diff --git a/packages/cultcache-rs/cultcache-rs-derive/src/lib.rs b/packages/cultcache-rs/cultcache-rs-derive/src/lib.rs
index 01e46f52..d172e4fd 100644
--- a/packages/cultcache-rs/cultcache-rs-derive/src/lib.rs
+++ b/packages/cultcache-rs/cultcache-rs-derive/src/lib.rs
@@ -12,6 +12,13 @@ use syn::parse_macro_input;
 #[proc_macro_derive(DatabaseEntry, attributes(cultcache))]
 pub fn derive_database_entry(input: TokenStream) -> TokenStream {
     let input = parse_macro_input!(input as DeriveInput);
+    match database_entry_codec(input) {
+        Ok(tokens) => tokens.into(),
+        Err(error) => error.to_compile_error().into(),
+    }
+}
+
+fn database_entry_codec(input: DeriveInput) -> syn::Result<proc_macro2::TokenStream> {
     let ident = input.ident.clone();
     let mut entry_type: Option<LitStr> = None;
     let mut schema_name: Option<LitStr> = None;
@@ -34,36 +41,28 @@ pub fn derive_database_entry(input: TokenStream) -> TokenStream {
                 Err(meta.error("unsupported cultcache attribute"))
             }
         });
-        if let Err(error) = parse_result {
-            return error.to_compile_error().into();
-        }
+        parse_result?;
     }
 
     let Some(entry_type) = entry_type else {
-        return syn::Error::new_spanned(
+        return Err(syn::Error::new_spanned(
             ident,
             "DatabaseEntry derive requires #[cultcache(type = \"...\")]",
-        )
-        .to_compile_error()
-        .into();
+        ));
     };
     let schema_name = schema_name.unwrap_or_else(|| LitStr::new(&ident.to_string(), ident.span()));
 
-    let slots = match named_slots(input.data, &ident, "DatabaseEntry") {
-        Ok(slots) => slots,
-        Err(error) => return error.to_compile_error().into(),
-    };
+    let slots = named_slots(input.data, &ident, "DatabaseEntry")?;
     let codec = slot_codec(&ident, &slots);
 
-    quote! {
+    Ok(quote! {
         impl ::cultcache_rs::DatabaseEntry for #ident {
             const TYPE: &'static str = #entry_type;
             const SCHEMA_NAME: &'static str = #schema_name;
         }
 
         #codec
-    }
-    .into()
+    })
 }
 
 /// A named struct whose fields carry `#[cultcache(key = N)]`: the same positional
@@ -72,18 +71,21 @@ pub fn derive_database_entry(input: TokenStream) -> TokenStream {
 #[proc_macro_derive(CultSlots, attributes(cultcache))]
 pub fn derive_cult_slots(input: TokenStream) -> TokenStream {
     let input = parse_macro_input!(input as DeriveInput);
-    let ident = input.ident.clone();
-    if !input.generics.params.is_empty() {
-        return syn::Error::new_spanned(ident, "CultSlots derive does not support generics")
-            .to_compile_error()
-            .into();
-    }
-    match named_slots(input.data, &ident, "CultSlots") {
-        Ok(slots) => slot_codec(&ident, &slots).into(),
+    match cult_slots_codec(input) {
+        Ok(tokens) => tokens.into(),
         Err(error) => error.to_compile_error().into(),
     }
 }
 
+fn cult_slots_codec(input: DeriveInput) -> syn::Result<proc_macro2::TokenStream> {
+    let ident = input.ident.clone();
+    if !input.generics.params.is_empty() {
+        return Err(syn::Error::new_spanned(ident, "CultSlots derive does not support generics"));
+    }
+    let slots = named_slots(input.data, &ident, "CultSlots")?;
+    Ok(slot_codec(&ident, &slots))
+}
+
 /// An enum written as `[key, armSlots]`. Every variant carries `#[cultcache(key = N)]`
 /// and is a unit (`[N, []]`) or a newtype over a type that serializes as an array.
 /// A key the enum does not declare is a decode error; there is no catch-all arm.
@@ -96,6 +98,11 @@ pub fn derive_cult_union(input: TokenStream) -> TokenStream {
     }
 }
 
+/// The largest array16 length: a slot record is written as a dense array, so a key above it would
+/// make every encode allocate and write an unbounded run of nils. Every runtime and the C# catalog
+/// refuse the same domain.
+const MAX_SLOT_KEY: usize = 65535;
+
 fn named_slots(data: Data, ident: &syn::Ident, owner: &str) -> syn::Result<Vec<CultCacheSlot>> {
     let fields = match data {
         Data::Struct(data) => match data.fields {
@@ -135,7 +142,14 @@ fn named_slots(data: Data, ident: &syn::Ident, owner: &str) -> syn::Result<Vec<C
                 if meta.path.is_ident("key") {
                     once(&key, &meta, "key")?;
                     let value: LitInt = meta.value()?.parse()?;
-                    key = Some(value.base10_parse()?);
+                    let slot: usize = value.base10_parse()?;
+                    if slot > MAX_SLOT_KEY {
+                        return Err(syn::Error::new_spanned(
+                            &value,
+                            format!("{owner} slot key {slot} is outside 0..={MAX_SLOT_KEY}"),
+                        ));
+                    }
+                    key = Some(slot);
                     Ok(())
                 } else if meta.path.is_ident("default") {
                     default = true;
@@ -261,12 +275,14 @@ fn slot_codec(ident: &syn::Ident, slots: &[CultCacheSlot]) -> proc_macro2::Token
     });
 
     quote! {
-        impl ::cultcache_rs::SlotRecord for #ident {}
+        impl ::cultcache_rs::SlotRecord for #ident {
+            fn __slots_array() {}
+        }
 
         impl ::serde::Serialize for #ident {
-            fn serialize<S>(&self, serializer: S) -> ::std::result::Result<S::Ok, S::Error>
+            fn serialize<__S>(&self, serializer: __S) -> ::std::result::Result<__S::Ok, __S::Error>
             where
-                S: ::serde::Serializer,
+                __S: ::serde::Serializer,
             {
                 use ::serde::ser::SerializeTuple;
                 let mut tuple = serializer.serialize_tuple(#max_slot + 1)?;
@@ -276,22 +292,22 @@ fn slot_codec(ident: &syn::Ident, slots: &[CultCacheSlot]) -> proc_macro2::Token
         }
 
         impl<'de> ::serde::Deserialize<'de> for #ident {
-            fn deserialize<D>(deserializer: D) -> ::std::result::Result<Self, D::Error>
+            fn deserialize<__D>(deserializer: __D) -> ::std::result::Result<Self, __D::Error>
             where
-                D: ::serde::Deserializer<'de>,
+                __D: ::serde::Deserializer<'de>,
             {
-                struct CultCacheVisitor;
+                struct __CultCacheVisitor;
 
-                impl<'de> ::serde::de::Visitor<'de> for CultCacheVisitor {
+                impl<'de> ::serde::de::Visitor<'de> for __CultCacheVisitor {
                     type Value = #ident;
 
                     fn expecting(&self, formatter: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
                         formatter.write_str(concat!("a CultCache array for ", stringify!(#ident)))
                     }
 
-                    fn visit_seq<A>(self, mut seq: A) -> ::std::result::Result<Self::Value, A::Error>
+                    fn visit_seq<__A>(self, mut seq: __A) -> ::std::result::Result<Self::Value, __A::Error>
                     where
-                        A: ::serde::de::SeqAccess<'de>,
+                        __A: ::serde::de::SeqAccess<'de>,
                     {
                         #(#option_declarations)*
                         let mut index = 0usize;
@@ -309,7 +325,7 @@ fn slot_codec(ident: &syn::Ident, slots: &[CultCacheSlot]) -> proc_macro2::Token
                     }
                 }
 
-                deserializer.deserialize_tuple(#max_slot + 1, CultCacheVisitor)
+                deserializer.deserialize_tuple(#max_slot + 1, __CultCacheVisitor)
             }
         }
     }
@@ -424,6 +440,7 @@ fn union_codec(input: DeriveInput) -> syn::Result<proc_macro2::TokenStream> {
     let unknown_message = format!("unknown arm key {{}} for union {enum_name}; declared keys: {declared}");
     let arity_message = format!("union {enum_name} is not a [key, slots] pair");
     let non_integer_message = format!("union {enum_name} has a non-integer arm key; declared keys: {declared}");
+    let nil_slots_message = format!("union {enum_name} arm {{}} has nil slots; declared keys: {declared}");
     let out_of_range_message = format!("union {enum_name} has an arm key outside Int32; declared keys: {declared}");
     let payload_checks = arms.iter().filter_map(|arm| arm.payload.as_ref()).map(|ty| {
         quote! { assert_slot_record::<#ty>(); }
@@ -455,19 +472,37 @@ fn union_codec(input: DeriveInput) -> syn::Result<proc_macro2::TokenStream> {
         if let Some(ty) = &arm.payload {
             quote! {
                 #key => {
-                    let arm = seq
-                        .next_element::<#ty>()?
-                        .ok_or_else(|| ::serde::de::Error::custom(#arity_message))?;
-                    #ident::#variant(arm)
+                    // A nil in the slots position is not an empty record.
+                    match seq.next_element::<::std::option::Option<#ty>>()? {
+                        ::std::option::Option::Some(::std::option::Option::Some(arm)) => #ident::#variant(arm),
+                        ::std::option::Option::Some(::std::option::Option::None) => {
+                            return ::std::result::Result::Err(::serde::de::Error::custom(
+                                format!(#nil_slots_message, #key),
+                            ));
+                        }
+                        ::std::option::Option::None => {
+                            return ::std::result::Result::Err(::serde::de::Error::custom(#arity_message));
+                        }
+                    }
                 }
             }
         } else {
             quote! {
                 #key => {
-                    seq.next_element::<[(); 0]>()
+                    match seq
+                        .next_element::<::std::option::Option<[(); 0]>>()
                         .map_err(|_| ::serde::de::Error::custom(#arity_message))?
-                        .ok_or_else(|| ::serde::de::Error::custom(#arity_message))?;
-                    #ident::#variant
+                    {
+                        ::std::option::Option::Some(::std::option::Option::Some(_)) => #ident::#variant,
+                        ::std::option::Option::Some(::std::option::Option::None) => {
+                            return ::std::result::Result::Err(::serde::de::Error::custom(
+                                format!(#nil_slots_message, #key),
+                            ));
+                        }
+                        ::std::option::Option::None => {
+                            return ::std::result::Result::Err(::serde::de::Error::custom(#arity_message));
+                        }
+                    }
                 }
             }
         }
@@ -483,9 +518,9 @@ fn union_codec(input: DeriveInput) -> syn::Result<proc_macro2::TokenStream> {
         };
 
         impl ::serde::Serialize for #ident {
-            fn serialize<S>(&self, serializer: S) -> ::std::result::Result<S::Ok, S::Error>
+            fn serialize<__S>(&self, serializer: __S) -> ::std::result::Result<__S::Ok, __S::Error>
             where
-                S: ::serde::Serializer,
+                __S: ::serde::Serializer,
             {
                 use ::serde::ser::SerializeTuple;
                 let mut tuple = serializer.serialize_tuple(2)?;
@@ -497,128 +532,137 @@ fn union_codec(input: DeriveInput) -> syn::Result<proc_macro2::TokenStream> {
         }
 
         impl<'de> ::serde::Deserialize<'de> for #ident {
-            fn deserialize<D>(deserializer: D) -> ::std::result::Result<Self, D::Error>
+            fn deserialize<__D>(deserializer: __D) -> ::std::result::Result<Self, __D::Error>
             where
-                D: ::serde::Deserializer<'de>,
+                __D: ::serde::Deserializer<'de>,
             {
                 // The reference classifies the key before the arm: an integer in Int32, an
                 // integer outside it, or not an integer. No input value is kept or echoed.
-                enum ArmKey {
+                enum __ArmKey {
                     Int(i64),
                     OutOfRange,
                     NonInteger,
                 }
-                struct ArmKeyProbe(ArmKey);
-                struct ArmKeyVisitor;
+                struct __ArmKeyProbe(__ArmKey);
+                struct __ArmKeyVisitor;
 
