diff --git a/crates/ghostty-apprt-gtk/examples/demo.rs b/crates/ghostty-apprt-gtk/examples/demo.rs
index aae6fa9..5e453f5 100644
--- a/crates/ghostty-apprt-gtk/examples/demo.rs
+++ b/crates/ghostty-apprt-gtk/examples/demo.rs
@@ -203,7 +203,7 @@ impl CoreSurface for DemoSurface {
     fn complete_clipboard_request(
         &self,
         _req: ClipboardRequest,
-        data: &str,
+        data: &[u8],
         _confirmed: bool,
     ) -> Result<(), ClipboardError> {
         log::info!("demo: paste of {} bytes", data.len());
diff --git a/crates/ghostty-apprt-gtk/src/class/application.rs b/crates/ghostty-apprt-gtk/src/class/application.rs
index 5a93169..d436a8f 100644
--- a/crates/ghostty-apprt-gtk/src/class/application.rs
+++ b/crates/ghostty-apprt-gtk/src/class/application.rs
@@ -87,11 +87,11 @@ pub struct WindowOverrides {
 
 /// Set an environment variable.
 #[allow(unsafe_code)]
-fn set_env(key: &str, value: &str) {
+fn set_env(key: &str, value: impl AsRef<std::ffi::OsStr>) {
     // SAFETY: only called during application construction on the main
     // thread, before GTK is initialized and before we spawn any child
     // processes; no other thread in this crate reads the environment.
-    unsafe { std::env::set_var(key, value) }
+    unsafe { std::env::set_var(key, value.as_ref()) }
 }
 
 /// Funnel GLib/GObject/GTK log messages into the `log` crate rather than
@@ -284,7 +284,7 @@ mod imp {
         pub css: RefCell<Option<CssProviders>>,
 
         /// Copy of the LANG variable provided by the system, if any.
-        pub saved_language: RefCell<Option<String>>,
+        pub saved_language: RefCell<Option<std::ffi::OsString>>,
     }
 
     #[glib::object_subclass]
@@ -398,9 +398,13 @@ impl Application {
             }
         };
 
-        let saved_language = std::env::var("LANG").ok();
+        // Keep the inherited bytes: LANG need not be UTF-8.
+        let saved_language = std::env::var_os("LANG");
         if let Some(language) = &config.language {
-            set_env("LANG", language);
+            set_env(
+                "LANG",
+                ghostty_os::osbytes::EncodedOs::from_encoded(language.clone()).to_os(),
+            );
         }
 
         // Set gettext global domain to be our app so that our unqualified
@@ -560,7 +564,7 @@ impl Application {
     }
 
     /// The original `LANG` Ghostty was launched with (None if unset).
-    pub fn saved_language(&self) -> Option<String> {
+    pub fn saved_language(&self) -> Option<std::ffi::OsString> {
         self.imp().saved_language.borrow().clone()
     }
 
diff --git a/crates/ghostty-apprt-gtk/src/class/command_palette.rs b/crates/ghostty-apprt-gtk/src/class/command_palette.rs
index c6b7ec6..2b4b115 100644
--- a/crates/ghostty-apprt-gtk/src/class/command_palette.rs
+++ b/crates/ghostty-apprt-gtk/src/class/command_palette.rs
@@ -110,7 +110,9 @@ mod command_imp {
         fn prop_get_title(&self) -> Option<String> {
             let mut data = self.data.borrow_mut();
             match &mut *data {
-                CommandData::Regular { command } => Some(command.title.clone()),
+                CommandData::Regular { command } => {
+                    Some(ghostty_os::osbytes::config_str_to_text(&command.title))
+                }
                 CommandData::Jump { surface, title, .. } => {
                     if let Some(t) = title {
                         return Some(t.clone());
@@ -125,7 +127,9 @@ mod command_imp {
         fn prop_get_description(&self) -> Option<String> {
             let mut data = self.data.borrow_mut();
             match &mut *data {
-                CommandData::Regular { command } => Some(command.description.clone()),
+                CommandData::Regular { command } => Some(ghostty_os::osbytes::config_str_to_text(
+                    &command.description,
+                )),
                 CommandData::Jump {
                     surface,
                     description,
diff --git a/crates/ghostty-apprt-gtk/src/class/surface.rs b/crates/ghostty-apprt-gtk/src/class/surface.rs
index 3aa6382..da26c85 100644
--- a/crates/ghostty-apprt-gtk/src/class/surface.rs
+++ b/crates/ghostty-apprt-gtk/src/class/surface.rs
@@ -180,6 +180,16 @@ impl Default for Surface {
 // ---------------------------------------------------------------------
 // Construction / lookup
 
+/// Restore the `LANG` the application was launched with (as bytes; it need
+/// not be UTF-8), or remove it if it was unset.
+fn restore_language(env: &mut ghostty_os::EnvMap, saved: Option<&std::ffi::OsStr>) {
+    if let Some(language) = saved {
+        env.insert("LANG", language);
+    } else {
+        env.remove("LANG");
+    }
+}
+
 impl Surface {
     /// Create a new surface widget (Zig: `Surface.new`).
     pub fn new(overrides: SurfaceOverrides) -> Self {
@@ -1441,11 +1451,7 @@ impl Surface {
         // Byte-preserving environment (Zig `internal_os.getEnvMap`).
         let mut env = ghostty_os::env::get_env_map();
 
-        if let Some(language) = application.saved_language() {
-            env.insert("LANG", language);
-        } else {
-            env.remove("LANG");
-        }
+        restore_language(&mut env, application.saved_language().as_deref());
 
         // Don't leak these GTK environment variables to child processes.
         env.remove("GDK_DEBUG");
@@ -2610,7 +2616,7 @@ mod clipboard {
             };
 
             let Some(core) = this.core() else { return };
-            match core.complete_clipboard_request(state, &text, false) {
+            match core.complete_clipboard_request(state, text.as_bytes(), false) {
                 Ok(()) => {}
                 Err(ClipboardError::UnsafePaste | ClipboardError::UnauthorizedPaste) => {
                     show_clipboard_confirmation(&this, state, &text, false);
@@ -2636,7 +2642,7 @@ mod clipboard {
         }
 
         let Some(core) = surface.core() else { return };
-        match core.complete_clipboard_request(ClipboardRequest::Paste, text, false) {
+        match core.complete_clipboard_request(ClipboardRequest::Paste, text.as_bytes(), false) {
             Ok(()) => {}
             Err(ClipboardError::UnsafePaste | ClipboardError::UnauthorizedPaste) => {
                 show_clipboard_confirmation(surface, ClipboardRequest::Paste, text, false);
@@ -2725,7 +2731,7 @@ mod clipboard {
                 // Undo the byte escape only for non-UTF-8 pastes.
                 core.complete_paste_bytes(&confirmed_paste_bytes(&text, encoded), true)
             } else {
-                core.complete_clipboard_request(req, &text, true)
+                core.complete_clipboard_request(req, text.as_bytes(), true)
             };
             if let Err(err) = result {
                 log::warn!("failed to complete clipboard request: {err}");
@@ -3379,7 +3385,11 @@ mod imp {
         fn search_changed(_overlay: &SearchOverlay, needle: Option<String>, this: &super::Surface) {
             let Some(surface) = this.core() else { return };
             if let Err(err) = surface.perform_binding_action(
-                &ghostty_input::binding::Action::Search(needle.unwrap_or_default()),
+                // Binding payloads are config strings: the needle is plain
+                // UI text, so escape it instead of decoding it.
+                &ghostty_input::binding::Action::Search(
+                    ghostty_os::osbytes::valid_str_to_config_string(&needle.unwrap_or_default()),
+                ),
             ) {
                 log::warn!("unable to perform search action err={err}");
             }
@@ -3908,6 +3918,20 @@ mod imp {
 mod tests {
     use super::*;
 
+    #[test]
+    #[allow(clippy::unwrap_used)]
+    fn inherited_lang_keeps_non_utf8_bytes() {
+        use std::os::unix::ffi::{OsStrExt, OsStringExt};
+
+        let mut env = ghostty_os::EnvMap::new();
+        env.insert("LANG", "zz");
+        let saved = std::ffi::OsString::from_vec(b"C\xff".to_vec());
+        restore_language(&mut env, Some(&saved));
+        assert_eq!(env.get("LANG").unwrap().as_bytes(), b"C\xff");
+        restore_language(&mut env, None);
+        assert!(env.get("LANG").is_none());
+    }
+
     #[test]
     fn confirmed_paste_decodes_only_encoded_text() {
         // Ordinary text containing the escape char + U+00FF stays intact.
diff --git a/crates/ghostty-apprt-gtk/src/class/window.rs b/crates/ghostty-apprt-gtk/src/class/window.rs
index d6876fe..8125f3a 100644
--- a/crates/ghostty-apprt-gtk/src/class/window.rs
+++ b/crates/ghostty-apprt-gtk/src/class/window.rs
@@ -313,7 +313,7 @@ mod imp {
                 // so that any applications inspecting the window states see
                 // an immediate title set when the window appears.
                 if let Some(title) = &config.title {
-                    obj.set_title(Some(title));
+                    obj.set_title(Some(&ghostty_os::osbytes::config_str_to_text(title)));
                 }
             }
 
diff --git a/crates/ghostty-apprt-gtk/src/css.rs b/crates/ghostty-apprt-gtk/src/css.rs
index 30055ad..73dfdba 100644
--- a/crates/ghostty-apprt-gtk/src/css.rs
+++ b/crates/ghostty-apprt-gtk/src/css.rs
@@ -215,6 +215,7 @@ pub fn runtime_css(config: &Config, gtk_416: bool) -> String {
     }
 
     if let Some(family) = &config.window_title_font_family {
+        let family = ghostty_os::osbytes::config_str_to_text(family);
         let _ = write!(
             out,
             ".window headerbar {{\n  font-family: \"{family}\";\n}}\n"
diff --git a/crates/ghostty-apprt-gtk/src/winproto/wayland.rs b/crates/ghostty-apprt-gtk/src/winproto/wayland.rs
index df1ac1f..f5786f1 100644
--- a/crates/ghostty-apprt-gtk/src/winproto/wayland.rs
+++ b/crates/ghostty-apprt-gtk/src/winproto/wayland.rs
@@ -524,7 +524,9 @@ impl Window {
             QuickTerminalLayer::Bottom => layer_shell::Layer::Bottom,
             QuickTerminalLayer::Background => layer_shell::Layer::Background,
         });
-        window.set_namespace(Some(config.gtk_quick_terminal_namespace.as_str()));
+        window.set_namespace(Some(
+            ghostty_os::osbytes::config_str_to_text(&config.gtk_quick_terminal_namespace).as_str(),
+        ));
 
         window.set_keyboard_mode(match config.quick_terminal_keyboard_interactivity {
                 QuickTerminalKeyboardInteractivity::None => layer_shell::KeyboardMode::None,
diff --git a/crates/ghostty-apprt/src/runtime_api.rs b/crates/ghostty-apprt/src/runtime_api.rs
index abbf9e7..4cba831 100644
--- a/crates/ghostty-apprt/src/runtime_api.rs
+++ b/crates/ghostty-apprt/src/runtime_api.rs
@@ -198,17 +198,13 @@ pub trait CoreSurface {
     fn complete_clipboard_request(
         &self,
         req: ClipboardRequest,
-        data: &str,
+        data: &[u8],
         confirmed: bool,
     ) -> Result<(), ClipboardError>;
     /// [`CoreSurface::complete_clipboard_request`] for a paste of raw bytes
-    /// (Zig data is `[]const u8`). The default converts lossily.
+    /// (Zig data is `[]const u8`).
     fn complete_paste_bytes(&self, data: &[u8], confirmed: bool) -> Result<(), ClipboardError> {
-        self.complete_clipboard_request(
-            ClipboardRequest::Paste,
-            &String::from_utf8_lossy(data),
-            confirmed,
-        )
+        self.complete_clipboard_request(ClipboardRequest::Paste, data, confirmed)
     }
     /// After the user confirms (`surface.config.clipboard_read = .allow`).
     fn set_clipboard_read_allowed(&self, allow: bool);
diff --git a/crates/ghostty-cli/src/show_face.rs b/crates/ghostty-cli/src/show_face.rs
index 2caa5d1..27c9499 100644
--- a/crates/ghostty-cli/src/show_face.rs
+++ b/crates/ghostty-cli/src/show_face.rs
@@ -100,7 +100,9 @@ fn run_args(
             continue;
         }
         if FIELDS.contains(&d.key.as_str()) {
-            writeln!(stderr, "config error: {d}")?;
+            stderr.write_all(&ghostty_os::osbytes::config_str_to_bytes(&format!(
+                "config error: {d}\n"
+            )))?;
             exit = true;
         }
     }
@@ -133,7 +135,9 @@ fn run_args(
         {
             continue;
         }
-        writeln!(stderr, "config error: {d}")?;
+        stderr.write_all(&ghostty_os::osbytes::config_str_to_bytes(&format!(
+            "config error: {d}\n"
+        )))?;
     }
 
     let font_grid_set = match SharedGridSet::new() {
diff --git a/crates/ghostty-cli/src/validate_config.rs b/crates/ghostty-cli/src/validate_config.rs
index b10e18f..a9265b3 100644
--- a/crates/ghostty-cli/src/validate_config.rs
+++ b/crates/ghostty-cli/src/validate_config.rs
@@ -53,7 +53,11 @@ fn run_inner(opts: &Options, stdout: &mut dyn Write) -> Result<u8, Error> {
 
     if !cfg.diagnostics.items().is_empty() {
         for diag in cfg.diagnostics.items() {
-            writeln!(stdout, "{diag}")?;
+            // Diagnostics embed config values in the reversible byte
+            // encoding; print the original bytes.
+            stdout.write_all(&ghostty_os::osbytes::config_str_to_bytes(&format!(
+                "{diag}\n"
+            )))?;
         }
         return Ok(1);
     }
@@ -105,6 +109,27 @@ mod tests {
         );
     }
 
