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..4d72c7f 100644
--- a/include/os_compat.h
+++ b/include/os_compat.h
@@ -111,6 +111,40 @@ 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 */
 
+/* --- UTF-8 --------------------------------------------------------------
+ * Encode one code point as UTF-8, independent of the C locale.
+ *
+ * wcrtomb() follows setlocale(). On Linux the environment normally selects a
+ * UTF-8 locale, but on Windows the C runtime maps setlocale(LC_ALL, "") to the
+ * ANSI code page -- 936 (GBK) on a Chinese system -- so wcrtomb() hands back
+ * GBK bytes where the protocol requires UTF-8, and the model receives
+ * mojibake. Windows' C runtime has no ".UTF-8" locale to switch to.
+ *
+ * Returns the number of bytes written (1..4), or 0 if the code point cannot be
+ * encoded. Surrogate halves (as they arrive on 16-bit wchar_t platforms for
+ * non-BMP code points) return 0 and are skipped. */
+int os_utf8_encode(unsigned long cp, char out[4]);
+
+/* Decode one UTF-8 sequence from s (at most avail bytes).
+ * Returns the number of bytes consumed (1..4) and stores the code point in
+ * *cp, or 0 if the bytes are not a valid, complete, non-overlong sequence.
+ *
+ * The counterpart of the encoder above, and needed for the same reason:
+ * mbrtowc() also follows setlocale(), so on Windows it would decode UTF-8
+ * text as GBK and the screen would show mojibake. */
+int os_utf8_decode(const char *s, size_t avail, unsigned long *cp);
+
+/* --- 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/llm_openai.c b/src/llm_openai.c
index fe18de3..84d896d 100644
--- a/src/llm_openai.c
+++ b/src/llm_openai.c
@@ -22,6 +22,45 @@
 #define ERRBODY_MAX 4096
 #define LLM_MAX_RETRIES 3
 
+/* Point libcurl at a CA bundle on Windows.
+ *
+ * An OpenSSL-based libcurl keeps the path it was built with -- MSYS2's is
+ * "/mingw64/etc/ssl/certs/ca-bundle.crt", which resolves to nothing once the
+ * binary runs outside an MSYS2 shell, and that build ignores CURL_CA_BUNDLE.
+ * Every HTTPS request then dies with CURLE_SSL_CACERT_BADFILE (77).
+ *
+ * Order: $CST_CA_BUNDLE, then ca-bundle.crt beside the executable. If neither
+ * exists nothing is set, so a libcurl linked against Schannel (the Windows
+ * certificate store) behaves exactly as before. */
+static void llm_apply_ca_bundle(CURL *curl) {
+#if defined(_WIN32)
+  static char ca[OS_PATH_MAX];
+  static int checked;
+  if (!checked) {
+    checked = 1;
+    const char *env = getenv("CST_CA_BUNDLE");
+    if (env && env[0]) {
+      snprintf(ca, sizeof(ca), "%s", env);
+    } else {
+      char dir[OS_PATH_MAX];
+      if (os_exe_dir(dir, sizeof(dir)) == 0)
+        os_path_join(ca, sizeof(ca), dir, "ca-bundle.crt");
+    }
+    if (ca[0]) {
+      FILE *f = fopen(ca, "rb");
+      if (f)
+        fclose(f);
+      else
+        ca[0] = '\0';
+    }
+  }
+  if (ca[0])
+    curl_easy_setopt(curl, CURLOPT_CAINFO, ca);
+#else
+  (void)curl;
+#endif
+}
+
 /* --- wire body ----------------------------------------------------------- */
 
