fix: use Zig's numeric parsing in protocol and escape-sequence parsers

Rust's str::parse accepts and rejects different inputs than Zig's
std.fmt.parseInt/parseFloat (underscores, base prefixes, signs). The
config layer already used the Zig ports; now the protocol parsers do
too (call site -> Zig reference -> status in final-Z12.md):
- zig_fmt moves into the leaf crate ghostty-zigfmt (ghostty-vt cannot
  depend on ghostty-os); ghostty-os re-exports it unchanged.
- ghostty-vt: kitty graphics keys, kitty text sizing, OSC 4/5/10-19
  color indices, hex color components and intensities, OSC 9 sleep and
  progress, OSC 133 exit codes, OSC 3008 fields, kitty color palette.
- ghostty-terminal: tmux layout, format variables and pane tabs.
- ghostty-os: URI ports and %XX decoding; ghostty-bench CLI arguments.
Expected values were cross-checked against the real Zig toolchain.

Test Plan:
- cargo test --workspace --no-fail-fast
- just check-macos (both), just check-windows, just check-wasm

Cargo.lock

@@ -1618,6 +1618,7 @@ dependencies = [
  "crossbeam-channel",
  "gettext-rs",
  "ghostty-vt",
+ "ghostty-zigfmt",
  "libc",
  "log",
  "nix 0.31.3",
@@ -1698,6 +1699,7 @@ dependencies = [
  "ghostty-os",
  "ghostty-unicode",
  "ghostty-vt",
+ "ghostty-zigfmt",
  "log",
  "onig",
  "png",
@@ -1740,6 +1742,7 @@ name = "ghostty-vt"
 version = "0.1.0"
 dependencies = [
  "base64 0.23.1",
+ "ghostty-zigfmt",
  "log",
  "memchr",
  "thiserror 2.0.21",
@@ -1766,6 +1769,10 @@ dependencies = [
  "temp-env",
 ]
 
+[[package]]
+name = "ghostty-zigfmt"
+version = "0.1.0"
+
 [[package]]
 name = "gio"
 version = "0.22.10"

Cargo.toml

@@ -56,6 +56,7 @@ ghostty-font = { path = "crates/ghostty-font" }
 ghostty-renderer = { path = "crates/ghostty-renderer" }
 ghostty-termio = { path = "crates/ghostty-termio" }
 ghostty-os = { path = "crates/ghostty-os" }
+ghostty-zigfmt = { path = "crates/ghostty-zigfmt" }
 ghostty-onig = { path = "crates/ghostty-onig" }
 ghostty-datastruct = { path = "crates/ghostty-datastruct" }
 ghostty-core = { path = "crates/ghostty-core" }

crates/ghostty-bench/src/args.rs

@@ -6,11 +6,38 @@ use std::{
     fs::File,
     io::{self, Read},
     path::{Path, PathBuf},
-    str::FromStr,
 };
 
 use ghostty_os::osbytes::{os_bytes, os_from_bytes};
 
+/// Typed option value parsed like `cli/args.zig` `parseIntoField`
+/// (`std.fmt.parseInt(T, v, 0)` / `std.fmt.parseFloat`).
+pub trait ZigArg: Sized {
+    fn parse_zig(v: &[u8]) -> Option<Self>;
+}
+
+macro_rules! zig_arg_int {
+    ($($t:ty),*) => {$(
+        impl ZigArg for $t {
+            fn parse_zig(v: &[u8]) -> Option<Self> {
+                ghostty_os::zig_fmt::parse_int::<$t>(v, 0).ok()
+            }
+        }
+    )*};
+}
+zig_arg_int!(u8, u16, u32, u64, usize, i8, i16, i32, i64, isize);
+
+impl ZigArg for f64 {
+    fn parse_zig(v: &[u8]) -> Option<Self> {
+        ghostty_os::zig_fmt::parse_float::<f64>(v).ok()
+    }
+}
+impl ZigArg for f32 {
+    fn parse_zig(v: &[u8]) -> Option<Self> {
+        ghostty_os::zig_fmt::parse_float::<f32>(v).ok()
+    }
+}
+
 /// Parsed `--key[=value]` arguments.
 #[derive(Debug, Default)]
 pub struct Args {
@@ -65,12 +92,10 @@ impl Args {
     }
 
     /// Take a typed value, falling back to `default`.
-    pub fn take<T: FromStr>(&mut self, key: &str, default: T) -> Result<T, String> {
+    pub fn take<T: ZigArg>(&mut self, key: &str, default: T) -> Result<T, String> {
         match self.take_os(key)? {
             None => Ok(default),
-            Some(v) => v
+            Some(v) => T::parse_zig(&os_bytes(&v))
-                .to_str()
-                .and_then(|s| s.parse().ok())
                 .ok_or_else(|| format!("invalid value for --{key}: {}", v.to_string_lossy())),
         }
     }
@@ -145,6 +170,19 @@ mod tests {
 
     use super::*;
 