+    #[cfg(unix)]
+    #[test]
+    fn diagnostics_print_original_bytes() {
+        use std::os::unix::ffi::OsStrExt;
+        let tmp = tempfile::tempdir().unwrap();
+        let path = tmp.path().join("config");
+        std::fs::write(&path, b"bad\xffkey = 1\n").unwrap();
+        let opts = Options {
+            config_file: Some(path.to_string_lossy().into_owned()),
+        };
+        let mut out = Vec::new();
+        assert_eq!(1, run_inner(&opts, &mut out).unwrap());
+        assert!(
+            out.windows(7).any(|w| w == b"bad\xffkey"),
+            "{:?}",
+            String::from_utf8_lossy(&out)
+        );
+        assert!(!out.contains(&0xef), "no escape encoding in the output");
+        let _ = path.as_os_str().as_bytes();
+    }
+
     #[test]
     fn validate_missing_file_is_error() {
         let opts = Options {
diff --git a/crates/ghostty-config/src/io.rs b/crates/ghostty-config/src/io.rs
index 11c33ab..97ea38b 100644
--- a/crates/ghostty-config/src/io.rs
+++ b/crates/ghostty-config/src/io.rs
@@ -2,6 +2,8 @@
 
 use std::fmt;
 
+use ghostty_os::osbytes::EncodedOs;
+
 use crate::{
     cli_args::{ArgError, ConfigField, split_tagged_union},
     formatter::EntryFormatter,
@@ -16,8 +18,11 @@ use crate::{
 /// might be produced in an intermediate state.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum ReadableIO {
+    /// Raw bytes in the reversible OS-byte encoding (decode with
+    /// `osbytes::config_str_to_bytes` after [`ReadableIO::clone_parsed`]).
     Raw(String),
-    Path(String),
+    /// A file path (decode with `EncodedOs::to_path` before opening).
+    Path(EncodedOs),
 }
 
 impl Default for ReadableIO {
@@ -34,7 +39,9 @@ impl ReadableIO {
         // Parsing can't fail because we validate it in parse_cli
         match self {
             ReadableIO::Raw(v) => ReadableIO::Raw(string::parse(v).unwrap_or_default()),
-            ReadableIO::Path(v) => ReadableIO::Path(string::parse(v).unwrap_or_default()),
+            ReadableIO::Path(v) => ReadableIO::Path(EncodedOs::from_encoded(
+                string::parse(v.as_encoded()).unwrap_or_default(),
+            )),
         }
     }
 }
@@ -55,7 +62,7 @@ impl ConfigField for ReadableIO {
         let (tag, rest) = split_tagged_union(input);
         *self = match tag {
             "raw" => ReadableIO::Raw(rest.to_string()),
-            "path" => ReadableIO::Path(rest.to_string()),
+            "path" => ReadableIO::Path(EncodedOs::from_encoded(rest)),
             _ => ReadableIO::Raw(input.to_string()),
         };
         Ok(())
@@ -63,8 +70,8 @@ impl ConfigField for ReadableIO {
 
     fn format_entry(&self, f: &mut EntryFormatter<'_>) -> fmt::Result {
         let (tag, v) = match self {
-            ReadableIO::Raw(v) => ("raw", v),
-            ReadableIO::Path(v) => ("path", v),
+            ReadableIO::Raw(v) => ("raw", v.as_str()),
+            ReadableIO::Path(v) => ("path", v.as_encoded()),
         };
         writeln!(f.writer, "{} = {tag}:{v}", f.name)
     }
@@ -126,7 +133,18 @@ mod tests {
         io.parse_cli(Some("raw:foo")).unwrap();
         assert_eq!(io, ReadableIO::Raw("foo".into()));
         io.parse_cli(Some("path:foo")).unwrap();
-        assert_eq!(io, ReadableIO::Path("foo".into()));
+        assert_eq!(io, ReadableIO::Path(EncodedOs::from_plain_text("foo")));
+    }
+
+    #[test]
+    fn readable_io_path_decodes_to_the_original_bytes() {
+        let mut io = ReadableIO::default();
+        let enc = EncodedOs::from_bytes(b"/tmp/\xffin").into_encoded();
+        io.parse_cli(Some(&format!("path:{enc}"))).unwrap();
+        let ReadableIO::Path(p) = io.clone_parsed() else {
+            panic!("path expected")
+        };
+        assert_eq!(p.to_bytes(), b"/tmp/\xffin");
     }
 
     #[test]
diff --git a/crates/ghostty-config/src/string.rs b/crates/ghostty-config/src/string.rs
index 6944fa0..f746bcc 100644
--- a/crates/ghostty-config/src/string.rs
+++ b/crates/ghostty-config/src/string.rs
@@ -75,8 +75,21 @@ fn parse_escape_sequence(bytes: &[u8], i: &mut usize) -> Option<u32> {
     Some(cp)
 }
 
+/// Parse a string literal into the original bytes (Zig `parse`'s output).
+/// `input` is a config string in the reversible OS-byte encoding; the result
+/// is decoded, so it can be written to the pty / OS directly.
+pub fn parse_bytes(input: &str) -> Result<Vec<u8>, StringError> {
+    parse(input).map(|s| ghostty_os::osbytes::config_str_to_bytes(&s))
+}
+
 /// Parse a string literal into a string. The string can contain
 /// any valid Zig string literal escape sequences.
+///
+/// The input and output are in the reversible OS-byte encoding (see
+/// `ghostty_os::osbytes`): bytes are copied through unchanged, and a
+/// code point produced by an escape is encoded as plain text, so a
+/// literal `U+FDD0` from `\u{fdd0}` stays distinct from the escape
+/// character. Use [`parse_bytes`] for the raw bytes.
 pub fn parse(bytes: &str) -> Result<String, StringError> {
     let b = bytes.as_bytes();
     let mut out = String::with_capacity(bytes.len());
@@ -95,7 +108,12 @@ pub fn parse(bytes: &str) -> Result<String, StringError> {
 
         let cp = parse_escape_sequence(b, &mut i).ok_or(StringError::InvalidString)?;
         // Surrogates cannot be encoded.
-        out.push(char::from_u32(cp).ok_or(StringError::InvalidString)?);
+        let ch = char::from_u32(cp).ok_or(StringError::InvalidString)?;
+        if ch == '\u{FDD0}' {
+            out.push_str("\u{FDD0}\u{FDD0}");
+        } else {
+            out.push(ch);
+        }
     }
     Ok(out)
 }
@@ -138,6 +156,24 @@ impl CodepointIterator<'_> {
 #[cfg(test)]
 #[allow(clippy::unwrap_used)]
 mod tests {
+    #[test]
+    fn parse_bytes_decodes_the_reversible_encoding() {
+        use ghostty_os::osbytes::EncodedOs;
+
+        use super::{parse, parse_bytes};
+
+        // Raw non-UTF-8 bytes pass through as the same bytes.
+        let enc = EncodedOs::from_bytes(b"a\xffb\\n").into_encoded();
+        assert_eq!(parse_bytes(&enc).unwrap(), b"a\xffb\n");
+        // A literal U+FDD0 (typed or via an escape) is that character.
+        let lit = EncodedOs::from_plain_text("\u{fdd0}").into_encoded();
+        assert_eq!(parse_bytes(&lit).unwrap(), "\u{fdd0}".as_bytes());
+        assert_eq!(parse_bytes("\\u{fdd0}").unwrap(), "\u{fdd0}".as_bytes());
+        // An escape producing U+FF is the UTF-8 encoding, not byte FF.
+        assert_eq!(parse_bytes("\\xff").unwrap(), "\u{ff}".as_bytes());
+        assert!(parse("\\u{d800}").is_err());
+    }
+
     use super::*;
 
     #[test]
diff --git a/crates/ghostty-core/src/surface/binding.rs b/crates/ghostty-core/src/surface/binding.rs
index 0493442..754b982 100644
--- a/crates/ghostty-core/src/surface/binding.rs
+++ b/crates/ghostty-core/src/surface/binding.rs
@@ -713,30 +713,35 @@ impl Surface {
                 // We need to send the CSI/ESC sequence as a single write
                 // request. If you split it across two then the shell can
                 // interpret it as two literals.
-                let full = if matches!(action, Action::Csi(_)) {
-                    format!("\x1b[{data}")
+                let mut full: Vec<u8> = if matches!(action, Action::Csi(_)) {
+                    b"\x1b[".to_vec()
                 } else {
-                    format!("\x1b{data}")
+                    b"\x1b".to_vec()
                 };
+                // The payload is a config string in the reversible OS-byte
+                // encoding: the pty gets the original bytes.
+                full.extend(ghostty_os::osbytes::config_str_to_bytes(data));
                 // Zig formats into a fixed 128 byte buffer (`NoSpaceLeft`).
                 if full.len() > 128 {
                     return Err("csi/esc binding sequence too long (max 128 bytes)".into());
                 }
-                self.queue_io(IoMessage::write_req(full.as_bytes()));
+                self.queue_io(IoMessage::write_req(full.as_slice()));
 
                 // CSI/ESC triggers a scroll.
                 self.scroll_to_bottom(&mut self.lock_terminal());
             }
 
             Action::Text(data) => {
-                let text = match ghostty_config::string::parse(data) {
+                // Decoded bytes: escapes produce UTF-8, everything else is
+                // copied byte for byte (Zig `config/string.zig`).
+                let text = match ghostty_config::string::parse_bytes(data) {
                     Ok(t) => t,
                     Err(err) => {
                         log::warn!("error parsing text binding text={data} err={err:?}");
                         return Ok(true);
                     }
                 };
-                self.queue_io(IoMessage::write_req(text.as_bytes()));
+                self.queue_io(IoMessage::write_req(text.as_slice()));
 
                 // Text triggers a scroll.
                 self.scroll_to_bottom(&mut self.lock_terminal());
@@ -817,7 +822,11 @@ impl Surface {
                     }
                 } else if let Some(s) = self.search.borrow().as_ref() {
                     s.handle
-                        .send(SearchMessage::ChangeNeedle(text.as_bytes().to_vec()));
+                        // The payload is a config string in the reversible
+                        // encoding: search for the original bytes.
+                        .send(SearchMessage::ChangeNeedle(
+                            ghostty_os::osbytes::config_str_to_bytes(text),
+                        ));
                     s.handle.wakeup();
                 }
             }
@@ -925,11 +934,15 @@ impl Surface {
             }
 
             Action::SetSurfaceTitle(v) => {
-                return Ok(self.perform(RtAction::SetTitle(SetTitle { title: v.clone() })));
+                return Ok(self.perform(RtAction::SetTitle(SetTitle {
+                    title: ghostty_os::osbytes::config_str_to_text(v),
+                })));
             }
 
             Action::SetTabTitle(v) => {
-                return Ok(self.perform(RtAction::SetTabTitle(SetTitle { title: v.clone() })));
+                return Ok(self.perform(RtAction::SetTabTitle(SetTitle {
+                    title: ghostty_os::osbytes::config_str_to_text(v),
+                })));
             }
 
             Action::ClearScreen => {
diff --git a/crates/ghostty-core/src/surface/clipboard.rs b/crates/ghostty-core/src/surface/clipboard.rs
index 82abc7a..6347e42 100644
--- a/crates/ghostty-core/src/surface/clipboard.rs
+++ b/crates/ghostty-core/src/surface/clipboard.rs
@@ -88,8 +88,9 @@ impl Surface {
 
         // The options we'll use for all formatting. We'll just override the
         // emit format.
-        let codepoint_map: Vec<formatter::CodepointMap> =
-            cfg.clipboard_codepoint_map.map.list.clone();
+        // Replacement strings are config strings (reversible byte encoding);
+        // the formatter produces text, so decode them to UTF-8 text first.
+        let codepoint_map = decode_codepoint_map(&cfg.clipboard_codepoint_map.map.list);
         let opts = formatter::Options {
             emit: Format::Plain, // We'll override this below
             unwrap: true,
@@ -195,20 +196,20 @@ impl Surface {
     pub fn complete_clipboard_request(
         &self,
         req: ClipboardRequest,
-        data: &str,
+        data: &[u8],
         confirmed: bool,
     ) -> Result<(), ClipboardError> {
         match req {
-            ClipboardRequest::Paste => self.complete_clipboard_paste(data.as_bytes(), confirmed),
+            ClipboardRequest::Paste => self.complete_clipboard_paste(data, confirmed),
 
             ClipboardRequest::Osc52Read(clipboard) => {
-                self.complete_clipboard_read_osc52(data.as_bytes(), clipboard, confirmed)
+                self.complete_clipboard_read_osc52(data, clipboard, confirmed)
             }
 
