diff --git a/src/font/CodepointResolver.zig b/src/font/CodepointResolver.zig
index a4f13c290..90fe2491c 100644
--- a/src/font/CodepointResolver.zig
+++ b/src/font/CodepointResolver.zig
@@ -24,7 +24,7 @@ const Face = font.Face;
 const Glyph = font.Glyph;
 const Library = font.Library;
 const Presentation = font.Presentation;
-const RenderOptions = font.face.RenderOptions;
+const RenderOptions = font.Glyph.RenderOptions;
 const SpriteFace = font.SpriteFace;
 const Style = font.Style;
 
diff --git a/src/font/Glyph.zig b/src/font/Glyph.zig
index f99370271..62c44e227 100644
--- a/src/font/Glyph.zig
+++ b/src/font/Glyph.zig
@@ -1,6 +1,9 @@
 //! Glyph is a single loaded glyph for a face.
 const Glyph = @This();
 
+const std = @import("std");
+const Metrics = @import("Metrics.zig");
+
 /// width of glyph in pixels
 width: u32,
 
@@ -17,3 +20,611 @@ offset_y: i32,
 /// be normalized to be between 0 and 1 prior to use in shaders.
 atlas_x: u32,
 atlas_y: u32,
+
+/// The size and position of a glyph.
+pub const Size = struct {
+    width: f64,
+    height: f64,
+    x: f64,
+    y: f64,
+};
+
+/// Metrics describing the authored glyph coordinate space.
+pub const DesignMetrics = struct {
+    /// Units-per-em for outline/design coordinates.
+    units_per_em: u32,
+
+    /// Authored advance width in design units.
+    advance_width: u32,
+
+    /// Authored line height in design units.
+    line_height: u32,
+};
+
+/// Additional options for rendering glyphs.
+pub const RenderOptions = struct {
+    /// The metrics that are defining the grid layout. These are usually
+    /// the metrics of the primary font face. The grid metrics are used
+    /// by the font face to better layout the glyph in situations where
+    /// the font is not exactly the same size as the grid.
+    grid_metrics: Metrics,
+
+    /// The number of grid cells this glyph will take up. This can be used
+    /// optionally by the rasterizer to better layout the glyph.
+    cell_width: ?u2 = null,
+
+    /// Constraint and alignment properties for the glyph. The rasterizer
+    /// should call the `constrain` function on this with the original size
+    /// and bearings of the glyph to get remapped values that the glyph
+    /// should be scaled/moved to.
+    constraint: Constraint = .none,
+
+    /// The number of cells, horizontally that the glyph is free to take up
+    /// when resized and aligned by `constraint`. This is usually 1, but if
+    /// there's whitespace to the right of the cell then it can be 2.
+    constraint_width: u2 = 1,
+
+    /// Thicken the glyph. This draws the glyph with a thicker stroke width.
+    /// This is purely an aesthetic setting.
+    ///
+    /// This only works with CoreText currently.
+    thicken: bool = false,
+
+    /// "Strength" of the thickening, between `0` and `255`.
+    /// Only has an effect when `thicken` is enabled.
+    ///
+    /// `0` does not correspond to *no* thickening,
+    /// just the *lightest* thickening available.
+    ///
+    /// CoreText only.
+    thicken_strength: u8 = 255,
+
+    /// See the `constraint` field.
+    pub const Constraint = struct {
+        /// Don't constrain the glyph in any way.
+        pub const none: Constraint = .{};
+
+        /// Sizing rule.
+        size: Constraint.Size = .none,
+
+        /// Vertical alignment rule.
+        align_vertical: Align = .none,
+        /// Horizontal alignment rule.
+        align_horizontal: Align = .none,
+
+        /// Top padding when resizing.
+        pad_top: f64 = 0.0,
+        /// Left padding when resizing.
+        pad_left: f64 = 0.0,
+        /// Right padding when resizing.
+        pad_right: f64 = 0.0,
+        /// Bottom padding when resizing.
+        pad_bottom: f64 = 0.0,
+
+        // Size and bearings of the glyph relative
+        // to the bounding box of its scale group.
+        relative_width: f64 = 1.0,
+        relative_height: f64 = 1.0,
+        relative_x: f64 = 0.0,
+        relative_y: f64 = 0.0,
+
+        /// Maximum aspect ratio (width/height) to allow when stretching.
+        max_xy_ratio: ?f64 = null,
+
+        /// Maximum number of cells horizontally to use.
+        max_constraint_width: u2 = 2,
+
+        /// What to use as the height metric when constraining the glyph and
+        /// the constraint width is 1,
+        height: Height = .cell,
+
+        pub const Size = enum {
+            /// Don't change the size of this glyph.
+            none,
+            /// Scale the glyph down if needed to fit within the bounds,
+            /// preserving aspect ratio.
+            fit,
+            /// Scale the glyph up or down to exactly match the bounds,
+            /// preserving aspect ratio.
+            cover,
+            /// Scale the glyph down if needed to fit within the bounds,
+            /// preserving aspect ratio. If the glyph doesn't cover a
+            /// single cell, scale up. If the glyph exceeds a single
+            /// cell but is within the bounds, do nothing.
+            /// (Nerd Font specific rule.)
+            fit_cover1,
+            /// Stretch the glyph to exactly fit the bounds in both
+            /// directions, disregarding aspect ratio.
+            stretch,
+        };
+
+        pub const Align = enum {
+            /// Don't move the glyph on this axis.
+            none,
+            /// Move the glyph so that its leading (bottom/left)
+            /// edge aligns with the leading edge of the axis.
+            start,
+            /// Move the glyph so that its trailing (top/right)
+            /// edge aligns with the trailing edge of the axis.
+            end,
+            /// Move the glyph so that it is centered on this axis.
+            center,
+            /// Move the glyph so that it is centered on this axis,
+            /// but always with respect to the first cell even for
+            /// multi-cell constraints. (Nerd Font specific rule.)
+            center1,
+        };
+
+        pub const Height = enum {
+            /// Use the full line height of the primary face for
+            /// constraining this glyph.
+            cell,
+            /// Use the icon height from the grid metrics for
+            /// constraining this glyph. Unlike `cell`, the value of
+            /// this height depends on both the constraint width and the
+            /// affected by the `adjust-icon-height` config option.
+            icon,
+        };
+
+        /// Returns true if the constraint does anything. If it doesn't,
+        /// because it neither sizes nor positions the glyph, then this
+        /// returns false.
+        pub inline fn doesAnything(self: Constraint) bool {
+            return self.size != .none or
+                self.align_horizontal != .none or
+                self.align_vertical != .none;
+        }
+
+        /// Apply this constraint to the provided glyph
+        /// size, given the available width and height.
+        pub fn constrain(
+            self: Constraint,
+            glyph: Glyph.Size,
+            metrics: Metrics,
+            /// Number of cells horizontally available for this glyph.
+            constraint_width: u2,
+        ) Glyph.Size {
+            if (!self.doesAnything()) return glyph;
+
+            switch (self.size) {
+                .stretch => {
+                    // Stretched glyphs are usually meant to align across cell
+                    // boundaries, which works best if they're scaled and
+                    // aligned to the grid rather than the face. This is most
+                    // easily done by inserting this little fib in the metrics.
+                    var m = metrics;
+                    m.face_width = @floatFromInt(m.cell_width);
+                    m.face_height = @floatFromInt(m.cell_height);
+                    m.face_y = 0.0;
+
+                    // Negative padding for stretched glyphs is a band-aid to
+                    // avoid gaps due to pixel rounding, but at the cost of
+                    // unsightly overlap artifacts. Since we scale and align to
+                    // the grid rather than the face, we don't need it.
+                    var c = self;
+                    c.pad_bottom = @max(0, c.pad_bottom);
+                    c.pad_top = @max(0, c.pad_top);
+                    c.pad_left = @max(0, c.pad_left);
+                    c.pad_right = @max(0, c.pad_right);
+
+                    return c.constrainInner(glyph, m, constraint_width);
+                },
+                else => return self.constrainInner(glyph, metrics, constraint_width),
+            }
+        }
+
+        fn constrainInner(
+            self: Constraint,
+            glyph: Glyph.Size,
+            metrics: Metrics,
+            constraint_width: u2,
+        ) Glyph.Size {
+            // For extra wide font faces, never stretch glyphs across two cells.
+            // This mirrors font_patcher.
+            const min_constraint_width: u2 = if ((self.size == .stretch) and (metrics.face_width > 0.9 * metrics.face_height))
+                1
+            else
+                @min(self.max_constraint_width, constraint_width);
+
+            // The bounding box for the glyph's scale group.
+            // Scaling and alignment rules are calculated for
+            // this box and then applied to the glyph.
+            var group: Glyph.Size = group: {
+                const group_width = glyph.width / self.relative_width;
+                const group_height = glyph.height / self.relative_height;
+                break :group .{
+                    .width = group_width,
+                    .height = group_height,
+                    .x = glyph.x - (group_width * self.relative_x),
+                    .y = glyph.y - (group_height * self.relative_y),
+                };
+            };
+
+            // Apply prescribed scaling, preserving the
+            // center bearings of the group bounding box
+            const width_factor, const height_factor = self.scale_factors(group, metrics, min_constraint_width);
+            const center_x = group.x + (group.width / 2);
+            const center_y = group.y + (group.height / 2);
+            group.width *= width_factor;
+            group.height *= height_factor;
+            group.x = center_x - (group.width / 2);
+            group.y = center_y - (group.height / 2);
+
+            // NOTE: font_patcher jumps through a lot of hoops at this
+            // point to ensure that the glyph remains within the target
+            // bounding box after rounding to font definition units.
+            // This is irrelevant here as we're not rounding, we're
+            // staying in f64 and heading straight to rendering.
+
+            // Apply prescribed alignment
+            group.y = self.aligned_y(group, metrics);
+            group.x = self.aligned_x(group, metrics, min_constraint_width);
+
+            // Transfer the scaling and alignment back to the glyph and return.
+            return .{
+                .width = width_factor * glyph.width,
+                .height = height_factor * glyph.height,
+                .x = group.x + (group.width * self.relative_x),
+                .y = group.y + (group.height * self.relative_y),
+            };
+        }
+
+        /// Return width and height scaling factors for this scaling group.
+        fn scale_factors(
+            self: Constraint,
+            group: Glyph.Size,
+            metrics: Metrics,
+            min_constraint_width: u2,
+        ) struct { f64, f64 } {
+            if (self.size == .none) {
+                return .{ 1.0, 1.0 };
+            }
+
+            const multi_cell = (min_constraint_width > 1);
+
+            const pad_width_factor = @as(f64, @floatFromInt(min_constraint_width)) - (self.pad_left + self.pad_right);
+            const pad_height_factor = 1 - (self.pad_bottom + self.pad_top);
+
+            const target_width = pad_width_factor * metrics.face_width;
+            const target_height = pad_height_factor * switch (self.height) {
+                .cell => metrics.face_height,
+                // Like font-patcher, the icon constraint height depends on the
+                // constraint width. Unlike font-patcher, the multi-cell
+                // icon_height may be different from face_height due to the
+                // `adjust-icon-height` config option.
+                .icon => if (multi_cell)
+                    metrics.icon_height
+                else
+                    metrics.icon_height_single,
+            };
+
+            var width_factor = target_width / group.width;
+            var height_factor = target_height / group.height;
+
+            switch (self.size) {
+                .none => unreachable,
+                .fit => {
+                    // Scale down to fit if needed
+                    height_factor = @min(1, width_factor, height_factor);
+                    width_factor = height_factor;
+                },
+                .cover => {
+                    // Scale to cover
+                    height_factor = @min(width_factor, height_factor);
+                    width_factor = height_factor;
+                },
+                .fit_cover1 => {
+                    // Scale down to fit or up to cover at least one cell
+                    // NOTE: This is similar to font_patcher's "pa" mode,
+                    // however, font_patcher will only do the upscaling
+                    // part if the constraint width is 1, resulting in
+                    // some icons becoming smaller when the constraint
+                    // width increases. You'd see icons shrinking when
+                    // opening up a space after them. This makes no
+                    // sense, so we've fixed the rule such that these
+                    // icons are scaled to the same size for multi-cell
+                    // constraints as they would be for single-cell.
+                    height_factor = @min(width_factor, height_factor);
