diff --git a/fastsim-core/fastsim-proc-macros/src/cumu_method_derive.rs b/fastsim-core/fastsim-proc-macros/src/cumu_method_derive.rs
index ed789c82..2cd1c385 100644
--- a/fastsim-core/fastsim-proc-macros/src/cumu_method_derive.rs
+++ b/fastsim-core/fastsim-proc-macros/src/cumu_method_derive.rs
@@ -15,16 +15,20 @@ pub(crate) fn cumu_method_derive(input: TokenStream) -> TokenStream {
         abort_call_site!("`SetCumulative` works only on Named Field structs.")
     };
 
-    let struct_has_state = fields.iter().any(|x| *x.ident.as_ref().unwrap() == "state");
-    let ident_str = ident.to_string();
-    let struct_is_state = ident_str.contains("State");
+    let struct_is_state = item_struct
+        .attrs
+        .iter()
+        .any(|attr| attr.path().is_ident("is_state"));
+    // A field is recursed into if it's marked `#[has_state]` (its type *contains* nested
+    // state) or `#[is_state]` (its type itself *is* a state struct). Both are handled
+    // identically today; the distinction is kept explicit to allow future divergence.
     let fields_with_state_vec: Vec<bool> = fields
         .iter()
         .map(|field| {
             field
                 .attrs
                 .iter()
-                .any(|attr| attr.path().is_ident("has_state"))
+                .any(|attr| attr.path().is_ident("has_state") || attr.path().is_ident("is_state"))
         })
         .collect();
 
@@ -106,24 +110,6 @@ pub(crate) fn cumu_method_derive(input: TokenStream) -> TokenStream {
                 }
             }
         });
-    } else if struct_has_state {
-        impl_block.extend::<TokenStream2>(quote! {
-            // this tells the compiler that the `SetCumulative` trait is not manually derived
-            #[automatically_derived]
-            impl SetCumulative for #ident {
-                fn set_cumulative<F: Fn() -> String>(&mut self, dt: si::Time, loc: F) -> anyhow::Result<()> {
-                    self.state.set_cumulative(dt, || format!("{}\n{}", loc(), format_dbg!()))?;
-                    #(self.#fields_with_state.set_cumulative(dt, || format!("{}\n{}", loc(), format_dbg!()))?;)*
-                    Ok(())
-                }
-
-                fn reset_cumulative<F: Fn() -> String>(&mut self, loc: F) -> anyhow::Result<()> {
-                    self.state.reset_cumulative(|| format!("{}\n{}", loc(), format_dbg!()))?;
-                    #(self.#fields_with_state.reset_cumulative(|| format!("{}\n{}", loc(), format_dbg!()))?;)*
-                    Ok(())
-                }
-            }
-        });
     } else {
         impl_block.extend::<TokenStream2>(quote! {
             // this tells the compiler that the `SetCumulative` trait is not manually derived
diff --git a/fastsim-core/fastsim-proc-macros/src/lib.rs b/fastsim-core/fastsim-proc-macros/src/lib.rs
index 83c4156a..a4cdbd76 100755
--- a/fastsim-core/fastsim-proc-macros/src/lib.rs
+++ b/fastsim-core/fastsim-proc-macros/src/lib.rs
@@ -39,16 +39,28 @@ pub fn history_vec_derive(input: TokenStream) -> TokenStream {
     history_vec_derive::history_vec_derive(input)
 }
 
-#[proc_macro_derive(StateMethods, attributes(has_state))]
+#[proc_macro_derive(StateMethods, attributes(has_state, is_state))]
 /// Generates remaining `StateMethods` child traits that work for struct and any
-/// nested fields with the `#[has_state]` attribute.
+/// nested fields marked `#[has_state]` or `#[is_state]`.
+///
+/// - `#[is_state]` on a struct itself marks it as a leaf state struct (e.g.
+///   `FooState`), which changes how that struct's own impls are generated.
+/// - `#[is_state]` on a field marks that the field's type *is* a state struct
+///   (e.g. a `state: FooState` field) and should be recursed into.
+/// - `#[has_state]` on a field marks that the field's type merely *contains*
+///   nested state (e.g. a sub-component like `fc: FuelConverter`) and should
+///   be recursed into.
+///
+/// `#[has_state]` and `#[is_state]` on a field are handled identically today;
+/// the distinction is kept explicit so the two cases can diverge later.
 pub fn state_methods_derive(input: TokenStream) -> TokenStream {
     sm_derive::state_methods_derive(input)
 }
 