+    #[test]
+    fn take_uses_zig_parse_int_base_0() {
+        let args = ["--terminal-rows=0x10", "--terminal-cols=1_0", "--p=0x1p-1"];
+        let mut a = Args::parse(args.map(OsString::from)).unwrap();
+        assert_eq!(a.take("terminal-rows", 0u16).unwrap(), 16);
+        assert_eq!(a.take("terminal-cols", 0u16).unwrap(), 10);
+        assert_eq!(a.take("p", 0f64).unwrap(), 0.5);
+        let mut a = Args::parse([OsString::from("--n=+5")]).unwrap();
+        assert_eq!(a.take("n", 0u16).unwrap(), 5);
+        let mut a = Args::parse([OsString::from("--n=-1")]).unwrap();
+        assert!(a.take("n", 0u16).is_err());
+    }
+
     #[test]
     fn data_path_keeps_non_utf8_bytes() {
         let arg = OsString::from_vec(b"--data=/tmp/a\xffb".to_vec());

crates/ghostty-os/Cargo.toml

@@ -5,6 +5,7 @@ edition.workspace = true
 license.workspace = true
 
 [dependencies]
+ghostty-zigfmt.workspace = true
 ghostty-vt.workspace = true
 log = "0.4.34"
 thiserror = "2.0.21"

crates/ghostty-os/src/lib.rs

@@ -32,7 +32,7 @@ pub mod file_type;
 #[cfg(unix)]
 pub mod flatpak;
 pub mod osbytes;
-pub mod zig_fmt;
+pub use ghostty_zigfmt as zig_fmt;
 #[cfg(windows)]
 pub mod flatpak {
     //! Flatpak does not exist on Windows.

crates/ghostty-os/src/uri.rs

@@ -89,9 +89,10 @@ fn percent_decode(s: &str) -> Cow<'_, [u8]> {
     while i < b.len() {
         if b[i] == b'%'
             && i + 2 < b.len()
-            && let (Some(h), Some(l)) = (hexval(b[i + 1]), hexval(b[i + 2]))
+            // `std.fmt.parseInt(u8, "XX", 16)`: accepts a sign and `_`.
+            && let Ok(v) = crate::zig_fmt::parse_int::<u8>(&b[i + 1..i + 3], 16)
         {
-            out.push((h << 4) | l);
+            out.push(v);
             i += 3;
             continue;
         }
@@ -101,15 +102,6 @@ fn percent_decode(s: &str) -> Cow<'_, [u8]> {
     Cow::Owned(out)
 }
 
-fn hexval(c: u8) -> Option<u8> {
-    match c {
-        b'0'..=b'9' => Some(c - b'0'),
-        b'a'..=b'f' => Some(c - b'a' + 10),
-        b'A'..=b'F' => Some(c - b'A' + 10),
-        _ => None,
-    }
-}
-
 /// `std.Uri`.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Uri<'a> {
@@ -217,8 +209,7 @@ fn parse_authority<'a>(authority: &'a str, uri: &mut Uri<'a>) -> Result<(), Pars
             // if not part of the V6 address field
             end_of_host = end_of_host.min(index);
             uri.port = Some(
-                authority[index + 1..]
+                crate::zig_fmt::parse_int::<u16>(&authority.as_bytes()[index + 1..], 10)
-                    .parse::<u16>()
                     .map_err(|_| ParseError::InvalidPort)?,
             );
         }
@@ -228,8 +219,7 @@ fn parse_authority<'a>(authority: &'a str, uri: &mut Uri<'a>) -> Result<(), Pars
         // if not part of the userinfo field
         end_of_host = end_of_host.min(index);
         uri.port = Some(
-            authority[index + 1..]
+            crate::zig_fmt::parse_int::<u16>(&authority.as_bytes()[index + 1..], 10)
-                .parse::<u16>()
                 .map_err(|_| ParseError::InvalidPort)?,
         );
     }
@@ -411,6 +401,24 @@ fn is_valid_mac_address(s: &str) -> bool {
 mod tests {
     use super::*;
 