-                impl<'de> ::serde::de::Visitor<'de> for ArmKeyVisitor {
-                    type Value = ArmKeyProbe;
+                impl<'de> ::serde::de::Visitor<'de> for __ArmKeyVisitor {
+                    type Value = __ArmKeyProbe;
 
                     fn expecting(&self, formatter: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
                         formatter.write_str("an integer arm key")
                     }
 
-                    fn visit_i64<E>(self, value: i64) -> ::std::result::Result<Self::Value, E> {
-                        Ok(ArmKeyProbe(ArmKey::Int(value)))
+                    fn visit_i64<__E>(self, value: i64) -> ::std::result::Result<Self::Value, __E> {
+                        Ok(__ArmKeyProbe(__ArmKey::Int(value)))
                     }
 
-                    fn visit_u64<E>(self, value: u64) -> ::std::result::Result<Self::Value, E> {
-                        Ok(ArmKeyProbe(match i64::try_from(value) {
-                            Ok(value) => ArmKey::Int(value),
-                            Err(_) => ArmKey::OutOfRange,
+                    fn visit_u64<__E>(self, value: u64) -> ::std::result::Result<Self::Value, __E> {
+                        Ok(__ArmKeyProbe(match i64::try_from(value) {
+                            Ok(value) => __ArmKey::Int(value),
+                            Err(_) => __ArmKey::OutOfRange,
                         }))
                     }
 
-                    fn visit_bool<E>(self, _: bool) -> ::std::result::Result<Self::Value, E> {
-                        Ok(ArmKeyProbe(ArmKey::NonInteger))
+                    fn visit_bool<__E>(self, _: bool) -> ::std::result::Result<Self::Value, __E> {
+                        Ok(__ArmKeyProbe(__ArmKey::NonInteger))
                     }
 
-                    fn visit_f64<E>(self, _: f64) -> ::std::result::Result<Self::Value, E> {
-                        Ok(ArmKeyProbe(ArmKey::NonInteger))
+                    fn visit_f64<__E>(self, _: f64) -> ::std::result::Result<Self::Value, __E> {
+                        Ok(__ArmKeyProbe(__ArmKey::NonInteger))
                     }
 
-                    fn visit_str<E>(self, _: &str) -> ::std::result::Result<Self::Value, E> {
-                        Ok(ArmKeyProbe(ArmKey::NonInteger))
+                    fn visit_str<__E>(self, _: &str) -> ::std::result::Result<Self::Value, __E> {
+                        Ok(__ArmKeyProbe(__ArmKey::NonInteger))
                     }
 
-                    fn visit_bytes<E>(self, _: &[u8]) -> ::std::result::Result<Self::Value, E> {
-                        Ok(ArmKeyProbe(ArmKey::NonInteger))
+                    fn visit_bytes<__E>(self, _: &[u8]) -> ::std::result::Result<Self::Value, __E> {
+                        Ok(__ArmKeyProbe(__ArmKey::NonInteger))
                     }
 
-                    fn visit_unit<E>(self) -> ::std::result::Result<Self::Value, E> {
-                        Ok(ArmKeyProbe(ArmKey::NonInteger))
+                    fn visit_unit<__E>(self) -> ::std::result::Result<Self::Value, __E> {
+                        Ok(__ArmKeyProbe(__ArmKey::NonInteger))
                     }
 
-                    fn visit_none<E>(self) -> ::std::result::Result<Self::Value, E> {
-                        Ok(ArmKeyProbe(ArmKey::NonInteger))
+                    fn visit_none<__E>(self) -> ::std::result::Result<Self::Value, __E> {
+                        Ok(__ArmKeyProbe(__ArmKey::NonInteger))
                     }
 
-                    fn visit_some<D>(self, deserializer: D) -> ::std::result::Result<Self::Value, D::Error>
+                    fn visit_some<__D>(self, deserializer: __D) -> ::std::result::Result<Self::Value, __D::Error>
                     where
-                        D: ::serde::Deserializer<'de>,
+                        __D: ::serde::Deserializer<'de>,
                     {
                         <::serde::de::IgnoredAny as ::serde::Deserialize>::deserialize(deserializer)?;
-                        Ok(ArmKeyProbe(ArmKey::NonInteger))
+                        Ok(__ArmKeyProbe(__ArmKey::NonInteger))
                     }
 
-                    fn visit_seq<A>(self, mut seq: A) -> ::std::result::Result<Self::Value, A::Error>
+                    // A MessagePack ext value (fixext, timestamp, ext8) reaches the visitor here.
+                    fn visit_newtype_struct<__D>(self, deserializer: __D) -> ::std::result::Result<Self::Value, __D::Error>
                     where
-                        A: ::serde::de::SeqAccess<'de>,
+                        __D: ::serde::Deserializer<'de>,
+                    {
+                        <::serde::de::IgnoredAny as ::serde::Deserialize>::deserialize(deserializer)?;
+                        Ok(__ArmKeyProbe(__ArmKey::NonInteger))
+                    }
+
+                    fn visit_seq<__A>(self, mut seq: __A) -> ::std::result::Result<Self::Value, __A::Error>
+                    where
+                        __A: ::serde::de::SeqAccess<'de>,
                     {
                         while seq.next_element::<::serde::de::IgnoredAny>()?.is_some() {}
-                        Ok(ArmKeyProbe(ArmKey::NonInteger))
+                        Ok(__ArmKeyProbe(__ArmKey::NonInteger))
                     }
 
-                    fn visit_map<A>(self, mut map: A) -> ::std::result::Result<Self::Value, A::Error>
+                    fn visit_map<__A>(self, mut map: __A) -> ::std::result::Result<Self::Value, __A::Error>
                     where
-                        A: ::serde::de::MapAccess<'de>,
+                        __A: ::serde::de::MapAccess<'de>,
                     {
                         while map.next_entry::<::serde::de::IgnoredAny, ::serde::de::IgnoredAny>()?.is_some() {}
-                        Ok(ArmKeyProbe(ArmKey::NonInteger))
+                        Ok(__ArmKeyProbe(__ArmKey::NonInteger))
                     }
                 }
 
-                impl<'de> ::serde::Deserialize<'de> for ArmKeyProbe {
-                    fn deserialize<D>(deserializer: D) -> ::std::result::Result<Self, D::Error>
+                impl<'de> ::serde::Deserialize<'de> for __ArmKeyProbe {
+                    fn deserialize<__D>(deserializer: __D) -> ::std::result::Result<Self, __D::Error>
                     where
-                        D: ::serde::Deserializer<'de>,
+                        __D: ::serde::Deserializer<'de>,
                     {
-                        deserializer.deserialize_any(ArmKeyVisitor)
+                        deserializer.deserialize_any(__ArmKeyVisitor)
                     }
                 }
 
-                struct CultUnionVisitor;
+                struct __CultUnionVisitor;
 
-                impl<'de> ::serde::de::Visitor<'de> for CultUnionVisitor {
+                impl<'de> ::serde::de::Visitor<'de> for __CultUnionVisitor {
                     type Value = #ident;
 
                     fn expecting(&self, formatter: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
                         formatter.write_str(concat!("a [key, slots] pair for union ", stringify!(#ident)))
                     }
 
-                    fn visit_seq<A>(self, mut seq: A) -> ::std::result::Result<Self::Value, A::Error>
+                    fn visit_seq<__A>(self, mut seq: __A) -> ::std::result::Result<Self::Value, __A::Error>
                     where
-                        A: ::serde::de::SeqAccess<'de>,
+                        __A: ::serde::de::SeqAccess<'de>,
                     {
                         if seq.size_hint().is_some_and(|len| len != 2) {
                             return ::std::result::Result::Err(::serde::de::Error::custom(#arity_message));
                         }
                         let key = match seq
-                            .next_element::<ArmKeyProbe>()
+                            .next_element::<__ArmKeyProbe>()
                             .map_err(|_| ::serde::de::Error::custom(#arity_message))?
                             .ok_or_else(|| ::serde::de::Error::custom(#arity_message))?
                             .0
                         {
-                            ArmKey::Int(wide) => match i32::try_from(wide) {
+                            __ArmKey::Int(wide) => match i32::try_from(wide) {
                                 Ok(key) => key,
                                 Err(_) => {
                                     return ::std::result::Result::Err(::serde::de::Error::custom(#out_of_range_message));
                                 }
                             },
-                            ArmKey::OutOfRange => {
+                            __ArmKey::OutOfRange => {
                                 return ::std::result::Result::Err(::serde::de::Error::custom(#out_of_range_message));
                             }
-                            ArmKey::NonInteger => {
+                            __ArmKey::NonInteger => {
                                 return ::std::result::Result::Err(::serde::de::Error::custom(#non_integer_message));
                             }
                         };
@@ -637,7 +681,7 @@ fn union_codec(input: DeriveInput) -> syn::Result<proc_macro2::TokenStream> {
                     }
                 }
 