 static size_t wire_tool_cap(const cst_llm_request_t *req) {
@@ -621,6 +660,7 @@ int cst_llm_chat_stream(const cst_llm_request_t *req, char *err,
       ret = -1;
       break;
     }
+    llm_apply_ca_bundle(curl);
     sse_reset(&s);
     s.curl = curl;
     if (rec_dir && rec_dir[0]) {
@@ -765,10 +805,11 @@ int cst_llm_list_models(const char *api_base, const char *api_key,
   }
   CURL *curl = curl_easy_init();
   if (!curl) {
-    snprintf(err, err_sz, "curl init failed");
-    return -1;
-  }
-  char url[1024];
+      snprintf(err, err_sz, "curl init failed");
+      return -1;
+    }
+    llm_apply_ca_bundle(curl);
+    char url[1024];
   size_t bl = strlen(api_base);
   while (bl && api_base[bl - 1] == '/')
     bl--;
diff --git a/src/os_compat.c b/src/os_compat.c
index 7fa3c73..a579117 100644
--- a/src/os_compat.c
+++ b/src/os_compat.c
@@ -1168,3 +1168,120 @@ void os_proc_kill(os_proc_t *p) {
   }
 }
 #endif
+
+/* --- UTF-8 ---------------------------------------------------------------
+ * See the declaration in os_compat.h for why wcrtomb() is not used. */
+int os_utf8_encode(unsigned long cp, char out[4]) {
+  if (cp < 0x80) {
+    out[0] = (char)cp;
+    return 1;
+  }
+  if (cp < 0x800) {
+    out[0] = (char)(0xC0 | (cp >> 6));
+    out[1] = (char)(0x80 | (cp & 0x3F));
+    return 2;
+  }
+  if (cp >= 0xD800 && cp <= 0xDFFF)
+    return 0; /* lone surrogate half from a 16-bit wchar_t */
+  if (cp < 0x10000) {
+    out[0] = (char)(0xE0 | (cp >> 12));
+    out[1] = (char)(0x80 | ((cp >> 6) & 0x3F));
+    out[2] = (char)(0x80 | (cp & 0x3F));
+    return 3;
+  }
+  if (cp <= 0x10FFFF) {
+    out[0] = (char)(0xF0 | (cp >> 18));
+    out[1] = (char)(0x80 | ((cp >> 12) & 0x3F));
+    out[2] = (char)(0x80 | ((cp >> 6) & 0x3F));
+    out[3] = (char)(0x80 | (cp & 0x3F));
+    return 4;
+  }
+  return 0;
+}
+
+/* See the declaration in os_compat.h for why mbrtowc() is not used. */
+int os_utf8_decode(const char *s, size_t avail, unsigned long *cp) {
+  const unsigned char *p = (const unsigned char *)s;
+  int need, i;
+  unsigned long v;
+  static const unsigned long MINV[4] = {0, 0x80, 0x800, 0x10000};
+
+  if (!s || !cp || avail == 0)
+    return 0;
+  if (p[0] < 0x80) {
+    *cp = p[0];
+    return 1;
+  }
+  if ((p[0] & 0xE0) == 0xC0) {
+    need = 1;
+    v = p[0] & 0x1Fu;
+  } else if ((p[0] & 0xF0) == 0xE0) {
+    need = 2;
+    v = p[0] & 0x0Fu;
+  } else if ((p[0] & 0xF8) == 0xF0) {
+    need = 3;
+    v = p[0] & 0x07u;
+  } else {
+    return 0; /* not a UTF-8 lead byte */
+  }
+  if (avail < (size_t)need + 1)
+    return 0; /* truncated */
+  for (i = 1; i <= need; i++) {
+    if ((p[i] & 0xC0) != 0x80)
+      return 0; /* bad continuation byte */
+    v = (v << 6) | (p[i] & 0x3Fu);
+  }
+  if (v < MINV[need] || (v >= 0xD800 && v <= 0xDFFF) || v > 0x10FFFF)
+    return 0; /* overlong, surrogate, or out of range */
+  *cp = v;
+  return need + 1;
+}
+
+/* --- 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 */
diff --git a/src/textsel.c b/src/textsel.c
index 0be8583..18bae3b 100644
--- a/src/textsel.c
+++ b/src/textsel.c
@@ -3,23 +3,29 @@
 #endif
 
 #include "cst_textsel.h"
+#include "os_compat.h"
 
