diff --git a/crates/ghostty-build-info/Cargo.toml b/crates/ghostty-build-info/Cargo.toml
new file mode 100644
index 0000000..0dc266e
--- /dev/null
+++ b/crates/ghostty-build-info/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "ghostty-build-info"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "Single source of truth for the derived build version, channel, commit and mode (Zig: build_config.zig)"
+
+[lints]
+workspace = true
diff --git a/crates/ghostty-build-info/build.rs b/crates/ghostty-build-info/build.rs
new file mode 100644
index 0000000..93a89c0
--- /dev/null
+++ b/crates/ghostty-build-info/build.rs
@@ -0,0 +1,144 @@
+//! Derives the build version metadata for [`ghostty_build_info`](crate):
+//!
+//! * the app version, release channel and commit (Zig: `build/Config.zig`
+//!   version detection + `build/GitVersion.zig`)
+//! * the build mode (Zig: `builtin.mode`)
+//!
+//! The version can be overridden with `GHOSTTY_VERSION_STRING` (Zig:
+//! `-Dversion-string`), which must be a semantic version.
+
+use std::{
+    path::{Path, PathBuf},
+    process::Command,
+};
+
+fn run(cmd: &str, args: &[&str], dir: &Path) -> Option<String> {
+    let out = Command::new(cmd)
+        .args(args)
+        .current_dir(dir)
+        .output()
+        .ok()?;
+    out.status
+        .success()
+        .then(|| String::from_utf8_lossy(&out.stdout).trim_end().to_string())
+}
+
+/// The `app-version` from `[workspace.metadata.ghostty]` of the workspace
+/// manifest (two levels above this crate).
+fn app_version(manifest_dir: &Path) -> (u64, u64, u64) {
+    let root = manifest_dir.join("../../Cargo.toml");
+    println!("cargo:rerun-if-changed={}", root.display());
+    let text = std::fs::read_to_string(root).unwrap_or_default();
+    let v = text
+        .lines()
+        .find_map(|l| l.trim().strip_prefix("app-version"))
+        .and_then(|l| l.split('"').nth(1))
+        .unwrap_or("1.3.1");
+    let mut it = v.split('.').map(|p| p.parse::<u64>().unwrap_or(0));
+    (
+        it.next().unwrap_or(1),
+        it.next().unwrap_or(3),
+        it.next().unwrap_or(1),
+    )
+}
+
+/// Returns `(version_string, commit)` mirroring `Config.zig`.
+fn detect_version(manifest_dir: &Path) -> (String, Option<String>) {
+    println!("cargo:rerun-if-env-changed=GHOSTTY_VERSION_STRING");
+    if let Ok(v) = std::env::var("GHOSTTY_VERSION_STRING") {
+        let commit = v.split_once('+').map(|(_, b)| b.to_string());
+        return (v, commit);
+    }
+
+    let (maj, min, pat) = app_version(manifest_dir);
+    let base = format!("{maj}.{min}.{pat}");
+
+    // Git unavailable or not a repository: unknown dev version.
+    let Some(branch) = run("git", &["rev-parse", "--abbrev-ref", "HEAD"], manifest_dir) else {
+        return (format!("{base}-dev+0000000"), Some("0000000".into()));
+    };
+    if let Some(git_dir) = run("git", &["rev-parse", "--absolute-git-dir"], manifest_dir) {
+        let git_dir = PathBuf::from(git_dir);
+        println!("cargo:rerun-if-changed={}", git_dir.join("HEAD").display());
+        println!("cargo:rerun-if-changed={}", git_dir.join("refs").display());
+        println!(
+            "cargo:rerun-if-changed={}",
+            git_dir.join("packed-refs").display()
+        );
+    }
+    let hash = run(
+        "git",
+        &[
+            "-c",
+            "log.showSignature=false",
+            "log",
+            "--pretty=format:%h",
+            "-n",
+            "1",
+        ],
+        manifest_dir,
+    )
+    .unwrap_or_default();
+    let tag = run(
+        "git",
+        &["describe", "--exact-match", "--tags"],
+        manifest_dir,
+    )
+    .filter(|t| !t.is_empty());
+
+    if let Some(tag) = tag {
+        // Tip releases behave just like any other pre-release so we skip.
+        if tag != "tip" {
+            assert!(
+                tag == format!("v{base}"),
+                "tagged releases must be in vX.Y.Z format matching the workspace app-version"
+            );
+            return (base, None);
+        }
+    }
+
+    // Slashes in branch names are replaced like in `GitVersion.zig`.
+    let branch = branch.replace('/', "-");
+    (format!("{base}-{branch}+{hash}"), Some(hash))
+}
+
+/// The Zig optimize-mode name for the Cargo profile being built.
+fn build_mode() -> &'static str {
+    // The profile name is the directory below `target/` that holds OUT_DIR.
+    let out = std::env::var("OUT_DIR").unwrap_or_default();
+    let profile = Path::new(&out)
+        .ancestors()
+        .skip_while(|p| p.file_name().is_none_or(|n| n != "build"))
+        .nth(1)
+        .and_then(|p| p.file_name())
+        .map(|n| n.to_string_lossy().into_owned())
+        .unwrap_or_default();
+    let debug_assertions = std::env::var_os("CARGO_CFG_DEBUG_ASSERTIONS").is_some();
+    let opt = std::env::var("OPT_LEVEL").unwrap_or_default();
+    if profile == "production" || !debug_assertions {
+        ".ReleaseFast"
+    } else if opt == "0" {
+        ".Debug"
+    } else {
+        ".ReleaseSafe"
+    }
+}
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+    let manifest_dir = PathBuf::from(std::env::var("CARGO_MANIFEST_DIR").unwrap_or_default());
+    let (version_string, commit) = detect_version(&manifest_dir);
+    // Release channel: stable unless the version has a pre-release part.
+    let channel = if version_string.contains('-') {
+        "tip"
+    } else {
+        "stable"
+    };
+    println!("cargo:rustc-env=GHOSTTY_VERSION_STRING={version_string}");
+    println!(
+        "cargo:rustc-env=GHOSTTY_COMMIT={}",
+        commit.unwrap_or_default()
+    );
+    println!("cargo:rustc-env=GHOSTTY_RELEASE_CHANNEL={channel}");
+    println!("cargo:rustc-env=GHOSTTY_BUILD_MODE={}", build_mode());
+}
diff --git a/crates/ghostty-build-info/src/lib.rs b/crates/ghostty-build-info/src/lib.rs
new file mode 100644
index 0000000..694b95e
--- /dev/null
+++ b/crates/ghostty-build-info/src/lib.rs
@@ -0,0 +1,106 @@
+//! Single source of truth for the build version metadata (Zig:
+//! `build_config.zig` `version`, `version_string`, `release_channel`, plus
+//! `builtin.mode`).
+//!
+//! Everything is derived at compile time by this crate's `build.rs` (git
+//! describe / workspace `app-version`, overridable with the
+//! `GHOSTTY_VERSION_STRING` environment variable). Consumers (XTVERSION,
+//! `TERM_PROGRAM_VERSION`, the About dialog, crash metadata, `+version`) must
+//! use these constants instead of hardcoding a version.
+
+/// `build_config.version_string`, e.g. `1.3.1` for tagged releases or
+/// `1.3.1-main+abc1234` for development builds.
+pub const VERSION_STRING: &str = env!("GHOSTTY_VERSION_STRING");
+
+/// The git commit hash (`build_config.version.build`), if the version has
+/// one (tagged releases do not).
+pub const COMMIT: Option<&str> = {
+    let c = env!("GHOSTTY_COMMIT");
+    if c.is_empty() { None } else { Some(c) }
+};
+
+/// `build_config.release_channel` tag name: `stable` or `tip`.
+pub const RELEASE_CHANNEL: &str = env!("GHOSTTY_RELEASE_CHANNEL");
+
+/// `{}` of `builtin.mode` (`.Debug`, `.ReleaseSafe`, `.ReleaseFast`).
+pub const BUILD_MODE: &str = env!("GHOSTTY_BUILD_MODE");
+
+/// A semantic version (Zig: `std.SemanticVersion`).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct SemanticVersion<'a> {
+    pub major: u64,
+    pub minor: u64,
+    pub patch: u64,
+    /// Pre-release part (after `-`), e.g. `main`.
+    pub pre: Option<&'a str>,
+    /// Build metadata (after `+`), e.g. the commit hash.
+    pub build: Option<&'a str>,
+}
+
+/// Parse `X.Y.Z[-pre][+build]`.
+pub fn parse(v: &str) -> Option<SemanticVersion<'_>> {
+    let (rest, build) = match v.split_once('+') {
+        Some((a, b)) => (a, Some(b)),
+        None => (v, None),
+    };
+    let (core, pre) = match rest.split_once('-') {
+        Some((a, b)) => (a, Some(b)),
+        None => (rest, None),
+    };
+    let mut it = core.split('.');
+    let major = it.next()?.parse().ok()?;
+    let minor = it.next()?.parse().ok()?;
+    let patch = it.next()?.parse().ok()?;
+    if it.next().is_some() {
+        return None;
+    }
+    Some(SemanticVersion {
+        major,
+        minor,
+        patch,
+        pre,
+        build,
+    })
+}
+
+/// `build_config.version` parsed from [`VERSION_STRING`].
+pub fn version() -> Option<SemanticVersion<'static>> {
+    parse(VERSION_STRING)
+}
+
+#[cfg(test)]
+mod tests {
+    #![allow(clippy::unwrap_used)]
+    use super::*;
+
+    #[test]
+    fn version_string_is_semver() {
+        let v = version().expect("derived version must be semantic");
+        assert!(v.major >= 1);
+        assert_eq!(v.build, COMMIT.filter(|_| v.build.is_some()));
+    }
+
+    #[test]
+    fn channel_matches_prerelease() {
+        let v = version().unwrap();
+        assert_eq!(RELEASE_CHANNEL == "tip", v.pre.is_some());
+    }
+
+    #[test]
+    fn parse_cases() {
+        assert_eq!(
+            parse("1.3.1"),
+            Some(SemanticVersion {
+                major: 1,
+                minor: 3,
+                patch: 1,
+                pre: None,
+                build: None
+            })
+        );
+        let v = parse("1.4.0-main+abc1234").unwrap();
+        assert_eq!((v.pre, v.build), (Some("main"), Some("abc1234")));
+        assert_eq!(parse("1.3"), None);
+        assert_eq!(parse("1.3.1.2"), None);
+    }
+}
diff --git a/crates/ghostty-cli/Cargo.toml b/crates/ghostty-cli/Cargo.toml
index 643863c..f6a32b9 100644
--- a/crates/ghostty-cli/Cargo.toml
+++ b/crates/ghostty-cli/Cargo.toml
@@ -6,6 +6,7 @@ license.workspace = true
 
 [dependencies]
 flate2 = "1.1.10"
+ghostty-build-info.workspace = true
 ghostty-apprt.workspace = true
 ghostty-config.workspace = true
 ghostty-crash.workspace = true
diff --git a/crates/ghostty-cli/build.rs b/crates/ghostty-cli/build.rs
index 83c603e..e2bbd20 100644
--- a/crates/ghostty-cli/build.rs
+++ b/crates/ghostty-cli/build.rs
@@ -1,130 +1,15 @@
 //! Build metadata for `+version` and the generated docs.
 //!
 //! * the compiler version (Zig: `builtin.zig_version_string`)
-//! * the app version, release channel and commit (Zig: `build/Config.zig`
-//!   version detection + `build/GitVersion.zig`)
-//! * the build mode (Zig: `builtin.mode`)
 //!
-//! The version can be overridden with `GHOSTTY_VERSION_STRING` (Zig:
-//! `-Dversion-string`), which must be a semantic version.
+//! The app version, channel, commit and build mode live in
+//! `ghostty-build-info` (single source of truth).
 
 use std::{
     path::{Path, PathBuf},
     process::Command,
 };
 
-fn run(cmd: &str, args: &[&str], dir: &Path) -> Option<String> {
-    let out = Command::new(cmd)
-        .args(args)
-        .current_dir(dir)
-        .output()
-        .ok()?;
-    out.status
-        .success()
-        .then(|| String::from_utf8_lossy(&out.stdout).trim_end().to_string())
-}
-
-/// The `app-version` from `[workspace.metadata.ghostty]` of the workspace
-/// manifest (two levels above this crate).
-fn app_version(manifest_dir: &Path) -> (u64, u64, u64) {
-    let root = manifest_dir.join("../../Cargo.toml");
-    println!("cargo:rerun-if-changed={}", root.display());
-    let text = std::fs::read_to_string(root).unwrap_or_default();
-    let v = text
-        .lines()
-        .find_map(|l| l.trim().strip_prefix("app-version"))
-        .and_then(|l| l.split('"').nth(1))
-        .unwrap_or("1.3.1");
-    let mut it = v.split('.').map(|p| p.parse::<u64>().unwrap_or(0));
-    (
-        it.next().unwrap_or(1),
-        it.next().unwrap_or(3),
-        it.next().unwrap_or(1),
-    )
-}
-
-/// Returns `(version_string, commit)` mirroring `Config.zig`.
-fn detect_version(manifest_dir: &Path) -> (String, Option<String>) {
-    println!("cargo:rerun-if-env-changed=GHOSTTY_VERSION_STRING");
-    if let Ok(v) = std::env::var("GHOSTTY_VERSION_STRING") {
-        let commit = v.split_once('+').map(|(_, b)| b.to_string());
-        return (v, commit);
-    }
-
-    let (maj, min, pat) = app_version(manifest_dir);
-    let base = format!("{maj}.{min}.{pat}");
-
-    // Git unavailable or not a repository: unknown dev version.
-    let Some(branch) = run("git", &["rev-parse", "--abbrev-ref", "HEAD"], manifest_dir) else {
-        return (format!("{base}-dev+0000000"), Some("0000000".into()));
-    };
-    if let Some(git_dir) = run("git", &["rev-parse", "--absolute-git-dir"], manifest_dir) {
-        let git_dir = PathBuf::from(git_dir);
-        println!("cargo:rerun-if-changed={}", git_dir.join("HEAD").display());
-        println!("cargo:rerun-if-changed={}", git_dir.join("refs").display());
-        println!(
-            "cargo:rerun-if-changed={}",
-            git_dir.join("packed-refs").display()
-        );
-    }
-    let hash = run(
-        "git",
-        &[
-            "-c",
-            "log.showSignature=false",
-            "log",
-            "--pretty=format:%h",
-            "-n",
-            "1",
-        ],
-        manifest_dir,
-    )
-    .unwrap_or_default();
-    let tag = run(
-        "git",
-        &["describe", "--exact-match", "--tags"],
-        manifest_dir,
-    )
-    .filter(|t| !t.is_empty());
-
-    if let Some(tag) = tag {
-        // Tip releases behave just like any other pre-release so we skip.
-        if tag != "tip" {
-            assert!(
-                tag == format!("v{base}"),
-                "tagged releases must be in vX.Y.Z format matching the workspace app-version"
-            );
-            return (base, None);
-        }
-    }
-
-    // Slashes in branch names are replaced like in `GitVersion.zig`.
-    let branch = branch.replace('/', "-");
-    (format!("{base}-{branch}+{hash}"), Some(hash))
-}
-
-/// The Zig optimize-mode name for the Cargo profile being built.
-fn build_mode() -> &'static str {
-    // The profile name is the directory below `target/` that holds OUT_DIR.
-    let out = std::env::var("OUT_DIR").unwrap_or_default();
-    let profile = Path::new(&out)
-        .ancestors()
-        .skip_while(|p| p.file_name().is_none_or(|n| n != "build"))
-        .nth(1)
-        .and_then(|p| p.file_name())
-        .map(|n| n.to_string_lossy().into_owned())
-        .unwrap_or_default();
-    let debug_assertions = std::env::var_os("CARGO_CFG_DEBUG_ASSERTIONS").is_some();
-    let opt = std::env::var("OPT_LEVEL").unwrap_or_default();
-    if profile == "production" || !debug_assertions {
-        ".ReleaseFast"
-    } else if opt == "0" {
-        ".Debug"
-    } else {
-        ".ReleaseSafe"
-    }
-}
-
 /// Generates the per-action help table (Zig: `helpgen.zig` `genActions`):
 /// the doc comment of each action's `run` function becomes
 /// `Action::help_text`.
@@ -175,20 +60,4 @@ fn main() {
         .and_then(|s| s.split_whitespace().nth(1).map(str::to_owned))
         .unwrap_or_else(|| "unknown".into());
     println!("cargo:rustc-env=GHOSTTY_RUSTC_VERSION={version}");
-
-    let manifest_dir = PathBuf::from(std::env::var("CARGO_MANIFEST_DIR").unwrap_or_default());
-    let (version_string, commit) = detect_version(&manifest_dir);
-    // Release channel: stable unless the version has a pre-release part.
-    let channel = if version_string.contains('-') {
-        "tip"
-    } else {
-        "stable"
-    };
-    println!("cargo:rustc-env=GHOSTTY_VERSION_STRING={version_string}");
-    println!(
-        "cargo:rustc-env=GHOSTTY_COMMIT={}",
-        commit.unwrap_or_default()
-    );
-    println!("cargo:rustc-env=GHOSTTY_RELEASE_CHANNEL={channel}");
-    println!("cargo:rustc-env=GHOSTTY_BUILD_MODE={}", build_mode());
 }
diff --git a/crates/ghostty-cli/src/action.rs b/crates/ghostty-cli/src/action.rs
index fdf8001..5ef856f 100644
--- a/crates/ghostty-cli/src/action.rs
+++ b/crates/ghostty-cli/src/action.rs
@@ -53,7 +53,7 @@ pub trait ActionEnum: Sized + Copy {
 