+                    if (multi_cell and (height_factor > 1)) {
+                        // Call back into this function with
+                        // constraint width 1 to get single-cell scale
+                        // factors. We use the height factor as width
+                        // could have been modified by max_xy_ratio.
+                        _, const single_height_factor = self.scale_factors(group, metrics, 1);
+                        height_factor = @max(1, single_height_factor);
+                    }
+                    width_factor = height_factor;
+                },
+                .stretch => {},
+            }
+
+            // Reduce aspect ratio if required
+            if (self.max_xy_ratio) |ratio| {
+                if (group.width * width_factor > group.height * height_factor * ratio) {
+                    width_factor = group.height * height_factor * ratio / group.width;
+                }
+            }
+
+            return .{ width_factor, height_factor };
+        }
+
+        /// Return vertical bearing for aligning this group
+        fn aligned_y(
+            self: Constraint,
+            group: Glyph.Size,
+            metrics: Metrics,
+        ) f64 {
+            if ((self.size == .none) and (self.align_vertical == .none)) {
+                // If we don't have any constraints affecting the vertical axis,
+                // we don't touch vertical alignment.
+                return group.y;
+            }
+            // We use face_height and offset by face_y, rather than
+            // using cell_height directly, to account for the asymmetry
+            // of the pixel cell around the face (a consequence of
+            // aligning the baseline with a pixel boundary rather than
+            // vertically centering the face).
+            const pad_bottom_dy = self.pad_bottom * metrics.face_height;
+            const pad_top_dy = self.pad_top * metrics.face_height;
+            const start_y = metrics.face_y + pad_bottom_dy;
+            const end_y = metrics.face_y + (metrics.face_height - group.height - pad_top_dy);
+            const center_y = (start_y + end_y) / 2;
+            return switch (self.align_vertical) {
+                // NOTE: Even if there is no prescribed alignment, we ensure
+                // that the group doesn't protrude outside the padded cell,
+                // since this is implied by every available size constraint. If
+                // the group is too high we fall back to centering, though if we
+                // hit the .none prong we always have self.size != .none, so
+                // this should never happen.
+                .none => if (end_y < start_y)
+                    center_y
+                else
+                    @max(start_y, @min(group.y, end_y)),
+                .start => start_y,
+                .end => end_y,
+                .center, .center1 => center_y,
+            };
+        }
+
+        /// Return horizontal bearing for aligning this group
+        fn aligned_x(
+            self: Constraint,
+            group: Glyph.Size,
+            metrics: Metrics,
+            min_constraint_width: u2,
+        ) f64 {
+            if ((self.size == .none) and (self.align_horizontal == .none)) {
+                // If we don't have any constraints affecting the horizontal
+                // axis, we don't touch horizontal alignment.
+                return group.x;
+            }
+            // For multi-cell constraints, we align relative to the span
+            // from the left edge of the first cell to the right edge of
+            // the last face cell assuming it's left-aligned within the
+            // rounded and adjusted pixel cell. Any horizontal offset to
+            // center the face within the grid cell is the responsibility
+            // of the backend-specific rendering code, and should be done
+            // after applying constraints.
+            const full_face_span = metrics.face_width + @as(f64, @floatFromInt((min_constraint_width - 1) * metrics.cell_width));
+            const pad_left_dx = self.pad_left * metrics.face_width;
+            const pad_right_dx = self.pad_right * metrics.face_width;
+            const start_x = pad_left_dx;
+            const end_x = full_face_span - group.width - pad_right_dx;
+            return switch (self.align_horizontal) {
+                // NOTE: Even if there is no prescribed alignment, we ensure
+                // that the glyph doesn't protrude outside the padded cell,
+                // since this is implied by every available size constraint. The
+                // left-side bound has priority if the group is too wide, though
+                // if we hit the .none prong we always have self.size != .none,
+                // so this should never happen.
+                .none => @max(start_x, @min(group.x, end_x)),
+                .start => start_x,
+                .end => @max(start_x, end_x),
+                .center => @max(start_x, (start_x + end_x) / 2),
+                // NOTE: .center1 implements the font_patcher rule of centering
+                // in the first cell even for multi-cell constraints. Since glyphs
+                // are not allowed to protrude to the left, this results in the
+                // left-alignment like .start when the glyph is wider than a cell.
+                .center1 => center1: {
+                    const end1_x = metrics.face_width - group.width - pad_right_dx;
+                    break :center1 @max(start_x, (start_x + end1_x) / 2);
+                },
+            };
+        }
+    };
+};
+
+test "Constraints" {
+    const comparison = @import("../datastruct/comparison.zig");
+    const getConstraint = @import("nerd_font_attributes.zig").getConstraint;
+    const GlyphSize = Size;
+
+    // Hardcoded data matches metrics from CoreText at size 12 and DPI 96.
+
+    // Define grid metrics (matches font-family = JetBrains Mono)
+    const metrics: Metrics = .{
+        .cell_width = 10,
+        .cell_height = 22,
+        .cell_baseline = 5,
+        .underline_position = 19,
+        .underline_thickness = 1,
+        .strikethrough_position = 12,
+        .strikethrough_thickness = 1,
+        .overline_position = 0,
+        .overline_thickness = 1,
+        .box_thickness = 1,
+        .cursor_thickness = 1,
+        .cursor_height = 22,
+        .icon_height = 21.12,
+        .icon_height_single = 44.48 / 3.0,
+        .face_width = 9.6,
+        .face_height = 21.12,
+        .face_y = 0.2,
+    };
+
+    // ASCII (no constraint).
+    {
+        const constraint: RenderOptions.Constraint = .none;
+
+        // BBox of 'x' from JetBrains Mono.
+        const glyph_x: GlyphSize = .{
+            .width = 6.784,
+            .height = 15.28,
+            .x = 1.408,
+            .y = 4.84,
+        };
+
+        // Any constraint width: do nothing.
+        inline for (.{ 1, 2 }) |constraint_width| {
+            try comparison.expectApproxEqual(
+                glyph_x,
+                constraint.constrain(glyph_x, metrics, constraint_width),
+            );
+        }
+    }
+
+    // Symbol (same constraint as hardcoded in Renderer.addGlyph).
+    {
+        const constraint: RenderOptions.Constraint = .{ .size = .fit };
+
+        // BBox of '■' (0x25A0 black square) from Iosevka.
+        // NOTE: This glyph is designed to span two cells.
+        const glyph_25A0: GlyphSize = .{
+            .width = 10.272,
+            .height = 10.272,
+            .x = 2.864,
+            .y = 5.304,
+        };
+
+        // Constraint width 1: scale down and shift to fit a single cell.
+        try comparison.expectApproxEqual(
+            GlyphSize{
+                .width = metrics.face_width,
+                .height = metrics.face_width,
+                .x = 0,
+                .y = 5.64,
+            },
+            constraint.constrain(glyph_25A0, metrics, 1),
+        );
+
+        // Constraint width 2: do nothing.
+        try comparison.expectApproxEqual(
+            glyph_25A0,
+            constraint.constrain(glyph_25A0, metrics, 2),
+        );
+    }
+
+    // Emoji (same constraint as hardcoded in SharedGrid.renderGlyph).
+    {
+        const constraint: RenderOptions.Constraint = .{
+            .size = .cover,
+            .align_horizontal = .center,
+            .align_vertical = .center,
+            .pad_left = 0.025,
+            .pad_right = 0.025,
+        };
+
+        // BBox of '🥸' (0x1F978) from Apple Color Emoji.
+        const glyph_1F978: GlyphSize = .{
+            .width = 20,
+            .height = 20,
+            .x = 0.46,
+            .y = 1,
+        };
+
+        // Constraint width 2: scale to cover two cells with padding, center;
+        try comparison.expectApproxEqual(
+            GlyphSize{
+                .width = 18.72,
+                .height = 18.72,
+                .x = 0.44,
+                .y = 1.4,
+            },
+            constraint.constrain(glyph_1F978, metrics, 2),
+        );
+    }
+
+    // Nerd Font default.
+    {
+        const constraint = getConstraint(0xea61).?;
+
+        // Verify that this is the constraint we expect.
+        try std.testing.expectEqual(.fit_cover1, constraint.size);
+        try std.testing.expectEqual(.icon, constraint.height);
+        try std.testing.expectEqual(.center1, constraint.align_horizontal);
+        try std.testing.expectEqual(.center1, constraint.align_vertical);
+
+        // BBox of '' (0xEA61 nf-cod-lightbulb) from Symbols Only.
+        // NOTE: This icon is part of a group, so the
+        // constraint applies to a larger bounding box.
+        const glyph_EA61: GlyphSize = .{
+            .width = 9.015625,
+            .height = 13.015625,
+            .x = 3.015625,
+            .y = 3.76525,
+        };
+
+        // Constraint width 1: scale and shift group to fit a single cell.
+        try comparison.expectApproxEqual(
+            GlyphSize{
+                .width = 7.2125,
+                .height = 10.4125,
+                .x = 0.8125,
+                .y = 5.950695224719102,
+            },
+            constraint.constrain(glyph_EA61, metrics, 1),
+        );
+
+        // Constraint width 2: no scaling; left-align and vertically center group.
+        try comparison.expectApproxEqual(
+            GlyphSize{
+                .width = glyph_EA61.width,
+                .height = glyph_EA61.height,
+                .x = 1.015625,
+                .y = 4.7483690308988775,
+            },
+            constraint.constrain(glyph_EA61, metrics, 2),
+        );
+    }
+
+    // Nerd Font stretch.
+    {
+        const constraint = getConstraint(0xe0c0).?;
+
+        // Verify that this is the constraint we expect.
+        try std.testing.expectEqual(.stretch, constraint.size);
+        try std.testing.expectEqual(.cell, constraint.height);
+        try std.testing.expectEqual(.start, constraint.align_horizontal);
+        try std.testing.expectEqual(.center1, constraint.align_vertical);
+
+        // BBox of ' ' (0xE0C0 nf-ple-flame_thick) from Symbols Only.
+        const glyph_E0C0: GlyphSize = .{
+            .width = 16.796875,
+            .height = 16.46875,
+            .x = -0.796875,
+            .y = 1.7109375,
+        };
+
+        // Constraint width 1: stretch and position to exactly cover one cell.
+        try comparison.expectApproxEqual(
+            GlyphSize{
+                .width = @floatFromInt(metrics.cell_width),
+                .height = @floatFromInt(metrics.cell_height),
+                .x = 0,
+                .y = 0,
+            },
+            constraint.constrain(glyph_E0C0, metrics, 1),
+        );
+
+        // Constraint width 1: stretch and position to exactly cover two cells.
+        try comparison.expectApproxEqual(
+            GlyphSize{
+                .width = @floatFromInt(2 * metrics.cell_width),
+                .height = @floatFromInt(metrics.cell_height),
+                .x = 0,
+                .y = 0,
+            },
+            constraint.constrain(glyph_E0C0, metrics, 2),
+        );
+    }
+}
diff --git a/src/font/Metrics.zig b/src/font/Metrics.zig
index a72cb7bee..60a300b0c 100644
--- a/src/font/Metrics.zig
+++ b/src/font/Metrics.zig
@@ -560,6 +560,8 @@ pub const Modifier = union(enum) {
     }
 
     test "formatConfig percent" {
+        if (comptime @import("terminal_options").artifact == .lib) return;
+
         const configpkg = @import("../config.zig");
         const testing = std.testing;
         var buf: std.Io.Writer.Allocating = .init(testing.allocator);
@@ -571,6 +573,8 @@ pub const Modifier = union(enum) {
     }
 
     test "formatConfig absolute" {
+        if (comptime @import("terminal_options").artifact == .lib) return;
+
         const configpkg = @import("../config.zig");
         const testing = std.testing;
         var buf: std.Io.Writer.Allocating = .init(testing.allocator);
diff --git a/src/font/SharedGrid.zig b/src/font/SharedGrid.zig
index 5fd729b30..18870aaba 100644
--- a/src/font/SharedGrid.zig
+++ b/src/font/SharedGrid.zig
@@ -33,7 +33,7 @@ const Library = font.Library;
 const Metrics = font.Metrics;
 const Presentation = font.Presentation;
 const Style = font.Style;
-const RenderOptions = font.face.RenderOptions;
+const RenderOptions = font.Glyph.RenderOptions;
 
 const log = std.log.scoped(.font_shared_grid);
 
diff --git a/src/font/face.zig b/src/font/face.zig
index d77253adf..aeb7280c7 100644
--- a/src/font/face.zig
+++ b/src/font/face.zig
@@ -2,7 +2,6 @@ const std = @import("std");
 const builtin = @import("builtin");
 const build_config = @import("../build_config.zig");
 const options = @import("main.zig").options;
-const Metrics = @import("main.zig").Metrics;
 const config = @import("../config.zig");
 const freetype = @import("face/freetype.zig");
 const coretext = @import("face/coretext.zig");
@@ -94,407 +93,6 @@ pub const Variation = struct {
     };
 };
 
