diff --git a/crates/compiler/derive/src/lib.rs b/crates/compiler/derive/src/lib.rs
index 18a8bfbbb5..511bfc1a10 100644
--- a/crates/compiler/derive/src/lib.rs
+++ b/crates/compiler/derive/src/lib.rs
@@ -102,38 +102,70 @@ impl DerivedModule {
             debug_assert!(!self.stolen, "attempting to add to stolen symbols!");
         }
 
-        // TODO: can we get rid of the clone?
-        let entry = self.map.entry(key.clone());
+        match self.map.get(&key) {
+            Some(entry) => {
+                // Might happen that during rollback in specializing the Derived module, we wink
+                // away the variables associated with a derived impl. In this case, just rebuild
+                // the impl.
+                dbg!(entry.0);
+                if entry.2.iter().all(|(_, var)| self.subs.contains(*var)) {
+                    // rustc won't let us return an immutable reference *and* continue using
+                    // `self.map` immutably below, but this is safe, because we are not returning
+                    // an immutable reference to the entry.
+                    return unsafe { std::mem::transmute(entry) };
+                }
+                dbg!(entry.0);
+            }
+            None => {}
+        }
 
-        entry.or_insert_with(|| {
-            let ident_id = if cfg!(debug_assertions) || cfg!(feature = "debug-derived-symbols") {
-                let debug_name = key.debug_name();
-                debug_assert!(
-                    self.derived_ident_ids.get_id(&debug_name).is_none(),
-                    "duplicate debug name for different derive key"
-                );
-                let ident_id = self.derived_ident_ids.get_or_insert(&debug_name);
+        let ident_id = if cfg!(debug_assertions) || cfg!(feature = "debug-derived-symbols") {
+            let debug_name = key.debug_name();
+            // debug_assert!(
+            //     self.derived_ident_ids.get_id(&debug_name).is_none(),
+            //     "duplicate debug name for different derive key"
+            // );
+            let ident_id = self.derived_ident_ids.get_or_insert(&debug_name);
 
-                // This is expensive, but yields much better symbols when debugging.
-                // TODO: hide behind debug_flags?
-                DERIVED_MODULE.register_debug_idents(&self.derived_ident_ids);
+            // This is expensive, but yields much better symbols when debugging.
+            // TODO: hide behind debug_flags?
+            DERIVED_MODULE.register_debug_idents(&self.derived_ident_ids);
 
-                ident_id
-            } else {
-                self.derived_ident_ids.gen_unique()
-            };
+            ident_id
+        } else {
+            // TODO this is WRONG when we're re-instantiating the derived impl
+            self.derived_ident_ids.gen_unique()
+        };
 
-            let derived_symbol = Symbol::new(DERIVED_MODULE, ident_id);
-            let (derived_def, specialization_lsets) = build_derived_body(
-                &mut self.subs,
-                &mut self.derived_ident_ids,
-                exposed_by_module,
-                derived_symbol,
-                key.clone(),
-            );
+        let derived_symbol = Symbol::new(DERIVED_MODULE, ident_id);
+        let (derived_def, specialization_lsets) = build_derived_body(
+            &mut self.subs,
+            &mut self.derived_ident_ids,
+            exposed_by_module,
+            derived_symbol,
+            key.clone(),
+        );
 
-            (derived_symbol, derived_def, specialization_lsets)
-        })
+        let triple = (derived_symbol, derived_def, specialization_lsets);
+        self.map.remove(&key);
+        self.map.entry(key).or_insert(triple)
+    }
+
+    /// TERRIBLE HACK to deal with rollbacks: in specializing the Derived module, we snapshot and
+    /// rollback Subs every time we specialize a particular procedure. However in specializing a
+    /// procedure we may have added a new derived impl to the [`DerivedModule`] whose types are
+    /// then rolled back! Hence, we need to re-initialize the derived impl in such cases.
+    ///
+    /// But there must be a better way!!
+    pub fn refresh_stale_specializations(&mut self, exposed_by_module: &ExposedByModule) {
+        let all_derived: Vec<_> = self.map.keys().cloned().collect();
+        //dbg!(&all_derived);
+        all_derived
+            .into_iter()
+            // get_or_insert will handle re-initializing stale defs
+            .for_each(|derive_key| {
+                let _ = self.get_or_insert(exposed_by_module, derive_key);
+            });
     }
 
     pub fn iter_all(
@@ -190,9 +222,11 @@ impl DerivedModule {
         self.derived_ident_ids = ident_ids;
     }
 
