diff --git a/crates/haku-cli/src/main.rs b/crates/haku-cli/src/main.rs
index 02f2770..5c506c0 100644
--- a/crates/haku-cli/src/main.rs
+++ b/crates/haku-cli/src/main.rs
@@ -6,7 +6,7 @@ use std::{error::Error, fmt::Display, io::BufRead};
 use haku::{
     bytecode::{Chunk, Defs},
     compiler::{compile_expr, Compiler, Source},
-    sexp::{parse_toplevel, Ast, Parser},
+    sexp::{parse_toplevel, Ast, Parser, SourceCode},
     system::System,
     value::{BytecodeLoc, Closure, FunctionName, Ref, Value},
     vm::{Vm, VmLimits},
@@ -16,6 +16,7 @@ fn eval(code: &str) -> Result<Value, Box<dyn Error>> {
     let mut system = System::new(1);
 
     let ast = Ast::new(1024);
+    let code = SourceCode::unlimited_len(code);
     let mut parser = Parser::new(ast, code);
     let root = parse_toplevel(&mut parser);
     let ast = parser.ast;
diff --git a/crates/haku-wasm/src/lib.rs b/crates/haku-wasm/src/lib.rs
index 92dc725..b510c29 100644
--- a/crates/haku-wasm/src/lib.rs
+++ b/crates/haku-wasm/src/lib.rs
@@ -12,7 +12,7 @@ use haku::{
         tiny_skia::{Pixmap, PremultipliedColorU8},
         Renderer, RendererLimits,
     },
-    sexp::{parse_toplevel, Ast, Parser},
+    sexp::{parse_toplevel, Ast, Parser, SourceCode},
     system::{ChunkId, System, SystemImage},
     value::{BytecodeLoc, Closure, FunctionName, Ref, Value},
     vm::{Exception, Vm, VmImage, VmLimits},
@@ -37,6 +37,7 @@ unsafe extern "C" fn haku_free(ptr: *mut u8, size: usize, align: usize) {
 
 #[derive(Debug, Clone, Copy)]
 struct Limits {
+    max_source_code_len: usize,
     max_chunks: usize,
     max_defs: usize,
     ast_capacity: usize,
@@ -53,6 +54,7 @@ struct Limits {
 impl Default for Limits {
     fn default() -> Self {
         Self {
+            max_source_code_len: 65536,
             max_chunks: 2,
             max_defs: 256,
             ast_capacity: 1024,
@@ -92,6 +94,7 @@ macro_rules! limit_setter {
     };
 }
 
+limit_setter!(max_source_code_len);
 limit_setter!(max_chunks);
 limit_setter!(max_defs);
 limit_setter!(ast_capacity);
@@ -193,6 +196,7 @@ unsafe extern "C" fn haku_exception_message_len(instance: *const Instance) -> u3
 #[repr(C)]
 enum StatusCode {
     Ok,
+    SourceCodeTooLong,
     ChunkTooBig,
     DiagnosticsEmitted,
     TooManyChunks,
@@ -223,6 +227,7 @@ extern "C" fn haku_is_exception(code: StatusCode) -> bool {
 extern "C" fn haku_status_string(code: StatusCode) -> *const i8 {
     match code {
         StatusCode::Ok => c"ok",
+        StatusCode::SourceCodeTooLong => c"source code is too long",
         StatusCode::ChunkTooBig => c"compiled bytecode is too large",
         StatusCode::DiagnosticsEmitted => c"diagnostics were emitted",
         StatusCode::TooManyChunks => c"too many registered bytecode chunks",
@@ -297,6 +302,10 @@ unsafe extern "C" fn haku_compile_brush(
 
     let code = core::str::from_utf8(slice::from_raw_parts(code, code_len as usize))
         .expect("invalid UTF-8");
+    let code = match SourceCode::limited_len(code, instance.limits.max_source_code_len) {
+        Some(code) => code,
+        None => return StatusCode::SourceCodeTooLong,
+    };
 
