diff --git a/src/main.rs b/src/main.rs
index 0469579..7cc4397 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -205,10 +205,41 @@ fn resolve_codex_home(cli_codex_home: Option<PathBuf>) -> Result<PathBuf> {
     resolve_codex_home_from_env(
         cli_codex_home,
         std::env::var_os("CODEX_HOME").map(PathBuf::from),
-        std::env::var_os("HOME").map(PathBuf::from),
+        home_base_from_env(
+            std::env::var_os("USERPROFILE").map(PathBuf::from),
+            std::env::var_os("HOME").map(PathBuf::from),
+            cfg!(windows),
+        ),
     )
 }
 
+/// Resolve the base directory the default `.codex` home hangs off of when
+/// neither `--codex-home` nor `CODEX_HOME` is set.
+///
+/// This mirrors how Codex itself locates its home via `dirs::home_dir()`: on
+/// Windows that resolves from `USERPROFILE` and never consults `HOME`, while on
+/// Unix it uses `HOME`. threadripper historically looked only at `HOME`, so any
+/// process that injects `HOME` (e.g. an app pointing it at `%APPDATA%\<app>`)
+/// sent us to the wrong `.codex` while Codex kept writing under
+/// `%USERPROFILE%\.codex`. Preferring `USERPROFILE` on Windows realigns the two;
+/// `HOME` stays as a fallback there for shells that only set it (Git Bash, MSYS).
+fn home_base_from_env(
+    userprofile: Option<PathBuf>,
+    home: Option<PathBuf>,
+    is_windows: bool,
+) -> Option<PathBuf> {
+    fn non_empty(path: PathBuf) -> Option<PathBuf> {
+        (!path.as_os_str().is_empty()).then_some(path)
+    }
+    if is_windows {
+        userprofile
+            .and_then(non_empty)
+            .or_else(|| home.and_then(non_empty))
+    } else {
+        home.and_then(non_empty)
+    }
+}
+
 fn resolve_codex_home_from_env(
     cli_codex_home: Option<PathBuf>,
     env_codex_home: Option<PathBuf>,
diff --git a/src/output.rs b/src/output.rs
index b2916da..93ec3cd 100644
--- a/src/output.rs
+++ b/src/output.rs
@@ -350,8 +350,12 @@ pub(crate) fn bytes_value_name(locale: Locale) -> &'static str {
 
 pub(crate) fn codex_home_help(locale: Locale) -> &'static str {
     match locale {
-        Locale::En => "Codex home directory. Defaults to CODEX_HOME, then $HOME/.codex.",
-        Locale::ZhHans => "Codex home 目录。默认值依次使用 CODEX_HOME、$HOME/.codex。",
+        Locale::En => {
+            "Codex home directory. Defaults to CODEX_HOME, then <home>/.codex (USERPROFILE on Windows, else HOME)."
+        }
+        Locale::ZhHans => {
+            "Codex home 目录。默认依次使用 CODEX_HOME、家目录下的 .codex(Windows 取 USERPROFILE,其余取 HOME)。"
+        }
     }
 }
 
diff --git a/src/tests.rs b/src/tests.rs
index 80660bb..d3bc53a 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -265,6 +265,65 @@ fn resolves_codex_home_from_cli_then_env_then_home() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn home_base_prefers_userprofile_on_windows() -> Result<()> {
+    // Windows: USERPROFILE wins and HOME is ignored even when both are set —
+    // this is the reported regression (HOME hijacked to %APPDATA%\<app>).
+    assert_eq!(
+        crate::home_base_from_env(
+            Some(PathBuf::from(r"C:\Users\alice")),
+            Some(PathBuf::from(r"C:\Users\alice\AppData\Roaming\SPB_16.6")),
+            true,
+        ),
+        Some(PathBuf::from(r"C:\Users\alice")),
+    );
+    // Windows: fall back to HOME when USERPROFILE is missing or empty.
+    assert_eq!(
+        crate::home_base_from_env(None, Some(PathBuf::from(r"C:\home")), true),
+        Some(PathBuf::from(r"C:\home")),
+    );
+    assert_eq!(
+        crate::home_base_from_env(Some(PathBuf::new()), Some(PathBuf::from(r"C:\home")), true),
+        Some(PathBuf::from(r"C:\home")),
+    );
+    // Windows: nothing usable -> None (caller then defaults to ".").
+    assert_eq!(crate::home_base_from_env(None, None, true), None);
+    assert_eq!(
+        crate::home_base_from_env(Some(PathBuf::new()), None, true),
+        None,
+    );
+
+    // Non-Windows: HOME is authoritative and USERPROFILE is ignored.
+    assert_eq!(
+        crate::home_base_from_env(
+            Some(PathBuf::from("/should/ignore")),
+            Some(PathBuf::from("/home/alice")),
+            false,
+        ),
+        Some(PathBuf::from("/home/alice")),
+    );
+    assert_eq!(
+        crate::home_base_from_env(Some(PathBuf::from("/should/ignore")), None, false),
+        None,
+    );
+
+    // End-to-end: with HOME hijacked but USERPROFILE intact, the default Codex
+    // home still resolves under the real user profile, not the app dir.
+    assert_eq!(
+        crate::resolve_codex_home_from_env(
+            None,
+            None,
+            crate::home_base_from_env(
+                Some(PathBuf::from(r"C:\Users\alice")),
+                Some(PathBuf::from(r"C:\Users\alice\AppData\Roaming\SPB_16.6")),
+                true,
+            ),
+        )?,
+        PathBuf::from(r"C:\Users\alice").join(".codex"),
+    );
+    Ok(())
+}
+
 #[test]
 fn resolves_sqlite_path_from_config_sqlite_home() -> Result<()> {
     let dir = tempfile::tempdir()?;