-                deserializer.deserialize_tuple(2, CultUnionVisitor)
+                deserializer.deserialize_tuple(2, __CultUnionVisitor)
             }
         }
     })
@@ -738,4 +782,54 @@ mod tests {
         let message = union_error("enum E { #[cultcache(key = \"1\")] A(Arm) }");
         assert!(message.contains("integer literal"), "{message}");
     }
+    fn entry_result(source: &str) -> syn::Result<proc_macro2::TokenStream> {
+        database_entry_codec(syn::parse_str(source).expect("the fixture parses"))
+    }
+
+    fn slots_result(source: &str) -> syn::Result<proc_macro2::TokenStream> {
+        cult_slots_codec(syn::parse_str(source).expect("the fixture parses"))
+    }
+
+    // Each attribute names a thing once; the second would silently win otherwise.
+    #[test]
+    fn a_repeated_type_or_schema_attribute_is_refused_for_that_reason() {
+        for source in [
+            "#[cultcache(type = \"a.one\")] #[cultcache(type = \"a.two\")] struct S { #[cultcache(key = 0)] id: String }",
+            "#[cultcache(type = \"a.one\", type = \"a.two\")] struct S { #[cultcache(key = 0)] id: String }",
+            "#[cultcache(type = \"a.one\", schema = \"s.one\")] #[cultcache(schema = \"s.two\")] struct S { #[cultcache(key = 0)] id: String }",
+            "#[cultcache(type = \"a.one\", schema = \"s.one\", schema = \"s.two\")] struct S { #[cultcache(key = 0)] id: String }",
+        ] {
+            let message = entry_result(source).expect_err(source).to_string();
+            assert!(message.contains("repeated `type`") || message.contains("repeated `schema`"), "{source}: {message}");
+        }
+        entry_result("#[cultcache(type = \"a.one\", schema = \"s.one\")] struct S { #[cultcache(key = 0)] id: String }")
+            .expect("one type and one schema are accepted");
+    }
+
+    #[test]
+    fn a_generic_slots_struct_and_a_missing_type_are_refused_for_their_reasons() {
+        let message = slots_result("struct S<T> { #[cultcache(key = 0)] id: T }").unwrap_err().to_string();
+        assert!(message.contains("does not support generics"), "{message}");
+        let message = entry_result("struct S { #[cultcache(key = 0)] id: String }").unwrap_err().to_string();
+        assert!(message.contains("requires #[cultcache(type"), "{message}");
+    }
+
+    // A slot record is a dense array: keys stop at the largest array16 length.
+    #[test]
+    fn the_slot_key_domain_ends_at_65535() {
+        slots_error_free("struct S { #[cultcache(key = 0)] a: String, #[cultcache(key = 65535)] b: String }");
+        for source in [
+            "struct S { #[cultcache(key = 65536)] a: String }",
+            "struct S { #[cultcache(key = 4294967296)] a: String }",
+            "struct S { #[cultcache(key = 0x10000)] a: String }",
+        ] {
+            let message = slots_error(source);
+            assert!(message.contains("outside 0..=65535"), "{source}: {message}");
+        }
+    }
+
+    fn slots_error_free(source: &str) {
+        let input: DeriveInput = syn::parse_str(source).expect("the fixture parses");
+        named_slots(input.data, &input.ident, "CultSlots").expect("the fixture is accepted");
+    }
 }
diff --git a/packages/cultcache-rs/src/lib.rs b/packages/cultcache-rs/src/lib.rs
index 0423c224..5332e8a4 100644
--- a/packages/cultcache-rs/src/lib.rs
+++ b/packages/cultcache-rs/src/lib.rs
@@ -99,6 +99,14 @@
 //! enum Bad { #[cultcache(key = 4)] A(Option<Arm>) }
 //! ```
 //!
+//! A hand-written `SlotRecord` impl, which only the derives provide:
+//!
+//! ```compile_fail,E0046
+//! use cultcache_rs::SlotRecord;
+//! struct Sneaky(String);
+//! impl SlotRecord for Sneaky {}
+//! ```
+//!
 //! A CultSlots field without a key, a repeated slot, a repeated key attribute, and a generic struct:
 //!
 //! ```compile_fail
@@ -163,7 +171,11 @@ pub trait DatabaseEntry: Serialize + DeserializeOwned + Clone + Send + 'static {
     label = "a CultUnion arm is a newtype over a #[derive(CultSlots)] struct",
     note = "a String, an Option or any other type is not written as an array of slots"
 )]
-pub trait SlotRecord {}
+pub trait SlotRecord {
+    /// Implemented only by the derives, which write the positional array the contract names.
+    #[doc(hidden)]
+    fn __slots_array();
+}
 
 pub trait CultCacheRegistry {
     fn register_entries(&self, cache: &mut CultCache) -> Result<()>;
@@ -484,6 +496,124 @@ fn describe_undecodable(error: &anyhow::Error) -> String {
     parts.join(": ")
 }
 
+/// The deepest a document payload may nest, on read and on write, in every CultCache runtime: every
+/// MessagePack array and map counts, read by a deserializer or skipped past it, and the outermost
+/// array is depth 1. A payload of this depth is accepted and one container deeper is refused. It is
+/// the C# reference's `CultMessagePackSecurity.MaximumPayloadDepth`.
+pub const MAX_PAYLOAD_DEPTH: usize = 256;
+
+/// A payload nests more containers than [`MAX_PAYLOAD_DEPTH`]. It names the depth reached and the
+/// limit, never payload bytes, and can be recovered from the `anyhow::Error` a read or write returns.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct PayloadTooDeep {
+    pub depth: usize,
+    pub limit: usize,
+}
+
+impl std::fmt::Display for PayloadTooDeep {
+    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(
+            formatter,
+            "payload nests {} MessagePack containers deep; the limit is {}",
+            self.depth, self.limit
+        )
+    }
+}
+
+impl std::error::Error for PayloadTooDeep {}
+
+/// Walks the MessagePack headers of one payload without recursion, counting every array and map and
+/// refusing the first container past [`MAX_PAYLOAD_DEPTH`]. Scalars are skipped by length and nothing
+/// is allocated per element. A payload that ends early is the decoder's to refuse.
+pub fn check_payload_depth(bytes: &[u8]) -> std::result::Result<(), PayloadTooDeep> {
+    let mut remaining = [0usize; MAX_PAYLOAD_DEPTH];
+    let mut depth = 0usize;
+    let mut position = 0usize;
+    while position < bytes.len() {
+        let Some((advance, children)) = element_header(bytes, position) else {
+            return Ok(());
+        };
+        position = position.saturating_add(advance);
+        if let Some(children) = children {
+            if depth == MAX_PAYLOAD_DEPTH {
+                return Err(PayloadTooDeep { depth: MAX_PAYLOAD_DEPTH + 1, limit: MAX_PAYLOAD_DEPTH });
+            }
+            if children > 0 {
+                remaining[depth] = children;
+                depth += 1;
+                continue;
+            }
+        }
+        // One element is complete; close every container it was the last element of.
+        while depth > 0 {
+            remaining[depth - 1] -= 1;
+            if remaining[depth - 1] != 0 {
+                break;
+            }
+            depth -= 1;
+        }
+        if depth == 0 {
+            return Ok(());
+        }
+    }
+    Ok(())
+}
+
+/// The element at `position`: how many bytes to advance past it (its whole length for a scalar, its
+/// header for a container) and, for a container, how many child elements follow. `None` when the
+/// payload ends inside the header.
+fn element_header(bytes: &[u8], position: usize) -> Option<(usize, Option<usize>)> {
+    // The big-endian length after the code byte.
+    let length = |width: usize| -> Option<usize> {
+        let field = bytes.get(position + 1..position + 1 + width)?;
+        Some(field.iter().fold(0usize, |length, byte| (length << 8) | usize::from(*byte)))
+    };
+    let code = bytes[position];
+    Some(match code {
+        0x00..=0x7f | 0xe0..=0xff | 0xc0..=0xc3 => (1, None),
+        0x80..=0x8f => (1, Some(2 * usize::from(code & 0x0f))),
+        0x90..=0x9f => (1, Some(usize::from(code & 0x0f))),
+        0xa0..=0xbf => (1 + usize::from(code & 0x1f), None),
+        0xc4 | 0xd9 => (2usize.saturating_add(length(1)?), None),
+        0xc5 | 0xda => (3usize.saturating_add(length(2)?), None),
+        0xc6 | 0xdb => (5usize.saturating_add(length(4)?), None),
+        0xc7 => (3usize.saturating_add(length(1)?), None),
+        0xc8 => (4usize.saturating_add(length(2)?), None),
+        0xc9 => (6usize.saturating_add(length(4)?), None),
+        0xca | 0xce | 0xd2 => (5, None),
+        0xcb | 0xcf | 0xd3 => (9, None),
+        0xcc | 0xd0 => (2, None),
+        0xcd | 0xd1 | 0xd4 => (3, None),
+        0xd5 => (4, None),
+        0xd6 => (6, None),
+        0xd7 => (10, None),
+        0xd8 => (18, None),
+        0xdc => (3, Some(length(2)?)),
+        0xdd => (5, Some(length(4)?)),
+        0xde => (3, Some(length(2)?.saturating_mul(2))),
+        0xdf => (5, Some(length(4)?.saturating_mul(2))),
+    })
+}
+
+/// The one decode of a document payload: the nesting bound first, then the deserializer, whose own
+/// depth counter (1024) is only a backstop. A failure keeps its kind and never quotes the bytes.
+/// Readers of stored envelope payloads outside this crate decode through it too.
+pub fn decode_payload<T: serde::de::DeserializeOwned>(payload: &[u8]) -> Result<T> {
+    check_payload_depth(payload)?;
+    rmp_serde::from_slice(payload).map_err(opaque_decode_error)
+}
+
+/// The nesting bound for an envelope admitted without a typed decode, from the same walk that
+/// [`decode_payload`] runs first. Names the entry's type and key, never the payload.
+fn require_payload_depth(entry: &CultCacheEnvelope) -> Result<()> {
+    check_payload_depth(&entry.payload).with_context(|| {
+        format!(
+            "refused CultCache envelope {:?} at key {:?}",
+            entry.r#type, entry.key
+        )
+    })
+}
+
 /// A decoder failure as an error that keeps only its kind. Every decode of stored bytes
 /// goes through this or [`describe_undecodable`], so no error text quotes a stored value.
 fn opaque_decode_error(error: rmp_serde::decode::Error) -> anyhow::Error {
@@ -2420,7 +2550,7 @@ impl CultCache {
         let Some(entry) = self.entries.get(&entry_id_parts(T::TYPE, key)) else {
             return Ok(None);
         };
-        let payload = rmp_serde::from_slice(&entry.payload).map_err(opaque_decode_error).with_context(|| {
+        let payload = decode_payload(&entry.payload).with_context(|| {
             format!(
                 "failed to decode CultCache entry {:?} at key {:?} as {}",
                 T::TYPE,
@@ -2453,7 +2583,7 @@ impl CultCache {
             if entry.r#type != T::TYPE {
                 continue;
             }
-            values.push(rmp_serde::from_slice(&entry.payload).map_err(opaque_decode_error).with_context(|| {
+            values.push(decode_payload(&entry.payload).with_context(|| {
                 format!(
                     "failed to decode CultCache entry {:?} at key {:?} as {}",
                     T::TYPE,
@@ -2474,7 +2604,7 @@ impl CultCache {
             }
             values.push((
                 entry.key.clone(),
-                rmp_serde::from_slice(&entry.payload).map_err(opaque_decode_error).with_context(|| {
+                decode_payload(&entry.payload).with_context(|| {
                     format!(
                         "failed to decode CultCache entry {:?} at key {:?} as {}",
                         T::TYPE,
@@ -2556,7 +2686,7 @@ impl CultCache {
         key: String,
         payload: Vec<u8>,
     ) -> Result<(CultCacheEnvelope, T)> {
-        let parsed: T = rmp_serde::from_slice(&payload).map_err(opaque_decode_error).with_context(|| {
+        let parsed: T = decode_payload(&payload).with_context(|| {
             format!(
                 "failed to validate CultCache entry {:?} at key {:?} as {}",
                 T::TYPE,
@@ -2599,6 +2729,7 @@ impl CultCache {
                     entry.r#type
                 ));
             }
+            require_payload_depth(entry)?;
             let Some(home) = self.home_index(&entry.r#type)? else {
                 continue;
             };
@@ -2643,7 +2774,7 @@ impl CultCache {
     /// claiming compile-time knowledge of its payload schema. This is the
     /// CultMesh/schema-registry boundary: callers own payload validation before
     /// admission; CultCache still enforces registered type, identity, timestamp,
-    /// routing, and persistence.
+    /// the payload nesting bound ([`MAX_PAYLOAD_DEPTH`]), routing, and persistence.
     pub fn put_raw_envelope(&mut self, entry: CultCacheEnvelope) -> Result<()> {
         if !self.definitions.contains_key(&entry.r#type) {
             return Err(anyhow!(
@@ -2663,6 +2794,7 @@ impl CultCache {
                 entry.r#type
             ));
         }
+        require_payload_depth(&entry)?;
         if let Some(home) = self.home_index(&entry.r#type)? {
             self.stores[home].store.push(&entry)?;
         }
@@ -2692,7 +2824,7 @@ impl CultCache {
             ));
         }
 
