diff --git a/CMakeLists.txt b/CMakeLists.txt
index ab7277e..ca607a3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -322,10 +322,12 @@ target_link_libraries(btl4 PRIVATE
     "${DXSDK}/Lib/x86/dxguid.lib"
     "${DXSDK}/Lib/x86/DxErr.lib"
     winmm dbghelp shell32 user32 gdi32 ole32 ws2_32)
-# /FORCE: the 1995 headers define free funcs/globals without inline/extern, so
-# identical symbols land in every TU (~124 LNK2005); MULTIPLE keeps the first.
-# UNRESOLVED tolerates the dead offline-tool factory in mech3.cpp.  See docs.
-target_link_options(btl4 PRIVATE /FORCE)
+# /FORCE:MULTIPLE: the 1995 headers define free funcs/globals without
+# inline/extern, so identical symbols land in every TU (~124 LNK2005);
+# MULTIPLE keeps the first.  UNRESOLVED is gone (BT412/v143): the dead
+# offline-tool factory in mech3.cpp is neutralized behind BT412_OFFLINE_TOOLS,
+# so any unresolved external is now a REAL link error, not a runtime AV.
+target_link_options(btl4 PRIVATE /FORCE:MULTIPLE)
 
 # Copy the third-party runtime DLLs next to the built exe.
 add_custom_command(TARGET btl4 POST_BUILD
diff --git a/README.md b/README.md
index 11c33d8..19f1952 100644
--- a/README.md
+++ b/README.md
@@ -50,9 +50,10 @@ CLAUDE.md           knowledge-base ROUTER — identity, protocols, quick-lookup,
 
 ## Prerequisites
 
-- **Visual Studio 2019 BuildTools** (MSVC v142, x86). The Community install on the original dev
-  box was broken, hence the explicit BuildTools instance in the configure line below; adjust to
-  your install.
+- **Visual Studio 2022 BuildTools** (MSVC v143, x86) — the BT412 toolchain of record (moved from
+  the 2019/v142 line at the 4.12 fork; v142 still builds if you swap the generator back). The
+  explicit BuildTools instance in the configure line below matches the dev box; adjust to your
+  install.
 - **CMake ≥ 3.20**.
 - **Legacy DirectX SDK (June 2010)** — the engine uses `d3dx9`/`dinput`/`dxerr`, removed from the
   modern Windows SDK. Default path `C:/Program Files (x86)/Microsoft DirectX SDK (June 2010)`;
@@ -65,16 +66,19 @@ the exe automatically at build time.
 ## Build (32-bit / Win32)
 
 ```
-cmake -S . -B build -G "Visual Studio 16 2019" -A Win32 ^
-      -DCMAKE_GENERATOR_INSTANCE="C:/Program Files (x86)/Microsoft Visual Studio/2019/BuildTools"
+cmake -S . -B build -G "Visual Studio 17 2022" -A Win32 ^
+      -DCMAKE_GENERATOR_INSTANCE="C:/Program Files (x86)/Microsoft Visual Studio/2022/BuildTools"
 cmake --build build --config Debug
 ```
 
-Must be **Win32** — the DirectX SDK link libs are `Lib/x86`. The link uses `/FORCE`: the 1995
-headers define free functions/globals without `inline`/`extern`, so identical symbols appear in
-many translation units (~124 `LNK2005`); `/FORCE:MULTIPLE` keeps the first. `UNRESOLVED` tolerates
-a dead offline-tool factory in `mech3.cpp` that is never called at runtime. (Cleanup task: move
-those definitions to single TUs + neutralize the dead factory, then drop `/FORCE`.)
+Must be **Win32** — the DirectX SDK link libs are `Lib/x86`. The link uses `/FORCE:MULTIPLE`: the
+1995 headers define free functions/globals without `inline`/`extern`, so identical symbols appear
+in many translation units (~124 `LNK2005`); `MULTIPLE` keeps the first. `UNRESOLVED` is gone as of
+4.12 — the v143 linker refuses to emit an image with unresolved externals even under `/FORCE`, so
+the dead offline-tool factory in `mech3.cpp` is neutralized behind `BT412_OFFLINE_TOOLS` (default
+off) and a genuinely unresolved symbol is now a hard link error instead of a hidden runtime AV.
+(Remaining cleanup task: move the multiply-defined globals to single TUs, then drop `/FORCE`
+entirely.)
 
