+    - Handler starts async work (e.g. authentication) before calling ws.prepare()
+    - Client disconnects while the handler is busy
+    - Handler then calls ws.prepare() → ConnectionResetError (not AssertionError)
+    """
+    handler_started = asyncio.Event()
+    captured_protocol = None
+
+    async def handler(request: web.Request) -> web.Response:
+        nonlocal captured_protocol
+        ws = web.WebSocketResponse()
+        captured_protocol = request._protocol
+        handler_started.set()
+        # Simulate async work (e.g., auth check) during which client disconnects.
+        await asyncio.sleep(0)
+        with pytest.raises(ConnectionResetError, match="Connection lost"):
+            await ws.prepare(request)
+        return web.Response(status=503)
+
+    app = web.Application()
+    app.router.add_route("GET", "/", handler)
+    client = await aiohttp_client(app)
+
+    request_task = asyncio.create_task(
+        client.session.get(
+            client.make_url("/"),
+            headers={
+                hdrs.UPGRADE: "websocket",
+                hdrs.CONNECTION: "Upgrade",
+                hdrs.SEC_WEBSOCKET_KEY: "dGhlIHNhbXBsZSBub25jZQ==",
+                hdrs.SEC_WEBSOCKET_VERSION: "13",
+            },
+        )
+    )
+
+    # Wait until the handler is running but has not yet called ws.prepare().
+    await handler_started.wait()
+    assert captured_protocol is not None
+
+    # Simulate the client disconnecting abruptly.
+    captured_protocol.force_close()
+
+    # Yield so the handler can resume and hit the ConnectionResetError.
+    await asyncio.sleep(0)
+
+    with contextlib.suppress(
+        aiohttp.ServerDisconnectedError, aiohttp.ClientConnectionResetError
+    ):
+        await request_task