fix(craft): harden ACP response matching for cached sessions

The cached ACPExecClient could emit keepalives forever on follow-up
messages because the PromptResponse was never matched. This adds:

- ID type-mismatch tolerance (str fallback for int/str id comparison)
- Guard against agent request ID collision (require no "method" field)
- PromptResponse via session/update notification handler
- Reader thread health check (detect dead WebSocket, stop keepalives)
- Buffer flush on reader thread exit (catch trailing PromptResponse)
- Diagnostic logging for every dequeued message and dropped messages

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/onyx/server/features/build/sandbox/kubernetes/internal/acp_exec_client.py

@@ -218,59 +218,76 @@ class ACPExecClient:
         buffer = ""
         packet_logger = get_packet_logger()
 
-        while not self._stop_reader.is_set():
-            if self._ws_client is None:
-                break
-
-            try:
-                if self._ws_client.is_open():
-                    # Read available data
-                    self._ws_client.update(timeout=0.1)
-
-                    # Read stdout (channel 1)
-                    data = self._ws_client.read_stdout(timeout=0.1)
-                    if data:
-                        buffer += data
-
-                        # Process complete lines
-                        while "\n" in buffer:
-                            line, buffer = buffer.split("\n", 1)
-                            line = line.strip()
-                            if line:
-                                try:
-                                    message = json.loads(line)
-                                    # Log the raw incoming message
-                                    packet_logger.log_jsonrpc_raw_message(
-                                        "IN", message, context="k8s"
-                                    )
-                                    self._response_queue.put(message)
-                                except json.JSONDecodeError:
-                                    packet_logger.log_raw(
-                                        "JSONRPC-PARSE-ERROR-K8S",
-                                        {
-                                            "raw_line": line[:500],
-                                            "error": "JSON decode failed",
-                                        },
-                                    )
-                                    logger.warning(
-                                        f"Invalid JSON from agent: {line[:100]}"
-                                    )
-
-                else:
-                    packet_logger.log_raw(
-                        "K8S-WEBSOCKET-CLOSED",
-                        {"pod": self._pod_name, "namespace": self._namespace},
-                    )
+        try:
+            while not self._stop_reader.is_set():
+                if self._ws_client is None:
                     break
 
-            except Exception as e:
-                if not self._stop_reader.is_set():
-                    packet_logger.log_raw(
-                        "K8S-READER-ERROR",
-                        {"error": str(e), "pod": self._pod_name},
+                try:
+                    if self._ws_client.is_open():
+                        # Read available data
+                        self._ws_client.update(timeout=0.1)
+
+                        # Read stdout (channel 1)
+                        data = self._ws_client.read_stdout(timeout=0.1)
+                        if data:
+                            buffer += data
+
+                            # Process complete lines
+                            while "\n" in buffer:
+                                line, buffer = buffer.split("\n", 1)
+                                line = line.strip()
+                                if line:
+                                    try:
+                                        message = json.loads(line)
+                                        # Log the raw incoming message
+                                        packet_logger.log_jsonrpc_raw_message(
+                                            "IN", message, context="k8s"
+                                        )
+                                        self._response_queue.put(message)
+                                    except json.JSONDecodeError:
+                                        packet_logger.log_raw(
+                                            "JSONRPC-PARSE-ERROR-K8S",
+                                            {
+                                                "raw_line": line[:500],
+                                                "error": "JSON decode failed",
+                                            },
+                                        )
+                                        logger.warning(
+                                            f"Invalid JSON from agent: {line[:100]}"
+                                        )
+
+                    else:
+                        packet_logger.log_raw(
+                            "K8S-WEBSOCKET-CLOSED",
+                            {"pod": self._pod_name, "namespace": self._namespace},
+                        )
+                        break
+
+                except Exception as e:
+                    if not self._stop_reader.is_set():
+                        packet_logger.log_raw(
+                            "K8S-READER-ERROR",
+                            {"error": str(e), "pod": self._pod_name},
+                        )
+                        logger.debug(f"Reader error: {e}")
+                    break
+        finally:
+            # Flush any remaining data in buffer (e.g., PromptResponse without
+            # trailing newline when the WebSocket closes)
+            remaining = buffer.strip()
+            if remaining:
+                try:
+                    message = json.loads(remaining)
+                    packet_logger.log_jsonrpc_raw_message(
+                        "IN", message, context="k8s-flush"
+                    )
+                    self._response_queue.put(message)
+                except json.JSONDecodeError:
+                    packet_logger.log_raw(
+                        "K8S-BUFFER-FLUSH-FAILED",
+                        {"remaining": remaining[:500]},
                     )
-                    logger.debug(f"Reader error: {e}")
-                break
 