-#[proc_macro_derive(SetCumulative, attributes(has_state))]
+#[proc_macro_derive(SetCumulative, attributes(has_state, is_state))]
 /// Generate `SetCumulative` trait impl that work for struct and any nested
-/// fields with the `#[has_state]` attribute.
+/// fields marked `#[has_state]` or `#[is_state]`. See [`state_methods_derive`]
+/// for how these attributes are used.
 pub fn cumu_method_derive(input: TokenStream) -> TokenStream {
     cumu_method_derive::cumu_method_derive(input)
 }
diff --git a/fastsim-core/fastsim-proc-macros/src/sm_derive.rs b/fastsim-core/fastsim-proc-macros/src/sm_derive.rs
index eef164f1..a7f36a22 100644
--- a/fastsim-core/fastsim-proc-macros/src/sm_derive.rs
+++ b/fastsim-core/fastsim-proc-macros/src/sm_derive.rs
@@ -1,4 +1,5 @@
 use crate::imports::*;
+use crate::utilities::TokenStreamIterator;
 
 lazy_static! {
     static ref ENERGY_REGEX: Regex = Regex::new(r"energy_(\w+)").unwrap();
@@ -15,23 +16,29 @@ pub(crate) fn state_methods_derive(input: TokenStream) -> TokenStream {
         abort_call_site!("`StateMethods` works only on Named Field structs.")
     };
 
-    let struct_has_state = fields.iter().any(|x| *x.ident.as_ref().unwrap() == "state");
-    let ident_str = ident.to_string();
-    let struct_is_state = ident_str.contains("State");
+    let struct_is_state = item_struct
+        .attrs
+        .iter()
+        .any(|attr| attr.path().is_ident("is_state"));
     let struct_has_save_interval = fields
         .iter()
         .any(|x| *x.ident.as_ref().unwrap() == "save_interval");
+    // A field is recursed into if it's marked `#[has_state]` (its type *contains* nested
+    // state) or `#[is_state]` (its type itself *is* a state struct). Both are handled
+    // identically today; the distinction is kept explicit to allow future divergence.
     let fields_with_state_vec: Vec<bool> = fields
         .iter()
         .map(|field| {
             field
                 .attrs
                 .iter()
-                .any(|attr| attr.path().is_ident("has_state"))
+                .any(|attr| attr.path().is_ident("has_state") || attr.path().is_ident("is_state"))
         })
         .collect();
 
-    // fields that contain nested `state` fields
+    // fields that participate in nested state-tracking, i.e. fields explicitly marked
+    // `#[has_state]` or `#[is_state]` (this includes the primary `state` field itself,
+    // when present, since it must also carry `#[is_state]`)
     let fields_with_state = fields
         .iter()
         .zip(fields_with_state_vec)
@@ -39,21 +46,29 @@ pub(crate) fn state_methods_derive(input: TokenStream) -> TokenStream {
         .map(|(f, _hsv)| f.ident.as_ref().unwrap())
         .collect::<Vec<_>>();
 
+    // whether this struct owns a primary `state: ...` field, as opposed to merely
+    // containing other has_state/is_state sub-component fields
+    let struct_has_state = fields_with_state.iter().any(|f| *f == "state");
+
+    // Types of fields tagged `#[is_state]` (as opposed to `#[has_state]`). Each such type
+    // is asserted below to implement the `IsState` marker trait, which is only implemented
+    // for structs that themselves derive `#[is_state]`. This turns a field mistagged
+    // `#[is_state]` (whose type isn't actually a state struct) into a compile error
+    // instead of silent drift, since both tags are otherwise handled identically by this
+    // derive. It does not catch the opposite mistake (`#[has_state]` on a field whose type
+    // happens to be a state struct) since that's not observably wrong today.
+    let is_state_field_types: Vec<&syn::Type> = fields
+        .iter()
+        .filter(|f| f.attrs.iter().any(|attr| attr.path().is_ident("is_state")))
+        .map(|f| &f.ty)
+        .collect();
+
     let all_fields = fields
         .iter()
         .map(|f| f.ident.as_ref().unwrap())
         .collect::<Vec<_>>();
 