+    #[test]
+    fn zig_parse_int_port_and_percent() {
+        let u = parse("http://host:8_0/p", ParseOptions::default()).unwrap();
+        assert_eq!(u.port, Some(80));
+        let u = parse("http://host:+80/p", ParseOptions::default()).unwrap();
+        assert_eq!(u.port, Some(80));
+        assert!(parse("http://host:0x50/p", ParseOptions::default()).is_err());
+        assert!(parse("http://host:65536/p", ParseOptions::default()).is_err());
+        assert!(parse("http://host:-1/p", ParseOptions::default()).is_err());
+        // `%XX` is decoded with `parseInt(u8, XX, 16)`: sign and nothing else.
+        let u = parse(
+            "file://localhost/a%+Ab%-0c%_1d%1_e%41",
+            ParseOptions::default(),
+        )
+        .unwrap();
+        assert_eq!(&*u.path.to_raw_bytes(), b"/a\x0ab\x00c%_1d%1_eA");
+    }
+
     const MAC: ParseOptions = ParseOptions {
         mac_address: true,
         raw_path: false,

crates/ghostty-terminal/Cargo.toml

@@ -11,6 +11,7 @@ flate2 = "1.1.10"
 ghostty-datastruct.workspace = true
 ghostty-unicode.workspace = true
 ghostty-vt.workspace = true
+ghostty-zigfmt.workspace = true
 log = "0.4.34"
 png = "0.18.1"
 thiserror = "2.0.21"

crates/ghostty-terminal/src/tmux/layout.rs

@@ -41,22 +41,8 @@ pub enum ParseError {
 }
 
 fn parse_usize(s: &[u8]) -> Result<usize, ParseError> {
-    // std.fmt.parseInt(usize, s, 10): optional '+', digits only.
+    // std.fmt.parseInt(usize, s, 10) (underscores, sign rules).
-    let s = s.strip_prefix(b"+").unwrap_or(s);
+    ghostty_zigfmt::parse_int::<usize>(s, 10).map_err(|_| ParseError::SyntaxError)
-    if s.is_empty() {
-        return Err(ParseError::SyntaxError);
-    }
-    let mut v: usize = 0;
-    for &c in s {
-        if !c.is_ascii_digit() {
-            return Err(ParseError::SyntaxError);
-        }
-        v = v
-            .checked_mul(10)
-            .and_then(|v| v.checked_add(usize::from(c - b'0')))
-            .ok_or(ParseError::SyntaxError)?;
-    }
-    Ok(v)
 }
 
 fn index_of_any(s: &[u8], set: &[u8]) -> Option<usize> {
@@ -191,6 +177,23 @@ impl Checksum {
 mod tests {
     use super::*;
 
+    #[test]
+    fn zig_parse_int_differences() {
+        // `std.fmt.parseInt(usize, _, 10)`: `_` separators, `+`/`-0` signs.
+        let l = p("1_0x2_0,+3,-0,4").unwrap();
+        assert_eq!((l.width, l.height, l.x, l.y), (10, 20, 3, 0));
+        assert_eq!(pane(&l), 4);
+        assert_eq!(p("1_x2,3,4,5").err(), Some(ParseError::SyntaxError));
+        assert_eq!(p("0x10x2,3,4,5").err(), Some(ParseError::SyntaxError));
+        assert_eq!(p("10x2,-1,4,5").err(), Some(ParseError::SyntaxError));
+        assert_eq!(parse_usize(b"1__0"), Ok(10));
+        assert_eq!(parse_usize(b"_1"), Err(ParseError::SyntaxError));
+        assert_eq!(
+            parse_usize(b"99999999999999999999999"),
+            Err(ParseError::SyntaxError)
+        );
+    }
+
     fn p(s: &str) -> Result<Layout, ParseError> {
         Layout::parse(s.as_bytes())
     }

crates/ghostty-terminal/src/tmux/output.rs

@@ -55,36 +55,11 @@ pub enum Value {
     Str(String),
 }
 
-/// Equivalent of `std.fmt.parseInt(usize, s, 10)`.
 fn parse_int(s: &str) -> Result<usize, VariableError> {
-    let b = s.as_bytes();
+    ghostty_zigfmt::parse_int::<usize>(s.as_bytes(), 10).map_err(|e| match e {
-    let (neg, digits) = match b.first() {
+        ghostty_zigfmt::ParseIntError::Overflow => VariableError::Overflow,
-        Some(b'-') => (true, &b[1..]),
+        ghostty_zigfmt::ParseIntError::InvalidCharacter => VariableError::InvalidCharacter,
-        Some(b'+') => (false, &b[1..]),
+    })
-        _ => (false, b),
-    };
-    if digits.is_empty() {
-        return Err(VariableError::InvalidCharacter);
-    }
-    let mut v: usize = 0;
-    let mut overflow = false;
-    for &c in digits {
-        if !c.is_ascii_digit() {
-            return Err(VariableError::InvalidCharacter);
-        }
-        match v
-            .checked_mul(10)
-            .and_then(|v| v.checked_add(usize::from(c - b'0')))
-        {
-            Some(n) => v = n,
-            None => overflow = true,
-        }
-    }
-    // Negative values (other than zero) overflow an unsigned type.
-    if overflow || (neg && v != 0) {
-        return Err(VariableError::Overflow);
-    }
-    Ok(v)
 }
 
 fn parse_prefixed(prefix: u8, value: &str) -> Result<usize, VariableError> {
@@ -320,6 +295,24 @@ mod tests {
 
     use super::*;
 