 #include <stdlib.h>
 #include <string.h>
 #include <wctype.h>
 
 int cst_utf8_decode(const char *s, size_t avail, wchar_t *wc, size_t *eaten) {
+  unsigned long cp = 0;
+  int n;
   if (avail == 0)
     return -1;
-  mbstate_t st;
-  memset(&st, 0, sizeof(st));
-  size_t n = mbrtowc(wc, s, avail, &st);
-  if (n == (size_t)-1 || n == (size_t)-2 || n == 0) {
-    *wc = (unsigned char)s[0];
+  /* os_utf8_decode(), not mbrtowc(): the latter follows setlocale(), which on
+   * Windows means the ANSI code page, so UTF-8 text would be decoded as GBK
+   * and rendered as mojibake. */
+  n = os_utf8_decode(s, avail, &cp);
+  if (n <= 0) {
+    /* Not a valid sequence: fall back to one raw byte, as before. */
+    *wc = (wchar_t)(unsigned char)s[0];
     *eaten = 1;
     return 0;
   }
-  *eaten = n;
+  *wc = (wchar_t)cp;
+  *eaten = (size_t)n;
   return 0;
 }
 
diff --git a/src/tui.c b/src/tui.c
index 2a1866a..c58c4a6 100644
--- a/src/tui.c
+++ b/src/tui.c
@@ -2966,12 +2966,10 @@ static void input_clear(tui_t *t) { input_set(t, ""); }
 
 static void input_put_wc(tui_t *t, wchar_t wc) {
   char mb[8];
-  mbstate_t st;
-  memset(&st, 0, sizeof(st));
-  size_t n = wcrtomb(mb, wc, &st);
-  if (n == (size_t)-1 || n == 0)
+  int n = os_utf8_encode((unsigned long)wc, mb);
+  if (n <= 0)
     return;
-  input_insert(t, mb, n);
+  input_insert(t, mb, (size_t)n);
 }
 
 static void input_put_byte(tui_t *t, char ch) { input_insert(t, &ch, 1); }
@@ -3600,16 +3598,14 @@ static int picker_run(tui_t *t, const char *title, char items[][128], int n,
       continue;
     }
     if (iswprint(wch)) {
-      char mb[8];
-      mbstate_t st;
-      memset(&st, 0, sizeof(st));
-      size_t k = wcrtomb(mb, (wchar_t)wch, &st);
-      if (k != (size_t)-1 && qlen + k < sizeof(query)) {
-        memcpy(query + qlen, mb, k);
-        qlen += k;
-        query[qlen] = 0;
-        sel = top = 0;
-      }
+          char mb[8];
+          int k = os_utf8_encode((unsigned long)(wchar_t)wch, mb);
+          if (k > 0 && qlen + (size_t)k < sizeof(query)) {
+            memcpy(query + qlen, mb, (size_t)k);
+            qlen += (size_t)k;
+            query[qlen] = 0;
+            sel = top = 0;
+          }
     }
   }
 
@@ -6143,15 +6139,13 @@ static void run_ask_modal(tui_t *t) {
       break;
     }
     if (iswprint(wch)) {
-      char mb[8];
-      mbstate_t st;
-      memset(&st, 0, sizeof(st));
-      size_t k = wcrtomb(mb, (wchar_t)wch, &st);
-      size_t n = strlen(t->ask_buf);
-      if (k != (size_t)-1 && n + k + 1 < sizeof(t->ask_buf)) {
-        memcpy(t->ask_buf + n, mb, k);
-        t->ask_buf[n + k] = 0;
-      }
+          char mb[8];
+          int k = os_utf8_encode((unsigned long)(wchar_t)wch, mb);
+          size_t n = strlen(t->ask_buf);
+          if (k > 0 && n + (size_t)k + 1 < sizeof(t->ask_buf)) {
+            memcpy(t->ask_buf + n, mb, (size_t)k);
+            t->ask_buf[n + k] = 0;
+          }
     }
   }
   curs_set(1);