             ClipboardRequest::Osc52Write(clipboard) => {
                 self.rt_surface.set_clipboard(
                     clipboard,
-                    &[("text/plain".to_owned(), data.as_bytes().to_vec())],
+                    &[("text/plain".to_owned(), data.to_vec())],
                     !confirmed,
                 );
                 Ok(())
@@ -407,3 +408,40 @@ impl Surface {
         self.complete_clipboard_paste(text, true)
     }
 }
+
+/// The clipboard codepoint map with string replacements decoded from the
+/// config encoding (a literal `U+FDD0` is restored; bytes that are not
+/// UTF-8 can't be represented in the formatter's text output).
+fn decode_codepoint_map(list: &[formatter::CodepointMap]) -> Vec<formatter::CodepointMap> {
+    list.iter()
+        .map(|e| formatter::CodepointMap {
+            range: e.range,
+            replacement: match &e.replacement {
+                formatter::Replacement::String(s) => {
+                    formatter::Replacement::String(ghostty_os::osbytes::config_str_to_text(s))
+                }
+                r @ formatter::Replacement::Codepoint(_) => r.clone(),
+            },
+        })
+        .collect()
+}
+
+#[cfg(test)]
+mod decode_map_tests {
+    use super::*;
+
+    #[test]
+    fn string_replacements_are_decoded() {
+        let enc = ghostty_os::osbytes::valid_str_to_config_string("a\u{fdd0}");
+        assert_ne!(enc, "a\u{fdd0}");
+        let list = [formatter::CodepointMap {
+            range: [1, 2],
+            replacement: formatter::Replacement::String(enc),
+        }];
+        let out = decode_codepoint_map(&list);
+        assert_eq!(
+            out[0].replacement,
+            formatter::Replacement::String("a\u{fdd0}".to_owned())
+        );
+    }
+}
diff --git a/crates/ghostty-core/src/surface/config.rs b/crates/ghostty-core/src/surface/config.rs
index 695998c..17bd9dc 100644
--- a/crates/ghostty-core/src/surface/config.rs
+++ b/crates/ghostty-core/src/surface/config.rs
@@ -55,7 +55,7 @@ impl DerivedConfig {
         let mut links = Vec::with_capacity(config.link.links.len());
         for link in &config.link.links {
             links.push(Link {
-                regex: Regex::new(&link.regex)?,
+                regex: Regex::new(&ghostty_os::osbytes::config_str_to_text(&link.regex))?,
                 action: link.action,
                 highlight: link.highlight,
             });
diff --git a/crates/ghostty-core/src/surface/mod.rs b/crates/ghostty-core/src/surface/mod.rs
index 0dc7712..785cc8e 100644
--- a/crates/ghostty-core/src/surface/mod.rs
+++ b/crates/ghostty-core/src/surface/mod.rs
@@ -459,7 +459,10 @@ impl Surface {
             // don't leak GHOSTTY_LOG to any subprocesses
             env.remove("GHOSTTY_LOG");
 
-            let mut ecfg = exec::Config::new(env, config.term.clone());
+            let mut ecfg = exec::Config::new(
+                env,
+                ghostty_os::osbytes::EncodedOs::from_encoded(config.term.clone()),
+            );
             ecfg.command = command.clone();
             ecfg.env_override = config.env.clone();
             ecfg.shell_integration = config.shell_integration;
@@ -604,7 +607,7 @@ impl Surface {
             rt_app.perform_action(
                 Target::Surface(id),
                 RtAction::SetTitle(SetTitle {
-                    title: title.clone(),
+                    title: ghostty_os::osbytes::config_str_to_text(title),
                 }),
             );
         } else if config.xdg_terminal_exec
@@ -617,7 +620,7 @@ impl Surface {
             rt_app.perform_action(
                 Target::Surface(id),
                 RtAction::SetTitle(SetTitle {
-                    title: cmd.string(),
+                    title: ghostty_os::osbytes::config_str_to_text(&cmd.string()),
                 }),
             );
         } else if let Some(cmd) = &command {
@@ -633,7 +636,7 @@ impl Surface {
                 rt_app.perform_action(
                     Target::Surface(id),
                     RtAction::SetTitle(SetTitle {
-                        title: first.clone(),
+                        title: ghostty_os::osbytes::config_str_to_text(first),
                     }),
                 );
             }
@@ -1303,7 +1306,7 @@ impl Surface {
         // configured title.
         if let Some(title) = &config.title {
             self.perform(RtAction::SetTitle(SetTitle {
-                title: title.clone(),
+                title: ghostty_os::osbytes::config_str_to_text(title),
             }));
         }
 
@@ -2062,7 +2065,7 @@ impl CoreSurface for Surface {
     fn complete_clipboard_request(
         &self,
         req: ClipboardRequest,
-        data: &str,
+        data: &[u8],
         confirmed: bool,
     ) -> Result<(), ClipboardError> {
         Surface::complete_clipboard_request(self, req, data, confirmed)
diff --git a/crates/ghostty-core/tests/common/mod.rs b/crates/ghostty-core/tests/common/mod.rs
index b737df9..de42da7 100644
--- a/crates/ghostty-core/tests/common/mod.rs
+++ b/crates/ghostty-core/tests/common/mod.rs
@@ -326,7 +326,10 @@ impl Harness {
                 .get(&cb_key(clipboard))
                 .cloned()
                 .unwrap_or_default();
-            match self.surface.complete_clipboard_request(req, &text, false) {
+            match self
+                .surface
+                .complete_clipboard_request(req, text.as_bytes(), false)
+            {
                 Ok(()) => {}
                 Err(ClipboardError::UnsafePaste | ClipboardError::UnauthorizedPaste) => {
                     self.rt
diff --git a/crates/ghostty-core/tests/e2e_bindings.rs b/crates/ghostty-core/tests/e2e_bindings.rs
index b0ab596..b886d4f 100644
--- a/crates/ghostty-core/tests/e2e_bindings.rs
+++ b/crates/ghostty-core/tests/e2e_bindings.rs
@@ -111,7 +111,7 @@ fn unbracketed_paste_converts_newlines_and_requires_confirmation_when_unsafe() {
     // Confirm it.
     let c = h.rt.confirmations.borrow()[0].clone();
     h.surface
-        .complete_clipboard_request(c.request, &c.text, true)
+        .complete_clipboard_request(c.request, c.text.as_bytes(), true)
         .unwrap();
     h.expect_input(b"rm -rf x\rls");
 }
@@ -367,6 +367,31 @@ fn text_csi_esc_and_ignore_bindings() {
     h.expect_input(b"hi!\x1b[1;5H\x1bx");
 }
 
+#[test]
+fn config_loaded_text_csi_esc_write_decoded_bytes() {
+    use ghostty_os::osbytes::EncodedOs;
+
+    let h = harness!(Harness::recorder("", &[]));
+    let enc = |raw: &[u8]| EncodedOs::from_bytes(raw).into_encoded();
+    // Byte FF and a literal U+FDD0 must reach the pty unchanged, not as
+    // the reversible escape encoding.
+    for (kind, raw, want) in [
+        ("text", &b"a\xffb"[..], b"a\xffb".to_vec()),
+        (
+            "text",
+            "\u{fdd0}".as_bytes(),
+            "\u{fdd0}".as_bytes().to_vec(),
+        ),
+        ("csi", &b"\xff"[..], b"\x1b[\xff".to_vec()),
+        ("esc", "\u{fdd0}".as_bytes(), b"\x1b\xef\xb7\x90".to_vec()),
+    ] {
+        h.mark();
+        let a = ghostty_input::binding::Action::parse(&format!("{kind}:{}", enc(raw))).unwrap();
+        assert!(h.surface.perform_binding_action(&a).unwrap());
+        h.expect_input(&want);
+    }
+}
+
 #[test]
 fn csi_binding_longer_than_the_zig_buffer_is_rejected() {
     let h = harness!(Harness::recorder("", &[]));
diff --git a/crates/ghostty-core/tests/e2e_term.rs b/crates/ghostty-core/tests/e2e_term.rs
index b6a0c5f..2db46c8 100644
--- a/crates/ghostty-core/tests/e2e_term.rs
+++ b/crates/ghostty-core/tests/e2e_term.rs
@@ -162,7 +162,7 @@ fn osc52_write_ask_requires_confirmation_and_deny_ignores() {
     // The user confirms: the apprt calls complete_clipboard_request.
     let c = h.rt.confirmations.borrow()[0].clone();
     h.surface
-        .complete_clipboard_request(c.request, &c.text, true)
+        .complete_clipboard_request(c.request, c.text.as_bytes(), true)
         .unwrap();
     assert_eq!(h.rt.writes.borrow().len(), 2);
 
@@ -208,7 +208,7 @@ fn osc52_read_requires_confirmation_by_default() {
     assert_eq!(h.recorded(), Vec::<u8>::new());
     let c = h.rt.confirmations.borrow()[0].clone();
     h.surface
-        .complete_clipboard_request(c.request, &c.text, true)
+        .complete_clipboard_request(c.request, c.text.as_bytes(), true)
         .unwrap();
     // base64("secret") = c2VjcmV0
     h.expect_input(b"\x1b]52;c;c2VjcmV0\x1b\\");
diff --git a/crates/ghostty-font/Cargo.toml b/crates/ghostty-font/Cargo.toml
index ec69a43..abe73a5 100644
--- a/crates/ghostty-font/Cargo.toml
+++ b/crates/ghostty-font/Cargo.toml
@@ -5,6 +5,7 @@ edition.workspace = true
 license.workspace = true
 
 [dependencies]
+ghostty-os.workspace = true
 tiny-skia = "0.12.0"
 png = "0.18.1"
 thiserror = "2.0.21"
@@ -33,9 +34,6 @@ harfbuzz-sys = "0.8.0"
 freetype-sys = { version = "0.23.0", features = ["bundled"] }
 harfbuzz-sys = { version = "0.8.0", features = ["bundled"] }
 
-[target.'cfg(target_arch = "wasm32")'.dependencies]
-ghostty-os.workspace = true
-
 # Apple targets use CoreText (`coretext` backend) by default; FreeType and
 # HarfBuzz are only linked for the `coretext-freetype` / `coretext-harfbuzz`
 # backends (see `build.rs` and the `[features]` table).
diff --git a/crates/ghostty-font/src/shared_grid_set.rs b/crates/ghostty-font/src/shared_grid_set.rs
index 58cd23f..c743a2a 100644
--- a/crates/ghostty-font/src/shared_grid_set.rs
+++ b/crates/ghostty-font/src/shared_grid_set.rs
@@ -501,7 +501,7 @@ impl Key {
         let mut descriptors: Vec<Descriptor> = Vec::new();
         for family in &config.font_family.list {
             descriptors.push(Descriptor {
-                family: Some(family.clone()),
+                family: Some(ghostty_os::osbytes::config_str_to_text(family)),
                 style: config.font_style.name_value().map(str::to_string),
                 size: font_size.points,
                 variations: config.font_variation.list.clone(),
@@ -520,7 +520,7 @@ impl Key {
         for family in &config.font_family_bold.list {
             let style = config.font_style_bold.name_value().map(str::to_string);
             descriptors.push(Descriptor {
-                family: Some(family.clone()),
+                family: Some(ghostty_os::osbytes::config_str_to_text(family)),
                 bold: style.is_none(),
                 style,
                 size: font_size.points,
@@ -531,7 +531,7 @@ impl Key {
         for family in &config.font_family_italic.list {
             let style = config.font_style_italic.name_value().map(str::to_string);
             descriptors.push(Descriptor {
-                family: Some(family.clone()),
+                family: Some(ghostty_os::osbytes::config_str_to_text(family)),
                 italic: style.is_none(),
                 style,
                 size: font_size.points,
@@ -545,7 +545,7 @@ impl Key {
                 .name_value()
                 .map(str::to_string);
             descriptors.push(Descriptor {
-                family: Some(family.clone()),
+                family: Some(ghostty_os::osbytes::config_str_to_text(family)),
                 bold: style.is_none(),
                 italic: style.is_none(),
                 style,
diff --git a/crates/ghostty-lib/src/embedded.rs b/crates/ghostty-lib/src/embedded.rs
index e81e3d3..f71e541 100644
--- a/crates/ghostty-lib/src/embedded.rs
+++ b/crates/ghostty-lib/src/embedded.rs
@@ -856,24 +856,32 @@ unsafe fn platform_view_from_c(
 
 /// Zig's `std.zig.stringEscape` with non-ASCII code points written as
 /// `\u{...}` (the config value is parsed as a Zig string; the bytes of
-/// valid UTF-8 round trip either way).
-fn string_escape(input: &str) -> String {
+/// valid UTF-8 round trip either way). Bytes that are not valid UTF-8 are
+/// emitted in the reversible OS-byte encoding, which the string parser
+/// copies through unchanged, so `string::parse_bytes(string_escape(b)) == b`.
+fn string_escape(input: &[u8]) -> String {
     let mut out = String::with_capacity(input.len());
-    for c in input.chars() {
-        match c {
-            '\n' => out.push_str("\\n"),
-            '\r' => out.push_str("\\r"),
-            '\t' => out.push_str("\\t"),
-            '\\' => out.push_str("\\\\"),
-            '"' => out.push_str("\\\""),
-            ' ' | '!' | '#'..='&' | '\'' | '('..='[' | ']'..='~' => out.push(c),
-            c if c.is_ascii() => {
-                let _ = write!(out, "\\x{:02x}", c as u32);
-            }
-            c => {
-                let _ = write!(out, "\\u{{{:x}}}", c as u32);
+    for chunk in input.utf8_chunks() {
+        for c in chunk.valid().chars() {
+            match c {
+                '\n' => out.push_str("\\n"),
+                '\r' => out.push_str("\\r"),
+                '\t' => out.push_str("\\t"),
+                '\\' => out.push_str("\\\\"),
+                '"' => out.push_str("\\\""),
+                ' ' | '!' | '#'..='&' | '\'' | '('..='[' | ']'..='~' => out.push(c),
+                c if c.is_ascii() => {
+                    let _ = write!(out, "\\x{:02x}", c as u32);
+                }
+                c => {
+                    let _ = write!(out, "\\u{{{:x}}}", c as u32);
+                }
             }
         }
+        for &b in chunk.invalid() {
+            out.push('\u{FDD0}');
+            out.push(char::from(b));
+        }
     }
     out
 }
@@ -939,9 +947,10 @@ unsafe fn apply_surface_options(config: &mut Config, opts: &ghostty_surface_conf
         // We need to escape the string because the "raw" field expects a
         // Zig string.
         config.input.list.clear();
-        config.input.list.push(ReadableIO::Raw(string_escape(
-            &osbytes::bytes_to_config_string(input.to_bytes()),
-        )));
+        config
+            .input
+            .list
+            .push(ReadableIO::Raw(string_escape(input.to_bytes())));
     }
 