+    #[test]
+    fn zig_parse_int_differences() {
+        assert_eq!(parse_int("1_0"), Ok(10));
+        assert_eq!(parse_int("+5"), Ok(5));
+        assert_eq!(parse_int("-0"), Ok(0));
+        assert_eq!(parse_int("-1"), Err(VariableError::Overflow));
+        assert_eq!(parse_int("0x10"), Err(VariableError::InvalidCharacter));
+        assert_eq!(parse_int("1_"), Err(VariableError::InvalidCharacter));
+        assert_eq!(parse_int(""), Err(VariableError::InvalidCharacter));
+        assert_eq!(
+            parse_int("99999999999999999999999"),
+            Err(VariableError::Overflow)
+        );
+        assert_eq!(PaneId.parse("%1_2"), Ok(Value::Usize(12)));
+        assert_eq!(WindowId.parse("@+7"), Ok(Value::Usize(7)));
+        assert_eq!(WindowWidth.parse("8_0"), Ok(Value::Usize(80)));
+    }
+
     fn flag(v: Variable) {
         assert_eq!(v.parse("1"), Ok(Value::Bool(true)));
         assert_eq!(v.parse("0"), Ok(Value::Bool(false)));

crates/ghostty-terminal/src/tmux/viewer.rs

@@ -976,7 +976,7 @@ impl Viewer {
             let tabs = data.str(Variable::PaneTabs);
             if !tabs.is_empty() {
                 for tab_str in tabs.split(',') {
-                    let Ok(col) = tab_str.parse::<usize>() else {
+                    let Ok(col) = ghostty_zigfmt::parse_int::<usize>(tab_str.as_bytes(), 10) else {
                         continue;
                     };
                     let Some(col_cell) = to_cell(col) else {

crates/ghostty-vt/Cargo.toml

@@ -5,6 +5,7 @@ edition.workspace = true
 license.workspace = true
 
 [dependencies]
+ghostty-zigfmt.workspace = true
 base64 = "0.23.1"
 log = "0.4.34"
 memchr = "2.8.3"

crates/ghostty-vt/src/color.rs

@@ -463,29 +463,9 @@ pub struct Rgb {
 /// Alias matching the Zig name.
 pub type RGB = Rgb;
 
-/// Zig `std.fmt.parseUnsigned(u16, value, 16)` semantics (optional leading
+/// Zig `std.fmt.parseUnsigned(u16, value, 16)`.
-/// `+`; a leading `-` is only valid if the value is zero; no underscores).
 fn parse_unsigned_hex_u16(value: &[u8]) -> Option<u16> {
-    let (neg, digits) = match value.first()? {
+    ghostty_zigfmt::parse_unsigned::<u16>(value, 16).ok()
-        b'+' => (false, &value[1..]),
-        b'-' => (true, &value[1..]),
-        _ => (false, value),
-    };
-    if digits.is_empty() {
-        return None;
-    }
-    let mut acc: u32 = 0;
-    for &c in digits {
-        let d = (c as char).to_digit(16)?;
-        acc = acc * 16 + d;
-        if acc > u16::MAX as u32 {
-            return None;
-        }
-    }
-    if neg && acc != 0 {
-        return None;
-    }
-    Some(acc as u16)
 }
 
 impl Rgb {
@@ -537,8 +517,7 @@ impl Rgb {
     ///
     /// The value should be between 0.0 and 1.0, inclusive.
     fn from_intensity(value: &[u8]) -> Result<u8, InvalidFormat> {
-        let s = std::str::from_utf8(value).map_err(|_| InvalidFormat)?;
+        let i: f64 = ghostty_zigfmt::parse_float::<f64>(value).map_err(|_| InvalidFormat)?;
-        let i: f64 = s.parse().map_err(|_| InvalidFormat)?;
         if !(0.0..=1.0).contains(&i) && !i.is_nan() {
             return Err(InvalidFormat);
         }
@@ -854,6 +833,30 @@ mod tests {
     #![allow(clippy::unwrap_used)]
     use super::*;
 
