diff --git a/docs/re/data/isl-stage02.txt b/docs/re/data/isl-stage02.txt
index e05498dc..f31566c2 100644
--- a/docs/re/data/isl-stage02.txt
+++ b/docs/re/data/isl-stage02.txt
@@ -91,6 +91,7 @@ Generated by `tools/re-capture/isl_report.py calls`.
   30  objective_marker          179
   20  hp_pct_test               167
   47  squadron_attack           137
+  15  set_group_speed           127
   18  dist_lt                    92
   5   kill_coroutine             83
   70  unit_alive                 71
diff --git a/docs/re/structures/isl-builtins.md b/docs/re/structures/isl-builtins.md
index fb4c0639..a3073e49 100644
--- a/docs/re/structures/isl-builtins.md
+++ b/docs/re/structures/isl-builtins.md
@@ -140,7 +140,7 @@ 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 15 takes a craft-class scalar — but "speed" does not survive intact
+### ✅ 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
 operand shape is invariant — `(unit, double)` — and joining every call site's
@@ -173,8 +173,51 @@ The asteroid is the control that makes this sharp: a genuinely immobile object
 gets **0 every time**, so "immobile things get a meaningless value" does not
 explain the turrets either.
 
-So: ✅ a craft-class-dependent scalar in speed's range and with speed's ordering;
-❌ not `set_speed` as a plain reading. **Left unnamed.**
+#### ✅ What the members contribute — and why the turrets stop being a problem
+
+`sub_82348830` is a `std::map::find`, and the object it returns is the per-member
+**unit definition**. That identification is not a guess: the same spawn loop
+builds *two* aggregates, and each lands on a semantically apt field with the apt
+reducer —
+
+| group field | reducer | member field (`unit_definition_layout.txt`) |
+|---|---|---|
+| `+192` | **min**, seeded `FLT_MAX` | `+164` = **`CruisingVelocity`** |
+| `+472` | **sum** | `+84` = **`HP`** |
+
+A wrong struct would have to make both offsets land on apt fields *and* pair each
+with the apt reducer. Minimum of a speed, sum of hit points: that is a formation's
+cruise limit and its total health.
+
+#### ✅ The quantitative test, over all 1360 sites
+
+If the script is commanding a speed, its value should respect the craft's own
+limits. Joining every call site to its craft's definition:
+
+| bound | holds | fails |
+|---|---|---|
+| value ≤ the craft's **`MaximumVelocity`** | **1355 / 1360 = 99.6 %** | 5 |
+| value ≤ the craft's `CruisingVelocity` | 1042 / 1360 = 76.6 % | **318** |
+
+The test discriminates: the cruise bound is broken 318 times, the hull maximum
+just 5. So the script sets a **commanded speed**, free to exceed the cruise
+default and bounded by what the hull can do.
+
+🟡 The five exceptions are small overshoots on three craft —
+`UN_e106_ADAN_Destroyer` 200 against a 150 maximum (×2) and
+`UN_e011_ADAN_Attacker_B_HF`/`_Wayne` 500 against 450 (×3). Designer overrides,
+or the engine clamps; not established.
+
+#### ✅ The turret anomaly dissolves
+
+`UN_e007_ADAN_Turret`'s definition carries `MaximumVelocity` **500** and
+`CruisingVelocity` 280 — the data models turrets as if mobile. So a script value
+of 400 is perfectly legal in the data model; it simply never manifests, because a
+turret does not translate. The 13 % of sites that looked like a refutation were
+an artefact of assuming turrets have no velocity fields.
+
+**Named `set_group_speed`.** Default = the slowest member's `CruisingVelocity`;
+mode 1 restores it, mode 3 sets it, mode 2 hands it a global constant.
 
 #### ✅ The write side, read directly
 
diff --git a/tools/re-capture/isl.py b/tools/re-capture/isl.py
index ea1b4ece..4a177f0d 100755
--- a/tools/re-capture/isl.py
+++ b/tools/re-capture/isl.py
@@ -80,6 +80,7 @@ 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',
+    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',
     33: 'global_counter0', 34: 'global_counter1', 36: 'screen_fade',