 ## Run
 
diff --git a/context/build-and-run.md b/context/build-and-run.md
index e242ba4..fb6ed51 100644
--- a/context/build-and-run.md
+++ b/context/build-and-run.md
@@ -13,20 +13,24 @@ The one top-level `CMakeLists.txt` builds `munga_engine` (engine) + `bt410_l4` (
 game lib) + `btl4.exe`. Full recipe + repo-layout map: `docs/PROGRESS_LOG.md §10a / §10a-bis` +
 `README.md`. Env-gate table: [[decomp-reference]] §6.
 
-## Build (Win32 / VS2019 BuildTools — the DXSDK link libs are Lib/x86)
+## Build (Win32 / VS2022 BuildTools — the DXSDK link libs are Lib/x86)
 ```
-# configure once:
-cmake -S C:\git\bt411 -B C:\git\bt411\build -G "Visual Studio 16 2019" -A Win32 \
-  -DCMAKE_GENERATOR_INSTANCE="C:/Program Files (x86)/Microsoft Visual Studio/2019/BuildTools"
+# configure once (BT412 toolchain of record, moved from VS2019/v142 at the 4.12 fork):
+cmake -S . -B build -G "Visual Studio 17 2022" -A Win32 \
+  -DCMAKE_GENERATOR_INSTANCE="C:/Program Files (x86)/Microsoft Visual Studio/2022/BuildTools"
 # build:
-cmake --build C:\git\bt411\build --config Debug
+cmake --build build --config Debug
 ```
 - Links DXSDK d3d9/d3dx9/dinput8 + OpenAL/libsndfile (`engine/lib/`). DXSDK June 2010 at
   `C:\Program Files (x86)\Microsoft DirectX SDK (June 2010)\` (overridable `-DDXSDK`). [T2]
-- Linker uses **`/FORCE`** — tolerates header-defined globals + the **dead offline-factory
-  unresolved externals in mech3.cpp** (`Mech::CreateSubsystemStream` references `void*`-signature
-  `CreateStreamedSubsystem` for every class; never called at runtime). ⚠ `/FORCE` also HIDES real
-  unresolved symbols as runtime AVs — see [[reconstruction-gotchas]] §6. [T2]
+- Linker uses **`/FORCE:MULTIPLE`** — tolerates the ~124 header-defined-global LNK2005s.
+  **`UNRESOLVED` is gone (BT412, 2026-07-14):** the v143 linker refuses to emit an image with
+  unresolved externals even under `/FORCE`, so the dead offline-tool ladders in `mech3.cpp`
+  (`Mech::CreateSubsystemStream` / `Mech::SubsystemDefaultData` — never called at runtime) are
+  compiled out behind `BT412_OFFLINE_TOOLS` (default off). A genuinely unresolved symbol is now a
+  hard LINK ERROR, not a hidden runtime AV — the [[reconstruction-gotchas]] §6 hazard class is
+  retired for the exe link. No other source changes were needed for v143 (verified: clean build,
+  solo DEV.EGG run, two-instance loopback MP with replication). [T2]
 - Editing an `engine/` file rebuilds the engine lib automatically (one project). A NEW member on a
   `DPLRenderer`/`d3d_OBJECT` class needs the game objs that embed its layout recompiled — delete
   stale objs if layout-mismatch corruption appears. [T2]
diff --git a/context/steamification.md b/context/steamification.md
index 6dec628..e4b1a14 100644
--- a/context/steamification.md
+++ b/context/steamification.md
@@ -30,7 +30,17 @@ pattern). Forked at BT411 `4e72f0c` (2026-07-14).
   `run/run.cmd` working-tree edit discarded (committed version is the documented solo
   launcher), identity pass (README/CLAUDE.md/CMake `project(bt412)`), this topic +
   roadmap authored.
-- Phase 1 (VS2022 v143): not started.
+- **Phase 1 (VS2022 v143) — DONE 2026-07-14.** Generator moved to
+  `Visual Studio 17 2022` (v143, Win32). One real change was forced: the v143 linker
+  fails the image on unresolved externals even under `/FORCE`, so the dead
+  offline-tool ladders in `game/reconstructed/mech3.cpp` are neutralized behind
+  `BT412_OFFLINE_TOOLS` (default off) and the exe now links `/FORCE:MULTIPLE`
+  (UNRESOLVED retired — real unresolved symbols are hard errors now). None of RP412's
+  other v143 source fixes (TIME.h, NETWORK.h, L4DINPUT, CAMMGR) were needed — BT's
+  tree was already v142-clean. Verified: clean build; solo DEV.EGG runs (gait/target/
+  31-subsystem ticks); two-instance loopback MP via btconsole.py — mesh forms
+  (console + 1502↔1602 game ports, "All connections completed!"), mission runs both
+  sides, each world ticks its master + the remote replicant. [T2]
 - Phases 2–7: not started — see the roadmap.
 
