diff --git a/crates/haku-wasm/src/lib.rs b/crates/haku-wasm/src/lib.rs
index 7417839..0e21b02 100644
--- a/crates/haku-wasm/src/lib.rs
+++ b/crates/haku-wasm/src/lib.rs
@@ -2,7 +2,7 @@
 
 extern crate alloc;
 
-use core::{alloc::Layout, mem, slice};
+use core::{alloc::Layout, mem, ptr, slice};
 
 use alloc::{boxed::Box, string::String, vec::Vec};
 use haku::{
@@ -12,7 +12,7 @@ use haku::{
     diagnostic::Diagnostic,
     lexer::{lex, Lexer},
     parser::{self, IntoAstError, Parser},
-    source::SourceCode,
+    source::{SourceCode, Span},
     token::Lexis,
 };
 use log::{debug, info};
@@ -449,6 +449,22 @@ unsafe extern "C" fn haku_compile_brush(
     );
     debug!("compiling: {closure_spec:?}");
 
+    debug!("bytecode: {:?}", chunk.bytecode);
+    {
+        let mut cursor = 0_usize;
+        for info in &chunk.span_info {
+            let slice = &chunk.bytecode[cursor..cursor + info.len as usize];
+            debug!(
+                "{:?} | 0x{:x} {:?} | {:?}",
+                info.span,
+                cursor,
+                slice,
+                info.span.slice(src.code),
+            );
+            cursor += info.len as usize;
+        }
+    }
+
     instance.compile_result2 = Some(CompileResult {
         defs_string: instance.defs.serialize_defs(),
         tags_string: instance.defs.serialize_tags(),
@@ -528,6 +544,25 @@ unsafe extern "C" fn haku_bytecode(instance: *const Instance) -> *const u8 {
     unwrap_compile_result(instance).chunk.bytecode.as_ptr()
 }
 
+#[unsafe(no_mangle)]
+unsafe extern "C" fn haku_bytecode_find_span(instance: *const Instance, pc: u16) -> *const Span {
+    let chunk = &unwrap_compile_result(instance).chunk;
+    match chunk.find_span(pc) {
+        Some(r) => r,
+        None => ptr::null(),
+    }
+}
+
+#[unsafe(no_mangle)]
+unsafe extern "C" fn haku_span_start(span: *const Span) -> u32 {
+    (*span).start
+}
+
+#[unsafe(no_mangle)]
+unsafe extern "C" fn haku_span_end(span: *const Span) -> u32 {
+    (*span).end
+}
+
 #[unsafe(no_mangle)]
 unsafe extern "C" fn haku_local_count(instance: *const Instance) -> u8 {
     unwrap_compile_result(instance).closure_spec.local_count
diff --git a/crates/haku/src/bytecode.rs b/crates/haku/src/bytecode.rs
index 60a6a35..026c1b3 100644
--- a/crates/haku/src/bytecode.rs
+++ b/crates/haku/src/bytecode.rs
@@ -1,10 +1,9 @@
-use core::{
-    fmt::{self, Display},
-    mem::transmute,
-};
+use core::fmt::{self, Display};
 
 use alloc::{borrow::ToOwned, string::String, vec::Vec};
 
+use crate::source::Span;
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 #[repr(u8)]
 pub enum Opcode {
@@ -56,6 +55,14 @@ pub const CAPTURE_CAPTURE: u8 = 1;
 #[derive(Debug, Clone)]
 pub struct Chunk {
     pub bytecode: Vec<u8>,
+    pub span_info: Vec<SpanRun>,
+    pub current_span: Span,
+}
+
+#[derive(Debug, Clone)]
+pub struct SpanRun {
+    pub span: Span,
+    pub len: u16,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -66,6 +73,8 @@ impl Chunk {
         if capacity <= (1 << 16) {
             Ok(Chunk {
                 bytecode: Vec::with_capacity(capacity),
+                span_info: Vec::new(),
+                current_span: Span::new(0, 0),
             })
         } else {
             Err(ChunkSizeError)
@@ -76,6 +85,16 @@ impl Chunk {
         Offset(self.bytecode.len() as u16)
     }
 
+    fn push_span_info(&mut self, span: Span, len: u16) {
+        if let Some(info) = self.span_info.last_mut() {
+            if info.span == span {
+                info.len += len;
+                return;
+            }
+        }
+        self.span_info.push(SpanRun { span, len });
+    }
+
     pub fn emit_bytes(&mut self, bytes: &[u8]) -> Result<Offset, EmitError> {
         if self.bytecode.len() + bytes.len() > self.bytecode.capacity() {
             return Err(EmitError);
@@ -83,6 +102,7 @@ impl Chunk {
 
         let offset = Offset(self.bytecode.len() as u16);
         self.bytecode.extend_from_slice(bytes);
+        self.push_span_info(self.current_span, bytes.len() as u16);
 
         Ok(offset)
     }
@@ -122,40 +142,15 @@ impl Chunk {
         self.patch_u16(offset, x.0);
     }
 
-    // NOTE: I'm aware these aren't the fastest implementations since they validate quite a lot
-    // during runtime, but this is just an MVP. It doesn't have to be blazingly fast.
-
-    pub fn read_u8(&self, pc: &mut usize) -> Result<u8, ReadError> {
-        let x = self.bytecode.get(*pc).copied();
-        *pc += 1;
-        x.ok_or(ReadError)
-    }
-
-    pub fn read_u16(&self, pc: &mut usize) -> Result<u16, ReadError> {
-        let xs = &self.bytecode[*pc..*pc + 2];
-        *pc += 2;
-        Ok(u16::from_le_bytes(xs.try_into().map_err(|_| ReadError)?))
-    }
-
-    pub fn read_u32(&self, pc: &mut usize) -> Result<u32, ReadError> {
-        let xs = &self.bytecode[*pc..*pc + 4];
-        *pc += 4;
-        Ok(u32::from_le_bytes(xs.try_into().map_err(|_| ReadError)?))
-    }
-
-    pub fn read_f32(&self, pc: &mut usize) -> Result<f32, ReadError> {
-        let xs = &self.bytecode[*pc..*pc + 4];
-        *pc += 4;
-        Ok(f32::from_le_bytes(xs.try_into().map_err(|_| ReadError)?))
-    }
-
-    pub fn read_opcode(&self, pc: &mut usize) -> Result<Opcode, ReadError> {
-        let x = self.read_u8(pc)?;
-        if x <= Opcode::Return as u8 {
-            Ok(unsafe { transmute::<u8, Opcode>(x) })
-        } else {
-            Err(ReadError)
+    pub fn find_span(&self, pc: u16) -> Option<&Span> {
+        let mut cur = 0;
+        for info in &self.span_info {
+            if pc >= cur && pc < cur + info.len {
+                return Some(&info.span);
+            }
+            cur += info.len;
         }
+        None
     }
 }
 
