diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ae72e8a9..464ea10c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -23,7 +23,7 @@ on:
 
 env:
   MCPP_SOURCE_REF: ${{ github.event.inputs.mcpp_ref || vars.MCPP_SOURCE_REF }}
-  MCPP_VERSION: 2026.9.18.3
+  MCPP_VERSION: 2026.9.20.1
   XLINGS_VERSION: v2026.8.17.2
   XLINGS_NON_INTERACTIVE: '1'
 
diff --git a/README.md b/README.md
index c7e0649d..7192566b 100644
--- a/README.md
+++ b/README.md
@@ -6,19 +6,27 @@ C library.
 
 ```toml
 [dependencies]
-openkal-llvm-runtime = "0.12.0"
+openkal-llvm-runtime = "0.13.0"
 ```
 
-> **Engine floor (mcpp 2026.9.18.3):** this version of this package pins
-> `openkal-musl 0.16.0` and inherits its `[c-abi]` declaration. The engine's
-> `cenv_probe::verify` strips host macros (`-U_WIN32` / `-U_WIN64` /
-> `-U__MINGW32__` / `-U__MINGW64__`) on Windows hosts before reading the
-> predefined macros that back the declaration, and `cenv::realise` forces
-> `-fno-short-wchar` on freestanding wchar. Older engines silently
-> misbuild this package on Windows × freestanding (the cross-build to
-> `riscv64-none-elf` etc. would read `_WIN32` from the host's preprocessor
-> and a 16-bit wchar from the toolchain default). Upgrade:
-> `xlings install mcpp --force`.
+> **Engine floor (mcpp 2026.9.20.1):** this version of this package pins
+> `openkal-musl 0.18.0` and inherits its `[c-abi]` declaration. The engine
+> assembles the verification probe's command line in one place that REFUSES
+> to run it without a target selection (`cenv_probe::assemble_argv`), and
+> `cenv::realise` forces `-fno-short-wchar` on freestanding wchar. Older
+> engines silently misbuild this package on freestanding cross-builds
+> (`riscv64-none-elf` and the rest): their probe ran with no `--target=` at
+> all, so the compiler answered for the machine it was running on.
+>
+> **The floor named here was 2026.9.18.3 and that was wrong.** It credited
+> that release with stripping host macros (`-U_WIN32` and the MinGW pair) on
+> Windows hosts before reading the predefined macros. The strip was real and
+> it protected nothing: there was no target substitution in the freestanding
+> argv for it to compensate for, and deleting the host's predefines deleted
+> the one piece of evidence that would have said so. A measurement that
+> removes its own disagreement reports an agreement it never established.
+>
+> Upgrade: `xlings install mcpp --force`.
 
 A C++ standard library is not portable in the way a program is. It is
 *configured* for one C library and compiled against that library's headers, and
diff --git a/mcpp.toml b/mcpp.toml
index aa6fc807..ee6b48d2 100644
--- a/mcpp.toml
+++ b/mcpp.toml
@@ -1,7 +1,7 @@
 [package]
 namespace   = "mcpplibs"
 name        = "openkal-llvm-runtime"
-version     = "0.12.0"
+version     = "0.13.0"
 description = "LLVM's C++ runtime libraries — libc++, libc++abi and libunwind — configured for openkal-musl rather than for a host C library."
 license     = "Apache-2.0"
 authors     = ["mcpplibs"]
@@ -216,7 +216,7 @@ sources = [
 cflags = ["-DDISABLE_AARCH64_FMV=1"]
 
 [dependencies]
-openkal-musl = "0.16.0"
+openkal-musl = "0.18.0"
 
 [build]