diff --git a/tests/websocket/test_close_code_handling.py b/tests/websocket/test_close_code_handling.py
new file mode 100644
index 000000000..72106bb45
--- /dev/null
+++ b/tests/websocket/test_close_code_handling.py
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+"""Close-code handling for the agent WebSocket connection.
+
+Platform contract (thenvoi-platform ``spec/websocket-api.draft.md``, the
+``agent_control`` channel): the ``supersede`` push is terminal
+(``retryable: false``) — after receiving it the SDK must stay down no matter
+which WebSocket close code is observed when the platform then drops the
+socket. Since Phoenix 1.8 the platform closes evicted agent sockets with
+close code 1001 ("going away"), and the push can be missed entirely when the
+close outruns the platform's short drain window — in that case the close
+code itself is the SDK's only signal, so its classification is pinned here
+(literal 1001, mirroring the platform's own close-code pin).
+
+These tests drive the real PHXChannelsClient supervisor against an
+in-process fake Phoenix server, so the assertions cover the actual
+reconnect loop rather than a mock of it.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import json
+import logging
+
+from phoenix_channels_python_client.client import PHXChannelsClient
+from phoenix_channels_python_client.client_types import ReconnectPolicy
+from websockets.asyncio.server import ServerConnection, serve
+from websockets.exceptions import ConnectionClosed
+
+from band.client.streaming import SupersedePayload, WebSocketClient
+
+logger = logging.getLogger(__name__)
+
+# Payload shape from the platform spec (Supersede Payload).
+SUPERSEDE_PAYLOAD: dict = {
+    "reason": "session.already_connected",
+    "message": "This connection has been superseded by a newer session for this agent.",
+    "retryable": False,
+    "retry_after": 30,
+    "target_socket_id": "agent_socket:agent-123",
+    "correlation_id": "evict-abc123",
+}
+
+# Generous ceiling on the client's first reconnect delay (base 0.5s, factor 2,
+# equal jitter → ≤1s for the first attempt). If no reconnect landed within
+# this window, the supervisor has stopped.
+RECONNECT_GRACE_S = 2.0
+
+
+class FakePhoenixServer:
+    """Minimal Phoenix Channels V2 server: acks joins, pushes, closes."""
+
+    def __init__(self) -> None:
+        self.connection_count = 0
+        # topic -> join_ref of the current subscription; pushes must carry it
+        # (the client drops pushes whose join_ref doesn't match the join).
+        self.joined_topics: dict[str, str] = {}
+        self._current: ServerConnection | None = None
+        self._connected = asyncio.Event()
+        self._server = None
+        self.url = ""
+
+    async def __aenter__(self) -> "FakePhoenixServer":
+        self._server = await serve(self._handler, "127.0.0.1", 0)
+        port = self._server.sockets[0].getsockname()[1]
+        self.url = f"ws://127.0.0.1:{port}/api/v1/socket/websocket"
+        return self
+
+    async def __aexit__(self, exc_type, exc_val, exc_tb) -> None:
+        self._server.close()
+        await self._server.wait_closed()
+
+    async def _handler(self, ws: ServerConnection) -> None:
+        self.connection_count += 1
+        self._current = ws
+        self._connected.set()
+        try:
+            async for raw in ws:
+                join_ref, ref, topic, event, _payload = json.loads(raw)
+                if event in {"phx_join", "phx_leave"}:
+                    reply = [
+                        join_ref,
+                        ref,
+                        topic,
+                        "phx_reply",
+                        {"status": "ok", "response": {}},
+                    ]
+                    await ws.send(json.dumps(reply))
+                    if event == "phx_join":
+                        self.joined_topics[topic] = join_ref
+                elif event == "heartbeat":
+                    reply = [
+                        None,
+                        ref,
+                        "phoenix",
+                        "phx_reply",
+                        {"status": "ok", "response": {}},
+                    ]
+                    await ws.send(json.dumps(reply))
+        except ConnectionClosed:
+            logger.debug("Fake server connection closed")
+
+    async def wait_for_connection(self, timeout: float = 5.0) -> None:
+        await asyncio.wait_for(self._connected.wait(), timeout=timeout)
+        self._connected.clear()
+
+    async def push(self, topic: str, event: str, payload: dict) -> None:
+        assert self._current is not None
+        join_ref = self.joined_topics.get(topic)
+        await self._current.send(json.dumps([join_ref, None, topic, event, payload]))
+
+    async def close_current(self, code: int, reason: str = "") -> None:
+        assert self._current is not None
+        await self._current.close(code=code, reason=reason)
+
+    def abort_current(self) -> None:
+        """Drop the TCP connection without a close frame (abnormal close)."""