+    // Zig `std.fmt.parseUnsigned` / `parseFloat` differences from Rust's.
+    #[test]
+    fn zig_parse_hex_u16_underscore_and_sign() {
+        assert_eq!(Some(0x1f), parse_unsigned_hex_u16(b"1_f"));
+        assert_eq!(Some(0x1f), parse_unsigned_hex_u16(b"1__f"));
+        assert_eq!(None, parse_unsigned_hex_u16(b"+f"));
+        assert_eq!(None, parse_unsigned_hex_u16(b"-0"));
+        assert_eq!(None, parse_unsigned_hex_u16(b"_1"));
+        assert_eq!(None, parse_unsigned_hex_u16(b"f_"));
+        assert_eq!(None, parse_unsigned_hex_u16(b"0x1"));
+    }
+
+    #[test]
+    fn zig_parse_float_intensity() {
+        // hex floats and `_` separators are valid Zig floats.
+        assert_eq!(Ok(127), Rgb::from_intensity(b"0x1p-1"));
+        assert_eq!(Ok(127), Rgb::from_intensity(b"0.5_0"));
+        assert!(Rgb::from_intensity(b"0_.5").is_err());
+        assert_eq!(Ok(127), Rgb::from_intensity(b"+.5"));
+        assert_eq!(Ok(255), Rgb::from_intensity(b"1."));
+        assert!(Rgb::from_intensity(b"1.5").is_err());
+        assert!(Rgb::from_intensity(b"").is_err());
+    }
+
     fn rgb(r: u8, g: u8, b: u8) -> Rgb {
         Rgb { r, g, b }
     }

crates/ghostty-vt/src/kitty/color.rs

@@ -166,16 +166,10 @@ impl Kind {
         if let Some(s) = Special::from_name(key) {
             return Some(Kind::Special(s));
         }
-        // Zig parseUnsigned(u8, key, 10): optional '+', digits only.
+        // Zig parseUnsigned(u8, key, 10).
-        let digits = match key.first()? {
+        ghostty_zigfmt::parse_unsigned::<u8>(key, 10)
-            b'+' => &key[1..],
+            .ok()
-            _ => key,
+            .map(Kind::Palette)
-        };
-        if digits.is_empty() || !digits.iter().all(u8::is_ascii_digit) {
-            return None;
-        }
-        let s = std::str::from_utf8(digits).ok()?;
-        s.parse::<u8>().ok().map(Kind::Palette)
     }
 }
 
@@ -233,6 +227,16 @@ mod tests {
     #![allow(clippy::unwrap_used)]
     use super::*;
 
+    #[test]
+    fn zig_parse_unsigned_palette_key() {
+        assert!(matches!(Kind::parse(b"1_0"), Some(Kind::Palette(10))));
+        assert!(Kind::parse(b"+5").is_none());
+        assert!(Kind::parse(b"-0").is_none());
+        assert!(Kind::parse(b"0x10").is_none());
+        assert!(Kind::parse(b"256").is_none());
+        assert!(Kind::parse(b"1_").is_none());
+    }
+
     #[test]
     fn request_records_round_trip() {
         let mut reqs = vec![];

crates/ghostty-vt/src/kitty/graphics_command.rs

@@ -24,45 +24,20 @@ pub enum Error {
     Overflow,
 }
 
-/// Mirrors `std.fmt.parseInt(T, buf, 10)` for 32-bit targets: optional sign,
+/// Mirrors `std.fmt.parseInt(u32|i32, buf, 10)` (Zig `kv` value parsing),
-/// at least one digit, range checked.
+/// including `_` digit separators and Zig's sign rules.
 fn parse_int(buf: &[u8], signed: bool) -> Result<u32, Error> {
-    // kv_temp is at most 11 bytes so an i64 always suffices unless there
+    use ghostty_zigfmt::ParseIntError;
-    // are overflow digits, which we detect via checked arithmetic.
+    let map = |e| match e {
-    let (neg, digits) = match buf.first() {
+        ParseIntError::Overflow => Error::Overflow,
-        Some(b'+') if buf.len() > 1 => (false, &buf[1..]),
+        ParseIntError::InvalidCharacter => Error::InvalidCharacter,
-        Some(b'-') if buf.len() > 1 => (true, &buf[1..]),
-        _ => (false, buf),
     };
-    if digits.is_empty() {
-        return Err(Error::InvalidCharacter);
-    }
-    let mut v: i64 = 0;
-    let mut overflow = false;
-    for &c in digits {
-        if !c.is_ascii_digit() {
-            return Err(Error::InvalidCharacter);
-        }
-        match v
-            .checked_mul(10)
-            .and_then(|x| x.checked_add(i64::from(c - b'0')))
-        {
-            Some(x) => v = x,
-            None => overflow = true,
-        }
-    }
-    if overflow {
-        return Err(Error::Overflow);
-    }
-    if neg {
-        v = -v;
-    }
     if signed {
-        i32::try_from(v)
+        ghostty_zigfmt::parse_int::<i32>(buf, 10)
             .map(|x| x as u32)
-            .map_err(|_| Error::Overflow)
+            .map_err(map)
     } else {
-        u32::try_from(v).map_err(|_| Error::Overflow)
+        ghostty_zigfmt::parse_int::<u32>(buf, 10).map_err(map)
     }
 }
 