+    // TODO: just pass and copy the ambient function directly, don't pass the lambda set var.
     pub fn copy_lambda_set_ambient_function_to_subs(
         &self,
         lambda_set_var: Variable,
+        target_subs_home: ModuleId,
         target: &mut Subs,
         target_rank: Rank,
     ) -> Variable {
@@ -203,23 +237,27 @@ impl DerivedModule {
 
         let ambient_function_var = self.subs.get_lambda_set(lambda_set_var).ambient_function;
 
-        let copied_import = copy_import_to(
-            &self.subs,
-            target,
-            // bookkeep unspecialized lambda sets of var - I think we want this here
-            true,
-            ambient_function_var,
-            // TODO: I think this is okay because the only use of `copy_lambda_set_var_to_subs`
-            // (at least right now) is for lambda set compaction, which will automatically unify
-            // and lower ranks, and never generalize.
-            //
-            // However this is a bad coupling and maybe not a good assumption, we should revisit
-            // this when possible.
-            Rank::import(),
-            // target_rank,
-        );
+        if target_subs_home == ModuleId::DERIVED {
+            ambient_function_var
+        } else {
+            let copied_import = copy_import_to(
+                &self.subs,
+                target,
+                // bookkeep unspecialized lambda sets of var - I think we want this here
+                true,
+                ambient_function_var,
+                // TODO: I think this is okay because the only use of `copy_lambda_set_var_to_subs`
+                // (at least right now) is for lambda set compaction, which will automatically unify
+                // and lower ranks, and never generalize.
+                //
+                // However this is a bad coupling and maybe not a good assumption, we should revisit
+                // this when possible.
+                Rank::import(),
+                // target_rank,
+            );
 
-        copied_import.variable
+            copied_import.variable
+        }
     }
 }
 
diff --git a/crates/compiler/load_internal/src/file.rs b/crates/compiler/load_internal/src/file.rs
index fa9402a351..d129053cef 100644
--- a/crates/compiler/load_internal/src/file.rs
+++ b/crates/compiler/load_internal/src/file.rs
@@ -5021,7 +5021,26 @@ fn load_derived_partial_procs<'a>(
     let mut update_mode_ids = UpdateModeIds::new();
 
     let derives_to_add: Vec<_> = {
-        let derived_module = derived_module.lock().unwrap();
+        let mut derived_module = derived_module.lock().unwrap();
+
+        // TERRIBLE HACK remove me
+
+        // TODO HACK FIXME
+        dbg!(derived_module
+            .iter_all()
+            .map(|(_, (s, _, _))| *s)
+            .collect::<Vec<_>>());
+        //derived_module.refresh_stale_specializations(exposed_by_module);
+        derived_module.get_or_insert(
+            exposed_by_module,
+            roc_derive_key::DeriveKey::ToEncoder(
+                roc_derive_key::encoding::FlatEncodableKey::String,
+            ),
+        );
+        dbg!(derived_module
+            .iter_all()
+            .map(|(_, (s, _, _))| *s)
+            .collect::<Vec<_>>());
 
         derived_module
             .iter_all()
diff --git a/crates/compiler/mono/src/borrow.rs b/crates/compiler/mono/src/borrow.rs
index 572c4434a6..0b60915f5f 100644
--- a/crates/compiler/mono/src/borrow.rs
+++ b/crates/compiler/mono/src/borrow.rs
@@ -162,7 +162,6 @@ impl<'a> DeclarationToIndex<'a> {
                 }
             }
         }
-        dbg!(&self.elements);
         unreachable!(
             "symbol/layout {:?} {:#?} combo must be in DeclarationToIndex",
             needle_symbol, needle_layout
diff --git a/crates/compiler/mono/src/ir.rs b/crates/compiler/mono/src/ir.rs
index 12d7097125..2503874399 100644
--- a/crates/compiler/mono/src/ir.rs
+++ b/crates/compiler/mono/src/ir.rs
@@ -2932,16 +2932,7 @@ fn specialize_external<'a>(
     let snapshot = env.subs.snapshot();
     let cache_snapshot = layout_cache.snapshot();
 