-        rmp_serde::from_slice(&entry.payload).map_err(opaque_decode_error).with_context(|| {
+        decode_payload(&entry.payload).with_context(|| {
             format!(
                 "failed to validate CultCache envelope {:?} at key {:?} as {}",
                 T::TYPE,
diff --git a/packages/cultcache-rs/tests/union_parity.rs b/packages/cultcache-rs/tests/union_parity.rs
index 1f0cdf31..da2b1a08 100644
--- a/packages/cultcache-rs/tests/union_parity.rs
+++ b/packages/cultcache-rs/tests/union_parity.rs
@@ -4,6 +4,11 @@
 use std::path::PathBuf;
 
 use cultcache_rs::CultCache;
+use cultcache_rs::CacheBackingStore;
+use cultcache_rs::CultCacheEnvelope;
+use cultcache_rs::MAX_PAYLOAD_DEPTH;
+use cultcache_rs::PayloadTooDeep;
+use cultcache_rs::check_payload_depth;
 use cultcache_rs::CultSlots;
 use cultcache_rs::CultUnion;
 use cultcache_rs::DatabaseEntry;
@@ -19,6 +24,14 @@ enum Node {
     Use(UseArm),
     #[cultcache(key = 5)]
     Image(ImageArm),
+    #[cultcache(key = 6)]
+    Wrap(WrapArm),
+}
+
+#[derive(Clone, Debug, PartialEq, CultSlots)]
+struct WrapArm {
+    #[cultcache(key = 0)]
+    inner: Option<std::boxed::Box<Node>>,
 }
 
 #[derive(Clone, Debug, PartialEq, CultSlots)]
@@ -74,6 +87,10 @@ enum Outcome {
     Completed(CompletedArm),
     #[cultcache(key = 2)]
     Denied,
+    #[cultcache(key = -2147483648)]
+    Floor,
+    #[cultcache(key = 2147483647)]
+    Ceiling,
 }
 
 #[derive(Clone, Debug, PartialEq, CultSlots)]
@@ -243,6 +260,15 @@ fn union_refusal_vectors() {
     assert_eq!((refused, read_only), (5, 4));
 }
 
+// A nil where the arm's slots belong is not an empty record, for an arm with slots and a unit arm.
+#[test]
+fn nil_slots_are_refused_naming_the_union_and_the_key() {
+    for (bytes, key) in [(vec![0x92, 0x01, 0xc0], "1"), (vec![0x92, 0x02, 0xc0], "2")] {
+        let message = rmp_serde::from_slice::<Outcome>(&bytes).unwrap_err().to_string();
+        assert!(message.contains("Outcome") && message.contains(&format!("arm {key} has nil slots")), "{message}");
+    }
+}
+
 #[test]
 fn an_unknown_arm_lists_the_declared_keys_and_never_echoes_the_payload() {
     // Shaped like a real arm, an array of strings, under an undeclared key.
@@ -393,7 +419,11 @@ mod arm_keys {
 
