diff --git a/src/ca2a_runtime/transport/a2a_sdk.py b/src/ca2a_runtime/transport/a2a_sdk.py
index 34a7b7d..030d513 100644
--- a/src/ca2a_runtime/transport/a2a_sdk.py
+++ b/src/ca2a_runtime/transport/a2a_sdk.py
@@ -263,8 +263,14 @@ def attach_to_sdk_message(message: Any, request: PeerRequest) -> Any:
     speak HTTP.
     """
     _require_protobuf()
-    merged = dict(metadata_from_sdk_message(message))
-    merged.update(a2a_adapter.attach_ca2a_metadata({}, request)["metadata"])
+    # Attach against the message's *current* metadata, not a throwaway ``{}``: the
+    # dict adapter clears an absent optional key (sealed_payload / caller_offer /
+    # holder_proof) with ``meta.pop(...)``, and those pops only bite when they run
+    # against the real metadata. Passing ``{}`` here would make them no-ops and let
+    # a reused Message ship a prior request's stale optional keys.
+    merged = a2a_adapter.attach_ca2a_metadata(
+        {"metadata": dict(metadata_from_sdk_message(message))}, request
+    )["metadata"]
     message.metadata.Clear()
     json_format.ParseDict(merged, message.metadata)
 
diff --git a/tests/unit/test_a2a_sdk_bridge.py b/tests/unit/test_a2a_sdk_bridge.py
index cc482b4..42ad19f 100644
--- a/tests/unit/test_a2a_sdk_bridge.py
+++ b/tests/unit/test_a2a_sdk_bridge.py
@@ -452,6 +452,52 @@ def test_attach_declares_the_extension_on_the_message() -> None:
     assert list(message.extensions).count(EXTENSION_URI) == 1
 
 
+def test_reused_message_clears_a_stale_sealed_payload() -> None:
+    """A reused SDK message must not carry a prior request's sealed ciphertext.
+
+    ``attach_to_sdk_message`` mutates in place, so an application that reuses one
+    ``Message`` across tasks would otherwise ship task N-1's ``sealed_payload`` as
+    task N's: cross-request payload confusion.
+    """
+    message = Message(message_id="m1")
+    a2a_sdk.attach_to_sdk_message(message, _request(sealed_payload=b"SECRET"))
+    a2a_sdk.attach_to_sdk_message(message, _request())
+    parsed = a2a_sdk.parse_sdk_message(message)
+    assert parsed is not None
+    assert parsed.sealed_payload is None
+
+
+def test_reused_message_clears_a_stale_caller_offer() -> None:
+    """A reused SDK message must not carry a prior request's caller offer."""
+    offer = ChannelOffer(
+        channel_public_key="k" * 43,
+        report=AttestationReport(
+            platform="software-only",
+            measurement="m",
+            public_key="k" * 43,
+            nonce="v1.123.abc.def",
+        ),
+    )
+    message = Message(message_id="m1")
+    a2a_sdk.attach_to_sdk_message(message, _request(caller_offer=offer))
+    a2a_sdk.attach_to_sdk_message(message, _request())
+    parsed = a2a_sdk.parse_sdk_message(message)
+    assert parsed is not None
+    assert parsed.caller_offer is None
+
+
+def test_reused_message_clears_a_stale_holder_proof() -> None:
+    """A reused SDK message must not carry a prior request's holder proof."""
+    chain, leaf_key = _chain_with_keys()
+    node = PeerNode(LocalPolicy.of({"read"}), trusted_root_issuers={chain[0].issuer})
+    message = Message(message_id="m1")
+    a2a_sdk.attach_to_sdk_message(message, _request(node=node, chain=chain, leaf_key=leaf_key))
+    a2a_sdk.attach_to_sdk_message(message, _request())
+    parsed = a2a_sdk.parse_sdk_message(message)
+    assert parsed is not None
+    assert parsed.holder_proof is None
+
+
 # --------------------------------------------------------------------------
 # Not-a-cA2A-message, and failing closed
 # --------------------------------------------------------------------------