-    let (self_step, self_reset_step): (TokenStream2, TokenStream2) = if struct_has_state {
-        (
-            quote! {
-                self.state.step(|| format!("{}\n{}", loc(), #ident_str))?;
-            },
-            quote! {
-                self.state.reset_step(|| format!("{}\n{}", loc(), #ident_str))?;
-            },
-        )
-    } else if struct_is_state {
+    let (self_step, self_reset_step): (TokenStream2, TokenStream2) = if struct_is_state {
         (
             quote! {
                 self.i.increment(1, || format_dbg!())?;
@@ -107,27 +122,6 @@ pub(crate) fn state_methods_derive(input: TokenStream) -> TokenStream {
                 }
             }
         });
-    } else if struct_has_state {
-        impl_block.extend::<TokenStream2>(quote! {
-            #[automatically_derived]
-            impl TrackedStateMethods for #ident {
-                fn check_and_reset<F: Fn() -> String>(&mut self, loc: F) -> anyhow::Result<()> {
-                    self.state.check_and_reset(|| format!("{}", loc()))?;
-                    #(
-                        self.#fields_with_state.check_and_reset(|| format!("{}\n    field in `{}` has not been updated", loc(), stringify!(#fields_with_state)))?;
-                    )*
-                    Ok(())
-                }
-
-                fn mark_fresh<F: Fn() -> String>(&mut self, loc: F) -> anyhow::Result<()> {
-                    self.state.mark_fresh(|| format!("{}", loc()))?;
-                    #(
-                        self.#fields_with_state.mark_fresh(|| format!("{}\n    field in `{}` has already been updated", loc(), stringify!(#fields_with_state)))?;
-                    )*
-                    Ok(())
-                }
-            }
-        });
     } else {
         impl_block.extend::<TokenStream2>(quote! {
             #[automatically_derived]
@@ -153,6 +147,31 @@ pub(crate) fn state_methods_derive(input: TokenStream) -> TokenStream {
         impl StateMethods for #ident {}
     });
 
+    if struct_is_state {
+        impl_block.extend::<TokenStream2>(quote! {
+            #[automatically_derived]
+            impl IsState for #ident {}
+        });
+    }
+
+    // Compile-time check that every `#[is_state]` field's type actually implements
+    // `IsState` (i.e. that type's own struct derives `StateMethods` with `#[is_state]`
+    // on it). `const _` items are anonymous, so this is safe to emit once per field with
+    // no naming collisions.
+    impl_block.extend::<TokenStream2>(
+        is_state_field_types
+            .iter()
+            .map(|ty| {
+                quote! {
+                    const _: fn() = || {
+                        fn assert_impl_is_state<T: IsState>() {}
+                        assert_impl_is_state::<#ty>();
+                    };
+                }
+            })
+            .concat(),
+    );
+
     if struct_has_save_interval {
         impl_block.extend::<TokenStream2>(quote! {
             #[automatically_derived]
diff --git a/fastsim-core/src/traits.rs b/fastsim-core/src/traits.rs
index 8c0f256a..d2db82b8 100644
--- a/fastsim-core/src/traits.rs
+++ b/fastsim-core/src/traits.rs
@@ -335,6 +335,12 @@ impl<T: Clone + Sub<T, Output = T> + Default> Diff<T> for Vec<T> {
     }
 }
 
+/// Marker trait implemented for leaf state structs (i.e. structs derived with
+/// `#[is_state]`, e.g. `FooState`). Used to give a compile error, rather than silent
+/// drift, when a field tagged `#[is_state]` in `#[derive(StateMethods)]` /
+/// `#[derive(SetCumulative)]` does not actually point at a state struct.
+pub trait IsState {}
+
 /// Super trait to ensure that related traits are implemented together
 pub trait StateMethods: SetCumulative + SaveState + Step + TrackedStateMethods {}
 
