Say goodbye before dropping the socket, and specify that

SPEC.md

@@ -485,6 +485,30 @@ A client should nonetheless be prepared to receive a **text** frame where it
 expected binary, and treat it as a fatal server-side error rather than
 attempting to unpack it.
 
+### 7.5 The client stopping
+
+Everything above is the server closing. A client may also stop first - it
+has collected the episodes it was asked for, or its operator interrupted it -
+and that is not an error. The server keeps no state that outlives the
+connection, so a client that disappears costs nothing beyond the feedback it
+had not yet sent.
+
+A client that is finished **SHOULD** send a WebSocket close frame with status
+**1000 (normal closure)** before dropping the socket, as RFC 6455 section
+5.5.1 asks. Nothing breaks without one - the server sees the connection end
+either way - but the difference is visible in its logs, as
+`ConnectionClosedError: no close frame received or sent` rather than a clean
+`ConnectionClosedOK`, and an operator reading those logs should not have to
+wonder whether a client crashed.
+
+`examples/conformance_server.py` reports a missing close frame as a note, not
+a violation, which is the level this rule deserves.
+
+> **Historical note.** Until 2026-09-11 the C++ reference client did not send
+> one. It went unnoticed because in every test until then the *server* ran
+> out of steps first and closed the connection itself; E7, where the client
+> finishes first, made it visible on all 45 runs.
+
 ---
 
 ## 8. Conformance checklist

examples/README.md

@@ -41,7 +41,7 @@ see whether the values it prints are the ones the server sent.
 |---|---|---|
 | Language | Python | C++17 |
 | Dependencies | `msgpack`, `websockets` | **none** |
-| Lines | 275 | 814 |
+| Lines | 275 | 843 |
 | Notably absent | numpy, and every `plugrl_*` package | libstdc++ and libc are the only links |
 
 The C++ one is the interesting case. It was written on a machine with no

examples/conformance_server.py

@@ -31,6 +31,7 @@
 
 import numpy as np
 import websockets.asyncio.server as ws_server
+import websockets.exceptions as ws_exceptions
 
 # Use the real codec, not a re-implementation: the point is to test a client
 # against what plugrl-server actually does.
@@ -245,6 +246,21 @@ async def handle(self, websocket) -> None:
                     self.check_feedback(payload)
                     awaiting = str(MessageType.INFER)
                     self.exchanges += 1
+        except ws_exceptions.ConnectionClosedOK:
+            # The client said goodbye and left. SPEC.md section 7.5 allows
+            # that at any point, so it is not a failure - a client that has
+            # collected the episodes it wanted is finished, and the server
+            # does not get to call that a protocol violation.
+            report.ok("7.5 a client that stops sends a close frame")
+        except ws_exceptions.ConnectionClosedError:
+            # It vanished without a close frame. Section 7.5 asks for one,
+            # and nothing breaks without it, so this is a note.
+            report.advise(
+                False,
+                "7.5 a client that stops sends a close frame",
+                "the connection dropped with no close frame; RFC 6455 asks "
+                "for one and SPEC.md section 7.5 repeats the ask",
+            )
         except Exception as exc:  # noqa: BLE001 - the report is the output
             report.fail("connection", f"{type(exc).__name__}: {exc}")
         finally:

examples/plugrl_client.cpp

@@ -445,7 +445,36 @@ class WebSocket {
     handshake(host, port);
   }
 
-  ~WebSocket() { if (fd_ >= 0) ::close(fd_); }
+  // RFC 6455 section 5.5.1: say goodbye before dropping the socket. Without
+  // this the server sees the connection vanish and reports
+  // `ConnectionClosedError: no close frame received or sent` - which is what
+  // it did on all 45 runs of E7, where the client finishes first. It only
+  // went unnoticed before because in every earlier test the *server* ran out
+  // of steps first and closed the connection itself.
+  void close_cleanly() {
+    if (fd_ < 0) return;
+    std::string frame;
+    frame.push_back(static_cast<char>(0x88));  // FIN + close opcode
+    frame.push_back(static_cast<char>(0x80 | 2));  // masked, 2-byte payload
+    uint8_t key[4];
+    for (int i = 0; i < 4; ++i) key[i] = static_cast<uint8_t>(rng_() & 0xff);
+    frame.append(reinterpret_cast<char*>(key), 4);
+    uint8_t status[2] = {0x03, 0xe8};  // 1000, normal closure
+    for (int i = 0; i < 2; ++i)
+      frame.push_back(static_cast<char>(status[i] ^ key[i % 4]));
+    try {
+      write_all(frame.data(), frame.size());
+    } catch (const std::exception&) {
+      // Already gone. Nothing useful to do on the way out.
+    }
+    ::close(fd_);
+    fd_ = -1;
+  }
+
+  ~WebSocket() {
+    close_cleanly();
+    if (fd_ >= 0) ::close(fd_);
+  }
 
   void send_binary(const std::string& payload) {
     std::string frame;