diff --git a/core/tests/host_guest.rs b/core/tests/host_guest.rs
new file mode 100644
index 0000000..64c8e47
--- /dev/null
+++ b/core/tests/host_guest.rs
@@ -0,0 +1,296 @@
+//! Host and guest driven by hand, message by message. These tests produce
+//! what the in-memory network of `session.rs` never does, because it lets
+//! every guest run each tick before the next one arrives: a guest that lags
+//! behind, a tick that gets lost, and joins that wait at the limits.
+
+use grovewright_core::command::Command;
+use grovewright_core::guest::{Arrival, Guest};
+use grovewright_core::host::{Host, Peer, MAX_ACTIONS_PER_TICK};
+use grovewright_core::map::Pos;
+use grovewright_core::mapgen::outcrop_origin;
+use grovewright_core::player::{PlayerId, PlayerKey};
+use grovewright_core::protocol::{Action, Refusal, ToGuest, ToHost, CHECK_INTERVAL_TICKS};
+use grovewright_core::world::{Event, World};
+
+const HOST_KEY: PlayerKey = [100; 16];
+
+/// A host whose own player is in a new world.
+fn started_host() -> Host {
+    let mut host = Host::new(World::new(5));
+    host.join_local(HOST_KEY, "Host");
+    host.step();
+    host
+}
+
+/// Connects a guest with a key made of `key` as `peer` and hands its
+/// greeting to the host.
+fn greet(host: &mut Host, peer: Peer, key: u8) -> Guest {
+    host.connect(peer);
+    let mut guest = Guest::new([key; 16], "Guest");
+    for bytes in guest.take_outbox() {
+        host.receive(peer, &bytes);
+    }
+    guest
+}
+
+/// Takes the host's whole outbox and keeps what is for `peer`.
+fn messages_for(host: &mut Host, peer: Peer) -> Vec<Vec<u8>> {
+    host.take_outbox()
+        .into_iter()
+        .filter(|(to, _)| *to == peer)
+        .map(|(_, bytes)| bytes)
+        .collect()
+}
+
+/// The host's answer to a greeting from `peer`: `None` for its world, or
+/// the reason it turned the guest away.
+fn answer_to(host: &mut Host, peer: Peer) -> Option<Refusal> {
+    let messages = messages_for(host, peer);
+    match ToGuest::decode(&messages[0]) {
+        Ok(ToGuest::World { .. }) => None,
+        Ok(ToGuest::Refused(refusal)) => Some(refusal),
+        other => panic!("unexpected answer {other:?}"),
+    }
+}
+
+/// A host and the guest Ada at peer 2, in the same state.
+fn host_with_ada() -> (Host, Guest) {
+    let mut host = started_host();
+    let mut ada = greet(&mut host, 2, 1);
+    for bytes in messages_for(&mut host, 2) {
+        ada.receive(&bytes).unwrap();
+    }
+    host.step();
+    for bytes in messages_for(&mut host, 2) {
+        ada.receive(&bytes).unwrap();
+    }
+    while ada.step().is_some() {}
+    assert!(ada.local_player().is_some());
+    assert_eq!(
+        ada.world().map(World::state_hash),
+        Some(host.world().state_hash())
+    );
+    (host, ada)
+}
+
+/// A world that knows players with keys made of 1 to `count`, all offline.
+fn world_with_offline_players(count: u8) -> World {
+    let mut world = World::new(5);
+    for key in 1..=count {
+        world.step(&[Command::Join {
+            key: [key; 16],
+            name: "Visitor".to_owned(),
+        }]);
+        world.step(&[Command::Leave {
+            player: PlayerId(key - 1),
+        }]);
+    }
+    world
+}
+
+#[test]
+fn the_hosts_own_queued_join_counts_for_the_online_limit() {
+    let mut host = Host::new(World::new(5));
+    host.join_local(HOST_KEY, "Host");
+    // No tick has run: the host's join is still queued.
+    let mut answers = Vec::new();
+    for key in 1..=4 {
+        let peer = Peer::from(key) + 1;
+        greet(&mut host, peer, key);
+        answers.push(answer_to(&mut host, peer));
+    }
+    assert_eq!(answers, [None, None, None, Some(Refusal::WorldFull)]);
+}
+
+#[test]
+fn queued_new_keys_count_for_the_limit_of_known_players_but_known_ones_do_not() {
+    let mut host = Host::new(world_with_offline_players(15));
+    greet(&mut host, 2, 3);
+    assert_eq!(answer_to(&mut host, 2), None, "a returning player");
+    greet(&mut host, 3, 200);
+    assert_eq!(
+        answer_to(&mut host, 3),
+        None,
+        "a new player fits as the 16th"
+    );
+    greet(&mut host, 4, 201);
+    assert_eq!(
+        answer_to(&mut host, 4),
+        Some(Refusal::TooManyPlayers),
+        "a second new player does not"
+    );
+
+    let events = host.step();
+    assert!(!events
+        .iter()
+        .any(|event| matches!(event, Event::JoinRefused { .. })));
+    assert_eq!(host.world().players().len(), 16);
+}
+
+#[test]
+fn a_queued_join_of_a_player_who_is_online_already_counts_once() {
+    let mut world = World::new(5);
+    let joins = [HOST_KEY, [1; 16], [2; 16]].map(|key| Command::Join {
+        key,
+        name: "Online".to_owned(),
+    });
+    world.step(&joins);
+    let mut host = Host::new(world);
+    // The host's player is online already; its queued join changes nothing.