@@ -183,15 +178,6 @@ impl Display for EmitError {
     }
 }
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct ReadError;
-
-impl Display for ReadError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "invalid bytecode: out of bounds read or invalid opcode")
-    }
-}
-
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
 pub struct DefId(u16);
 
diff --git a/crates/haku/src/compiler.rs b/crates/haku/src/compiler.rs
index 7b69e96..a095dcb 100644
--- a/crates/haku/src/compiler.rs
+++ b/crates/haku/src/compiler.rs
@@ -1,9 +1,14 @@
 use core::{
     error::Error,
     fmt::{self, Display},
+    mem::take,
 };
 
-use alloc::vec::Vec;
+use alloc::{
+    format,
+    string::{String, ToString},
+    vec::Vec,
+};
 
 use crate::{
     ast::{Ast, NodeId, NodeKind},
@@ -42,6 +47,11 @@ pub struct Compiler<'a> {
     pub chunk: &'a mut Chunk,
     pub diagnostics: Vec<Diagnostic>,
     scopes: Vec<Scope<'a>>,
+    // NOTE: This mechanism is kind of non-obvious at first, but keep in mind this is the name of
+    // any functions emitted within the current scope---not of the current function.
+    // The convention is to set this to the current def's name, and any functions inside the current
+    // function get λ as a suffix. The more nesting, the more lambdas---e.g. myFunctionλλ.
+    function_name: String,
 }
 
 #[derive(Debug, Clone, Copy)]
@@ -60,6 +70,8 @@ impl<'a> Compiler<'a> {
                 captures: Vec::new(),
                 let_count: 0,
             }]),
+            // For unnamed functions within the brush's toplevel.
+            function_name: "(brush)λ".into(),
         }
     }
 
@@ -86,7 +98,10 @@ impl<'a> Compiler<'a> {
 type CompileResult<T = ()> = Result<T, CompileError>;
 
 pub fn compile_expr<'a>(c: &mut Compiler<'a>, src: &Source<'a>, node_id: NodeId) -> CompileResult {
-    match src.ast.kind(node_id) {
+    let previous_span = c.chunk.current_span;
+    c.chunk.current_span = src.ast.span(node_id);
+
+    let result = match src.ast.kind(node_id) {
         // The nil node is special, as it inhabits node ID 0.
         NodeKind::Nil => {
             unreachable!("Nil node should never be emitted (ParenEmpty is used for nil literals)")
@@ -121,7 +136,11 @@ pub fn compile_expr<'a>(c: &mut Compiler<'a>, src: &Source<'a>, node_id: NodeId)
         // Error nodes are ignored, because for each error node an appropriate parser
         // diagnostic is emitted anyways.
         NodeKind::Error => Ok(()),
-    }
+    };
+
+    c.chunk.current_span = previous_span;
+
+    result
 }
 
 fn compile_nil(c: &mut Compiler) -> CompileResult {
@@ -518,7 +537,17 @@ fn compile_lambda<'a>(c: &mut Compiler<'a>, src: &Source<'a>, node_id: NodeId) -
 
     c.chunk.emit_opcode(Opcode::Function)?;
     c.chunk.emit_u8(param_count)?;
-    let after_offset = c.chunk.emit_u16(0)?;
+    let then_offset = c.chunk.emit_u16(0)?;
+
+    let function_name = &c.function_name[0..c.function_name.len().min(u8::MAX as usize)];
+    c.chunk.emit_u8(function_name.len() as u8)?;
+    c.chunk.emit_bytes(function_name.as_bytes())?;
+
+    // Swap the current name for a name with a λ at the end.
+    // This is to make lambdas within the current lambda shown as a different functions in
+    // stack traces.
+    let previous_function_name = take(&mut c.function_name);
+    c.function_name = format!("{previous_function_name}λ");
 
     c.scopes.push(Scope {
         locals,
@@ -528,8 +557,10 @@ fn compile_lambda<'a>(c: &mut Compiler<'a>, src: &Source<'a>, node_id: NodeId) -
     compile_expr(c, src, body)?;
     c.chunk.emit_opcode(Opcode::Return)?;
 
-    let after = u16::try_from(c.chunk.bytecode.len()).expect("chunk is too large");
-    c.chunk.patch_u16(after_offset, after);
+    c.function_name = previous_function_name;
+
+    let then = u16::try_from(c.chunk.bytecode.len()).expect("chunk is too large");
+    c.chunk.patch_u16(then_offset, then);
 
     let scope = c.scopes.pop().unwrap();
     let let_count = u8::try_from(scope.let_count).unwrap_or_else(|_| {
@@ -580,6 +611,7 @@ fn compile_toplevel<'a>(c: &mut Compiler<'a>, src: &Source<'a>, node_id: NodeId)
             ));
         }
 
+        c.chunk.current_span = src.ast.span(toplevel_expr);
         match compile_toplevel_expr(c, src, toplevel_expr)? {
             ToplevelExpr::Def => (),
             ToplevelExpr::Result if result_expr.is_none() => result_expr = Some(toplevel_expr),
@@ -676,10 +708,20 @@ fn compile_def<'a>(c: &mut Compiler<'a>, src: &Source<'a>, node_id: NodeId) -> C
     // zero def instead.
     let def_id = c.defs.get_def(name).unwrap_or_default();
 
+    let previous_function_name = take(&mut c.function_name);
+    if src.ast.kind(right) == NodeKind::Lambda {
+        c.function_name = name.to_string();
+    } else {
+        // Name for lambdas within the current def, which is not a lambda itself.
+        c.function_name = format!("{name}λ");
+    }
+
     compile_expr(c, src, right)?;
     c.chunk.emit_opcode(Opcode::SetDef)?;
     c.chunk.emit_u16(def_id.to_u16())?;
 
