diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2a83f8a..5d4a218 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -18,12 +18,17 @@ endif()
 # Separate archive for MinGW so Linux host builds keep libsashidown.a
 if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
   set(SASHIDOWN_A "${SD_DIR}/libsashidown-mingw.a")
-  set(_MINGW_AR x86_64-w64-mingw32-ar)
+  # Use whatever archiver CMake found for the active toolchain. Hardcoding
+  # "x86_64-w64-mingw32-ar" only works on cross-build hosts, where binutils
+  # installs prefixed names; MSYS2's mingw-w64 toolchain ships plain ar.exe
+  # and ranlib.exe, so the hardcoded names are not found.
+  set(_MINGW_AR "${CMAKE_AR}")
+  set(_MINGW_RANLIB "${CMAKE_RANLIB}")
   add_custom_command(
     OUTPUT "${SASHIDOWN_A}"
     COMMAND ${CMAKE_MAKE_PROGRAM} -C "${SD_DIR}" clean
     COMMAND ${CMAKE_MAKE_PROGRAM} -C "${SD_DIR}" libsashidown.a
-            CC=${CMAKE_C_COMPILER} AR=${_MINGW_AR} RANLIB=x86_64-w64-mingw32-ranlib
+            CC=${CMAKE_C_COMPILER} AR=${_MINGW_AR} RANLIB=${_MINGW_RANLIB}
     COMMAND ${CMAKE_COMMAND} -E copy_if_different
             "${SD_DIR}/libsashidown.a" "${SASHIDOWN_A}"
     COMMAND ${CMAKE_MAKE_PROGRAM} -C "${SD_DIR}" clean
@@ -111,9 +116,11 @@ endif()
 if(CST_BUILD_TUI)
   if(WIN32)
     # Prefer PDCurses (pdcurses / curses). Users may set PDCurses_ROOT.
+    # PATH_SUFFIXES also covers the layout MSYS2's mingw-w64-pdcurses uses,
+    # which puts the headers under include/pdcurses/ rather than include/.
     find_path(PDCURSES_INCLUDE_DIR curses.h
       HINTS ENV PDCurses_ROOT "${PDCurses_ROOT}"
-      PATH_SUFFIXES include)
+      PATH_SUFFIXES include pdcurses include/pdcurses)
     find_library(PDCURSES_LIBRARY
       NAMES pdcurses pdcurses.dll curses
       HINTS ENV PDCurses_ROOT "${PDCurses_ROOT}"
@@ -121,7 +128,11 @@ if(CST_BUILD_TUI)
     if(PDCURSES_INCLUDE_DIR AND PDCURSES_LIBRARY)
       target_include_directories(cst-agent PRIVATE ${PDCURSES_INCLUDE_DIR})
       target_link_libraries(cst-agent PRIVATE ${PDCURSES_LIBRARY})
-      target_compile_definitions(cst-agent PRIVATE CST_USE_PDCURSES=1)
+      # PDCurses suffixes its exported symbols with an ABI tag (endwin_u64_4400
+      # etc.). MSYS2 builds the UTF-8 wide variant, so PDC_FORCE_UTF8 has to be
+      # defined or curses.h maps to the w64 names and the link fails.
+      target_compile_definitions(cst-agent PRIVATE
+        CST_USE_PDCURSES=1 PDC_FORCE_UTF8=1)
       message(STATUS "TUI: PDCurses (${PDCURSES_LIBRARY})")
     else()
       message(FATAL_ERROR "CST_BUILD_TUI=ON on Windows requires PDCurses (set PDCurses_ROOT)")
@@ -143,9 +154,9 @@ else()
 endif()
 
 if(CMAKE_SYSTEM_NAME STREQUAL "Windows" OR WIN32)
-  # curl Schannel + networking
+  # curl Schannel + networking; winmm is needed by PDCurses (PlaySoundW)
   target_link_libraries(cst-agent PRIVATE
-    ws2_32 crypt32 secur32 bcrypt advapi32)
+    ws2_32 crypt32 secur32 bcrypt advapi32 winmm)
 endif()
 
 # --- tests ---
diff --git a/docs/README-windows.md b/docs/README-windows.md
index a4e6a3c..b2099d4 100644
--- a/docs/README-windows.md
+++ b/docs/README-windows.md
@@ -41,6 +41,35 @@ cmake --build build-win-tui
 Copy `build-win-tui/cst-agent.exe`, `third_party/busybox/busybox.exe`, and
 `libwinpthread-1.dll` into one folder.
 
