diff --git a/Cargo.lock b/Cargo.lock
index e7ae1d0e1..13b4f60fe 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -958,6 +958,13 @@ dependencies = [
  "craft-fonts-build",
 ]
 
+[[package]]
+name = "craft-gcode"
+version = "0.1.0"
+dependencies = [
+ "thiserror 2.0.21",
+]
+
 [[package]]
 name = "craft-heif"
 version = "0.1.0"
diff --git a/Cargo.toml b/Cargo.toml
index 3177250c9..fdfcf3b67 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -34,6 +34,7 @@ craft-ecproj-format = { path = "crates/ecproj-format" }
 craft-font-catalog = { path = "crates/font-catalog" }
 craft-font-manager = { path = "crates/font-manager" }
 craft-font-store = { path = "crates/font-store" }
+craft-gcode = { path = "crates/gcode" }
 craft-heif = { path = "crates/heif" }
 craft-fonts-build = { path = "crates/fonts-build" }
 craft-i18n = { path = "crates/i18n" }
diff --git a/README.md b/README.md
index 6b64e33e7..460b45bea 100644
--- a/README.md
+++ b/README.md
@@ -71,6 +71,8 @@ craft-<name> = { git = "https://github.com/storytold/craft-libs", rev = "<commit
 - [`craft-fonts-build`](crates/fonts-build/README.md): embeds the optional craft-fonts faces
   from an app's `build.rs` (`CRAFT_FONTS_DIR`, `CRAFT_FONTS_REQUIRED`, script filters) as a table
   of `EmbeddedFace`s. No dependencies; the app picks the faces.
+- [`craft-gcode`](crates/gcode/src/lib.rs): Grbl-style G-code writer: polylines in
+  millimetres to `G0`/`G1` moves with laser power and feed, for cutting lasers and plotters.
 - [`craft-heif`](crates/heif/README.md): optional HEIF/HEIC still-image decoder, from LightCraft:
   heic-rs reads the container and HEVC pictures, and this crate converts to 16-bit RGB to match
   libheif (VUI range and matrix, chroma upsampling, `imir`, ICC on grid tiles). Also orientation,
diff --git a/crates/gcode/Cargo.toml b/crates/gcode/Cargo.toml
new file mode 100644
index 000000000..44b58e501
--- /dev/null
+++ b/crates/gcode/Cargo.toml
@@ -0,0 +1,14 @@
+[package]
+name = "craft-gcode"
+description = "Grbl-style G-code writer for laser cutters and plotters: polylines in millimetres to G0/G1 moves"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+repository.workspace = true
+
+[lints]
+workspace = true
+
+[dependencies]
+thiserror = { workspace = true }
diff --git a/crates/gcode/src/lib.rs b/crates/gcode/src/lib.rs
new file mode 100644
index 000000000..4bd817c23
--- /dev/null
+++ b/crates/gcode/src/lib.rs
@@ -0,0 +1,313 @@
+//! A Grbl-style G-code writer for cutting lasers and plotters.
+//!
+//! Modality: `G21` millimetres, `G90` absolute, feed in mm/min. Each contour is one cut: rapid
+//! (`G0`) to the start with the beam off, power on, `G1` along the path, power off. Laser
+//! machines take `M4` (dynamic power) plus an `S` power value in the conventional 0–1000 range,
+//! or `M3` for controllers that only model a spindle ([`LaserMode`]). A contour can be traced in
+//! several passes, with an optional power ramp. Coordinates are absolute millimetres, exactly as
+//! given.
+//!
+//! Flattening curves, choosing what to cut and at what power, and machine or material presets
+//! are the caller's business. Depends on no other Craft crate.
+#![deny(clippy::unwrap_used, clippy::expect_used, clippy::panic, clippy::unimplemented, clippy::todo, clippy::unreachable)]
+#![forbid(unsafe_code)]
+
+use std::fmt::Write as _;
+
+#[derive(Debug, thiserror::Error, PartialEq)]
+pub enum Error {
+    /// A setting or power no machine should be sent. Names the knob and the value.
+    #[error("{0}")]
+    Settings(String),
+    #[error("a vertex is not finite")]
+    NotFinite,
+}
+
+/// Most repeated passes a job may ask for: past this the settings are a mistake, not a thick
+/// material.
+pub const MAX_PASSES: u32 = 10;
+
+/// Highest power Grbl's `S` word conventionally accepts.
+pub const MAX_POWER: u32 = 1000;
+
+/// How the controller is told to fire.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum LaserMode {
+    /// `M4`: power tracks motion (Grbl laser mode). Rapids go dark on their own, but the writer
+    /// still switches off explicitly so the output is safe on any sender.