+    c.function_name = previous_function_name;
+
     Ok(())
 }
 
diff --git a/crates/haku2/src/haku2.zig b/crates/haku2/src/haku2.zig
index 111b77e..15dc3b1 100644
--- a/crates/haku2/src/haku2.zig
+++ b/crates/haku2/src/haku2.zig
@@ -22,7 +22,10 @@ pub const std_options: std.Options = .{
 };
 
 pub fn enableLogScope(scope: @TypeOf(.enum_literal)) bool {
-    if (scope == .vm) return false else return true;
+    if (scope == .vm)
+        return false
+    else
+        return true;
 }
 
 // Allocator
@@ -124,14 +127,21 @@ export fn haku2_vm_run_main(
     chunk.* = bytecode.Chunk{
         .bytecode = code[0..code_len],
     };
-    const closure = value.Closure{
-        .chunk = chunk,
-        .start = 0,
-        .param_count = 0,
-        .local_count = local_count,
-        .captures = &[_]value.Value{},
+    const closure = scratch.allocator().create(value.Closure) catch {
+        vm.outOfMemory() catch {};
+        return false;
     };
-    vm.run(scratch.allocator(), &closure, vm.stack_top) catch return false;
+    closure.* = value.Closure{
+        .name = "(brush)",
+        .param_count = 0,
+        .impl = .{ .bytecode = .{
+            .chunk = chunk,
+            .start = 0,
+            .local_count = local_count,
+            .captures = &[_]value.Value{},
+        } },
+    };
+    vm.run(scratch.allocator(), closure, vm.stack_top) catch return false;
     return true;
 }
 
@@ -182,6 +192,33 @@ export fn haku2_vm_exception_render(vm: *const Vm, buffer: [*]u8) void {
     _ = exn.format(buffer[0..exn.len], &exn.args);
 }
 
+export fn haku2_vm_stackframe_count(vm: *const Vm) usize {
+    return vm.stackFrameCount();
+}
+
+export fn haku2_vm_stackframe_is_system(vm: *const Vm, index: usize) bool {
+    const stack_frame = vm.stackFrame(index);
+    return stack_frame.closure.impl == .system;
+}
+
+export fn haku2_vm_stackframe_pc(vm: *const Vm, index: usize) i32 {
+    const stack_frame = vm.stackFrame(index);
+    if (stack_frame.closure.impl == .bytecode)
+        if (stack_frame.ip) |ip|
+            return stack_frame.closure.programCounter(ip);
+    return -1; // no return address
+}
+
+export fn haku2_vm_stackframe_function_name_len(vm: *const Vm, index: usize) usize {
+    const stack_frame = vm.stackFrame(index);
+    return stack_frame.closure.name.len;
+}
+
+export fn haku2_vm_stackframe_function_name(vm: *const Vm, index: usize) [*]const u8 {
+    const stack_frame = vm.stackFrame(index);
+    return stack_frame.closure.name.ptr;
+}
+
 // Renderer
 
 export fn haku2_render(vm: *Vm, canvas: *Canvas, max_depth: usize) bool {
diff --git a/crates/haku2/src/system.zig b/crates/haku2/src/system.zig
index 464d1ba..e165031 100644
--- a/crates/haku2/src/system.zig
+++ b/crates/haku2/src/system.zig
@@ -62,7 +62,12 @@ pub const Context = struct {
     }
 };
 
-pub const Fn = *const fn (Context) Vm.Error!Value;
+pub const Fn = struct {
+    pub const Impl = *const fn (Context) Vm.Error!Value;
+
+    closure: value.Closure,
+    impl: Impl,
+};
 pub const FnTable = [256]Fn;
 
 fn invalid(cx: Context) !Value {
@@ -150,13 +155,24 @@ fn intoReturn(cx: Context, any: anytype) Vm.Error!Value {
     };
 }
 
+fn rawFn(name: []const u8, func: Fn.Impl) Fn {
+    return .{
+        .closure = .{
+            .name = name,
+            .param_count = 0, // TODO
+            .impl = .{ .system = void{} },
+        },
+        .impl = func,
+    };
+}
+
 /// Erase a well-typed function into a function that operates on raw values.
 /// The erased function performs all the necessary conversions automatically.
 ///
 /// Note that the argument order is important---function arguments go first, then context (such as
 /// the VM or the allocator.) Otherwise argument indices will not match up.
 fn erase(comptime name: []const u8, comptime func: anytype) Fn {
-    return Erased(name, func).call;
+    return rawFn(name, Erased(name, func).call);
 }
 
 fn countParams(comptime func: anytype) usize {
@@ -191,7 +207,7 @@ const SparseFn = struct { u8, Fn };
 const SparseFnTable = []const SparseFn;
 
 fn makeFnTable(init: SparseFnTable) FnTable {
-    var table = [_]Fn{invalid} ** @typeInfo(FnTable).array.len;
+    var table = [_]Fn{rawFn("(invalid)", invalid)} ** @typeInfo(FnTable).array.len;
     for (init) |entry| {
         const index, const func = entry;
         table[index] = func;
@@ -249,12 +265,12 @@ pub const fns = makeFnTable(&[_]SparseFn{
     .{ 0x44, erase("<=", lessOrEqual) },
     .{ 0x45, erase(">", greater) },
     .{ 0x46, erase(">=", greaterOrEqual) },
-    .{ 0x80, vec },
+    .{ 0x80, rawFn("vec", vec) },
     .{ 0x81, erase("vecX", vecX) },
     .{ 0x82, erase("vecY", vecY) },
     .{ 0x83, erase("vecZ", vecZ) },
     .{ 0x84, erase("vecW", vecW) },
-    .{ 0x85, rgba },
+    .{ 0x85, rawFn("rgba", rgba) },
     .{ 0x86, erase("rgbaR", rgbaR) },
     .{ 0x87, erase("rgbaG", rgbaG) },
     .{ 0x88, erase("rgbaB", rgbaB) },
diff --git a/crates/haku2/src/value.zig b/crates/haku2/src/value.zig
index 27ed963..2de5d2e 100644
--- a/crates/haku2/src/value.zig
+++ b/crates/haku2/src/value.zig
@@ -77,7 +77,7 @@ pub const Value = union(enum) {
             inline .tag, .number => |x| try std.fmt.format(writer, "{d}", .{x}),
             inline .vec4, .rgba => |x| try std.fmt.format(writer, "{s}{d}", .{ @tagName(value), x }),
             .ref => |ref| switch (ref.*) {
-                .closure => |c| try std.fmt.format(writer, "function({})", .{c.param_count}),
+                .closure => |c| try std.fmt.format(writer, "function {s}", .{c.name}),
                 .list => |l| {
                     try std.fmt.formatBuf("[", options, writer);
                     for (l, 0..) |elem, i| {
@@ -128,11 +128,31 @@ pub const Ref = union(enum) {
 };
 
