diff --git a/src/lean_spec/subspecs/xmss/containers.py b/src/lean_spec/subspecs/xmss/containers.py
index 32467d517..565f6bfa6 100644
--- a/src/lean_spec/subspecs/xmss/containers.py
+++ b/src/lean_spec/subspecs/xmss/containers.py
@@ -8,7 +8,7 @@
 from __future__ import annotations
 
 from collections.abc import Mapping
-from typing import TYPE_CHECKING, NamedTuple
+from typing import TYPE_CHECKING, NamedTuple, override
 
 from pydantic import model_serializer
 
@@ -16,6 +16,7 @@
 
 from ...types import Bytes32, Uint64
 from ...types.container import Container
+from .constants import TARGET_CONFIG
 from .subtree import HashSubTree
 from .types import (
     HashDigestList,
@@ -62,10 +63,12 @@ class Signature(Container):
     - rho: Vector[Fp, RAND_LEN_FE]
     - hashes: List[Vector[Fp, HASH_DIGEST_LENGTH], NODE_LIST_LIMIT]
 
-    This is a variable-size Container because path and hashes are variable-size
-    fields. The field dimensions are determined by the scheme parameters, so in
-    practice every valid signature serializes to the same byte count, but the SSZ
-    type system correctly classifies it as variable-size.
+    Although the fields are internally variable-size SSZ types, every valid
+    signature serializes to exactly `SIGNATURE_LEN_BYTES`. This class overrides
+    `is_fixed_size()` to report as fixed-size so that parent containers treat
+    it as an opaque byte blob. This avoids leaking internal structure (field
+    count, offset layout) into the wire format, keeping the signature scheme
+    an implementation detail that can evolve independently.
     """
 
     path: HashTreeOpening
@@ -75,6 +78,23 @@ class Signature(Container):
     hashes: HashDigestList
     """The one-time signature itself: a list of intermediate Winternitz chain hashes."""
 
+    @classmethod
+    @override
+    def is_fixed_size(cls) -> bool:
+        """
+        Report as fixed-size for cross-client SSZ interoperability.
+
+        Ream serializes XMSS signatures as `FixedBytes<3112>`, so parent
+        containers must inline the bytes without an offset pointer.
+        """
+        return True
+
+    @classmethod
+    @override
+    def get_byte_length(cls) -> int:
+        """Return the fixed byte length of the SSZ-encoded signature."""
+        return TARGET_CONFIG.SIGNATURE_LEN_BYTES
+
     @model_serializer(mode="plain", when_used="json")
     def _serialize_as_bytes(self) -> str:
         """Serialize as hex-encoded SSZ bytes for JSON output."""
diff --git a/tests/consensus/devnet/ssz/test_consensus_containers.py b/tests/consensus/devnet/ssz/test_consensus_containers.py
index e18963998..0dc2a51f6 100644
--- a/tests/consensus/devnet/ssz/test_consensus_containers.py
+++ b/tests/consensus/devnet/ssz/test_consensus_containers.py
@@ -2,6 +2,7 @@
 
 import pytest
 from consensus_testing import SSZTestFiller
+from consensus_testing.keys import create_dummy_signature
 
 from lean_spec.subspecs.containers import (
     AggregatedAttestation,
@@ -33,7 +34,6 @@
     JustifiedSlots,
     Validators,
 )
-from lean_spec.subspecs.xmss import Signature
 from lean_spec.subspecs.xmss.aggregation import AggregatedSignatureProof
 from lean_spec.types import Boolean, ByteListMiB, Bytes32, Bytes52, Uint64
 
@@ -59,16 +59,6 @@ def _typical_attestation_data() -> AttestationData:
     return AttestationData(slot=Slot(100), head=head, target=target, source=source)
 
 
-# Empty signature: path=[], rho=zeros, hashes=[]
-EMPTY_SIGNATURE_BYTES = bytes.fromhex(
-    "24000000000000000000000000000000000000000000000000000000000000002800000004000000"
-)
-
-
-def _empty_signature() -> Signature:
-    return Signature.decode_bytes(EMPTY_SIGNATURE_BYTES)
-
-
 # --- Checkpoint ---
 
 
@@ -127,7 +117,7 @@ def test_signed_attestation_minimal(ssz: SSZTestFiller) -> None:
         value=SignedAttestation(
             validator_id=ValidatorIndex(0),
             data=_zero_attestation_data(),
-            signature=_empty_signature(),
+            signature=create_dummy_signature(),
         ),
     )
 
@@ -286,7 +276,7 @@ def test_block_signatures_empty(ssz: SSZTestFiller) -> None:
         type_name="BlockSignatures",
         value=BlockSignatures(
             attestation_signatures=AttestationSignatures(data=[]),
-            proposer_signature=_empty_signature(),
+            proposer_signature=create_dummy_signature(),
         ),
     )
 
@@ -304,7 +294,7 @@ def test_block_signatures_with_attestation(ssz: SSZTestFiller) -> None:
                     )
                 ]
             ),
-            proposer_signature=_empty_signature(),
+            proposer_signature=create_dummy_signature(),
         ),
     )
 
@@ -325,7 +315,7 @@ def test_signed_block_with_attestation_minimal(ssz: SSZTestFiller) -> None:
     message = BlockWithAttestation(block=block, proposer_attestation=attestation)
     signature = BlockSignatures(
         attestation_signatures=AttestationSignatures(data=[]),
-        proposer_signature=_empty_signature(),
+        proposer_signature=create_dummy_signature(),
     )
     ssz(
         type_name="SignedBlockWithAttestation",
diff --git a/tests/lean_spec/subspecs/xmss/test_cross_client_compat.py b/tests/lean_spec/subspecs/xmss/test_cross_client_compat.py
index 995c00db0..f65e7dc6f 100644
--- a/tests/lean_spec/subspecs/xmss/test_cross_client_compat.py
+++ b/tests/lean_spec/subspecs/xmss/test_cross_client_compat.py
@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+from typing import override
+
 import pytest
 
 from lean_spec.subspecs.containers.slot import Slot
@@ -35,6 +37,18 @@ class ProdSignature(Container):
     rho: Randomness
     hashes: ProdHashDigestList
 
+    @classmethod
+    @override
+    def is_fixed_size(cls) -> bool:
+        """Report as fixed-size, matching ream's `FixedBytes<3112>`."""
+        return True
+
+    @classmethod
+    @override
+    def get_byte_length(cls) -> int:
+        """Return the prod signature byte length."""
+        return PROD_CONFIG.SIGNATURE_LEN_BYTES
+
 
 REAM_PUBLIC_KEY_HEX = (
     "7bbaf95bd653c827b5775e00b973b24d50ab4743db3373244f29c95fdf4ccc62"