+    #[default]
+    Dynamic,
+    /// `M3`: constant power while moving, for controllers that only understand a spindle on/off.
+    Constant,
+}
+
+impl LaserMode {
+    /// The G-code word that turns the tool on.
+    pub fn on_command(self) -> &'static str {
+        match self {
+            Self::Dynamic => "M4",
+            Self::Constant => "M3",
+        }
+    }
+
+    /// Parse the CLI/protocol spelling.
+    pub fn parse(value: &str) -> Option<Self> {
+        match value.trim().to_ascii_lowercase().as_str() {
+            "dynamic" | "m4" => Some(Self::Dynamic),
+            "constant" | "m3" => Some(Self::Constant),
+            _ => None,
+        }
+    }
+}
+
+/// How every contour of a program is traced. Power is given per contour.
+#[derive(Debug, Clone, PartialEq)]
+pub struct Settings {
+    /// Feed rate while cutting, millimetres per minute.
+    pub feed_mm_per_min: f64,
+    /// How many times each contour is traced, 1–[`MAX_PASSES`].
+    pub passes: u32,
+    /// Spread the power across passes so the last one runs at full power. Useful on material
+    /// that scorches at full power in one hit.
+    pub ramp_power: bool,
+    /// Which on/off command the controller understands.
+    pub mode: LaserMode,
+}
+
+impl Default for Settings {
+    fn default() -> Self {
+        Self { feed_mm_per_min: 1000.0, passes: 1, ramp_power: false, mode: LaserMode::Dynamic }
+    }
+}
+
+impl Settings {
+    /// Checks every knob is inside its range.
+    pub fn validate(&self) -> Result<(), Error> {
+        // Written to three decimals: a smaller rate would come out as `F0`, which Grbl rejects.
+        if !self.feed_mm_per_min.is_finite() || self.feed_mm_per_min < 0.001 {
+            return Err(Error::Settings(format!("feed rate {} mm/min is not a finite rate of at least 0.001", self.feed_mm_per_min)));
+        }
+        if self.passes == 0 || self.passes > MAX_PASSES {
+            return Err(Error::Settings(format!("passes {} is outside the 1–{MAX_PASSES} range", self.passes)));
+        }
+        Ok(())
+    }
+
+    /// Power for one pass of a contour running at `base` power. Without a ramp every pass is at
+    /// `base`; with one, pass `n` of `N` gets `base × n / N`, so the last pass is the full-power
+    /// pass.
+    pub fn power_for_pass(&self, base: u32, pass: u32) -> u32 {
+        if !self.ramp_power || self.passes <= 1 || pass >= self.passes {
+            return base;
+        }
+        let scaled = u64::from(base) * u64::from(pass) / u64::from(self.passes);
+        let scaled = u32::try_from(scaled).unwrap_or(base);
+        // A pass at S0 would trace the path with the beam off.
+        scaled.max(1).min(base)
+    }
+}
+
+/// Checks a power is one the `S` word accepts; `what` names it in the error ("power",
+/// "engrave power").
+pub fn check_power(what: &str, power: u32) -> Result<(), Error> {
+    if power > MAX_POWER {
+        return Err(Error::Settings(format!("{what} {power} is outside the 0–{MAX_POWER} range")));
+    }
+    Ok(())
+}
+
+/// Builds one program: [`Writer::new`] writes the header, [`Writer::finish`] the footer.
+#[derive(Debug)]
+pub struct Writer {
+    out: String,
+    settings: Settings,
+}
+
+impl Writer {
+    /// Starts a program with `banner` as its first comment line.
+    pub fn new(settings: Settings, banner: &str) -> Result<Writer, Error> {
+        settings.validate()?;
+        let mut w = Writer { out: String::with_capacity(4096), settings };
+        w.comment(banner);
+        let _ = writeln!(w.out, "G21 ; millimetres");
+        let _ = writeln!(w.out, "G90 ; absolute coordinates");
+        let _ = writeln!(w.out, "G17 ; XY plane");
+        // Start dark: the first move is a rapid to the first contour.
+        let _ = writeln!(w.out, "{} S0 ; laser off until the first cut", w.settings.mode.on_command());
+        Ok(w)
+    }
+
+    /// A comment line. Control characters become spaces: a line break in `text` (a layer name
+    /// from a file, say) would otherwise start a line the machine runs as a command. Grbl acts
+    /// on its real-time bytes (`!` feed hold, `?`, `~` and everything past ASCII) wherever they
+    /// arrive, comments included, so those become `_`.