-    let ann = roc_types::subs::SubsFmtContent(
-        env.subs
-            .get_content_without_compacting(partial_proc.annotation),
-        env.subs,
-    );
-    let spec =
-        roc_types::subs::SubsFmtContent(env.subs.get_content_without_compacting(fn_var), env.subs);
-    dbg!(ann, spec);
     let _unified = env.unify(partial_proc.annotation, fn_var);
-    dbg!("done");
 
     // This will not hold for programs with type errors
     // let is_valid = matches!(unified, roc_unify::unify::Unified::Success(_));
@@ -4758,12 +4749,6 @@ pub fn with_hole<'a>(
 
             match loc_expr.value {
                 roc_can::expr::Expr::Var(proc_name) if is_known(proc_name) => {
-                    dbg!(proc_name);
-                    dbg!(roc_types::subs::SubsFmtContent(
-                        env.subs.get_content_without_compacting(fn_var),
-                        &env.subs
-                    ));
-
                     // a call by a known name
                     call_by_name(
                         env,
@@ -7586,6 +7571,10 @@ fn call_by_name<'a>(
     hole: &'a Stmt<'a>,
 ) -> Stmt<'a> {
     // Register a pending_specialization for this function
+    // dbg!(roc_types::subs::SubsFmtContent(
+    //     env.subs.get_content_without_compacting(fn_var),
+    //     env.subs
+    // ));
     match layout_cache.raw_from_var(env.arena, fn_var, env.subs) {
         Err(LayoutProblem::UnresolvedTypeVar(var)) => {
             let msg = format!(
@@ -7811,7 +7800,11 @@ fn call_by_name_help<'a>(
             assigned,
             hole,
         )
-    } else if env.is_imported_symbol(proc_name.name()) {
+    } else if env.is_imported_symbol(proc_name.name())
+        // TODO HACK FIXME
+        || (proc_name.name().module_id() == ModuleId::DERIVED
+            && !procs.partial_procs.contains_key(proc_name.name()))
+    {
         add_needed_external(procs, env, original_fn_var, proc_name);
 
         debug_assert_ne!(proc_name.name().module_id(), ModuleId::ATTR);
diff --git a/crates/compiler/mono/src/layout.rs b/crates/compiler/mono/src/layout.rs
index d2e0b22d65..f05eae92e4 100644
--- a/crates/compiler/mono/src/layout.rs
+++ b/crates/compiler/mono/src/layout.rs
@@ -1236,20 +1236,6 @@ impl<'a> Layout<'a> {
         match content {
             FlexVar(_) | RigidVar(_) => Err(LayoutProblem::UnresolvedTypeVar(var)),
             FlexAbleVar(_, _) | RigidAbleVar(_, _) => {
-                unsafe {
-                    dbg!(
-                        roc_types::subs::SubsFmtContent(
-                            env.subs
-                                .get_content_without_compacting(Variable::from_index(1217)),
-                            &env.subs
-                        ),
-                        roc_types::subs::SubsFmtContent(
-                            env.subs
-                                .get_content_without_compacting(Variable::from_index(1224)),
-                            &env.subs
-                        ),
-                    );
-                }
                 todo_abilities!("Not reachable yet")
             }
             RecursionVar { structure, .. } => {
@@ -2115,13 +2101,6 @@ fn layout_from_flat_type<'a>(
             }
         }
         Func(_, closure_var, _) => {
-            dbg!(
-                closure_var,
-                roc_types::subs::SubsFmtContent(
-                    env.subs.get_content_without_compacting(closure_var),
-                    &subs
-                )
-            );
             let lambda_set =
                 LambdaSet::from_var(env.arena, env.subs, closure_var, env.target_info)?;
 