     // Wait after command
@@ -1935,7 +1944,9 @@ pub unsafe extern "C" fn ghostty_surface_binding_action(
     }
     // SAFETY: `action` points to `len` readable bytes.
     let bytes = unsafe { std::slice::from_raw_parts(action.cast::<u8>(), len) };
-    let action_str = String::from_utf8_lossy(bytes);
+    // The action text is bytes (Zig `[]const u8`); the parser works on the
+    // reversible OS-byte encoding so a `text:` payload keeps invalid UTF-8.
+    let action_str = ghostty_os::osbytes::bytes_to_config_string(bytes);
     let action = match BindingAction::parse(&action_str) {
         Ok(a) => a,
         Err(err) => {
@@ -1976,13 +1987,14 @@ pub unsafe extern "C" fn ghostty_surface_complete_clipboard_request(
     let Some(core) = s.core() else {
         return;
     };
-    let str = str.to_string_lossy();
+    // The clipboard contents are bytes (Zig `[:0]const u8`), not text.
+    let str = str.to_bytes();
     // SAFETY: the pointer is a `Box<ClipboardRequest>` leaked by
     // `clipboard_request`; it stays alive when confirmation is requested.
     let req = unsafe { *state.cast::<ClipboardRequest>() };
 
     // Attempt to complete the request, but we may request confirmation.
-    match core.complete_clipboard_request(req, &str, confirmed) {
+    match core.complete_clipboard_request(req, str, confirmed) {
         Err(
             ghostty_apprt::runtime_api::ClipboardError::UnsafePaste
             | ghostty_apprt::runtime_api::ClipboardError::UnauthorizedPaste,
@@ -1993,10 +2005,10 @@ pub unsafe extern "C" fn ghostty_surface_complete_clipboard_request(
                     ghostty_apprt::structs::ClipboardRequestType::Osc52Read => 1,
                     ghostty_apprt::structs::ClipboardRequestType::Osc52Write => 2,
                 };
-                let c = CString::new(str.replace('\0', "")).unwrap_or_default();
-                // SAFETY: embedder callback; `state` is preserved for the
+                // SAFETY: embedder callback; `text` is the caller's NUL
+                // terminated string and `state` is preserved for the
                 // confirmation route.
-                unsafe { cb(s.rt.userdata, c.as_ptr(), state, ty) };
+                unsafe { cb(s.rt.userdata, text, state, ty) };
             }
             return;
         }
@@ -2172,6 +2184,23 @@ mod tests {
     use super::*;
     use crate::types::ghostty_env_var_s;
 
+    #[test]
+    fn initial_input_escape_round_trips_bytes() {
+        for raw in [
+            &b"a\xffb\n"[..],
+            "t\u{fdd0}\u{e9}\"\\".as_bytes(),
+            b"\xfd\xfd\x1b[0m",
+            "\u{fdd0}\u{ff}".as_bytes(),
+        ] {
+            let esc = string_escape(raw);
+            assert_eq!(
+                ghostty_config::string::parse_bytes(&esc).unwrap(),
+                raw,
+                "{esc:?}"
+            );
+        }
+    }
+
     /// Surface options are raw C bytes in Zig; non-UTF-8 values must reach
     /// the config unchanged (reversible byte escape).
     #[test]
diff --git a/crates/ghostty-lib/tests/surface.rs b/crates/ghostty-lib/tests/surface.rs
index 50ef5b3..e316eda 100644
--- a/crates/ghostty-lib/tests/surface.rs
+++ b/crates/ghostty-lib/tests/surface.rs
@@ -516,6 +516,32 @@ fn selection_and_clipboard_round_trip() {
     unsafe { ghostty_surface_free(s) };
 }
 
+/// A child that prints the hex of the first two bytes it receives raw.
+const HEX_TWO: &str = "stty raw -echo; dd bs=1 count=2 2>/dev/null | od -An -tx1 | tr -d ' \\n'; printf '\\nHEX-DONE\\n'; sleep 5";
+
+#[test]
+fn clipboard_completion_and_binding_action_keep_raw_bytes() {
+    // Clipboard contents are bytes: 0xFF must reach the child as 0xFF.
+    let h = host!("");
+    let s = h.surface(HEX_TWO);
+    let act = b"paste_from_clipboard";
+    assert!(unsafe { ghostty_surface_binding_action(s, act.as_ptr().cast(), act.len()) });
+    let (_, state) = rec(|r| r.reads.last().copied()).expect("clipboard read");
+    let pasted = CString::new(&b"a\xff"[..]).unwrap();
+    unsafe {
+        ghostty_surface_complete_clipboard_request(s, pasted.as_ptr(), state as *mut c_void, false);
+    };
+    h.wait_screen(s, "61ff");
+    unsafe { ghostty_surface_free(s) };
+
+    // Binding action text is bytes too: `text:` followed by 0xFF.
+    let s = h.surface(HEX_TWO);
+    let act = b"text:b\xff";
+    assert!(unsafe { ghostty_surface_binding_action(s, act.as_ptr().cast(), act.len()) });
+    h.wait_screen(s, "62ff");
+    unsafe { ghostty_surface_free(s) };
+}
+
 #[test]
 fn osc_52_and_title_and_pwd_actions() {
     let h = host!("clipboard-write = allow\n");
diff --git a/crates/ghostty-os/src/env.rs b/crates/ghostty-os/src/env.rs
index 40b600f..075667e 100644
--- a/crates/ghostty-os/src/env.rs
+++ b/crates/ghostty-os/src/env.rs
@@ -12,20 +12,6 @@ const DELIMITER: char = ';';
 #[cfg(not(windows))]
 const DELIMITER: char = ':';
 
-/// Uppercase one UTF-16 code unit like Windows' `RtlUpcaseUnicodeChar`
-/// (Unicode simple case mapping, per unit): surrogate halves and characters
-/// whose uppercase is not a single BMP code unit are unchanged.
-fn upcase_unit(u: u16) -> u16 {
-    let Some(c) = char::from_u32(u32::from(u)) else {
-        return u; // lone surrogate half
-    };
-    let mut up = c.to_uppercase();
-    match (up.next(), up.next()) {
-        (Some(m), None) => u16::try_from(u32::from(m)).unwrap_or(u),
-        _ => u,
-    }
-}
-
 /// The comparison form of an environment key on Windows: its WTF-16 code
 /// units, uppercased per unit. Never lossy: distinct lone surrogates stay
 /// distinct.
@@ -33,7 +19,7 @@ fn upcase_unit(u: u16) -> u16 {
 fn windows_key(key: &OsStr) -> Vec<u16> {
     crate::osbytes::wtf8_to_wide(&crate::osbytes::os_bytes(key))
         .into_iter()
-        .map(upcase_unit)
+        .map(crate::windows_nls::upcase_w)
         .collect()
 }
 
@@ -400,20 +386,26 @@ mod tests {
 
 #[cfg(test)]
 mod windows_key_tests {
+    #[cfg(unix)]
     use super::*;
 
     fn key(units: &[u16]) -> Vec<u16> {
-        units.iter().copied().map(upcase_unit).collect()
+        units
+            .iter()
+            .copied()
+            .map(crate::windows_nls::upcase_w)
+            .collect()
     }
 
     #[test]
-    fn upcase_is_unicode_simple_per_unit() {
-        assert_eq!(upcase_unit(u16::from(b'a')), u16::from(b'A'));
-        assert_eq!(upcase_unit(0x00E9), 0x00C9); // e acute
-        assert_eq!(upcase_unit(0x03C9), 0x03A9); // omega
-        assert_eq!(upcase_unit(0x00DF), 0x00DF); // sharp s has no simple upper
-        assert_eq!(upcase_unit(0xD800), 0xD800);
-        assert_eq!(upcase_unit(0xDFFF), 0xDFFF);
+    fn keys_use_the_windows_table_not_unicode_mappings() {
+        // U+1F80 and U+1F88 are the same key under Windows/Zig's table even
+        // though Unicode's uppercase of U+1F80 is a two-unit sequence.
+        assert_eq!(key(&[0x1F80]), key(&[0x1F88]));
+        // The Georgian letters gained uppercase after the table froze.
+        assert_ne!(key(&[0x10D0]), key(&[0x1C90]));
+        // U+00DF stays distinct from "SS" (and from U+1E9E).
+        assert_ne!(key(&[0xDF]), key(&[0x53, 0x53]));
     }
 
     #[test]
diff --git a/crates/ghostty-os/src/lib.rs b/crates/ghostty-os/src/lib.rs
index efef642..59891a2 100644
--- a/crates/ghostty-os/src/lib.rs
+++ b/crates/ghostty-os/src/lib.rs
@@ -111,6 +111,7 @@ pub mod uri;
 pub mod wasm;
 #[cfg(windows)]
 pub mod windows;
+pub mod windows_nls;
 
 #[cfg(target_arch = "wasm32")]
 mod unsupported;
diff --git a/crates/ghostty-os/src/locale.rs b/crates/ghostty-os/src/locale.rs
index 650a30b..654ca8c 100644
--- a/crates/ghostty-os/src/locale.rs
+++ b/crates/ghostty-os/src/locale.rs
@@ -5,7 +5,10 @@
 //! comments): `setlocale(LC_ALL, "")`, then with `LANG` unset, then
 //! `en_US.UTF-8` (exported as `LANG`). On Darwin (`target_vendor = "apple"`)
 //! `LANG`/`LANGUAGE` are additionally pre-populated from Cocoa when unset.
-//! Other targets (Windows, wasm) have no `setlocale` handling.
+//! Windows runs the same chain against the C runtime: `setlocale` (with
+//! Zig's literal `LC_ALL = 6`, which the UCRT rejects, so the chain proceeds
+//! exactly as it does in Zig) and `_putenv_s` for the `LANG` updates.
+//! wasm has no `setlocale` handling.
 
 /// The locale Ghostty falls back to (and exports as `LANG`) when the
 /// environment's locale can't be set.
@@ -14,14 +17,14 @@ pub const FALLBACK_LOCALE: &str = "en_US.UTF-8";
 /// Ensure that the locale is set (`ensureLocale`).
 /// Returns `Some("en_US.UTF-8")` if we had to fall back to it (and exported
 /// it as `LANG`), otherwise `None`.
-#[cfg(unix)]
+#[cfg(any(unix, windows))]
 pub fn ensure_locale() -> Option<String> {
     posix::ensure_locale()
 }
 
-/// Ensure that the locale is set: nothing to do on targets without libc
-/// locales (Windows, wasm).
-#[cfg(not(unix))]
+/// Ensure that the locale is set: nothing to do on targets without a C
+/// runtime locale (wasm).
+#[cfg(not(any(unix, windows)))]
 pub fn ensure_locale() -> Option<String> {
     None
 }
@@ -53,15 +56,52 @@ pub fn preferred_language_value<S: AsRef<str>>(preferred: &[S]) -> Option<String
     (!out.is_empty()).then_some(out)
 }
 
-#[cfg(unix)]
+#[cfg(any(unix, windows))]
 #[allow(unsafe_code)]
 mod posix {
+    #[cfg(windows)]
+    use std::ffi::c_char;
     use std::ffi::{CStr, CString};
 
     use super::FALLBACK_LOCALE;
     #[cfg(target_vendor = "apple")]
     use super::{format_lang, preferred_language_value};
 
+    /// Zig's `LC_ALL` constant (`os/locale.zig`, "from locale.h"). It is
+    /// the value the Zig code passes on every platform, Windows included.
+    #[cfg(windows)]
+    const LC_ALL: i32 = 6;
+
+    #[cfg(windows)]
+    unsafe extern "C" {
+        fn setlocale(category: i32, locale: *const c_char) -> *mut c_char;
+        fn _putenv_s(name: *const c_char, value: *const c_char) -> i32;
+    }
+
+    #[cfg(windows)]
+    fn setlocale_all(locale: &str) -> Option<String> {
+        let c = CString::new(locale).ok()?;
+        // SAFETY: valid NUL-terminated string; the C runtime returns a
+        // pointer to a static string (or null) which we copy immediately.
+        unsafe {
+            let r = setlocale(LC_ALL, c.as_ptr());
+            (!r.is_null()).then(|| CStr::from_ptr(r).to_string_lossy().into_owned())
+        }
+    }
+
+    /// Zig's Windows `setenv`/`unsetenv`: `_putenv_s` (an empty value
+    /// removes the variable).
+    #[cfg(windows)]
+    fn putenv_s(key: &str, value: &str) {
+        let (Ok(k), Ok(v)) = (CString::new(key), CString::new(value)) else {
+            return;
+        };
+        // SAFETY: both pointers are valid NUL-terminated strings that the
+        // C runtime copies; called during early startup like the Zig code.
+        unsafe { _putenv_s(k.as_ptr(), v.as_ptr()) };
+    }
+
+    #[cfg(unix)]
     fn setlocale_all(locale: &str) -> Option<String> {
         let c = CString::new(locale).ok()?;
         // SAFETY: valid NUL-terminated string; setlocale returns a pointer to
@@ -72,12 +112,24 @@ mod posix {
         }
     }
 
+    #[cfg(windows)]
+    fn set_env(key: &str, value: &str) {
+        putenv_s(key, value);
+    }
+
+    #[cfg(windows)]
+    fn unset_env(key: &str) {
+        putenv_s(key, "");
+    }
+
+    #[cfg(unix)]
     fn set_env(key: &str, value: &str) {
         // SAFETY: called during early process startup (locale setup) before
         // other threads read the environment, mirroring the Zig code.
         unsafe { std::env::set_var(key, value) };
     }
 