-/// The size and position of a glyph.
-pub const GlyphSize = struct {
-    width: f64,
-    height: f64,
-    x: f64,
-    y: f64,
-};
-
-/// Additional options for rendering glyphs.
-pub const RenderOptions = struct {
-    /// The metrics that are defining the grid layout. These are usually
-    /// the metrics of the primary font face. The grid metrics are used
-    /// by the font face to better layout the glyph in situations where
-    /// the font is not exactly the same size as the grid.
-    grid_metrics: Metrics,
-
-    /// The number of grid cells this glyph will take up. This can be used
-    /// optionally by the rasterizer to better layout the glyph.
-    cell_width: ?u2 = null,
-
-    /// Constraint and alignment properties for the glyph. The rasterizer
-    /// should call the `constrain` function on this with the original size
-    /// and bearings of the glyph to get remapped values that the glyph
-    /// should be scaled/moved to.
-    constraint: Constraint = .none,
-
-    /// The number of cells, horizontally that the glyph is free to take up
-    /// when resized and aligned by `constraint`. This is usually 1, but if
-    /// there's whitespace to the right of the cell then it can be 2.
-    constraint_width: u2 = 1,
-
-    /// Thicken the glyph. This draws the glyph with a thicker stroke width.
-    /// This is purely an aesthetic setting.
-    ///
-    /// This only works with CoreText currently.
-    thicken: bool = false,
-
-    /// "Strength" of the thickening, between `0` and `255`.
-    /// Only has an effect when `thicken` is enabled.
-    ///
-    /// `0` does not correspond to *no* thickening,
-    /// just the *lightest* thickening available.
-    ///
-    /// CoreText only.
-    thicken_strength: u8 = 255,
-
-    /// See the `constraint` field.
-    pub const Constraint = struct {
-        /// Don't constrain the glyph in any way.
-        pub const none: Constraint = .{};
-
-        /// Sizing rule.
-        size: Size = .none,
-
-        /// Vertical alignment rule.
-        align_vertical: Align = .none,
-        /// Horizontal alignment rule.
-        align_horizontal: Align = .none,
-
-        /// Top padding when resizing.
-        pad_top: f64 = 0.0,
-        /// Left padding when resizing.
-        pad_left: f64 = 0.0,
-        /// Right padding when resizing.
-        pad_right: f64 = 0.0,
-        /// Bottom padding when resizing.
-        pad_bottom: f64 = 0.0,
-
-        // Size and bearings of the glyph relative
-        // to the bounding box of its scale group.
-        relative_width: f64 = 1.0,
-        relative_height: f64 = 1.0,
-        relative_x: f64 = 0.0,
-        relative_y: f64 = 0.0,
-
-        /// Maximum aspect ratio (width/height) to allow when stretching.
-        max_xy_ratio: ?f64 = null,
-
-        /// Maximum number of cells horizontally to use.
-        max_constraint_width: u2 = 2,
-
-        /// What to use as the height metric when constraining the glyph and
-        /// the constraint width is 1,
-        height: Height = .cell,
-
-        pub const Size = enum {
-            /// Don't change the size of this glyph.
-            none,
-            /// Scale the glyph down if needed to fit within the bounds,
-            /// preserving aspect ratio.
-            fit,
-            /// Scale the glyph up or down to exactly match the bounds,
-            /// preserving aspect ratio.
-            cover,
-            /// Scale the glyph down if needed to fit within the bounds,
-            /// preserving aspect ratio. If the glyph doesn't cover a
-            /// single cell, scale up. If the glyph exceeds a single
-            /// cell but is within the bounds, do nothing.
-            /// (Nerd Font specific rule.)
-            fit_cover1,
-            /// Stretch the glyph to exactly fit the bounds in both
-            /// directions, disregarding aspect ratio.
-            stretch,
-        };
-
-        pub const Align = enum {
-            /// Don't move the glyph on this axis.
-            none,
-            /// Move the glyph so that its leading (bottom/left)
-            /// edge aligns with the leading edge of the axis.
-            start,
-            /// Move the glyph so that its trailing (top/right)
-            /// edge aligns with the trailing edge of the axis.
-            end,
-            /// Move the glyph so that it is centered on this axis.
-            center,
-            /// Move the glyph so that it is centered on this axis,
-            /// but always with respect to the first cell even for
-            /// multi-cell constraints. (Nerd Font specific rule.)
-            center1,
-        };
-
-        pub const Height = enum {
-            /// Use the full line height of the primary face for
-            /// constraining this glyph.
-            cell,
-            /// Use the icon height from the grid metrics for
-            /// constraining this glyph. Unlike `cell`, the value of
-            /// this height depends on both the constraint width and the
-            /// affected by the `adjust-icon-height` config option.
-            icon,
-        };
-
-        /// Returns true if the constraint does anything. If it doesn't,
-        /// because it neither sizes nor positions the glyph, then this
-        /// returns false.
-        pub inline fn doesAnything(self: Constraint) bool {
-            return self.size != .none or
-                self.align_horizontal != .none or
-                self.align_vertical != .none;
-        }
-
-        /// Apply this constraint to the provided glyph
-        /// size, given the available width and height.
-        pub fn constrain(
-            self: Constraint,
-            glyph: GlyphSize,
-            metrics: Metrics,
-            /// Number of cells horizontally available for this glyph.
-            constraint_width: u2,
-        ) GlyphSize {
-            if (!self.doesAnything()) return glyph;
-
-            switch (self.size) {
-                .stretch => {
-                    // Stretched glyphs are usually meant to align across cell
-                    // boundaries, which works best if they're scaled and
-                    // aligned to the grid rather than the face. This is most
-                    // easily done by inserting this little fib in the metrics.
-                    var m = metrics;
-                    m.face_width = @floatFromInt(m.cell_width);
-                    m.face_height = @floatFromInt(m.cell_height);
-                    m.face_y = 0.0;
-
-                    // Negative padding for stretched glyphs is a band-aid to
-                    // avoid gaps due to pixel rounding, but at the cost of
-                    // unsightly overlap artifacts. Since we scale and align to
-                    // the grid rather than the face, we don't need it.
-                    var c = self;
-                    c.pad_bottom = @max(0, c.pad_bottom);
-                    c.pad_top = @max(0, c.pad_top);
-                    c.pad_left = @max(0, c.pad_left);
-                    c.pad_right = @max(0, c.pad_right);
-
-                    return c.constrainInner(glyph, m, constraint_width);
-                },
-                else => return self.constrainInner(glyph, metrics, constraint_width),
-            }
-        }
-
-        fn constrainInner(
-            self: Constraint,
-            glyph: GlyphSize,
-            metrics: Metrics,
-            constraint_width: u2,
-        ) GlyphSize {
-            // For extra wide font faces, never stretch glyphs across two cells.
-            // This mirrors font_patcher.
-            const min_constraint_width: u2 = if ((self.size == .stretch) and (metrics.face_width > 0.9 * metrics.face_height))
-                1
-            else
-                @min(self.max_constraint_width, constraint_width);
-
-            // The bounding box for the glyph's scale group.
-            // Scaling and alignment rules are calculated for
-            // this box and then applied to the glyph.
-            var group: GlyphSize = group: {
-                const group_width = glyph.width / self.relative_width;
-                const group_height = glyph.height / self.relative_height;
-                break :group .{
-                    .width = group_width,
-                    .height = group_height,
-                    .x = glyph.x - (group_width * self.relative_x),
-                    .y = glyph.y - (group_height * self.relative_y),
-                };
-            };
-
-            // Apply prescribed scaling, preserving the
-            // center bearings of the group bounding box
-            const width_factor, const height_factor = self.scale_factors(group, metrics, min_constraint_width);
-            const center_x = group.x + (group.width / 2);
-            const center_y = group.y + (group.height / 2);
-            group.width *= width_factor;
-            group.height *= height_factor;
-            group.x = center_x - (group.width / 2);
-            group.y = center_y - (group.height / 2);
-
-            // NOTE: font_patcher jumps through a lot of hoops at this
-            // point to ensure that the glyph remains within the target
-            // bounding box after rounding to font definition units.
-            // This is irrelevant here as we're not rounding, we're
-            // staying in f64 and heading straight to rendering.
-
-            // Apply prescribed alignment
-            group.y = self.aligned_y(group, metrics);
-            group.x = self.aligned_x(group, metrics, min_constraint_width);
-
-            // Transfer the scaling and alignment back to the glyph and return.
-            return .{
-                .width = width_factor * glyph.width,
-                .height = height_factor * glyph.height,
-                .x = group.x + (group.width * self.relative_x),
-                .y = group.y + (group.height * self.relative_y),
-            };
-        }
-
-        /// Return width and height scaling factors for this scaling group.
-        fn scale_factors(
-            self: Constraint,
-            group: GlyphSize,
-            metrics: Metrics,
-            min_constraint_width: u2,
-        ) struct { f64, f64 } {
-            if (self.size == .none) {
-                return .{ 1.0, 1.0 };
-            }
-
-            const multi_cell = (min_constraint_width > 1);
-
-            const pad_width_factor = @as(f64, @floatFromInt(min_constraint_width)) - (self.pad_left + self.pad_right);
-            const pad_height_factor = 1 - (self.pad_bottom + self.pad_top);
-
-            const target_width = pad_width_factor * metrics.face_width;
-            const target_height = pad_height_factor * switch (self.height) {
-                .cell => metrics.face_height,
-                // Like font-patcher, the icon constraint height depends on the
-                // constraint width. Unlike font-patcher, the multi-cell
-                // icon_height may be different from face_height due to the
-                // `adjust-icon-height` config option.
-                .icon => if (multi_cell)
-                    metrics.icon_height
-                else
-                    metrics.icon_height_single,
-            };
-
-            var width_factor = target_width / group.width;
-            var height_factor = target_height / group.height;
-
-            switch (self.size) {
-                .none => unreachable,
-                .fit => {
-                    // Scale down to fit if needed
-                    height_factor = @min(1, width_factor, height_factor);
-                    width_factor = height_factor;
-                },
-                .cover => {
-                    // Scale to cover
-                    height_factor = @min(width_factor, height_factor);
-                    width_factor = height_factor;
-                },
-                .fit_cover1 => {
-                    // Scale down to fit or up to cover at least one cell
-                    // NOTE: This is similar to font_patcher's "pa" mode,
-                    // however, font_patcher will only do the upscaling
-                    // part if the constraint width is 1, resulting in
-                    // some icons becoming smaller when the constraint
-                    // width increases. You'd see icons shrinking when
-                    // opening up a space after them. This makes no
-                    // sense, so we've fixed the rule such that these
-                    // icons are scaled to the same size for multi-cell
-                    // constraints as they would be for single-cell.
-                    height_factor = @min(width_factor, height_factor);
-                    if (multi_cell and (height_factor > 1)) {