     #[test]
     fn a_key_that_is_not_an_integer_is_its_own_class_and_never_echoed() {
-        let keys: [&[u8]; 7] = [
+        let keys: [&[u8]; 10] = [
+            // A MessagePack ext: fixext 1, the timestamp extension and ext 8.
+            &[0xd4, 0x01, 0x00],
+            &[0xd6, 0xff, 0x00, 0x00, 0x00, 0x00],
+            &[0xc7, 0x02, 0x05, b'C', b'A'],
             &[0xc0],
             &[0xc3],
             &[0xcb, 0x3f, 0xf0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00],
@@ -467,3 +497,449 @@ mod serde_enum_bytes {
         assert_eq!(rmp_serde::from_slice::<PlainEnums>(AT_BASE).unwrap(), value);
     }
 }
+
+// Names a derive's generated code must not capture: the generic parameters and the local items of the
+// expansion, used as user type names.
+mod generated_names {
+    use cultcache_rs::CultSlots;
+    use cultcache_rs::CultUnion;
+    use cultcache_rs::DatabaseEntry;
+
+    #[derive(Clone, Debug, PartialEq, CultSlots)]
+    struct D {
+        #[cultcache(key = 0)]
+        name: String,
+    }
+
+    #[derive(Clone, Debug, PartialEq, CultSlots)]
+    struct S {
+        #[cultcache(key = 0)]
+        count: i32,
+    }
+
+    #[derive(Clone, Debug, PartialEq, CultSlots)]
+    struct ArmKey {
+        #[cultcache(key = 0)]
+        id: i32,
+    }
+
+    #[derive(Clone, Debug, PartialEq, CultUnion)]
+    enum A {
+        #[cultcache(key = 1)]
+        D(D),
+        #[cultcache(key = 2)]
+        S(S),
+        #[cultcache(key = 3)]
+        ArmKey(ArmKey),
+        #[cultcache(key = 4)]
+        E,
+    }
+
+    #[derive(Clone, Debug, PartialEq, CultUnion)]
+    enum CultUnionVisitor {
+        #[cultcache(key = 1)]
+        A(ArmKey),
+    }
+
+    #[derive(Clone, Debug, PartialEq, DatabaseEntry)]
+    #[cultcache(type = "tests.names")]
+    struct E {
+        #[cultcache(key = 0)]
+        union: A,
+        #[cultcache(key = 1)]
+        other: CultUnionVisitor,
+    }
+
+    #[test]
+    fn types_named_like_generated_generics_and_locals_round_trip() {
+        let value = E {
+            union: A::D(D { name: "n".into() }),
+            other: CultUnionVisitor::A(ArmKey { id: 9 }),
+        };
+        let bytes = rmp_serde::to_vec(&value).unwrap();
+        assert_eq!(rmp_serde::from_slice::<E>(&bytes).unwrap(), value);
+        for union in [A::S(S { count: 2 }), A::ArmKey(ArmKey { id: 3 }), A::E] {
+            let bytes = rmp_serde::to_vec(&union).unwrap();
+            assert_eq!(rmp_serde::from_slice::<A>(&bytes).unwrap(), union);
+        }
+    }
+}
+
+// The reference's CultUnionGuard classes, from bounds-outcomes.msgpack; depth entries go through the
+// cache because the nesting rule lives in its one decode.
+const CANARY: &str = "CANARY-union-golden-4c2a";
+
+fn envelope(key: &str, payload: &[u8]) -> CultCacheEnvelope {
+    CultCacheEnvelope {
+        key: key.into(),
+        r#type: UnionSurface::TYPE.into(),
+        payload: payload.to_vec(),
+        stored_at: "2026-01-01T00:00:00Z".into(),
+        schema_id: None,
+    }
+}
+
+// A payload past the bound cannot be admitted through the cache, so a store file written behind its
+// back stands in for one a peer or an older runtime left.
+fn through_cache(payload: &[u8]) -> Result<UnionSurface, String> {
+    let temp = tempfile::tempdir().unwrap();
+    let path = temp.path().join("foreign.cc");
+    SingleFileMessagePackBackingStore::new(&path).push(&envelope("k", payload)).unwrap();
+    let mut cache = CultCache::new();
+    cache.register_entry_type::<UnionSurface>().unwrap();
+    cache.add_generic_backing_store(SingleFileMessagePackBackingStore::new(&path)).unwrap();
+    cache.pull_all_backing_stores().unwrap();
+    match cache.get::<UnionSurface>("k") {
+        Ok(value) => Ok(value.expect("the envelope was stored")),
+        Err(error) => Err(format!("{error:#}")),
+    }
+}
+
+#[test]
+fn bounds_vectors() {
+    let entries: Vec<serde_json::Value> = rmp_serde::from_slice(&vector("bounds-outcomes.msgpack")).unwrap();
+    assert_eq!(entries[0], serde_json::json!(["limit", 256]));
+    assert_eq!(MAX_PAYLOAD_DEPTH, 256);
+    let (mut accepted, mut refused, mut read_only) = (0, 0, 0);
+    for entry in &entries[1..] {
+        let fields: Vec<&str> = entry.as_array().unwrap().iter().filter_map(|field| field.as_str()).collect();
+        let (file, outcome, union, detail) = (fields[0], fields[1], fields[2], fields[3]);
+        let bytes = vector(file);
+        let result = if union == "UnionSurface" { through_cache(&bytes) } else { decode(&bytes) };
+        match outcome {
+            "accepted" => {
+                accepted += 1;
+                let value = result.unwrap_or_else(|message| panic!("{file}: {message}"));
+                assert_eq!(rmp_serde::to_vec(&value).unwrap(), vector(detail), "{file}");
+                match file {
+                    "arm-key-int32-min.msgpack" => assert_eq!(value.outcomes.last(), Some(&Outcome::Floor)),
+                    "arm-key-int32-max.msgpack" | "arm-key-wide-coding.msgpack" => {
+                        assert_eq!(value.outcomes.last(), Some(&Outcome::Ceiling))
+                    }
+                    _ => {}
+                }
+            }
+            "read-only" => {
+                read_only += 1;
+                assert_eq!(result.unwrap_or_else(|message| panic!("{file}: {message}")), constructed(), "{file}");
+            }
+            "refused" => {
+                refused += 1;
+                let message = result.expect_err(file);
+                // The cache path names the entry type in its context; the union names itself.
+                let names = if union == "UnionSurface" { UnionSurface::TYPE } else { union };
+                assert!(message.contains(names), "{file}: {message}");
+                let class = match detail {
+                    "key-out-of-range" => "outside Int32",
+                    "key-not-integer" => "non-integer",
+                    "nil-slots" => "arm 1 has nil slots",
+                    "depth" => "the limit is 256",
+                    other => panic!("{file}: unknown class {other}"),
+                };
+                assert!(message.contains(class), "{file}: {message}");
+                assert!(!message.contains(CANARY), "{file} echoed a value: {message}");
+            }
+            other => panic!("{file}: unknown outcome {other}"),
+        }
+    }
+    assert_eq!((accepted, refused, read_only), (4, 7, 1));
+}
+
+fn wrap_chain(wraps: usize, leaf: Node) -> UnionSurface {
+    let mut node = leaf;
+    for _ in 0..wraps {
+        node = Node::Wrap(WrapArm { inner: Some(std::boxed::Box::new(node)) });
+    }
+    UnionSurface { root: Some(node), ..constructed() }
+}
+
+// Each Wrap adds a pair and a slots array; the surface array is depth 1, so a Box leaf (pair, slots,
+// children) ends at 2 * (wraps + 1) + 2 and a Text leaf at 2 * (wraps + 1) + 1.
+#[test]
+fn the_constructed_chains_write_the_vector_bytes() {
+    let at_limit = wrap_chain(126, boxed("leaf", vec![]));
+    assert_eq!(rmp_serde::to_vec(&at_limit).unwrap(), vector("depth-at-limit.msgpack"));
+    let past_limit = wrap_chain(127, text("leaf", CANARY));
+    assert_eq!(rmp_serde::to_vec(&past_limit).unwrap(), vector("depth-past-limit.msgpack"));
+}
+
+#[test]
+fn the_canary_is_in_the_committed_bytes() {
+    let bytes = vector("depth-past-limit.msgpack");
+    assert!(bytes.windows(CANARY.len()).any(|window| window == CANARY.as_bytes()));
+}
+
+#[test]
+fn write_refused_past_limit() {
+    let temp = tempfile::tempdir().unwrap();
+    let path = temp.path().join("depth.cc");
+    let mut cache = CultCache::new();
+    cache.register_entry_type::<UnionSurface>().unwrap();
+    cache.add_generic_backing_store(SingleFileMessagePackBackingStore::new(&path)).unwrap();
+
+    let at_limit = wrap_chain(126, boxed("leaf", vec![]));
+    assert_eq!(cache.put("at", &at_limit).unwrap(), at_limit);
+    let stored = cache.snapshot_envelopes();
+    assert_eq!(stored.len(), 1);
+    assert_eq!(stored[0].payload, vector("depth-at-limit.msgpack"));
+    let file_before = std::fs::read(&path).unwrap();
+
+    let error = cache.put("past", &wrap_chain(127, text("leaf", CANARY))).unwrap_err();
+    assert_eq!(error.downcast_ref::<PayloadTooDeep>(), Some(&PayloadTooDeep { depth: 257, limit: 256 }));
+    assert!(!format!("{error:#}").contains(CANARY), "{error:#}");
+    assert_eq!(cache.get::<UnionSurface>("past").unwrap(), None);
+    assert_eq!(cache.snapshot_envelopes(), stored);
+    assert_eq!(std::fs::read(&path).unwrap(), file_before);
+}
+
+// Every door an envelope enters the store by holds the one bound: the typed put above, and the raw
+// doors a peer's bytes arrive through. Three nestings: union arms and maps
+// written by the production encoder, and the arrays of the committed vector.
+#[derive(Clone, Debug, PartialEq, serde::Serialize, serde::Deserialize)]
+struct MapNest {
+    inner: Option<std::boxed::Box<MapNest>>,
+}
+
+#[derive(Clone, Debug, PartialEq, DatabaseEntry)]
+#[cultcache(type = "tests.map_doc", schema = "tests.map_doc.v1")]
+struct MapDoc {
+    #[cultcache(key = 0)]
+    root: Option<MapNest>,
+}
+
+// The document array is depth 1 and each MapNest is one map.
+fn map_doc(maps: usize) -> MapDoc {
+    let mut nest = MapNest { inner: None };
+    for _ in 1..maps {
+        nest = MapNest { inner: Some(std::boxed::Box::new(nest)) };
+    }
+    MapDoc { root: Some(nest) }
+}
+
+#[test]
+fn maps_count_one_level_each_on_every_write_door() {
+    let at_limit = map_doc(MAX_PAYLOAD_DEPTH - 1);
+    let past_limit = map_doc(MAX_PAYLOAD_DEPTH);
+    let at_bytes = rmp_serde::to_vec_named(&at_limit).unwrap();
+    let past_bytes = rmp_serde::to_vec_named(&past_limit).unwrap();
+    assert_eq!(check_payload_depth(&at_bytes), Ok(()));
+    assert_eq!(check_payload_depth(&past_bytes), Err(TOO_DEEP));
+
+    let mut cache = CultCache::new();
+    cache.register_entry_type::<MapDoc>().unwrap();
+    let (prepared, parsed) = cache.prepare_entry_named("at", &at_limit).unwrap();
+    assert_eq!((prepared.payload.as_slice(), &parsed), (at_bytes.as_slice(), &at_limit));
+    cache.put_prepared_batch(vec![prepared]).unwrap();
+    assert_eq!(cache.get::<MapDoc>("at").unwrap(), Some(at_limit));
+    let error = cache.prepare_entry_named("past", &past_limit).unwrap_err();
+    assert_eq!(error.downcast_ref::<PayloadTooDeep>(), Some(&TOO_DEEP));
+}
+
+fn raw(type_name: &str, key: &str, payload: Vec<u8>) -> CultCacheEnvelope {
+    CultCacheEnvelope { r#type: type_name.into(), ..envelope(key, &payload) }
+}
+
+#[test]
+fn the_raw_doors_hold_the_bound_for_arms_maps_and_arrays() {
+    let arms_at = rmp_serde::to_vec(&wrap_chain(126, boxed("leaf", vec![]))).unwrap();
+    let arms_past = rmp_serde::to_vec(&wrap_chain(127, text("leaf", CANARY))).unwrap();
+    let maps_at = rmp_serde::to_vec_named(&map_doc(MAX_PAYLOAD_DEPTH - 1)).unwrap();
+    let maps_past = rmp_serde::to_vec_named(&map_doc(MAX_PAYLOAD_DEPTH)).unwrap();
+    let arrays_at = vector("depth-at-limit.msgpack");
+    let arrays_past = vector("depth-past-limit.msgpack");
+    let cases = [
+        (UnionSurface::TYPE, "arms", arms_at, arms_past),
+        (MapDoc::TYPE, "maps", maps_at, maps_past),
+        (UnionSurface::TYPE, "arrays", arrays_at, arrays_past),
+    ];
+    for (type_name, name, at, past) in cases {
+        let temp = tempfile::tempdir().unwrap();
+        let path = temp.path().join("doors.cc");
+        let mut cache = CultCache::new();
+        cache.register_entry_type::<UnionSurface>().unwrap();
+        cache.register_entry_type::<MapDoc>().unwrap();
+        cache.add_generic_backing_store(SingleFileMessagePackBackingStore::new(&path)).unwrap();
+        cache.put_raw_envelope(raw(type_name, "at-raw", at.clone())).unwrap();
+        cache.put_prepared_batch(vec![raw(type_name, "at-batch", at.clone())]).unwrap();
+        let stored = cache.snapshot_envelopes();
+        let file_before = std::fs::read(&path).unwrap();
+
+        let refusals = [
+            cache.put_raw_envelope(raw(type_name, "past-raw", past.clone())).unwrap_err(),
+            // The valid entry staged beside it must not land either.
+            cache
+                .put_prepared_batch(vec![raw(type_name, "ok-staged", at.clone()), raw(type_name, "past-batch", past.clone())])
+                .unwrap_err(),
+        ];
+        for error in refusals {
+            assert_eq!(error.downcast_ref::<PayloadTooDeep>(), Some(&TOO_DEEP), "{name}");
+            let message = format!("{error:#}");
+            assert!(message.contains(type_name) && !message.contains(CANARY), "{name}: {message}");
+        }
+        assert_eq!(cache.snapshot_envelopes(), stored, "{name}");
+        assert_eq!(std::fs::read(&path).unwrap(), file_before, "{name}");
+    }
+}
+
+// A debug build on a 2 MiB thread carries 405 containers of this document; the bound keeps every
+// accepted payload below that, and every refusal is an error, not an abort.
+#[test]
+fn stack_budget_debug_2mib() {
+    std::thread::Builder::new()
+        .stack_size(2 * 1024 * 1024)
+        .spawn(|| {
+            assert_eq!(
+                through_cache(&vector("depth-at-limit.msgpack")).unwrap(),
+                wrap_chain(126, boxed("leaf", vec![]))
+            );
+            let refused = through_cache(&vector("depth-past-limit.msgpack")).unwrap_err();
+            assert!(refused.contains("the limit is 256"), "{refused}");
+            let mut cache = CultCache::new();
+            cache.register_entry_type::<UnionSurface>().unwrap();
+            let value = wrap_chain(126, boxed("leaf", vec![]));
+            assert_eq!(cache.put("at", &value).unwrap(), value);
+            assert_eq!(cache.get::<UnionSurface>("at").unwrap(), Some(value));
+            let mut deep = vec![0x91u8; 100_000];
+            deep.push(0xc0);
+            let refused = through_cache(&deep).unwrap_err();
+            assert!(refused.contains("the limit is 256"), "{refused}");
+        })
+        .unwrap()
+        .join()
+        .unwrap();
+}
+
+// The walk is iterative: 100000 nested arrays are refused on a 256 KiB stack.
+#[test]
+fn the_walk_does_not_recurse() {
+    std::thread::Builder::new()
+        .stack_size(256 * 1024)
+        .spawn(|| {
+            let mut deep = vec![0x91u8; 100_000];
+            deep.push(0xc0);
+            assert_eq!(check_payload_depth(&deep), Err(PayloadTooDeep { depth: 257, limit: 256 }));
+        })
+        .unwrap()
+        .join()
+        .unwrap();
+}
+
+fn nested(open: &[u8], levels: usize, leaf: &[u8]) -> Vec<u8> {
+    let mut bytes = open.repeat(levels);
+    bytes.extend_from_slice(leaf);
+    bytes
+}
+
+const TOO_DEEP: PayloadTooDeep = PayloadTooDeep { depth: 257, limit: 256 };
+
+// Every container header width counts one level, and the bound sits between 256 and 257 for each.
+#[test]
+fn every_container_header_counts_one_level() {
+    let openers: [(&str, &[u8]); 6] = [
+        ("fixarray", &[0x91]),
+        ("array16", &[0xdc, 0x00, 0x01]),
+        ("array32", &[0xdd, 0x00, 0x00, 0x00, 0x01]),
+        ("fixmap", &[0x81, 0x01]),
+        ("map16", &[0xde, 0x00, 0x01, 0x01]),
+        ("map32", &[0xdf, 0x00, 0x00, 0x00, 0x01, 0x01]),
+    ];
+    for (name, open) in openers {
+        assert_eq!(check_payload_depth(&nested(open, 256, &[0xc0])), Ok(()), "{name} at the limit");
+        assert_eq!(check_payload_depth(&nested(open, 257, &[0xc0])), Err(TOO_DEEP), "{name} past the limit");
+    }
+    // An empty container still counts the level it sits on.
+    assert_eq!(check_payload_depth(&nested(&[0x91], 255, &[0x90])), Ok(()));
+    assert_eq!(check_payload_depth(&nested(&[0x91], 256, &[0x90])), Err(TOO_DEEP));
+    assert_eq!(check_payload_depth(&nested(&[0x91], 255, &[0x80])), Ok(()));
+    assert_eq!(check_payload_depth(&nested(&[0x91], 256, &[0x80])), Err(TOO_DEEP));
+}
+
+// Every scalar and container header is followed by the chain that tests the bound. The skipped bytes
+// are filled with the array marker, so a skip that stops short reads them as containers and a skip
+// that runs long eats the chain: either way the chain's depth stops reading as 256 and 257.
+#[test]
+fn every_element_is_skipped_by_its_length_and_counted_by_its_children() {
+    const FILL: u8 = 0x91;
+    let filled = |head: &[u8], bytes: usize| -> Vec<u8> { [head.to_vec(), vec![FILL; bytes]].concat() };
+    let long = |code: u8, width: usize, length: usize, extra: usize| -> Vec<u8> {
+        let size = length.to_be_bytes();
+        filled(&[&[code][..], &size[size.len() - width..]].concat(), length + extra)
+    };
+    let mut elements: Vec<Vec<u8>> = vec![
+        vec![0x00],
+        vec![0xff],
+        vec![0xc0],
+        vec![0xc2],
+        vec![0xc3],
+        filled(&[0xcc], 1),
+        filled(&[0xcd], 2),
+        filled(&[0xce], 4),
+        filled(&[0xcf], 8),
+        filled(&[0xd0], 1),
+        filled(&[0xd1], 2),
+        filled(&[0xd2], 4),
+        filled(&[0xd3], 8),
+        filled(&[0xca], 4),
+        filled(&[0xcb], 8),
+        filled(&[0xa3], 3),
+        filled(&[0xbf], 31),
+        filled(&[0xd4], 2),
+        filled(&[0xd5], 3),
+        filled(&[0xd6], 5),
+        filled(&[0xd7], 9),
+        filled(&[0xd8], 17),
+    ];
+    for (code, width) in [(0xd9, 1), (0xc4, 1), (0xda, 2), (0xc5, 2), (0xdb, 4), (0xc6, 4)] {
+        for length in [3, 200, 256, 65536] {
+            if length < (1usize << (8 * width)) {
+                elements.push(long(code, width, length, 0));
+            }
+        }
+    }
+    // An ext carries a type byte after its length.
+    for (code, width) in [(0xc7, 1), (0xc8, 2), (0xc9, 4)] {
+        for length in [3, 200, 256, 65536] {
+            if length < (1usize << (8 * width)) {
+                elements.push(long(code, width, length, 1));
+            }
+        }
+    }
+    // Containers whose children are scalars, so a wrong child count shifts everything after them.
+    elements.push(vec![0x92, 0x00, 0x00]);
+    elements.push(vec![0x93, 0x00, 0x00, 0x00]);
+    elements.push(vec![0x82, 0x00, 0x00, 0x00, 0x00]);
+    elements.push(vec![0x83, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]);
+    elements.push([vec![0xdc, 0x01, 0x00], vec![0x00; 256]].concat());
+    elements.push([vec![0xdd, 0x00, 0x00, 0x00, 0x03], vec![0x00; 3]].concat());
+    elements.push([vec![0xde, 0x00, 0x03], vec![0x00; 6]].concat());
+    elements.push([vec![0xde, 0x01, 0x00], vec![0x00; 512]].concat());
+    elements.push([vec![0xdf, 0x00, 0x00, 0x00, 0x03], vec![0x00; 6]].concat());
+    elements.push(vec![0xdc, 0x00, 0x00]);
+    elements.push(vec![0xdf, 0x00, 0x00, 0x00, 0x00]);
+
+    let with_chain = |levels: usize| {
+        let mut bytes = vec![0xdd];
+        bytes.extend_from_slice(&u32::try_from(elements.len() + 1).unwrap().to_be_bytes());
+        for element in &elements {
+            bytes.extend_from_slice(element);
+        }
+        bytes.extend(nested(&[0x91], levels, &[0xc0]));
+        bytes
+    };
+    assert_eq!(check_payload_depth(&with_chain(255)), Ok(()));
+    assert_eq!(check_payload_depth(&with_chain(256)), Err(TOO_DEEP));
+}
+
+// An ending cut short is the decoder's to refuse, never a panic or a depth error.
+#[test]
+fn a_payload_cut_short_is_not_a_depth_error() {
+    for bytes in [&[0xdc, 0x00][..], &[0xdd, 0, 0], &[0xd9], &[0xc5, 0x00], &[0xde], &[0xdf, 0, 0, 0], &[0x91], &[]] {
+        assert_eq!(check_payload_depth(bytes), Ok(()), "{bytes:02x?}");
+    }
+    assert_eq!(check_payload_depth(&[0xdb, 0xff, 0xff, 0xff, 0xff, b'a']), Ok(()));
+}
+
+#[test]
+fn the_refusal_names_depth_and_limit_only() {
+    assert_eq!(TOO_DEEP.to_string(), "payload nests 257 MessagePack containers deep; the limit is 256");
+}
diff --git a/packages/cultmesh-rs/src/lib.rs b/packages/cultmesh-rs/src/lib.rs
index 1af0d906..6965e2d5 100644
--- a/packages/cultmesh-rs/src/lib.rs
+++ b/packages/cultmesh-rs/src/lib.rs
@@ -1733,6 +1733,114 @@ mod tests {
         Ok(())
     }
 