 /// Detect the action from the process CLI args (Zig: `detectArgs`).
 pub fn detect_args<E: ActionEnum>() -> Result<Option<E>, DetectError> {
-    detect_iter(ghostty_os::args::collect_lossy())
+    detect_iter(ghostty_os::args::collect_config_strings())
 }
 
 /// Detect the action from any iterator. Each iterator value should yield
diff --git a/crates/ghostty-cli/src/build_info.rs b/crates/ghostty-cli/src/build_info.rs
index 489eb3c..bfb8bda 100644
--- a/crates/ghostty-cli/src/build_info.rs
+++ b/crates/ghostty-cli/src/build_info.rs
@@ -6,6 +6,10 @@
 
 use std::sync::OnceLock;
 
+/// Re-export of the derived version constants (`VERSION_STRING`, `COMMIT`,
+/// `RELEASE_CHANNEL`, `BUILD_MODE`) from `ghostty-build-info`.
+pub use ghostty_build_info as derived;
+
 /// A `major.minor.micro` version (Zig: `gtk_version.zig` / `adw_version.zig`).
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct Version {
@@ -49,11 +53,11 @@ pub struct BuildInfo {
 impl Default for BuildInfo {
     fn default() -> Self {
         BuildInfo {
-            // Derived by `build.rs` (git describe / workspace app-version).
-            version_string: env!("GHOSTTY_VERSION_STRING").to_string(),
-            commit: Some(env!("GHOSTTY_COMMIT").to_string()).filter(|c| !c.is_empty()),
-            release_channel: env!("GHOSTTY_RELEASE_CHANNEL").to_string(),
-            build_mode: env!("GHOSTTY_BUILD_MODE").to_string(),
+            // Derived by `ghostty-build-info` (single source of truth).
+            version_string: ghostty_build_info::VERSION_STRING.to_string(),
+            commit: ghostty_build_info::COMMIT.map(str::to_string),
+            release_channel: ghostty_build_info::RELEASE_CHANNEL.to_string(),
+            build_mode: ghostty_build_info::BUILD_MODE.to_string(),
             app_runtime: ".gtk".to_string(),
             font_backend: ".fontconfig_freetype".to_string(),
             renderer: "renderer.generic.Renderer(renderer.OpenGL)".to_string(),
diff --git a/crates/ghostty-cli/src/edit_config.rs b/crates/ghostty-cli/src/edit_config.rs
index a6ef00a..3efc05a 100644
--- a/crates/ghostty-cli/src/edit_config.rs
+++ b/crates/ghostty-cli/src/edit_config.rs
@@ -93,8 +93,7 @@ fn exec_editor(stderr: &mut dyn Write, path: &str) -> Result<u8, Error> {
     let Some(editor) = find_editor() else {
         // If we don't have `$EDITOR` set then we can't do anything
         // but we can still print a helpful message.
-        write!(
-            stderr,
+        let msg = String::from(
             "The $EDITOR or $VISUAL environment variable is not set or is empty.\n\
              This environment variable is required to edit the Ghostty configuration\n\
              via this CLI command.\n\
@@ -104,17 +103,20 @@ fn exec_editor(stderr: &mut dyn Write, path: &str) -> Result<u8, Error> {
              \n\
              If you prefer to edit the configuration file another way,\n\
              you can find the configuration file at the following path:\n\
-             \n"
-        )?;
-
+             \n",
+        );
         // Output the path using the OSC8 sequence so that it is linked.
-        writeln!(stderr, "\x1b]8;;file://{path}\x1b\\{path}\x1b]8;;\x1b\\")?;
+        // (Paths are raw bytes: decode the reversible escape when writing.)
+        let msg = format!("{msg}\x1b]8;;file://{path}\x1b\\{path}\x1b]8;;\x1b\\\n");
+        stderr.write_all(&ghostty_os::osbytes::config_str_to_bytes(&msg))?;
         return Ok(1);
     };
 
     // Flush before exec so no output is lost.
     let _ = stderr.flush();
-    let err = std::process::Command::new(&editor).arg(path).exec();
+    let err = std::process::Command::new(ghostty_os::osbytes::config_str_to_os(&editor))
+        .arg(ghostty_os::osbytes::config_str_to_os(path))
+        .exec();
 
     // If we reached this point then exec failed.
     let err = match err.kind() {
@@ -122,8 +124,7 @@ fn exec_editor(stderr: &mut dyn Write, path: &str) -> Result<u8, Error> {
         std::io::ErrorKind::PermissionDenied => "error.AccessDenied".to_string(),
         _ => err.to_string(),
     };
-    write!(
-        stderr,
+    let msg = format!(
         "Failed to execute the editor. Error code={err}.\n\
          \n\
          This is usually due to the executable path not existing, invalid\n\
@@ -132,6 +133,7 @@ fn exec_editor(stderr: &mut dyn Write, path: &str) -> Result<u8, Error> {
          \n\
          Editor: {editor}\n\
          Path: {path}\n"
-    )?;
+    );
+    stderr.write_all(&ghostty_os::osbytes::config_str_to_bytes(&msg))?;
     Ok(1)
 }
diff --git a/crates/ghostty-cli/src/ghostty.rs b/crates/ghostty-cli/src/ghostty.rs
index ee3b580..49e10f6 100644
--- a/crates/ghostty-cli/src/ghostty.rs
+++ b/crates/ghostty-cli/src/ghostty.rs
@@ -243,6 +243,28 @@ pub fn detect(args: &[String]) -> Result<Option<Action>, DetectError> {
     detect_iter::<Action, _, _>(args.iter())
 }
 
+/// Convert raw OS arguments (Zig `[]const u8` argv) into the argument strings
+/// used by the CLI layers. No bytes are lost: invalid UTF-8 bytes use the
+/// reversible escape encoding of `ghostty_os::osbytes`, which the config
+/// layer (`cli_args`) decodes back to the original bytes.
+#[must_use]
+pub fn os_args_to_strings(args: &[std::ffi::OsString]) -> Vec<String> {
+    args.iter()
+        .map(|a| ghostty_os::osbytes::os_to_config_string(a))
+        .collect()
+}
+
+/// [`detect`] over raw OS arguments.
+pub fn detect_os(args: &[std::ffi::OsString]) -> Result<Option<Action>, DetectError> {
+    detect(&os_args_to_strings(args))
+}
+
+/// [`run_from_args`] over raw OS arguments (byte-preserving).
+#[must_use]
+pub fn run_from_os_args(args: &[std::ffi::OsString]) -> Option<u8> {
+    run_from_args(&os_args_to_strings(args))
+}
+
 /// The CLI entrypoint logic of `main_ghostty.zig` for actions: detect the
 /// action from `args` (the full argv), print the errors Ghostty prints for
 /// invalid invocations and run the action.
@@ -338,6 +360,23 @@ mod tests {
         );
     }
 
+    #[cfg(unix)]
+    #[test]
+    fn os_args_keep_non_utf8_bytes_and_still_dispatch() {
+        use std::os::unix::ffi::OsStringExt;
+        let args = vec![
+            std::ffi::OsString::from("ghostty"),
+            std::ffi::OsString::from_vec(b"--title=\xff".to_vec()),
+            std::ffi::OsString::from("+version"),
+        ];
+        assert_eq!(Some(Action::Version), detect_os(&args).unwrap());
+        let strings = os_args_to_strings(&args);
+        assert_eq!(
+            ghostty_os::osbytes::config_str_to_bytes(&strings[1]),
+            b"--title=\xff"
+        );
+    }
+
     #[test]
     fn names_round_trip() {
         for a in Action::ALL {
diff --git a/crates/ghostty-cli/src/list_themes.rs b/crates/ghostty-cli/src/list_themes.rs
index 2957ea8..f9631ab 100644
--- a/crates/ghostty-cli/src/list_themes.rs
+++ b/crates/ghostty-cli/src/list_themes.rs
@@ -15,6 +15,12 @@ use ghostty_config::{
     config_struct,
     theme::{Location, LocationIterator},
 };
+use ghostty_os::osbytes::{
+    config_str_to_bytes,
+    config_str_to_path,
+    os_to_config_string,
+    path_to_config_string,
+};
 
 use crate::{error::Error, tui};
 
@@ -121,7 +127,7 @@ pub fn collect_themes(resources_dir: Option<&str>) -> Vec<ThemeListElement> {
     let mut themes = vec![];
     let mut it = LocationIterator::new(resources_dir);
     while let Some((location, dir)) = it.next_location() {
-        let rd = match std::fs::read_dir(&dir) {
+        let rd = match std::fs::read_dir(config_str_to_path(&dir)) {
             Ok(rd) => rd,
             Err(e) if e.kind() == std::io::ErrorKind::NotFound => continue,
             Err(e) => {
@@ -137,15 +143,14 @@ pub fn collect_themes(resources_dir: Option<&str>) -> Vec<ThemeListElement> {
             if !(ft.is_file() || ft.is_symlink()) {
                 continue;
             }
-            let name = entry.file_name().to_string_lossy().into_owned();
+            // Entry names are raw bytes: keep them via the reversible escape.
+            let raw_name = entry.file_name();
+            let name = os_to_config_string(&raw_name);
             if name == ".DS_Store" {
                 continue;
             }
             themes.push(ThemeListElement {
-                path: std::path::Path::new(&dir)
-                    .join(&name)
-                    .to_string_lossy()
-                    .into_owned(),
+                path: path_to_config_string(&config_str_to_path(&dir).join(&raw_name)),
                 location,
                 theme: name,
                 rank: None,
@@ -170,8 +175,8 @@ pub fn should_include_theme(theme_filter: ColorScheme, theme_config: &Config) ->
 
 /// Make `path` absolute (`Config::load_file` requires absolute paths).
 pub(crate) fn absolute_path(path: &str) -> String {
-    std::path::absolute(path)
-        .map_or_else(|_| path.to_string(), |p| p.to_string_lossy().into_owned())
+    std::path::absolute(config_str_to_path(path))
+        .map_or_else(|_| path.to_string(), |p| path_to_config_string(&p))
 }
 
 fn resources_dir() -> Option<String> {
@@ -273,11 +278,14 @@ pub fn write_plain(
             continue;
         }
         let loc = location_name(theme.location);
-        if opts.path {
-            writeln!(out, "{} ({loc}) {}", theme.theme, theme.path)?;
+        // Names/paths are raw bytes in Zig: decode the reversible escape so
+        // non-UTF-8 bytes are written back unchanged.
+        let line = if opts.path {
+            format!("{} ({loc}) {}\n", theme.theme, theme.path)
         } else {
-            writeln!(out, "{} ({loc})", theme.theme)?;
-        }
+            format!("{} ({loc})\n", theme.theme)
+        };
+        out.write_all(&config_str_to_bytes(&line))?;
     }
     Ok(())
 }
@@ -297,6 +305,29 @@ mod tests {
         tmp
     }
 
+    #[cfg(unix)]
+    #[test]
+    fn collect_themes_keeps_non_utf8_names() {
+        use std::os::unix::ffi::OsStrExt;
+        let tmp = tempfile::tempdir().unwrap();
+        let dir = tmp.path().join("themes");
+        std::fs::create_dir_all(&dir).unwrap();
+        let raw = std::ffi::OsStr::from_bytes(b"th\xffeme");
+        std::fs::write(dir.join(raw), "background = #000000\n").unwrap();
+        let res = tmp.path().to_string_lossy().into_owned();
+        let themes: Vec<_> = collect_themes(Some(&res))
+            .into_iter()
+            .filter(|t| t.location == Location::Resources)
+            .collect();
+        assert_eq!(1, themes.len());
+        assert_eq!(config_str_to_bytes(&themes[0].theme), b"th\xffeme".to_vec());
+        // The path points at the real file and the listing loads it.
+        assert_eq!(config_str_to_path(&themes[0].path), dir.join(raw));
+        let mut out = Vec::new();
+        write_plain(&mut out, &themes, &Options::default()).unwrap();
+        assert_eq!(out, b"th\xffeme (resources)\n");
+    }
+
     #[test]
     fn collect_sort_and_filter() {
         let tmp = theme_dir(&[
diff --git a/crates/ghostty-cli/src/list_themes/preview.rs b/crates/ghostty-cli/src/list_themes/preview.rs
index e87e739..7c99bd7 100644
--- a/crates/ghostty-cli/src/list_themes/preview.rs
+++ b/crates/ghostty-cli/src/list_themes/preview.rs
@@ -1408,14 +1408,13 @@ fn error_name(e: &ghostty_config::config::Error) -> String {
 pub fn resolve_auto_theme_path() -> Result<String, Error> {
     let main_cfg_path = ghostty_config::file_load::preferred_default_file_path()
         .map_err(|e| Error::Other(e.to_string()))?;
-    let base_dir = std::path::Path::new(&main_cfg_path)
+    let main_cfg_path = ghostty_os::osbytes::config_str_to_path(&main_cfg_path);
+    let base_dir = main_cfg_path
         .parent()
         .ok_or_else(|| Error::Other("BadPathName".to_string()))?;
-    Ok(base_dir
-        .join("auto")
-        .join("theme.ghostty")
-        .to_string_lossy()
-        .into_owned())
+    Ok(ghostty_os::osbytes::path_to_config_string(
+        &base_dir.join("auto").join("theme.ghostty"),
+    ))
 }
 