-                        // Call back into this function with
-                        // constraint width 1 to get single-cell scale
-                        // factors. We use the height factor as width
-                        // could have been modified by max_xy_ratio.
-                        _, const single_height_factor = self.scale_factors(group, metrics, 1);
-                        height_factor = @max(1, single_height_factor);
-                    }
-                    width_factor = height_factor;
-                },
-                .stretch => {},
-            }
-
-            // Reduce aspect ratio if required
-            if (self.max_xy_ratio) |ratio| {
-                if (group.width * width_factor > group.height * height_factor * ratio) {
-                    width_factor = group.height * height_factor * ratio / group.width;
-                }
-            }
-
-            return .{ width_factor, height_factor };
-        }
-
-        /// Return vertical bearing for aligning this group
-        fn aligned_y(
-            self: Constraint,
-            group: GlyphSize,
-            metrics: Metrics,
-        ) f64 {
-            if ((self.size == .none) and (self.align_vertical == .none)) {
-                // If we don't have any constraints affecting the vertical axis,
-                // we don't touch vertical alignment.
-                return group.y;
-            }
-            // We use face_height and offset by face_y, rather than
-            // using cell_height directly, to account for the asymmetry
-            // of the pixel cell around the face (a consequence of
-            // aligning the baseline with a pixel boundary rather than
-            // vertically centering the face).
-            const pad_bottom_dy = self.pad_bottom * metrics.face_height;
-            const pad_top_dy = self.pad_top * metrics.face_height;
-            const start_y = metrics.face_y + pad_bottom_dy;
-            const end_y = metrics.face_y + (metrics.face_height - group.height - pad_top_dy);
-            const center_y = (start_y + end_y) / 2;
-            return switch (self.align_vertical) {
-                // NOTE: Even if there is no prescribed alignment, we ensure
-                // that the group doesn't protrude outside the padded cell,
-                // since this is implied by every available size constraint. If
-                // the group is too high we fall back to centering, though if we
-                // hit the .none prong we always have self.size != .none, so
-                // this should never happen.
-                .none => if (end_y < start_y)
-                    center_y
-                else
-                    @max(start_y, @min(group.y, end_y)),
-                .start => start_y,
-                .end => end_y,
-                .center, .center1 => center_y,
-            };
-        }
-
-        /// Return horizontal bearing for aligning this group
-        fn aligned_x(
-            self: Constraint,
-            group: GlyphSize,
-            metrics: Metrics,
-            min_constraint_width: u2,
-        ) f64 {
-            if ((self.size == .none) and (self.align_horizontal == .none)) {
-                // If we don't have any constraints affecting the horizontal
-                // axis, we don't touch horizontal alignment.
-                return group.x;
-            }
-            // For multi-cell constraints, we align relative to the span
-            // from the left edge of the first cell to the right edge of
-            // the last face cell assuming it's left-aligned within the
-            // rounded and adjusted pixel cell. Any horizontal offset to
-            // center the face within the grid cell is the responsibility
-            // of the backend-specific rendering code, and should be done
-            // after applying constraints.
-            const full_face_span = metrics.face_width + @as(f64, @floatFromInt((min_constraint_width - 1) * metrics.cell_width));
-            const pad_left_dx = self.pad_left * metrics.face_width;
-            const pad_right_dx = self.pad_right * metrics.face_width;
-            const start_x = pad_left_dx;
-            const end_x = full_face_span - group.width - pad_right_dx;
-            return switch (self.align_horizontal) {
-                // NOTE: Even if there is no prescribed alignment, we ensure
-                // that the glyph doesn't protrude outside the padded cell,
-                // since this is implied by every available size constraint. The
-                // left-side bound has priority if the group is too wide, though
-                // if we hit the .none prong we always have self.size != .none,
-                // so this should never happen.
-                .none => @max(start_x, @min(group.x, end_x)),
-                .start => start_x,
-                .end => @max(start_x, end_x),
-                .center => @max(start_x, (start_x + end_x) / 2),
-                // NOTE: .center1 implements the font_patcher rule of centering
-                // in the first cell even for multi-cell constraints. Since glyphs
-                // are not allowed to protrude to the left, this results in the
-                // left-alignment like .start when the glyph is wider than a cell.
-                .center1 => center1: {
-                    const end1_x = metrics.face_width - group.width - pad_right_dx;
-                    break :center1 @max(start_x, (start_x + end1_x) / 2);
-                },
-            };
-        }
-    };
-};
-
 test {
     @import("std").testing.refAllDecls(@This());
 }
@@ -512,197 +110,3 @@ test "Variation.Id: slnt should be 1936486004" {
     try testing.expectEqual(@as(u32, 1936486004), @as(u32, @bitCast(id)));
     try testing.expectEqualStrings("slnt", &(id.str()));
 }
-
-test "Constraints" {
-    const comparison = @import("../datastruct/comparison.zig");
-    const getConstraint = @import("nerd_font_attributes.zig").getConstraint;
-
-    // Hardcoded data matches metrics from CoreText at size 12 and DPI 96.
-
-    // Define grid metrics (matches font-family = JetBrains Mono)
-    const metrics: Metrics = .{
-        .cell_width = 10,
-        .cell_height = 22,
-        .cell_baseline = 5,
-        .underline_position = 19,
-        .underline_thickness = 1,
-        .strikethrough_position = 12,
-        .strikethrough_thickness = 1,
-        .overline_position = 0,
-        .overline_thickness = 1,
-        .box_thickness = 1,
-        .cursor_thickness = 1,
-        .cursor_height = 22,
-        .icon_height = 21.12,
-        .icon_height_single = 44.48 / 3.0,
-        .face_width = 9.6,
-        .face_height = 21.12,
-        .face_y = 0.2,
-    };
-
-    // ASCII (no constraint).
-    {
-        const constraint: RenderOptions.Constraint = .none;
-
-        // BBox of 'x' from JetBrains Mono.
-        const glyph_x: GlyphSize = .{
-            .width = 6.784,
-            .height = 15.28,
-            .x = 1.408,
-            .y = 4.84,
-        };
-
-        // Any constraint width: do nothing.
-        inline for (.{ 1, 2 }) |constraint_width| {
-            try comparison.expectApproxEqual(
-                glyph_x,
-                constraint.constrain(glyph_x, metrics, constraint_width),
-            );
-        }
-    }
-
-    // Symbol (same constraint as hardcoded in Renderer.addGlyph).
-    {
-        const constraint: RenderOptions.Constraint = .{ .size = .fit };
-
-        // BBox of '■' (0x25A0 black square) from Iosevka.
-        // NOTE: This glyph is designed to span two cells.
-        const glyph_25A0: GlyphSize = .{
-            .width = 10.272,
-            .height = 10.272,
-            .x = 2.864,
-            .y = 5.304,
-        };
-
-        // Constraint width 1: scale down and shift to fit a single cell.
-        try comparison.expectApproxEqual(
-            GlyphSize{
-                .width = metrics.face_width,
-                .height = metrics.face_width,
-                .x = 0,
-                .y = 5.64,
-            },
-            constraint.constrain(glyph_25A0, metrics, 1),
-        );
-
-        // Constraint width 2: do nothing.
-        try comparison.expectApproxEqual(
-            glyph_25A0,
-            constraint.constrain(glyph_25A0, metrics, 2),
-        );
-    }
-
-    // Emoji (same constraint as hardcoded in SharedGrid.renderGlyph).
-    {
-        const constraint: RenderOptions.Constraint = .{
-            .size = .cover,
-            .align_horizontal = .center,
-            .align_vertical = .center,
-            .pad_left = 0.025,
-            .pad_right = 0.025,
-        };
-
-        // BBox of '🥸' (0x1F978) from Apple Color Emoji.
-        const glyph_1F978: GlyphSize = .{
-            .width = 20,
-            .height = 20,
-            .x = 0.46,
-            .y = 1,
-        };
-
-        // Constraint width 2: scale to cover two cells with padding, center;
-        try comparison.expectApproxEqual(
-            GlyphSize{
-                .width = 18.72,
-                .height = 18.72,
-                .x = 0.44,
-                .y = 1.4,
-            },
-            constraint.constrain(glyph_1F978, metrics, 2),
-        );
-    }
-
-    // Nerd Font default.
-    {
-        const constraint = getConstraint(0xea61).?;
-
-        // Verify that this is the constraint we expect.
-        try std.testing.expectEqual(.fit_cover1, constraint.size);
-        try std.testing.expectEqual(.icon, constraint.height);
-        try std.testing.expectEqual(.center1, constraint.align_horizontal);
-        try std.testing.expectEqual(.center1, constraint.align_vertical);
-
-        // BBox of '' (0xEA61 nf-cod-lightbulb) from Symbols Only.
-        // NOTE: This icon is part of a group, so the
-        // constraint applies to a larger bounding box.
-        const glyph_EA61: GlyphSize = .{
-            .width = 9.015625,
-            .height = 13.015625,
-            .x = 3.015625,
-            .y = 3.76525,
-        };
-
-        // Constraint width 1: scale and shift group to fit a single cell.
-        try comparison.expectApproxEqual(
-            GlyphSize{
-                .width = 7.2125,
-                .height = 10.4125,
-                .x = 0.8125,
-                .y = 5.950695224719102,
-            },
-            constraint.constrain(glyph_EA61, metrics, 1),
-        );
-
-        // Constraint width 2: no scaling; left-align and vertically center group.
-        try comparison.expectApproxEqual(
-            GlyphSize{
-                .width = glyph_EA61.width,
-                .height = glyph_EA61.height,
-                .x = 1.015625,
-                .y = 4.7483690308988775,
-            },
-            constraint.constrain(glyph_EA61, metrics, 2),
-        );
-    }
-
-    // Nerd Font stretch.
-    {
-        const constraint = getConstraint(0xe0c0).?;
-
-        // Verify that this is the constraint we expect.
-        try std.testing.expectEqual(.stretch, constraint.size);
-        try std.testing.expectEqual(.cell, constraint.height);
-        try std.testing.expectEqual(.start, constraint.align_horizontal);
-        try std.testing.expectEqual(.center1, constraint.align_vertical);
-
-        // BBox of ' ' (0xE0C0 nf-ple-flame_thick) from Symbols Only.
-        const glyph_E0C0: GlyphSize = .{
-            .width = 16.796875,
-            .height = 16.46875,
-            .x = -0.796875,
-            .y = 1.7109375,
-        };
-
-        // Constraint width 1: stretch and position to exactly cover one cell.
-        try comparison.expectApproxEqual(
-            GlyphSize{
-                .width = @floatFromInt(metrics.cell_width),
-                .height = @floatFromInt(metrics.cell_height),
-                .x = 0,
-                .y = 0,
-            },
-            constraint.constrain(glyph_E0C0, metrics, 1),
-        );
-
-        // Constraint width 1: stretch and position to exactly cover two cells.
-        try comparison.expectApproxEqual(
-            GlyphSize{
-                .width = @floatFromInt(2 * metrics.cell_width),
-                .height = @floatFromInt(metrics.cell_height),
-                .x = 0,
-                .y = 0,
-            },
-            constraint.constrain(glyph_E0C0, metrics, 2),
-        );
-    }
-}
diff --git a/src/font/face/coretext.zig b/src/font/face/coretext.zig
index 1d1333882..cd8bc7d8b 100644
--- a/src/font/face/coretext.zig
+++ b/src/font/face/coretext.zig
@@ -291,7 +291,7 @@ pub const Face = struct {
         alloc: Allocator,
         atlas: *font.Atlas,
         glyph_index: u32,
-        opts: font.face.RenderOptions,
+        opts: font.Glyph.RenderOptions,
     ) !font.Glyph {
         var glyphs = [_]macos.graphics.Glyph{@intCast(glyph_index)};
 