+    #[derive(serde::Serialize)]
+    struct Nest {
+        inner: Option<Box<Nest>>,
+    }
+
+    // `levels` maps deep, written by the serializer the cache itself uses.
+    fn nest(levels: usize) -> Vec<u8> {
+        let mut value = Nest { inner: None };
+        for _ in 1..levels {
+            value = Nest { inner: Some(Box::new(value)) };
+        }
+        rmp_serde::to_vec_named(&value).unwrap()
+    }
+
+    fn peer_document(key: &str, payload: Vec<u8>) -> cultnet_rs::CultNetRawDocumentRecord {
+        cultnet_rs::CultNetRawDocumentRecord {
+            schema_id: "cultmesh.test.note.v0".to_string(),
+            record_key: key.to_string(),
+            stored_at: "2026-07-03T00:00:00Z".to_string(),
+            payload_encoding: cultnet_rs::CultNetRawPayloadEncoding::Messagepack,
+            payload,
+            source_runtime_id: None,
+            source_agent_id: None,
+            source_role: None,
+            tags: None,
+        }
+    }
+
+    // Serves `documents` as the catalog's snapshot until the returned flag is set.
+    fn serve_snapshot(
+        documents: Vec<cultnet_rs::CultNetRawDocumentRecord>,
+    ) -> Result<(
+        SocketAddr,
+        std::sync::Arc<std::sync::atomic::AtomicBool>,
+        thread::JoinHandle<Result<()>>,
+    )> {
+        let socket = UdpSocket::bind("127.0.0.1:0")?;
+        let target = socket.local_addr()?;
+        let stop = std::sync::Arc::new(std::sync::atomic::AtomicBool::new(false));
+        let server_stop = stop.clone();
+        let server = thread::spawn(move || -> Result<()> {
+            let mut server = CultMeshRudpDocumentServer::new(
+                socket,
+                |_receipt: CultMeshRudpRawDocumentReceipt| Ok(()),
+                move |_query: &CultMeshRudpSnapshotQuery| Ok(documents.clone()),
+                CultMeshSystemClock::default(),
+                CultMeshRudpDocumentServerOptions::default(),
+            )?;
+            while !server_stop.load(std::sync::atomic::Ordering::SeqCst) {
+                server.poll_once()?;
+                thread::sleep(Duration::from_millis(1));
+            }
+            Ok(())
+        });
+        Ok((target, stop, server))
+    }
+
+    fn pull(
+        node: &mut CultMeshNode,
+        documents: Vec<cultnet_rs::CultNetRawDocumentRecord>,
+    ) -> Result<usize> {
+        let (target, stop, server) = serve_snapshot(documents)?;
+        let pulled = node.pull_rudp_catalog_snapshot(CultMeshRudpSnapshotOptions {
+            target,
+            runtime_id: "pull-test".into(),
+            message_id: String::new(),
+            connect_timeout: Duration::from_millis(500),
+            response_timeout: Duration::from_secs(2),
+            poll_interval: Duration::from_millis(2),
+            resend_delay_ms: 5,
+            schema_ids: None,
+            record_keys: None,
+        });
+        stop.store(true, std::sync::atomic::Ordering::SeqCst);
+        server.join().expect("server thread should not panic")?;
+        pulled
+    }
+
+    #[test]
+    fn a_pulled_peer_payload_nested_past_the_bound_is_refused_and_nothing_is_persisted() -> Result<()> {
+        let temp = tempfile::tempdir()?;
+        let store_path = temp.path().join("cultmesh.cc");
+        let mut node = CultMesh::create_node(&store_path, TestDocuments, Default::default())?;
+
+        let limit = cultcache_rs::MAX_PAYLOAD_DEPTH;
+        let pulled = pull(
+            &mut node,
+            vec![peer_document("a", nest(limit)), peer_document("b", nest(limit))],
+        )?;
+        assert_eq!(pulled, 2);
+        node.flush()?;
+        let held = std::fs::read(&store_path)?;
+
+        let error = pull(&mut node, vec![peer_document("past", nest(limit + 1))]).unwrap_err();
+        assert_eq!(
+            error.downcast_ref::<cultcache_rs::PayloadTooDeep>(),
+            Some(&cultcache_rs::PayloadTooDeep { depth: limit + 1, limit })
+        );
+        node.flush()?;
+        assert_eq!(std::fs::read(&store_path)?, held);
+        let reloaded = CultMesh::create_node(&store_path, TestDocuments, Default::default())?;
+        let mut keys: Vec<_> =
+            reloaded.cache().snapshot().into_iter().map(|entry| entry.key).collect();
+        keys.sort();
+        assert_eq!(keys, vec!["a".to_string(), "b".to_string()]);
+        Ok(())
+    }
+
     #[test]
     fn stream_catalog_negotiates_and_publishes_latest_shared_memory_frame() -> Result<()> {
         let mut catalog = CultMesh::create_stream_catalog();
diff --git a/packages/cultnet-rs/src/contracts.rs b/packages/cultnet-rs/src/contracts.rs
index 1808c62f..6d9a2d77 100644
--- a/packages/cultnet-rs/src/contracts.rs
+++ b/packages/cultnet-rs/src/contracts.rs
@@ -604,11 +604,7 @@ pub fn decode_cultnet_message_from_slice(
     bytes: &[u8],
     contract: CultNetWireContract,
 ) -> Result<CultNetMessage> {
-    if contract == CultNetWireContract::CultNetSchemaV0 {
-        let wire_value: rmpv::Value = rmp_serde::from_slice(bytes)?;
-        return parse_cultnet_message(&wire_value, contract);
-    }
-    let wire_value: rmpv::Value = rmp_serde::from_slice(bytes)?;
+    let wire_value: rmpv::Value = cultcache_rs::decode_payload(bytes)?;
     parse_cultnet_message(&wire_value, contract)
 }
 
