diff --git a/plan/README.md b/plan/README.md
index 722da43..c26c57d 100644
--- a/plan/README.md
+++ b/plan/README.md
@@ -57,6 +57,7 @@ frontmatter keys every other epic carries, run the script, stage both.
 | [training](training.md) | Weights fitted from recorded play | blocked | features, harness |
 | [observability](observability.md) | Read back what the bot believed and chose | open | beliefs |
 | [artillery](artillery.md) | A shell fired this round lands next round | open | ev, beliefs |
+| [battle-armor](battle-armor.md) | Battle armor plays, mounted and on foot | open | unit-types, candidates |
 | [bridges](bridges.md) | A bridge is a floor above the ground | open | protocol |
 | [buildings](buildings.md) | Buildings are terrain the bot can see, enter and destroy | open | los, protocol |
 | [candidates](candidates.md) | Propose positions, not four verbs | open | los, ev, features |
diff --git a/plan/battle-armor.md b/plan/battle-armor.md
new file mode 100644
index 0000000..d59751d
--- /dev/null
+++ b/plan/battle-armor.md
@@ -0,0 +1,76 @@
+---
+id: battle-armor
+title: Battle armor plays, mounted and on foot
+status: open
+dependsOn: [unit-types, candidates]
+exitCriterion: >
+  A force with battle armor in it mounts, rides, dismounts and attacks without
+  any of the four taking an inert default, and `sds spread` shows the mount
+  decision reading something that moves.
+---
+
+# battle-armor
+
+Princess cannot field mounted battle armor at all. That makes this the first
+piece of play where "what would MegaMek's bot do" is not even a question, and
+the only authority is what the game permits.
+
+`sds ba-probe` asks it, and `scenarios/ba/observed.txt` is the answer on MegaMek
+0.51.00. `tests/test_battle_armor.py` pins it.
+
+## What the probe settled
+
+- **`hasBattleArmorHandles()` is not the gate.** Every Mek in the matrix answers
+  yes, a non-omni Atlas and a quad Goliath included, and every one of them
+  reports four or five transporters. The accessor whose name is the question is
+  not the answer to it.
+- **`Entity.canLoad` is the gate, and what it turns on is omni.** The Clan
+  OmniMek accepts all three squads; the two non-omni carriers refuse all three,
+  with and without the elevation check. Every squad in the matrix answers
+  `canDoMechanizedBA()` yes, so the squad is not what varies.
+- **The refusal is not advisory.** `Entity.load` on a non-omni throws
+  `IllegalArgumentException`. A candidate that offered that mount would take the
+  phase down rather than be ignored.
+- **`MovePath.isMoveLegal()` does not ask the carrier.** It accepts a `MOUNT`
+  step onto a carrier whose `canLoad` refuses the squad. The path gate is
+  checking movement points and terrain and nothing else.
+- **The mount step is free, and reaches one hex.** `getMpUsed` for the mount is
+  the walk's own cost in every cell, and mounting is accepted from the carrier's
+  hex and from an adjacent one. Every refusal in the matrix is the squad running
+  out of points before it arrives - the same walk without the mount step is
+  refused in exactly the same cells.
+- **`Entity.load` writes only the carrier's half of the link.** The squad's own
+  `transportId` is still -1 afterwards; in a game the server sets the other
+  half. Both halves have to be set before any predicate that reads the squad
+  means anything.
+- **A mounted squad gets no turn of its own.** With both halves set,
+  `GameTurn.isValidEntity` refuses it and `isSelectableThisTurn` agrees. This is
+  the one that decides the shape of the work: while a squad is aboard, its
+  reachable set is the carrier's and only the carrier's, so the two candidate
+  sets are never crossed. The coupling is a sequence, not a product.
+- **And the path gate says otherwise a third time.** `isMoveLegal` still accepts
+  a `FORWARDS` step for a mounted squad the turn generator will never call on.
+
+## What it needs
+
+- Legal movement for a squad on foot, in the observation. Battle armor walks one
+  or two and jumps three, so its candidate set is small and its terrain column
+  is not a Mek's.
+- The mount as an intent. It is a decision about two units taken on one unit's
+  turn, and the force is where the pair is known.
+- The dismount, which is the carrier's `UNLOAD` step or the squad's `DISCONNECT`
+  - the probe accepts both and the difference between them has not been
+  measured under a server.
+- Swarm and leg attacks: `getSwarmAttackerId`, `getSwarmTargetId` and
+  `checkDislodgeSwarmers` are the state, and the attacks themselves are weapons
+  (`SwarmAttack`, `LegAttack`, `StopSwarmAttack`) rather than physical actions.
+  Not probed yet.
+- Ordinary weapon fire from a squad, which should need nothing new, and has not
+  been checked.
+
+## Open questions for jmm
+
+- Is a non-omni carrier ever legal for mechanised battle armor under a rules
+  option this probe did not turn on, or is omni the whole of it?
+- Swarming is a decision to give up a unit's mobility for a turn or more. Is
+  that a goal, or a candidate scored like any other?