 ## The seams (what plugs in where) [T0 unless noted]
@@ -60,6 +70,23 @@ pattern). Forked at BT411 `4e72f0c` (2026-07-14).
 
 ## Known merge risks (from the 3-way BT411 / RP411-baseline / RP412 comparison)
 
+**Quantified 2026-07-14 [T1]** — BT-vs-RP411-baseline divergence measured with
+`diff --strip-trailing-cr` (baseline extracted from RP412 history, `git show
+3b8b729:MUNGA_L4/<f>`); RP412-side change sizes from `git diff 3b8b729..HEAD`:
+
+| File | BT vs baseline | RP412 change | Treatment |
+|------|----------------|--------------|-----------|
+| `L4RIO.h` | identical | 125 lines (RIOBase split) | RP412 verbatim |
+| `L4RIO.cpp` | identical | 0 | nothing to do |
+| `L4CTRL.h`, `L4NET.H`, `L4PLASMA.*`, `L4KEYBD.h` | identical | small | RP412 verbatim |
+| `L4CTRL.cpp` | 29 diff lines | 17 (PadRIO hook) | apply RP412 diff |
+| `L4GREND.cpp` | 6 diff lines | 15 (PlasmaScreen selector) | apply RP412 diff |
+| `L4VIDEO.cpp` | **2725** diff lines (BT render work) | only 14 | apply RP412 diff — small despite BT divergence |
+| `L4VIDEO.h` | 29 diff lines | 7 | apply RP412 diff |
+| `L4NET.CPP` | 23 diff lines (BT_NET_TRACE ×5) | 345 (seam refactor) | **MEDIUM** — apply seam refactor, re-site traces |
+| `L4VB16.cpp` | **514** diff lines (dev-gauge dock) | 350 (split view) | **HIGH** — true hand-merge |
+| `L4VB16.h` | 14 diff lines | 32 | hand-merge with L4VB16.cpp |
+
 - **`L4VB16.cpp` — HIGH.** RP412 rebuilt ~3846–4046 (splitViews/canvas/present); BT
   rebuilt the same window-management region for dev-gauge docking (`BT_DEV_GAUGES`,
   `BT_DEV_GAUGES_WINDOW`, `BT_DEV_GAUGES_DOCK`, `BT_GAUGE_SCALE`, `BT_GAUGE_SEC_ROT`,
@@ -67,8 +94,6 @@ pattern). Forked at BT411 `4e72f0c` (2026-07-14).
 - **`L4NET.CPP` — MEDIUM.** RP412 rerouted all ~22–26 Winsock call sites through
   `NetTransport_Get()`; BT's 5 `BT_NET_TRACE` diagnostic blocks sit exactly on those
   send/receive sites and must be re-sited onto the seam calls.
-- **`L4RIO.*`, `L4CTRL.cpp`, `L4GREND.cpp` — LOW.** BT never customized these
-  (verified: zero BT_ hits); take RP412's versions after the 3-way check.
 - **CMake**: engine sources are explicit per-file lines (no globs) — every ported
   `.cpp` needs its own `add_library` line or it silently doesn't compile. New deps:
   `xinput9_1_0.lib` (PadRIO), C++/WinRT (KeyLight, per-file C++17), Steamworks SDK +
diff --git a/game/reconstructed/mech3.cpp b/game/reconstructed/mech3.cpp
index b04e42b..deae01f 100644
--- a/game/reconstructed/mech3.cpp
+++ b/game/reconstructed/mech3.cpp
@@ -96,9 +96,11 @@
 //   Those ~21 classes live in sibling modules not visible here (and PPC /
 //   GaussRifle / SubsystemMessageManager have no reconstructed class at all),
 //   so the uniform factory surface is declared here as minimal stubs.  The
-//   bodies/data are resolved at link against the real subsystem modules; these
-//   declarations only let the dispatch compile.  Kept LOCAL so they never
-//   collide with the real sibling classes.
+//   bodies/data were NEVER reconstructed anywhere -- under v142 the link
+//   left them unresolved and /FORCE tolerated it (the path is dead at
+//   runtime); the v143 linker does not, so the dispatch ladders below are
+//   compiled out unless BT412_OFFLINE_TOOLS is defined.  Kept LOCAL so they
+//   never collide with the real sibling classes if ever reconstructed.
 //===========================================================================//
 typedef Mech::SharedData SharedData;	// namespace-scope alias (== Entity__SharedData)
 