diff --git a/packages/cultnet-rs/src/media_stream_wire.rs b/packages/cultnet-rs/src/media_stream_wire.rs
index 5c89a8c3..6b3d0d85 100644
--- a/packages/cultnet-rs/src/media_stream_wire.rs
+++ b/packages/cultnet-rs/src/media_stream_wire.rs
@@ -211,31 +211,31 @@ pub fn decode_media_wire_record(payload: &[u8]) -> Result<GameCultMediaWireRecor
     let record = match document.schema_id.as_str() {
         GAMECULT_MEDIA_VIDEO_ACCESS_UNIT_SCHEMA => {
             let record: GameCultMediaVideoAccessUnitRecord =
-                rmp_serde::from_slice(&document.payload)?;
+                cultcache_rs::decode_payload(&document.payload)?;
             validate_video_record(&record)?;
             GameCultMediaWireRecord::Video(record)
         }
         GAMECULT_MEDIA_VIDEO_PARITY_SHARD_SCHEMA => {
             let record: GameCultMediaVideoParityShardRecord =
-                rmp_serde::from_slice(&document.payload)?;
+                cultcache_rs::decode_payload(&document.payload)?;
             validate_video_parity_record(&record)?;
             GameCultMediaWireRecord::VideoParity(record)
         }
         GAMECULT_MEDIA_AUDIO_PACKET_SCHEMA => {
             let record: GameCultMediaAudioPacketRecord =
-                rmp_serde::from_slice(&document.payload)?;
+                cultcache_rs::decode_payload(&document.payload)?;
             validate_audio_record(&record)?;
             GameCultMediaWireRecord::Audio(record)
         }
         GAMECULT_MEDIA_AUDIO_PARITY_SHARD_SCHEMA => {
             let record: GameCultMediaAudioParityShardRecord =
-                rmp_serde::from_slice(&document.payload)?;
+                cultcache_rs::decode_payload(&document.payload)?;
             validate_audio_parity_record(&record)?;
             GameCultMediaWireRecord::AudioParity(record)
         }
         GAMECULT_MEDIA_RECEIVER_FEEDBACK_SCHEMA => {
             let record: GameCultMediaReceiverFeedbackRecord =
-                rmp_serde::from_slice(&document.payload)?;
+                cultcache_rs::decode_payload(&document.payload)?;
             validate_feedback_record(&record)?;
             GameCultMediaWireRecord::Feedback(record)
         }
diff --git a/packages/cultnet-rs/src/provider_session.rs b/packages/cultnet-rs/src/provider_session.rs
index 9965b0ef..2584120f 100644
--- a/packages/cultnet-rs/src/provider_session.rs
+++ b/packages/cultnet-rs/src/provider_session.rs
@@ -325,7 +325,7 @@ pub fn decode_provider_connect_evidence(
             "Provider Connect evidence must be a non-empty MessagePack map"
         ));
     }
-    let evidence: CultMeshProviderConnectEvidenceWire = rmp_serde::from_slice(payload)?;
+    let evidence: CultMeshProviderConnectEvidenceWire = cultcache_rs::decode_payload(payload)?;
     evidence.validate()?;
     Ok(evidence)
 }
@@ -338,7 +338,7 @@ where
     let bytes = STANDARD
         .decode(payload)
         .map_err(|error| anyhow!("Provider-session payload must be MessagePack base64: {error}"))?;
-    let value: T = rmp_serde::from_slice(&bytes)?;
+    let value: T = cultcache_rs::decode_payload(&bytes)?;
     value.validate()?;
     Ok(value)
 }
@@ -353,7 +353,7 @@ pub fn decode_provider_command_document(
             document.schema_id
         ));
     }
-    let command: CultMeshProviderCommandWire = rmp_serde::from_slice(&document.payload)?;
+    let command: CultMeshProviderCommandWire = cultcache_rs::decode_payload(&document.payload)?;
     command.validate()?;
     Ok(command)
 }
diff --git a/packages/cultnet-rs/src/replication.rs b/packages/cultnet-rs/src/replication.rs
index 7073f1a9..7b070fbd 100644
--- a/packages/cultnet-rs/src/replication.rs
+++ b/packages/cultnet-rs/src/replication.rs
@@ -522,7 +522,7 @@ impl CultNetDocumentRegistry {
         envelope: &CultCacheEnvelope,
     ) -> Result<CultNetDocumentRecord<Value>> {
         let binding = self.require_binding(&envelope.r#type)?;
-        let payload: Value = rmp_serde::from_slice(&envelope.payload).with_context(|| {
+        let payload: Value = cultcache_rs::decode_payload(&envelope.payload).with_context(|| {
             format!(
                 "failed to decode CultCache envelope {:?} at {:?} as generic CultNet payload",
                 envelope.r#type, envelope.key
diff --git a/packages/cultnet-rs/src/service_identity.rs b/packages/cultnet-rs/src/service_identity.rs
index 6f3e58d8..3d37adfa 100644
--- a/packages/cultnet-rs/src/service_identity.rs
+++ b/packages/cultnet-rs/src/service_identity.rs
@@ -306,7 +306,7 @@ fn open_service_identity_envelopes<P: ServiceIdentityProfile>(
     {
         bail!("service identity store belongs to a different profile or schema");
     }
-    let entry: ServiceIdentityPrivateEntry = rmp_serde::from_slice(&envelope.payload)
+    let entry: ServiceIdentityPrivateEntry = cultcache_rs::decode_payload(&envelope.payload)
         .context("service identity private payload is malformed MessagePack")?;
     validate_private::<P>(&entry)?;
     if entry.protector_kind != platform_protector_kind() || entry.assurance != platform_assurance()
@@ -741,6 +741,34 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn a_credential_payload_nested_past_the_bound_is_refused_by_the_cache_walk() -> Result<()> {
+        let limit = cultcache_rs::MAX_PAYLOAD_DEPTH;
+        let credential = |levels: usize| -> Result<Vec<u8>> {
+            let envelope = CultCacheEnvelope {
+                key: IdunnServiceIdentity::PRIVATE_KEY.into(),
+                r#type: IdunnServiceIdentity::PRIVATE_TYPE.into(),
+                payload: [vec![0x91u8; levels], vec![0xc0]].concat(),
+                stored_at: "2026-10-11T00:00:00Z".into(),
+                schema_id: Some(IdunnServiceIdentity::PRIVATE_SCHEMA.into()),
+            };
+            Ok(rmp_serde::to_vec(&vec![envelope])?)
+        };
+        let past = open_service_identity_credential_bytes::<IdunnServiceIdentity>(&credential(limit + 1)?)
+            .err()
+            .expect("a payload past the bound is refused");
+        assert_eq!(
+            past.downcast_ref::<cultcache_rs::PayloadTooDeep>(),
+            Some(&cultcache_rs::PayloadTooDeep { depth: limit + 1, limit })
+        );
+        // At the bound the walk lets the payload through to the decoder, which refuses its shape.
+        let at = open_service_identity_credential_bytes::<IdunnServiceIdentity>(&credential(limit)?)
+            .err()
+            .expect("the payload is not a credential");
+        assert!(at.downcast_ref::<cultcache_rs::PayloadTooDeep>().is_none());
+        Ok(())
+    }
+
     #[test]
     fn immutable_credential_reader_opens_without_a_path_and_retains_signing_authority() -> Result<()>
     {
diff --git a/packages/cultnet-rs/tests/cultnet.rs b/packages/cultnet-rs/tests/cultnet.rs
index 2f4572d9..cac4d9dd 100644
--- a/packages/cultnet-rs/tests/cultnet.rs
+++ b/packages/cultnet-rs/tests/cultnet.rs
@@ -1,6 +1,7 @@
 use anyhow::Result;
 use chrono::Duration;
 use chrono::Utc;
+use cultcache_rs::CacheBackingStore;
 use cultcache_rs::CultCache;
 use cultcache_rs::DatabaseEntry;
 use cultcache_rs::SingleFileMessagePackBackingStore;
@@ -1778,6 +1779,49 @@ fn document_registry_advertises_mutation_contracts_with_bindings() -> Result<()>
     Ok(())
 }
 
