diff --git a/.changeset/kms-router-one-vendor-type.md b/.changeset/kms-router-one-vendor-type.md
new file mode 100644
index 000000000..a6511e9a0
--- /dev/null
+++ b/.changeset/kms-router-one-vendor-type.md
@@ -0,0 +1,5 @@
+---
+'@cipherstash/auth': patch
+---
+
+A key router now holds ZeroKMS and at most one key vendor backend by its type. The ZeroKMS side of a router must be a ZeroKMS key source, and a router is not one, so a router that holds a second router does not compile. No runtime behaviour changes.
diff --git a/docs/plans/2026-11-05-key-vendor-backends-decisions.md b/docs/plans/2026-11-05-key-vendor-backends-decisions.md
index 8a4f6a49e..6796faaa1 100644
--- a/docs/plans/2026-11-05-key-vendor-backends-decisions.md
+++ b/docs/plans/2026-11-05-key-vendor-backends-decisions.md
@@ -365,9 +365,15 @@ the alternatives and the reason.
   in `stack-kms-vendor`. A keyset id or name that a vendor record has goes
   to the vendor (a local record name wins over a ZeroKMS keyset of the same
   name); everything else, and the default keyset, goes to ZeroKMS. The type
-  holds exactly one vendor; `VendorKeySource::new` refuses records of two
-  vendors with `RecordError::SecondVendor`, and a record of another vendor
-  than the backend with `RecordError::ForeignVendor`.
+  holds at most one vendor: its ZeroKMS side must implement the marker
+  `stack_kms::ZeroKmsKeySource`, which no router implements, so
+  `KeyRouter<KeyRouter<_, Aws>, Gcp>` does not compile (a `compile_fail`
+  doc test shows it). This was not true when first written: the ZeroKMS
+  side took any `DataKeySource`, so a router could hold a second router; the
+  review after CIP-5313 found it and the marker closes it.
+  `VendorKeySource::new` refuses records of two vendors with
+  `RecordError::SecondVendor`, and a record of another vendor than the
+  backend with `RecordError::ForeignVendor`.
 - **Decision:** the static key backend (`StaticKeyVendor`) wraps with
   AES-256-GCM under a random 96-bit nonce: `version(1)=1 ‖ nonce(12) ‖
   ciphertext ‖ tag`. It holds several keys by reference, for rotation.
@@ -1810,6 +1816,10 @@ ticket that recorded it.
 - CIP-5311 → CIP-5313: the capability matrix on the Rust side.
 - CIP-5312 → CIP-5313: the CLI README section for `stash keyset` (the docs
   site is still open, above).
+- **Review after CIP-5313 (FIXED):** "at most one vendor" was not held by
+  the type: `KeyRouter`'s ZeroKMS side took any `DataKeySource`, so a router
+  could nest a second router and vendor. The ZeroKMS side must now implement
+  `stack_kms::ZeroKmsKeySource`, which no router implements.
 - **CIP-5313 (FIXED, defect):** a rewrap of a wrapped-data-key record to
   another vendor key made its stored values unreadable. `KeysetRecord::rewrap`
   now refuses it (`RecordError::RewrapToAnotherKey`; `REWRAP_REFUSED` in
diff --git a/languages/golang/encrypt/guest/src/zerokms.rs b/languages/golang/encrypt/guest/src/zerokms.rs
index 29dc187fd..2b011efb8 100644
--- a/languages/golang/encrypt/guest/src/zerokms.rs
+++ b/languages/golang/encrypt/guest/src/zerokms.rs
@@ -27,6 +27,9 @@ pub enum MaybeZeroKms<Z> {
     Absent,
 }
 
+// MaybeZeroKms stands for ZeroKMS (present or not), never for a router.
+impl<Z: stack_kms::ZeroKmsKeySource> stack_kms::ZeroKmsKeySource for MaybeZeroKms<Z> {}
+
 /// The cause of [`no_zerokms`]: its type is what the status layer finds to
 /// report `STATUS_NO_ZEROKMS`.
 #[derive(Debug, Clone, Copy)]
diff --git a/packages/stack-kms-vendor/src/router.rs b/packages/stack-kms-vendor/src/router.rs
index 59507722e..de083c5c8 100644
--- a/packages/stack-kms-vendor/src/router.rs
+++ b/packages/stack-kms-vendor/src/router.rs
@@ -5,7 +5,7 @@ use std::borrow::Cow;
 use stack_kms::{
     DataKey, DataKeySource, DataKeyWithTag, Error, GenerateKeyPayload, IdentifiedBy, IndexKey,
     IndexKeySource, KeyBackend, KeyMode, LoadedIndexKey, RetrieveKeyPayload, SelectedKeyset,
-    UnverifiedContext,
+    UnverifiedContext, ZeroKmsKeySource,
 };
 use uuid::Uuid;
 