+## Build natively on Windows (MSYS2)
+
+The CMake build also works directly on Windows with MSYS2's mingw-w64
+toolchain -- no separate Linux host needed:
+
+```bash
+# in an MSYS2 MINGW64 shell
+pacman -S --needed --noconfirm \
+  mingw-w64-x86_64-gcc mingw-w64-x86_64-cmake mingw-w64-x86_64-make \
+  mingw-w64-x86_64-curl mingw-w64-x86_64-pdcurses
+
+cd agent
+cmake -B build-win -G "MinGW Makefiles" -DCMAKE_BUILD_TYPE=Release
+cmake --build build-win --target cst-agent -j"$(nproc)"
+```
+
+`CST_BUILD_TUI=ON` (the default) picks up MSYS2's PDCurses automatically,
+including its `include/pdcurses/` layout; set `-DPDCurses_ROOT=...` only for
+a custom PDCurses.
+
+Two differences from the cross-build release layout above:
+
+- **Extra DLLs.** MSYS2's curl package is the shared OpenSSL build, so the
+  executable needs its whole dependency chain beside it. Collect it with
+  `ldd build-win/cst-agent.exe | grep mingw64`.
+- **Only build the `cst-agent` target.** Some sources under `tests/` are
+  POSIX-only (two-argument `mkdir`, `setenv`, `arpa/inet.h`, `symlink`) and
+  do not compile with MinGW. The main program is unaffected.
+
 ## Run
 
 ```bat
diff --git a/include/os_compat.h b/include/os_compat.h
index 51ac12e..0ef9a57 100644
--- a/include/os_compat.h
+++ b/include/os_compat.h
@@ -111,6 +111,17 @@ void os_localtime(const time_t *t, struct tm *out);
 int os_getpid(void);
 int os_chmod_private(const char *path); /* best-effort 0600; no-op/ok on Win */
 
+/* --- libc gaps on Windows ------------------------------------------------
+ * MinGW-w64 provides neither strcasestr() (a GNU extension) nor
+ * setenv()/unsetenv() (POSIX): they are simply absent from its headers.
+ * src/tui.c uses all three. Implemented in os_compat.c for _WIN32 only,
+ * so the Unix builds keep using the libc versions. */
+#if defined(_WIN32)
+char *strcasestr(const char *haystack, const char *needle);
+int setenv(const char *name, const char *value, int overwrite);
+int unsetenv(const char *name);
+#endif
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/src/os_compat.c b/src/os_compat.c
index 7fa3c73..6b941ae 100644
--- a/src/os_compat.c
+++ b/src/os_compat.c
@@ -1168,3 +1168,52 @@ void os_proc_kill(os_proc_t *p) {
   }
 }
 #endif
+
+/* --- libc gaps on Windows -----------------------------------------------
+ * strcasestr() is a GNU extension and setenv()/unsetenv() are POSIX; glibc
+ * has all three, MinGW-w64 has none of them. src/tui.c needs them (command
+ * filtering and ESCDELAY), so the Windows build supplies its own.
+ *
+ * The case folding here is ASCII-only, unlike glibc's locale-aware version.
+ * That is deliberate: these are used to match command and setting names,
+ * which are ASCII.
+ */
+#if defined(_WIN32)
+
+static int ascii_lower(int c) {
+  return (c >= 'A' && c <= 'Z') ? c + ('a' - 'A') : c;
+}
+
+char *strcasestr(const char *haystack, const char *needle) {
+  if (!haystack || !needle)
+    return NULL;
+  if (!*needle)
+    return (char *)haystack;
+  for (; *haystack; haystack++) {
+    const char *h = haystack;
+    const char *n = needle;
+    while (*h && *n && ascii_lower((unsigned char)*h) == ascii_lower((unsigned char)*n)) {
+      h++;
+      n++;
+    }
+    if (!*n)
+      return (char *)haystack;
+  }
+  return NULL;
+}
+
+int setenv(const char *name, const char *value, int overwrite) {
+  if (!name || !*name || strchr(name, '='))
+    return -1;
+  if (!overwrite && getenv(name))
+    return 0;
+  return _putenv_s(name, value ? value : "");
+}
+
+int unsetenv(const char *name) {
+  if (!name || !*name || strchr(name, '='))
+    return -1;
+  return _putenv_s(name, "");
+}
+
+#endif /* _WIN32 */