diff --git a/src/core/HostShell.lua b/src/core/HostShell.lua
index 41068ed7..fe19021c 100644
--- a/src/core/HostShell.lua
+++ b/src/core/HostShell.lua
@@ -429,8 +429,22 @@ function HostShell.httpPost(url, body, contentType, userAgent, maxTime)
   userAgent = userAgent or "gen1recomp"
   if HostShell.haveCurl() then
     -- io.popen is one-way on Lua/LuaJIT: its mode is "r" or "w", never
-    -- "rw".  Stage the request body so the response can stay on a read pipe.
-    local bodyPath = os.tmpname()
+    -- "rw".  Stage the request body so the response can stay on a read
+    -- pipe.  The staging directory comes from the OS temp contract, never
+    -- tmpnam(): the CRT's tmpnam() can return a name relative to the process
+    -- working directory, and a game installed under Program Files has no
+    -- writable CWD -- io.open would fail before curl ever runs and postLog
+    -- would silently drop the send.  TEMP/TMP are per-user writable on
+    -- Windows; TMPDIR (with /tmp fallback) covers POSIX.  No love.filesystem:
+    -- the sandbox-era transport stays on plain io/os.
+    local function stagingPath()
+      local dir = os.getenv("TEMP") or os.getenv("TMP")
+      if not dir or dir == "" then dir = os.getenv("TMPDIR") or "/tmp" end
+      local sep = dir:find("\\") and "\\" or "/"
+      return dir .. sep .. ("gen1recomp-post-%d.tmp"):format(
+        (os.time() % 1000000) * 100 + math.random(0, 99))
+    end
+    local bodyPath = stagingPath()
     local bodyFile, bodyOpenErr = io.open(bodyPath, "wb")
     if not bodyFile then
       pcall(os.remove, bodyPath)
diff --git a/tests/engine/host_shell_postlog.lua b/tests/engine/host_shell_postlog.lua
index 2261aec2..a15fb22d 100644
--- a/tests/engine/host_shell_postlog.lua
+++ b/tests/engine/host_shell_postlog.lua
@@ -12,13 +12,13 @@ local check, eq = T.check, T.eq
 local HostShell = require("src.core.HostShell")
 
 local MARK = "\n__gen1recomp_http__"
-local BODY_PATH = "/tmp/gen1recomp-postlog-body-test"
+local STAGE_DIR = "/tmp/gen1recomp-postlog-stage"
 local URL = "https://logs.example.com/logs"
 local BODY = "debug log body\n"
 
 local realOpen = io.open
 local realPopen = io.popen
-local realTmpname = os.tmpname
+local realGetenv = os.getenv
 local realRemove = os.remove
 local realHaveCurl = HostShell.haveCurl
 
@@ -26,7 +26,12 @@ local openedPath, openedMode, writtenBody
 local popenCommand, popenMode, removedPath
 
 HostShell.haveCurl = function() return true end
-os.tmpname = function() return BODY_PATH end
+os.getenv = function(name)
+  if name == "TEMP" or name == "TMP" or name == "TMPDIR" then
+    return STAGE_DIR
+  end
+  return realGetenv(name)
+end
 os.remove = function(path)
   removedPath = path
   return true
@@ -55,21 +60,22 @@ local ok, err = HostShell.httpPost(URL, BODY, "text/plain", "gen1recomp-mod/test
 
 io.open = realOpen
 io.popen = realPopen
-os.tmpname = realTmpname
+os.getenv = realGetenv
 os.remove = realRemove
 HostShell.haveCurl = realHaveCurl
 
 eq(ok, true, "a desktop POST succeeds through the read-only response pipe: " .. tostring(err))
-eq(openedPath, BODY_PATH, "the request body is written to a temporary file")
+check(type(openedPath) == "string" and openedPath:find(STAGE_DIR .. "/gen1recomp-post-", 1, true) == 1, "the request body is staged under the OS temp dir")
+check(openedPath and openedPath:sub(-4) == ".tmp", "the staged body carries a .tmp name")
 eq(openedMode, "wb", "the temporary request body is opened for binary writing")
 eq(writtenBody, BODY, "the complete log body is staged")
 eq(popenMode, "r", "curl is opened in the supported read-only mode")
 check(popenCommand:find("--data-binary", 1, true) ~= nil,
   "curl reads the staged body with --data-binary")
-check(popenCommand:find(BODY_PATH, 1, true) ~= nil,
+check(openedPath and popenCommand:find(openedPath, 1, true) ~= nil,
   "curl receives the temporary body path")
 check(popenCommand:find(BODY, 1, true) == nil,
   "the log body is not placed directly in the command line")
-eq(removedPath, BODY_PATH, "the temporary request body is removed")
+eq(removedPath, openedPath, "the staged request body is removed")
 
 T.finish("host shell postlog")