diff --git a/core/libs/prx/libc/src/Math.cpp b/core/libs/prx/libc/src/Math.cpp
index 0d1bd3bbb4..3ee11e1da4 100644
--- a/core/libs/prx/libc/src/Math.cpp
+++ b/core/libs/prx/libc/src/Math.cpp
@@ -1,4 +1,5 @@
 #include <mutex>
+#include <array>
 #include <bit>
 #include <cstddef>
 #include <cstdint>
@@ -792,7 +793,10 @@ SonyDconst _LSnan_nid_postfix {{1, 0, 0, 0x8000, 0x7fff, 0, 0, 0}};
 
 int APS5_VABI fesetenv_nid_postfix(const GuestFloatingPointEnvironment* environment) {
     if (environment == nullptr) throw std::invalid_argument("fesetenv: null environment");
-    if ((environment->mxcsr & 0xffff0000u) != 0) throw std::invalid_argument("fesetenv: unsupported MXCSR bits");
+    alignas(16) std::array<std::uint32_t, 128> state;
+    asm volatile("fxsave %0" : "=m"(state));
+    const auto supported = state[7] != 0 ? state[7] : 0xffbfu;
+    if ((environment->mxcsr & ~supported) != 0) throw std::invalid_argument("fesetenv: unsupported MXCSR bits");
     asm volatile("fldenv %0" : : "m"(*environment) : "st", "st(1)", "st(2)", "st(3)", "st(4)", "st(5)", "st(6)", "st(7)");
     _mm_setcsr(environment->mxcsr);
     return 0;
diff --git a/core/libs/tests/GuestSetEnv.cpp b/core/libs/tests/GuestSetEnv.cpp
index e6fee83bc6..ed61a9717a 100644
--- a/core/libs/tests/GuestSetEnv.cpp
+++ b/core/libs/tests/GuestSetEnv.cpp
@@ -146,13 +146,18 @@ void BoundsAndErrors() {
     *__error_nid_postfix()=123;
     try { setEnvironment(nullptr); } catch(const std::invalid_argument&) { rejected=true; }
     Require(rejected && Same(Read(),original) && *__error_nid_postfix()==123,"null rejected without state or errno change");
-    for(unsigned reserved:{0x10000u,0x80000000u,0xffff0000u}) {
+    alignas(16) std::array<std::uint32_t,128> extended;
+    asm volatile("fxsave %0" : "=m"(extended));
+    const auto supported=extended[7] ? extended[7] : 0xffbfu;
+    for(unsigned bit=0;bit<32;++bit) {
+        const auto reserved=1u<<bit;
+        if((supported&reserved)!=0) continue;
         auto desired=original;
         desired.x87[0]^=0xc00;
         desired.mxcsr|=reserved;
         rejected=false;
         try { setEnvironment(&desired); } catch(const std::invalid_argument&) { rejected=true; }
-        Require(rejected && Same(Read(),original) && *__error_nid_postfix()==123,"unsupported MXCSR rejected before x87 changes");
+        Require(rejected && Same(Read(),original) && *__error_nid_postfix()==123,"host-unsupported MXCSR rejected before x87 changes");
     }
 }
 
diff --git a/docs/dev/debt/libc.md b/docs/dev/debt/libc.md
index 31a3e52b0d..89461bbe38 100644
--- a/docs/dev/debt/libc.md
+++ b/docs/dev/debt/libc.md
@@ -2,7 +2,7 @@
 
 ### Unknown function info
 
-- [fesetenv](../../../core/libs/prx/libc/src/Math.cpp) (libSceLibcInternal, implemented in libc, NID `Q-bLp+b-RVY`) uses the 32-byte amd64 environment and x87/MXCSR restore sequence in the [public SDK fenv.h](https://github.com/ps5-payload-dev/sdk/blob/a7685ce8e97ce1f8a4f38253a41d54241c9124f1/include/freebsd/fenv.h). The console layout, behavior and module/library versions are unmeasured. Null pointers and reserved MXCSR bits throw before changing state; console fault behavior for invalid environments is unverified. The separate `__fe_dfl_env` object remains missing and its console initializer is unknown.
+- [fesetenv](../../../core/libs/prx/libc/src/Math.cpp) (libSceLibcInternal, implemented in libc, NID `Q-bLp+b-RVY`) uses the 32-byte amd64 environment and x87/MXCSR restore sequence in the [public SDK fenv.h](https://github.com/ps5-payload-dev/sdk/blob/a7685ce8e97ce1f8a4f38253a41d54241c9124f1/include/freebsd/fenv.h). The console layout, behavior and module/library versions are unmeasured. Null pointers and MXCSR bits unsupported by the current host's `FXSAVE` mask throw before changing state; a zero mask uses the architectural `0xffbf` fallback. Console fault behavior for invalid environments is unverified. The separate `__fe_dfl_env` object remains missing and its console initializer is unknown.
 
 - [strtonum](../../../core/libs/prx/libc/src/StringExtensions.cpp) (libc) is unverified on the console. Its signature, decimal parsing, inclusive bounds, error strings, error precedence and errno preservation follow [FreeBSD 11.4 strtonum.c](https://github.com/freebsd/freebsd-src/blob/8c639945a44588f6cfdddfb057bb5aad88735da0/lib/libc/stdlib/strtonum.c). Parsing uses the host `strtoll` with base 10.