diff --git a/core/src/protocol.rs b/core/src/protocol.rs
index f8b855e..ec97936 100644
--- a/core/src/protocol.rs
+++ b/core/src/protocol.rs
@@ -19,16 +19,20 @@ use crate::world::TICKS_PER_SECOND;
 /// Version of the messages. Raise it whenever a message changes.
 pub const PROTOCOL_VERSION: u32 = 1;
 /// Longest message a guest may send, in bytes. The longest one is a greeting
-/// with a name of full length, which needs 65 bytes.
+/// with a name of full length, which needs 65 bytes. Greetings of every
+/// future version must fit as well: a longer one is rejected unread instead
+/// of being recognised as another version.
 pub const MAX_GUEST_MESSAGE_BYTES: usize = 128;
 /// Ticks between two state checksums in the stream of ticks: one second.
 pub const CHECK_INTERVAL_TICKS: u64 = TICKS_PER_SECOND as u64;
 
+// Never changes, see the module documentation.
 const TO_HOST_HELLO: u8 = 1;
 const TO_HOST_ACT: u8 = 2;
 const TO_HOST_DIVERGED: u8 = 3;
 
 const TO_GUEST_WORLD: u8 = 1;
+// Never changes, see the module documentation.
 const TO_GUEST_REFUSED: u8 = 2;
 const TO_GUEST_TICK: u8 = 3;
 
@@ -75,7 +79,8 @@ impl Action {
     }
 }
 
-/// Why a host turned a guest away.
+/// Why a host turned a guest away. The codes never change, because games of
+/// other versions read them.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum Refusal {
     /// The guest's game speaks another version of these messages.
diff --git a/core/tests/golden.rs b/core/tests/golden.rs
index f87f09e..684c376 100644
--- a/core/tests/golden.rs
+++ b/core/tests/golden.rs
@@ -12,7 +12,7 @@ use grovewright_core::command::Command;
 use grovewright_core::hash::fnv1a64;
 use grovewright_core::map::{Map, Pos, Terrain, Tile};
 use grovewright_core::player::PlayerId;
-use grovewright_core::protocol::{Action, Refusal, ToGuest, ToHost};
+use grovewright_core::protocol::{Action, Refusal, ToGuest, ToHost, PROTOCOL_VERSION};
 use grovewright_core::replay::Recording;
 use grovewright_core::rules::{
     end_hash, frozen_offline_script, frozen_script, rules_id, RULES_SEED,
@@ -162,6 +162,45 @@ fn the_protocol_format_is_unchanged() {
         fnv1a64(&bytes),
         0xc263_69c0_f892_247b,
         "the messages between host and guests changed; raise \
-         PROTOCOL_VERSION in core/src/protocol.rs; then update this value"
+         PROTOCOL_VERSION in core/src/protocol.rs; then update this value. \
+         The start of a greeting and the refusal must never change, see \
+         the_greeting_start_and_the_refusal_never_change"
     );
 }
+
+/// Covers the promise that games of every version can still tell each other
+/// that they do not fit: a greeting starts with its tag and the version, and
+/// a refusal is its tag and the code of its reason. Unlike every other part
+/// of the messages, these stay the same when PROTOCOL_VERSION is raised.
+#[test]
+fn the_greeting_start_and_the_refusal_never_change() {
+    let hello = ToHost::Hello {
+        rules: 1,
+        key: [3; 16],
+        name: "Ada".to_owned(),
+    }
+    .encode();
+    let mut start = vec![1];
+    start.extend(PROTOCOL_VERSION.to_le_bytes());
+    assert_eq!(
+        hello[..start.len()],
+        start[..],
+        "a greeting starts with the tag 1 and the version; this never \
+         changes, not even with a new PROTOCOL_VERSION"
+    );
+    let refusals = [
+        (Refusal::OtherProtocol, 1),
+        (Refusal::OtherRules, 2),
+        (Refusal::WorldFull, 3),
+        (Refusal::TooManyPlayers, 4),
+        (Refusal::AlreadyPlaying, 5),
+    ];
+    for (refusal, code) in refusals {
+        assert_eq!(
+            ToGuest::Refused(refusal).encode(),
+            vec![2, code],
+            "a refusal is the tag 2 and the code of its reason; this never \
+             changes, not even with a new PROTOCOL_VERSION"
+        );
+    }
+}