diff --git a/src/font/face/freetype.zig b/src/font/face/freetype.zig
index 528f72d52..d04fe1809 100644
--- a/src/font/face/freetype.zig
+++ b/src/font/face/freetype.zig
@@ -393,7 +393,7 @@ pub const Face = struct {
     }
 
     /// Get a rect that represents the position and size of the loaded glyph.
-    fn getGlyphSize(glyph: freetype.c.FT_GlyphSlot) font.face.GlyphSize {
+    fn getGlyphSize(glyph: freetype.c.FT_GlyphSlot) font.Glyph.Size {
         // If we're dealing with an outline glyph then we get the
         // outline's bounding box instead of using the built-in
         // metrics, since that's more precise and allows better
@@ -427,7 +427,7 @@ pub const Face = struct {
         alloc: Allocator,
         atlas: *font.Atlas,
         glyph_index: u32,
-        opts: font.face.RenderOptions,
+        opts: font.Glyph.RenderOptions,
     ) !Glyph {
         self.ft_mutex.lock();
         defer self.ft_mutex.unlock();
diff --git a/src/font/face/web_canvas.zig b/src/font/face/web_canvas.zig
index b4f9f5d5d..a36a482dd 100644
--- a/src/font/face/web_canvas.zig
+++ b/src/font/face/web_canvas.zig
@@ -189,7 +189,7 @@ pub const Face = struct {
         alloc: Allocator,
         atlas: *font.Atlas,
         glyph_index: u32,
-        opts: font.face.RenderOptions,
+        opts: font.Glyph.RenderOptions,
     ) !font.Glyph {
         _ = opts;
 
diff --git a/src/font/glyf_rasterize.zig b/src/font/glyf_rasterize.zig
index 43dde026f..f321214bc 100644
--- a/src/font/glyf_rasterize.zig
+++ b/src/font/glyf_rasterize.zig
@@ -9,21 +9,10 @@ const assert = std.debug.assert;
 const Allocator = std.mem.Allocator;
 const z2d = @import("z2d");
 
-const face = @import("face.zig");
+const Glyph = @import("Glyph.zig");
 const glyf = @import("opentype/glyf.zig");
 
-/// Metrics describing the authored glyf coordinate space, since
-/// a glyf table doesn't contain this on its own.
-pub const DesignMetrics = struct {
-    /// Units-per-em for outline/design coordinates.
-    units_per_em: u32,
-
-    /// Authored advance width in design units.
-    advance_width: u32,
-
-    /// Authored line height in design units.
-    line_height: u32,
-};
+const DesignMetrics = Glyph.DesignMetrics;
 
 /// An owned, tightly packed alpha8 bitmap.
 pub const Bitmap = struct {
@@ -52,7 +41,7 @@ pub const Error = Allocator.Error || z2d.Path.Error || z2d.painter.FillError;
 ///
 /// The returned bitmap is always `grid_metrics.cell_width * cell_width` by
 /// `grid_metrics.cell_height`. `opts.constraint` is applied using the same
-/// `face.RenderOptions.Constraint` machinery used by the platform font
+/// `RenderOptions.Constraint` machinery used by the platform font
 /// backends.
 ///
 /// The caller owns the returned bitmap.
@@ -60,7 +49,7 @@ pub fn rasterize(
     alloc: Allocator,
     outline: glyf.Glyf.Outline,
     design: DesignMetrics,
-    opts: face.RenderOptions,
+    opts: Glyph.RenderOptions,
 ) Error!Bitmap {
     assert(design.units_per_em > 0);
     assert(design.advance_width > 0);
@@ -207,16 +196,16 @@ const Placement = struct {
 
     /// Bottom edge of the rasterized outline bounds in bitmap pixels, measured
     /// from the bitmap's bottom edge. This matches the cell-relative y axis
-    /// used by font.face.GlyphSize and is converted to z2d's y-down axis when
+    /// used by font.Glyph.Size and is converted to z2d's y-down axis when
     /// points are transformed.
     y: f64,
 
     /// Width of the rasterized outline bounds in bitmap pixels after applying
-    /// font.face.RenderOptions.Constraint.
+    /// font.Glyph.RenderOptions.Constraint.
     width: f64,
 
     /// Height of the rasterized outline bounds in bitmap pixels after applying
-    /// font.face.RenderOptions.Constraint.
+    /// font.Glyph.RenderOptions.Constraint.
     height: f64,
 
     /// Full bitmap height in pixels, used to convert cell-relative y-up-ish
@@ -240,7 +229,7 @@ const Placement = struct {
     fn init(
         bounds: Bounds,
         design: DesignMetrics,
-        opts: face.RenderOptions,
+        opts: Glyph.RenderOptions,
     ) Placement {
         // Start with protocol-like design units mapped so that the em square
         // occupies one cell. This makes units_per_em the scale reference and
@@ -253,7 +242,7 @@ const Placement = struct {
         // Convert the decoded point bounds into the same pixel coordinate space
         // expected by RenderOptions.Constraint. This rectangle is the visible
         // outline bounds, not the full advance/line-height layout box.
-        const glyph: face.GlyphSize = .{
+        const glyph: Glyph.Size = .{
             .width = bounds.width() * scale,
             .height = bounds.height() * scale,
             .x = bounds.x_min * scale,
@@ -268,7 +257,7 @@ const Placement = struct {
         // Apply the same fit/cover/stretch/alignment/padding rules used by
         // normal font rendering. The result is still the outline bounds, but
         // placed as if its containing advance/line-height box was constrained.
-        const constraint: face.RenderOptions.Constraint = constraint: {
+        const constraint: Glyph.RenderOptions.Constraint = constraint: {
             var constraint = opts.constraint;
             if (group_width > 0 and group_height > 0) {
                 // Tell Constraint that `glyph` is a sub-rectangle of the
diff --git a/src/font/nerd_font_attributes.zig b/src/font/nerd_font_attributes.zig
index f4a19d963..fc9884964 100644
--- a/src/font/nerd_font_attributes.zig
+++ b/src/font/nerd_font_attributes.zig
@@ -4,7 +4,7 @@
 //! This file provides info extracted from the nerd fonts patcher script,
 //! specifying the scaling/positioning attributes of various glyphs.
 
-const Constraint = @import("face.zig").RenderOptions.Constraint;
+const Constraint = @import("Glyph.zig").RenderOptions.Constraint;
 
 /// Get the constraints for the provided codepoint.
 pub fn getConstraint(cp: u21) ?Constraint {
diff --git a/src/font/opentype/glyf.zig b/src/font/opentype/glyf.zig
index 194385345..912b5fe84 100644
--- a/src/font/opentype/glyf.zig
+++ b/src/font/opentype/glyf.zig
@@ -672,6 +672,9 @@ fn testAppendHeader(
 }
 
 test "glyf" {
+    // lib-vt source archives intentionally exclude full Ghostty font fixtures.
+    if (comptime @import("terminal_options").artifact == .lib) return error.SkipZigTest;
+
     const testing = std.testing;
     const alloc = testing.allocator;
     // Cozette because it doesn't have any hinting.
@@ -874,6 +877,9 @@ test "glyf: decode contour ending at max point index" {
 }
 
 test "glyf: reject glyphs with instructions and composite glyphs" {
+    // lib-vt source archives intentionally exclude full Ghostty font fixtures.
+    if (comptime @import("terminal_options").artifact == .lib) return error.SkipZigTest;
+
     const testing = std.testing;
     const alloc = testing.allocator;
     const test_font = @import("../embedded.zig").jetbrains_mono;
@@ -908,6 +914,9 @@ test "glyf: reject glyphs with instructions and composite glyphs" {
 }
 
 test "glyf: reject truncated" {
+    // lib-vt source archives intentionally exclude full Ghostty font fixtures.
+    if (comptime @import("terminal_options").artifact == .lib) return error.SkipZigTest;
+
     const testing = std.testing;
     const alloc = testing.allocator;
     // Cozette because it doesn't have any hinting.
@@ -926,6 +935,9 @@ test "glyf: reject truncated" {
 }
 
 test "glyf: reject endpoints out of order" {
+    // lib-vt source archives intentionally exclude full Ghostty font fixtures.
+    if (comptime @import("terminal_options").artifact == .lib) return error.SkipZigTest;
+
     const testing = std.testing;
     const alloc = testing.allocator;
     // Cozette because it doesn't have any hinting.
@@ -952,6 +964,9 @@ test "glyf: reject endpoints out of order" {
 }
 
 test "glyf: reject too many points" {
+    // lib-vt source archives intentionally exclude full Ghostty font fixtures.
+    if (comptime @import("terminal_options").artifact == .lib) return error.SkipZigTest;
+
     const testing = std.testing;
     const alloc = testing.allocator;
     // Cozette because it doesn't have any hinting.
diff --git a/src/font/opentype/sfnt.zig b/src/font/opentype/sfnt.zig
index 9373cda03..55a9a0bdf 100644
--- a/src/font/opentype/sfnt.zig
+++ b/src/font/opentype/sfnt.zig
@@ -285,6 +285,9 @@ pub const SFNT = struct {
 const native_endian = @import("builtin").target.cpu.arch.endian();
 
 test "parse font" {
+    // lib-vt source archives intentionally exclude full Ghostty font fixtures.
+    if (comptime @import("terminal_options").artifact == .lib) return error.SkipZigTest;
+
     const testing = std.testing;
     const alloc = testing.allocator;
 
@@ -298,6 +301,9 @@ test "parse font" {
 }
 
 test "get table" {
+    // lib-vt source archives intentionally exclude full Ghostty font fixtures.
+    if (comptime @import("terminal_options").artifact == .lib) return error.SkipZigTest;
+
     const testing = std.testing;
     const alloc = testing.allocator;
 
diff --git a/src/font/sprite/Face.zig b/src/font/sprite/Face.zig
index 596a92044..f5df2d421 100644
--- a/src/font/sprite/Face.zig
+++ b/src/font/sprite/Face.zig
@@ -176,7 +176,7 @@ pub fn renderGlyph(
     alloc: Allocator,
     atlas: *font.Atlas,
     cp: u32,
-    opts: font.face.RenderOptions,
+    opts: font.Glyph.RenderOptions,
 ) !font.Glyph {
     if (std.debug.runtime_safety) {
         if (!self.hasCodepoint(cp, null)) {
diff --git a/src/terminal/apc/glyph.zig b/src/terminal/apc/glyph.zig
index 67eb5163f..727e1ab4d 100644
--- a/src/terminal/apc/glyph.zig
+++ b/src/terminal/apc/glyph.zig
@@ -152,7 +152,13 @@ const std = @import("std");
 
 pub const request = @import("glyph/request.zig");
 pub const response = @import("glyph/response.zig");
+pub const execute = @import("glyph/execute.zig").execute;
 