+    host.join_local(HOST_KEY, "Host");
+    greet(&mut host, 2, 3);
+    assert_eq!(answer_to(&mut host, 2), None, "the fourth player fits");
+}
+
+#[test]
+fn the_count_of_a_guests_actions_starts_again_in_every_tick() {
+    let (mut host, mut ada) = host_with_ada();
+    for tick in 0..3 {
+        for x in 0..10 {
+            ada.act(Action::Dig {
+                pos: Pos::new(x, 1),
+            });
+        }
+        for bytes in ada.take_outbox() {
+            host.receive(2, &bytes);
+        }
+        host.step();
+        let counts: Vec<usize> = messages_for(&mut host, 2)
+            .iter()
+            .filter_map(|bytes| match ToGuest::decode(bytes) {
+                Ok(ToGuest::Tick { commands, .. }) => Some(commands.len()),
+                _ => None,
+            })
+            .collect();
+        assert_eq!(counts, [MAX_ACTIONS_PER_TICK as usize], "tick {tick}");
+    }
+}
+
+#[test]
+fn a_tick_carries_the_number_of_the_tick_before_it_runs() {
+    let (mut host, mut ada) = host_with_ada();
+    let before = host.world().tick();
+    host.step();
+    let messages = messages_for(&mut host, 2);
+    let Ok(ToGuest::Tick { tick, .. }) = ToGuest::decode(&messages[0]) else {
+        panic!("not a tick");
+    };
+    assert_eq!(tick, before);
+    for bytes in messages {
+        ada.receive(&bytes).unwrap();
+    }
+    ada.step().unwrap();
+    assert_eq!(ada.world().map(World::tick), Some(before + 1));
+}
+
+#[test]
+fn a_lagging_guest_that_diverges_runs_nothing_until_the_world_comes_and_drops_older_ticks() {
+    let (mut host, mut ada) = host_with_ada();
+
+    // The host's player hits the rock, but Ada's copy of that tick loses the
+    // command, as if her machine had run it differently.
+    host.act_local(Action::Dig {
+        pos: outcrop_origin(host.world().map()),
+    });
+    host.step();
+    for bytes in messages_for(&mut host, 2) {
+        let bytes = match ToGuest::decode(&bytes) {
+            Ok(ToGuest::Tick { tick, check, .. }) => ToGuest::Tick {
+                tick,
+                commands: vec![],
+                check,
+            }
+            .encode(),
+            _ => bytes,
+        };
+        ada.receive(&bytes).unwrap();
+    }
+    while ada.step().is_some() {}
+    assert!(!ada.has_diverged());
+
+    // Thirty more ticks arrive before Ada runs any of them.
+    for _ in 0..30 {
+        host.step();
+        for bytes in messages_for(&mut host, 2) {
+            ada.receive(&bytes).unwrap();
+        }
+    }
+    assert!(ada.pending_ticks() >= 30);
+
+    // She catches up and trips over the next checksum.
+    while ada.step().is_some() && !ada.has_diverged() {}
+    assert!(ada.has_diverged());
+    let waiting = ada.pending_ticks();
+    assert!(waiting > 0, "newer ticks are still waiting");
+    assert!(ada.step().is_none(), "no tick runs after a divergence");
+    assert_eq!(ada.pending_ticks(), waiting, "and none is used up");
+
+    // Her report names her tick after the step that showed the divergence.
+    let reports = ada.take_outbox();
+    assert_eq!(reports.len(), 1);
+    let Ok(ToHost::Diverged { tick, hash }) = ToHost::decode(&reports[0]) else {
+        panic!("not a divergence report");
+    };
+    assert_eq!(Some(tick), ada.world().map(World::tick));
+    assert_eq!(tick % CHECK_INTERVAL_TICKS, 0);
+    host.receive(2, &reports[0]);
+    let divergences = host.take_divergences();
+    assert_eq!(divergences.len(), 1);
+    assert_eq!((divergences[0].tick, divergences[0].hash), (tick, hash));
+
+    // The host has gone on meanwhile. Its world comes first, then later ticks.
+    let world_tick = host.world().tick();
+    let answer = messages_for(&mut host, 2);
+    assert_eq!(answer.len(), 1);
+    host.step();
+    host.step();
+    let later = messages_for(&mut host, 2);
+    assert_eq!(later.len(), 2);
+
+    assert_eq!(
+        ada.receive(&answer[0]),
+        Ok(Arrival::World { replaced: true })
+    );
+    assert_eq!(ada.pending_ticks(), 0, "the older ticks are dropped");
+    assert_eq!(ada.world().map(World::tick), Some(world_tick));
+    assert!(!ada.has_diverged());
+    for bytes in later {
+        ada.receive(&bytes).unwrap();
+    }
+    while ada.step().is_some() {}
+    assert_eq!(
+        ada.world().map(World::state_hash),
+        Some(host.world().state_hash())
+    );
+    assert!(ada.take_diverged_recording().is_some());
+}
+
+#[test]
+fn a_missing_tick_makes_the_guest_ask_for_the_world() {
+    let (mut host, mut ada) = host_with_ada();
+    host.step();
+    // This tick never arrives.
+    messages_for(&mut host, 2);
+    host.step();
+    for bytes in messages_for(&mut host, 2) {
+        ada.receive(&bytes).unwrap();
+    }
+    assert!(ada.step().is_none());
+    assert!(ada.has_diverged());
+    let reports = ada.take_outbox();
+    assert_eq!(reports.len(), 1);
+
+    host.receive(2, &reports[0]);
+    for bytes in messages_for(&mut host, 2) {
+        assert_eq!(ada.receive(&bytes), Ok(Arrival::World { replaced: true }));
+    }
+    host.step();
+    for bytes in messages_for(&mut host, 2) {
+        ada.receive(&bytes).unwrap();
+    }
+    while ada.step().is_some() {}
+    assert_eq!(
+        ada.world().map(World::state_hash),
+        Some(host.world().state_hash())
+    );
+}