diff --git a/build.zig b/build.zig
index 8d6faa5158..596adde0cc 100644
--- a/build.zig
+++ b/build.zig
@@ -140,7 +140,7 @@ pub fn build(b: *std.Build) void {
             .name = "builtin_compiler",
             .root_source_file = b.path("src/build/builtin_compiler/main.zig"),
             .target = b.graph.host, // this runs at build time on the *host* machine!
-            .optimize = .Debug, // No need to optimize - only compiles builtin modules
+            .optimize = .ReleaseFast, // Must match playground optimization to ensure struct compatibility
             // Note: libc linking is handled by add_tracy below (required when tracy is enabled)
         });
 
@@ -234,7 +234,7 @@ pub fn build(b: *std.Build) void {
         .name = "builtin_compiler",
         .root_source_file = b.path("src/build/builtin_compiler/main.zig"),
         .target = b.graph.host,
-        .optimize = .Debug,
+        .optimize = .ReleaseFast, // Must match playground optimization to ensure struct compatibility
     });
 
     builtin_compiler_exe_force.root_module.addImport("base", roc_modules.base);
diff --git a/src/canonicalize/ModuleEnv.zig b/src/canonicalize/ModuleEnv.zig
index ae6094d99f..82223397e1 100644
--- a/src/canonicalize/ModuleEnv.zig
+++ b/src/canonicalize/ModuleEnv.zig
@@ -1385,12 +1385,12 @@ pub fn getSourceLine(self: *const Self, region: Region) ![]const u8 {
     return self.common.getSourceLine(region);
 }
 
-/// Serialized representation of ModuleEnv
+/// Serialized representation of ModuleEnv.
+/// NOTE: Field order matters for cross-platform compatibility! Keep `module_kind` at the end.
 pub const Serialized = struct {
     gpa: [2]u64, // Reserve space for allocator (vtable ptr + context ptr), provided during deserialization
     common: CommonEnv.Serialized,
     types: TypeStore.Serialized,
-    module_kind: ModuleKind, // Must match field order in Self
     all_defs: CIR.Def.Span,
     all_statements: CIR.Statement.Span,
     exports: CIR.Def.Span,
@@ -1400,6 +1400,7 @@ pub const Serialized = struct {
     module_name: [2]u64, // Reserve space for slice (ptr + len), provided during deserialization
     diagnostics: CIR.Diagnostic.Span,
     store: NodeStore.Serialized,
+    module_kind: ModuleKind,
 
     /// Serialize a ModuleEnv into this Serialized struct, appending data to the writer
     pub fn serialize(