@@ -1158,6 +1133,21 @@ mod tests {
 
     use super::*;
 
+    #[test]
+    fn zig_parse_int_values() {
+        // `_` separators, explicit `+`, negative zero for unsigned.
+        let c = parse("a=p,i=1_0,I=+7,r=-0,z=-1_0").unwrap();
+        let Control::Display(d) = c.control else {
+            panic!("expected display")
+        };
+        assert_eq!((d.image_id, d.image_number, d.rows, d.z), (10, 7, 0, -10));
+        // Signed keys accept negatives, unsigned do not.
+        assert!(matches!(parse("a=p,i=-1"), Err(Error::Overflow)));
+        assert!(matches!(parse("a=p,i=0x10"), Err(Error::InvalidCharacter)));
+        assert!(matches!(parse("a=p,i=1_"), Err(Error::InvalidCharacter)));
+        assert!(matches!(parse("a=p,i=4294967296"), Err(Error::Overflow)));
+    }
+
     fn parse(input: &str) -> Result<Command, Error> {
         let mut p = Parser::new();
         for c in input.bytes() {

crates/ghostty-vt/src/osc/parsers/color.rs

@@ -141,30 +141,10 @@ fn tokenize(buf: &[u8]) -> impl Iterator<Item = &[u8]> {
     buf.split(|&c| c == b';').filter(|t| !t.is_empty())
 }
 
-/// Zig `std.fmt.parseInt(u9, s, 10)`: optional sign, digits only.
+/// Zig `std.fmt.parseInt(u9, s, 10)`.
 fn parse_u9(s: &[u8]) -> Option<u16> {
-    let (neg, digits) = match s.first()? {
+    let v = ghostty_zigfmt::parse_int::<i32>(s, 10).ok()?;
-        b'+' => (false, &s[1..]),
+    u16::try_from(v).ok().filter(|v| *v <= 511)
-        b'-' => (true, &s[1..]),
-        _ => (false, s),
-    };
-    if digits.is_empty() {
-        return None;
-    }
-    let mut acc: u32 = 0;
-    for &c in digits {
-        if !c.is_ascii_digit() {
-            return None;
-        }
-        acc = acc * 10 + (c - b'0') as u32;
-        if acc > 511 {
-            return None;
-        }
-    }
-    if neg && acc != 0 {
-        return None;
-    }
-    Some(acc as u16)
 }
 
 /// Parse any color operation string. This should NOT include the operation
@@ -542,6 +522,21 @@ mod tests {
     #![allow(clippy::unwrap_used)]
     use super::*;
 
+    #[test]
+    fn zig_parse_u9_differences() {
+        assert_eq!(Some(10), parse_u9(b"1_0"));
+        assert_eq!(Some(500), parse_u9(b"5_0_0"));
+        assert_eq!(Some(5), parse_u9(b"+5"));
+        assert_eq!(Some(0), parse_u9(b"-0"));
+        assert_eq!(None, parse_u9(b"-1"));
+        assert_eq!(None, parse_u9(b"0x10"));
+        assert_eq!(None, parse_u9(b"1_"));
+        assert_eq!(None, parse_u9(b"_1"));
+        assert_eq!(Some(511), parse_u9(b"511"));
+        assert_eq!(None, parse_u9(b"512"));
+        assert_eq!(None, parse_u9(b"99999999999"));
+    }
+
     const RED: Rgb = Rgb { r: 255, g: 0, b: 0 };
     const BLUE: Rgb = Rgb { r: 0, g: 0, b: 255 };
 

crates/ghostty-vt/src/osc/parsers/context_signal.rs

@@ -216,9 +216,8 @@ impl Field {
                     if value.iter().any(|c| !c.is_ascii_digit()) {
                         return None;
                     }
-                    std::str::from_utf8(value)
+                    ghostty_zigfmt::parse_int::<u64>(value, 10)
                         .ok()
-                        .and_then(|s| s.parse::<u64>().ok())
                         .map(FieldValue::U64)
                 }
                 // String fields