 pub const CommandParser = request.CommandParser;
 pub const Request = request.Request;
 pub const Response = response.Response;
+pub const Glossary = @import("glyph/Glossary.zig");
+
+test {
+    std.testing.refAllDecls(@This());
+}
diff --git a/src/terminal/apc/glyph/AGENTS.md b/src/terminal/apc/glyph/AGENTS.md
new file mode 100644
index 000000000..c81029c0f
--- /dev/null
+++ b/src/terminal/apc/glyph/AGENTS.md
@@ -0,0 +1,9 @@
+# Glyph Protocol
+
+- The specification source of truth is:
+  <https://github.com/raphamorim/rio/blob/main/specs/glyph-protocol.md>
+- A summary of the specification is available in
+  `src/terminal/apc/glyph.zig` at the top.
+- Reference the specification whenever any changes are made to
+  this folder. Prefer the local specification over fetching the
+  latest, unless it is lacking information.
diff --git a/src/terminal/apc/glyph/Glossary.zig b/src/terminal/apc/glyph/Glossary.zig
new file mode 100644
index 000000000..28458dbb2
--- /dev/null
+++ b/src/terminal/apc/glyph/Glossary.zig
@@ -0,0 +1,444 @@
+/// Glossary is the per-terminal storage for Glyph Protocol
+/// codepoints. We use the word Glossary to match up with the spec which
+/// also uses this word.
+const Glossary = @This();
+
+const std = @import("std");
+const assert = std.debug.assert;
+const Allocator = std.mem.Allocator;
+const CircBuf = @import("../../../datastruct/circ_buf.zig").CircBuf;
+const FontGlyph = @import("../../../font/Glyph.zig");
+const Glyf = @import("../../../font/opentype/glyf.zig").Glyf;
+
+const request = @import("request.zig");
+const RegisterReq = request.Request.Register;
+
+const DesignMetrics = FontGlyph.DesignMetrics;
+const Constraint = FontGlyph.RenderOptions.Constraint;
+
+/// Maximum entries allowed in the glossary before eviction.
+/// Defined by the specification.
+pub const max_entries = 1024;
+
+/// An empty glossary with no registered glyphs.
+pub const empty: Glossary = .{ .entries = .empty };
+
+/// Errors that can occur while registering a glossary entry.
+pub const RegisterError = Allocator.Error || error{OutOfNamespace};
+
+/// Errors that can occur while clearing glossary entries.
+pub const ClearError = error{OutOfNamespace};
+
+/// The set of entries in the glossary keyed by the codepoint.
+///
+/// The array hash map preserves insertion order and has O(N)
+/// orderedRemove, so we use it as a FIFO too for eviction when
+/// the glossary is full. Since the specification limits the protocol
+/// to 1024 maximum entries, ordered removal should never be that
+/// expensive.
+///
+/// I'm also operating under the assumption that full glossaries
+/// for a session will be rare, so the eviction cost shouldn't
+/// happen regularly.
+entries: std.AutoArrayHashMapUnmanaged(u21, Entry),
+
+/// Release all glyph entries and hash map storage owned by the glossary.
+pub fn deinit(self: *Glossary, alloc: Allocator) void {
+    for (self.entries.values()) |*entry| entry.deinit(alloc);
+    self.entries.deinit(alloc);
+    self.* = undefined;
+}
+
+/// Register the given glyph entry.
+///
+/// This will act according to the glyph specification
+pub fn register(
+    self: *Glossary,
+    alloc: Allocator,
+    cp: u21,
+    entry: Entry,
+) RegisterError!void {
+    // Validate codepoint according to spec.
+    if (!isPrivateUse(cp)) return error.OutOfNamespace;
+
+    const gop = try self.entries.getOrPut(alloc, cp);
+    if (gop.found_existing) {
+        // Found an existing entry, we need to shift the FIFO so
+        // that this is now the most recent (at the end). This is
+        // O(N) but N is usually small and max N is bounded by the spec.
+        gop.value_ptr.*.deinit(alloc);
+        assert(self.entries.orderedRemove(cp));
+
+        // We already had enough capacity for this key before removing it, so
+        // reinserting the replacement cannot require another allocation.
+        self.entries.putAssumeCapacity(cp, entry);
+        return;
+    }
+
+    // Array hash maps preserve insertion order so always immediately insert.
+    gop.value_ptr.* = entry;
+
+    // Fast, typical path: we fit within the glossary, just return.
+    if (self.entries.count() <= max_entries) return;
+
+    // Slow path: we need to evict.
+    self.entries.values()[0].deinit(alloc);
+    self.entries.orderedRemoveAt(0);
+}
+
+/// Delete a single entry from the glossary. If the entry doesn't exist,
+/// then this does nothing and is safe.
+pub fn delete(
+    self: *Glossary,
+    alloc: Allocator,
+    cp: u21,
+) ClearError!void {
+    if (!isPrivateUse(cp)) return error.OutOfNamespace;
+    const kv = self.entries.fetchOrderedRemove(cp) orelse return;
+    var entry = kv.value;
+    entry.deinit(alloc);
+}
+
+/// Clear all entries from the glossary and free up any underlying
+/// storage.
+pub fn clearAndFree(self: *Glossary, alloc: Allocator) void {
+    for (self.entries.values()) |*entry| entry.deinit(alloc);
+    self.entries.deinit(alloc);
+    self.entries = .empty;
+}
+
+/// Contains returns true if the codepoint is covered by the glossary.
+pub fn contains(self: *Glossary, cp: u21) bool {
+    return self.entries.contains(cp);
+}
+
+/// A single glyph registration entry.
+pub const Entry = struct {
+    /// Stored glyph payload variants.
+    pub const Glyph = union(enum) {
+        glyf: Glyf.Outline,
+    };
+
+    /// The glyph itself. The tagged union only has glyf right now but
+    /// will eventually expand to support COLR and maybe other formats.
+    /// These are stored as raw outlines; rasterization is delayed to
+    /// renderers. The outlines have been validated.
+    glyph: Glyph,
+
+    /// Authored metrics for the glyph's design coordinate space.
+    design: DesignMetrics,
+
+    /// Unicode cell width requested by the registration.
+    width: request.Width,
+
+    /// Normalized scale, alignment, and padding behavior for rasterization.
+    constraint: Constraint,
+
+    /// Errors that can occur while constructing a glossary entry from a
+    /// register request.
+    pub const InitError = RegisterReq.DecodeError || error{
+        /// The register request is missing a required option or has an invalid
+        /// explicitly-provided option value.
+        InvalidOptions,
+
+        /// The requested payload format is not supported by this glossary.
+        UnsupportedFormat,
+    };
+
+    /// Initialize a glossary entry from a register request.
+    ///
+    /// This validates the request fields needed to construct the entry,
+    /// decodes the base64 glyph payload, and stores the decoded outline. The
+    /// returned entry owns decoded glyph memory and must be released with
+    /// `deinit`.
+    pub fn init(alloc: Allocator, req: RegisterReq) Entry.InitError!Entry {
+        // Validate format
+        const fmt = req.get(.fmt) orelse return error.InvalidOptions;
+        const design: DesignMetrics = .{
+            .units_per_em = req.get(.upm) orelse return error.InvalidOptions,
+            .advance_width = req.get(.aw) orelse return error.InvalidOptions,
+            .line_height = req.get(.lh) orelse return error.InvalidOptions,
+        };
+        if (design.units_per_em == 0 or
+            design.advance_width == 0 or
+            design.line_height == 0) return error.InvalidOptions;
+        const width = req.get(.width) orelse return error.InvalidOptions;
+
+        // Get our constraints
+        const constraint = try constraintFromRegister(req);
+
+        // Decode the payload into some usable glyph format for
+        // future rasterization.
+        const glyph: Glyph = switch (fmt) {
+            .glyf => .{ .glyf = try req.decodeGlyfPayload(alloc) },
+            .colrv0, .colrv1 => return error.UnsupportedFormat,
+        };
+
+        // No more errors, since we never do glyph cleanup above.
+        errdefer comptime unreachable;
+
+        return .{
+            .glyph = glyph,
+            .design = design,
+            .width = width,
+            .constraint = constraint,
+        };
+    }
+
+    /// Release memory owned by this entry.
+    pub fn deinit(self: *Entry, alloc: Allocator) void {
+        switch (self.glyph) {
+            .glyf => |*outline| outline.deinit(alloc),
+        }
+        self.* = undefined;
+    }
+
+    /// Return the renderer constraint for a register request.
+    ///
+    /// Glyph Protocol §8.5 defines sizing, alignment, and padding in terms of
+    /// the authored extent and render span. This function is the single
+    /// normalization point for how protocol sizing choices map to the
+    /// renderer-neutral constraint stored here.
+    fn constraintFromRegister(
+        req: RegisterReq,
+    ) error{InvalidOptions}!Constraint {
+        // Register.get applies the Glyph Protocol §6.1 defaults when options
+        // are omitted: size=height, align=center,center, and pad=0,0,0,0.
+        const size = req.get(.size) orelse return error.InvalidOptions;
+        const alignment = req.get(.@"align") orelse return error.InvalidOptions;
+        const pad = req.get(.pad) orelse return error.InvalidOptions;
+
+        return .{
+            .size = switch (size) {
+                // The rasterizer's base transform already maps the design em
+                // to the cell height. That is the closest existing behavior to
+                // the protocol's default height-driven mode.
+                .height => .none,
+                // There is no width-driven, aspect-preserving constraint mode
+                // today. Leave the base transform intact rather than forcing a
+                // fit/contain policy that would unexpectedly prevent overflow.
+                .advance => .none,
+                // Constraint.cover currently scales preserving aspect ratio to
+                // the available bounds, which is the best existing match for
+                // the protocol's contain mode.
+                .contain => .cover,
+                // There is no true protocol-cover equivalent that chooses the
+                // larger axis scale, so use the nearest named renderer policy.
+                .cover => .cover,
+                .stretch => .stretch,
+            },
+            .align_horizontal = switch (alignment.horizontal) {
+                .start => .start,
+                .center => .center,
+                .end => .end,
+            },
+            .align_vertical = switch (alignment.vertical) {
+                .start => .start,
+                .center => .center,
+                .end => .end,
+                // The current constraint API has no baseline alignment mode.
+                // Start is the closest stable default because the glyf
+                // rasterizer's coordinate model already treats y=0 as the
+                // baseline/bottom before constraints are applied.
+                .baseline => .start,
+            },
+            .pad_top = pad.top,
+            .pad_right = pad.right,
+            .pad_bottom = pad.bottom,
+            .pad_left = pad.left,
+        };
+    }
+};
+
+/// Return true if `cp` is in one of the Unicode Private Use Areas.
+fn isPrivateUse(cp: u21) bool {
+    return (cp >= 0xE000 and cp <= 0xF8FF) or
+        (cp >= 0xF0000 and cp <= 0xFFFFD) or
+        (cp >= 0x100000 and cp <= 0x10FFFD);
+}
+
+fn testParseRegister(alloc: Allocator, data: []const u8) !RegisterReq {
+    const raw = try alloc.dupe(u8, data);
+    errdefer alloc.free(raw);
+
+    const req = try request.Request.parse(alloc, raw);
+    switch (req) {
+        .register => |reg| return reg,
+        else => unreachable,
+    }
+}
+
+// Base64-encoded glyf payload from the "glyf: decode triangle" test in
+// font/opentype/glyf.zig. This is a real simple-glyph record with one contour
+// and three on-curve points.
+const test_triangle_glyf_payload = "AAEAZABkA4QDhAACAAABAQEB9P5wAyADhPzgAAA=";
+
+fn testRegisterReq(alloc: Allocator, cp: u21) !RegisterReq {
+    const data = try std.fmt.allocPrint(
+        alloc,
+        "r;cp={x};upm=2048;aw=1024;lh=1536;width=2;size=stretch;align=end,start;pad=0.1,0.2,0.3,0.4;{s}",
+        .{ cp, test_triangle_glyf_payload },
+    );
+    errdefer alloc.free(data);
+
+    const req = try request.Request.parse(alloc, data);
+    switch (req) {
+        .register => |reg| return reg,
+        else => unreachable,
+    }
+}
+
+fn testRegisterEntry(alloc: Allocator, cp: u21) !Entry {
+    const req = try testRegisterReq(alloc, cp);
+    defer alloc.free(req.raw);
+    return try Entry.init(alloc, req);
+}
+
+test "Entry init decodes glyf payload and applies register fields" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const req = try testRegisterReq(alloc, 0xE000);
+    defer alloc.free(req.raw);
+
+    var entry = try Entry.init(alloc, req);
+    defer entry.deinit(alloc);
+
+    try testing.expectEqual(@as(u32, 2048), entry.design.units_per_em);
+    try testing.expectEqual(@as(u32, 1024), entry.design.advance_width);
+    try testing.expectEqual(@as(u32, 1536), entry.design.line_height);
+    try testing.expectEqual(request.Width.wide, entry.width);
+    try testing.expectEqual(Constraint.Size.stretch, entry.constraint.size);
+    try testing.expectEqual(Constraint.Align.end, entry.constraint.align_horizontal);
+    try testing.expectEqual(Constraint.Align.start, entry.constraint.align_vertical);
+    try testing.expectEqual(@as(f64, 0.1), entry.constraint.pad_top);
+    try testing.expectEqual(@as(f64, 0.2), entry.constraint.pad_right);
+    try testing.expectEqual(@as(f64, 0.3), entry.constraint.pad_bottom);
+    try testing.expectEqual(@as(f64, 0.4), entry.constraint.pad_left);
+
+    try testing.expectEqual(@as(usize, 3), entry.glyph.glyf.points.len);
+    try testing.expectEqual(@as(usize, 1), entry.glyph.glyf.contours.len);
+}
+
+test "Entry init rejects invalid register payload" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const req = try testParseRegister(alloc, "r;cp=e000;%%%not-base64%%%");
+    defer alloc.free(req.raw);
+
+    try testing.expectError(error.MalformedPayload, Entry.init(alloc, req));
+}
+
+test "Glossary register overwrites and moves entry to newest position" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    try glossary.register(alloc, 0xE000, try testRegisterEntry(alloc, 0xE000));