diff --git a/fastsim-core/src/vehicle/cabin.rs b/fastsim-core/src/vehicle/cabin.rs
index 3e671f4e..36417c4d 100644
--- a/fastsim-core/src/vehicle/cabin.rs
+++ b/fastsim-core/src/vehicle/cabin.rs
@@ -168,6 +168,7 @@ pub struct LumpedCabin {
     /// cabin width, modeled as a flat plate
     pub width: si::Length,
     #[serde(default)]
+    #[is_state]
     pub state: LumpedCabinState,
     #[serde(default, skip_serializing_if = "LumpedCabinStateHistoryVec::is_empty")]
     pub history: LumpedCabinStateHistoryVec,
@@ -327,6 +328,7 @@ impl LumpedCabin {
 )]
 #[cfg_attr(feature = "pyo3", pyclass(module = "fastsim", subclass, eq))]
 #[serde(deny_unknown_fields)]
+#[is_state]
 pub struct LumpedCabinState {
     /// time step counter
     pub i: TrackedState<usize>,
diff --git a/fastsim-core/src/vehicle/conv.rs b/fastsim-core/src/vehicle/conv.rs
index 26ffcf1c..5c3824d2 100644
--- a/fastsim-core/src/vehicle/conv.rs
+++ b/fastsim-core/src/vehicle/conv.rs
@@ -23,6 +23,7 @@ pub struct DfcoControls {
     pub save_interval: Option<usize>,
     /// current state of control variables
     #[serde(default)]
+    #[is_state]
     pub state: DfcoState,
     /// history of current state
     #[serde(default, skip_serializing_if = "DfcoStateHistoryVec::is_empty")]
@@ -149,6 +150,7 @@ impl DfcoControls {
 )]
 #[non_exhaustive]
 #[serde(deny_unknown_fields)]
+#[is_state]
 pub struct DfcoState {
     /// time step index
     pub i: TrackedState<usize>,
@@ -612,6 +614,7 @@ pub struct ConvStartStopControl {
     pub save_interval: Option<usize>,
     /// current state of control variables
     #[serde(default)]
+    #[is_state]
     pub state: ConvStartStopState,
     /// history of current state
     #[serde(
@@ -741,6 +744,7 @@ impl ConvStartStopControl {
 )]
 #[non_exhaustive]
 #[serde(deny_unknown_fields)]
+#[is_state]
 pub struct ConvStartStopState {
     /// time step index
     pub i: TrackedState<usize>,
diff --git a/fastsim-core/src/vehicle/hev.rs b/fastsim-core/src/vehicle/hev.rs
index b5c88b44..72cdd29a 100644
--- a/fastsim-core/src/vehicle/hev.rs
+++ b/fastsim-core/src/vehicle/hev.rs
@@ -556,6 +556,7 @@ impl Mass for HybridElectricVehicle {
 )]
 #[non_exhaustive]
 #[serde(deny_unknown_fields)]
+#[is_state]
 pub struct RGWDBState {
     /// time step index
     pub i: TrackedState<usize>,
@@ -902,6 +903,7 @@ pub struct RESGreedyWithDynamicBuffers {
     pub temp_fc_allowed_off: Option<si::Temperature>,
     /// current state of control variables
     #[serde(default)]
+    #[is_state]
     pub state: RGWDBState,
     /// history of current state
     #[serde(default, skip_serializing_if = "RGWDBStateHistoryVec::is_empty")]
@@ -1219,6 +1221,7 @@ for an HEV equipped with thermal models or superfluous otherwise",
 )]
 #[non_exhaustive]
 #[serde(deny_unknown_fields)]
+#[is_state]
 pub struct StartStopState {
     /// time step index
     pub i: TrackedState<usize>,
@@ -1295,6 +1298,7 @@ pub struct HEVStartStopControl {
     pub save_interval: Option<usize>,
     /// current state of control variables
     #[serde(default)]
+    #[is_state]
     pub state: StartStopState,
     /// history of current state
     #[serde(default, skip_serializing_if = "StartStopStateHistoryVec::is_empty")]
diff --git a/fastsim-core/src/vehicle/hvac/hvac_sys_for_lumped_cabin.rs b/fastsim-core/src/vehicle/hvac/hvac_sys_for_lumped_cabin.rs
index 73c4e98b..2d1a310b 100644