 pub const Closure = struct {
-    chunk: *const bytecode.Chunk,
-    start: bytecode.Loc,
+    pub const Impl = union(enum) {
+        bytecode: Bytecode,
+        // Currently unimplemented; only used as a discriminator for system functions on
+        // the call stack that do not have bytecode.
+        system: void,
+
+        pub const Bytecode = struct {
+            chunk: *const bytecode.Chunk,
+            start: bytecode.Loc,
+            local_count: u8,
+            captures: []Value,
+        };
+    };
+
+    name: []const u8,
     param_count: u8,
-    local_count: u8,
-    captures: []Value,
+    impl: Impl,
+
+    pub fn bytecodePtr(c: *const Closure, pc: usize) [*]const u8 {
+        return c.impl.bytecode.chunk.bytecode[c.impl.bytecode.start + pc ..].ptr;
+    }
+
+    pub fn programCounter(c: *const Closure, ip: [*]const u8) u16 {
+        return @truncate(ip - c.impl.bytecode.chunk.bytecode.ptr);
+    }
 };
 
 pub const List = []Value;
diff --git a/crates/haku2/src/vm.zig b/crates/haku2/src/vm.zig
index 970d672..c08c8e3 100644
--- a/crates/haku2/src/vm.zig
+++ b/crates/haku2/src/vm.zig
@@ -19,6 +19,10 @@ call_stack_top: u32 = 0,
 defs: []Value = &.{},
 fuel: u32 = 0, // NOTE: VM must be refueled via reset() before running code
 exception: ?Exception = null,
+// closure, ip are for exception reporting. These variables are read-only, system functions
+// cannot affect control flow using them.
+closure: ?*const value.Closure = null,
+ip: ?[*]const u8 = null,
 
 pub const Limits = struct {
     stack_capacity: usize = 256,
@@ -27,7 +31,7 @@ pub const Limits = struct {
 
 pub const CallFrame = struct {
     closure: *const value.Closure,
-    ip: [*]const u8,
+    return_addr: ?[*]const u8,
     bottom: u32,
 };
 
@@ -59,8 +63,10 @@ pub fn reset(vm: *Vm, fuel: u32) void {
     vm.exception = null;
 }
 
+// NOTE: When the VM throws an exception, it must be reset. There's no resuming from an exception.
 pub fn throw(vm: *Vm, comptime fmt: []const u8, args: anytype) Error {
     log.info("throw: fmt={s}", .{fmt});
+    log.debug("implicit stack frame: closure={?any} ip={?*}", .{ vm.closure, vm.ip });
 
     const Args = @TypeOf(args);
     const max_args_size = @sizeOf(@TypeOf(vm.exception.?.args));
@@ -92,6 +98,45 @@ pub fn outOfMemory(vm: *Vm) Error {
     return vm.throw("out of memory", .{});
 }
 
+// NOTE: Different from CallFrame, this is a helper type for obtaining stack traces.
+pub const StackFrame = struct {
+    closure: *const value.Closure,
+    ip: ?[*]const u8,
+};
+
+pub fn stackFrameCount(vm: *const Vm) usize {
+    if (vm.closure != null) {
+        return vm.call_stack_top + 1;
+    } else {
+        return vm.call_stack_top;
+    }
+}
+
+pub fn stackFrame(vm: *const Vm, index: usize) StackFrame {
+    if (vm.closure) |closure| {
+        if (index == 0) {
+            return .{
+                .closure = closure,
+                .ip = vm.ip,
+            };
+        } else {
+            // NOTE: Minus two, because remember index == 0 is occupied by the current stack frame.
+            const call_frame_index = vm.call_stack_top - 1 - (index - 1);
+            const call_frame = vm.call_stack[call_frame_index];
+            return .{
+                .closure = call_frame.closure,
+                .ip = call_frame.return_addr,
+            };
+        }
+    } else {
+        const call_frame = vm.call_stack[vm.call_stack_top - 1 - index];
+        return .{
+            .closure = call_frame.closure,
+            .ip = call_frame.return_addr,
+        };
+    }
+}
+
 /// Debug assertion for bytecode validity.
 /// In future versions, this may become disabled in release builds.
 fn validateBytecode(vm: *Vm, ok: bool, comptime fmt: []const u8, args: anytype) Error!void {
@@ -162,16 +207,22 @@ pub fn def(vm: *Vm, index: u16) Error!*Value {
 
 // Utility struct for storing and restoring the VM's state variables across FFI boundaries.
 const Context = struct {
+    ip: ?[*]const u8,
+    closure: ?*const value.Closure,
     fuel: u32,
 };
 
 fn restoreContext(vm: *Vm) Context {
     return .{
+        .ip = vm.ip,
+        .closure = vm.closure,
         .fuel = vm.fuel,
     };
 }
 
 fn storeContext(vm: *Vm, context: Context) void {
+    vm.ip = context.ip;
+    vm.closure = context.closure;
     vm.fuel = context.fuel;
 }
 
@@ -181,7 +232,8 @@ inline fn read(comptime T: type, ip: *[*]const u8) T {
     return result;
 }
 
-inline fn readOpcode(ip: *[*]const u8) bytecode.Opcode {
+inline fn readOpcode(vm: *Vm, ip: *[*]const u8) bytecode.Opcode {
+    vm.ip = ip.*;
     const opcode: bytecode.Opcode = @enumFromInt(read(u8, ip));
     log.debug("OP {*} {}", .{ ip.*, opcode });
     return opcode;
@@ -200,53 +252,59 @@ pub fn run(
 ) Error!void {
     log.debug("BEGIN RUN {}", .{init_closure});
 
+    // NOTE: This will need swapping out once we implement first-class system functions, because
+    // system functions should be able to call themselves (like: map (range 1 10) sin)
+    debug.assert(init_closure.impl == .bytecode);
+
     var closure = init_closure;
-    var ip: [*]const u8 = closure.chunk.bytecode[closure.start..].ptr;
+    var ip: [*]const u8 = closure.bytecodePtr(0);
     var bottom = init_bottom;
     var fuel = vm.fuel;
 