 /// `writeAutoThemeFile`.
@@ -1424,10 +1423,14 @@ pub fn write_auto_theme_file(path: Option<&str>, theme_name: &str) -> Result<(),
         Some(p) => p.to_string(),
         None => resolve_auto_theme_path()?,
     };
-    if let Some(dir) = std::path::Path::new(&auto_path).parent() {
+    let auto_path = ghostty_os::osbytes::config_str_to_path(&auto_path);
+    if let Some(dir) = auto_path.parent() {
         std::fs::create_dir_all(dir)?;
     }
-    std::fs::write(&auto_path, format!("theme = {theme_name}\n"))?;
+    std::fs::write(
+        &auto_path,
+        ghostty_os::osbytes::config_str_to_bytes(&format!("theme = {theme_name}\n")),
+    )?;
     Ok(())
 }
 
@@ -1734,7 +1737,11 @@ fn run_loop(app: &mut Preview) -> Result<(), Error> {
                 match effect {
                     Effect::Copy(text) => {
                         let out = terminal.backend_mut();
-                        write!(out, "\x1b]52;c;{}\x1b\\", base64(text.as_bytes()))?;
+                        write!(
+                            out,
+                            "\x1b]52;c;{}\x1b\\",
+                            base64(&ghostty_os::osbytes::config_str_to_bytes(&text))
+                        )?;
                         out.flush()?;
                     }
                 }
diff --git a/crates/ghostty-cli/src/new_window.rs b/crates/ghostty-cli/src/new_window.rs
index 792e575..b3b589f 100644
--- a/crates/ghostty-cli/src/new_window.rs
+++ b/crates/ghostty-cli/src/new_window.rs
@@ -59,11 +59,12 @@ impl Options {
             let expanded =
                 ghostty_os::expand_home(stripped).map_err(|e| ArgError::Other(e.to_string()))?;
             let realpath =
-                std::fs::canonicalize(&expanded).map_err(|e| ArgError::Other(e.to_string()))?;
+                std::fs::canonicalize(ghostty_os::osbytes::config_str_to_path(&expanded))
+                    .map_err(|e| ArgError::Other(e.to_string()))?;
             self.working_directory_seen = true;
             return Ok(Some(format!(
                 "--working-directory={}",
-                realpath.to_string_lossy()
+                ghostty_os::osbytes::path_to_config_string(&realpath)
             )));
         }
 
@@ -192,8 +193,13 @@ pub fn run(args: &[String]) -> Result<u8, Error> {
     if !opts.working_directory_seen {
         // This should be inserted at the beginning of the list, just in case `-e` was used.
         let wd = std::fs::canonicalize(".")?;
-        opts.arguments
-            .insert(0, format!("--working-directory={}", wd.to_string_lossy()));
+        opts.arguments.insert(
+            0,
+            format!(
+                "--working-directory={}",
+                ghostty_os::osbytes::path_to_config_string(&wd)
+            ),
+        );
     }
 
     let target = match opts.class {
diff --git a/crates/ghostty-cli/src/validate_config.rs b/crates/ghostty-cli/src/validate_config.rs
index f8d0fcc..1237843 100644
--- a/crates/ghostty-cli/src/validate_config.rs
+++ b/crates/ghostty-cli/src/validate_config.rs
@@ -39,8 +39,10 @@ fn run_inner(opts: &Options, stdout: &mut dyn Write) -> Result<u8, Error> {
 
     // If a config path is passed, validate it, otherwise validate default configs
     if let Some(config_path) = &opts.config_file {
-        let abs_path = std::fs::canonicalize(config_path)?;
-        cfg.load_file(&abs_path.to_string_lossy())?;
+        // Byte-preserving: the option string carries non-UTF-8 bytes via the
+        // reversible `osbytes` escape and the canonical path may too.
+        let abs_path = std::fs::canonicalize(ghostty_os::osbytes::config_str_to_path(config_path))?;
+        cfg.load_file(&ghostty_os::osbytes::path_to_config_string(&abs_path))?;
         cfg.load_recursive_files()?;
     } else {
         cfg = Config::load()?;
@@ -109,4 +111,21 @@ mod tests {
         };
         assert!(run_inner(&opts, &mut Vec::new()).is_err());
     }
+
+    #[cfg(unix)]
+    #[test]
+    fn validate_symlink_to_non_utf8_target() {
+        use std::os::unix::ffi::OsStrExt;
+        let tmp = tempfile::tempdir().unwrap();
+        let target = tmp.path().join(std::ffi::OsStr::from_bytes(b"cfg\xff"));
+        std::fs::write(&target, "font-size = 14\n").unwrap();
+        let link = tmp.path().join("link");
+        std::os::unix::fs::symlink(&target, &link).unwrap();
+        let opts = Options {
+            config_file: Some(link.to_string_lossy().into_owned()),
+        };
+        let mut out = Vec::new();
+        assert_eq!(0, run_inner(&opts, &mut out).unwrap());
+        assert_eq!(out.len(), 0);
+    }
 }
diff --git a/crates/ghostty-config/src/cli_args.rs b/crates/ghostty-config/src/cli_args.rs
index bae91a3..55d1e9e 100644
--- a/crates/ghostty-config/src/cli_args.rs
+++ b/crates/ghostty-config/src/cli_args.rs
@@ -343,10 +343,21 @@ pub fn parse_auto_struct_with(
 
 /// Decode a double-quoted Zig string literal (including the quotes).
 /// Returns `None` on failure. Bytes produced by `\x` escapes that do not form
-/// valid UTF-8 are converted lossily.
+/// valid UTF-8 stay raw bytes: they are carried through the reversible
+/// `ghostty_os::osbytes` escape encoding (`\xFF` -> `U+FDD0 U+00FF`, decoded back
+/// to `0xFF` by `config_str_to_bytes`). See [`parse_zig_string_literal_bytes`]
+/// for the raw bytes.
 #[must_use]
 pub fn parse_zig_string_literal(s: &str) -> Option<String> {
-    let b = s.as_bytes();
+    parse_zig_string_literal_bytes(s).map(|b| ghostty_os::osbytes::bytes_to_config_string(&b))
+}
+
+/// Byte-exact [`parse_zig_string_literal`] (Zig `std.zig.string_literal`
+/// decoding yields a `[]u8`).
+#[must_use]
+pub fn parse_zig_string_literal_bytes(s: &str) -> Option<Vec<u8>> {
+    let b = ghostty_os::osbytes::config_str_to_bytes(s);
+    let b = b.as_slice();
     if b.len() < 2 || b[0] != b'"' || b[b.len() - 1] != b'"' {
         return None;
     }
@@ -395,7 +406,7 @@ pub fn parse_zig_string_literal(s: &str) -> Option<String> {
         }
         i += 1;
     }
-    Some(String::from_utf8_lossy(&out).into_owned())
+    Some(out)
 }
 
 /// Parse a "packed struct" value (comma separated names, optionally
@@ -652,8 +663,13 @@ impl<I: Iterator<Item = String>> ArgIter for ArgsIterator<I> {
 /// Create an args iterator for the process args. This will skip argv0.
 #[must_use]
 pub fn args_iterator() -> ArgsIterator<std::iter::Skip<std::vec::IntoIter<String>>> {
-    // Byte-safe: never panics on non-UTF-8 arguments.
-    ArgsIterator::new(ghostty_os::args::collect_lossy().into_iter().skip(1))
+    // Byte-safe: non-UTF-8 arguments keep their bytes (reversible escape
+    // encoding, see `ghostty_os::osbytes`).
+    ArgsIterator::new(
+        ghostty_os::args::collect_config_strings()
+            .into_iter()
+            .skip(1),
+    )
 }
 
 /// An iterator valid for arg parsing from a slice.
@@ -791,7 +807,8 @@ impl<R: BufRead> LineIterator<R> {
 
             self.line += 1;
 
-            let line = String::from_utf8_lossy(&buf);
+            // Raw bytes survive via the reversible escape encoding.
+            let line = ghostty_os::osbytes::bytes_to_config_string(&buf);
             // Trim any whitespace (including CR) around it
             let trim = line.trim_matches(|c| c == ' ' || c == '\t' || c == '\r');
 
@@ -1533,4 +1550,42 @@ mod tests {
         );
         assert_eq!(parse_zig_string_literal("\"\\q\""), None);
     }
+
+    #[test]
+    fn zig_string_literal_keeps_raw_bytes() {
+        // `\xFF` is not valid UTF-8 and must stay the single byte 0xFF.
+        assert_eq!(
+            parse_zig_string_literal_bytes("\"a\\xFFb\"").unwrap(),
+            vec![b'a', 0xff, b'b']
+        );
+        let s = parse_zig_string_literal("\"a\\xFFb\"").unwrap();
+        assert_eq!(
+            ghostty_os::osbytes::config_str_to_bytes(&s),
+            vec![b'a', 0xff, b'b']
+        );
+        assert!(!s.contains('\u{fffd}'));
+        // Valid multi-byte sequences spelled with escapes still decode.
+        assert_eq!(
+            parse_zig_string_literal("\"\\xC3\\xA9\"").unwrap(),
+            "\u{e9}"
+        );
+        // Raw (unescaped) invalid input bytes survive the escape encoding.
+        let raw = ghostty_os::osbytes::bytes_to_config_string(b"\"x\xfey\"");
+        assert_eq!(
+            parse_zig_string_literal_bytes(&raw).unwrap(),
+            vec![b'x', 0xfe, b'y']
+        );
+    }
+
+    #[test]
+    fn line_iterator_keeps_invalid_utf8_bytes() {
+        let data: &[u8] = b"title = a\xffb\n";
+        let mut it = LineIterator::new(data);
+        let line = it.next().unwrap();
+        assert_eq!(
+            ghostty_os::osbytes::config_str_to_bytes(&line),
+            b"--title=a\xffb".to_vec()
+        );
+        assert!(!line.contains('\u{fffd}'));
+    }
 }
diff --git a/crates/ghostty-config/src/config.rs b/crates/ghostty-config/src/config.rs
index a41cbab..6f90072 100644
--- a/crates/ghostty-config/src/config.rs
+++ b/crates/ghostty-config/src/config.rs
@@ -3877,7 +3877,7 @@ impl Config {
     ///
     /// `path` must be resolved and absolute.
     pub fn load_file(&mut self, path: &str) -> Result<(), Error> {
-        assert!(std::path::Path::new(path).is_absolute());
+        assert!(ghostty_os::osbytes::config_str_to_path(path).is_absolute());
         let file = match crate::file_load::open(path) {
             Ok(f) => f,
             Err(crate::file_load::OpenFileError::NotAFile) => {
@@ -3916,9 +3916,9 @@ impl Config {
         }
         let mut iter = cli_args::LineIterator::with_path(reader, path);
         self.load_iter(&mut iter)?;
-        let dir = std::path::Path::new(path)
+        let dir = ghostty_os::osbytes::config_str_to_path(path)
             .parent()
-            .map(|p| p.to_string_lossy().into_owned())
+            .map(ghostty_os::osbytes::path_to_config_string)
             .unwrap_or_default();
         self.expand_paths(&dir)?;
         Ok(())
@@ -4067,7 +4067,7 @@ impl Config {
                 // a command to execute.
                 let mut builder: Vec<String> = Vec::new();
                 for arg in args {
-                    let copy = arg.to_string_lossy().into_owned();
+                    let copy = ghostty_os::osbytes::bytes_to_config_string(arg.to_bytes());
                     self.replay_steps.push(ReplayStep::Arg(copy.clone()));
                     builder.push(copy);
                 }
@@ -4126,7 +4126,7 @@ impl Config {
         // Any paths referenced from the CLI are relative to the current working
         // directory.
         let cwd = std::fs::canonicalize(".")?;
-        self.expand_paths(&cwd.to_string_lossy())?;
+        self.expand_paths(&ghostty_os::osbytes::path_to_config_string(&cwd))?;
         Ok(())
     }
 
@@ -4170,7 +4170,7 @@ impl Config {
 
             // All paths should already be absolute at this point because
             // they're fixed up after each load.
-            assert!(std::path::Path::new(&path).is_absolute());
+            assert!(ghostty_os::osbytes::config_str_to_path(&path).is_absolute());
 
             // We must only load a unique file once
             if !loaded.insert(path.clone()) {
@@ -4183,7 +4183,7 @@ impl Config {
                 continue;
             }
 
-            let file = match std::fs::File::open(&path) {
+            let file = match std::fs::File::open(ghostty_os::osbytes::config_str_to_path(&path)) {
                 Ok(f) => f,
                 Err(err) => {
                     if err.kind() != std::io::ErrorKind::NotFound || !optional {
diff --git a/crates/ghostty-config/src/file_load.rs b/crates/ghostty-config/src/file_load.rs
index a1f702b..e983744 100644
--- a/crates/ghostty-config/src/file_load.rs
+++ b/crates/ghostty-config/src/file_load.rs
@@ -59,7 +59,7 @@ pub fn legacy_default_app_support_path() -> Result<String, xdg::Error> {
 #[cfg(target_os = "macos")]
 fn app_support_dir(sub_path: &str) -> Result<String, xdg::Error> {
     ghostty_os::macos::app_support_dir(sub_path)
-        .map(|p| p.to_string_lossy().into_owned())
+        .map(|p| ghostty_os::osbytes::path_to_config_string(&p))
         .map_err(|_| xdg::Error::AppleApiFailed)
 }
 
@@ -159,9 +159,10 @@ impl std::error::Error for OpenFileError {}
 /// if it exists and is non-empty. This also constrains the possible
 /// errors to a smaller set that we can explicitly handle.
 pub fn open(path: &str) -> Result<File, OpenFileError> {
-    assert!(std::path::Path::new(path).is_absolute());
+    assert!(ghostty_os::osbytes::config_str_to_path(path).is_absolute());
 
-    let file = File::open(path).map_err(|err| {
+    // `path` may carry the reversible byte escape of non-UTF-8 names.
+    let file = File::open(ghostty_os::osbytes::config_str_to_os(path)).map_err(|err| {
         if err.kind() == std::io::ErrorKind::NotFound {
             OpenFileError::FileNotFound
         } else {
diff --git a/crates/ghostty-config/src/path.rs b/crates/ghostty-config/src/path.rs
index 614d604..c879e9d 100644
--- a/crates/ghostty-config/src/path.rs
+++ b/crates/ghostty-config/src/path.rs
@@ -2,9 +2,11 @@
 
 use std::{
     fmt,
-    path::{Component, Path as StdPath, PathBuf},
+    path::{Component, PathBuf},
 };
 
+use ghostty_os::osbytes::{config_str_to_path, path_to_config_string};
+
 use crate::{
     c_value::CValue,
     cli_args::{ArgError, ConfigField},
@@ -32,7 +34,7 @@ impl Default for Path {
 