--- a/fastsim-core/src/vehicle/hvac/hvac_sys_for_lumped_cabin.rs
+++ b/fastsim-core/src/vehicle/hvac/hvac_sys_for_lumped_cabin.rs
@@ -32,6 +32,7 @@ pub struct HVACSystemForLumpedCabin {
     pub pwr_aux_for_hvac_max: si::Power,
     /// coefficient of performance of vapor compression cycle
     #[serde(default)]
+    #[is_state]
     pub state: HVACSystemForLumpedCabinState,
     #[serde(
         default,
@@ -507,6 +508,7 @@ impl SerdeAPI for CabinHeatSource {}
 #[cfg_attr(feature = "pyo3", pyclass(module = "fastsim", subclass, eq))]
 #[serde(default)]
 #[serde(deny_unknown_fields)]
+#[is_state]
 pub struct HVACSystemForLumpedCabinState {
     /// time step counter
     pub i: TrackedState<usize>,
diff --git a/fastsim-core/src/vehicle/hvac/hvac_sys_for_lumped_cabin_and_res.rs b/fastsim-core/src/vehicle/hvac/hvac_sys_for_lumped_cabin_and_res.rs
index f1285c93..43d77f6d 100644
--- a/fastsim-core/src/vehicle/hvac/hvac_sys_for_lumped_cabin_and_res.rs
+++ b/fastsim-core/src/vehicle/hvac/hvac_sys_for_lumped_cabin_and_res.rs
@@ -54,6 +54,7 @@ pub struct HVACSystemForLumpedCabinAndRES {
     pub pwr_aux_for_hvac_res_max: si::Power,
     /// coefficient of performance of vapor compression cycle
     #[serde(default)]
+    #[is_state]
     pub state: HVACSystemForLumpedCabinAndRESState,
     #[serde(
         default,
@@ -1160,6 +1161,7 @@ impl HVACSystemForLumpedCabinAndRES {
 #[serde(default)]
 #[cfg_attr(feature = "pyo3", pyclass(module = "fastsim", subclass, eq))]
 #[serde(deny_unknown_fields)]
+#[is_state]
 pub struct HVACSystemForLumpedCabinAndRESState {
     /// time step counter
     pub i: TrackedState<usize>,
diff --git a/fastsim-core/src/vehicle/powertrain/electric_machine.rs b/fastsim-core/src/vehicle/powertrain/electric_machine.rs
index fe323864..4f2d1292 100755
--- a/fastsim-core/src/vehicle/powertrain/electric_machine.rs
+++ b/fastsim-core/src/vehicle/powertrain/electric_machine.rs
@@ -39,6 +39,7 @@ pub struct ElectricMachine {
     pub save_interval: Option<usize>,
     /// struct for tracking current state
     #[serde(default)]
+    #[is_state]
     pub state: ElectricMachineState,
     /// Custom vector of [Self::state]
     #[serde(
@@ -873,7 +874,7 @@ impl EMBuilder {
 #[serde(default)]
 #[serde(deny_unknown_fields)]
 #[cfg_attr(feature = "pyo3", pyclass(module = "fastsim", subclass, eq))]
-
+#[is_state]
 pub struct ElectricMachineState {
     /// time step index
     pub i: TrackedState<usize>,
diff --git a/fastsim-core/src/vehicle/powertrain/fuel_converter.rs b/fastsim-core/src/vehicle/powertrain/fuel_converter.rs
index e2f8eea3..a9727dd8 100755
--- a/fastsim-core/src/vehicle/powertrain/fuel_converter.rs
+++ b/fastsim-core/src/vehicle/powertrain/fuel_converter.rs
@@ -40,6 +40,7 @@ pub struct FuelConverter {
     pub pwr_idle_fuel: si::Power,
     /// struct for tracking current state
     #[serde(default)]
+    #[is_state]
     pub state: FuelConverterState,
     /// Custom vector of [Self::state]
     #[serde(
@@ -561,6 +562,7 @@ pub struct FCBuilder {
 #[serde(default)]
 #[serde(deny_unknown_fields)]
 #[cfg_attr(feature = "pyo3", pyclass(module = "fastsim", subclass, eq))]
+#[is_state]
 pub struct FuelConverterState {
     /// time step index
     pub i: TrackedState<usize>,
@@ -791,6 +793,7 @@ pub struct FuelConverterThermal {
     pub fc_eff_model: FCTempEffModel,
     /// struct for tracking current state
     #[serde(default)]