+    pub fn comment(&mut self, text: &str) {
+        let safe = |c: char| match c {
+            c if c.is_control() => ' ',
+            '!' | '?' | '~' => '_',
+            c if !c.is_ascii() => '_',
+            c => c,
+        };
+        let text: String = text.chars().map(safe).collect();
+        let _ = writeln!(self.out, "; {text}");
+    }
+
+    /// One contour along `points` (x, y in millimetres) at `power`, traced once per pass. A
+    /// closed contour ends where it started; a single point is a minimal in-and-out dot; an
+    /// empty one writes nothing. A refused contour writes nothing either.
+    pub fn contour(&mut self, points: &[(f64, f64)], closed: bool, power: u32) -> Result<(), Error> {
+        check_power("power", power)?;
+        let Some(&first) = points.first() else {
+            return Ok(());
+        };
+        if points.iter().any(|p| !p.0.is_finite() || !p.1.is_finite()) {
+            return Err(Error::NotFinite);
+        }
+        let mut path = points.to_vec();
+        if points.len() < 2 {
+            path.extend([(first.0 + 0.001, first.1), first]);
+        } else if closed {
+            path.push(first);
+        }
+        let on = self.settings.mode.on_command();
+        let passes = self.settings.passes;
+        for pass in 1..=passes {
+            let _ = writeln!(self.out, "G0 X{:.4} Y{:.4}", first.0, first.1);
+            let power = self.settings.power_for_pass(power, pass);
+            if passes > 1 {
+                let _ = writeln!(self.out, "{on} S{power} ; pass {pass} of {passes}");
+            } else {
+                let _ = writeln!(self.out, "{on} S{power}");
+            }
+            for p in path.iter().skip(1) {
+                let _ = writeln!(self.out, "G1 X{:.4} Y{:.4} F{}", p.0, p.1, (self.settings.feed_mm_per_min * 1000.0).round() / 1000.0);
+            }
+            // Never rapid with the beam on, whatever the mode or controller.
+            let _ = writeln!(self.out, "M5");
+        }
+        Ok(())
+    }
+
+    /// Turns the tool off, returns to the origin and gives the program.
+    pub fn finish(mut self) -> String {
+        let _ = writeln!(self.out, "M5 ; tool off");
+        let _ = writeln!(self.out, "G0 X0 Y0 ; return to origin");
+        self.out
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn program(settings: Settings, points: &[(f64, f64)], closed: bool) -> String {
+        let mut w = Writer::new(settings, "test").unwrap();
+        w.contour(points, closed, 500).unwrap();
+        w.finish()
+    }
+
+    #[test]
+    fn a_line_is_a_whole_program() {
+        assert_eq!(
+            program(Settings::default(), &[(0.0, 0.0), (10.0, 0.0)], false),
+            "; test\nG21 ; millimetres\nG90 ; absolute coordinates\nG17 ; XY plane\nM4 S0 ; laser off until the first cut\n\
+             G0 X0.0000 Y0.0000\nM4 S500\nG1 X10.0000 Y0.0000 F1000\nM5\nM5 ; tool off\nG0 X0 Y0 ; return to origin\n"
+        );
+    }
+
+    #[test]
+    fn closed_loops_return_to_start_and_a_point_is_a_dot() {
+        let g = program(Settings::default(), &[(1.0, 2.0), (10.0, 2.0), (10.0, 10.0)], true);
+        assert!(g.contains("G1 X10.0000 Y10.0000 F1000\nG1 X1.0000 Y2.0000 F1000\nM5\n"), "{g}");
+        // Closed or not, a dot is the same in-and-out move.