 /// Lexically resolve `base/path` like `std.fs.path.resolve`.
 fn resolve_lexical(base: &str, path: &str) -> String {
-    let joined = PathBuf::from(base).join(path);
+    let joined = config_str_to_path(base).join(config_str_to_path(path));
     let mut out = PathBuf::new();
     for c in joined.components() {
         match c {
@@ -43,7 +45,7 @@ fn resolve_lexical(base: &str, path: &str) -> String {
             other => out.push(other.as_os_str()),
         }
     }
-    out.to_string_lossy().into_owned()
+    path_to_config_string(&out)
 }
 
 impl Path {
@@ -109,12 +111,15 @@ impl Path {
     /// absolute path. Errors are added to `diags` if they occur; an `Err` is
     /// only returned if the base directory cannot be opened.
     pub fn expand(&mut self, base: &str, diags: &mut DiagnosticList) -> std::io::Result<()> {
-        assert!(StdPath::new(base).is_absolute());
+        // Config strings carry non-UTF-8 bytes via a reversible escape
+        // encoding; filesystem calls use the decoded OS paths.
+        let base_path = config_str_to_path(base);
+        assert!(base_path.is_absolute());
 
         let path = self.path().to_string();
 
         // If it is already absolute we can ignore it.
-        if path.is_empty() || StdPath::new(&path).is_absolute() {
+        if path.is_empty() || config_str_to_path(&path).is_absolute() {
             return Ok(());
         }
 
@@ -143,11 +148,11 @@ impl Path {
         }
 
         // Make sure the base directory can be opened.
-        std::fs::read_dir(base)?;
+        std::fs::read_dir(&base_path)?;
 
-        let full = StdPath::new(base).join(&path);
+        let full = base_path.join(config_str_to_path(&path));
         let abs = match std::fs::canonicalize(&full) {
-            Ok(p) => p.to_string_lossy().into_owned(),
+            Ok(p) => path_to_config_string(&p),
             Err(err) if err.kind() == std::io::ErrorKind::NotFound => {
                 // The file doesn't exist. Try to resolve the relative path
                 // another way.
@@ -374,4 +379,23 @@ mod tests {
         assert_eq!(p, Path::Optional("/abs/path".into()));
         assert!(diags.is_empty());
     }
+
+    #[cfg(unix)]
+    #[test]
+    fn expand_symlink_to_non_utf8_target_round_trips() {
+        use std::os::unix::ffi::OsStrExt;
+        let tmp = ghostty_os::TempDir::init().unwrap();
+        let base = std::fs::canonicalize(tmp.path()).unwrap();
+        let target = base.join(std::ffi::OsStr::from_bytes(b"t\xffarget"));
+        std::fs::write(&target, b"x").unwrap();
+        std::os::unix::fs::symlink(&target, base.join("link")).unwrap();
+
+        let mut p = Path::Required("link".into());
+        let mut diags = DiagnosticList::default();
+        p.expand(&path_to_config_string(&base), &mut diags).unwrap();
+        assert_eq!(diags.items().len(), 0);
+        // The canonical (non-UTF-8) target is preserved byte for byte.
+        assert_eq!(config_str_to_path(p.path()), target);
+        assert!(std::fs::metadata(config_str_to_path(p.path())).is_ok());
+    }
 }
diff --git a/crates/ghostty-config/src/theme.rs b/crates/ghostty-config/src/theme.rs
index e55db70..31f1d00 100644
--- a/crates/ghostty-config/src/theme.rs
+++ b/crates/ghostty-config/src/theme.rs
@@ -3,7 +3,14 @@
 //! Zig reads the resources dir from global state; here it is passed
 //! explicitly (`resources_dir`, the value of `ResourcesDir.app()`).
 
-use std::{fs::File, path::Path};
+use std::fs::File;
+
+use ghostty_os::osbytes::{
+    config_str_to_os,
+    config_str_to_path,
+    os_to_config_string,
+    path_to_config_string,
+};
 
 use crate::diagnostics::{Diagnostic, DiagnosticList};
 
@@ -35,12 +42,9 @@ impl Location {
             .ok(),
             Location::Resources => {
                 let base = resources_dir?;
-                Some(
-                    Path::new(base)
-                        .join("themes")
-                        .to_string_lossy()
-                        .into_owned(),
-                )
+                Some(path_to_config_string(
+                    &config_str_to_path(base).join("themes"),
+                ))
             }
         }
     }
@@ -161,7 +165,7 @@ pub fn open(
     diags: &mut DiagnosticList,
 ) -> Option<OpenedTheme> {
     // Absolute themes are loaded a different path.
-    if Path::new(theme).is_absolute() {
+    if config_str_to_path(theme).is_absolute() {
         let file = open_absolute(theme, diags)?;
         if !check_regular(&file, theme, diags) {
             return None;
@@ -172,9 +176,9 @@ pub fn open(
         });
     }
 
-    let basename = Path::new(theme)
+    let basename = config_str_to_path(theme)
         .file_name()
-        .map(|s| s.to_string_lossy().into_owned())
+        .map(os_to_config_string)
         .unwrap_or_default();
     if theme != basename {
         diag(
@@ -190,8 +194,8 @@ pub fn open(
     // one that exists.
     let mut it = LocationIterator::new(resources_dir);
     while let Some((_, dir)) = it.next_location() {
-        let path = Path::new(&dir).join(theme).to_string_lossy().into_owned();
-        match File::open(&path) {
+        let path = path_to_config_string(&config_str_to_path(&dir).join(config_str_to_os(theme)));
+        match File::open(config_str_to_path(&path)) {
             Ok(file) => {
                 if !check_regular(&file, theme, diags) {
                     return None;
@@ -215,7 +219,7 @@ pub fn open(
     // our iterator, reiterate over in order to build a better error message.
     it.reset();
     while let Some((_, dir)) = it.next_location() {
-        let path = Path::new(&dir).join(theme).to_string_lossy().into_owned();
+        let path = path_to_config_string(&config_str_to_path(&dir).join(config_str_to_os(theme)));
         diag(
             diags,
             format!("theme \"{theme}\" not found, tried path \"{path}\""),
@@ -229,7 +233,7 @@ pub fn open(
 /// then messages will be appended to the given error list and None is
 /// returned.
 pub fn open_absolute(theme: &str, diags: &mut DiagnosticList) -> Option<File> {
-    match File::open(theme) {
+    match File::open(config_str_to_path(theme)) {
         Ok(f) => Some(f),
         Err(err) => {
             if err.kind() == std::io::ErrorKind::NotFound {
diff --git a/crates/ghostty-os/src/args.rs b/crates/ghostty-os/src/args.rs
index 2e9c029..11e61ac 100644
--- a/crates/ghostty-os/src/args.rs
+++ b/crates/ghostty-os/src/args.rs
@@ -5,9 +5,28 @@
 //! don't panic).
 
 /// Returns an iterator over the command line arguments.
+///
+/// If [`set_argv`] was called (the embedded `ghostty_init`, Zig:
+/// `std.os.argv = argv[0..argc]`) those arguments are returned instead of
+/// the process arguments.
 #[cfg(not(target_os = "macos"))]
-pub fn iterator() -> std::env::ArgsOs {
-    std::env::args_os()
+pub fn iterator() -> std::vec::IntoIter<std::ffi::OsString> {
+    match ARGV_OVERRIDE.lock().ok().and_then(|g| g.clone()) {
+        Some(v) => v.into_iter(),
+        None => std::env::args_os().collect::<Vec<_>>().into_iter(),
+    }
+}
+
+static ARGV_OVERRIDE: std::sync::Mutex<Option<Vec<std::ffi::OsString>>> =
+    std::sync::Mutex::new(None);
+
+/// Replace the process arguments seen by [`iterator`] (Zig's assignment to
+/// `std.os.argv` in `ghostty_init`). Has no effect on macOS, where the
+/// arguments always come from `NSProcessInfo` (as in Zig).
+pub fn set_argv(args: Vec<std::ffi::OsString>) {
+    if let Ok(mut g) = ARGV_OVERRIDE.lock() {
+        *g = Some(args);
+    }
 }
 
 /// Returns an iterator over the command line arguments.
@@ -27,15 +46,25 @@ pub fn iterator() -> std::vec::IntoIter<std::ffi::OsString> {
         .into_iter()
 }
 
-/// Collect the process arguments as `String`s without ever panicking on
-/// non-UTF-8 bytes (invalid sequences become U+FFFD). The CLI/config layers
-/// are string based; `+action` dispatch only depends on the ASCII prefix.
-pub fn collect_lossy() -> Vec<String> {
-    lossy(iterator())
+/// Collect the process arguments as raw OS strings (no conversion, never
+/// panics on non-UTF-8 bytes).
+pub fn collect_os() -> Vec<std::ffi::OsString> {
+    iterator().collect()
 }
 
-fn lossy(it: impl Iterator<Item = std::ffi::OsString>) -> Vec<String> {
-    it.map(|a| a.to_string_lossy().into_owned()).collect()
+/// Collect the process arguments as raw byte vectors (Zig `[]const u8`).
+pub fn collect_bytes() -> Vec<Vec<u8>> {
+    iterator().map(|a| crate::osbytes::os_bytes(&a)).collect()
+}
+
+/// Collect the process arguments as `String`s using the reversible
+/// byte-escape encoding of [`crate::osbytes`]: no bytes are lost, invalid
+/// UTF-8 bytes are carried as private-use escapes that the config layer
+/// decodes back to the original bytes.
+pub fn collect_config_strings() -> Vec<String> {
+    iterator()
+        .map(|a| crate::osbytes::os_to_config_string(&a))
+        .collect()
 }
 
 #[cfg(test)]
@@ -50,13 +79,13 @@ mod tests {
 
     #[cfg(unix)]
     #[test]
-    fn lossy_does_not_panic_on_non_utf8() {
+    fn config_strings_keep_non_utf8_bytes() {
         use std::os::unix::ffi::OsStringExt;
-        let args = vec![
-            std::ffi::OsString::from("ghostty"),
-            std::ffi::OsString::from_vec(vec![b'+', b'x', 0xff]),
-        ];
-        let out = lossy(args.into_iter());
-        assert_eq!(out, vec!["ghostty".to_owned(), "+x\u{fffd}".to_owned()]);
+        let a = std::ffi::OsString::from_vec(vec![b'+', b'x', 0xff]);
+        let s = crate::osbytes::os_to_config_string(&a);
+        assert_eq!(
+            crate::osbytes::config_str_to_bytes(&s),
+            vec![b'+', b'x', 0xff]
+        );
     }
 }
diff --git a/crates/ghostty-os/src/cgroup.rs b/crates/ghostty-os/src/cgroup.rs
index 9768532..db7786b 100644
--- a/crates/ghostty-os/src/cgroup.rs
+++ b/crates/ghostty-os/src/cgroup.rs
@@ -4,8 +4,14 @@ use std::io::Read;
 /// into a caller-provided buffer; callers there use path-sized buffers).
 const READ_LIMIT: u64 = 4096;
 
-/// Returns the path to the cgroup for the given pid.
+/// Returns the path to the cgroup for the given pid. Non-UTF-8 bytes are
+/// carried via [`crate::osbytes`]; see [`current_os`] for the raw bytes.
 pub fn current(pid: u32) -> Option<String> {
+    current_os(pid).map(|p| crate::osbytes::os_to_config_string(&p))
+}
+
+/// Byte-preserving [`current`].
+pub fn current_os(pid: u32) -> Option<std::ffi::OsString> {
     // Read our cgroup by opening /proc/<pid>/cgroup and reading the first
     // line. The first line will look something like this:
     // 0::/user.slice/user-1000.slice/session-1.scope
@@ -21,15 +27,15 @@ pub fn current(pid: u32) -> Option<String> {
         .iter()
         .rposition(|b| !matches!(b, b' ' | b'\r' | b'\n'))
         .map_or(0, |i| i + 1);
-    Some(String::from_utf8_lossy(&rest[..end]).into_owned())
+    Some(crate::osbytes::os_from_bytes(rest[..end].to_vec()))
 }
 
 /// True if the final path component of `pid`'s cgroup equals `expected`.
 pub fn scope_matches(pid: u32, expected: &str) -> bool {
-    current(pid)
-        .as_deref()
-        .and_then(|raw| raw.rsplit_once('/'))
-        .is_some_and(|(_, last)| last == expected)
+    current_os(pid).is_some_and(|raw| {
+        let raw = crate::osbytes::os_bytes(&raw);
+        raw.rsplit(|&b| b == b'/').next() == Some(expected.as_bytes()) && raw.contains(&b'/')
+    })
 }
 
 /// Poll (every 25ms, at most 250ms) until `pid`'s cgroup ends in `expected`.
@@ -61,4 +67,15 @@ mod tests {
         }
         assert!(current(u32::MAX).is_none());
     }
+
+    #[test]
+    fn scope_matches_requires_slash_and_last_component() {
+        // Our own cgroup path always matches its own last component.
+        if let Some(p) = current(std::process::id())
+            && let Some((_, last)) = p.rsplit_once('/')
+        {
+            assert!(scope_matches(std::process::id(), last));
+        }
+        assert!(!scope_matches(u32::MAX, "x"));
+    }
 }
diff --git a/crates/ghostty-os/src/env.rs b/crates/ghostty-os/src/env.rs
index c4c8d0b..1ab261c 100644
--- a/crates/ghostty-os/src/env.rs
+++ b/crates/ghostty-os/src/env.rs
@@ -235,10 +235,24 @@ pub fn prepend_env(current: &str, value: &str) -> String {
     format!("{value}{DELIMITER}{current}")
 }
 
-/// Gets the value of an environment variable, or None if not found
-/// (or not valid unicode).
+/// Gets the raw value of an environment variable, or None if not found.
+/// A present value that is not valid UTF-8 is returned as-is (never treated
+/// as absent).
+pub fn getenv_os(key: &str) -> Option<OsString> {
+    std::env::var_os(key)
+}
+
+/// Gets the raw value of an environment variable. Returns None if not found
+/// or the value is empty.
+pub fn getenv_not_empty_os(key: &str) -> Option<OsString> {
+    getenv_os(key).filter(|v| !v.is_empty())
+}
+
+/// Gets the value of an environment variable, or None if not found. A
+/// non-UTF-8 value is still present: its invalid bytes are carried through
+/// the reversible [`crate::osbytes`] encoding.
 pub fn getenv(key: &str) -> Option<String> {
-    std::env::var(key).ok()
+    getenv_os(key).map(|v| crate::osbytes::os_to_config_string(&v))
 }
 
 /// Gets the value of an environment variable. Returns None if not found or
@@ -294,6 +308,19 @@ mod tests {
         assert_eq!(append_env("a:b", "foo"), format!("a:b{DELIMITER}foo"));
     }
 
+    #[cfg(unix)]
+    #[test]
+    fn getenv_non_utf8_is_present() {
+        let v = OsString::from_vec(vec![b'a', 0xff]);
+        temp_env::with_var("GHOSTTY_RS_T3", Some(&v), || {
+            assert_eq!(getenv_os("GHOSTTY_RS_T3").unwrap(), v);
+            assert_eq!(getenv_not_empty_os("GHOSTTY_RS_T3").unwrap(), v);
+            let s = getenv("GHOSTTY_RS_T3").unwrap();
+            assert_eq!(crate::osbytes::config_str_to_bytes(&s), vec![b'a', 0xff]);
+            assert!(getenv_not_empty("GHOSTTY_RS_T3").is_some());
+        });
+    }
+
     #[cfg(unix)]
     #[test]
     fn os_variants_preserve_bytes() {
diff --git a/crates/ghostty-os/src/file.rs b/crates/ghostty-os/src/file.rs
index 2edba9c..d7ed730 100644
--- a/crates/ghostty-os/src/file.rs
+++ b/crates/ghostty-os/src/file.rs
@@ -70,19 +70,41 @@ pub fn restore_max_files(lim: Rlimit) {
     let _ = setrlimit(Resource::Nofile, lim);
 }
 