+    #[is_state]
     pub state: FuelConverterThermalState,
     /// Custom vector of [Self::state]
     #[serde(
@@ -1137,6 +1140,7 @@ impl Default for FuelConverterThermal {
 #[serde(default)]
 #[cfg_attr(feature = "pyo3", pyclass(module = "fastsim", subclass, eq))]
 #[serde(deny_unknown_fields)]
+#[is_state]
 pub struct FuelConverterThermalState {
     /// time step index
     pub i: TrackedState<usize>,
diff --git a/fastsim-core/src/vehicle/powertrain/reversible_energy_storage.rs b/fastsim-core/src/vehicle/powertrain/reversible_energy_storage.rs
index f96dd07b..aada81da 100644
--- a/fastsim-core/src/vehicle/powertrain/reversible_energy_storage.rs
+++ b/fastsim-core/src/vehicle/powertrain/reversible_energy_storage.rs
@@ -38,6 +38,7 @@ pub struct ReversibleEnergyStorage {
     pub max_soc: si::Ratio,
     /// struct for tracking current state
     #[serde(default)]
+    #[is_state]
     pub state: ReversibleEnergyStorageState,
     /// Custom vector of [Self::state]
     #[serde(
@@ -874,6 +875,7 @@ pub enum SpecificEnergySideEffect {
 #[cfg_attr(feature = "pyo3", pyclass(module = "fastsim", subclass, eq))]
 #[serde(default)]
 /// ReversibleEnergyStorage state variables
+#[is_state]
 pub struct ReversibleEnergyStorageState {
     // limits
     /// max output power for propulsion during positive traction
@@ -1122,6 +1124,7 @@ pub struct RESLumpedThermal {
     pub conductance_to_cab: si::ThermalConductance,
     /// current state
     #[serde(default)]
+    #[is_state]
     pub state: RESLumpedThermalState,
     /// history of state
     #[serde(
@@ -1248,6 +1251,7 @@ impl RESLumpedThermal {
 )]
 #[cfg_attr(feature = "pyo3", pyclass(module = "fastsim", subclass, eq))]
 #[serde(deny_unknown_fields)]
+#[is_state]
 pub struct RESLumpedThermalState {
     /// time step index
     pub i: TrackedState<usize>,
diff --git a/fastsim-core/src/vehicle/powertrain/transmission.rs b/fastsim-core/src/vehicle/powertrain/transmission.rs
index 1d89028b..6c1bfa34 100644
--- a/fastsim-core/src/vehicle/powertrain/transmission.rs
+++ b/fastsim-core/src/vehicle/powertrain/transmission.rs
@@ -15,6 +15,7 @@ pub struct Transmission {
     pub eff_interp: InterpolatorEnum<f64>,
     /// struct for tracking current state
     #[serde(default)]
+    #[is_state]
     pub state: TransmissionState,
     /// Custom vector of [Self::state]
     #[serde(default, skip_serializing_if = "TransmissionStateHistoryVec::is_empty")]
@@ -215,6 +216,7 @@ impl Mass for Transmission {
 #[non_exhaustive]
 #[serde(default)]
 #[serde(deny_unknown_fields)]
+#[is_state]
 pub struct TransmissionState {
     /// time step index
     pub i: TrackedState<usize>,
diff --git a/fastsim-core/src/vehicle/vehicle_model.rs b/fastsim-core/src/vehicle/vehicle_model.rs
index d54dfe37..d768e31d 100644
--- a/fastsim-core/src/vehicle/vehicle_model.rs
+++ b/fastsim-core/src/vehicle/vehicle_model.rs
@@ -75,6 +75,7 @@ pub struct Vehicle {
     pub(crate) save_interval: Option<usize>,
     /// current state of vehicle
     #[serde(default)]
+    #[is_state]
     pub state: VehicleState,
     /// Vector-like history of [Self::state]
     #[serde(default, skip_serializing_if = "VehicleStateHistoryVec::is_empty")]
@@ -1300,6 +1301,7 @@ impl Vehicle {
 #[non_exhaustive]
 #[serde(default)]
 #[serde(deny_unknown_fields)]
+#[is_state]
 pub struct VehicleState {
     /// time step index
     pub i: TrackedState<usize>,