+        for closed in [false, true] {
+            let dot = program(Settings::default(), &[(5.0, 5.0)], closed);
+            assert!(dot.contains("G0 X5.0000 Y5.0000\nM4 S500\nG1 X5.0010 Y5.0000 F1000\nG1 X5.0000 Y5.0000 F1000\nM5\n"), "{dot}");
+        }
+        assert!(!program(Settings::default(), &[], true).contains("G1 "));
+    }
+
+    #[test]
+    fn rapids_never_run_with_the_beam_on() {
+        let settings = Settings { passes: 3, mode: LaserMode::Constant, ..Settings::default() };
+        let mut w = Writer::new(settings, "test").unwrap();
+        w.contour(&[(0.0, 0.0), (10.0, 0.0)], false, 500).unwrap();
+        w.contour(&[(20.0, 0.0), (30.0, 0.0), (30.0, 5.0)], true, 200).unwrap();
+        let g = w.finish();
+        let mut lit = false;
+        for line in g.lines().filter(|l| !l.starts_with(';')) {
+            if line.starts_with("M3 S0") || line.starts_with("M5") {
+                lit = false;
+            } else if line.starts_with("M3 ") {
+                lit = true;
+            } else if line.starts_with("G0") {
+                assert!(!lit, "rapid with the beam on:\n{g}");
+            }
+        }
+        assert!(!g.contains("M4"), "{g}");
+        assert_eq!(g.matches("G1 X10.0000 Y0.0000 F1000").count(), 3, "{g}");
+        assert_eq!(g.matches("M3 S500 ; pass ").count(), 3, "{g}");
+    }
+
+    #[test]
+    fn ramp_power_climbs_to_the_set_power() {
+        let ramp = Settings { passes: 4, ramp_power: true, ..Settings::default() };
+        assert_eq!([1, 2, 3, 4].map(|pass| ramp.power_for_pass(800, pass)), [200, 400, 600, 800]);
+        let flat = Settings { passes: 4, ..Settings::default() };
+        assert_eq!(flat.power_for_pass(800, 1), 800);
+        // A ramped pass never drops to S0, which would trace with the beam off.
+        let fine = Settings { passes: 10, ramp_power: true, ..Settings::default() };
+        assert_eq!(fine.power_for_pass(3, 1), 1);
+        let g = program(ramp, &[(0.0, 0.0), (1.0, 0.0)], false);
+        assert!(g.contains("M4 S125 ; pass 1 of 4") && g.contains("M4 S500 ; pass 4 of 4"), "{g}");
+    }
+
+    #[test]
+    fn bad_settings_powers_and_vertices_are_errors() {
+        for feed in [0.0, -1.0, f64::NAN, f64::INFINITY] {
+            assert!(matches!(Writer::new(Settings { feed_mm_per_min: feed, ..Settings::default() }, ""), Err(Error::Settings(_))));
+        }
+        for passes in [0, MAX_PASSES + 1] {
+            assert!(matches!(Settings { passes, ..Settings::default() }.validate(), Err(Error::Settings(_))));
+        }
+        assert_eq!(check_power("engrave power", 1001), Err(Error::Settings("engrave power 1001 is outside the 0–1000 range".into())));
+        let mut w = Writer::new(Settings::default(), "").unwrap();
+        assert!(matches!(w.contour(&[(0.0, 0.0), (1.0, 1.0)], false, MAX_POWER + 1), Err(Error::Settings(_))));
+        assert_eq!(w.contour(&[(0.0, 0.0), (f64::NAN, 1.0)], false, 500), Err(Error::NotFinite));
+        // Nothing of a refused contour is written, not even its travel move.
+        assert!(!w.finish().contains("G0 X0.0000"));
+    }
+
+    #[test]
+    fn grbl_real_time_bytes_never_reach_the_program() {
+        let mut w = Writer::new(Settings::default(), "stop! now? ~ café").unwrap();
+        w.comment("\u{18}reset");
+        let g = w.finish();
+        assert!(g.starts_with("; stop_ now_ _ caf_\n"), "{g}");
+        assert!(g.is_ascii() && !g.contains(['!', '?', '~', '\u{18}']), "{g}");
+    }
+
+    #[test]
+    fn a_fractional_feed_is_written_not_rounded_to_zero() {
+        let slow = Settings { feed_mm_per_min: 0.25, ..Settings::default() };
+        assert!(program(slow, &[(0.0, 0.0), (1.0, 0.0)], false).contains(" F0.25\n"));
+        assert!(program(Settings::default(), &[(0.0, 0.0), (1.0, 0.0)], false).contains(" F1000\n"));
+        assert!(matches!(Writer::new(Settings { feed_mm_per_min: 0.0004, ..Settings::default() }, ""), Err(Error::Settings(_))));
+    }
+
+    #[test]
+    fn a_line_break_in_a_comment_cannot_start_a_command() {
+        let mut w = Writer::new(Settings::default(), "banner").unwrap();
+        w.comment("layer x\nM3 S1000\r\nG1 X9");
+        let g = w.finish();
+        assert!(g.contains("; layer x M3 S1000  G1 X9\n"), "{g}");
+        assert!(g.lines().all(|l| !l.starts_with("M3")));
+    }
+
+    #[test]
+    fn laser_modes_parse_their_spellings() {
+        assert_eq!(LaserMode::parse(" M3 "), Some(LaserMode::Constant));
+        assert_eq!(LaserMode::parse("Dynamic"), Some(LaserMode::Dynamic));
+        assert_eq!(LaserMode::parse("m5"), None);
+    }
+}