-    for (0..closure.local_count) |_| {
+    vm.closure = closure;
+    vm.ip = ip;
+
+    for (0..closure.impl.bytecode.local_count) |_| {
         try vm.push(.nil);
     }
 
-    const call_bottom = vm.call_stack_top;
     try vm.pushCall(.{
         .closure = closure,
-        .ip = ip,
+        .return_addr = null, // nowhere to return to
         .bottom = bottom,
     });
 
-    next: switch (readOpcode(&ip)) {
+    next: switch (vm.readOpcode(&ip)) {
         .nil => {
             try vm.consumeFuel(&fuel, 1);
             try vm.push(.nil);
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .false => {
             try vm.consumeFuel(&fuel, 1);
             try vm.push(.false);
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .true => {
             try vm.consumeFuel(&fuel, 1);
             try vm.push(.true);
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .tag => {
             try vm.consumeFuel(&fuel, 1);
             const tag_id: value.TagId = @enumFromInt(read(u16, &ip));
             try vm.push(.{ .tag = tag_id });
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .number => {
             try vm.consumeFuel(&fuel, 1);
             const number: f32 = @bitCast(read(u32, &ip));
             try vm.push(.{ .number = number });
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .rgba => {
@@ -256,7 +314,7 @@ pub fn run(
             const b = read(u8, &ip);
             const a = read(u8, &ip);
             try vm.push(.{ .rgba = value.rgbaFrom8(value.Rgba8{ r, g, b, a }) });
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .local => {
@@ -264,7 +322,7 @@ pub fn run(
             const index = read(u8, &ip);
             const l = try vm.local(bottom, index);
             try vm.push(l.*);
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .set_local => {
@@ -273,16 +331,17 @@ pub fn run(
             const new = try vm.pop();
             const l = try vm.local(bottom, index);
             l.* = new;
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .capture => {
             try vm.consumeFuel(&fuel, 1);
             const index = read(u8, &ip);
-            try vm.validateBytecode(index < closure.captures.len, "capture index out of bounds", .{});
-            const capture = closure.captures[index];
+            const captures = closure.impl.bytecode.captures;
+            try vm.validateBytecode(index < captures.len, "capture index out of bounds", .{});
+            const capture = captures[index];
             try vm.push(capture);
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .def => {
@@ -290,7 +349,7 @@ pub fn run(
             const index = read(u16, &ip);
             const d = try vm.def(index);
             try vm.push(d.*);
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .set_def => {
@@ -299,7 +358,7 @@ pub fn run(
             const new = try vm.pop();
             const d = try vm.def(index);
             d.* = new;
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .list => {
@@ -312,62 +371,70 @@ pub fn run(
             const ref = allocator.create(value.Ref) catch return vm.outOfMemory();
             ref.* = .{ .list = list };
             try vm.push(.{ .ref = ref });
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .function => {
-            try vm.consumeFuel(&fuel, 1);
             const param_count = read(u8, &ip);
             const then = read(u16, &ip);
+            const name_len = read(u8, &ip);
+            const name = ip[0..name_len];
+            ip += name_len;
             const body = ip;
-            ip = closure.chunk.bytecode[then..].ptr;
+            ip = closure.bytecodePtr(then);
 
             const local_count = read(u8, &ip);
             const capture_count = read(u8, &ip);
 
+            try vm.consumeFuel(&fuel, 1 + capture_count);
             const captures = allocator.alloc(Value, capture_count) catch return vm.outOfMemory();
             for (captures) |*capture| {
                 const capture_kind = read(u8, &ip);
                 const index = read(u8, &ip);
+                vm.ip = ip;
                 capture.* = switch (capture_kind) {
                     bytecode.capture_local => (try vm.local(bottom, index)).*,
                     bytecode.capture_capture => blk: {
-                        try vm.validateBytecode(index < closure.captures.len, "capture index out of bounds", .{});
-                        break :blk closure.captures[index];
+                        const closureCaptures = closure.impl.bytecode.captures;
+                        try vm.validateBytecode(index < closureCaptures.len, "capture index out of bounds", .{});
+                        break :blk closureCaptures[index];
                     },
                     else => .nil,
                 };
             }
 
             const new_closure = value.Closure{
-                .chunk = closure.chunk,
-                .start = @truncate(body - closure.chunk.bytecode.ptr),
+                .name = name,
                 .param_count = param_count,
-                .local_count = local_count,
-                .captures = captures,
+                .impl = .{ .bytecode = .{
+                    .chunk = closure.impl.bytecode.chunk,
+                    .start = @truncate(body - closure.bytecodePtr(0)),
+                    .local_count = local_count,
+                    .captures = captures,
+                } },
             };
             const ref = allocator.create(value.Ref) catch return vm.outOfMemory();
             ref.* = .{ .closure = new_closure };
             try vm.push(.{ .ref = ref });
 
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .jump => {
-            try vm.consumeFuel(&fuel, 1);
             const offset = read(u16, &ip);
-            ip = closure.chunk.bytecode[offset..].ptr;
-            continue :next readOpcode(&ip);
+            try vm.consumeFuel(&fuel, 1);
+            ip = closure.bytecodePtr(offset);
+            continue :next vm.readOpcode(&ip);
         },
 
         .jump_if_not => {
-            try vm.consumeFuel(&fuel, 1);
             const offset = read(u16, &ip);
+            try vm.consumeFuel(&fuel, 1);
             const condition = try vm.pop();
             if (!condition.isTruthy()) {
-                ip = closure.chunk.bytecode[offset..].ptr;
+                ip = closure.bytecodePtr(offset);
             }
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .field => {
@@ -389,27 +456,31 @@ pub fn run(
             }
 
             if (found_index) |index| {
-                try vm.validateBytecode(index < closure.captures.len, "field index out of bounds", .{});
-                const field = closure.captures[index];
+                const fields = closure.impl.bytecode.captures;
+                try vm.validateBytecode(index < fields.len, "field index out of bounds", .{});
+                const field = fields[index];
                 try vm.push(field);
             } else {
                 return vm.throw("field with this name does not exist", .{});
             }
 
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .call => {
-            try vm.consumeFuel(&fuel, 1);
-
             const arg_count = read(u8, &ip);
 
+            try vm.consumeFuel(&fuel, 1);
+
             const function_value = try vm.pop();
             if (function_value != .ref or function_value.ref.* != .closure) {
                 return vm.throw("attempt to call a value that is not a function", .{});
             }
             const called_closure = &function_value.ref.closure;
 