+    #[cfg(unix)]
     fn unset_env(key: &str) {
         // SAFETY: see `set_env`.
         unsafe { std::env::remove_var(key) };
diff --git a/crates/ghostty-os/src/osbytes.rs b/crates/ghostty-os/src/osbytes.rs
index b7857a4..4bf6a41 100644
--- a/crates/ghostty-os/src/osbytes.rs
+++ b/crates/ghostty-os/src/osbytes.rs
@@ -174,6 +174,28 @@ pub fn wtf8_to_wide(b: &[u8]) -> Vec<u16> {
     out
 }
 
+/// The bytes are not well-formed WTF-8 (Zig: `error.InvalidWtf8` /
+/// `error.InvalidUtf8` from `std.unicode.utf8ToUtf16Le*`).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, thiserror::Error)]
+#[error("invalid wtf-8")]
+pub struct InvalidWtf8;
+
+/// Strict [`wtf8_to_wide`]: malformed input is an error instead of U+FFFD.
+/// Lone surrogates encoded as 3-byte sequences are accepted (WTF-8). This is
+/// the conversion used where Zig's is fallible (process launch).
+pub fn try_wtf8_to_wide(b: &[u8]) -> Result<Vec<u16>, InvalidWtf8> {
+    let wide = wtf8_to_wide(b);
+    // The lossy decoder yields U+FFFD only for malformed bytes; a genuine
+    // U+FFFD in the input is the 3-byte sequence EF BF BD. Re-encoding is
+    // the cheapest exact check: it reproduces the input iff it was
+    // well-formed.
+    if wide_to_wtf8(&wide) == b {
+        Ok(wide)
+    } else {
+        Err(InvalidWtf8)
+    }
+}
+
 /// OS string to reversible `String` (see the module docs). Lossless on every
 /// platform (Windows goes through WTF-8).
 pub fn os_to_config_string(s: &OsStr) -> String {
@@ -187,6 +209,14 @@ pub fn valid_str_to_config_string(s: &str) -> String {
     bytes_to_config_string(s.as_bytes())
 }
 
+/// Decode a config string for a UI-text boundary that requires UTF-8 (window
+/// titles, CSS font names, toolkit namespaces): the original bytes, with
+/// anything that is not valid UTF-8 replaced by U+FFFD. A literal `U+FDD0`
+/// in the text is restored (not shown as the doubled escape).
+pub fn config_str_to_text(s: &str) -> String {
+    String::from_utf8_lossy(&config_str_to_bytes(s)).into_owned()
+}
+
 /// Inverse of [`os_to_config_string`].
 pub fn config_str_to_os(s: &str) -> OsString {
     os_from_bytes(config_str_to_bytes(s))
@@ -286,6 +316,33 @@ impl EncodedOs {
         wtf8_to_wide(&self.to_bytes())
     }
 
+    /// Strict [`EncodedOs::to_wide`] for process launch: malformed WTF-8 is
+    /// an error (Zig's `utf8ToUtf16Le*` fail) instead of becoming U+FFFD.
+    pub fn try_to_wide(&self) -> Result<Vec<u16>, InvalidWtf8> {
+        try_wtf8_to_wide(&self.to_bytes())
+    }
+
+    /// [`EncodedOs::try_to_wide`] plus a trailing NUL.
+    pub fn try_to_wide_nul(&self) -> Result<Vec<u16>, InvalidWtf8> {
+        let mut w = self.try_to_wide()?;
+        w.push(0);
+        Ok(w)
+    }
+
+    /// Strict [`EncodedOs::to_os`]: on Windows malformed WTF-8 is an error;
+    /// elsewhere every byte string is a valid OS string.
+    pub fn try_to_os(&self) -> Result<OsString, InvalidWtf8> {
+        #[cfg(windows)]
+        {
+            use std::os::windows::ffi::OsStringExt;
+            Ok(OsString::from_wide(&self.try_to_wide()?))
+        }
+        #[cfg(not(windows))]
+        {
+            Ok(self.to_os())
+        }
+    }
+
     /// [`EncodedOs::to_wide`] plus a trailing NUL.
     #[must_use]
     pub fn to_wide_nul(&self) -> Vec<u16> {
@@ -508,6 +565,39 @@ mod tests {
         assert_eq!(wtf8_to_wide(&config_str_to_bytes(&s)), wide);
     }
 
+    #[test]
+    fn config_str_to_text_restores_literal_escape_char() {
+        assert_eq!(
+            config_str_to_text(&valid_str_to_config_string("a\u{fdd0}b")),
+            "a\u{fdd0}b"
+        );
+        assert_eq!(
+            config_str_to_text(&bytes_to_config_string(b"a\xffb")),
+            "a\u{fffd}b"
+        );
+    }
+
+    #[test]
+    fn strict_wtf8_rejects_malformed_input() {
+        assert_eq!(try_wtf8_to_wide(b"a\xffb"), Err(InvalidWtf8));
+        assert_eq!(try_wtf8_to_wide(b"\xe0\x80\x80"), Err(InvalidWtf8));
+        assert_eq!(try_wtf8_to_wide(b"\xc3"), Err(InvalidWtf8));
+        // A real U+FFFD and lone surrogates are well-formed.
+        assert_eq!(try_wtf8_to_wide("\u{FFFD}".as_bytes()), Ok(vec![0xFFFD]));
+        assert_eq!(try_wtf8_to_wide(b"\xed\xa0\x80"), Ok(vec![0xD800]));
+        assert_eq!(
+            try_wtf8_to_wide("\u{e9}\u{1F600}".as_bytes()),
+            Ok(vec![0xE9, 0xD83D, 0xDE00])
+        );
+        let bad = EncodedOs::from_bytes(b"C:\\\xff");
+        assert_eq!(bad.try_to_wide(), Err(InvalidWtf8));
+        assert_eq!(bad.try_to_wide_nul(), Err(InvalidWtf8));
+        // The lossy form still substitutes (non-launch uses).
+        assert_eq!(bad.to_wide().last(), Some(&0xFFFD));
+        let ok = EncodedOs::from_plain_text("C:\\x");
+        assert_eq!(ok.try_to_wide_nul().unwrap().last(), Some(&0));
+    }
+
     #[test]
     fn wtf8_invalid_bytes_become_replacement() {
         assert_eq!(wtf8_to_wide(b"a\xffb"), vec![0x61, 0xFFFD, 0x62]);
diff --git a/crates/ghostty-os/src/windows_nls.rs b/crates/ghostty-os/src/windows_nls.rs
new file mode 100644
index 0000000..2171e82
--- /dev/null
+++ b/crates/ghostty-os/src/windows_nls.rs
@@ -0,0 +1,201 @@
+//! Windows' per-code-unit uppercase mapping (`RtlUpcaseUnicodeChar`), a port
+//! of `std/os/windows/nls.zig` (`upcaseW` and its `uppercase_table`).
+//!
+//! The table is the locale-independent `l_intl.nls` data, unchanged since
+//! Windows 8.1 and therefore out of sync with the Unicode standard. Zig's
+//! `EnvMap` hashes and compares Windows environment keys with it, so this is
+//! what decides whether two keys name the same variable.
+
+/// This corresponds to the uppercase table within the locale-independent
+/// l_intl.nls data (found at system32\l_intl.nls).
+const UPPERCASE_TABLE: [u16; 2544] = [
+    272, 288, 304, 320, 336, 352, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256,
+    256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 368, 384, 400, 256, 416, 256, 256, 432, 256,
+    256, 256, 256, 256, 256, 256, 448, 464, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256,
+    256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256,
+    256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256,
+    256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256,
+    256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256,
+    256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256,
+    256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 480, 496, 256, 256, 256,
+    256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256,
+    256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256,
+    256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256,
+    256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256, 256,
+    256, 256, 256, 256, 256, 256, 256, 256, 512, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528,
+    528, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528, 544, 560, 528, 528, 528, 576, 528,
+    528, 592, 608, 624, 640, 656, 672, 688, 704, 720, 736, 752, 768, 784, 800, 816, 832, 848, 864,
+    880, 896, 912, 928, 944, 960, 976, 992, 1008, 1024, 528, 528, 528, 528, 528, 528, 528, 528,
+    528, 528, 1040, 528, 528, 1056, 528, 528, 1072, 1088, 1104, 1120, 1136, 1152, 528, 528, 528,
+    1168, 1184, 1200, 1216, 1232, 1248, 1264, 1280, 1296, 1312, 1328, 1344, 1360, 1376, 1392, 1408,
+    528, 528, 528, 1424, 1440, 1456, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528,
+    528, 528, 1472, 528, 528, 528, 528, 528, 528, 528, 528, 1488, 1504, 1520, 1536, 1552, 1568,
+    1584, 1600, 1616, 1632, 1648, 1664, 1680, 1696, 1712, 1728, 1744, 1760, 1776, 1792, 1808, 1824,
+    1840, 1856, 1872, 1888, 1904, 1920, 1936, 1952, 1968, 1984, 528, 528, 528, 528, 2000, 528, 528,
+    2016, 2032, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528,
+    528, 528, 528, 528, 2048, 2064, 528, 528, 528, 528, 2080, 2096, 2112, 2128, 2144, 2160, 2176,
+    2192, 2208, 2224, 2240, 2256, 528, 2272, 2288, 2304, 528, 528, 528, 528, 528, 528, 528, 528,
+    528, 528, 528, 528, 528, 528, 528, 528, 528, 2320, 2336, 2352, 528, 2368, 2384, 528, 528, 528,
+    528, 528, 528, 528, 528, 2400, 2416, 2432, 2448, 2464, 2480, 2496, 528, 528, 528, 528, 528,
+    528, 528, 528, 528, 528, 528, 2512, 2528, 528, 528, 528, 528, 528, 528, 528, 528, 528, 528, 0,
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 65504, 65504, 65504, 65504, 65504, 65504,
+    65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504,
+    65504, 65504, 65504, 65504, 65504, 65504, 65504, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+    0, 0, 0, 0, 0, 0, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504,
+    65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 0, 65504,
+    65504, 65504, 65504, 65504, 65504, 65504, 121, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 0, 0, 65535, 0, 65535, 0, 65535, 0, 0, 65535, 0, 65535, 0, 65535, 0, 65535,
+    0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 0,
+    65535, 0, 65535, 0, 65535, 0, 195, 0, 0, 65535, 0, 65535, 0, 0, 65535, 0, 0, 0, 65535, 0, 0, 0,
+    0, 0, 65535, 0, 0, 97, 0, 0, 0, 65535, 163, 0, 0, 0, 130, 0, 0, 65535, 0, 65535, 0, 65535, 0,
+    0, 65535, 0, 0, 0, 0, 65535, 0, 0, 65535, 0, 0, 0, 65535, 0, 65535, 0, 0, 65535, 0, 0, 0,
+    65535, 0, 56, 0, 0, 0, 0, 0, 0, 65534, 0, 0, 65534, 0, 0, 65534, 0, 65535, 0, 65535, 0, 65535,
+    0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 65457, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 0, 0, 65534, 0, 65535, 0, 0, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 0, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 0, 0, 0, 0, 0, 0, 0, 65535, 0, 0, 0, 0, 0, 65535, 0, 0, 0, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 10783, 10780, 0, 65326, 65330, 0, 65331, 65331, 0, 65334,
+    0, 65333, 0, 0, 0, 0, 65331, 0, 0, 65329, 0, 0, 0, 0, 65327, 65325, 0, 10743, 0, 0, 0, 65325,
+    0, 10749, 65323, 0, 0, 65322, 0, 0, 0, 0, 0, 0, 0, 10727, 0, 0, 65318, 0, 0, 65318, 0, 0, 0, 0,
+    65318, 65467, 65319, 65319, 65465, 0, 0, 0, 0, 0, 65317, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 65535, 0, 65535, 0, 0, 0, 65535, 0, 0, 0,
+    130, 130, 130, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 65498, 65499, 65499, 65499, 0, 65504,
+    65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504,
+    65504, 65504, 65504, 0, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65472,
+    65473, 65473, 0, 0, 0, 0, 0, 0, 0, 0, 65528, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535,
+    0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 0, 7, 0, 0, 0, 0, 0,
+    65535, 0, 0, 65535, 0, 0, 0, 0, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504,
+    65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504,
+    65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65456, 65456, 65456,
+    65456, 65456, 65456, 65456, 65456, 65456, 65456, 65456, 65456, 65456, 65456, 65456, 65456, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 0, 0, 0, 0, 0,
+    0, 0, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535,
+    0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 65521, 0, 65535, 0, 65535,
+    0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 65488, 65488,
+    65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488,
+    65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488,
+    65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 0, 0, 0, 0, 0, 0, 0, 0,
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 35332, 0, 0, 0, 3814, 0, 0, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 0, 0,
+    0, 0, 0, 0, 0, 0, 0, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 8, 8, 8, 8, 8, 8, 8, 8, 0, 0, 0, 0, 0, 0, 0, 0,
+    8, 8, 8, 8, 8, 8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 8, 8, 8, 8, 8, 8, 8, 8, 0, 0, 0, 0, 0, 0, 0, 0,
+    8, 8, 8, 8, 8, 8, 8, 8, 0, 0, 0, 0, 0, 0, 0, 0, 8, 8, 8, 8, 8, 8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+    0, 8, 0, 8, 0, 8, 0, 8, 0, 0, 0, 0, 0, 0, 0, 0, 8, 8, 8, 8, 8, 8, 8, 8, 0, 0, 0, 0, 0, 0, 0, 0,
+    74, 74, 86, 86, 86, 86, 100, 100, 128, 128, 112, 112, 126, 126, 0, 0, 8, 8, 8, 8, 8, 8, 8, 8,
+    0, 0, 0, 0, 0, 0, 0, 0, 8, 8, 8, 8, 8, 8, 8, 8, 0, 0, 0, 0, 0, 0, 0, 0, 8, 8, 8, 8, 8, 8, 8, 8,
+    0, 0, 0, 0, 0, 0, 0, 0, 8, 8, 0, 9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 9, 0, 0, 0, 0,
+    0, 0, 0, 0, 0, 0, 0, 0, 8, 8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 8, 8, 0, 0, 0, 7, 0, 0,
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+    0, 0, 0, 0, 0, 0, 65508, 0, 65520, 65520, 65520, 65520, 65520, 65520, 65520, 65520, 65520,
+    65520, 65520, 65520, 65520, 65520, 65520, 65520, 0, 0, 0, 0, 65535, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+    0, 0, 65510, 65510, 65510, 65510, 65510, 65510, 65510, 65510, 65510, 65510, 65510, 65510,
+    65510, 65510, 65510, 65510, 65510, 65510, 65510, 65510, 65510, 65510, 65510, 65510, 65510,
+    65510, 0, 0, 0, 0, 0, 0, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488,
+    65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488,
+    65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488,
+    65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 65488, 0, 0, 65535, 0, 0,
+    0, 54741, 54744, 0, 65535, 0, 65535, 0, 65535, 0, 0, 0, 0, 0, 0, 65535, 0, 0, 65535, 0, 0, 0,
+    0, 0, 0, 0, 0, 0, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+    0, 0, 0, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272,
+    58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272,
+    58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 58272, 0,
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 0, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 0, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 0, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0,
+    65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+    0, 65535, 0, 65535, 0, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 65535, 0, 0, 0, 0, 65535, 0,
+    0, 0, 0, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504,
+    65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504, 65504,
+    65504, 0, 0, 0, 0, 0,
+];
+
+/// Cross-platform implementation of `ntdll.RtlUpcaseUnicodeChar`.
+/// Transforms the UTF-16 code unit in `c` to its uppercased version if there
+/// is one. Otherwise, returns `c` unmodified.
+#[must_use]
+pub fn upcase_w(c: u16) -> u16 {
+    if c < u16::from(b'a') {
+        return c;
+    }
+    if c <= u16::from(b'z') {
+        return c - (u16::from(b'a') - u16::from(b'A'));
+    }
+    if c >= 0xC0 {
+        let mut offset: u16 = 0;
+
+        offset = offset.wrapping_add(u16::from((c >> 8) as u8));
+        offset = UPPERCASE_TABLE[usize::from(offset)];
+        offset = offset.wrapping_add((c >> 4) & 0xF);
+        offset = UPPERCASE_TABLE[usize::from(offset)];
+        offset = offset.wrapping_add(c & 0xF);
+        offset = UPPERCASE_TABLE[usize::from(offset)];
+
+        return c.wrapping_add(offset);
+    }
+    c
+}
+
+#[cfg(test)]
+mod tests {
+    use super::upcase_w;
+
+    #[test]
+    fn ascii_and_latin() {
+        assert_eq!(upcase_w(u16::from(b'a')), u16::from(b'A'));
+        assert_eq!(upcase_w(u16::from(b'Z')), u16::from(b'Z'));
+        assert_eq!(upcase_w(u16::from(b'_')), u16::from(b'_'));
+        assert_eq!(upcase_w(0xE9), 0xC9); // e acute
+        assert_eq!(upcase_w(0xFF), 0x178); // y diaeresis
+        assert_eq!(upcase_w(0xDF), 0xDF); // sharp s has no simple uppercase
+    }
+
+    #[test]
+    fn matches_the_windows_table_not_modern_unicode() {
+        // Windows folds the Greek prosgegrammeni forms U+1F80 -> U+1F88.
+        assert_eq!(upcase_w(0x1F88), 0x1F88);
+        assert_eq!(upcase_w(0x1F80), 0x1F88);
+        // U+1E9E (capital sharp s) and U+A64A-era additions are unchanged.
+        assert_eq!(upcase_w(0x1E9E), 0x1E9E);
+        // Georgian Mtavruli (U+1C90) is newer than the table: its lowercase
+        // U+10D0 maps to nothing under Windows but to U+1C90 in Unicode.
+        assert_eq!(upcase_w(0x10D0), 0x10D0);
+        // Surrogate halves are untouched.
+        assert_eq!(upcase_w(0xD800), 0xD800);
+        assert_eq!(upcase_w(0xDFFF), 0xDFFF);
+    }
+}
diff --git a/crates/ghostty-renderer/src/link.rs b/crates/ghostty-renderer/src/link.rs
index b9638e1..c46c115 100644
--- a/crates/ghostty-renderer/src/link.rs
+++ b/crates/ghostty-renderer/src/link.rs
@@ -43,7 +43,7 @@ impl Set {
         let mut links = Vec::with_capacity(config.len());
         for link in config {
             links.push(Link {
-                regex: Regex::new(&link.regex)?,
+                regex: Regex::new(&ghostty_os::osbytes::config_str_to_text(&link.regex))?,
                 highlight: link.highlight,
             });
         }
diff --git a/crates/ghostty-terminal/Cargo.toml b/crates/ghostty-terminal/Cargo.toml
index 18371e9..04abf44 100644
--- a/crates/ghostty-terminal/Cargo.toml
+++ b/crates/ghostty-terminal/Cargo.toml
@@ -25,7 +25,7 @@ ghostty-onig.workspace = true
 [target.'cfg(not(any(target_arch = "wasm32", windows)))'.dependencies]
 rustix = { version = "1.1.5", features = ["shm", "fs"] }
 