-/// Return the recommended path for temporary files.
-pub fn alloc_tmp_dir() -> Option<String> {
+/// Resource limit placeholder on platforms without `rlimit` (Zig:
+/// `rlimit = if (@hasDecl(posix.system, "rlimit")) posix.rlimit else struct {}`).
+#[cfg(not(unix))]
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
+pub struct Rlimit;
+
+/// Unsupported-platform fallback: returns `None` like Zig.
+#[cfg(not(unix))]
+pub fn fix_max_files() -> Option<Rlimit> {
+    None
+}
+
+/// Unsupported-platform fallback: no-op like Zig.
+#[cfg(not(unix))]
+pub fn restore_max_files(_lim: Rlimit) {}
+
+/// Return the recommended path for temporary files (byte-preserving).
+pub fn alloc_tmp_dir_os() -> Option<std::path::PathBuf> {
     if cfg!(windows) {
         // TODO: what is a good fallback path on windows? (same as Zig)
-        return std::env::var_os("TMP").map(|v| v.to_string_lossy().into_owned());
+        return std::env::var_os("TMP").map(Into::into);
     }
     if let Some(v) = std::env::var_os("TMPDIR") {
-        return Some(v.to_string_lossy().into_owned());
+        return Some(v.into());
     }
     if let Some(v) = std::env::var_os("TMP") {
-        return Some(v.to_string_lossy().into_owned());
+        return Some(v.into());
     }
-    Some("/tmp".to_owned())
+    Some("/tmp".into())
+}
+
+/// Return the recommended path for temporary files. Non-UTF-8 bytes are
+/// carried via [`crate::osbytes`]; see [`alloc_tmp_dir_os`].
+pub fn alloc_tmp_dir() -> Option<String> {
+    alloc_tmp_dir_os().map(|p| crate::osbytes::os_to_config_string(p.as_os_str()))
 }
 
 #[cfg(all(test, unix))]
@@ -103,6 +125,18 @@ mod tests {
         });
     }
 
+    #[test]
+    fn tmp_dir_non_utf8() {
+        use std::os::unix::ffi::{OsStrExt, OsStringExt};
+        let v = std::ffi::OsString::from_vec(b"/tmp/\xff".to_vec());
+        temp_env::with_vars([("TMPDIR", Some(v.clone())), ("TMP", None)], || {
+            assert_eq!(
+                alloc_tmp_dir_os().unwrap().as_os_str().as_bytes(),
+                b"/tmp/\xff"
+            );
+        });
+    }
+
     #[cfg(unix)]
     #[test]
     fn fix_and_restore_max_files() {
diff --git a/crates/ghostty-os/src/homedir.rs b/crates/ghostty-os/src/homedir.rs
index bb4884a..d91ac26 100644
--- a/crates/ghostty-os/src/homedir.rs
+++ b/crates/ghostty-os/src/homedir.rs
@@ -1,6 +1,8 @@
+use std::ffi::{OsStr, OsString};
 #[cfg(unix)]
 use std::process::{Command, Stdio};
 
+use crate::osbytes;
 #[cfg(unix)]
 use crate::passwd;
 
@@ -24,16 +26,29 @@ const MAX_PATH_BYTES: usize = 4096;
 
 /// Determine the home directory for the currently executing user. This
 /// is generally an expensive process so the value should be cached.
+///
+/// Non-UTF-8 bytes are preserved through [`crate::osbytes`]; use
+/// [`home_os`] for the raw OS string.
 pub fn home() -> Result<Option<String>, Error> {
     home_with_capacity(MAX_PATH_BYTES)
 }
 
+/// Byte-preserving [`home`].
+pub fn home_os() -> Result<Option<OsString>, Error> {
+    home_os_with_capacity(MAX_PATH_BYTES)
+}
+
 /// Like [`home`] but emulates the fixed output buffer of the Zig version:
 /// errors with `BufferTooSmall` if the result is longer than `cap` bytes.
-#[cfg(unix)]
 pub fn home_with_capacity(cap: usize) -> Result<Option<String>, Error> {
-    let check = |s: String| {
-        if cap < s.len() {
+    Ok(home_os_with_capacity(cap)?.map(|h| osbytes::os_to_config_string(&h)))
+}
+
+/// Byte-preserving [`home_with_capacity`].
+#[cfg(unix)]
+pub fn home_os_with_capacity(cap: usize) -> Result<Option<OsString>, Error> {
+    let check = |s: OsString| {
+        if cap < osbytes::os_bytes(&s).len() {
             Err(Error::BufferTooSmall)
         } else {
             Ok(Some(s))
@@ -42,7 +57,7 @@ pub fn home_with_capacity(cap: usize) -> Result<Option<String>, Error> {
 
     // First: if we have a HOME env var, then we use that.
     if let Some(result) = std::env::var_os("HOME") {
-        return check(result.to_string_lossy().into_owned());
+        return check(result);
     }
 
     // On macOS: [NSFileManager defaultManager].homeDirectoryForCurrentUser.path
@@ -53,12 +68,12 @@ pub fn home_with_capacity(cap: usize) -> Result<Option<String>, Error> {
         let manager = NSFileManager::defaultManager();
         let home_url = manager.homeDirectoryForCurrentUser();
         if let Some(path) = home_url.path() {
-            return check(path.to_string());
+            return check(OsString::from(path.to_string()));
         }
     }
 
     // We try passwd. This doesn't work on multi-user mac but we try it anyways.
-    if let Ok(pw) = passwd::get()
+    if let Ok(pw) = passwd::get_os()
         && let Some(result) = pw.home
     {
         return check(result);
@@ -73,8 +88,7 @@ pub fn home_with_capacity(cap: usize) -> Result<Option<String>, Error> {
     if let Ok(run) = run
         && run.status.success()
     {
-        let out = String::from_utf8_lossy(&run.stdout);
-        return check(trim_space(&out).to_owned());
+        return check(osbytes::os_from_bytes(trim_space(&run.stdout).to_vec()));
     }
 
     Ok(None)
@@ -83,12 +97,10 @@ pub fn home_with_capacity(cap: usize) -> Result<Option<String>, Error> {
 /// Windows: `%HOMEDRIVE%%HOMEPATH%` (`homeWindows`). Returns `None` if
 /// either variable is missing.
 #[cfg(windows)]
-pub fn home_with_capacity(cap: usize) -> Result<Option<String>, Error> {
-    home_windows(
-        std::env::var("HOMEDRIVE").ok().as_deref(),
-        std::env::var("HOMEPATH").ok().as_deref(),
-        cap,
-    )
+pub fn home_os_with_capacity(cap: usize) -> Result<Option<OsString>, Error> {
+    let drive = std::env::var_os("HOMEDRIVE").map(|v| v.to_string_lossy().into_owned());
+    let path = std::env::var_os("HOMEPATH").map(|v| v.to_string_lossy().into_owned());
+    Ok(home_windows(drive.as_deref(), path.as_deref(), cap)?.map(OsString::from))
 }
 
 /// Pure part of [`home_with_capacity`] on Windows.
@@ -108,8 +120,14 @@ fn home_windows(
 }
 
 #[cfg(unix)]
-fn trim_space(input: &str) -> &str {
-    input.trim_matches([' ', '\n', '\t'])
+fn trim_space(input: &[u8]) -> &[u8] {
+    let is_ws = |b: &u8| matches!(b, b' ' | b'\n' | b'\t');
+    let start = input.iter().position(|b| !is_ws(b)).unwrap_or(input.len());
+    let end = input
+        .iter()
+        .rposition(|b| !is_ws(b))
+        .map_or(start, |i| i + 1);
+    &input[start..end]
 }
 
 /// Expands a path that starts with a tilde (~) to the home directory of
@@ -121,31 +139,41 @@ pub fn expand_home(path: &str) -> Result<String, ExpandError> {
     expand_home_with_capacity(path, MAX_PATH_BYTES)
 }
 
+/// Byte-preserving [`expand_home`].
+pub fn expand_home_os(path: &OsStr) -> Result<OsString, ExpandError> {
+    expand_home_os_with_capacity(path, MAX_PATH_BYTES)
+}
+
 /// Like [`expand_home`] with an explicit output capacity (emulating the
 /// caller-provided buffer of the Zig version).
+pub fn expand_home_with_capacity(path: &str, cap: usize) -> Result<String, ExpandError> {
+    expand_home_os_with_capacity(&osbytes::config_str_to_os(path), cap)
+        .map(|p| osbytes::os_to_config_string(&p))
+}
+
+/// Windows: `~/` is not an idiom generally used on Windows.
 #[cfg(windows)]
-pub fn expand_home_with_capacity(path: &str, _cap: usize) -> Result<String, ExpandError> {
-    // `~/` is not an idiom generally used on Windows
+pub fn expand_home_os_with_capacity(path: &OsStr, _cap: usize) -> Result<OsString, ExpandError> {
     Ok(path.to_owned())
 }
 
 /// See the Windows variant above.
 #[cfg(unix)]
-pub fn expand_home_with_capacity(path: &str, cap: usize) -> Result<String, ExpandError> {
-    if !path.starts_with("~/") {
+pub fn expand_home_os_with_capacity(path: &OsStr, cap: usize) -> Result<OsString, ExpandError> {
+    let bytes = osbytes::os_bytes(path);
+    if !bytes.starts_with(b"~/") {
         return Ok(path.to_owned());
     }
-    let Ok(Some(home_dir)) = home_with_capacity(cap) else {
+    let Ok(Some(home_dir)) = home_os_with_capacity(cap) else {
         return Err(ExpandError::HomeDetectionFailed);
     };
-    let rest = &path[1..]; // Skip the ~
-    let expanded_len = home_dir.len() + rest.len();
-
-    if expanded_len > cap {
+    let mut out = osbytes::os_bytes(&home_dir);
+    let rest = &bytes[1..]; // Skip the ~
+    if out.len() + rest.len() > cap {
         return Err(ExpandError::BufferTooSmall);
     }
-
-    Ok(format!("{home_dir}{rest}"))
+    out.extend_from_slice(rest);
+    Ok(osbytes::os_from_bytes(out))
 }
 
 #[cfg(test)]
@@ -211,6 +239,20 @@ mod tests_unix {
         });
     }
 
+    #[test]
+    fn home_preserves_non_utf8_env() {
+        use std::os::unix::ffi::{OsStrExt, OsStringExt};
+        let h = OsString::from_vec(b"/home/\xffu".to_vec());
+        temp_env::with_var("HOME", Some(&h), || {
+            assert_eq!(home_os().unwrap().unwrap(), h);
+            let e = expand_home_os(OsStr::from_bytes(b"~/x\xfe")).unwrap();
+            assert_eq!(e.as_bytes(), b"/home/\xffu/x\xfe");
+            // The String API round-trips through the escape encoding.
+            let s = expand_home("~/y").unwrap();
+            assert_eq!(osbytes::config_str_to_bytes(&s), b"/home/\xffu/y");
+        });
+    }
+
     #[test]
     fn home_prefers_env() {
         temp_env::with_var("HOME", Some("/some/home"), || {
diff --git a/crates/ghostty-os/src/i18n.rs b/crates/ghostty-os/src/i18n.rs
index 0a26d63..a311c05 100644
--- a/crates/ghostty-os/src/i18n.rs
+++ b/crates/ghostty-os/src/i18n.rs
@@ -79,26 +79,49 @@ pub fn tr(msgid: &str) -> String {
 /// a POSIX-compliant value.
 ///
 /// The Zig version is a thin layer over gnulib's
-/// `_libintl_locale_name_canonicalize` (only linked when building libintl
-/// from source, i.e. macOS). That function isn't available here, so this is
-/// a pure-Rust approximation of the cases that matter: BCP-47 style tags
-/// have `-` replaced with `_` and the Chinese script tags are mapped to
-/// their POSIX region equivalents. Encodings are NOT parsed out (like the
-/// original, `en_US.UTF-8` becomes `en_US.UTF_8`).
+/// `_libintl_locale_name_canonicalize` (the Mac OS X flavour of
+/// `gl_locale_name_canonicalize`), after fixing up some `zh` locales. This
+/// ports that function exactly, including its legacy-name, language-tag and
+/// script tables ([`crate::i18n_canon_table`]). Encodings are NOT parsed out
+/// (like the original, `en_US.UTF-8` becomes `en_US.UTF_8`).
 pub fn canonicalize_locale(locale: &str) -> String {
     // Fix zh locales
     if let Some(fixed) = fix_zh_locale(locale) {
         return fixed.to_owned();
     }
 
-    // Emulate gnulib's mapping of bare zh script tags.
-    match locale {
-        "zh-Hans" => return "zh_CN".to_owned(),
-        "zh-Hant" => return "zh_TW".to_owned(),
-        _ => {}
+    libintl_locale_name_canonicalize(locale)
+}
+
+/// `gl_locale_name_canonicalize` (gnulib `localename-unsafe.c`).
+fn libintl_locale_name_canonicalize(name: &str) -> String {
+    use crate::i18n_canon_table::{LANGTAG_TABLE, LEGACY_TABLE, SCRIPT_TABLE};
+
+    // Step 1: Convert using legacy_table.
+    if name.as_bytes().first().is_some_and(u8::is_ascii_uppercase)
+        && let Some((_, unixy)) = LEGACY_TABLE.iter().find(|(l, _)| *l == name)
+    {
+        return (*unixy).to_owned();
     }
 
-    locale.replace('-', "_")
+    // Step 2: Convert using langtag_table and script_table.
+    if name.len() == 7 && name.as_bytes()[2] == b'-' {
+        if let Some((_, unixy)) = LANGTAG_TABLE.iter().find(|(l, _)| *l == name) {
+            return (*unixy).to_owned();
+        }
+
+        // `name.get(3..)` is None only when byte 3 is not a char boundary,
+        // in which case no script entry can match.
+        if let Some(script) = name.get(3..)
+            && let Some((_, unixy)) = SCRIPT_TABLE.iter().find(|(s, _)| *s == script)
+        {
+            // name[2] = '@'; strcpy (name + 3, unixy)
+            return format!("{}@{unixy}", &name[..2]);
+        }
+    }
+
+    // Step 3: Convert new-style dash to Unix underscore.
+    name.replace('-', "_")
 }
 
 /// Handles some zh locales canonicalization because internal libintl
@@ -183,6 +206,37 @@ mod tests {
         assert_eq!("en_US.UTF_8", canonicalize_locale("en_US.UTF-8"));
     }
 
