fix(tui): promote windows terminal diff ansi16 to truecolor

Windows Terminal supports 24-bit color but `supports-color` often
reports only ANSI-16 because `COLORTERM` is not set. Introduce
`diff_color_level_for_terminal`, a pure mapping from detected color
level to intended diff palette, that promotes to truecolor when
Windows Terminal is detected.

Detection uses two independent signals — `TerminalName::WindowsTerminal`
(from `terminal_info()`) and raw `WT_SESSION` presence — so the
promotion fires even if one detection path changes. When `WT_SESSION`
is present, promotion is unconditional (including `Unknown` level);
without it, only ANSI-16 is promoted for named WindowsTerminal
sessions. Setting `FORCE_COLOR` suppresses both paths to preserve
explicit user intent.

All env reads stay in the thin `diff_color_level()` wrapper; the core
function is pure for testability. Six unit tests cover every branch.

codex-rs/core/src/terminal.rs

@@ -249,6 +249,7 @@ pub fn terminal_info() -> TerminalInfo {
 ///   type is split on whitespace to extract a program name plus optional version (for example,
 ///   `ghostty 1.2.3`), while the client term name becomes the `TERM` capability string.
 /// - Otherwise, `TERM_PROGRAM` (plus `TERM_PROGRAM_VERSION`) drives the detected terminal name.
+///   This means `TERM_PROGRAM` can mask later probes (for example `WT_SESSION`).
 /// - Next, terminal-specific variables (WEZTERM, iTerm2, Apple Terminal, kitty, etc.) are checked.
 /// - Finally, `TERM` is used as the capability fallback with `TerminalName::Unknown`.
 ///

codex-rs/tui/src/diff_render.rs

@@ -83,6 +83,8 @@ use crate::terminal_palette::indexed_color;
 use crate::terminal_palette::rgb_color;
 use crate::terminal_palette::stdout_color_level;
 use codex_core::git_info::get_git_repo_root;
+use codex_core::terminal::TerminalName;
+use codex_core::terminal::terminal_info;
 use codex_protocol::protocol::FileChange;
 
 /// Classifies a diff line for gutter sign rendering and style selection.
@@ -110,6 +112,14 @@ enum DiffTheme {
     Light,
 }
 
+/// Palette depth the diff renderer will target.
+///
+/// This is the *renderer's own* notion of color depth, derived from — but not
+/// identical to — the raw [`StdoutColorLevel`] reported by `supports-color`.
+/// The indirection exists because some terminals (notably Windows Terminal)
+/// advertise only ANSI-16 support while actually rendering truecolor sequences
+/// correctly; [`diff_color_level_for_terminal`] promotes those cases so the
+/// diff output uses the richer palette.
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 enum DiffColorLevel {
     TrueColor,
@@ -876,11 +886,75 @@ fn diff_theme() -> DiffTheme {
     diff_theme_for_bg(default_bg())
 }
 