     def stop(self) -> None:
         """Stop the exec session and clean up."""
@@ -465,7 +482,63 @@ class ACPExecClient:
             try:
                 message_data = self._response_queue.get(timeout=min(remaining, 1.0))
                 last_event_time = time.time()  # Reset keepalive timer on event
+
+                # Diagnostic: log every dequeued message with id comparison
+                msg_id = message_data.get("id")
+                msg_method = message_data.get("method")
+                msg_keys = list(message_data.keys())
+                logger.debug(
+                    f"[ACP-DIAG] Dequeued message: id={msg_id} (type={type(msg_id).__name__}), "
+                    f"method={msg_method}, keys={msg_keys}, "
+                    f"request_id={request_id} (type={type(request_id).__name__}), "
+                    f"id_match={msg_id == request_id}"
+                )
             except Empty:
+                # Check if reader thread is still alive (equivalent to
+                # process.poll() in the local agent client). If the reader
+                # thread died, the WebSocket connection is gone and no more
+                # data will arrive — break instead of emitting keepalives
+                # forever.
+                if (
+                    self._reader_thread is not None
+                    and not self._reader_thread.is_alive()
+                ):
+                    # Drain any final messages the reader flushed before dying
+                    while not self._response_queue.empty():
+                        try:
+                            final_msg = self._response_queue.get_nowait()
+                            if final_msg.get("id") == request_id:
+                                if "error" in final_msg:
+                                    error_data = final_msg["error"]
+                                    yield Error(
+                                        code=error_data.get("code", -1),
+                                        message=error_data.get(
+                                            "message", "Unknown error"
+                                        ),
+                                    )
+                                else:
+                                    result = final_msg.get("result", {})
+                                    try:
+                                        yield PromptResponse.model_validate(result)
+                                    except ValidationError:
+                                        pass
+                                break
+                        except Empty:
+                            break
+
+                    packet_logger.log_raw(
+                        "ACP-CONNECTION-LOST-K8S",
+                        {
+                            "session_id": session_id,
+                            "events_yielded": events_yielded,
+                        },
+                    )
+                    logger.warning(
+                        f"Reader thread died for session {session_id}, "
+                        "ending message stream"
+                    )
+                    break
+
                 # Check if we need to send an SSE keepalive
                 idle_time = time.time() - last_event_time
                 if idle_time >= SSE_KEEPALIVE_INTERVAL:
@@ -480,8 +553,21 @@ class ACPExecClient:
                     last_event_time = time.time()  # Reset after yielding keepalive
                 continue
 
-            # Check for response to our prompt request
-            if message_data.get("id") == request_id:
+            # Check for response to our prompt request.
+            # A JSON-RPC response has "id" but no "method" field.
+            # Use str() comparison as a fallback — some ACP servers may echo
+            # the id back as a string even though we sent it as an integer.
+            msg_id = message_data.get("id")
+            is_response = "method" not in message_data and (
+                msg_id == request_id
+                or (msg_id is not None and str(msg_id) == str(request_id))
+            )
+            if is_response and msg_id != request_id:
+                logger.warning(
+                    f"[ACP] ID type mismatch: got {type(msg_id).__name__}({msg_id}), "
+                    f"expected {type(request_id).__name__}({request_id})"
+                )
+            if is_response:
                 if "error" in message_data:
                     error_data = message_data["error"]
                     packet_logger.log_jsonrpc_response(
@@ -533,12 +619,31 @@ class ACPExecClient:
                     context="k8s",
                 )
 
