diff --git a/tests/integration/messages_endpoint/test_beta_gated_fields_wire.py b/tests/integration/messages_endpoint/test_beta_gated_fields_wire.py
new file mode 100644
index 00000000000..87540b3fc90
--- /dev/null
+++ b/tests/integration/messages_endpoint/test_beta_gated_fields_wire.py
@@ -0,0 +1,155 @@
+import json
+import re
+import uuid
+from collections.abc import Iterator, Mapping
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Final
+
+import pytest
+from integration._support.client import Gateway, gateway_from_environment, list_value, object_value
+from integration._support.process import owned_proxy
+from integration._support.wire import Reply, Request, Wire, wire_server
+from pydantic import JsonValue, TypeAdapter
+
+pytestmark: Final = pytest.mark.shares_worker_state
+
+ANTHROPIC_MODEL: Final = "beta-gated-anthropic"
+FOUNDRY_MODEL: Final = "beta-gated-foundry"
+CLEAR_AT_BETA: Final = "mid-conversation-system-clear-at-2026-08-21"
+_JSON_OBJECT: Final = TypeAdapter(dict[str, JsonValue])
+_NONCE: Final = re.compile(rb"nonce-([0-9a-f]{32})")
+
+
+def nonce() -> str:
+    return f"nonce-{uuid.uuid4().hex}"
+
+
+def claude_reply(request: Request) -> Reply:
+    match: Final = _NONCE.search(request.body)
+    assert match is not None, request.body[:200]
+    identity: Final = match.group(1).decode()
+    return Reply(
+        body=json.dumps(
+            {
+                "id": f"msg_{identity}",
+                "type": "message",
+                "role": "assistant",
+                "model": "claude-opus-4-8",
+                "content": [{"type": "text", "text": f"answered {identity}"}],
+                "stop_reason": "end_turn",
+                "stop_sequence": None,
+                "usage": {"input_tokens": 12, "output_tokens": 3},
+            }
+        ).encode()
+    )
+
+
+def respond(request: Request) -> Reply:
+    if request.method != "POST":
+        return Reply(body=b'{"object":"list","data":[]}')
+    return claude_reply(request)
+
+
+def deployment(name: str, model: str, api_base: str) -> dict[str, JsonValue]:
+    return {
+        "model_name": name,
+        "litellm_params": {"model": model, "api_base": api_base, "api_key": f"synthetic-{name}-key"},
+    }
+
+
+def write_config(directory: Path, wire_url: str) -> Path:
+    path: Final = directory / f"beta-gated-{uuid.uuid4().hex}.json"
+    path.write_text(
+        json.dumps(
+            {
+                "model_list": [
+                    deployment(ANTHROPIC_MODEL, "anthropic/claude-opus-4-8", f"{wire_url}/anthropic"),
+                    deployment(FOUNDRY_MODEL, "azure_ai/claude-opus-4-8", f"{wire_url}/foundry"),
+                ],
+                "general_settings": {
+                    "master_key": "os.environ/LITELLM_MASTER_KEY",
+                    "database_url": "os.environ/DATABASE_URL",
+                },
+            }
+        )
+    )
+    return path
+
+
+@dataclass(frozen=True, slots=True)
+class Rig:
+    gateway: Gateway
+    wire: Wire
+
+    def send(self, model: str, request_nonce: str) -> tuple[int, bytes]:
+        response: Final = self.gateway.client.post(
+            "/v1/messages",
+            json={
+                "model": model,
+                "max_tokens": 64,
+                "messages": [
+                    {"role": "user", "content": f"hi {request_nonce}"},
+                    {"role": "assistant", "content": "Hello"},
+                    {"role": "user", "content": "how are you"},
+                    {"role": "system", "content": "Answer in French", "clear_at": "next_user_message"},
+                ],
+            },
+            headers={"Authorization": f"Bearer {self.gateway.key}", "anthropic-beta": CLEAR_AT_BETA},
+            timeout=120,
+        )
+        return response.status_code, response.content
+
+    def received(self, request_nonce: str) -> Request:
+        requests: Final = tuple(request for request in self.wire.drain() if request_nonce.encode() in request.body)
+        assert len(requests) == 1, [request.target for request in requests]
+        return requests[0]
+
+
+@pytest.fixture(scope="module")
+def rig(tmp_path_factory: pytest.TempPathFactory) -> Iterator[Rig]:
+    directory: Final = tmp_path_factory.mktemp("beta-gated")
+    with (
+        gateway_from_environment() as shared,
+        wire_server(respond) as wire,
+        owned_proxy(shared, directory, {}, config=write_config(directory, wire.url), workers=1) as gateway,
+    ):
+        yield Rig(gateway, wire)
+
+
+def reply_text(status: int, raw: bytes) -> str:
+    assert status == 200, raw[:500]
+    return "".join(str(object_value(block)["text"]) for block in list_value(_JSON_OBJECT.validate_json(raw)["content"]))
+
+
+def beta_header(headers: Mapping[str, str]) -> str:
+    return ",".join(value for key, value in headers.items() if key.lower() == "anthropic-beta")
+
+
+def system_message(body: bytes) -> dict[str, JsonValue]:
+    messages: Final = list_value(_JSON_OBJECT.validate_json(body)["messages"])
+    system: Final = [object_value(m) for m in messages if object_value(m).get("role") == "system"]
+    assert len(system) == 1, messages
+    return system[0]
+
+
+def test_a_leg_whose_row_lacks_the_beta_sends_neither_the_beta_nor_clear_at(rig: Rig) -> None:
+    request_nonce: Final = nonce()
+    status, raw = rig.send(FOUNDRY_MODEL, request_nonce)
+    assert reply_text(status, raw) == f"answered {request_nonce.removeprefix('nonce-')}"
+    upstream: Final = rig.received(request_nonce)
+    assert CLEAR_AT_BETA not in beta_header(upstream.headers), upstream.headers
+    assert system_message(upstream.body) == {"role": "system", "content": "Answer in French"}
+
+
+def test_a_leg_that_carries_the_beta_forwards_clear_at_untouched(rig: Rig) -> None:
+    request_nonce: Final = nonce()
+    status, raw = rig.send(ANTHROPIC_MODEL, request_nonce)
+    assert reply_text(status, raw) == f"answered {request_nonce.removeprefix('nonce-')}"
+    upstream: Final = rig.received(request_nonce)
+    assert CLEAR_AT_BETA in beta_header(upstream.headers), upstream.headers
+    assert system_message(upstream.body) == {
+        "role": "system",
+        "content": "Answer in French",
+        "clear_at": "next_user_message",
+    }