diff --git a/README.md b/README.md
index 1a691e7..b5614e9 100644
--- a/README.md
+++ b/README.md
@@ -17,6 +17,12 @@ vertex fetch optimization, simplification, meshlets, spatial ordering, the
 vertex, index and meshlet codecs — is vendored byte for byte from the 1.3
 release and built with the package; nothing else is linked.
 
+The library is built with `-ffp-contract=off` and without fast math, so the
+same input gives the same output bit for bit on every platform: with the
+platform's own default arm64 fuses a multiply and an add into one rounding
+and the meshlets of a scattered 48 × 48 sheet differ from x86-64's (from
+1.3.1; 1.3 kept the platform's default).
+
 ## Modules
 
 - `import mcpplibs.mwrap.meshoptimizer;`: the whole API (118 names: 9 types, 20 enumerators, 89 functions), re-exporting the one below
@@ -27,7 +33,7 @@ release and built with the package; nothing else is linked.
 ## Getting started
 
 ```sh
-mcpp new lod --template mcpplibs.mwrap.meshoptimizer@1.3
+mcpp new lod --template mcpplibs.mwrap.meshoptimizer@1.3.1
 cd lod
 mcpp run
 ```
@@ -37,7 +43,7 @@ and simplifies it to a tenth of its triangles.
 
 ```toml
 [dependencies]
-mcpplibs.mwrap.meshoptimizer = "1.3"
+mcpplibs.mwrap.meshoptimizer = "1.3.1"
 ```
 
 ```cpp
diff --git a/README.zh.md b/README.zh.md
index dc5233d..27daeb9 100644
--- a/README.zh.md
+++ b/README.zh.md
@@ -15,6 +15,10 @@
 简化、meshlet、空间排序、顶点／索引／meshlet 编解码——逐字节取自 1.3 发布包并随本包
 构建，不链接其他东西。
 
+库以 `-ffp-contract=off`、不开 fast math 编译，同样的输入在每个平台上逐位得到同样的
+输出：按平台默认，arm64 会把乘与加合成一次舍入，打乱的 48 × 48 网格得到的 meshlet
+与 x86-64 不同（1.3.1 起；1.3 用的是平台默认）。
+
 ## Modules
 
 - `import mcpplibs.mwrap.meshoptimizer;`：全部 API（118 个名字：9 个类型、20 个枚举值、89 个函数），并再导出下面这个
@@ -25,7 +29,7 @@
 ## 快速开始
 
 ```sh
-mcpp new lod --template mcpplibs.mwrap.meshoptimizer@1.3
+mcpp new lod --template mcpplibs.mwrap.meshoptimizer@1.3.1
 cd lod
 mcpp run
 ```
@@ -34,7 +38,7 @@ mcpp run
 
 ```toml
 [dependencies]
-mcpplibs.mwrap.meshoptimizer = "1.3"
+mcpplibs.mwrap.meshoptimizer = "1.3.1"
 ```
 
 ```cpp
diff --git a/mcpp.toml b/mcpp.toml
index a93cf8f..5dc5efa 100644
--- a/mcpp.toml
+++ b/mcpp.toml
@@ -2,7 +2,7 @@
 [package]
 namespace   = "mcpplibs.mwrap"
 name        = "meshoptimizer"
-version     = "1.3"
+version     = "1.3.1"
 description = "meshoptimizer 1.3 C++ modules for mcpp (import mcpplibs.mwrap.meshoptimizer;): mesh indexing, vertex cache / overdraw / fetch optimization, simplification, meshlets, and the vertex / index codecs"
 license     = "Apache-2.0"
 authors     = ["mcpp-community"]
@@ -17,6 +17,10 @@ kind = "lib"
 # (upstream/documents.json: the release tarball, each file's checksum).
 sources      = ["src/*.cppm", "upstream/meshoptimizer/src/*.cpp"]
 include_dirs = ["upstream/meshoptimizer/src"]
+# Every platform makes the same meshlets, clusters and simplifications: the
+# compiler fuses no multiply and add into one rounding (arm64's default does,
+# x86-64's cannot without FMA), so no float decision differs by platform.
+cxxflags     = ["-ffp-contract=off", "-fno-fast-math"]
 
 [dev-dependencies]
 compat.gtest = "1.15.2"