     let ast = Ast::new(instance.limits.ast_capacity);
     let mut parser = Parser::new(ast, code);
diff --git a/crates/haku/src/compiler.rs b/crates/haku/src/compiler.rs
index 4a0e65c..a6966f3 100644
--- a/crates/haku/src/compiler.rs
+++ b/crates/haku/src/compiler.rs
@@ -7,12 +7,12 @@ use alloc::vec::Vec;
 
 use crate::{
     bytecode::{Chunk, DefError, Defs, EmitError, Opcode, CAPTURE_CAPTURE, CAPTURE_LOCAL},
-    sexp::{Ast, NodeId, NodeKind, Span},
+    sexp::{Ast, NodeId, NodeKind, SourceCode, Span},
     system::System,
 };
 
 pub struct Source<'a> {
-    pub code: &'a str,
+    pub code: &'a SourceCode,
     pub ast: &'a Ast,
     pub system: &'a System,
 }
diff --git a/crates/haku/src/sexp.rs b/crates/haku/src/sexp.rs
index 96892b3..9a2b8b2 100644
--- a/crates/haku/src/sexp.rs
+++ b/crates/haku/src/sexp.rs
@@ -1,4 +1,4 @@
-use core::{cell::Cell, fmt};
+use core::{cell::Cell, fmt, ops::Deref};
 
 use alloc::vec::Vec;
 
@@ -13,8 +13,40 @@ impl Span {
         Self { start, end }
     }
 
-    pub fn slice<'a>(&self, source: &'a str) -> &'a str {
-        &source[self.start..self.end]
+    pub fn slice<'a>(&self, source: &'a SourceCode) -> &'a str {
+        &source.code[self.start..self.end]
+    }
+}
+
+/// Source code string with a verified size limit.
+/// An exact size limit is not enforced by this type - it only ensures the string isn't longer than
+/// intended, to not stall the parser for an unexpected amount of time.
+#[derive(Debug, PartialEq, Eq)]
+#[repr(transparent)]
+pub struct SourceCode {
+    code: str,
+}
+
+impl SourceCode {
+    pub fn limited_len(code: &str, max_len: usize) -> Option<&Self> {
+        if code.len() <= max_len {
+            Some(Self::unlimited_len(code))
+        } else {
+            None
+        }
+    }
+
+    pub fn unlimited_len(code: &str) -> &Self {
+        // SAFETY: SourceCode is a transparent wrapper around str, so converting between them is safe.
+        unsafe { core::mem::transmute(code) }
+    }
+}
+
+impl Deref for SourceCode {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        &self.code
     }
 }
 
@@ -94,7 +126,7 @@ impl Ast {
 
     pub fn write(
         &self,
-        source: &str,
+        source: &SourceCode,
         node_id: NodeId,
         w: &mut dyn fmt::Write,
         mode: AstWriteMode,
@@ -102,7 +134,7 @@ impl Ast {
         #[allow(clippy::too_many_arguments)]
         fn write_list(
             ast: &Ast,
-            source: &str,
+            source: &SourceCode,
             w: &mut dyn fmt::Write,
             mode: AstWriteMode,
             mut head: NodeId,
@@ -131,7 +163,7 @@ impl Ast {
         // NOTE: Separated out to a separate function in case we ever want to introduce auto-indentation.
         fn write_rec(
             ast: &Ast,
-            source: &str,
+            source: &SourceCode,
             w: &mut dyn fmt::Write,
             mode: AstWriteMode,
             node_id: NodeId,
@@ -177,7 +209,7 @@ pub struct NodeAllocError;
 
 pub struct Parser<'a> {
     pub ast: Ast,
-    input: &'a str,
+    input: &'a SourceCode,
     position: usize,
     fuel: Cell<usize>,
     alloc_error: NodeId,
@@ -186,7 +218,7 @@ pub struct Parser<'a> {
 impl<'a> Parser<'a> {
     const FUEL: usize = 256;
 