+            // NOTE: Will need replacing for first-class system functions.
+            debug.assert(called_closure.impl == .bytecode);
+
             if (arg_count != called_closure.param_count) {
                 return vm.throw(
                     "function expects {} arguments, but it received {}",
@@ -419,7 +490,7 @@ pub fn run(
 
             const call_frame = CallFrame{
                 .closure = closure,
-                .ip = ip,
+                .return_addr = ip,
                 .bottom = bottom,
             };
 
@@ -427,29 +498,61 @@ pub fn run(
             try vm.validateBytecode(overflow == 0, "not enough values on the stack for arguments", .{});
 
             closure = called_closure;
-            ip = closure.chunk.bytecode[closure.start..].ptr;
+            ip = closure.bytecodePtr(0);
             bottom = new_bottom;
 
-            for (0..closure.local_count) |_| {
+            // pushCall before setting vm.closure and vm.ip, such that stack overflow errors are
+            // reported at the call site rather than within the called function.
+            try vm.pushCall(call_frame);
+
+            // Remember to update the closure used for exception reporting, since readOpcode does
+            // not do that as it does with ip.
+            // ip also needs updating, because the topmost stack frame will assume that vm.ip is
+            // based in the new closure.
+            // We want errors for no stack space being left for locals to appear at the start of the
+            // function in source code, not earlier or later. Hence we do this before local slots
+            // are pushed onto the stack.
+            vm.closure = closure;
+            vm.ip = ip;
+
+            for (0..closure.impl.bytecode.local_count) |_| {
                 try vm.push(.nil);
             }
 
-            try vm.pushCall(call_frame);
-
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         .system => {
-            try vm.consumeFuel(&fuel, 1);
-
             const index = read(u8, &ip);
             const arg_count = read(u8, &ip);
-            const system_fn = system.fns[index];
+            const system_fn = &system.fns[index];
 
-            log.debug("system index={} arg_count={} system_fn={p}", .{ index, arg_count, system_fn });
+            try vm.consumeFuel(&fuel, 1);
 
-            vm.storeContext(.{ .fuel = fuel });
-            const result = try system_fn(.{
+            log.debug("system index={} arg_count={} system_fn={s}", .{ index, arg_count, system_fn.closure.name });
+
+            // Push the current call frame onto the stack, such that the stack trace displays it
+            // in addition to the ongoing system call.
+            try vm.pushCall(.{
+                .closure = closure,
+                .return_addr = ip,
+                .bottom = bottom,
+            });
+
+            // Also now, push the system function onto the stack, because nulling out vm.closure
+            // and vm.ip removes the extra "current function" stack entry.
+            try vm.pushCall(.{
+                .closure = &system_fn.closure,
+                .return_addr = null,
+                .bottom = vm.stack_top - arg_count,
+            });
+
+            vm.storeContext(.{
+                .fuel = fuel,
+                .closure = null,
+                .ip = null,
+            });
+            const result = try system_fn.impl(.{
                 .vm = vm,
                 .allocator = allocator,
                 .args = vm.stack[vm.stack_top - arg_count .. vm.stack_top],
@@ -460,7 +563,16 @@ pub fn run(
             vm.stack_top -= arg_count;
             try vm.push(result);
 
-            continue :next readOpcode(&ip);
+            // Restore the exception handling ip and closure to what it was.
+            log.debug("system restore", .{});
+            vm.ip = ip;
+            vm.closure = closure;
+
+            // Remove the temporary call frames.
+            _ = try vm.popCall(); // systemClosure
+            _ = try vm.popCall(); // closure
+
+            continue :next vm.readOpcode(&ip);
         },
 
         .ret => {
@@ -474,26 +586,28 @@ pub fn run(
                 "called function popped too many values. bottom={} stack_top={}",
                 .{ bottom, vm.stack_top },
             );
-            try vm.validateBytecode(
-                call_bottom <= vm.call_stack_top,
-                "called function popped too many call frames. call_bottom={} call_stack_top={}",
-                .{ call_bottom, vm.call_stack_top },
-            );
 
             vm.stack_top = bottom;
             try vm.push(result);
 
-            if (vm.call_stack_top == call_bottom) {
-                // If this is the last call frame from this `run` instance, return from the function.
-                vm.storeContext(Context{ .fuel = fuel });
+            if (call_frame.return_addr) |addr| {
+                closure = call_frame.closure;
+                ip = addr;
+                bottom = call_frame.bottom;
+
+                vm.closure = closure;
+                vm.ip = ip;
+            } else {
+                // If there's nowhere to return to, stop interpreting.
+                vm.storeContext(Context{
+                    .closure = closure,
+                    .ip = ip,
+                    .fuel = fuel,
+                });
                 break :next;
             }
 
-            closure = call_frame.closure;
-            ip = call_frame.ip;
-            bottom = call_frame.bottom;
-
-            continue :next readOpcode(&ip);
+            continue :next vm.readOpcode(&ip);
         },
 
         _ => |invalid_opcode| {
@@ -531,11 +645,14 @@ pub fn runDotter(
     }) catch return vm.outOfMemory();
     const ref = allocator.create(value.Ref) catch return vm.outOfMemory();
     ref.* = value.Ref{ .closure = .{
-        .chunk = &system.record_dotter,
-        .start = 0,
+        .name = "dotter",
         .param_count = 1,
-        .local_count = 0,
-        .captures = data,
+        .impl = .{ .bytecode = .{
+            .chunk = &system.record_dotter,
+            .start = 0,
+            .local_count = 0,
+            .captures = data,
+        } },
     } };
 
     const bottom = vm.stack_top;
diff --git a/static/base.css b/static/base.css
index 1843701..6a9ede5 100644
--- a/static/base.css
+++ b/static/base.css
@@ -2,6 +2,7 @@
 
 :root {
     --color-text: #111;
+    --color-text-dim: #777;
     --color-error: #db344b;
 
     --color-brand-blue: #40b1f4;
@@ -190,6 +191,12 @@ pre:has(code) {
     overflow: auto;
 }
 
+/* Abbreviations */
+
+abbr {
+    cursor: help;
+}
+
 /* Icons */
 
 :root {
diff --git a/static/brush-editor.js b/static/brush-editor.js
index 41fe576..7d77433 100644
--- a/static/brush-editor.js
+++ b/static/brush-editor.js
@@ -1,4 +1,4 @@
-import { CodeEditor } from "rkgk/code-editor.js";
+import { CodeEditor, Selection } from "rkgk/code-editor.js";
 import { SaveData } from "rkgk/framework.js";
 import { builtInPresets } from "rkgk/brush-box.js";
 