+#[test]
+fn a_stored_payload_nested_past_the_bound_is_not_replicated_as_a_document() -> Result<()> {
+    let temp = tempfile::tempdir()?;
+    let store = temp.path().join("foreign.msgpack");
+    let limit = cultcache_rs::MAX_PAYLOAD_DEPTH;
+    let envelope = |key: &str, levels: usize| cultcache_rs::CultCacheEnvelope {
+        key: key.to_string(),
+        r#type: GhostlightAgentStateFixture::TYPE.to_string(),
+        payload: [vec![0x91u8; levels], vec![0xc0]].concat(),
+        stored_at: "2026-10-11T00:00:00Z".to_string(),
+        schema_id: None,
+    };
+    let mut registry = CultNetDocumentRegistry::new();
+    registry.register(CultNetDocumentBinding::for_entry::<GhostlightAgentStateFixture>(
+        "ghostlight.agent_state.v0".to_string(),
+    ));
+    // A store written by something that does not hold the bound.
+    let mut backing = SingleFileMessagePackBackingStore::new(&store);
+    backing.push(&envelope("at", limit))?;
+    let mut cache = CultCache::new();
+    cache.register_entry_type::<GhostlightAgentStateFixture>()?;
+    cache.add_generic_backing_store(SingleFileMessagePackBackingStore::new(&store))?;
+    cache.pull_all_backing_stores()?;
+
+    let at_limit = registry.create_snapshot_response(&cache, "at", None, None)?;
+    let CultNetMessage::SnapshotResponse { documents, .. } = at_limit else {
+        panic!("expected a snapshot response");
+    };
+    assert_eq!(documents.len(), 1);
+
+    backing.push(&envelope("past", limit + 1))?;
+    let mut cache = CultCache::new();
+    cache.register_entry_type::<GhostlightAgentStateFixture>()?;
+    cache.add_generic_backing_store(SingleFileMessagePackBackingStore::new(&store))?;
+    cache.pull_all_backing_stores()?;
+    let error = registry.create_snapshot_response(&cache, "past", None, None).unwrap_err();
+    assert_eq!(
+        error.downcast_ref::<cultcache_rs::PayloadTooDeep>(),
+        Some(&cultcache_rs::PayloadTooDeep { depth: limit + 1, limit })
+    );
+    Ok(())
+}
+
 #[test]
 fn document_registry_replicates_typed_cultcache_state() -> Result<()> {
     let temp = tempfile::tempdir()?;
diff --git a/packages/cultnet-rs/tests/payload_depth.rs b/packages/cultnet-rs/tests/payload_depth.rs
new file mode 100644
index 00000000..d810e2de
--- /dev/null
+++ b/packages/cultnet-rs/tests/payload_depth.rs
@@ -0,0 +1,378 @@
+//! Every cultnet-rs decoder of a wire payload that can carry free-form or skipped content holds the
+//! shared 256 nesting bound through `cultcache_rs::decode_payload`: 256 is accepted and 257 is refused
+//! with the typed refusal, for arrays, maps and union arms, and a 2000-deep payload is an error on a
+//! 2 MiB stack, not an abort.
+
+use anyhow::{Error, Result};
+use base64::Engine;
+use base64::engine::general_purpose::STANDARD;
+use cultcache_rs::{CultSlots, CultUnion, MAX_PAYLOAD_DEPTH, PayloadTooDeep};
+use cultnet_rs::*;
+use serde::Serialize;
+
+const CANARY: &str = "canary-secret-payload-text";
+const TOO_DEEP: PayloadTooDeep = PayloadTooDeep { depth: MAX_PAYLOAD_DEPTH + 1, limit: MAX_PAYLOAD_DEPTH };
+
+#[derive(Clone, Debug, PartialEq, CultUnion)]
+enum Node {
+    #[cultcache(key = 0)]
+    Box(BoxArm),
+    #[cultcache(key = 1)]
+    Text(TextArm),
+    #[cultcache(key = 6)]
+    Wrap(WrapArm),
+}
+
+#[derive(Clone, Debug, PartialEq, CultSlots)]
+struct BoxArm {
+    #[cultcache(key = 0)]
+    id: String,
+    #[cultcache(key = 1)]
+    children: Vec<Node>,
+}
+
+#[derive(Clone, Debug, PartialEq, CultSlots)]
+struct TextArm {
+    #[cultcache(key = 0)]
+    id: String,
+    #[cultcache(key = 1)]
+    content: String,
+}
+
+#[derive(Clone, Debug, PartialEq, CultSlots)]
+struct WrapArm {
+    #[cultcache(key = 0)]
+    inner: Option<std::boxed::Box<Node>>,
+}
+
+#[derive(Serialize)]
+struct MapNest {
+    inner: Option<std::boxed::Box<MapNest>>,
+}
+
+/// One value `depth` containers deep, from the production encoder.
+fn arrays(depth: usize) -> Vec<u8> {
+    let mut value = rmpv::Value::String(CANARY.into());
+    for _ in 0..depth {
+        value = rmpv::Value::Array(vec![value]);
+    }
+    rmp_serde::to_vec(&value).unwrap()
+}
+
+fn maps(depth: usize) -> Vec<u8> {
+    let mut nest = MapNest { inner: None };
+    for _ in 1..depth {
+        nest = MapNest { inner: Some(std::boxed::Box::new(nest)) };
+    }
+    rmp_serde::to_vec_named(&nest).unwrap()
+}
+
+// Each Wrap adds a pair and a slots array: a Text leaf is 2 * wraps + 2 deep, a Box leaf 2 * wraps + 3.
+fn arms(depth: usize) -> Vec<u8> {
+    let (mut node, wraps) = if depth % 2 == 0 {
+        (Node::Text(TextArm { id: "leaf".into(), content: CANARY.into() }), (depth - 2) / 2)
+    } else {
+        (Node::Box(BoxArm { id: "leaf".into(), children: vec![] }), (depth - 3) / 2)
+    };
+    for _ in 0..wraps {
+        node = Node::Wrap(WrapArm { inner: Some(std::boxed::Box::new(node)) });
+    }
+    rmp_serde::to_vec(&node).unwrap()
+}
+
+type Nesting = fn(usize) -> Vec<u8>;
+const NESTINGS: [(&str, Nesting); 3] = [("arrays", arrays), ("maps", maps), ("arms", arms)];
+
+/// The hole at 2000 containers, from marker bytes: the encoders recurse too.
+fn deep_marker_payload() -> Vec<u8> {
+    let mut bytes = vec![0x91; 2000];
+    bytes.push(0xc0);
+    bytes
+}
+
+/// Swaps the trailing nil of a document for `nested`: the free field is its last.
+fn replace_trailing_nil(mut document: Vec<u8>, nested: &[u8]) -> Vec<u8> {
+    assert_eq!(document.pop(), Some(0xc0));
+    document.extend_from_slice(nested);
+    document
+}
+
+/// Adds one `x` entry holding `nested` to a MessagePack map: the unknown key a named-map record skips.
+fn add_unknown_entry(mut map: Vec<u8>, nested: &[u8]) -> Vec<u8> {
+    match map[0] {
+        header @ 0x80..=0x8e => map[0] = header + 1,
+        0xde => {
+            let count = u16::from_be_bytes([map[1], map[2]]) + 1;
+            map[1..3].copy_from_slice(&count.to_be_bytes());
+        }
+        other => panic!("not a small map: {other:#x}"),
+    }
+    map.extend_from_slice(&[0xa1, b'x']);
+    map.extend_from_slice(nested);
+    map
+}
+
+/// Adds one trailing slot to a positional record array: the unknown slot the derive skips.
+fn add_trailing_slot(mut array: Vec<u8>, nested: &[u8]) -> Vec<u8> {
+    match array[0] {
+        header @ 0x90..=0x9e => array[0] = header + 1,
+        0x9f => array.splice(0..1, [0xdc, 0x00, 0x10]).for_each(drop),
+        0xdc => {
+            let count = u16::from_be_bytes([array[1], array[2]]) + 1;
+            array[1..3].copy_from_slice(&count.to_be_bytes());
+        }
+        other => panic!("not a small array: {other:#x}"),
+    }
+    array.extend_from_slice(nested);
+    array
+}
+
+type Decode = fn(&[u8]) -> Result<()>;
+/// A payload whose free or skipped part is `nested`, deep by `1 + depth(nested)`.
+type Frame = Box<dyn Fn(&[u8]) -> Vec<u8>>;
+
+struct Decoder {
+    name: &'static str,
+    frame: Frame,
+    decode: Decode,
+}
+
+fn command_document(payload: Vec<u8>) -> CultNetRawDocumentRecord {
+    CultNetRawDocumentRecord {
+        schema_id: CULTMESH_PROVIDER_COMMAND_SCHEMA.to_string(),
+        record_key: "command-1".to_string(),
+        stored_at: "unix:1000".to_string(),
+        payload_encoding: CultNetRawPayloadEncoding::Messagepack,
+        payload,
+        source_runtime_id: None,
+        source_agent_id: None,
+        source_role: None,
+        tags: None,
+    }
+}
+
+fn command_with_nil() -> Vec<u8> {
+    rmp_serde::to_vec_named(&CultMeshProviderCommandWire {
+        command_id: "command-1".into(),
+        command_kind: "reload".into(),
+        provider_id: "aetheria".into(),
+        service_instance_id: "aetheria-1".into(),
+        payload: rmpv::Value::Nil,
+    })
+    .unwrap()
+}
+
+fn receipt_with_nil() -> Vec<u8> {
+    rmp_serde::to_vec_named(&CultMeshProviderCommandReceiptWire {
+        receipt_id: "receipt-1".into(),
+        command_id: "command-1".into(),
+        command_kind: "reload".into(),
+        provider_id: "aetheria".into(),
+        service_instance_id: "aetheria-1".into(),
+        state: CultMeshProviderReceiptStateWire::Applied,
+        completed_at_utc: "2026-01-01T00:00:00Z".into(),
+        result: Some(rmpv::Value::Nil),
+        error: None,
+    })
+    .unwrap()
+}
+
+fn evidence_bytes() -> Vec<u8> {
+    encode_provider_connect_evidence(&CultMeshProviderConnectEvidenceWire {
+        client_session_id: "client-1".into(),
+        session_token: None,
+    })
+    .unwrap()
+}
+
+fn hello_bytes() -> Vec<u8> {
+    let hello = CultNetMessage::Hello {
+        runtime_id: "peer".into(),
+        runtime_kind: "rust-worker".into(),
+        agent_id: None,
+        role: None,
+        display_name: None,
+        supported_document_types: None,
+        supported_mutation_contracts: None,
+        supported_message_versions: None,
+        transport_profiles: None,
+        supports_schema_catalog: None,
+    };
+    encode_cultnet_message_to_vec(&hello, CultNetWireContract::CultNetSchemaV0).unwrap()
+}
+
+fn media_records() -> Vec<GameCultMediaWireRecord> {
+    vec![
+        GameCultMediaWireRecord::Video(GameCultMediaVideoAccessUnitRecord {
+            stream_id: "s".into(),
+            session_id: "x".into(),
+            frame_id: 7,
+            codec: "h264".into(),
+            pts_ticks: 90_000,
+            duration_ticks: 3_000,
+            timebase_num: 1,
+            timebase_den: 90_000,
+            keyframe: true,
+            dependency_frame_id: None,
+            deadline_ticks: 108_000,
+            chunk_index: 0,
+            chunk_count: 1,
+            payload: vec![1, 2, 3],
+        }),
+        GameCultMediaWireRecord::VideoParity(GameCultMediaVideoParityShardRecord {
+            stream_id: "s".into(),
+            session_id: "x".into(),
+            frame_id: 1,
+            codec: "h264".into(),
+            pts_ticks: 0,
+            duration_ticks: 1,
+            timebase_num: 1,
+            timebase_den: 90_000,
+            keyframe: false,
+            dependency_frame_id: Some(0),
+            deadline_ticks: 1,
+            chunk_count: 4,
+            fec_scheme: "rs-gf256-v1".into(),
+            block_index: 0,
+            block_count: 1,
+            block_data_start: 0,
+            block_data_count: 4,
+            parity_index: 1,
+            parity_count: 2,
+            shard_payload_bytes: 3,
+            last_chunk_payload_bytes: 2,
+            payload: vec![1, 2, 3],
+        }),
+        GameCultMediaWireRecord::AudioParity(GameCultMediaAudioParityShardRecord {
+            stream_id: "s".into(),
+            session_id: "x".into(),
+            codec: "opus".into(),
+            fec_scheme: "rs-gf256-v1".into(),
+            base_packet_id: 8,
+            base_pts_ticks: 0,
+            packet_duration_ticks: 960,
+            timebase_num: 1,
+            timebase_den: 48_000,
+            deadline_ticks: 4_000,
+            data_shard_count: 4,
+            parity_index: 0,
+            parity_shard_count: 2,
+            shard_payload_bytes: 3,
+            payload: vec![4, 5, 6],
+        }),
+        GameCultMediaWireRecord::Audio(GameCultMediaAudioPacketRecord {
+            stream_id: "s".into(),
+            session_id: "x".into(),
+            packet_id: 9,
+            codec: "opus".into(),
+            pts_ticks: 0,
+            duration_ticks: 960,
+            timebase_num: 1,
+            timebase_den: 48_000,
+            deadline_ticks: 1,
+            payload: vec![4, 5, 6],
+        }),
+        GameCultMediaWireRecord::Feedback(GameCultMediaReceiverFeedbackRecord {
+            stream_id: "s".into(),
+            session_id: "x".into(),
+            receiver_id: "obs".into(),
+            highest_decodable_frame_id: Some(41),
+            missing_frame_ids: vec![42],
+            late_frame_ids: vec![43],
+            requested_keyframe: true,
+            jitter_us: 500,
+            decode_queue_us: 2_000,
+            observed_at: "unix:1000".into(),
+            missing_video_chunk_keys: vec!["42:1".into()],
+        }),
+    ]
+}
+
+/// A valid media wire message whose record carries one more, unknown slot holding `nested`.
+fn media_wire_with_unknown_slot(record: &GameCultMediaWireRecord, nested: &[u8]) -> Vec<u8> {
+    let provenance =
+        MediaWireProvenance { stored_at: "unix:1000", runtime_id: "peer", role: "media", producer: "peer" };
+    let wire = encode_media_wire_record(record, provenance).unwrap();
+    let message = decode_cultnet_message_from_slice(&wire, CultNetWireContract::CultNetSchemaV0).unwrap();
+    let CultNetMessage::DocumentPutRaw { message_id, mut document } = message else { unreachable!() };
+    document.payload = add_trailing_slot(document.payload, nested);
+    let message = CultNetMessage::DocumentPutRaw { message_id, document };
+    encode_cultnet_message_to_vec(&message, CultNetWireContract::CultNetSchemaV0).unwrap()
+}
+
+const MEDIA_NAMES: [&str; 5] = [
+    "media video record",
+    "media video parity record",
+    "media audio parity record",
+    "media audio record",
+    "media feedback record",
+];
+
+fn decoders() -> Vec<Decoder> {
+    vec![
+        Decoder {
+            name: "provider command document (a free Value field)",
+            frame: Box::new(|nested| replace_trailing_nil(command_with_nil(), nested)),
+            decode: |payload| decode_provider_command_document(&command_document(payload.to_vec())).map(drop),
+        },
+        Decoder {
+            name: "provider session payload (a receipt's free Value field)",
+            frame: Box::new(|nested| replace_trailing_nil(receipt_with_nil(), nested)),
+            decode: |payload| {
+                decode_provider_session_payload::<CultMeshProviderCommandReceiptWire>(&STANDARD.encode(payload))
+                    .map(drop)
+            },
+        },
+        Decoder {
+            name: "provider connect evidence (an unknown key a named map skips)",
+            frame: Box::new(|nested| add_unknown_entry(evidence_bytes(), nested)),
+            decode: |payload| decode_provider_connect_evidence(payload).map(drop),
+        },
+        Decoder {
+            name: "CultNet message (a free rmpv::Value tree)",
+            frame: Box::new(|nested| add_unknown_entry(hello_bytes(), nested)),
+            decode: |payload| decode_cultnet_message_from_slice(payload, CultNetWireContract::CultNetSchemaV0).map(drop),
+        },
+    ]
+    .into_iter()
+    .chain(media_records().into_iter().zip(MEDIA_NAMES).map(|(record, name)| Decoder {
+        name,
+        frame: Box::new(move |nested| media_wire_with_unknown_slot(&record, nested)),
+        decode: |wire| decode_media_wire_record(wire).map(drop),
+    }))
+    .collect()
+}
+
+fn refusal(error: &Error) -> Option<PayloadTooDeep> {
+    error.downcast_ref::<PayloadTooDeep>().copied()
+}
+
+// The document is one container itself, so the free part nests 255 at the limit and 256 past it.
+#[test]
+fn every_payload_decoder_accepts_256_and_refuses_257_for_arrays_maps_and_arms() {
+    for decoder in decoders() {
+        for (kind, nesting) in NESTINGS {
+            let at = (decoder.frame)(&nesting(MAX_PAYLOAD_DEPTH - 1));
+            let past = (decoder.frame)(&nesting(MAX_PAYLOAD_DEPTH));
+            (decoder.decode)(&at).unwrap_or_else(|error| panic!("{} {kind} at the limit: {error:#}", decoder.name));
+            let error = (decoder.decode)(&past).expect_err(&format!("{} {kind} past the limit", decoder.name));
+            assert_eq!(refusal(&error), Some(TOO_DEEP), "{} {kind}: {error:#}", decoder.name);
+            assert!(!format!("{error:#}").contains(CANARY), "{} {kind} echoed the payload", decoder.name);
+        }
+    }
+}
+
+#[test]
+fn a_2000_deep_payload_is_a_typed_refusal_on_a_2_mib_stack() {
+    let handle = std::thread::Builder::new()
+        .stack_size(2 * 1024 * 1024)
+        .spawn(|| {
+            for decoder in decoders() {
+                let deep = (decoder.frame)(&deep_marker_payload());
+                let error = (decoder.decode)(&deep).expect_err(decoder.name);
+                assert_eq!(refusal(&error), Some(TOO_DEEP), "{}: {error:#}", decoder.name);
+            }
+        })
+        .unwrap();
+    handle.join().expect("a decoder aborted or panicked on a 2 MiB stack");
+}