-    pub fn new(mut ast: Ast, input: &'a str) -> Self {
+    pub fn new(mut ast: Ast, input: &'a SourceCode) -> Self {
         let alloc_error = ast
             .alloc(Node {
                 span: Span::new(0, 0),
@@ -412,12 +444,13 @@ mod tests {
         expected: &str,
     ) -> Result<(), Box<dyn Error>> {
         let ast = Ast::new(16);
-        let mut p = Parser::new(ast, source);
+        let code = SourceCode::unlimited_len(source);
+        let mut p = Parser::new(ast, code);
         let node = f(&mut p);
         let ast = p.ast;
 
         let mut s = String::new();
-        ast.write(source, node, &mut s, AstWriteMode::Spans)?;
+        ast.write(code, node, &mut s, AstWriteMode::Spans)?;
 
         assert_eq!(s, expected);
 
diff --git a/crates/haku/tests/language.rs b/crates/haku/tests/language.rs
index 594d549..787c46f 100644
--- a/crates/haku/tests/language.rs
+++ b/crates/haku/tests/language.rs
@@ -3,7 +3,7 @@ use std::error::Error;
 use haku::{
     bytecode::{Chunk, Defs},
     compiler::{compile_expr, Compiler, Source},
-    sexp::{self, Ast, Parser},
+    sexp::{self, Ast, Parser, SourceCode},
     system::System,
     value::{BytecodeLoc, Closure, FunctionName, Ref, RefId, Value},
     vm::{Vm, VmLimits},
@@ -13,6 +13,7 @@ fn eval(code: &str) -> Result<Value, Box<dyn Error>> {
     let mut system = System::new(1);
 
     let ast = Ast::new(1024);
+    let code = SourceCode::unlimited_len(code);
     let mut parser = Parser::new(ast, code);
     let root = sexp::parse_toplevel(&mut parser);
     let ast = parser.ast;
diff --git a/crates/rkgk/src/haku.rs b/crates/rkgk/src/haku.rs
index 6a5e4ae..6f4aa19 100644
--- a/crates/rkgk/src/haku.rs
+++ b/crates/rkgk/src/haku.rs
@@ -8,7 +8,7 @@ use haku::{
     bytecode::{Chunk, Defs, DefsImage},
     compiler::{Compiler, Source},
     render::{tiny_skia::Pixmap, Renderer, RendererLimits},
-    sexp::{Ast, Parser},
+    sexp::{Ast, Parser, SourceCode},
     system::{ChunkId, System, SystemImage},
     value::{BytecodeLoc, Closure, FunctionName, Ref, Value},
     vm::{Vm, VmImage, VmLimits},
@@ -22,6 +22,7 @@ use crate::schema::Vec2;
 // because we do some dynamic typing magic over on the JavaScript side to automatically call all
 // the appropriate functions for setting these limits on the client side.
 pub struct Limits {
+    pub max_source_code_len: usize,
     pub max_chunks: usize,
     pub max_defs: usize,
     pub ast_capacity: usize,
@@ -88,6 +89,8 @@ impl Haku {
         self.reset();
 
         let ast = Ast::new(self.limits.ast_capacity);
+        let code = SourceCode::limited_len(code, self.limits.max_source_code_len)
+            .ok_or_eyre("source code is too long")?;
         let mut parser = Parser::new(ast, code);
         let root = haku::sexp::parse_toplevel(&mut parser);
         let ast = parser.ast;
diff --git a/rkgk.toml b/rkgk.toml
index 572feb2..92c2eb1 100644
--- a/rkgk.toml
+++ b/rkgk.toml
@@ -33,6 +33,9 @@ interval_seconds = 10
 # Technically clients may override these settings with some hackery, but then the server may not
 # register changes they make to the canvas.
 
+# Maximum length of source code.
+max_source_code_len = 65536
+
 # Maximum amount of source code chunks.
 # This should be at least 2, to allow for loading in a standard library chunk.
 max_chunks = 2