@@ -33,6 +33,10 @@ export class BrushEditor extends HTMLElement {
 
         this.codeEditor = this.appendChild(
             new CodeEditor([
+                {
+                    className: "layer-syntax",
+                    render: (code, element) => this.#renderSyntax(code, element),
+                },
                 {
                     className: "layer-error-squiggles",
                     render: (code, element) => this.#renderErrorSquiggles(code, element),
@@ -116,8 +120,32 @@ export class BrushEditor extends HTMLElement {
         } else if (result.errorKind == "plain") {
             this.errorHeader.textContent = result.message;
         } else if (result.errorKind == "exception") {
-            // TODO: This should show a stack trace.
-            this.errorArea.textContent = `an exception occurred: ${result.message}`;
+            let renderer = new ErrorException(result);
+            let squiggles = renderer.prepareSquiggles();
+
+            this.codeEditor.rebuildLineMap();
+            this.errorSquiggles = this.#computeErrorSquiggles(
+                this.codeEditor.lineMap,
+                squiggles.diagnostics,
+            );
+            this.codeEditor.renderLayer("layer-error-squiggles");
+
+            renderer.addEventListener(".functionNameMouseEnter", (event) => {
+                squiggles.highlightSegment(event.frameIndex);
+            });
+
+            renderer.addEventListener(".functionNameMouseLeave", () => {
+                squiggles.highlightSegment(null);
+            });
+
+            renderer.addEventListener(".functionNameClick", (event) => {
+                let span = event.frame.span;
+                this.codeEditor.textArea.focus();
+                this.codeEditor.setSelection(new Selection(span.start, span.end));
+            });
+
+            this.errorArea.replaceChildren();
+            this.errorArea.appendChild(renderer);
         } else {
             console.warn(`unknown error kind: ${result.errorKind}`);
             this.errorHeader.textContent = "(unknown error kind)";
@@ -206,6 +234,10 @@ export class BrushEditor extends HTMLElement {
                         let spanElement = lineElement.appendChild(document.createElement("span"));
                         spanElement.classList.add("squiggle", "squiggle-error");
                         spanElement.textContent = text;
+
+                        for (let diagnostic of segment.diagnostics) {
+                            diagnostic.customizeSegment?.(diagnostic, spanElement);
+                        }
                     }
                 }
             } else {
@@ -213,6 +245,151 @@ export class BrushEditor extends HTMLElement {
             }
         }
     }
+
+    #renderSyntax(lines, element) {
+        // TODO: Syntax highlighting.
+        // Right now we just render a layer of black text to have the text visible in the code editor.
+        element.textContent = lines.string;
+    }
 }
 
 customElements.define("rkgk-brush-editor", BrushEditor);
+
+class ErrorException extends HTMLElement {
+    constructor(result) {
+        super();
+        this.result = result;
+    }
+
+    prepareSquiggles() {
+        let diagnostics = [];
+        let segments = [];
+
+        const customizeSegment = (diagnostic, segment) => {
+            // The control flow here is kind of spaghetti, but basically this fills out `segments`
+            // as the squiggles are being rendered.
+            // Once squiggles are rendered, you're free to call highlightSegment(i).
+            segments.push({
+                frameIndex: diagnostic.frameIndex,
+                colorIndex: diagnostic.colorIndex,
+                segment,
+            });
+            if (diagnostic.colorIndex != null) {
+                segment.classList.add("squiggle-colored");
+                segment.style.setProperty("--color-index", diagnostic.colorIndex);
+            }
+        };
+
+        for (let i = 0; i < this.result.stackTrace.length; ++i) {
+            let frame = this.result.stackTrace[i];
+            if (frame.span != null) {
+                diagnostics.push({
+                    start: frame.span.start,
+                    end: frame.span.end,
+
+                    customizeSegment,
+                    frameIndex: i,
+                    colorIndex: i / this.result.stackTrace.length,
+                });
+            }
+        }
+
+        return {
+            diagnostics,
+            highlightSegment(frameIndex) {
+                let justHighlighted = new Set();
+                for (let entry of segments.values()) {
+                    let segment = entry.segment;
+                    if (entry.frameIndex == frameIndex) {
+                        segment.classList.add("squiggle-highlighted");
+                        // This logic exists so that a specific segment can display on top of
+                        // another one if it's highlighted.
+                        segment.style.setProperty("--highlight-color-index", entry.colorIndex);
+                        justHighlighted.add(segment);
+                    } else if (!justHighlighted.has(segment)) {
+                        segment.classList.remove("squiggle-highlighted");
+                    }
+                }
+            },
+        };
+    }
+
+    connectedCallback() {
+        let message = this.appendChild(document.createElement("p"));
+        message.classList.add("message");
+        message.textContent = this.result.message;
+
+        let stackTrace = this.appendChild(document.createElement("ol"));
+        stackTrace.classList.add("stack-trace");
+        for (let i = 0; i < this.result.stackTrace.length; ++i) {
+            let frame = this.result.stackTrace[i];
+            let line = stackTrace.appendChild(document.createElement("li"));
+
+            let [_, name, lambdasString] = frame.functionName.match(/([^λ]+)(λ*)/);
+
+            let inCaption = line.appendChild(document.createElement("span"));
+            inCaption.classList.add("in");
+            inCaption.innerText = "in ";
+
+            let functionName = line.appendChild(document.createElement("button"));
+            functionName.classList.add("function-name");
+            functionName.disabled = true;
+            functionName.innerText = name;
+            if (name == "(brush)") {
+                functionName.classList.add("function-name-brush");
+                functionName.title = "Top-level brush code";
+            }
+            let lambdas = functionName.appendChild(document.createElement("abbr"));
+            lambdas.classList.add("lambdas");
+            lambdas.textContent = lambdasString;
+            lambdas.title =
+                "λ - unnamed function\n" +
+                "Symbolizes functions that were not given a name.\n" +
+                "Each additional λ is one level of nesting inside another function.";
+
+            if (frame.isSystem) {
+                line.append(" ");
+                let system = line.appendChild(document.createElement("abbr"));
+                system.classList.add("system");
+                system.innerText = "(built-in)";
+                system.title =
+                    "This function is built into rakugaki and does not exist in your brush's source code, so you can't see it in the editor.";
+            }
+
+            if (frame.span != null) {
+                functionName.disabled = false;
+                functionName.classList.add("source-link");
+                functionName.style.setProperty("--color-index", i / this.result.stackTrace.length);
+
+                functionName.addEventListener("mouseenter", () => {
+                    this.dispatchEvent(
+                        Object.assign(new Event(".functionNameMouseEnter"), {
+                            frameIndex: i,
+                            frame,
+                        }),
+                    );
+                });
+
+                functionName.addEventListener("mouseleave", () => {
+                    this.dispatchEvent(
+                        Object.assign(new Event(".functionNameMouseLeave"), {
+                            frameIndex: i,
+                            frame,
+                        }),
+                    );
+                });
+
+                functionName.addEventListener("click", () => {
+                    this.dispatchEvent(
+                        Object.assign(new Event(".functionNameClick"), {
+                            frameIndex: i,
+                            frame,
+                        }),
+                    );
+                });
+            }
+        }
+    }
+}
+
+customElements.define("rkgk-brush-editor-error-exception", ErrorException);
diff --git a/static/haku.js b/static/haku.js
index ad9e3c5..7d4a66a 100644
--- a/static/haku.js
+++ b/static/haku.js
@@ -244,7 +244,7 @@ export class Haku {
         w2.haku2_vm_destroy(this.#pVm2);
         w2.haku2_defs_destroy(this.#pDefs2);
         w2.haku2_limits_destroy(this.#pLimits2);
-        w2.haku_dealloc(this.#bytecode2.ptr);
+        freeString2(this.#bytecode2);
     }
 