+    try glossary.register(alloc, 0xE001, try testRegisterEntry(alloc, 0xE001));
+    try glossary.register(alloc, 0xE000, try testRegisterEntry(alloc, 0xE000));
+
+    try testing.expectEqual(@as(usize, 2), glossary.entries.count());
+    try testing.expectEqual(@as(u21, 0xE001), glossary.entries.keys()[0]);
+    try testing.expectEqual(@as(u21, 0xE000), glossary.entries.keys()[1]);
+}
+
+test "Glossary register evicts oldest entry" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    for (0..max_entries + 1) |i| {
+        const cp: u21 = @intCast(0xE000 + i);
+        try glossary.register(alloc, cp, try testRegisterEntry(alloc, cp));
+    }
+
+    try testing.expectEqual(@as(usize, max_entries), glossary.entries.count());
+    try testing.expect(!glossary.entries.contains(0xE000));
+    try testing.expect(glossary.entries.contains(0xE001));
+    try testing.expect(glossary.entries.contains(0xE000 + max_entries));
+}
+
+test "Glossary register rejects non-PUA codepoint" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    var entry = try testRegisterEntry(alloc, 0xE000);
+    defer entry.deinit(alloc);
+    try testing.expectError(error.OutOfNamespace, glossary.register(alloc, 'A', entry));
+}
+
+test "Glossary delete removes one PUA slot and ignores empty PUA slot" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    try glossary.register(alloc, 0xE000, try testRegisterEntry(alloc, 0xE000));
+    try glossary.register(alloc, 0xE001, try testRegisterEntry(alloc, 0xE001));
+
+    try glossary.delete(alloc, 0xE000);
+    try testing.expectEqual(@as(usize, 1), glossary.entries.count());
+    try testing.expect(!glossary.contains(0xE000));
+    try testing.expect(glossary.contains(0xE001));
+
+    try glossary.delete(alloc, 0xE000);
+    try testing.expectEqual(@as(usize, 1), glossary.entries.count());
+    try testing.expect(glossary.contains(0xE001));
+}
+
+test "Glossary delete rejects non-PUA codepoint" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    try glossary.register(alloc, 0xE000, try testRegisterEntry(alloc, 0xE000));
+    try testing.expectError(error.OutOfNamespace, glossary.delete(alloc, 'A'));
+    try testing.expectEqual(@as(usize, 1), glossary.entries.count());
+    try testing.expect(glossary.contains(0xE000));
+}
+
+test "Glossary clearAndFree removes all slots and remains reusable" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    try glossary.register(alloc, 0xE000, try testRegisterEntry(alloc, 0xE000));
+    try glossary.register(alloc, 0xE001, try testRegisterEntry(alloc, 0xE001));
+
+    glossary.clearAndFree(alloc);
+    try testing.expectEqual(@as(usize, 0), glossary.entries.count());
+    try testing.expect(!glossary.contains(0xE000));
+    try testing.expect(!glossary.contains(0xE001));
+
+    try glossary.register(alloc, 0xE002, try testRegisterEntry(alloc, 0xE002));
+    try testing.expectEqual(@as(usize, 1), glossary.entries.count());
+    try testing.expect(glossary.contains(0xE002));
+}
+
+test "Glossary contains reports registered slots" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    try testing.expect(!glossary.contains(0xE000));
+
+    try glossary.register(alloc, 0xE000, try testRegisterEntry(alloc, 0xE000));
+    try testing.expect(glossary.contains(0xE000));
+    try testing.expect(!glossary.contains(0xE001));
+}
diff --git a/src/terminal/apc/glyph/execute.zig b/src/terminal/apc/glyph/execute.zig
new file mode 100644
index 000000000..059029a62
--- /dev/null
+++ b/src/terminal/apc/glyph/execute.zig
@@ -0,0 +1,367 @@
+const std = @import("std");
+const Allocator = std.mem.Allocator;
+
+const request = @import("request.zig");
+const response = @import("response.zig");
+const Glossary = @import("Glossary.zig");
+const Request = request.Request;
+const Response = response.Response;
+
+const log = std.log.scoped(.glyph);
+
+/// Payload formats we support. Hardcoded because the support is
+/// fixed.
+pub const supported_formats: response.Response.Support.Formats = .{
+    .glyf = true,
+};
+
+/// Execute a Glyph protocol request against the given state.
+///
+/// This will never fail, but the response may indiciate an error and
+/// the terminal state may not be updated to reflect the command. This will
+/// never put the terminal in a corrupt or non-recoverable state.
+///
+/// For example, allocation errors can happen, but they're wrapped up in
+/// an out of memory response.
+///
+/// Query responses only report glossary coverage. Callers that can determine
+/// system font coverage must update the returned query response before sending
+/// it to the client.
+pub fn execute(
+    alloc: Allocator,
+    glossary: *Glossary,
+    req: *const Request,
+) ?Response {
+    log.debug("executing glyph protocol request: {t}", .{req.*});
+    return switch (req.*) {
+        .support => .{ .support = .{ .fmt = supported_formats } },
+        .query => |qry| query(glossary, qry),
+        .register => |reg| register(alloc, glossary, reg),
+        .clear => |clr| clear(alloc, glossary, clr),
+    };
+}
+
+fn query(
+    glossary: *Glossary,
+    qry: Request.Query,
+) ?Response {
+    const cp = qry.get(.cp) orelse return null;
+    return .{ .query = .{
+        .cp = cp,
+        .status = .{
+            .glossary = glossary.contains(cp),
+        },
+    } };
+}
+
+fn register(
+    alloc: Allocator,
+    glossary: *Glossary,
+    reg: Request.Register,
+) ?Response {
+    const reply = reg.get(.reply) orelse .all;
+    const cp = registerFallible(alloc, glossary, reg) catch |err| return switch (reply) {
+        .none => null,
+        .all, .failures => .{ .register = .{
+            .cp = reg.get(.cp) orelse 0,
+            .status = .err,
+            .reason = switch (err) {
+                error.OutOfMemory => .{ .other = "out_of_memory" },
+                error.OutOfNamespace => .out_of_namespace,
+                error.PayloadTooLarge => .payload_too_large,
+                error.MalformedPayload => .malformed_payload,
+                error.CompositeUnsupported => .composite_unsupported,
+                error.HintingUnsupported => .hinting_unsupported,
+                error.InvalidOptions,
+                error.UnsupportedFormat,
+                => .malformed_payload,
+            },
+        } },
+    };
+
+    return switch (reply) {
+        .none, .failures => null,
+        .all => .{ .register = .{ .cp = cp } },
+    };
+}
+
+fn registerFallible(
+    alloc: Allocator,
+    glossary: *Glossary,
+    reg: Request.Register,
+) (Glossary.Entry.InitError || Glossary.RegisterError)!u21 {
+    const cp = reg.get(.cp) orelse
+        return error.MalformedPayload;
+
+    var entry = try Glossary.Entry.init(alloc, reg);
+    errdefer entry.deinit(alloc);
+
+    try glossary.register(alloc, cp, entry);
+    return cp;
+}
+
+fn clear(
+    alloc: Allocator,
+    glossary: *Glossary,
+    clr: Request.Clear,
+) ?Response {
+    if (clr.get(.cp)) |cp| {
+        glossary.delete(alloc, cp) catch |err| return .{ .clear = .{
+            .status = .err,
+            .reason = switch (err) {
+                error.OutOfNamespace => "out_of_namespace",
+            },
+        } };
+    } else if (clr.has(.cp)) {
+        return .{ .clear = .{
+            .status = .err,
+            .reason = "malformed_payload",
+        } };
+    } else {
+        glossary.clearAndFree(alloc);
+    }
+
+    return .{ .clear = .{} };
+}
+
+fn testParse(alloc: Allocator, data: []const u8) !Request {
+    var parser = request.CommandParser.init(alloc, 1024 * 1024);
+    defer parser.deinit();
+    for (data) |byte| try parser.feed(byte);
+    return try parser.complete(alloc);
+}
+
+fn testExecute(alloc: Allocator, glossary: *Glossary, req: *const Request) ?Response {
+    return execute(alloc, glossary, req);
+}
+
+test "execute register stores glyph and returns success" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    var req = try testParse(alloc, "r;cp=e0a0;AAAAAAAAAAAAAA==");
+    defer req.deinit(alloc);
+
+    try testing.expectEqual(Response{
+        .register = .{ .cp = 0xE0A0 },
+    }, testExecute(alloc, &glossary, &req).?);
+    try testing.expect(glossary.contains(0xE0A0));
+}
+
+test "execute register reply failures suppresses success" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    var req = try testParse(alloc, "r;cp=e0a0;reply=2;AAAAAAAAAAAAAA==");
+    defer req.deinit(alloc);
+
+    try testing.expect(testExecute(alloc, &glossary, &req) == null);
+    try testing.expect(glossary.contains(0xE0A0));
+}
+
+test "execute register reply none suppresses failure" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    var req = try testParse(alloc, "r;cp=41;reply=0;%%%not-base64%%%");
+    defer req.deinit(alloc);
+
+    try testing.expect(testExecute(alloc, &glossary, &req) == null);
+    try testing.expect(!glossary.contains('A'));
+}
+
+test "execute register rejects non-PUA" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    var req = try testParse(alloc, "r;cp=41;AAAAAAAAAAAAAA==");
+    defer req.deinit(alloc);
+
+    try testing.expectEqual(Response{
+        .register = .{
+            .cp = 'A',
+            .status = .err,
+            .reason = .out_of_namespace,
+        },
+    }, testExecute(alloc, &glossary, &req).?);
+    try testing.expect(!glossary.contains('A'));
+}
+
+test "execute register reports malformed payload" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    var req = try testParse(alloc, "r;cp=e0a0;%%%not-base64%%%");
+    defer req.deinit(alloc);
+
+    try testing.expectEqual(Response{
+        .register = .{
+            .cp = 0xE0A0,
+            .status = .err,
+            .reason = .malformed_payload,
+        },
+    }, testExecute(alloc, &glossary, &req).?);
+    try testing.expect(!glossary.contains(0xE0A0));
+}
+
+test "execute clear removes all glyphs" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    var reg1 = try testParse(alloc, "r;cp=e0a0;AAAAAAAAAAAAAA==");
+    defer reg1.deinit(alloc);
+    _ = testExecute(alloc, &glossary, &reg1);
+
+    var reg2 = try testParse(alloc, "r;cp=e0a1;AAAAAAAAAAAAAA==");
+    defer reg2.deinit(alloc);
+    _ = testExecute(alloc, &glossary, &reg2);
+
+    var req = try testParse(alloc, "c");
+    defer req.deinit(alloc);
+
+    try testing.expectEqual(Response{ .clear = .{} }, testExecute(alloc, &glossary, &req).?);
+    try testing.expect(!glossary.contains(0xE0A0));
+    try testing.expect(!glossary.contains(0xE0A1));
+}
+
+test "execute clear removes one glyph" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    var reg1 = try testParse(alloc, "r;cp=e0a0;AAAAAAAAAAAAAA==");
+    defer reg1.deinit(alloc);
+    _ = testExecute(alloc, &glossary, &reg1);
+
+    var reg2 = try testParse(alloc, "r;cp=e0a1;AAAAAAAAAAAAAA==");
+    defer reg2.deinit(alloc);
+    _ = testExecute(alloc, &glossary, &reg2);
+
+    var req = try testParse(alloc, "c;cp=e0a0");
+    defer req.deinit(alloc);
+
+    try testing.expectEqual(Response{ .clear = .{} }, testExecute(alloc, &glossary, &req).?);
+    try testing.expect(!glossary.contains(0xE0A0));
+    try testing.expect(glossary.contains(0xE0A1));
+}
+
+test "execute clear rejects non-PUA" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    var req = try testParse(alloc, "c;cp=41");
+    defer req.deinit(alloc);
+
+    try testing.expectEqual(Response{
+        .clear = .{
+            .status = .err,
+            .reason = "out_of_namespace",
+        },
+    }, testExecute(alloc, &glossary, &req).?);
+}
+
+test "execute clear rejects malformed cp without clearing glossary" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    var reg1 = try testParse(alloc, "r;cp=e0a0;AAAAAAAAAAAAAA==");
+    defer reg1.deinit(alloc);
+    _ = testExecute(alloc, &glossary, &reg1);
+
+    var reg2 = try testParse(alloc, "r;cp=e0a1;AAAAAAAAAAAAAA==");
+    defer reg2.deinit(alloc);
+    _ = testExecute(alloc, &glossary, &reg2);
+
+    for ([_][]const u8{ "c;cp=zz", "c;cp=", "c;cp=200000" }) |data| {
+        var req = try testParse(alloc, data);
+        defer req.deinit(alloc);
+
+        try testing.expectEqual(Response{
+            .clear = .{
+                .status = .err,
+                .reason = "malformed_payload",
+            },
+        }, testExecute(alloc, &glossary, &req).?);
+        try testing.expect(glossary.contains(0xE0A0));
+        try testing.expect(glossary.contains(0xE0A1));
+    }
+}
+
+test "execute query reports no coverage" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    var req = try testParse(alloc, "q;cp=e0a0");
+    defer req.deinit(alloc);
+
+    try testing.expectEqual(Response{
+        .query = .{
+            .cp = 0xE0A0,
+            .status = .{},
+        },
+    }, testExecute(alloc, &glossary, &req).?);
+}
+
+test "execute query reports glossary coverage" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    var reg = try testParse(alloc, "r;cp=e0a0;AAAAAAAAAAAAAA==");
+    defer reg.deinit(alloc);
+    _ = testExecute(alloc, &glossary, &reg);
+
+    var req = try testParse(alloc, "q;cp=e0a0");
+    defer req.deinit(alloc);
+
+    try testing.expectEqual(Response{
+        .query = .{
+            .cp = 0xE0A0,
+            .status = .{ .glossary = true },
+        },
+    }, testExecute(alloc, &glossary, &req).?);
+}
+
+test "execute query without cp returns no response" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var glossary: Glossary = .empty;
+    defer glossary.deinit(alloc);
+
+    var req = try testParse(alloc, "q;foo=bar");
+    defer req.deinit(alloc);
+
+    try testing.expect(testExecute(alloc, &glossary, &req) == null);
+}
diff --git a/src/terminal/apc/glyph/request.zig b/src/terminal/apc/glyph/request.zig
index 4c50525fc..b33a28413 100644
--- a/src/terminal/apc/glyph/request.zig
+++ b/src/terminal/apc/glyph/request.zig
@@ -1,6 +1,11 @@
 const std = @import("std");
 const Allocator = std.mem.Allocator;
 const assert = std.debug.assert;
+const Glyf = @import("../../../font/opentype/glyf.zig").Glyf;
+
+/// Maximum decoded glyph payload size accepted by the protocol.
+/// This is documented in the spec.
+const max_payload_size = 64 * 1024; // 64 KiB
 