-[target.'cfg(target_vendor = "apple")'.dependencies]
+[target.'cfg(any(target_vendor = "apple", windows))'.dependencies]
 ghostty-os.workspace = true
 
 [dev-dependencies]
diff --git a/crates/ghostty-terminal/src/kitty/graphics_image.rs b/crates/ghostty-terminal/src/kitty/graphics_image.rs
index 28b0de6..bb7411a 100644
--- a/crates/ghostty-terminal/src/kitty/graphics_image.rs
+++ b/crates/ghostty-terminal/src/kitty/graphics_image.rs
@@ -11,24 +11,40 @@ use std::{
     time::Instant,
 };
 
-/// wasm32 has no `std::os::unix`; paths are treated as UTF-8 there.
+/// wasm32 has no `std::os::unix`; on Windows `as_encoded_bytes` is WTF-8.
 #[cfg(not(unix))]
 trait OsStrExt {
-    fn from_bytes(bytes: &[u8]) -> &std::ffi::OsStr;
     fn as_bytes(&self) -> &[u8];
 }
 
 #[cfg(not(unix))]
 impl OsStrExt for std::ffi::OsStr {
-    fn from_bytes(bytes: &[u8]) -> &std::ffi::OsStr {
-        std::ffi::OsStr::new(std::str::from_utf8(bytes).unwrap_or(""))
-    }
-
     fn as_bytes(&self) -> &[u8] {
         self.as_encoded_bytes()
     }
 }
 
+/// The OS path named by protocol payload bytes. Unix paths are the bytes;
+/// on Windows they are WTF-8 (lone surrogates are valid, malformed input is
+/// not a path); elsewhere (wasm) they must be UTF-8.
+#[cfg(unix)]
+#[allow(clippy::unnecessary_wraps)] // same signature as the fallible targets
+fn os_path_from_bytes(bytes: &[u8]) -> Option<std::ffi::OsString> {
+    Some(std::ffi::OsStr::from_bytes(bytes).to_owned())
+}
+
+#[cfg(windows)]
+fn os_path_from_bytes(bytes: &[u8]) -> Option<std::ffi::OsString> {
+    ghostty_os::osbytes::EncodedOs::from_bytes(bytes)
+        .try_to_os()
+        .ok()
+}
+
+#[cfg(not(any(unix, windows)))]
+fn os_path_from_bytes(bytes: &[u8]) -> Option<std::ffi::OsString> {
+    std::str::from_utf8(bytes).ok().map(Into::into)
+}
+
 use ghostty_vt::kitty::graphics_command::{
     Command,
     Display,
@@ -137,9 +153,11 @@ impl LoadingImage {
         match t.medium {
             TransmissionMedium::Direct => unreachable!("handled above"),
             TransmissionMedium::File | TransmissionMedium::TemporaryFile => {
-                let path = match std::fs::canonicalize(Path::new(std::ffi::OsStr::from_bytes(
-                    &cmd.data,
-                ))) {
+                let Some(os_path) = os_path_from_bytes(&cmd.data) else {
+                    log::warn!("failed to get absolute path: BadPathName");
+                    return Err(Error::InvalidData);
+                };
+                let path = match std::fs::canonicalize(Path::new(&os_path)) {
                     Ok(p) => p,
                     Err(err) => {
                         log::warn!("failed to get absolute path: {err}");
@@ -298,7 +316,10 @@ impl LoadingImage {
             }
         }
 
-        let os_path = Path::new(std::ffi::OsStr::from_bytes(path));
+        let Some(os_path) = os_path_from_bytes(path) else {
+            return Err(Error::InvalidData);
+        };
+        let os_path = Path::new(&os_path);
         let result = self.read_file_inner(os_path, offset, size);
         if temporary && let Err(err) = std::fs::remove_file(os_path) {
             log::warn!("failed to delete temporary file: {err}");
@@ -662,6 +683,24 @@ pub struct Rect {
 
 #[cfg(test)]
 mod tests {
+    #[test]
+    fn payload_path_bytes_are_kept() {
+        // Unix keeps arbitrary bytes; Windows accepts WTF-8 (lone
+        // surrogates) and rejects malformed input instead of using an
+        // empty path.
+        #[cfg(unix)]
+        {
+            let p = super::os_path_from_bytes(b"/tmp/\xff-tty-graphics-protocol").unwrap();
+            assert_eq!(p.as_encoded_bytes(), b"/tmp/\xff-tty-graphics-protocol");
+        }
+        #[cfg(windows)]
+        {
+            assert!(super::os_path_from_bytes(b"C:\\\xed\xa0\x80").is_some());
+            assert!(super::os_path_from_bytes(b"C:\\\xff").is_none());
+        }
+        assert!(super::os_path_from_bytes(b"plain").is_some());
+    }
+
     #[cfg(unix)]
     #[test]
     fn temp_dir_check_keeps_non_utf8_tmpdir_bytes() {
diff --git a/crates/ghostty-termio/src/command_windows.rs b/crates/ghostty-termio/src/command_windows.rs
index ca3e04f..6d96283 100644
--- a/crates/ghostty-termio/src/command_windows.rs
+++ b/crates/ghostty-termio/src/command_windows.rs
@@ -50,6 +50,8 @@ use crate::windows_args::{self, create_windows_env_block, windows_create_command
 /// Errors from [`Command::start`].
 #[derive(Debug, thiserror::Error)]
 pub enum StartError {
+    #[error("malformed wtf-8 in a path, argument or environment value")]
+    InvalidWtf8(#[from] ghostty_os::osbytes::InvalidWtf8),
     #[error("string or environment is not valid unicode")]
     Encode(#[from] windows_args::EncodeError),
     #[error("windows error: {0}")]
@@ -131,16 +133,22 @@ impl Command {
     pub fn start(&mut self) -> Result<(), StartError> {
         // `path` and `args` are the config command's argv strings (encoded
         // OS strings); decode them to WTF-16 for the Windows API.
-        let application_w = EncodedOs::from_encoded(&self.path).to_wide_nul();
-        let cwd_w = self.cwd.as_ref().map(EncodedOs::to_wide_nul);
+        // Like Zig's fallible `utf8ToUtf16Le*`, malformed input fails the
+        // launch instead of selecting a U+FFFD-substituted path or argument.
+        let application_w = EncodedOs::from_encoded(&self.path).try_to_wide_nul()?;
+        let cwd_w = self
+            .cwd
+            .as_ref()
+            .map(EncodedOs::try_to_wide_nul)
+            .transpose()?;
         let mut command_line_w = if self.args.is_empty() {
             None
         } else {
             let wide: Vec<Vec<u16>> = self
                 .args
                 .iter()
-                .map(|a| EncodedOs::from_encoded(a).to_wide())
-                .collect();
+                .map(|a| EncodedOs::from_encoded(a).try_to_wide())
+                .collect::<Result<_, _>>()?;
             let mut line = windows_create_command_line_wide(&wide);
             line.push(0);
             Some(line)
diff --git a/crates/ghostty-termio/src/exec.rs b/crates/ghostty-termio/src/exec.rs
index ede533f..ddee46e 100644
--- a/crates/ghostty-termio/src/exec.rs
+++ b/crates/ghostty-termio/src/exec.rs
@@ -109,7 +109,7 @@ pub struct Config {
     pub cursor_blink: Option<bool>,
     pub working_directory: Option<EncodedOs>,
     pub resources_dir: Option<EncodedOs>,
-    pub term: String,
+    pub term: EncodedOs,
 
     pub rt_pre_exec_info: RtPreExecInfo,
     pub rt_post_fork_info: RtPostForkInfo,
@@ -124,7 +124,7 @@ pub struct Config {
 
 impl Config {
     /// A config with Zig-struct-style defaults for everything optional.
-    pub fn new(env: EnvMap, term: impl Into<String>) -> Config {
+    pub fn new(env: EnvMap, term: EncodedOs) -> Config {
         Config {
             command: None,
             env,
@@ -134,7 +134,7 @@ impl Config {
             cursor_blink: None,
             working_directory: None,
             resources_dir: None,
-            term: term.into(),
+            term,
             rt_pre_exec_info: RtPreExecInfo::default(),
             rt_post_fork_info: RtPostForkInfo::default(),
             create_scope: None,
@@ -801,7 +801,8 @@ impl Subprocess {
         // For now, we just look up a bundled dir but in the future we should
         // also load the terminfo database and look for it.
         if let Some(base) = &cfg.resources_dir {
-            env.insert("TERM", cfg.term.clone());
+            // The configured TERM is an encoded OS string: export its bytes.
+            env.insert("TERM", cfg.term.to_os());
             env.insert("COLORTERM", "truecolor");
 
