diff --git a/gateway/relay/adapter.py b/gateway/relay/adapter.py
index 03529c3f99..01b4cedce6 100644
--- a/gateway/relay/adapter.py
+++ b/gateway/relay/adapter.py
@@ -95,6 +95,17 @@ def _event_ids(event) -> Tuple[Optional[str], Optional[str]]:
     return message_id, getattr(event.source, "chat_id", None)
 
 
+def _profile_from_session_key(session_key: str) -> Optional[str]:
+    """Named profile encoded in an ``agent:<ns>:...`` session key; None for the legacy ``agent:main``
+    namespace (single-profile gateway) so the wire frame stays byte-identical there."""
+    parts = (session_key or "").split(":")
+    if len(parts) < 2 or parts[0] != "agent" or not parts[1]:
+        return None
+    from gateway.session import profile_from_session_key_namespace
+    profile = profile_from_session_key_namespace(parts[1])
+    return None if profile == "default" else profile
+
+
 class RelayAdapter(BasePlatformAdapter):
     """Generic relay adapter advertising a connector-negotiated capability profile."""
 
@@ -125,6 +136,10 @@ class RelayAdapter(BasePlatformAdapter):
         # platforms on one WS and a reply must egress through the platform the
         # inbound came from. Empty for a single-platform gateway (connector default).
         self._platform_by_chat: Dict[str, str] = {}
+        # chat_id -> Hermes profile the connector routed the inbound to (multiplex mode). Echoed
+        # on every outbound frame's metadata so the connector can stamp the SAME profile on the
+        # next passthrough_forward for that chat; empty on a single-profile gateway.
+        self._profile_by_chat: Dict[str, str] = {}
         # Chats the connector has refused (see the terminal-decline latch).
         # chat_id -> (thread_id, initial_name) of the auto-thread the CONNECTOR
         # created for our latest send; read by the semantic thread-rename lane.
@@ -1042,6 +1057,7 @@ class RelayAdapter(BasePlatformAdapter):
             for attr, cache in (
                 ("user_id", self._dm_user_by_chat), ("scope_id", self._scope_by_chat),
                 ("chat_type", self._chat_type_by_chat),
+                ("profile", self.__dict__.setdefault("_profile_by_chat", {})),
             ):
                 value = getattr(src, attr, None)
                 if value:
@@ -1059,7 +1075,11 @@ class RelayAdapter(BasePlatformAdapter):
         first and only falls back to user_id on a route miss, so carrying both never
         overrides routing-table resolution."""
         meta: Dict[str, Any] = dict(metadata or {})
-        for key, cache in (("scope_id", self._scope_by_chat), ("user_id", self._dm_user_by_chat)):
+        # ``getattr``: relay tests build bare adapters via ``__new__`` without ``__init__``.
+        for key, cache in (
+            ("scope_id", self._scope_by_chat), ("user_id", self._dm_user_by_chat),
+            ("profile", getattr(self, "_profile_by_chat", {})),
+        ):
             if not meta.get(key):
                 value = cache.get(str(chat_id))
                 if value:
@@ -1698,13 +1718,20 @@ class RelayAdapter(BasePlatformAdapter):
         # default routes it.
         prefix = kind.split(".", 1)[0] if kind and "." in kind else None
         follow_up_platform = prefix if prefix and self.fronts_platform(prefix) else None
+        follow_up_metadata = dict(metadata or {})
+        # The session key names the profile namespace the interaction ran under; carry it so the
+        # connector's next passthrough_forward for this interaction routes to the same profile.
+        if not follow_up_metadata.get("profile"):
+            profile = _profile_from_session_key(session_key)
+            if profile:
+                follow_up_metadata["profile"] = profile
         result = await self._transport.send_follow_up(
             {
                 "op": "follow_up",
                 "session_key": session_key,
                 "kind": kind,
                 "content": content,
-                "metadata": metadata or {},
+                "metadata": follow_up_metadata,
             },
             platform=follow_up_platform,
         )
diff --git a/tests/gateway/relay/test_relay_passthrough.py b/tests/gateway/relay/test_relay_passthrough.py
index a8e27e4335..e19c6d6a37 100644
--- a/tests/gateway/relay/test_relay_passthrough.py
+++ b/tests/gateway/relay/test_relay_passthrough.py
@@ -266,3 +266,42 @@ async def test_dm_interaction_keys_as_discord_dm(adapter, monkeypatch):
     assert ev.source.delivered_via_upstream_relay is True
 