+/// Return the [`DiffColorLevel`] for the current terminal session.
+///
+/// This is the environment-reading adapter: it samples runtime signals
+/// (`supports-color` level, terminal name, `WT_SESSION`, and `FORCE_COLOR`)
+/// and forwards them to [`diff_color_level_for_terminal`].
+///
+/// Keeping env reads in this thin wrapper lets
+/// [`diff_color_level_for_terminal`] stay pure and easy to unit test.
 fn diff_color_level() -> DiffColorLevel {
-    match stdout_color_level() {
+    diff_color_level_for_terminal(
+        stdout_color_level(),
+        terminal_info().name,
+        std::env::var_os("WT_SESSION").is_some(),
+        has_force_color_override(),
+    )
+}
+
+/// Returns whether `FORCE_COLOR` is explicitly set.
+fn has_force_color_override() -> bool {
+    std::env::var_os("FORCE_COLOR").is_some()
+}
+
+/// Map a raw [`StdoutColorLevel`] to a [`DiffColorLevel`], promoting
+/// ANSI-16 to truecolor when running inside Windows Terminal.
+///
+/// This helper is intentionally pure (no env access) so tests can validate
+/// the policy table by passing explicit inputs.
+///
+/// Windows Terminal fully supports 24-bit color but the `supports-color`
+/// crate often reports only ANSI-16 there because no `COLORTERM` variable
+/// is set.  We detect Windows Terminal two ways — via `terminal_name`
+/// (parsed from `WT_SESSION` / `TERM_PROGRAM` by `terminal_info()`) and
+/// via the raw `has_wt_session` flag.
+///
+/// These signals are intentionally not equivalent: `terminal_name` is a
+/// derived classification with `TERM_PROGRAM` precedence, so `WT_SESSION`
+/// can be present while `terminal_name` is not `WindowsTerminal`.
+///
+/// When `WT_SESSION` is present, we promote to truecolor unconditionally
+/// unless `FORCE_COLOR` is set. This keeps Windows Terminal rendering rich
+/// by default while preserving explicit `FORCE_COLOR` user intent.
+///
+/// Outside `WT_SESSION`, only ANSI-16 is promoted for identified
+/// `WindowsTerminal` sessions; `Unknown` stays conservative.
+fn diff_color_level_for_terminal(
+    stdout_level: StdoutColorLevel,
+    terminal_name: TerminalName,
+    has_wt_session: bool,
+    has_force_color_override: bool,
+) -> DiffColorLevel {
+    if has_wt_session && !has_force_color_override {
+        return DiffColorLevel::TrueColor;
+    }
+
+    let base = match stdout_level {
         StdoutColorLevel::TrueColor => DiffColorLevel::TrueColor,
         StdoutColorLevel::Ansi256 => DiffColorLevel::Ansi256,
         StdoutColorLevel::Ansi16 | StdoutColorLevel::Unknown => DiffColorLevel::Ansi16,
+    };
+
+    // Outside `WT_SESSION`, keep the existing Windows Terminal promotion for
+    // ANSI-16 sessions that likely support truecolor.
+    if base == DiffColorLevel::Ansi16
+        && terminal_name == TerminalName::WindowsTerminal
+        && !has_force_color_override
+    {
+        DiffColorLevel::TrueColor
+    } else {
+        base
     }
 }
 
@@ -1608,6 +1682,84 @@ mod tests {
         assert_eq!(lines[1].style.bg, Some(rgb_color(LIGHT_TC_ADD_LINE_BG_RGB)));
     }
 
+    #[test]
+    fn windows_terminal_promotes_ansi16_to_truecolor_for_diffs() {
+        assert_eq!(
+            diff_color_level_for_terminal(
+                StdoutColorLevel::Ansi16,
+                TerminalName::WindowsTerminal,
+                false,
+                false,
+            ),
+            DiffColorLevel::TrueColor
+        );
+    }
+
+    #[test]
+    fn wt_session_promotes_ansi16_to_truecolor_for_diffs() {
+        assert_eq!(
+            diff_color_level_for_terminal(
+                StdoutColorLevel::Ansi16,
+                TerminalName::Unknown,
+                true,
+                false,
+            ),
+            DiffColorLevel::TrueColor
+        );
+    }
+
+    #[test]
+    fn non_windows_terminal_keeps_ansi16_diff_palette() {
+        assert_eq!(
+            diff_color_level_for_terminal(
+                StdoutColorLevel::Ansi16,
+                TerminalName::WezTerm,
+                false,
+                false,
+            ),
+            DiffColorLevel::Ansi16
+        );
+    }
+
+    #[test]
+    fn wt_session_promotes_unknown_color_level_to_truecolor() {
+        assert_eq!(
+            diff_color_level_for_terminal(
+                StdoutColorLevel::Unknown,
+                TerminalName::WindowsTerminal,
+                true,
+                false,
+            ),
+            DiffColorLevel::TrueColor
+        );
+    }
+
+    #[test]
+    fn explicit_force_override_keeps_ansi16_on_windows_terminal() {
+        assert_eq!(
+            diff_color_level_for_terminal(
+                StdoutColorLevel::Ansi16,
+                TerminalName::WindowsTerminal,
+                false,
+                true,
+            ),
+            DiffColorLevel::Ansi16
+        );
+    }
+
+    #[test]
+    fn explicit_force_override_keeps_ansi256_on_windows_terminal() {
+        assert_eq!(
+            diff_color_level_for_terminal(
+                StdoutColorLevel::Ansi256,
+                TerminalName::WindowsTerminal,
+                true,
+                true,
+            ),
+            DiffColorLevel::Ansi256
+        );
+    }
+
     #[test]
     fn add_diff_uses_path_extension_for_highlighting() {
         let mut changes: HashMap<PathBuf, FileChange> = HashMap::new();