+        assert self._current is not None
+        self._current.transport.abort()
+
+
+async def test_supersede_push_is_terminal_regardless_of_close_code():
+    """After the supersede push, the SDK stays down even on a retryable close code.
+
+    The server closes with 1011 (server error), which the reconnect
+    classification would otherwise retry — the push must win.
+    """
+    async with FakePhoenixServer() as server:
+        superseded = asyncio.Event()
+        ws = WebSocketClient(server.url, "test-key", "agent-123")
+
+        async def on_supersede(payload: SupersedePayload) -> None:
+            # Mirrors BandLink._on_supersede: the push is authoritative.
+            ws.record_terminal_disconnect(payload.to_disconnect_reason())
+            superseded.set()
+
+        async with ws:
+            await server.wait_for_connection()
+            await ws.join_agent_control_channel("agent-123", on_supersede=on_supersede)
+            assert "agent_control:agent-123" in server.joined_topics
+
+            await server.push("agent_control:agent-123", "supersede", SUPERSEDE_PAYLOAD)
+            await asyncio.wait_for(superseded.wait(), timeout=5.0)
+
+            await server.close_current(1011, "internal error")
+            await asyncio.sleep(RECONNECT_GRACE_S)
+
+            assert server.connection_count == 1, (
+                "SDK reconnected after terminal supersede"
+            )
+            assert ws.last_disconnect_reason is not None
+            assert ws.last_disconnect_reason.retryable is False
+            assert ws.last_disconnect_reason.reason == "session.already_connected"
+
+
+async def test_missed_supersede_close_1001_does_not_reconnect():
+    """Eviction close code 1001 keeps the SDK down when the push was missed.
+
+    Phoenix 1.8 closes evicted agent sockets with 1001. If the socket closes
+    before the supersede push is delivered, the close code is the only
+    signal — the SDK must not reconnect on it (pinned literal, matching the
+    platform's close-code pin).
+    """
+    async with FakePhoenixServer() as server:
+        supersede_calls: list[SupersedePayload] = []
+        ws = WebSocketClient(server.url, "test-key", "agent-123")
+
+        async def on_supersede(payload: SupersedePayload) -> None:
+            supersede_calls.append(payload)
+
+        async with ws:
+            await server.wait_for_connection()
+            await ws.join_agent_control_channel("agent-123", on_supersede=on_supersede)
+
+            await server.close_current(1001, "going away")
+            await asyncio.sleep(RECONNECT_GRACE_S)
+
+            assert server.connection_count == 1, (
+                "SDK reconnected on eviction close 1001"
+            )
+            assert not supersede_calls
+            assert ws.last_disconnect_reason is None
+
+
+async def test_abnormal_close_without_supersede_reconnects():
+    """A close with no code (abnormal drop) still reconnects — the accepted worst case.
+
+    When neither the supersede push nor a 1000/1001 close frame arrives (for
+    example an intermediary drops the TCP connection), the SDK cannot know it
+    was superseded and retries. The platform bounds this churn by
+    re-superseding and rate-limiting reconnects after an eviction.
+    """
+    async with FakePhoenixServer() as server:
+        ws = WebSocketClient(server.url, "test-key", "agent-123")
+
+        async def on_supersede(payload: SupersedePayload) -> None:
+            return None
+
+        async with ws:
+            await server.wait_for_connection()
+            await ws.join_agent_control_channel("agent-123", on_supersede=on_supersede)
+
+            server.joined_topics.clear()
+            server.abort_current()
+            await server.wait_for_connection(timeout=5.0)
+
+            assert server.connection_count == 2
+            # Let the rejoin settle before shutdown so teardown doesn't race
+            # the in-flight reconnect.
+            async with asyncio.timeout(5.0):
+                while "agent_control:agent-123" not in server.joined_topics:
+                    await asyncio.sleep(0.05)
+
+
+def test_default_reconnect_policy_treats_eviction_closes_as_terminal():
+    """Pin the close-code classification the SDK relies on.
+
+    WebSocketClient constructs PHXChannelsClient without a custom
+    ReconnectPolicy, so these defaults are load-bearing: 1000/1001 must not
+    reconnect (eviction path), while codeless and abnormal closes may.
+    """
+    assert ReconnectPolicy().reconnect_on_normal_close is False
+
+    client = PHXChannelsClient("ws://localhost/api/v1/socket/websocket", "test-key")
+    assert client._classify_disconnect(1000, "").should_reconnect is False
+    assert client._classify_disconnect(1001, "going away").should_reconnect is False
+    # Codeless / abnormal closes still reconnect (see the worst-case test above).
+    assert client._classify_disconnect(None, "").should_reconnect is True
+    assert client._classify_disconnect(1006, "").should_reconnect is True