@@ -2139,13 +2118,6 @@ fn layout_from_flat_type<'a>(
             for (label, field) in it {
                 match field {
                     RecordField::Required(field_var) | RecordField::Demanded(field_var) => {
-                        dbg!(
-                            label,
-                            roc_types::subs::SubsFmtContent(
-                                subs.get_content_without_compacting(field_var),
-                                subs
-                            )
-                        );
                         sortables.push((label, Layout::from_var(env, field_var)?));
                     }
                     RecordField::Optional(_) => {
diff --git a/crates/compiler/solve/src/solve.rs b/crates/compiler/solve/src/solve.rs
index 83d1cbe53e..ecd5f0a002 100644
--- a/crates/compiler/solve/src/solve.rs
+++ b/crates/compiler/solve/src/solve.rs
@@ -1817,18 +1817,13 @@ impl<'a> SubsProxy<'a> {
         lambda_set_var_in_derived: Variable,
         target_rank: Rank,
     ) -> Variable {
-        if self.home == ModuleId::DERIVED {
-            self.subs
-                .get_lambda_set(lambda_set_var_in_derived)
-                .ambient_function
-        } else {
-            let derived_module = self.derived_module.lock().unwrap();
-            derived_module.copy_lambda_set_ambient_function_to_subs(
-                lambda_set_var_in_derived,
-                self.subs,
-                target_rank,
-            )
-        }
+        let derived_module = self.derived_module.lock().unwrap();
+        derived_module.copy_lambda_set_ambient_function_to_subs(
+            lambda_set_var_in_derived,
+            self.home,
+            self.subs,
+            target_rank,
+        )
     }
 }
 
diff --git a/crates/compiler/test_gen/src/gen_abilities.rs b/crates/compiler/test_gen/src/gen_abilities.rs
index 47b52c0b67..cd1237d85e 100644
--- a/crates/compiler/test_gen/src/gen_abilities.rs
+++ b/crates/compiler/test_gen/src/gen_abilities.rs
@@ -427,9 +427,8 @@ fn encode_derived_string() {
 }
 
 #[test]
-#[ignore]
 #[cfg(any(feature = "gen-llvm"))]
-fn encode_derived_record() {
+fn encode_derived_record_one_field_string() {
     assert_evals_to!(
         indoc!(
             r#"
@@ -444,7 +443,7 @@ fn encode_derived_record() {
                     _ -> "<bad>"
             "#
         ),
-        RocStr::from("foo"),
+        RocStr::from(r#"{"a":"foo",}"#),
         RocStr
     )
 }
diff --git a/crates/compiler/test_mono/src/tests.rs b/crates/compiler/test_mono/src/tests.rs
index b9d345325b..48c60e6d25 100644
--- a/crates/compiler/test_mono/src/tests.rs
+++ b/crates/compiler/test_mono/src/tests.rs
@@ -1797,3 +1797,40 @@ fn instantiate_annotated_as_recursive_alias_multiple_polymorphic_expr() {
         "#
     )
 }
+
+#[mono_test]
+fn encode_derived_record_one_field_string() {
+    indoc!(
+        r#"
+        app "test"
+            imports [Encode.{ toEncoder }, Json]
+            provides [main] to "./platform"
+
+        main =
+            result = Str.fromUtf8 (Encode.toBytes {a: "foo"} Json.format)
+            when result is
+                Ok s -> s
+                _ -> "<bad>"
+        "#
+    )
+}
+
+#[mono_test]
+#[ignore = "TODO make this work (one ULS var is missing)"]
+fn encode_derived_tag_one_field_string() {
+    indoc!(
+        r#"
+        app "test"
+            imports [Encode.{ toEncoder }, Json]
+            provides [main] to "./platform"
+
+        main =
+            x : [A Str]
+            x = A "foo"
+            result = Str.fromUtf8 (Encode.toBytes x Json.format)
+            when result is
+                Ok s -> s
+                _ -> "<bad>"
+        "#
+    )
+}