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diff --git a/docs/TESTING.md b/docs/TESTING.md
index 1a1d369..23e3a19 100644
--- a/docs/TESTING.md
+++ b/docs/TESTING.md
@@ -180,6 +180,18 @@ Use an 80-column terminal because several output lines are width-sensitive.
    one row with `winnings = 0`, `details.result = "abandoned"` and
    `details.reason = "server_restart"`.
 
+## Manual version handshake check
+
+1. Connect a client and server from the same release: the client connects and
+   the server log shows the `HELLO` exchange.
+2. Connect a 4.x client to a 3.x server (or the reverse): the client must print
+   `Version mismatch: client ..., server ...` and never reach the login prompt.
+3. With a deliberately skew-built server (PATCH differs) the connection is
+   accepted and, when the server's version is newer, the client prints
+   `A newer version is available`.
+4. Before the handshake, `openssl s_client -quiet -connect host:port` followed
+   by `PING` must answer `ERROR HELLO required`.
+
 ## Manual identity / credential checklist
 
 1. Connect through one address, `register` and `login`. `hosts` shows the