 
+
+
+@pytest.mark.asyncio
+async def test_routed_profile_round_trips_on_every_egress_frame(adapter, monkeypatch):
+    """Relay passthrough round-trip keeps ``profile`` (#88715 phase 5): the profile the connector
+    stamped on an inbound interaction is echoed on the chat's outbound frames and on the
+    ``follow_up`` addressed by the routed session key, so the connector can stamp it on the NEXT
+    passthrough_forward for that chat; a single-profile gateway emits no ``profile`` key at all."""
+    await adapter.connect()
+    stub = adapter._transport
+    monkeypatch.setattr(adapter, "handle_message", _noop_handle)
+
+    fwd = _interaction_forward(
+        {
+            "id": "interaction-3", "type": 2, "channel_id": "chan-9", "guild_id": "guild-7",
+            "data": {"name": "summarize"}, "member": {"user": {"id": "user-3", "username": "ben"}},
+        },
+        profile="reviewer",
+    )
+    await stub.push_passthrough(fwd, buffer_id=None)
+    await adapter.send("chan-9", "done")
+    await adapter.send_follow_up(
+        session_key="agent:reviewer:discord:group:chan-9", kind="discord.interaction_token", content="x")
+    assert stub.sent[-1]["metadata"]["profile"] == "reviewer"
+    assert stub.follow_ups[-1]["metadata"]["profile"] == "reviewer"
+
+    # Legacy namespace / unrouted chat: byte-identical frames, no profile key.
+    await stub.push_passthrough(_interaction_forward({
+        "id": "interaction-4", "type": 2, "channel_id": "chan-1", "guild_id": "guild-7",
+        "data": {"name": "summarize"}, "member": {"user": {"id": "user-3"}}}), buffer_id=None)
+    await adapter.send("chan-1", "done")
+    await adapter.send_follow_up(
+        session_key="agent:main:discord:group:chan-1", kind="discord.interaction_token", content="x")
+    assert "profile" not in stub.sent[-1]["metadata"]
+    assert "profile" not in stub.follow_ups[-1]["metadata"]
+
+
+async def _noop_handle(event):
+    return None
diff --git a/website/docs/developer-guide/relay-connector-contract.md b/website/docs/developer-guide/relay-connector-contract.md
index 1a7552de68..bc8ffa9fb3 100644
--- a/website/docs/developer-guide/relay-connector-contract.md
+++ b/website/docs/developer-guide/relay-connector-contract.md
@@ -423,6 +423,16 @@ The gateway calls the transport with action dicts. Source of truth:
 `get_chat_info(chat_id)` is a separate proxied call returning at least
 `{name, type}`.
 
+**`metadata.profile` (multiplex round-trip).** Every chat-addressed outbound
+frame's `metadata` carries the tenant discriminators the gateway captured from
+the inbound (`scope_id`, `user_id`) and, on a multiplexed gateway, the Hermes
+`profile` the connector routed that chat's inbound to; `follow_up` frames carry
+the profile encoded in their `session_key` namespace. The connector MUST stamp
+the same `profile` on the next `passthrough_forward` / `inbound` for that chat or
+interaction, so a Discord button press after a slash command lands in the same
+profile's session (`gateway/relay/adapter.py::_with_scope`, `send_follow_up`).
+Single-profile gateways never emit the key — frames stay byte-identical.
+
 **`send_media` (Phase 2 media egress).** Media crosses the wire BY REFERENCE:
 `source_url` is either (a) a **connector re-host** the gateway previously
 uploaded via `POST {connector}/relay/media` (raw bytes body, `Content-Type` +