docs: a fourth rule a first implementation gets wrong

docs/protocol/index.md

@@ -40,7 +40,7 @@ msgpack maps with a `message_type` field. Arrays travel as
 `dtype` is a numpy typestr: a byte-order character, a kind character, and an
 item size. Parsing it takes about ten lines in any language.
 
-## Three rules a first implementation usually gets wrong
+## Four rules a first implementation usually gets wrong
 
 **Messages strictly alternate.** `infer`, `action`, `feedback`, `infer`, and
 so on. The server's connection handler is straight-line code with no
@@ -57,6 +57,14 @@ server routes feedback by environment index.
 **The reward is the sum over the chunk.** Not the last step's. A client that
 reports the final step's reward trains a different MDP, and nothing fails.
 
+**A reconnect starts from nothing.** Everything the server knows about an
+environment — its previous observation, the policy step state, its done
+flags — lives for exactly one connection. A client that reconnects must drop
+any `feedback` it was holding: the transition it describes can no longer be
+completed, and sending it puts a transition built from an empty observation
+into the training buffer. Nothing on either side reports an error when that
+happens, which is what makes it worth stating.
+
 ## Checking an implementation
 
 `plugrl-protocol` ships a server that grades a client against the

docs/protocol/index.zh.md

@@ -35,7 +35,7 @@ PlugRL 把一次训练拆成两个进程。**训练服务端**持有策略与学
 `dtype` 是 numpy 的 typestr：一个字节序字符、一个类型字符、一个元素字节数。
 用任何语言解析它大约十行代码。
 
-## 三条最容易实现错的规则
+## 四条最容易实现错的规则
 
 **消息严格交替。** `infer`、`action`、`feedback`、`infer`……服务端的连接处理
 是一段没有分发器的顺序代码，所以连发两个 `infer` 的客户端会让第二个被当成
@@ -49,6 +49,11 @@ PlugRL 把一次训练拆成两个进程。**训练服务端**持有策略与学
 **奖励是整个动作块上的求和**，不是最后一步的奖励。只汇报最后一步的客户端会在
 一个不同的 MDP 上训练，而且不会有任何东西报错。
 
+**重连意味着从零开始。** 服务端关于一个环境的全部记忆 —— 上一帧观测、策略的
+step state、终止标志 —— 只活在一条连接里。重连的客户端必须丢弃手上未发出的
+`feedback`：它描述的那次转移已经无法补全，发出去只会往训练缓冲里塞一条由空
+观测拼出来的转移。这件事发生时两端都不会报错，所以才值得单独写一条。
+
 ## 检验一个实现
 
 `plugrl-protocol` 附带一个服务端，它按规范逐条给客户端打分，有违规就以非零码退出：