     setBrush(code) {
@@ -332,12 +332,49 @@ export class Haku {
         return exn;
     }
 
+    #stackTrace() {
+        let count = w2.haku2_vm_stackframe_count(this.#pVm2);
+        let trace = [];
+        for (let i = 0; i < count; ++i) {
+            let isSystem = w2.haku2_vm_stackframe_is_system(this.#pVm2, i) != 0;
+            let pc = w2.haku2_vm_stackframe_pc(this.#pVm2, i);
+
+            let span = null;
+            if (!isSystem && pc > 0) {
+                // NOTE: We find the span at (pc - 1), because stack frames' program counters are
+                // situated at where control flow _ought_ to return, and not where the function call
+                // is located. Therefore, to pull the program counter back into the function call,
+                // we subtract 1.
+                let pSpan = w.haku_bytecode_find_span(this.#pInstance, Math.max(0, pc - 1));
+                if (pSpan != 0) {
+                    span = {
+                        start: w.haku_span_start(pSpan),
+                        end: w.haku_span_end(pSpan),
+                    };
+                }
+            }
+
+            trace.push({
+                isSystem,
+                pc,
+                span,
+                functionName: readString(
+                    memory2,
+                    w2.haku2_vm_stackframe_function_name_len(this.#pVm2, i),
+                    w2.haku2_vm_stackframe_function_name(this.#pVm2, i),
+                ),
+            });
+        }
+        return trace;
+    }
+
     #exceptionResult() {
         return {
             status: "error",
             errorKind: "exception",
             description: "Runtime error",
             message: this.#exceptionMessage(),
+            stackTrace: this.#stackTrace(),
         };
     }
 
diff --git a/static/index.css b/static/index.css
index ebcb5c2..4ff0072 100644
--- a/static/index.css
+++ b/static/index.css
@@ -1,7 +1,7 @@
 /* index.css - styles for index.html and generally main parts of the app
    For shared styles (such as color definitions) check out base.css. */
 
-* {
+html {
     /* On the main page, we don't really want to permit selecting things.
        It comes off as janky-looking. */
     user-select: none;
@@ -412,22 +412,47 @@ rkgk-code-editor {
 
             & > .squiggle-error {
                 text-decoration-color: var(--color-error);
+
+                &.squiggle-colored {
+                    --color: oklch(40% 100% calc(var(--color-index) * 300));
+                    text-decoration-color: var(--color);
+                    background-color: oklch(from var(--color) 60% c h / 0.13);
+
+                    &.squiggle-highlighted {
+                        --highlight-color-index: var(--color-index);
+                        text-decoration: none;
+                        background-color: oklch(
+                            40% 100% calc(var(--highlight-color-index) * 300)
+                        );
+                        color: white;
+                        font-weight: bold;
+                    }
+                }
             }
         }
     }
 
+    & > .layer-syntax {
+        white-space: pre-wrap;
+    }
+
     & > textarea {
         display: block;
         width: calc(100% - var(--gutter-width));
+
         margin: 0;
         margin-left: var(--gutter-width);
         padding: 0;
-        box-sizing: border-box;
+        border: none;
+
         overflow: hidden;
         resize: none;
+
         white-space: pre-wrap;
-        border: none;
+
         background: none;
+        color: transparent;
+        caret-color: var(--color-text);
 
         &:focus {
             /* The outline is displayed on the parent element to also surround the gutter. */
@@ -490,6 +515,67 @@ rkgk-brush-editor.rkgk-panel {
         margin: 0;
         color: var(--color-error);
         white-space: pre-wrap;
+
+        user-select: text;
+
+        max-height: 20em;
+        overflow-y: auto;
+    }
+}
+
+rkgk-brush-editor-error-exception {
+    & > .message {
+        margin: 4px 0;
+    }
+
+    & > .stack-trace {
+        margin: 0;
+        margin-left: 4ch;
+
+        color: var(--color-text);
+
+        & > li {
+            &::marker {
+                color: var(--color-text-dim);
+            }
+
+            & > button.function-name {
+                border: none;
+                border-radius: 0;
+                padding: 0;
+                background: none;
+
+                user-select: text;
+
+                font-weight: bold;
+
+                &:not(:disabled) {
+                    text-decoration: underline;
+                    cursor: pointer;
+                }
+
+                &:disabled {
+                    opacity: 100%;
+                }
+
+                &.source-link {
+                    --color-index: 0; /* set by JavaScript */
+                    --color: oklch(40% 100% calc(var(--color-index) * 300));
+
+                    color: var(--color);
+
+                    &:hover {
+                        text-decoration: none;
+                        background-color: var(--color);
+                        color: white;
+                    }
+                }
+            }
+
+            & > .system {
+                color: var(--color-text-dim);
+            }
+        }
     }
 }