+    #[test]
+    fn canonicalize_locale_libintl_tables() {
+        // langtag_table
+        assert_eq!("pa_IN", canonicalize_locale("pa-Guru"));
+        assert_eq!("pa_PK", canonicalize_locale("pa-Arab"));
+        assert_eq!("az", canonicalize_locale("az-Latn"));
+        assert_eq!("sr", canonicalize_locale("sr-Cyrl"));
+        assert_eq!("kk", canonicalize_locale("kk-Cyrl"));
+        assert_eq!("ga", canonicalize_locale("ga-dots"));
+        // 8 bytes long: the `strlen == 7` guard skips the tables (upstream quirk).
+        assert_eq!("yue_Hans", canonicalize_locale("yue-Hans"));
+        // script_table (name[2] = '@')
+        assert_eq!("sr@latin", canonicalize_locale("sr-Latn"));
+        assert_eq!("uz@arabic", canonicalize_locale("uz-Arab"));
+        assert_eq!("mn@mongolian", canonicalize_locale("mn-Mong"));
+        // legacy_table (only for names starting uppercase)
+        assert_eq!("pt_BR", canonicalize_locale("Brazilian Portuguese"));
+        assert_eq!("zh_TW", canonicalize_locale("Traditional Chinese"));
+        assert_eq!("de", canonicalize_locale("German"));
+        assert_eq!("yi", canonicalize_locale("Yiddish"));
+        assert_eq!("english", canonicalize_locale("english"));
+        // dash fallback
+        assert_eq!("pt_BR", canonicalize_locale("pt-BR"));
+        assert_eq!("en_US", canonicalize_locale("en-US"));
+        // non-ASCII input of length 7 must not panic
+        assert_eq!(
+            "\u{e9}\u{e9}\u{e9}x",
+            canonicalize_locale("\u{e9}\u{e9}\u{e9}x")
+        );
+    }
+
     #[test]
     fn locales_map_and_static_locale() {
         assert!(is_supported_locale("zh_CN"));
diff --git a/crates/ghostty-os/src/i18n_canon_table.rs b/crates/ghostty-os/src/i18n_canon_table.rs
new file mode 100644
index 0000000..a2c61d7
--- /dev/null
+++ b/crates/ghostty-os/src/i18n_canon_table.rs
@@ -0,0 +1,167 @@
+//! Tables for [`crate::i18n::canonicalize_locale`], transcribed from gnulib's
+//! `localename-unsafe.c` (`gl_locale_name_canonicalize`, Mac OS X flavour) as
+//! bundled in gettext (`_libintl_locale_name_canonicalize`).
+
+/// Legacy (NeXTstep inherited) Mac OS X English names to Unix names
+/// (gnulib `localename-unsafe.c`, `legacy_table`).
+pub(crate) const LEGACY_TABLE: [(&str, &str); 131] = [
+    ("Afrikaans", "af"),
+    ("Albanian", "sq"),
+    ("Amharic", "am"),
+    ("Arabic", "ar"),
+    ("Armenian", "hy"),
+    ("Assamese", "as"),
+    ("Aymara", "ay"),
+    ("Azerbaijani", "az"),
+    ("Basque", "eu"),
+    ("Belarusian", "be"),
+    ("Belorussian", "be"),
+    ("Bengali", "bn"),
+    ("Brazilian Portugese", "pt_BR"),
+    ("Brazilian Portuguese", "pt_BR"),
+    ("Breton", "br"),
+    ("Bulgarian", "bg"),
+    ("Burmese", "my"),
+    ("Byelorussian", "be"),
+    ("Catalan", "ca"),
+    ("Chewa", "ny"),
+    ("Chichewa", "ny"),
+    ("Chinese", "zh"),
+    ("Chinese, Simplified", "zh_CN"),
+    ("Chinese, Traditional", "zh_TW"),
+    ("Chinese, Tradtional", "zh_TW"),
+    ("Croatian", "hr"),
+    ("Czech", "cs"),
+    ("Danish", "da"),
+    ("Dutch", "nl"),
+    ("Dzongkha", "dz"),
+    ("English", "en"),
+    ("Esperanto", "eo"),
+    ("Estonian", "et"),
+    ("Faroese", "fo"),
+    ("Farsi", "fa"),
+    ("Finnish", "fi"),
+    ("Flemish", "nl_BE"),
+    ("French", "fr"),
+    ("Galician", "gl"),
+    ("Gallegan", "gl"),
+    ("Georgian", "ka"),
+    ("German", "de"),
+    ("Greek", "el"),
+    ("Greenlandic", "kl"),
+    ("Guarani", "gn"),
+    ("Gujarati", "gu"),
+    ("Hawaiian", "haw"),
+    ("Hebrew", "he"),
+    ("Hindi", "hi"),
+    ("Hungarian", "hu"),
+    ("Icelandic", "is"),
+    ("Indonesian", "id"),
+    ("Inuktitut", "iu"),
+    ("Irish", "ga"),
+    ("Italian", "it"),
+    ("Japanese", "ja"),
+    ("Javanese", "jv"),
+    ("Kalaallisut", "kl"),
+    ("Kannada", "kn"),
+    ("Kashmiri", "ks"),
+    ("Kazakh", "kk"),
+    ("Khmer", "km"),
+    ("Kinyarwanda", "rw"),
+    ("Kirghiz", "ky"),
+    ("Korean", "ko"),
+    ("Kurdish", "ku"),
+    ("Latin", "la"),
+    ("Latvian", "lv"),
+    ("Lithuanian", "lt"),
+    ("Macedonian", "mk"),
+    ("Malagasy", "mg"),
+    ("Malay", "ms"),
+    ("Malayalam", "ml"),
+    ("Maltese", "mt"),
+    ("Manx", "gv"),
+    ("Marathi", "mr"),
+    ("Moldavian", "mo"),
+    ("Mongolian", "mn"),
+    ("Nepali", "ne"),
+    ("Norwegian", "nb"),
+    ("Nyanja", "ny"),
+    ("Nynorsk", "nn"),
+    ("Oriya", "or"),
+    ("Oromo", "om"),
+    ("Panjabi", "pa"),
+    ("Pashto", "ps"),
+    ("Persian", "fa"),
+    ("Polish", "pl"),
+    ("Portuguese", "pt"),
+    ("Portuguese, Brazilian", "pt_BR"),
+    ("Punjabi", "pa"),
+    ("Pushto", "ps"),
+    ("Quechua", "qu"),
+    ("Romanian", "ro"),
+    ("Ruanda", "rw"),
+    ("Rundi", "rn"),
+    ("Russian", "ru"),
+    ("Sami", "se_NO"),
+    ("Sanskrit", "sa"),
+    ("Scottish", "gd"),
+    ("Serbian", "sr"),
+    ("Simplified Chinese", "zh_CN"),
+    ("Sindhi", "sd"),
+    ("Sinhalese", "si"),
+    ("Slovak", "sk"),
+    ("Slovenian", "sl"),
+    ("Somali", "so"),
+    ("Spanish", "es"),
+    ("Sundanese", "su"),
+    ("Swahili", "sw"),
+    ("Swedish", "sv"),
+    ("Tagalog", "tl"),
+    ("Tajik", "tg"),
+    ("Tajiki", "tg"),
+    ("Tamil", "ta"),
+    ("Tatar", "tt"),
+    ("Telugu", "te"),
+    ("Thai", "th"),
+    ("Tibetan", "bo"),
+    ("Tigrinya", "ti"),
+    ("Tongan", "to"),
+    ("Traditional Chinese", "zh_TW"),
+    ("Turkish", "tr"),
+    ("Turkmen", "tk"),
+    ("Uighur", "ug"),
+    ("Ukrainian", "uk"),
+    ("Urdu", "ur"),
+    ("Uzbek", "uz"),
+    ("Vietnamese", "vi"),
+    ("Welsh", "cy"),
+    ("Yiddish", "yi"),
+];
+
+/// New-style language tags (ISO 639 + ISO 15924) to Unix names (`langtag_table`).
+pub(crate) const LANGTAG_TABLE: [(&str, &str); 16] = [
+    ("az-Latn", "az"),
+    ("bs-Latn", "bs"),
+    ("ga-dots", "ga"),
+    ("kk-Cyrl", "kk"),
+    ("mn-Cyrl", "mn"),
+    ("ms-Latn", "ms"),
+    ("pa-Arab", "pa_PK"),
+    ("pa-Guru", "pa_IN"),
+    ("sr-Cyrl", "sr"),
+    ("tg-Cyrl", "tg"),
+    ("tk-Cyrl", "tk"),
+    ("tt-Cyrl", "tt"),
+    ("uz-Latn", "uz"),
+    ("yue-Hans", "yue"),
+    ("zh-Hans", "zh_CN"),
+    ("zh-Hant", "zh_TW"),
+];
+
+/// Script names (ISO 15924) to Unix conventions (`script_table`).
+pub(crate) const SCRIPT_TABLE: [(&str, &str); 4] = [
+    ("Arab", "arabic"),
+    ("Cyrl", "cyrillic"),
+    ("Latn", "latin"),
+    ("Mong", "mongolian"),
+];
diff --git a/crates/ghostty-os/src/lib.rs b/crates/ghostty-os/src/lib.rs
index 51fd3df..da4a702 100644
--- a/crates/ghostty-os/src/lib.rs
+++ b/crates/ghostty-os/src/lib.rs
@@ -30,6 +30,7 @@ pub mod file;
 pub mod file_type;
 #[cfg(unix)]
 pub mod flatpak;
+pub mod osbytes;
 #[cfg(windows)]
 pub mod flatpak {
     //! Flatpak does not exist on Windows.
@@ -45,6 +46,8 @@ pub mod homedir;
 pub mod hostname;
 #[cfg(not(target_arch = "wasm32"))]
 pub mod i18n;
+#[cfg(not(target_arch = "wasm32"))]
+mod i18n_canon_table;
 pub mod i18n_locales;
 #[cfg(not(target_arch = "wasm32"))]
 pub mod kernel_info;
@@ -73,10 +76,23 @@ pub mod passwd {
         pub name: Option<String>,
     }
 
+    /// Byte-preserving entry (same shape as the Unix one).
+    #[derive(Debug, Clone, Default, PartialEq, Eq)]
+    pub struct EntryOs {
+        pub shell: Option<std::ffi::OsString>,
+        pub home: Option<std::ffi::OsString>,
+        pub name: Option<std::ffi::OsString>,
+    }
+
     /// No passwd database on Windows: always an empty entry.
     pub fn get() -> std::io::Result<Entry> {
         Ok(Entry::default())
     }
+
+    /// See [`get`].
+    pub fn get_os() -> std::io::Result<EntryOs> {
+        Ok(EntryOs::default())
+    }
 }
 #[cfg(not(target_arch = "wasm32"))]
 pub mod path;
@@ -123,13 +139,13 @@ pub use env::{
     get_env_map,
     getenv,
     getenv_not_empty,
+    getenv_not_empty_os,
+    getenv_os,
     prepend_env,
     prepend_env_os,
 };
 #[cfg(not(target_arch = "wasm32"))]
-pub use file::alloc_tmp_dir;
-#[cfg(unix)]
-pub use file::{Rlimit, fix_max_files, restore_max_files};
+pub use file::{Rlimit, alloc_tmp_dir, alloc_tmp_dir_os, fix_max_files, restore_max_files};
 #[cfg(unix)]
 pub use flatpak::FlatpakHostCommand;
 #[cfg(not(target_arch = "wasm32"))]
diff --git a/crates/ghostty-os/src/osbytes.rs b/crates/ghostty-os/src/osbytes.rs
new file mode 100644
index 0000000..73fdce8
--- /dev/null
+++ b/crates/ghostty-os/src/osbytes.rs
@@ -0,0 +1,224 @@
+//! Byte-preserving conversions between OS strings and Rust `String`s.
+//!
+//! Zig treats paths, arguments and environment values as byte slices. Rust
+//! `String` cannot hold invalid UTF-8, so the String based layers (the config
+//! system) use a reversible encoding that is a true bijection on byte
+//! strings:
+//!
+//! * the escape character is the Unicode noncharacter `U+FDD0` (never used
+//!   in real text);
+//! * every byte that is not part of a valid UTF-8 sequence (always
+//!   `0x80..=0xFF`) is encoded as `U+FDD0` followed by the char `U+00bb`
+//!   (`bb` = the byte), e.g. `0xFF` becomes `U+FDD0 U+00FF`;
+//! * a literal `U+FDD0` in valid input is escaped as `U+FDD0 U+FDD0`.
+//!
+//! Hence every valid UTF-8 string without `U+FDD0` is encoded unchanged, and
+//! `config_str_to_bytes(bytes_to_config_string(x)) == x` for all byte strings.
+
+use std::ffi::{OsStr, OsString};
+
+const ESC: char = '\u{FDD0}';
+
+/// Encode arbitrary bytes into a `String`, escaping invalid UTF-8 bytes.
+pub fn bytes_to_config_string(bytes: &[u8]) -> String {
+    let mut out = String::with_capacity(bytes.len());
+    for chunk in bytes.utf8_chunks() {
+        for c in chunk.valid().chars() {
+            out.push(c);
+            if c == ESC {
+                out.push(ESC);
+            }
+        }
+        for &b in chunk.invalid() {
+            out.push(ESC);
+            out.push(char::from(b));
+        }
+    }
+    out
+}
+
+/// Inverse of [`bytes_to_config_string`].
+pub fn config_str_to_bytes(s: &str) -> Vec<u8> {
+    let mut out = Vec::with_capacity(s.len());
+    let mut chars = s.chars().peekable();
+    while let Some(c) = chars.next() {
+        if c == ESC {
+            match chars.peek().copied() {
+                Some(ESC) => {
+                    chars.next();
+                }
+                Some(n) if (0x80..=0xFF).contains(&(n as u32)) => {
+                    chars.next();
+                    out.push(n as u32 as u8);
+                    continue;
+                }
+                // Not produced by the encoder: keep the char as-is.
+                _ => {}
+            }
+        }
+        let mut buf = [0u8; 4];
+        out.extend_from_slice(c.encode_utf8(&mut buf).as_bytes());
+    }
+    out
+}
+
+/// The raw bytes of an OS string (Unix; lossless). On other platforms the
+/// string is converted via its UTF-8 (WTF-8) form.
+#[cfg(unix)]
+pub fn os_bytes(s: &OsStr) -> Vec<u8> {
+    use std::os::unix::ffi::OsStrExt;
+    s.as_bytes().to_vec()
+}
+
+#[cfg(not(unix))]
+pub fn os_bytes(s: &OsStr) -> Vec<u8> {
+    s.to_string_lossy().into_owned().into_bytes()
+}
+
+/// Build an OS string from raw bytes.
+#[cfg(unix)]
+pub fn os_from_bytes(b: Vec<u8>) -> OsString {
+    use std::os::unix::ffi::OsStringExt;
+    OsString::from_vec(b)
+}
+
+#[cfg(not(unix))]
+pub fn os_from_bytes(b: Vec<u8>) -> OsString {
+    OsString::from(String::from_utf8_lossy(&b).into_owned())
+}
+
+/// OS string to reversible `String` (see the module docs).
+pub fn os_to_config_string(s: &OsStr) -> String {
+    #[cfg(unix)]
+    {
+        use std::os::unix::ffi::OsStrExt;
+        bytes_to_config_string(s.as_bytes())
+    }
+    #[cfg(not(unix))]
+    {
+        s.to_string_lossy().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))
+}
+
+/// Path to reversible `String` (see the module docs).
+pub fn path_to_config_string(p: &std::path::Path) -> String {
+    os_to_config_string(p.as_os_str())
+}
+
+/// Inverse of [`path_to_config_string`].
+pub fn config_str_to_path(s: &str) -> std::path::PathBuf {
+    std::path::PathBuf::from(config_str_to_os(s))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn roundtrip_invalid_bytes() {
+        let raw = vec![b'a', 0xff, b'b', 0xc3, 0xa9, 0xc3];
+        let s = bytes_to_config_string(&raw);
+        assert_eq!(s, "a\u{fdd0}\u{ff}b\u{e9}\u{fdd0}\u{c3}");
+        assert_eq!(config_str_to_bytes(&s), raw);
+    }
+
+    #[test]
+    fn valid_utf8_unchanged() {
+        let s = "h\u{e9}llo \u{1f600}";
+        assert_eq!(bytes_to_config_string(s.as_bytes()), s);
+        assert_eq!(config_str_to_bytes(s), s.as_bytes());
+        // Nerd Font icons, including the old escape range, are untouched.
+        let icons = "\u{f500}\u{f780}\u{f7ff}\u{fd46}\u{e0b0}";
+        assert_eq!(bytes_to_config_string(icons.as_bytes()), icons);
+        assert_eq!(config_str_to_bytes(icons), icons.as_bytes());
+    }
+
+    #[test]
+    fn literal_escape_char_is_doubled() {
+        let s = "a\u{fdd0}\u{ff}";
+        let e = bytes_to_config_string(s.as_bytes());
+        assert_eq!(e, "a\u{fdd0}\u{fdd0}\u{ff}");
+        assert_eq!(config_str_to_bytes(&e), s.as_bytes());
+    }
+
+    /// Tiny xorshift generator (no external rand dependency).
+    fn rng(seed: &mut u64) -> u64 {
+        *seed ^= *seed << 13;
+        *seed ^= *seed >> 7;
+        *seed ^= *seed << 17;
+        *seed
+    }
+
+    #[test]
+    fn property_bytes_round_trip() {
+        // Fragments that stress the boundaries: icon ranges, the escape
+        // char, its doubled form, and truncated/invalid sequences.
+        let frags: [&[u8]; 12] = [
+            "\u{f780}".as_bytes(),
+            "\u{f7ff}".as_bytes(),
+            "\u{f500}".as_bytes(),
+            "\u{fd46}".as_bytes(),
+            "\u{fdd0}".as_bytes(),
+            "\u{fdd0}\u{fdd0}".as_bytes(),
+            "\u{fdd0}\u{ff}".as_bytes(),
+            "\u{1f600}".as_bytes(),
+            b"\xef\xb7",
+            b"\xff",
+            b"\xc3",
+            b"abc",
+        ];
+        let mut seed = 0x9E37_79B9_7F4A_7C15u64;
+        for _ in 0..5000 {
+            let mut x = Vec::new();
+            for _ in 0..(rng(&mut seed) % 12) {
+                match rng(&mut seed) % 3 {
+                    0 => x.push(rng(&mut seed) as u8),
+                    _ => x.extend_from_slice(frags[(rng(&mut seed) % 12) as usize]),
+                }
+            }
+            let enc = bytes_to_config_string(&x);
+            assert_eq!(config_str_to_bytes(&enc), x, "bytes {x:?}");
+            if let Ok(valid) = std::str::from_utf8(&x)
+                && !valid.contains('\u{fdd0}')
+            {
+                assert_eq!(enc, valid);
+            }
+        }
+    }
+
+    #[test]
+    fn property_valid_strings_round_trip() {
+        // Every string (valid UTF-8) decodes back to itself after encoding.
+        let mut seed = 0x1234_5678_9ABC_DEF1u64;
+        for _ in 0..5000 {
+            let mut s = String::new();
+            for _ in 0..(rng(&mut seed) % 10) {
+                let cp = match rng(&mut seed) % 4 {
+                    0 => 0xF500 + (rng(&mut seed) % 0x847) as u32,
+                    1 => 0xFDD0,
+                    2 => 0x80 + (rng(&mut seed) % 0x80) as u32,
+                    _ => 0x20 + (rng(&mut seed) % 0x5f) as u32,
+                };
+                s.push(char::from_u32(cp).unwrap());
+            }
+            let enc = bytes_to_config_string(s.as_bytes());
+            assert_eq!(config_str_to_bytes(&enc), s.as_bytes(), "{s:?}");
+            if !s.contains('\u{fdd0}') {
+                assert_eq!(enc, s);
+            }
+        }
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn os_roundtrip() {
+        let o = os_from_bytes(vec![0xff, b'/', 0x80]);
+        let s = os_to_config_string(&o);
+        assert_eq!(config_str_to_os(&s), o);
+    }
+}
diff --git a/crates/ghostty-os/src/passwd.rs b/crates/ghostty-os/src/passwd.rs
index 6f28a99..b903949 100644
--- a/crates/ghostty-os/src/passwd.rs
+++ b/crates/ghostty-os/src/passwd.rs
@@ -1,5 +1,8 @@
+use std::ffi::OsString;
+
 #[cfg(any(target_os = "linux", target_os = "android"))]
 use crate::flatpak::is_flatpak;
+use crate::osbytes;
 