crates/ghostty-vt/src/osc/parsers/kitty_text_sizing.rs

@@ -67,28 +67,8 @@ enum UpdateError {
 
 /// Equivalent of Zig `std.fmt.parseInt(u4, value, 10)`.
 fn parse_u4(value: &[u8]) -> Option<u8> {
-    let (neg, digits) = match value.first() {
+    let v = ghostty_zigfmt::parse_int::<i32>(value, 10).ok()?;
-        Some(b'+') => (false, &value[1..]),
+    u8::try_from(v).ok().filter(|v| *v <= 15)
-        Some(b'-') => (true, &value[1..]),
-        _ => (false, value),
-    };
-    if digits.is_empty() {
-        return None;
-    }
-    let mut acc: u32 = 0;
-    for &c in digits {
-        if !c.is_ascii_digit() {
-            return None;
-        }
-        acc = acc * 10 + (c - b'0') as u32;
-        if acc > 15 {
-            return None;
-        }
-    }
-    if neg && acc != 0 {
-        return None;
-    }
-    Some(acc as u8)
 }
 
 impl OSC<'_> {
@@ -213,6 +193,20 @@ pub fn parse<S: ByteStorage>(
 mod tests {
     use super::*;
 
+    #[test]
+    fn zig_parse_u4_differences() {
+        assert_eq!(Some(10), parse_u4(b"1_0"));
+        assert_eq!(Some(5), parse_u4(b"+5"));
+        assert_eq!(Some(0), parse_u4(b"-0"));
+        assert_eq!(Some(15), parse_u4(b"15"));
+        assert_eq!(None, parse_u4(b"16"));
+        assert_eq!(None, parse_u4(b"5_0_0"));
+        assert_eq!(None, parse_u4(b"-1"));
+        assert_eq!(None, parse_u4(b"0x1"));
+        assert_eq!(None, parse_u4(b"1_"));
+        assert_eq!(None, parse_u4(b"99999999999"));
+    }
+
     fn run(input: &[u8]) -> Option<Command<'static>> {
         let mut p = Parser::init(false);
         for &ch in input {

crates/ghostty-vt/src/osc/parsers/osc9.rs

@@ -12,8 +12,8 @@ use crate::osc::{
 };
 
 /// Parse a base-10 unsigned integer from raw bytes (Zig `parseUnsigned`).
-fn parse_unsigned<T: std::str::FromStr>(bytes: &[u8]) -> Option<T> {
+fn parse_unsigned<T: ghostty_zigfmt::ZigInt>(bytes: &[u8]) -> Option<T> {
-    std::str::from_utf8(bytes).ok()?.parse().ok()
+    ghostty_zigfmt::parse_unsigned::<T>(bytes, 10).ok()
 }
 
 /// Parse OSC 9, which could be an iTerm2 notification or a ConEmu extension.
@@ -221,6 +221,18 @@ fn conemu_inner(data: &[u8]) -> Option<Command<'_>> {
 mod tests {
     use super::*;
 
+    #[test]
+    fn zig_parse_unsigned_differences() {
+        assert_eq!(run("9;1;4_2"), Command::ConemuSleep { duration_ms: 42 });
+        assert_eq!(run("9;1;+42"), Command::ConemuSleep { duration_ms: 100 });
+        assert_eq!(run("9;1;-0"), Command::ConemuSleep { duration_ms: 100 });
+        assert_eq!(run("9;1;0x10"), Command::ConemuSleep { duration_ms: 100 });
+        assert_eq!(run("9;1;70000"), Command::ConemuSleep { duration_ms: 100 });
+        assert_eq!(parse_unsigned::<usize>(b"5_0"), Some(50));
+        assert_eq!(parse_unsigned::<usize>(b"+5"), None);
+        assert_eq!(parse_unsigned::<usize>(b"5_"), None);
+    }
+
     fn run(input: &str) -> Command<'static> {
         let mut p = Parser::init(false);
         for ch in input.bytes() {

crates/ghostty-vt/src/osc/parsers/semantic_prompt.rs

@@ -223,9 +223,8 @@ impl OptionKey {
             // If we're looking for exit_code we special case it.
             // as the first value.
             if self == OptionKey::ExitCode {
-                return std::str::from_utf8(full)
+                return ghostty_zigfmt::parse_int::<i32>(full, 10)
                     .ok()
-                    .and_then(|s| s.parse::<i32>().ok())
                     .map(OptionValue::Int);
             }
 
@@ -428,6 +427,16 @@ pub fn parse<S: ByteStorage>(
 mod tests {
     use super::*;
 
+    #[test]
+    fn zig_parse_int_exit_code() {
+        assert_eq!(run("133;D;1_0").unwrap().exit_code(), Some(10));
+        assert_eq!(run("133;D;+5").unwrap().exit_code(), Some(5));
+        assert_eq!(run("133;D;-5").unwrap().exit_code(), Some(-5));
+        assert_eq!(run("133;D;0x10").unwrap().exit_code(), None);
+        assert_eq!(run("133;D;1_").unwrap().exit_code(), None);
+        assert_eq!(run("133;D;99999999999").unwrap().exit_code(), None);
+    }
+
     fn run(input: &str) -> Option<Command<'static>> {
         let mut p = Parser::init(false);
         for ch in input.bytes() {

crates/ghostty-zigfmt/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "ghostty-zigfmt"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "Exact ports of Zig std.fmt.parseInt / parseUnsigned / parseFloat"
+
+[dependencies]
+
+[lints]
+workspace = true

crates/ghostty-zigfmt/src/lib.rs

@@ -8,6 +8,8 @@
 //! standard library once the Zig grammar has accepted the input (Zig's
 //! algorithm is also correctly rounded); hex floats follow `convertHex`.
 