@@ -17,8 +17,24 @@ use crate::vendor::KeyVendorBackend;
 ///
 /// A cipher holds ZeroKMS and **at most one** key vendor backend. The type
 /// has room for one or none: [`new`](Self::new) gives it one, and
-/// [`zerokms_only`](Self::zerokms_only) none. [`VendorKeySource::new`]
-/// refuses records that name a second vendor
+/// [`zerokms_only`](Self::zerokms_only) none. The ZeroKMS side must be a
+/// [`ZeroKmsKeySource`], which a router is not, so a router cannot hold a
+/// second router and its vendor:
+///
+/// ```compile_fail
+/// use stack_kms::FakeDataKeySource;
+/// use stack_kms_vendor::{FakeVendor, KeyRouter, VendorKeySource};
+///
+/// fn nested(
+///     inner: KeyRouter<FakeDataKeySource, FakeVendor>,
+///     vendor: VendorKeySource<FakeVendor>,
+/// ) {
+///     // A router is not a ZeroKmsKeySource.
+///     let _ = KeyRouter::new(inner, vendor);
+/// }
+/// ```
+///
+/// [`VendorKeySource::new`] refuses records that name a second vendor
 /// ([`RecordError::SecondVendor`](crate::RecordError::SecondVendor)).
 ///
 /// Routing:
@@ -52,7 +68,7 @@ impl<Z, B> std::fmt::Debug for KeyRouter<Z, B> {
     }
 }
 
-impl<Z, B: KeyVendorBackend> KeyRouter<Z, B> {
+impl<Z: ZeroKmsKeySource, B: KeyVendorBackend> KeyRouter<Z, B> {
     /// Route between `zerokms` and `vendor`.
     pub fn new(zerokms: Z, vendor: VendorKeySource<B>) -> Self {
         Self {
@@ -71,7 +87,9 @@ impl<Z, B: KeyVendorBackend> KeyRouter<Z, B> {
             vendor_default: false,
         }
     }
+}
 
+impl<Z, B: KeyVendorBackend> KeyRouter<Z, B> {
     /// Make naming no keyset select the vendor's default keyset (its only
     /// record, or the one set with
     /// [`VendorKeySource::with_default_keyset`]), not ZeroKMS's. For a
diff --git a/packages/stack-kms/src/key_source.rs b/packages/stack-kms/src/key_source.rs
index 1e007f758..3b00975e7 100644
--- a/packages/stack-kms/src/key_source.rs
+++ b/packages/stack-kms/src/key_source.rs
@@ -255,6 +255,18 @@ impl LoadedIndexKey {
     }
 }
 
+/// A key source that stands for ZeroKMS: the ZeroKMS side of a router that
+/// adds a key vendor backend (`stack_kms_vendor::KeyRouter`).
+///
+/// It is a marker. The router asks for it so that its ZeroKMS side cannot be
+/// another router, which would put two key vendor backends in one cipher
+/// (ADR-0009: ZeroKMS and at most one vendor). Implement it only for a
+/// source that talks to ZeroKMS, or that takes its place in a test. Never
+/// implement it for a source that routes to a key vendor backend.
+pub trait ZeroKmsKeySource {}
+
+impl<C, Conn> ZeroKmsKeySource for crate::StackKms<C, Conn> {}
+
 impl<C, Conn> DataKeySource for crate::StackKms<C, Conn>
 where
     C: stack_auth::AuthStrategyBounds,
@@ -403,6 +415,8 @@ mod fake {
         }
     }
 
+    impl super::ZeroKmsKeySource for FakeDataKeySource {}
+
     impl DataKeySource for FakeDataKeySource {
         async fn generate_keys(
             &self,
@@ -529,6 +543,8 @@ mod deterministic {
         }
     }
 
+    impl super::ZeroKmsKeySource for DeterministicSource {}
+
     impl DataKeySource for DeterministicSource {
         async fn generate_keys(
             &self,
diff --git a/packages/stack-kms/src/lib.rs b/packages/stack-kms/src/lib.rs
index 375a863e1..1de089a2f 100644
--- a/packages/stack-kms/src/lib.rs
+++ b/packages/stack-kms/src/lib.rs
@@ -149,7 +149,9 @@ pub use backend::{KeyBackend, KeyMode, KeyVendor};
 pub use key_source::DeterministicSource;
 #[cfg(feature = "test-support")]
 pub use key_source::FakeDataKeySource;
-pub use key_source::{DataKeySource, IndexKeySource, KeyLifetime, LoadedIndexKey, SelectedKeyset};
+pub use key_source::{
+    DataKeySource, IndexKeySource, KeyLifetime, LoadedIndexKey, SelectedKeyset, ZeroKmsKeySource,
+};
 
 // Key providers
 pub use key_provider::{