+                prompt_complete = False
                 for event in self._process_session_update(update):
                     events_yielded += 1
                     # Log each yielded event
                     event_type = self._get_event_type_name(event)
                     packet_logger.log_acp_event_yielded(event_type, event)
                     yield event
+                    # If PromptResponse arrived via notification, break
+                    if isinstance(event, PromptResponse):
+                        prompt_complete = True
+                        break
+
+                if prompt_complete:
+                    # Log completion summary
+                    elapsed_ms = (time.time() - start_time) * 1000
+                    packet_logger.log_raw(
+                        "ACP-SEND-MESSAGE-COMPLETE-K8S",
+                        {
+                            "session_id": session_id,
+                            "events_yielded": events_yielded,
+                            "elapsed_ms": elapsed_ms,
+                            "via": "session_update_notification",
+                        },
+                    )
+                    break
 
             # Handle requests from agent - send error response
             elif "method" in message_data and "id" in message_data:
@@ -552,6 +657,15 @@ class ACPExecClient:
                     f"Method not supported: {message_data['method']}",
                 )
 
+            else:
+                # Message didn't match any handler — silently dropped
+                logger.warning(
+                    f"[ACP-DIAG] Dropped message: id={message_data.get('id')}, "
+                    f"method={message_data.get('method')}, "
+                    f"keys={list(message_data.keys())}, "
+                    f"request_id={request_id}"
+                )
+
     def _get_event_type_name(self, event: ACPEvent) -> str:
         """Get the type name for an ACP event."""
         if isinstance(event, AgentMessageChunk):
@@ -641,6 +755,20 @@ class ACPExecClient:
                     {"update_type": update_type, "error": str(e), "update": update},
                 )
 
+        elif update_type == "prompt_response":
+            # Some ACP versions send PromptResponse as a session/update notification
+            # rather than (or in addition to) a JSON-RPC response.
+            logger.info(
+                "[ACP] Received prompt_response via session/update notification"
+            )
+            try:
+                yield PromptResponse.model_validate(update)
+            except ValidationError as e:
+                packet_logger.log_raw(
+                    "ACP-VALIDATION-ERROR-K8S",
+                    {"update_type": update_type, "error": str(e), "update": update},
+                )
+
         elif update_type == "available_commands_update":
             # Skip command updates
             packet_logger.log_raw(

backend/onyx/server/features/build/sandbox/kubernetes/kubernetes_sandbox_manager.py

@@ -353,6 +353,10 @@ class KubernetesSandboxManager(SandboxManager):
         self._agent_instructions_template_path = build_dir / "AGENTS.template.md"
         self._skills_path = Path(__file__).parent / "docker" / "skills"
 
+        # Track ACP exec clients in memory - keyed by (sandbox_id, session_id) tuple
+        # Each session within a sandbox has its own ACP client (WebSocket connection)
+        self._acp_clients: dict[tuple[UUID, UUID], ACPExecClient] = {}
+
         logger.info(
             f"KubernetesSandboxManager initialized: "
             f"namespace={self._namespace}, image={self._image}"
@@ -1161,6 +1165,20 @@ done
         Args:
             sandbox_id: The sandbox ID to terminate
         """
+        # Stop all ACP clients for this sandbox (keyed by (sandbox_id, session_id))
+        clients_to_stop = [
+            (key, cl) for key, cl in self._acp_clients.items() if key[0] == sandbox_id
+        ]
+        for key, cl in clients_to_stop:
+            try:
+                cl.stop()
+                del self._acp_clients[key]
+            except Exception as e:
+                logger.warning(
+                    f"Failed to stop ACP client for sandbox {sandbox_id}, "
+                    f"session {key[1]}: {e}"
+                )
+
         # Clean up Kubernetes resources (needs string for pod/service names)
         self._cleanup_kubernetes_resources(str(sandbox_id))
 
