diff --git a/docs/PROTOCOL.md b/docs/PROTOCOL.md
index 8a42a9e..6f10539 100644
--- a/docs/PROTOCOL.md
+++ b/docs/PROTOCOL.md
@@ -42,8 +42,8 @@ not called even though `SdsClient` inherits it.
 Sent before the first observation.
 
 ```json
-{"type": "board", "width": 16, "height": 17,
- "hexes": [{"x": 3, "y": 4, "level": 1, "terrain": {"woods": 1}}]}
+{"type": "board", "width": 16, "height": 17, "kinds": 24,
+ "hexes": [{"x": 3, "y": 4, "level": 1, "terrain": {"woods": 1}, "kind": 2}]}
 ```
 
 Sparse: a hex at level 0 with no terrain is omitted. Coordinates are MegaMek's
@@ -52,11 +52,17 @@ zero-based `(x, y)`, not the one-based numbers printed on a map sheet.
 Terrain that changes mid-fight — fire, smoke, a collapsed building — is **not**
 tracked. A bot that cares must currently do without.
 
-`level` and `terrain` are everything movement needs from a hex: `crates/sds-core/src/pathfind.rs`
-derives MegaMek's per-hex movement cost from the terrain names and their levels,
-and its floor from `level` less the `water` depth. What movement cannot get from
-the board is a property of the *unit*, which is why `movementMode` and
-`maxElevationChange` are on the observation.
+`kind` is which of the board's distinct hexes this is: hexes with the same
+terrain share one, `kinds` is how many there are, and **kind 0 is the clear hex**
+— which is what every hex the sparse message leaves out is. It exists so a
+unit's `terrain` answers can be one entry per kind rather than one per hex; a
+thousand-hex board is a few dozen kinds. A hex's *level* is not part of its
+kind, because neither answer reads it.
+
+`level` and `terrain` are what a hex costs geometrically: `crates/sds-core/src/pathfind.rs`
+takes its floor from `level` less the `water` depth. What terrain *costs*, and
+whether a mover may enter it at all, is a property of the mover and is on the
+unit as `terrain` — see below.
 
 ### `observation` — host to bot, before each decision
 
@@ -69,6 +75,7 @@ the board is a property of the *unit*, which is why `movementMode` and
             "weight": 55.0, "bv": 1272, "bvOriginal": 1387,
             "walkMp": 5, "runMp": 8, "jumpMp": 5,
             "movementMode": "BIPED", "maxElevationChange": 2,
+            "terrain": {"mp": [0, 1, 2], "barred": [false, false, true]},
             "armor": 128, "armorMax": 144, "internal": 91, "internalMax": 91,
             "heat": 4, "heatCapacity": 10, "heatBuildup": 2,
             "explodableAmmo": true,
@@ -156,6 +163,24 @@ them throws more than one shot: `Single`/`Ultra` for an Ultra autocannon,
 alternatives `modes` carries — one declaration, more projectiles, rather than a
 different projectile — and `currentMode` says which the mount is set to now.
 
+`terrain` is what this board's hexes cost this mover and which of them are shut
+to it, one entry per board `kind`, in kind order. `mp` is
+`Hex.movementCost(entity)` — MegaMek's own per-mover branch, so a tracked tank
+pays two for the swamp a Mek pays one for and a hovercraft pays nothing for
+either. `barred` is `Entity.isLocationProhibited`, which is the half a cost
+model cannot express: heavy woods to a tracked tank, any woods to a hovercraft,
+ultra-heavy woods to a Mek, water at any depth to a battle armor squad. Every
+one of those hexes costs less than the mover's walk allowance, so a bot that
+reads only `mp` proposes the hex and the server answers with a refused move.
+
+The answers are per whole hex and not per terrain, because they do not
+decompose: water bars a squad at any depth, and water with ice on it does not.
+A unit with no `terrain` — an observation recorded before the field existed — is
+read as the ported biped Mek column, which is what every bot had before.
+Movement is the same rule the weapons follow: a mover is its properties, never
+its name, and a movement mode nobody has thought about comes out right because
+the question was put to it.
+
 `locations` is armour and structure where damage actually lands. The totals
 above it cannot answer a fire decision — a shot hits one location, so "does this
 allocation open the centre torso" needs the centre torso. `rearArmor` and