             // Assume that the resources directory is adjacent to the terminfo
@@ -1586,6 +1587,18 @@ mod tests {
         );
     }
 
+    #[test]
+    fn configured_term_is_exported_decoded() {
+        use std::os::unix::ffi::OsStrExt;
+        let dir = ghostty_os::TempDir::init().unwrap();
+        for raw in [&b"x\xfft"[..], "t\u{fdd0}".as_bytes()] {
+            let mut cfg = Config::new(EnvMap::new(), EncodedOs::from_bytes(raw));
+            cfg.resources_dir = Some(EncodedOs::from_path(&dir.path()));
+            let exec = Exec::new(cfg).unwrap();
+            assert_eq!(exec.env().unwrap().get("TERM").unwrap().as_bytes(), raw);
+        }
+    }
+
     #[test]
     fn exec_command_darwin_hushlogin_with_non_utf8_home() {
         use std::os::unix::ffi::OsStrExt;
diff --git a/crates/ghostty-termio/src/exec_windows.rs b/crates/ghostty-termio/src/exec_windows.rs
index 070fdde..eca4960 100644
--- a/crates/ghostty-termio/src/exec_windows.rs
+++ b/crates/ghostty-termio/src/exec_windows.rs
@@ -79,6 +79,10 @@ pub enum Error {
     Start(#[from] command::StartError),
     #[error(transparent)]
     ShellIntegration(#[from] shell_integration::Error),
+    /// A configured path, argument or environment value is not well-formed
+    /// WTF-8 (Zig's process launch conversions fail on such input).
+    #[error(transparent)]
+    InvalidWtf8(#[from] ghostty_os::osbytes::InvalidWtf8),
     #[error("io error: {0}")]
     Io(#[from] io::Error),
     #[error("failed to get WINDIR, cannot run shell command")]
@@ -102,12 +106,12 @@ pub struct Config {
     pub cursor_blink: Option<bool>,
     pub working_directory: Option<EncodedOs>,
     pub resources_dir: Option<EncodedOs>,
-    pub term: String,
+    pub term: EncodedOs,
 }
 
 impl Config {
     /// A config with Zig-struct-style defaults for everything optional.
-    pub fn new(env: EnvMap, term: impl Into<String>) -> Config {
+    pub fn new(env: EnvMap, term: EncodedOs) -> Config {
         Config {
             command: None,
             env,
@@ -117,7 +121,7 @@ impl Config {
             cursor_blink: None,
             working_directory: None,
             resources_dir: None,
-            term: term.into(),
+            term,
         }
     }
 }
@@ -585,13 +589,20 @@ fn write_thread_main(handle: &OwnedHandle, rx: &Receiver<Vec<u8>>) {
 
 /// Insert the config `env` overrides, decoding the encoded key/value text
 /// to OS strings (WTF-16 on Windows) like the Unix implementation.
-fn apply_env_overrides(env: &mut EnvMap, overrides: &RepeatableStringMap) {
+///
+/// Malformed WTF-8 fails (Zig's conversions for the environment block are
+/// fallible) instead of being substituted with U+FFFD.
+fn apply_env_overrides(
+    env: &mut EnvMap,
+    overrides: &RepeatableStringMap,
+) -> Result<(), ghostty_os::osbytes::InvalidWtf8> {
     for (k, v) in overrides.iter() {
         env.insert(
-            ghostty_os::osbytes::config_str_to_os(k),
-            ghostty_os::osbytes::config_str_to_os(v),
+            EncodedOs::from_encoded(k).try_to_os()?,
+            EncodedOs::from_encoded(v).try_to_os()?,
         );
     }
+    Ok(())
 }
 
 struct Subprocess {
@@ -617,18 +628,20 @@ impl Subprocess {
             log::info!("found Ghostty resources dir: {}", dir.as_encoded());
             // The resources dir is an encoded OS string: decode it for the
             // child's environment.
-            env.insert("GHOSTTY_RESOURCES_DIR", dir.to_os());
+            env.insert("GHOSTTY_RESOURCES_DIR", dir.try_to_os()?);
         }
 
         // Set our TERM var. This is a bit complicated because we want to use
         // the ghostty TERM value but we want to only do that if we have
         // ghostty in the TERMINFO database.
         if let Some(base) = &cfg.resources_dir {
-            env.insert("TERM", cfg.term.clone());
+            // The configured TERM is an encoded OS string: export its bytes.
+            env.insert("TERM", cfg.term.try_to_os()?);
             env.insert("COLORTERM", "truecolor");
 
             // Assume that the resources directory is adjacent to the terminfo
             // database
+            base.try_to_os()?; // malformed WTF-8 fails like Zig
             env.insert("TERMINFO", terminfo_dir(base));
         } else {
             env.insert("TERM", "xterm-256color");
@@ -698,7 +711,7 @@ impl Subprocess {
         };
 
         // Add the environment variables that override any others.
-        apply_env_overrides(&mut env, &cfg.env_override);
+        apply_env_overrides(&mut env, &cfg.env_override)?;
 
         // Build our args list. If we fail to find the shell, we'd still like
         // to try to run something so Ghostty can launch.
@@ -720,7 +733,7 @@ impl Subprocess {
 
         // Propagate the current working directory (CWD) to the shell.
         if let Some(pwd) = &cwd {
-            env.insert("PWD", pwd.to_os());
+            env.insert("PWD", pwd.try_to_os()?);
         }
 
         Ok(Subprocess {
@@ -948,7 +961,7 @@ mod tests {
         m.parse_cli(Some(&format!("A={lone}"))).unwrap();
         m.parse_cli(Some(&format!("{lit}=v"))).unwrap();
         let mut env = EnvMap::new();
-        super::apply_env_overrides(&mut env, &m);
+        super::apply_env_overrides(&mut env, &m).unwrap();
         assert_eq!(
             env.get("A").unwrap(),
             std::ffi::OsString::from_wide(&[0x61, 0xD800])
@@ -956,6 +969,17 @@ mod tests {
         assert_eq!(env.get("K\u{fdd0}").unwrap(), "v");
     }
 
+    #[test]
+    fn malformed_wtf8_env_overrides_fail() {
+        use ghostty_config::cli_args::ConfigField;
+
+        let bad = EncodedOs::from_bytes(b"a\xff").into_encoded();
+        let mut m = RepeatableStringMap::default();
+        m.parse_cli(Some(&format!("A={bad}"))).unwrap();
+        let mut env = EnvMap::new();
+        assert!(super::apply_env_overrides(&mut env, &m).is_err());
+    }
+
     #[test]
     fn terminfo_dir_is_sibling_of_resources_and_lossless() {
         use std::os::windows::ffi::{OsStrExt, OsStringExt};
diff --git a/crates/ghostty-termio/src/shell_integration.rs b/crates/ghostty-termio/src/shell_integration.rs
index 3cf8046..1c62c37 100644
--- a/crates/ghostty-termio/src/shell_integration.rs
+++ b/crates/ghostty-termio/src/shell_integration.rs
@@ -263,7 +263,9 @@ fn setup_bash(
 
     env.insert("GHOSTTY_BASH_INJECT", inject);
     if let Some(v) = rcfile {
-        env.insert("GHOSTTY_BASH_RCFILE", v);
+        // The intercepted argument is an encoded OS string; the env var gets
+        // its original bytes (Zig inserts the argument slice unchanged).
+        env.insert("GHOSTTY_BASH_RCFILE", EncodedOs::from_encoded(v).to_os());
     }
 
     // In POSIX mode, HISTFILE defaults to ~/.sh_history, so unless we're
@@ -671,6 +673,24 @@ mod tests {
         );
     }
 
+    #[cfg(unix)]
+    #[test]
+    fn bash_rcfile_keeps_original_bytes() {
+        use std::os::unix::ffi::OsStrExt;
+
+        let res = TmpResourcesDir::new(Shell::Bash);
+        for raw in [&b"/home/x/\xffrc"[..], "/home/x/\u{fdd0}rc".as_bytes()] {
+            let mut env = EnvMap::new();
+            let command = Command::Direct(vec![
+                "bash".to_owned(),
+                "--rcfile".to_owned(),
+                EncodedOs::from_bytes(raw).into_encoded(),
+            ]);
+            setup_bash(&command, &res.enc(), &mut env).unwrap();
+            assert_eq!(env.get("GHOSTTY_BASH_RCFILE").unwrap().as_bytes(), raw);
+        }
+    }
+
     #[test]
     fn bash_histfile() {
         let res = TmpResourcesDir::new(Shell::Bash);
diff --git a/crates/ghostty-termio/src/stream_handler.rs b/crates/ghostty-termio/src/stream_handler.rs
index 7271d61..37524c0 100644
--- a/crates/ghostty-termio/src/stream_handler.rs
+++ b/crates/ghostty-termio/src/stream_handler.rs
@@ -348,7 +348,7 @@ impl TerminalStream {
         let st = &mut self.state;
         st.osc_color_report_format = config.osc_color_report_format;
         st.clipboard_write = config.clipboard_write;
-        st.enquiry_response = config.enquiry_response.clone().into_bytes();
+        st.enquiry_response.clone_from(&config.enquiry_response);
         st.default_cursor_style = config.cursor_style;
         st.default_cursor_blink = config.cursor_blink;
 
diff --git a/crates/ghostty-termio/src/termio.rs b/crates/ghostty-termio/src/termio.rs
index 1d94e1f..34a48dc 100644
--- a/crates/ghostty-termio/src/termio.rs
+++ b/crates/ghostty-termio/src/termio.rs
@@ -108,7 +108,8 @@ pub struct DerivedConfig {
     pub background: Color,
     pub osc_color_report_format: OSCColorReportFormat,
     pub clipboard_write: ClipboardAccess,
-    pub enquiry_response: String,
+    /// The decoded bytes of `enquiry-response` (written to the pty as is).
+    pub enquiry_response: Vec<u8>,
     pub conditional_state: conditional::State,
 }
 
@@ -142,7 +143,7 @@ impl DerivedConfig {
             background: config.background,
             osc_color_report_format: config.osc_color_report_format,
             clipboard_write: config.clipboard_write,
-            enquiry_response: config.enquiry_response.clone(),
+            enquiry_response: ghostty_os::osbytes::config_str_to_bytes(&config.enquiry_response),
             conditional_state: config.conditional_state,
         }
     }
@@ -175,8 +176,9 @@ impl ThreadEnterState {
             match item {
                 ReadableIO::Raw(v) => out.push(ghostty_os::osbytes::config_str_to_bytes(v)),
                 ReadableIO::Path(path) => {
-                    let f = std::fs::File::open(path).map_err(|err| {
-                        log::warn!("failed to open input file={path} err={err}");
+                    // The path is an encoded OS string: decode it to open.
+                    let f = std::fs::File::open(path.to_path()).map_err(|err| {
+                        log::warn!("failed to open input file={} err={err}", path.as_encoded());
                         Error::InputNotFound
                     })?;
                     let mut buf = Vec::new();
@@ -419,7 +421,7 @@ impl Termio {
             renderer_wakeup: renderer_wakeup.clone(),
             default_cursor_style: config.cursor_style,
             default_cursor_blink: config.cursor_blink,
-            enquiry_response: config.enquiry_response.clone().into_bytes(),
+            enquiry_response: config.enquiry_response.clone(),
             osc_color_report_format: config.osc_color_report_format,
             clipboard_write: config.clipboard_write,
         });
@@ -791,3 +793,26 @@ fn set_image_limits(term: &mut Terminal, limit: usize) {
         }
     }
 }
+
+#[cfg(test)]
+mod derived_config_tests {
+    use super::*;
+
+    #[test]
+    fn enquiry_response_is_decoded_to_bytes() {
+        let mut cfg = Config {
+            enquiry_response: ghostty_os::osbytes::EncodedOs::from_bytes(
+                "a\u{fdd0}\u{ff}\u{e9}".as_bytes(),
+            )
+            .into_encoded(),
+            ..Config::default()
+        };
+        assert_eq!(
+            DerivedConfig::new(&cfg).enquiry_response,
+            "a\u{fdd0}\u{ff}\u{e9}".as_bytes()
+        );
+        let raw = b"x\xffy";
+        cfg.enquiry_response = ghostty_os::osbytes::EncodedOs::from_bytes(raw).into_encoded();
+        assert_eq!(DerivedConfig::new(&cfg).enquiry_response, raw);
+    }
+}
diff --git a/crates/ghostty-termio/tests/exec_integration.rs b/crates/ghostty-termio/tests/exec_integration.rs
index 65f303c..490c8f8 100644
--- a/crates/ghostty-termio/tests/exec_integration.rs
+++ b/crates/ghostty-termio/tests/exec_integration.rs
@@ -103,7 +103,10 @@ fn start(command: ConfigCommand, tweak: impl FnOnce(&mut Config)) -> Fixture {
     let mut config = base_config();
     tweak(&mut config);
 
