diff --git a/crates/compiler/derive/src/lib.rs b/crates/compiler/derive/src/lib.rs
index b947a677ed..4d9bf1ce50 100644
--- a/crates/compiler/derive/src/lib.rs
+++ b/crates/compiler/derive/src/lib.rs
@@ -3,6 +3,8 @@
 use std::iter::once;
 use std::sync::{Arc, Mutex};
 
+use roc_can::abilities::SpecializationLambdaSets;
+use roc_can::expr::Expr;
 use roc_can::pattern::Pattern;
 use roc_can::{def::Def, module::ExposedByModule};
 use roc_collections::MutMap;
@@ -31,7 +33,7 @@ pub fn synth_var(subs: &mut Subs, content: Content) -> Variable {
 /// Map of [`DeriveKey`]s to their derived symbols.
 #[derive(Debug, Default)]
 pub struct DerivedModule {
-    map: MutMap<DeriveKey, (Symbol, Def)>,
+    map: MutMap<DeriveKey, (Symbol, Def, SpecializationLambdaSets)>,
     subs: Subs,
     derived_ident_ids: IdentIds,
 
@@ -45,14 +47,25 @@ pub struct StolenFromDerived {
     pub ident_ids: IdentIds,
 }
 
+pub(crate) struct DerivedBody {
+    pub body: Expr,
+    pub body_type: Variable,
+    /// mapping of lambda set region -> the specialization lambda set for this derived body
+    pub specialization_lambda_sets: SpecializationLambdaSets,
+}
+
 fn build_derived_body(
     derived_subs: &mut Subs,
     derived_ident_ids: &mut IdentIds,
     exposed_by_module: &ExposedByModule,
     derived_symbol: Symbol,
     derive_key: DeriveKey,
-) -> Def {
-    let (body, var) = match derive_key {
+) -> (Def, SpecializationLambdaSets) {
+    let DerivedBody {
+        body,
+        body_type,
+        specialization_lambda_sets,
+    } = match derive_key {
         DeriveKey::ToEncoder(to_encoder_key) => {
             let mut env = encoding::Env {
                 subs: derived_subs,
@@ -64,13 +77,15 @@ fn build_derived_body(
         DeriveKey::Decoding => todo!(),
     };
 
-    Def {
+    let def = Def {
         loc_pattern: Loc::at_zero(Pattern::Identifier(derived_symbol)),
         loc_expr: Loc::at_zero(body),
-        expr_var: var,
-        pattern_vars: once((derived_symbol, var)).collect(),
+        expr_var: body_type,
+        pattern_vars: once((derived_symbol, body_type)).collect(),
         annotation: None,
-    }
+    };
+
+    (def, specialization_lambda_sets)
 }
 
 impl DerivedModule {
@@ -78,7 +93,7 @@ impl DerivedModule {
         &mut self,
         exposed_by_module: &ExposedByModule,
         key: DeriveKey,
-    ) -> &(Symbol, Def) {
+    ) -> &(Symbol, Def, SpecializationLambdaSets) {
         #[cfg(debug_assertions)]
         {
             debug_assert!(!self.stolen, "attempting to add to stolen symbols!");
@@ -106,7 +121,7 @@ impl DerivedModule {
             };
 
             let derived_symbol = Symbol::new(DERIVED_MODULE, ident_id);
-            let derived_def = build_derived_body(
+            let (derived_def, specialization_lsets) = build_derived_body(
                 &mut self.subs,
                 &mut self.derived_ident_ids,
                 exposed_by_module,
@@ -114,11 +129,13 @@ impl DerivedModule {
                 key.clone(),
             );
 
-            (derived_symbol, derived_def)
+            (derived_symbol, derived_def, specialization_lsets)
         })
     }
 
-    pub fn iter_all(&self) -> impl Iterator<Item = (&DeriveKey, &(Symbol, Def))> {
+    pub fn iter_all(
+        &self,
+    ) -> impl Iterator<Item = (&DeriveKey, &(Symbol, Def, SpecializationLambdaSets))> {
         self.map.iter()
     }