 /// Entry that is retrieved from the passwd API. This only contains the fields
 /// we care about.
@@ -10,19 +13,45 @@ pub struct Entry {
     pub name: Option<String>,
 }
 
-/// Get the passwd entry for the currently executing user.
+/// Byte-preserving [`Entry`] (Zig slices are arbitrary bytes).
+#[derive(Debug, Clone, Default, PartialEq, Eq)]
+pub struct EntryOs {
+    pub shell: Option<OsString>,
+    pub home: Option<OsString>,
+    pub name: Option<OsString>,
+}
+
+impl From<EntryOs> for Entry {
+    fn from(e: EntryOs) -> Self {
+        let f = |v: Option<OsString>| v.map(|v| osbytes::os_to_config_string(&v));
+        Entry {
+            shell: f(e.shell),
+            home: f(e.home),
+            name: f(e.name),
+        }
+    }
+}
+
+/// Get the passwd entry for the currently executing user. Non-UTF-8 bytes
+/// are carried via the reversible [`crate::osbytes`] encoding; see
+/// [`get_os`] for the raw bytes.
 pub fn get() -> std::io::Result<Entry> {
+    get_os().map(Entry::from)
+}
+
+/// Byte-preserving [`get`].
+pub fn get_os() -> std::io::Result<EntryOs> {
     let uid = nix::unistd::getuid();
     let user = match nix::unistd::User::from_uid(uid) {
         Ok(Some(user)) => user,
         Ok(None) => {
             // Future: let's check if a better shell is available like zsh
             log::warn!("no pw entry to detect default shell, will default to 'sh'");
-            return Ok(Entry::default());
+            return Ok(EntryOs::default());
         }
         Err(e) => {
             log::warn!("error retrieving pw entry code={e}");
-            return Ok(Entry::default());
+            return Ok(EntryOs::default());
         }
     };
 
@@ -35,10 +64,10 @@ pub fn get() -> std::io::Result<Entry> {
         return host_entry(&user.name);
     }
 
-    Ok(Entry {
-        shell: Some(user.shell.to_string_lossy().into_owned()),
-        home: Some(user.dir.to_string_lossy().into_owned()),
-        name: Some(user.name),
+    Ok(EntryOs {
+        shell: Some(user.shell.into_os_string()),
+        home: Some(user.dir.into_os_string()),
+        name: Some(OsString::from(user.name)),
     })
 }
 
@@ -49,7 +78,7 @@ pub fn get() -> std::io::Result<Entry> {
 /// `Pty`) depends on this crate, so a minimal openpt/ptsname pair is opened
 /// here with rustix instead. Like Zig, only `shell` and `home` are filled in.
 #[cfg(any(target_os = "linux", target_os = "android"))]
-fn host_entry(name: &str) -> std::io::Result<Entry> {
+fn host_entry(name: &str) -> std::io::Result<EntryOs> {
     use std::io::Read;
 
     use rustix::{
@@ -112,17 +141,20 @@ fn host_entry(name: &str) -> std::io::Result<Entry> {
         }
     }
 
-    Ok(parse_getent(&String::from_utf8_lossy(&output)))
+    Ok(parse_getent(&output))
 }
 
-#[cfg(any(target_os = "linux", target_os = "android"))]
 /// Shell and home are the last two `:`-separated fields of the output.
 #[cfg(any(target_os = "linux", target_os = "android"))]
-fn parse_getent(output: &str) -> Entry {
-    let mut it = output.trim_end_matches([' ', '\r', '\n']).rsplit(':');
-    let shell = it.next().map(str::to_owned);
-    let home = it.next().map(str::to_owned);
-    Entry {
+fn parse_getent(output: &[u8]) -> EntryOs {
+    let end = output
+        .iter()
+        .rposition(|b| !matches!(b, b' ' | b'\r' | b'\n'))
+        .map_or(0, |i| i + 1);
+    let mut it = output[..end].rsplit(|&b| b == b':');
+    let shell = it.next().map(|b| osbytes::os_from_bytes(b.to_vec()));
+    let home = it.next().map(|b| osbytes::os_from_bytes(b.to_vec()));
+    EntryOs {
         shell,
         home,
         name: None,
@@ -136,12 +168,24 @@ mod tests {
     #[cfg(any(target_os = "linux", target_os = "android"))]
     #[test]
     fn parse_getent_output() {
-        let e = parse_getent("mitchell:x:1000:1000:Mitchell:/home/mitchell:/bin/zsh\r\n");
-        assert_eq!(e.shell.as_deref(), Some("/bin/zsh"));
-        assert_eq!(e.home.as_deref(), Some("/home/mitchell"));
+        let e = parse_getent(b"mitchell:x:1000:1000:Mitchell:/home/mitchell:/bin/zsh\r\n");
+        assert_eq!(e.shell.as_deref(), Some(std::ffi::OsStr::new("/bin/zsh")));
+        assert_eq!(
+            e.home.as_deref(),
+            Some(std::ffi::OsStr::new("/home/mitchell"))
+        );
         assert_eq!(e.name, None);
     }
 
+    #[cfg(any(target_os = "linux", target_os = "android"))]
+    #[test]
+    fn parse_getent_keeps_non_utf8() {
+        use std::os::unix::ffi::OsStrExt;
+        let e = parse_getent(b"u:x:1:1::/home/\xff:/bin/\xfesh\n");
+        assert_eq!(e.shell.unwrap().as_bytes(), b"/bin/\xfesh");
+        assert_eq!(e.home.unwrap().as_bytes(), b"/home/\xff");
+    }
+
     #[test]
     fn get_entry() {
         // We should be able to get an entry
diff --git a/crates/ghostty-os/src/path.rs b/crates/ghostty-os/src/path.rs
index 7eeaad3..01ca94c 100644
--- a/crates/ghostty-os/src/path.rs
+++ b/crates/ghostty-os/src/path.rs
@@ -1,6 +1,6 @@
-use std::io;
 #[cfg(unix)]
 use std::os::unix::fs::PermissionsExt;
+use std::{ffi::OsStr, io, path::PathBuf};
 
 /// `std.fs.path.delimiter` / `sep`.
 #[cfg(windows)]
@@ -25,28 +25,44 @@ pub enum Error {
     Io(#[from] io::Error),
 }
 
-/// Search for "cmd" in the PATH and return the absolute path.
+/// Search for "cmd" in the PATH and return the absolute path. Non-UTF-8
+/// bytes are carried via [`crate::osbytes`]; see [`expand_os`].
 pub fn expand(cmd: &str) -> Result<Option<String>, Error> {
+    let cmd = crate::osbytes::config_str_to_os(cmd);
+    Ok(expand_os(&cmd)?.map(|p| crate::osbytes::os_to_config_string(p.as_os_str())))
+}
+
+/// Byte-preserving [`expand`]: `$PATH` and the command are handled as raw
+/// bytes, so executables in directories with non-UTF-8 names are found.
+pub fn expand_os(cmd: &OsStr) -> Result<Option<PathBuf>, Error> {
+    let cmd_bytes = crate::osbytes::os_bytes(cmd);
+
     // If the command already contains a slash, then we return it as-is
     // because it is assumed to be absolute or relative.
-    if cmd.contains('/') {
-        return Ok(Some(cmd.to_owned()));
+    if cmd_bytes.contains(&b'/') {
+        return Ok(Some(PathBuf::from(cmd)));
     }
 
     let Some(path_var) = std::env::var_os("PATH") else {
         return Ok(None);
     };
-    let path_var = path_var.to_string_lossy();
+    let path_var = crate::osbytes::os_bytes(&path_var);
 
     let mut seen_eacces = false;
-    for search_path in path_var.split(DELIMITER).filter(|s| !s.is_empty()) {
+    for search_path in path_var
+        .split(|&b| b == DELIMITER as u8)
+        .filter(|s| !s.is_empty())
+    {
         // We need enough space in our path buffer to store this
-        let path_len = search_path.len() + cmd.len() + 1;
+        let path_len = search_path.len() + cmd_bytes.len() + 1;
         if MAX_PATH_BYTES < path_len {
             return Err(Error::PathTooLong);
         }
 
-        let full_path = format!("{search_path}{SEP}{cmd}");
+        let mut full = search_path.to_vec();
+        full.push(SEP as u8);
+        full.extend_from_slice(&cmd_bytes);
+        let full_path = PathBuf::from(crate::osbytes::os_from_bytes(full));
 
         // Stat it
         let f = match std::fs::File::open(&full_path) {
@@ -127,4 +143,23 @@ mod tests {
             assert!(expand("adir").unwrap().is_none());
         });
     }
+
+    #[cfg(unix)]
+    #[test]
+    fn expand_finds_executable_in_non_utf8_dir() {
+        use std::os::unix::{
+            ffi::{OsStrExt, OsStringExt},
+            fs::PermissionsExt,
+        };
+        let td = crate::TempDir::init().unwrap();
+        let dir = td.path().join(OsStr::from_bytes(b"d\xffir"));
+        std::fs::create_dir(&dir).unwrap();
+        let exe = dir.join("mytool");
+        std::fs::write(&exe, b"#!/bin/sh\n").unwrap();
+        std::fs::set_permissions(&exe, std::fs::Permissions::from_mode(0o755)).unwrap();
+        let pv = std::ffi::OsString::from_vec(dir.as_os_str().as_bytes().to_vec());
+        temp_env::with_var("PATH", Some(&pv), || {
+            assert_eq!(expand_os(OsStr::new("mytool")).unwrap(), Some(exe.clone()));
+        });
+    }
 }
diff --git a/crates/ghostty-os/src/resourcesdir.rs b/crates/ghostty-os/src/resourcesdir.rs
index d98d278..ec39798 100644
--- a/crates/ghostty-os/src/resourcesdir.rs
+++ b/crates/ghostty-os/src/resourcesdir.rs
@@ -1,6 +1,14 @@
-use std::{io, path::Path};
+use std::{
+    ffi::OsString,
+    io,
+    path::{Path, PathBuf},
+};
 
-use crate::path_join;
+use crate::osbytes::{config_str_to_os, os_to_config_string};
+
+fn enc(p: &Path) -> String {
+    os_to_config_string(p.as_os_str())
+}
 
 #[derive(Debug, Clone, Default, PartialEq, Eq)]
 pub struct ResourcesDir {
@@ -25,6 +33,17 @@ impl ResourcesDir {
     pub fn host(&self) -> Option<&str> {
         self.host_path.as_deref().or(self.app_path.as_deref())
     }
+
+    /// Byte-preserving [`ResourcesDir::app`]: the string fields carry
+    /// non-UTF-8 bytes via [`crate::osbytes`], this decodes them.
+    pub fn app_os(&self) -> Option<PathBuf> {
+        self.app().map(|p| PathBuf::from(config_str_to_os(p)))
+    }
+
+    /// Byte-preserving [`ResourcesDir::host`].
+    pub fn host_os(&self) -> Option<PathBuf> {
+        self.host().map(|p| PathBuf::from(config_str_to_os(p)))
+    }
 }
 