@@ -711,6 +713,20 @@ void
 		NotationFile				*subsystem_file,	// param_8
 		const ResourceDirectories	*directories)		// param_9
 {
+#if !BT412_OFFLINE_TOOLS
+	//-----------------------------------------------------------------------
+	// BT412 NEUTRALIZED (offline authoring path -- never called at runtime;
+	// resources ship prebuilt in BTL4.RES).  The ~21 per-subsystem
+	// CreateStreamedSubsystem statics were never reconstructed, and the
+	// VS2022 (v143) linker fails the image on unresolved externals even
+	// under /FORCE (the v142 linker tolerated them).  The authentic
+	// dispatch ladder is preserved under BT412_OFFLINE_TOOLS for the day
+	// the offline tool path is reconstructed.
+	//-----------------------------------------------------------------------
+	DEBUG_STREAM << "[offline] Mech::CreateSubsystemStream(" << type_name
+		<< ") reached with BT412_OFFLINE_TOOLS off\n" << std::flush;
+	return -1;
+#else
 	int	result;
 
 	if      (Streq(type_name, "HeatSinkClassID"))
@@ -765,6 +781,7 @@ void
 	}
 
 	return (result == 0) ? -1 : 1;
+#endif	// BT412_OFFLINE_TOOLS
 }
 
 
@@ -909,6 +926,8 @@ fail:
 	Mech::SubsystemDefaultData(const char *type_name)
 {
 	if (Streq(type_name, ""))						return &Mech::DefaultData;					// 0050bde4 (DAT_0050dc1f == "")
+#if BT412_OFFLINE_TOOLS	// BT412 NEUTRALIZED with the factory above -- the per-subsystem
+						// DefaultData statics were never reconstructed (no callers at runtime).
 	if (Streq(type_name, "ControlsMapper"))			return &MechControlsMapper::DefaultData;	// 0050ee00
 	if (Streq(type_name, "AmmoBinClassID"))			return &AmmoBin::DefaultData;				// 005125bc
 	if (Streq(type_name, "HeatSinkClassID"))		return &HeatSink::DefaultData;				// 0050e580
@@ -930,6 +949,7 @@ fail:
 	if (Streq(type_name, "GaussRifleClassID"))		return &GaussRifle::DefaultData;			// 00512478 (shares MissileLauncher table)
 	if (Streq(type_name, "ControlsMapper"))			return &MechControlsMapper::DefaultData;	// 0050ee00 (2nd alias)
 	if (Streq(type_name, "MechTech"))				return &MechTech::DefaultData;				// 0050e270
+#endif	// BT412_OFFLINE_TOOLS
 	return 0;
 }