diff --git a/Cargo.lock b/Cargo.lock
index 32f2fcf..757924e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -161,7 +161,7 @@ dependencies = [
 
 [[package]]
 name = "embedded-command-macros"
-version = "0.5.1"
+version = "0.5.2"
 dependencies = [
  "Inflector",
  "proc-macro2",
@@ -367,7 +367,7 @@ dependencies = [
  "cortex-m-rt",
  "defmt",
  "derive_more",
- "embedded-command-macros 0.5.1",
+ "embedded-command-macros 0.5.2",
  "fill-array",
  "heapless",
  "itertools",
diff --git a/macros/Cargo.toml b/macros/Cargo.toml
index 2ea9958..5d66a36 100644
--- a/macros/Cargo.toml
+++ b/macros/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "embedded-command-macros"
-version = "0.5.1"
+version = "0.5.2"
 edition = "2024"
 description = "Macros for the embedded command crate family."
 license = "CC-BY-NC-SA-4.0"
diff --git a/macros/src/serac/vanilla.rs b/macros/src/serac/vanilla.rs
index e167d9f..1ce60ea 100644
--- a/macros/src/serac/vanilla.rs
+++ b/macros/src/serac/vanilla.rs
@@ -134,7 +134,7 @@ fn serialize_struct(s: DataStruct, info: &BodyInfo) -> TokenStream2 {
     }
 }
 
-fn size_of_struct(s: DataStruct, info: &BodyInfo) -> (TokenStream2, HashSet<Type>) {
+fn size_of_struct(s: DataStruct, info: &BodyInfo) -> (TokenStream2, Vec<Type>) {
     let types: Vec<_> = s.fields.iter().map(|field| field.ty.clone()).collect();
     let path = &info.path;
 
@@ -144,10 +144,18 @@ fn size_of_struct(s: DataStruct, info: &BodyInfo) -> (TokenStream2, HashSet<Type
         } else {
             quote! { #( <#types as #path::Size>::SIZE )+* }
         },
-        HashSet::from_iter(types),
+        first_occurrences(types),
     )
 }
 
+/// Keeps the first occurrence of each type, in order, so the generated `where` bounds are the same
+/// on every run.
+fn first_occurrences(mut types: Vec<Type>) -> Vec<Type> {
+    let mut seen = HashSet::new();
+    types.retain(|ty| seen.insert(ty.clone()));
+    types
+}
+
 fn serialize_enum(e: DataEnum, info: &BodyInfo, repr: Type) -> TokenStream2 {
     let implementer = &info.ident;
     let path = &info.path;
@@ -309,8 +317,8 @@ fn serialize_enum(e: DataEnum, info: &BodyInfo, repr: Type) -> TokenStream2 {
     }
 }
 
-fn size_of_enum(e: DataEnum, info: &BodyInfo, repr: Type) -> (TokenStream2, HashSet<Type>) {
-    let mut types = HashSet::new();
+fn size_of_enum(e: DataEnum, info: &BodyInfo, repr: Type) -> (TokenStream2, Vec<Type>) {
+    let mut types = Vec::new();
 
     let path = &info.path;
     let sizes: Vec<_> = e
@@ -345,7 +353,7 @@ fn size_of_enum(e: DataEnum, info: &BodyInfo, repr: Type) -> (TokenStream2, Hash
 
             max + <#repr as #path::Size>::SIZE
         }},
-        types,
+        first_occurrences(types),
     )
 }
 
@@ -405,3 +413,78 @@ pub fn impl_size(item: TokenStream) -> TokenStream {
     }
     .into()
 }
+
+#[cfg(test)]
+mod tests {
+    use syn::{Data, DeriveInput, Type, parse_quote};
+
+    use super::{size_of_enum, size_of_struct};
+    use crate::serac::BodyInfo;
+
+    #[test]
+    fn struct_bounds_follow_field_order() {
+        let item: DeriveInput = parse_quote! {
+            struct S {
+                a: A,
+                b: B,
+                c: C,
+                d: D,
+                e: E,
+                f: F,
+                g: G,
+                h: H,
+                a_again: A,
+            }
+        };
+        let info = info(&item);
+        let Data::Struct(s) = item.data else {
+            unreachable!("expected item to be a struct")
+        };
+
+        let (.., types) = size_of_struct(s, &info);
+
+        assert_eq!(types, a_to_h());
+    }
+
+    #[test]
+    fn enum_bounds_follow_variant_order() {
+        let item: DeriveInput = parse_quote! {
+            #[repr(u8)]
+            enum E {
+                V0(A, B),
+                V1,
+                V2 { c: C, d: D },
+                V3(E, F, G, H, A),
+            }
+        };
+        let info = info(&item);
+        let Data::Enum(e) = item.data else {
+            unreachable!("expected item to be an enum")
+        };
+
+        let (.., types) = size_of_enum(e, &info, parse_quote! { u8 });
+
+        assert_eq!(types, a_to_h());
+    }
+
+    fn info(item: &DeriveInput) -> BodyInfo {
+        BodyInfo {
+            ident: item.ident.clone(),
+            generics: item.generics.clone(),
+            path: parse_quote! { serac },
+        }
+    }
+
+    fn a_to_h() -> Vec<Type> {
+        vec![
+            parse_quote! { A },
+            parse_quote! { B },
+            parse_quote! { C },
+            parse_quote! { D },
+            parse_quote! { E },
+            parse_quote! { F },
+            parse_quote! { G },
+            parse_quote! { H },
+        ]
+    }
+}