 /// Sentinels we look for in the path to know we've found the resources
@@ -50,7 +69,7 @@ const DEV_SENTINELS: [&str; 1] = ["terminfo/x/xterm-ghostty"];
 /// Zig's `builtin.mode == .Debug` is mapped to `cfg!(debug_assertions)`.
 pub fn resources_dir() -> io::Result<ResourcesDir> {
     let exe = std::env::current_exe().ok();
-    let env = std::env::var("GHOSTTY_RESOURCES_DIR").ok();
+    let env = std::env::var_os("GHOSTTY_RESOURCES_DIR");
     Ok(resources_dir_impl(
         cfg!(debug_assertions),
         exe.as_deref(),
@@ -58,7 +77,7 @@ pub fn resources_dir() -> io::Result<ResourcesDir> {
     ))
 }
 
-fn resources_dir_impl(debug: bool, exe: Option<&Path>, env: Option<String>) -> ResourcesDir {
+fn resources_dir_impl(debug: bool, exe: Option<&Path>, env: Option<OsString>) -> ResourcesDir {
     // Use the GHOSTTY_RESOURCES_DIR environment variable in release builds.
     //
     // In debug builds we try using terminfo detection first instead, since
@@ -67,7 +86,7 @@ fn resources_dir_impl(debug: bool, exe: Option<&Path>, env: Option<String>) -> R
     // freshly built ones under zig-out/share/ghostty.
     if !debug && let Some(dir) = env.as_ref().filter(|d| !d.is_empty()) {
         return ResourcesDir {
-            app_path: Some(dir.clone()),
+            app_path: Some(os_to_config_string(dir)),
             host_path: None,
         };
     }
@@ -82,14 +101,12 @@ fn resources_dir_impl(debug: bool, exe: Option<&Path>, env: Option<String>) -> R
     // bundle as we expect it.
     while let Some(dir) = exe.parent() {
         exe = dir;
-        let dir_str = dir.to_string_lossy();
-
         // On MacOS, we look for the app bundle path.
         #[cfg(target_vendor = "apple")]
         for sentinel in SENTINELS {
-            if let Some(v) = maybe_dir(&dir_str, "Contents/Resources", sentinel) {
+            if let Some(v) = maybe_dir_os(dir, "Contents/Resources", sentinel) {
                 return ResourcesDir {
-                    app_path: Some(path_join(&[&v, "ghostty"])),
+                    app_path: Some(enc(&v.join("ghostty"))),
                     host_path: None,
                 };
             }
@@ -97,9 +114,9 @@ fn resources_dir_impl(debug: bool, exe: Option<&Path>, env: Option<String>) -> R
 
         // We look for a /usr/share style path.
         for sentinel in SENTINELS {
-            if let Some(v) = maybe_dir(&dir_str, "share", sentinel) {
+            if let Some(v) = maybe_dir_os(dir, "share", sentinel) {
                 return ResourcesDir {
-                    app_path: Some(path_join(&[&v, "ghostty"])),
+                    app_path: Some(enc(&v.join("ghostty"))),
                     host_path: None,
                 };
             }
@@ -110,7 +127,7 @@ fn resources_dir_impl(debug: bool, exe: Option<&Path>, env: Option<String>) -> R
     // fallback and use the provided resources dir.
     if debug && let Some(dir) = env.filter(|d| !d.is_empty()) {
         return ResourcesDir {
-            app_path: Some(dir),
+            app_path: Some(os_to_config_string(&dir)),
             host_path: None,
         };
     }
@@ -122,7 +139,7 @@ fn resources_dir_impl(debug: bool, exe: Option<&Path>, env: Option<String>) -> R
     // lives inside this workspace.
     if debug && let Some(dir) = dev_resources_dir(exe_start) {
         return ResourcesDir {
-            app_path: Some(dir),
+            app_path: Some(enc(&dir)),
             host_path: None,
         };
     }
@@ -137,7 +154,7 @@ fn workspace_root() -> std::path::PathBuf {
 
 /// Returns `<workspace>/resources` if `exe` is inside the workspace this
 /// crate was built from and the directory contains our sentinel.
-fn dev_resources_dir(exe: &Path) -> Option<String> {
+fn dev_resources_dir(exe: &Path) -> Option<PathBuf> {
     let root = workspace_root().canonicalize().ok()?;
     let exe = exe.canonicalize().unwrap_or_else(|_| exe.to_path_buf());
     if !exe.starts_with(&root) {
@@ -148,7 +165,15 @@ fn dev_resources_dir(exe: &Path) -> Option<String> {
         .iter()
         .chain(DEV_SENTINELS.iter())
         .any(|s| res.join(s).exists())
-        .then(|| res.to_string_lossy().into_owned())
+        .then_some(res)
+}
+
+/// Byte-preserving [`maybe_dir`].
+pub fn maybe_dir_os(base: &Path, sub: &str, suffix: &str) -> Option<PathBuf> {
+    let dir = base.join(sub);
+    // Folder doesn't exist. If a different error happens its okay
+    // we just ignore it and move on.
+    dir.join(suffix).exists().then_some(dir)
 }
 
 /// Little helper to check if the "base/sub/suffix" directory exists and
@@ -204,7 +229,7 @@ mod tests {
     fn env_in_release_wins_and_in_debug_is_fallback() {
         let td = make_tree();
         let exe = td.path().join("bin/ghostty");
-        let env = Some("/env/res".to_owned());
+        let env = Some(OsString::from("/env/res"));
         assert_eq!(
             resources_dir_impl(false, Some(&exe), env.clone()).app(),
             Some("/env/res")
@@ -220,7 +245,7 @@ mod tests {
         );
         // Empty env is ignored.
         assert_eq!(
-            resources_dir_impl(false, None, Some(String::new())).app(),
+            resources_dir_impl(false, None, Some(OsString::new())).app(),
             None
         );
     }
@@ -238,6 +263,23 @@ mod tests {
         assert_eq!(r.app(), None);
     }
 
+    #[test]
+    fn non_utf8_env_dir_is_honored_and_exe_dir_detected() {
+        use std::os::unix::ffi::{OsStrExt, OsStringExt};
+        let env = OsString::from_vec(b"/env/\xffres".to_vec());
+        let r = resources_dir_impl(false, None, Some(env.clone()));
+        assert_eq!(r.app_os().unwrap().as_os_str(), env.as_os_str());
+
+        // Executable lives under a directory with a non-UTF-8 name.
+        let td = crate::TempDir::init().unwrap();
+        let root = td.path().join(std::ffi::OsStr::from_bytes(b"r\xfeoot"));
+        std::fs::create_dir_all(root.join("share/terminfo/x")).unwrap();
+        std::fs::write(root.join("share/terminfo/x/xterm-ghostty"), b"").unwrap();
+        std::fs::create_dir_all(root.join("bin")).unwrap();
+        let r = resources_dir_impl(true, Some(&root.join("bin/ghostty")), None);
+        assert_eq!(r.app_os().unwrap(), root.join("share/ghostty"));
+    }
+
     #[test]
     fn maybe_dir_checks_suffix() {
         let td = make_tree();
diff --git a/crates/ghostty-os/src/temp_dir.rs b/crates/ghostty-os/src/temp_dir.rs
index db27290..cae10f6 100644
--- a/crates/ghostty-os/src/temp_dir.rs
+++ b/crates/ghostty-os/src/temp_dir.rs
@@ -8,7 +8,7 @@ use std::{
     path::{Path, PathBuf},
 };
 
-use crate::file::alloc_tmp_dir;
+use crate::file::alloc_tmp_dir_os;
 
 // The amount of random bytes to get to determine our filename.
 const RANDOM_BYTES: usize = 16;
@@ -89,8 +89,8 @@ fn fill_random(buf: &mut [u8]) -> io::Result<()> {
 impl TempDir {
     /// Create the temporary directory.
     pub fn init() -> io::Result<TempDir> {
-        let parent = match alloc_tmp_dir() {
-            Some(p) => PathBuf::from(p),
+        let parent = match alloc_tmp_dir_os() {
+            Some(p) => p,
             None => std::env::current_dir()?,
         };
         // Verify the parent can be opened (Zig opens it as a Dir).
diff --git a/docs/rewrite/notes/final-Z1.md b/docs/rewrite/notes/final-Z1.md
new file mode 100644
index 0000000..8dfa871
--- /dev/null
+++ b/docs/rewrite/notes/final-Z1.md
@@ -0,0 +1,79 @@
+# Z1 final pass: os / config / cli / root main (byte fidelity, i18n, build info)
+
+Owner: Z1. Crates: `ghostty-os`, `ghostty-config`, `ghostty-cli`, new `ghostty-build-info`, root `src/main.rs`.
+
+## Byte fidelity design
+
+Zig paths/args/env are byte slices. The config system is `&str`/`String` based
+throughout, so rewriting it to a byte type was not feasible without changing
+hundreds of `ConfigField` impls. Instead:
+
+* `ghostty_os::osbytes` is the single conversion module:
+  * `bytes_to_config_string` / `config_str_to_bytes`, `os_to_config_string` /
+    `config_str_to_os`, `path_to_config_string` / `config_str_to_path`,
+    `os_bytes`, `os_from_bytes`.
+  * The encoding is a true bijection on byte strings. Escape char = noncharacter
+    `U+FDD0`; an invalid byte `b` becomes `U+FDD0` + char `U+00bb` (`0xFF` ->
+    `U+FDD0 U+00FF`); a literal `U+FDD0` in valid input becomes `U+FDD0 U+FDD0`.
+    Valid UTF-8 without `U+FDD0` is encoded unchanged (Nerd Font icons, incl.
+    `U+F500..U+FD46`, are safe), and `decode(encode(x)) == x` for all bytes.
+    Property tests (random bytes and strings, icon ranges, escape char) cover this.
+* Every os function that returned `String` now has a byte-exact OS-typed twin
+  (`*_os`), and the `String` function is a thin wrapper using the reversible
+  encoding (a present non-UTF-8 value is never "absent" and never U+FFFD).
+  Existing callers keep compiling.
+* The config/cli layers decode back to OS strings at every filesystem / exec /
+  stdout boundary they own.
+
+## Audit rows fixed
+
+| Row | Change |
+|---|---|
+| main_ghostty argv | `src/main.rs` collects `Vec<OsString>` (`os::args::collect_os`), dispatches with `ghostty_cli::ghostty::{detect_os, run_from_os_args}` (reversible encoding; no lossy). `collect_lossy` removed; `collect_bytes`, `collect_config_strings` added. |
+| cli/args.zig | `ArgsIterator` via `args::collect_config_strings`; `LineIterator` no longer `from_utf8_lossy`; `parse_zig_string_literal_bytes` returns raw bytes (`\xFF` stays `0xFF`), `parse_zig_string_literal` returns the reversible string. |
+| Config.zig | xdg-terminal-exec args and `cwd` keep bytes; `load_file`, config-file open, `load_reader` parent dir decode/encode paths. |
+| os/env | `getenv_os`, `getenv_not_empty_os`; `getenv*` return `Some` for non-UTF-8 values. |
+| os/homedir | `home_os`, `home_os_with_capacity`, `expand_home_os[_with_capacity]`; HOME, passwd and `cd && pwd` bytes kept (shell output trimmed as bytes). |
+| os/path | `path::expand_os` ($PATH split and lookup as bytes). |
+| os/passwd | `EntryOs`, `get_os` (native `shell`/`home`, flatpak `getent` output parsed as bytes). `Entry`/`get` unchanged shape (encoded strings). |
+| os/resourcesdir | `GHOSTTY_RESOURCES_DIR` read via `var_os`; exe-relative probing uses `Path`s (`maybe_dir_os`); `ResourcesDir::{app_os, host_os}`. |
+| os/file | `alloc_tmp_dir_os` (TMPDIR/TMP bytes, used by `TempDir`); `Rlimit` (unit struct), `fix_max_files` (-> `None`), `restore_max_files` (no-op) now exist on non-Unix targets like Zig's fallbacks. |
+| os/cgroup | `current_os`; `scope_matches` compares bytes. |
+| config/path | canonicalization, `resolve_lexical`, `theme.rs` locations use decoded paths and keep canonical (symlink-resolved) non-UTF-8 targets. |
+| cli/validate_config | canonicalizes the decoded option path and passes the reversible string to `load_file`. |
+| cli/list_themes | directory entries keep raw bytes (names/paths), plain listing writes raw bytes, `read_dir` on decoded dir; preview auto-theme file/clipboard copy decode bytes. |
+| cli edit_config / new_window | exec editor / canonical working directory keep bytes. |
+| os/i18n | `canonicalize_locale` is now an exact port of gnulib `gl_locale_name_canonicalize` (macOS flavour used by `_libintl_locale_name_canonicalize`): legacy table (131 names), langtag table (16), script table (4), `strlen==7` guard, dash->underscore; tables in `i18n_canon_table.rs` generated from gettext 1.0's `localename-unsafe.c`. `pa-Guru` -> `pa_IN`, `sr-Latn` -> `sr@latin`, `Brazilian Portuguese` -> `pt_BR`. |
+
+Not touched (other owners): `ghostty-lib` `ghostty_init`/`config_capi`/`embedded` lossy
+conversions, GTK file drop, `Surface::pwd()` -- Z2. Z2 may use the `osbytes`
+helpers and `*_os` functions; `Entry`/`ResourcesDir` string fields carry the
+reversible encoding, so use `config_str_to_os` before using them as paths.
+
+## Build info (single source of truth) -- Z1 owns, nobody else edits
+
+New crate `crates/ghostty-build-info` (no deps, `build.rs` derived; moved out of
+`ghostty-cli/build.rs`, which now only keeps the action help table and
+`GHOSTTY_RUSTC_VERSION`). Added to `[workspace.dependencies]`.
+
+```rust
+ghostty_build_info::VERSION_STRING   // &str  "1.3.1" or "1.3.1-<branch>+<hash>"; env GHOSTTY_VERSION_STRING overrides
+ghostty_build_info::COMMIT           // Option<&str>
+ghostty_build_info::RELEASE_CHANNEL  // "stable" | "tip"
+ghostty_build_info::BUILD_MODE       // ".Debug" | ".ReleaseSafe" | ".ReleaseFast"
+ghostty_build_info::{version(), parse(), SemanticVersion}
+```
+
+`ghostty_cli::build_info::Default` and `ghostty_cli::build_info::derived`
+re-export/use it. For Z2: add `ghostty-build-info.workspace = true` to
+termio/apprt-gtk/crash and use `VERSION_STRING` for XTVERSION
+(`stream_handler.rs`), `TERM_PROGRAM_VERSION` (`exec*.rs`), the About dialog
+(`window.rs`), and crash `release` (`sentry.rs`; `RELEASE_CHANNEL`/`COMMIT`
+for tags).
+
+## Tests added
+
+`osbytes` (3), env non-UTF-8, homedir non-UTF-8, path non-UTF-8 dir lookup, passwd getent bytes,
+resourcesdir non-UTF-8 env + exe dir, file tmpdir bytes, cgroup, i18n tables,
+cli_args (`\xFF` raw byte, LineIterator), config path symlink to non-UTF-8 target,
+validate_config symlink, list_themes non-UTF-8 entry, ghostty `os_args`, build-info (3).