+#![cfg_attr(test, allow(clippy::unwrap_used, clippy::expect_used, clippy::panic))]
+
 /// `std.fmt.ParseIntError`.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum ParseIntError {

docs/rewrite/notes/final-Z12.md

@@ -0,0 +1,32 @@
+# Z12: Zig-exact numeric parsing in protocol / non-config code
+
+`zig_fmt` moved to the new leaf crate `crates/ghostty-zigfmt` (no deps);
+`ghostty_os::zig_fmt` is now `pub use ghostty_zigfmt as zig_fmt`, so existing
+callers are unchanged. `ghostty-vt` and `ghostty-terminal` depend on it
+directly (ghostty-os depends on ghostty-vt, so vt could not use os).
+Expected values in the new tests were cross-checked with the real `zig 0.16`
+(`std.fmt.parseInt/parseUnsigned/parseFloat`).
+
+| Rust call site | Zig reference | Status |
+|---|---|---|
+| vt `kitty/graphics_command.rs::parse_int` | graphics_command.zig:262 `parseInt(i32/u32, _, 10)` | fixed (was hand-rolled: no `_`) |
+| vt `osc/parsers/kitty_text_sizing.rs::parse_u4` | kitty_text_sizing.zig:52 `parseInt(u4,10)` | fixed |
+| vt `osc/parsers/color.rs::parse_u9` | parsers/color.zig:176,249 `parseInt(u9,10)` | fixed |
+| vt `color.rs::parse_unsigned_hex_u16` | color.zig:505 `parseUnsigned(u16,16)` | fixed (Rust wrongly allowed `+`, `-0`) |
+| vt `color.rs::from_intensity` | color.zig:483 `parseFloat(f64)` | fixed (`str::parse` -> `parse_float`: hex floats, `_`) |
+| vt `osc/parsers/osc9.rs::parse_unsigned` | osc9.zig:27,195 `parseUnsigned(u16/usize,10)` | fixed (was `str::parse`, accepted `+`) |
+| vt `osc/parsers/semantic_prompt.rs` exit_code | semantic_prompt.zig:155 `parseInt(i32,10)` | fixed |
+| vt `osc/parsers/context_signal.rs` pid/status | context_signal.zig:167 (digit pre-check, `parseInt(u64)`) | fixed |
+| vt `kitty/color.rs::Kind::parse` | kitty/color.zig:51 `parseUnsigned(u8,10)` | fixed (no `+` accepted) |
+| vt `tmux/control.rs::digits` | control.zig regex `[0-9]+` then `parseInt(usize)` | unchanged: regex guarantees digits only |
+| terminal `tmux/layout.rs::parse_usize` | layout.zig:99-163 `parseInt(usize,10)` | fixed (`_`, `-0`) |
+| terminal `tmux/output.rs::parse_int` | output.zig:200-214 | fixed (`_`) |
+| terminal `tmux/viewer.rs` pane_tabs | viewer.zig:1026 `parseInt(usize,10) catch continue` | fixed |
+| os `uri.rs` port | std.Uri `parseInt(u16,10)` | fixed (`_`, no behaviour change for `+`) |
+| os `uri.rs::percent_decode` | std.Uri formatRaw `parseInt(u8,"XX",16)` | fixed (accepts `%+A`, `%-0`; rejects nothing new) |
+| bench `args.rs::take` | cli/args.zig:429 `parseInt(T,_,0)` / `parseFloat` | fixed (base 0 prefixes, `_`, hex float) |
+| vt `parser.rs` / `stream.rs` param_acc | Parser.zig:353, stream.zig:673 saturating digit loop | already identical |
+| font `shaper/feature.rs` | feature.zig:133-153 digit loop (u32 mul) | already identical |
+| config / input binding / cli ssh-cache / os desktop | Z11 | already using zig_fmt |
+| vt `x11_color.rs`, `build.rs` (rgb.txt) | x11_color.zig:30 comptime on fixed data | n/a (data file) |
+| cli `list_themes/preview.rs` COLORFGBG, core `GHOSTTY_DEBUG_SCREENSHOT_MS`, apprt-gtk debug_input, build-info/fake_version, renderer shadertoy | no Zig counterpart (Rust-only) | left on `str::parse` |