diff --git a/docs/re/data/isl-stage02.txt b/docs/re/data/isl-stage02.txt
index f31566c..38a362a 100644
--- a/docs/re/data/isl-stage02.txt
+++ b/docs/re/data/isl-stage02.txt
@@ -93,6 +93,7 @@ Generated by `tools/re-capture/isl_report.py calls`.
   47  squadron_attack           137
   15  set_group_speed           127
   18  dist_lt                    92
+  12  activate_unit              86
   5   kill_coroutine             83
   70  unit_alive                 71
   56  unit_relation              52
diff --git a/docs/re/structures/isl-builtins.md b/docs/re/structures/isl-builtins.md
index a3073e4..0e920f8 100644
--- a/docs/re/structures/isl-builtins.md
+++ b/docs/re/structures/isl-builtins.md
@@ -140,6 +140,47 @@ and spread checks but its maximum *exceeds* the table — flag indices run 0..31
 against symbol tables as small as 40 — so it is excluded, and the disassembler
 now declines to resolve it rather than printing an invented name.
 
+### ✅ Built-in 12 is `activate_unit` — 1197 sites, all 28 stages
+
+The highest-traffic unnamed built-in. `sub_822659F0`, read directly:
+
+* indexes `[phase+324]`'s record array by the slot-4 symbol;
+* **returns 0 immediately when the live object `[record+4]` is NULL** — so it
+  registers an object that already exists; it does **not** spawn one;
+* sets **`[record+16] = 2`**, the documented *active* state that every unit
+  predicate tests;
+* stores `sub_82301118`'s packed result into `[record+20]` (low 16 bits) and
+  `[record+24]` (high 16) — member counts;
+* posts interpreter opcode **513** (`0xAB0201BA`) either way; the slot-8 mode
+  (1 in 999 sites, 0 in 198) only decides whether `cmd+20` is also set to 1.
+
+#### ✅ The ordering test — 517 / 517
+
+If this activates a unit for the script, no predicate should ever test a unit
+before it. Over all 28 stages, for every (stage, unit) pair having both an
+`activate_unit` call and a predicate (`unit_state`, `hp_pct_test`, `unit_alive`,
+`unit_hp_pct`, `dist_lt`, `unit_relation`) on that same unit:
+
+| | count |
+|---|---|
+| `activate_unit` comes first | **517** |
+| a predicate comes first | **0** |
+
+🟡 That is *file* order, not proven execution order — coroutines mean the two can
+in principle interleave. But 517 with zero exceptions across 28 stages is not a
+coincidence.
+
+344 units are tested without ever being activated (they are live from mission
+start) and 203 are activated without being tested.
+
+#### ❌ It is NOT the survival-percentage baseline
+
+The tempting closure: `squad_survival_pct` (id 24) reports current ÷ initial, and
+`activate_unit` snapshots counts, so the snapshot must be the baseline. **It is
+not.** Built-in 24 reads `[record+16]` for the state and then calls
+`sub_823011B0` and `sub_82301118` on the **live object** — it never touches
+`+20`/`+24`. ❔ What reads those two fields is unidentified.
+
 ### ✅ Built-in 15 is `set_group_speed` — the group's commanded speed
 
 `15` is the biggest unnamed built-in: **1360 sites across 27 of 28 stages**. Its
diff --git a/tools/re-capture/isl.py b/tools/re-capture/isl.py
index 4a177f0..eec1518 100755
--- a/tools/re-capture/isl.py
+++ b/tools/re-capture/isl.py
@@ -79,7 +79,8 @@ KIND = {0: 'global', 1: 'imm', 2: 'special', 3: 'local'}
 BUILTIN = {
     1: 'start_coroutine', 2: 'deploy_squadron', 3: 'move_order', 4: 'wait_s',
     5: 'kill_coroutine', 6: 'END_PHASE', 8: 'set_flag', 9: 'read_freg',
-    10: 'random', 11: 'end_coroutine', 13: 'play_se', 14: 'play_bgm',
+    10: 'random', 11: 'end_coroutine', 12: 'activate_unit',
+    13: 'play_se', 14: 'play_bgm',
     15: 'set_group_speed',
     17: 'wait_frames', 18: 'dist_lt', 20: 'hp_pct_test', 24: 'squad_survival_pct',
     26: 'damage_unit', 30: 'objective_marker', 31: 'objective_marker_at_route',