 /// Stateful parser for a single glyph APC payload after the `25a1;` prefix.
 pub const CommandParser = struct {
@@ -114,12 +119,12 @@ pub const Request = union(enum) {
         payload_idx: usize,
 
         /// Initialize a register command from owned raw command bytes.
-        pub fn init(raw: []const u8) Register {
-            assert(raw.len >= 2);
-            assert(raw[0] == 'r');
-            assert(raw[1] == ';');
-            const payload_idx = std.mem.lastIndexOfScalar(u8, raw, ';').?;
-            assert(payload_idx > 1);
+        pub fn init(raw: []const u8) ?Register {
+            if (raw.len < 2) return null;
+            if (raw[0] != 'r') return null;
+            if (raw[1] != ';') return null;
+            const payload_idx = std.mem.lastIndexOfScalar(u8, raw, ';') orelse return null;
+            if (payload_idx <= 1) return null;
 
             return .{
                 .raw = raw,
@@ -242,6 +247,65 @@ pub const Request = union(enum) {
                 self.raw[self.payload_idx + 1 ..];
         }
 
+        /// Errors that can occur while decoding a register glyph payload.
+        pub const DecodeError = Allocator.Error || error{
+            /// The decoded payload exceeds the protocol limit.
+            PayloadTooLarge,
+
+            /// The payload could not be decoded or parsed as the declared format.
+            MalformedPayload,
+
+            /// The glyf payload is composite, which the protocol forbids.
+            CompositeUnsupported,
+
+            /// The glyf payload contains hinting instructions, which the
+            /// protocol forbids.
+            HintingUnsupported,
+        };
+
+        /// Decode this request's base64 glyf payload into an owned outline.
+        pub fn decodeGlyfPayload(self: Register, alloc: Allocator) DecodeError!Glyf.Outline {
+            // Prep base64 decoding, initial validation.
+            const Decoder = std.base64.standard.Decoder;
+            const payload_bytes = self.payload();
+            const size = Decoder.calcSizeForSlice(payload_bytes) catch
+                return error.MalformedPayload;
+            if (size > max_payload_size) return error.PayloadTooLarge;
+
+            // Max payload size is reasonable for stack and its likely
+            // we'll have stack space. We don't use much stack space in
+            // the future function calls either, so try a stack allocator
+            // here and fallback to heap as necessary.
+            var data_stack = std.heap.stackFallback(
+                max_payload_size,
+                alloc,
+            );
+            const data_alloc = data_stack.get();
+            const data = try data_alloc.alloc(u8, size);
+            defer data_alloc.free(data);
+
+            // Base64 decode
+            Decoder.decode(data, payload_bytes) catch
+                return error.MalformedPayload;
+
+            // Glyf.Entry borrows from `data`, but only for the duration of the
+            // decode call below. Glyf.Entry.decode returns an owned Outline, so
+            // it is safe to free `data` before returning that outline.
+            const glyf_entry = Glyf.Entry.init(data) catch return error.MalformedPayload;
+            return glyf_entry.decode(alloc) catch |err| switch (err) {
+                error.OutOfMemory => error.OutOfMemory,
+                // Unsupported fields
+                error.CompositeNotSupported => error.CompositeUnsupported,
+                error.InstructionsNotSupported => error.HintingUnsupported,
+                // Various semantic issues
+                error.EndOfStream,
+                error.EndPointsOutOfOrder,
+                error.TooManyPoints,
+                error.CoordinateOverflow,
+                => error.MalformedPayload,
+            };
+        }
+
         /// Return the raw option portion of a valid register command.
         fn rawOptions(self: Register) []const u8 {
             assert(self.raw.len >= 2);
@@ -286,6 +350,12 @@ pub const Request = union(enum) {
                     .cp => std.fmt.parseInt(u21, value, 16) catch null,
                 };
             }
+
+            /// Return whether the option is present in the raw option string,
+            /// independent of whether its value can be decoded.
+            pub fn present(comptime self: Option, raw: []const u8) bool {
+                return optionValue(raw, self.key()) != null;
+            }
         };
 
         /// Lazily decode a clear option on demand.
@@ -293,6 +363,11 @@ pub const Request = union(enum) {
             return option.read(self.rawOptions());
         }
 
+        /// Return whether a clear option was provided, even if malformed.
+        pub fn has(self: Clear, comptime option: Option) bool {
+            return option.present(self.rawOptions());
+        }
+
         /// Return the raw option portion of a valid clear command.
         fn rawOptions(self: Clear) []const u8 {
             assert(self.raw.len >= 2);
@@ -320,7 +395,7 @@ pub const Request = union(enum) {
                 return .support;
             },
             'q' => .{ .query = .init(raw) },
-            'r' => .{ .register = .init(raw) },
+            'r' => .{ .register = Register.init(raw) orelse return error.InvalidFormat },
             'c' => .{ .clear = .init(raw) },
             else => error.InvalidFormat,
         };
@@ -690,6 +765,39 @@ test "register command with invalid payload" {
     try testing.expectEqualStrings("%%%not-base64%%%", cmd.register.payload());
 }
 
+test "register command rejects missing payload separator" {
+    const testing = std.testing;
+
+    for ([_][]const u8{ "r", "r;cp=e0a0", "r;foo" }) |data| {
+        try testing.expectError(
+            error.InvalidFormat,
+            testParse(testing.allocator, data),
+        );
+    }
+}
+
+test "register decodes glyf payload" {
+    const testing = std.testing;
+
+    var cmd = try testParse(testing.allocator, "r;cp=e0a0;AAAAAAAAAAAAAA==");
+    defer cmd.deinit(testing.allocator);
+
+    var outline = try cmd.register.decodeGlyfPayload(testing.allocator);
+    defer outline.deinit(testing.allocator);
+
+    try testing.expectEqual(@as(usize, 0), outline.points.len);
+    try testing.expectEqual(@as(usize, 0), outline.contours.len);
+}
+
+test "register rejects malformed glyf payload" {
+    const testing = std.testing;
+
+    var cmd = try testParse(testing.allocator, "r;cp=e0a0;%%%not-base64%%%");
+    defer cmd.deinit(testing.allocator);
+
+    try testing.expectError(error.MalformedPayload, cmd.register.decodeGlyfPayload(testing.allocator));
+}
+
 test "register response without payload" {
     const testing = std.testing;
 
@@ -713,9 +821,23 @@ test "clear command" {
     defer cmd.deinit(testing.allocator);
 
     try testing.expect(cmd == .clear);
+    try testing.expect(cmd.clear.has(.cp));
     try testing.expectEqual(@as(u21, 0xE0A0), cmd.clear.get(.cp).?);
 }
 
+test "clear command tracks malformed cp presence" {
+    const testing = std.testing;
+
+    for ([_][]const u8{ "c;cp=zz", "c;cp=", "c;cp=200000" }) |data| {
+        var cmd = try testParse(testing.allocator, data);
+        defer cmd.deinit(testing.allocator);
+
+        try testing.expect(cmd == .clear);
+        try testing.expect(cmd.clear.has(.cp));
+        try testing.expect(cmd.clear.get(.cp) == null);
+    }
+}
+
 test "invalid command" {
     const testing = std.testing;
 
diff --git a/src/terminal/apc/glyph/response.zig b/src/terminal/apc/glyph/response.zig
index 4ed52b0b2..5b72495d7 100644
--- a/src/terminal/apc/glyph/response.zig
+++ b/src/terminal/apc/glyph/response.zig
@@ -303,11 +303,11 @@ test "register reason names" {
     const testing = std.testing;
     const Reason = Response.Register.Reason;
 
-    try testing.expectEqualStrings("out_of_namespace", Reason.out_of_namespace.name());
-    try testing.expectEqualStrings("composite_unsupported", Reason.composite_unsupported.name());
-    try testing.expectEqualStrings("hinting_unsupported", Reason.hinting_unsupported.name());
-    try testing.expectEqualStrings("malformed_payload", Reason.malformed_payload.name());
-    try testing.expectEqualStrings("payload_too_large", Reason.payload_too_large.name());
+    try testing.expectEqualStrings("out_of_namespace", (Reason{ .out_of_namespace = {} }).name());
+    try testing.expectEqualStrings("composite_unsupported", (Reason{ .composite_unsupported = {} }).name());
+    try testing.expectEqualStrings("hinting_unsupported", (Reason{ .hinting_unsupported = {} }).name());
+    try testing.expectEqualStrings("malformed_payload", (Reason{ .malformed_payload = {} }).name());
+    try testing.expectEqualStrings("payload_too_large", (Reason{ .payload_too_large = {} }).name());
     try testing.expectEqualStrings("future_reason", (Reason{ .other = "future_reason" }).name());
 }