-    let mut ecfg = exec::Config::new(ghostty_os::env::get_env_map(), "xterm-256color");
+    let mut ecfg = exec::Config::new(
+        ghostty_os::env::get_env_map(),
+        ghostty_os::osbytes::EncodedOs::from_plain_text("xterm-256color"),
+    );
     ecfg.command = Some(command);
     ecfg.shell_integration = ShellIntegration::None;
     // The default config has `linux-cgroup = single-instance` +
@@ -360,9 +363,9 @@ fn initial_input_is_written_after_start() {
 #[test]
 fn missing_input_file_shows_error_and_thread_keeps_draining() {
     let mut f = start(sh("true"), |c| {
-        c.input
-            .list
-            .push(ReadableIO::Path("/nonexistent/ghostty-input".into()));
+        c.input.list.push(ReadableIO::Path(
+            ghostty_os::osbytes::EncodedOs::from_plain_text("/nonexistent/ghostty-input"),
+        ));
     });
     f.wait_text("A configured `input` path was not found");
     assert!(
@@ -575,7 +578,10 @@ fn change_config_message_updates_palette_and_colors() {
 fn process_output_manual_api() {
     // Termio can also be fed manually (no child involved).
     let config = base_config();
-    let mut ecfg = exec::Config::new(ghostty_os::EnvMap::new(), "xterm-256color");
+    let mut ecfg = exec::Config::new(
+        ghostty_os::EnvMap::new(),
+        ghostty_os::osbytes::EncodedOs::from_plain_text("xterm-256color"),
+    );
     ecfg.command = Some(sh("true"));
     let (tx, _rx) = unbounded();
     let termio = Termio::init(Options {
diff --git a/crates/ghostty-termio/tests/flatpak_exec.rs b/crates/ghostty-termio/tests/flatpak_exec.rs
index eb9aad6..8fb2df2 100644
--- a/crates/ghostty-termio/tests/flatpak_exec.rs
+++ b/crates/ghostty-termio/tests/flatpak_exec.rs
@@ -108,7 +108,10 @@ fn start(command: ConfigCommand, tweak: impl FnOnce(&mut Config)) -> Fixture {
     let mut config = base_config();
     tweak(&mut config);
 
-    let mut ecfg = exec::Config::new(ghostty_os::env::get_env_map(), "xterm-256color");
+    let mut ecfg = exec::Config::new(
+        ghostty_os::env::get_env_map(),
+        ghostty_os::osbytes::EncodedOs::from_plain_text("xterm-256color"),
+    );
     ecfg.command = Some(command);
     ecfg.shell_integration = ShellIntegration::None;
     // The default config has `linux-cgroup = single-instance` +
diff --git a/docs/rewrite/notes/final-Z9.md b/docs/rewrite/notes/final-Z9.md
new file mode 100644
index 0000000..484a633
--- /dev/null
+++ b/docs/rewrite/notes/final-Z9.md
@@ -0,0 +1,96 @@
+# Z9: RV7 gaps and the string-field decode audit
+
+Builds on `final-Z7.md` / `final-Z8.md` (`EncodedOs`, the reversible `U+FDD0`
+escape in `ghostty_os::osbytes`). Contract used below:
+
+* **encoded** = a config/CLI `String` in the reversible encoding. It must be
+  decoded (`EncodedOs::{to_os,to_path,to_bytes,to_wide}`,
+  `config_str_to_bytes`, or for UTF-8-only UI text `config_str_to_text`) before it
+  reaches the OS, the pty, the clipboard, or C.
+* **plain** = real text; it may only become an encoded value through
+  `from_plain_text` / `valid_str_to_config_string` (escapes a literal `U+FDD0`).
+
+New helpers: `osbytes::{InvalidWtf8, try_wtf8_to_wide, config_str_to_text}`,
+`EncodedOs::{try_to_wide, try_to_wide_nul, try_to_os}`,
+`ghostty_config::string::parse_bytes`, `ghostty_os::windows_nls::upcase_w`.
+
+## 1. The nine RV7 gaps
+
+| # | Fix | Test |
+|---|---|---|
+| 1 Windows env key casing | `windows_nls.rs` is a port of `std/os/windows/nls.zig` (the 2544-entry `l_intl.nls` table and `upcaseW`); `EnvMap` keys use it per UTF-16 unit instead of `char::to_uppercase` | `windows_nls::tests::*`, `env::windows_key_tests::keys_use_the_windows_table_not_unicode_mappings` (U+1F80 == U+1F88, Georgian distinct, no `ß` == `SS`) |
+| 2 Windows locale init | `locale.rs` `posix` module now also builds on Windows: `setlocale` with Zig's literal `LC_ALL = 6` (the UCRT rejects it exactly like Zig, so the chain continues) and `_putenv_s` for the `LANG` updates (unsafe with SAFETY comments). `ensure_locale` is only a no-op on wasm | the mock-`Sys` chain tests run everywhere; Windows glue compiled by `check-windows` |
+| 3 bash `--rcfile` | `GHOSTTY_BASH_RCFILE` = `EncodedOs::from_encoded(arg).to_os()` | `bash_rcfile_keeps_original_bytes` (byte FF, literal U+FDD0) |
+| 4 configured `TERM` | termio `Config.term` is now `EncodedOs`; Unix exports `to_os()`, Windows `try_to_os()?` | `configured_term_is_exported_decoded` |
+| 5 GTK inherited `LANG` | `saved_language` is `Option<OsString>` read with `var_os`; the `language` config override is decoded; `set_env` takes `AsRef<OsStr>`; env restore factored into `restore_language` | `inherited_lang_keeps_non_utf8_bytes` |
+| 6 Windows Kitty paths | `os_path_from_bytes`: Unix bytes, Windows WTF-8 via `EncodedOs::try_to_os` (lone surrogates ok, malformed -> `InvalidData`, no empty-path fallback) | `payload_path_bytes_are_kept` |
+| 7 malformed WTF-8 | `try_wtf8_to_wide` (strict; lossy `wtf8_to_wide` kept for non-launch use). `Command::start` (path, cwd, argv) uses `try_to_wide*` -> `StartError::InvalidWtf8`; the Windows exec env block/overrides/`GHOSTTY_RESOURCES_DIR`/`TERM`/`TERMINFO`/`PWD` use `try_to_os` -> `Error::InvalidWtf8` | `strict_wtf8_rejects_malformed_input`, `malformed_wtf8_env_overrides_fail` (cfg(windows)) |
+| 8 embedded C API | `ghostty_surface_complete_clipboard_request` passes bytes (`CoreSurface::complete_clipboard_request` / `Surface::complete_clipboard_request` now take `&[u8]`; the confirm callback gets the caller's `text` pointer unchanged); `ghostty_surface_binding_action` encodes the bytes with `bytes_to_config_string` before `Action::parse` | `clipboard_completion_and_binding_action_keep_raw_bytes` (child receives `61 ff` / `62 ff`) |
+| 9 text/csi/esc bindings | `Text` uses new `string::parse_bytes`; `Csi`/`Esc` write `config_str_to_bytes(data)` (128-byte limit applies to bytes). `string::parse` now escapes a literal `U+FDD0` that an escape sequence produces | `config_loaded_text_csi_esc_write_decoded_bytes`, `string::tests::parse_bytes_decodes_the_reversible_encoding` |
+
+## 2. Systematic audit: config fields -> consumers -> decode status
+
+"OK" = decoded at the boundary (or is the typed `EncodedOs`/bytes); "plain" =
+guaranteed text with no OS meaning; "FIXED" = changed in Z9.
+
+| Field / type | Consumers that reach OS / pty / clipboard / C | Status |
+|---|---|---|
+| `language` (String) | GTK `LANG` override | FIXED (decoded; was raw encoded) |
+| `font-family*` (RepeatableString) | `Descriptor.family` -> fontconfig / CoreText | FIXED (`config_str_to_text` in `shared_grid_set::Key::new`; font names are UTF-8 for every backend) |
+| `font-feature` | HarfBuzz/CoreText feature tags | plain (ASCII tags parsed from text) |
+| `font-variation*`, `font-codepoint-map`, `adjust-*`, `font-style*` | descriptors/metrics | plain (tags, numbers, names) |
+| `clipboard-codepoint-map` replacement | clipboard text from the formatter | FIXED (`decode_codepoint_map`; the formatter emits text, so non-UTF-8 replacement bytes can only become U+FFFD, a literal U+FDD0 is restored) |
+| `title` | `SetTitle` action (all apprts, C `set_title`) | FIXED (`config_str_to_text` in core + GTK window) |
+| `class` | GTK application id (`id_is_valid`) | plain (must be an ASCII-ish app id; encoded junk is rejected as invalid) |
+| `window-title-font-family` | GTK CSS | FIXED (decoded) |
+| `gtk-quick-terminal-namespace` | layer-shell namespace (`&str` API) | FIXED (decoded text) |
+| `macos-custom-icon` | only via C `ghostty_config_get` | OK (`config_capi::cstring_value` decodes) |
+| `term` | child env `TERM` (Unix, Windows) | FIXED (`EncodedOs` through `exec::Config`) |
+| `enquiry-response` | pty write on ENQ | FIXED (was `String::into_bytes()` of the encoded text; `DerivedConfig.enquiry_response` is now decoded `Vec<u8>`; test `enquiry_response_is_decoded_to_bytes`) |
+| `command`, `initial-command` (`Command`) | exec argv/shell string, Windows `CreateProcessW`, flatpak `ay`, macOS login | OK (decoded in `command.rs` `config_str_to_bytes`; Windows strict `try_to_wide`); window title from argv[0]/`xdg-terminal-exec` is FIXED (decoded text) |
+| `env` (RepeatableStringMap) | child environment | OK (Unix `config_str_to_os`; Windows strict `try_to_os`) |
+| `input` (RepeatableReadableIO) | pty write; file read | `Raw`: OK (decoded after `clone_parsed`). `Path`: FIXED, now typed `EncodedOs` (was opened undecoded); embedded `initial_input` escape works on bytes (`string_escape`) so `parse_bytes(string_escape(b)) == b`; test `initial_input_escape_round_trips_bytes`, `readable_io_path_decodes_to_the_original_bytes` |
+| `link` regex | onig regex over terminal text | FIXED (decoded text; a pattern byte >0x7F that is not UTF-8 cannot match terminal text anyway) |
+| `working-directory`, `background-image`, `custom-shader`, `gtk-custom-css`, `bell-audio-path`, `config-file`, `theme` | filesystem | OK (typed `EncodedOs` since Z7) |
+| `command-palette-entry` title/description | GTK palette labels | FIXED (decoded text); its `action` follows the keybind row |
+| `keybind` payloads: `text`, `csi`, `esc` | pty | FIXED (gap 9) |
+| `keybind` `search:` | search needle bytes | FIXED (decoded; the GTK search entry encodes its plain UI text with `valid_str_to_config_string`) |
+| `keybind` `set_surface_title`, `set_tab_title` | `SetTitle` action | FIXED (decoded text) |
+| `keybind` `activate_key_table*` | table-name lookup (config vs config) | plain/consistent (both sides encoded) |
+| `keybind` `cursor_key` | pty | not config-parsable (raw `String` built in code) |
+| `selection-word-chars` | codepoints | plain |
+| `Diagnostic.{key,message}` | `+validate-config` stdout, `+show-face` stderr | FIXED (print decoded bytes); GUI diagnostics are display text |
+| CLI/C: `ghostty_init` argv, `ghostty_config_load_file`, surface options (`working_directory`, `command`, `env_vars`, `initial_input`), `ghostty_surface_text`, clipboard completion, binding action, `ghostty_config_get`/`open_path` | | OK (bytes in, encoded or bytes) |
+| C key event `text`, preedit | `String` (lossy) | documented limit: UTF-8 text by the `ghostty.h` contract (IME/key text); would need a byte `KeyEvent.utf8` across `ghostty-input` |
+
+## 3. OS data entering the system (byte preserving)
+
+| Source | Entry point | Status |
+|---|---|---|
+| process environment | `std::env::var_os` / `vars_os` everywhere (`get_env_map`, `getenv`, `homedir`, `xdg`, `shell_from_env`, TMPDIR/TMP, GTK `LANG`, `PATH`); `std::env::var` remains only for ASCII-only developer switches (`GHOSTTY_LOG`, `RUST_LOG`, `COLORFGBG`, `GHOSTTY_DEBUG_*`) | OK |
+| argv | `os::args` (`args_os` / C `ghostty_init` bytes) -> `collect_config_strings` | OK |
+| config files, theme files | read as bytes -> encoded strings | OK |
+| passwd (`getpwuid`), `uname` | bytes -> `EncodedOs` (hostname: text for comparison) | OK |
+| OSC 7 pwd, OSC 8 URIs, kitty file paths | bytes kept (`set_pwd(&[u8])`, `os_path_from_bytes`) | OK |
+| GTK file drops | `os_bytes(path)` into the shell-escaped paste | OK |
+| clipboard (C API, OSC 52, GTK text) | bytes (`&[u8]` in `complete_clipboard_request`; GTK text is UTF-8) | OK |
+| D-Bus / flatpak | cwd and argv `ay` bytes; environment is `a{ss}` (UTF-8 by D-Bus type) | known limit |
+| `+new-window` over D-Bus | arguments are GVariant strings | known limit (UTF-8 by type, as Zig) |
+| Windows APIs | WTF-16 via `EncodedOs::{from_wide,to_wide}`; process launch is strict | OK |
+
+## 4. Typed where practical
+
+`exec::Config.term` (`EncodedOs`), `DerivedConfig.enquiry_response` (`Vec<u8>`),
+`ReadableIO::Path` (`EncodedOs`), `ghostty_apprt::CoreSurface::complete_clipboard_request`
+(`&[u8]`). The remaining `String` config fields are either UI/plain text (above)
+or go through the generic config machinery (macro, formatter, C value) where a
+newtype would touch every field; each consumer is listed and decoded at its
+boundary instead.
+
+## 5. Gate
+
+`cargo test --workspace`, `clippy --workspace --all-targets -D warnings`,
+`cargo +nightly fmt --all`, `just check-macos` (both targets), `just check-windows`,
+`just check-wasm`, `+show-config --default --docs` vs `/usr/bin/ghostty` (only the
+`gtk-opengl-debug` line differs): see the final report. Windows and macOS code is
+compile-checked, not run natively.