@@ -1403,6 +1421,18 @@ echo "Session workspace setup complete"
             nextjs_port: Optional port where Next.js server is running (unused in K8s,
                         we use PID file instead)
         """
+        # Stop ACP client for this session
+        client_key = (sandbox_id, session_id)
+        acp_client = self._acp_clients.pop(client_key, None)
+        if acp_client:
+            try:
+                acp_client.stop()
+                logger.debug(f"Stopped ACP client for session {session_id}")
+            except Exception as e:
+                logger.warning(
+                    f"Failed to stop ACP client for session {session_id}: {e}"
+                )
+
         pod_name = self._get_pod_name(str(sandbox_id))
         session_path = f"/workspace/sessions/{session_id}"
 
@@ -1830,24 +1860,41 @@ echo "Session config regeneration complete"
         pod_name = self._get_pod_name(str(sandbox_id))
         session_path = f"/workspace/sessions/{session_id}"
 
-        # Log ACP client creation
-        packet_logger.log_acp_client_start(
-            sandbox_id, session_id, session_path, context="k8s"
-        )
+        # Get or create ACP client for this session
+        client_key = (sandbox_id, session_id)
+        client = self._acp_clients.get(client_key)
 
-        exec_client = ACPExecClient(
-            pod_name=pod_name,
-            namespace=self._namespace,
-            container="sandbox",
-        )
+        if client is None or not client.is_running:
+            # Clean up stale client if it exists but is no longer running
+            if client is not None:
+                try:
+                    client.stop()
+                except Exception:
+                    pass
+
+            # Log ACP client creation
+            packet_logger.log_acp_client_start(
+                sandbox_id, session_id, session_path, context="k8s"
+            )
+            logger.info(
+                f"Creating new ACP client for sandbox {sandbox_id}, session {session_id}"
+            )
+
+            # Create and start ACP client for this session
+            client = ACPExecClient(
+                pod_name=pod_name,
+                namespace=self._namespace,
+                container="sandbox",
+            )
+            client.start(cwd=session_path)
+            self._acp_clients[client_key] = client
 
         # Log the send_message call at sandbox manager level
         packet_logger.log_session_start(session_id, sandbox_id, message)
 
         events_count = 0
         try:
-            exec_client.start(cwd=session_path)
-            for event in exec_client.send_message(message):
+            for event in client.send_message(message):
                 events_count += 1
                 yield event
 
@@ -1884,10 +1931,6 @@ echo "Session config regeneration complete"
                 events_count=events_count,
             )
             raise
-        finally:
-            exec_client.stop()
-            # Log client stop
-            packet_logger.log_acp_client_stop(sandbox_id, session_id, context="k8s")
 
     def list_directory(
         self, sandbox_id: UUID, session_id: UUID, path: str

backend/tests/unit/onyx/server/features/build/sandbox/kubernetes/test_acp_client_caching.py

@@ -0,0 +1,360 @@
+"""Unit tests for ACPExecClient caching behavior in KubernetesSandboxManager.
+
+These tests verify that the KubernetesSandboxManager correctly caches
+ACPExecClient instances per (sandbox_id, session_id) pair, reuses them
+across send_message calls, replaces dead clients, and cleans them up
+on terminate/cleanup.
+
+All external dependencies (K8s, WebSockets, packet logging) are mocked.
+"""
+
+from collections.abc import Generator
+from typing import Any
+from unittest.mock import MagicMock
+from unittest.mock import patch
+from uuid import UUID
+from uuid import uuid4
+
+import pytest
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+# The fully-qualified path to the module under test, used for patching
+_K8S_MODULE = "onyx.server.features.build.sandbox.kubernetes.kubernetes_sandbox_manager"
+_ACP_CLIENT_CLASS = f"{_K8S_MODULE}.ACPExecClient"
+_GET_PACKET_LOGGER = f"{_K8S_MODULE}.get_packet_logger"
+
+
+def _make_mock_event() -> MagicMock:
+    """Create a mock ACP event."""
+    return MagicMock(name="mock_acp_event")
+
+
+def _make_mock_client(is_running: bool = True) -> MagicMock:
+    """Create a mock ACPExecClient with configurable is_running property."""
+    mock_client = MagicMock()
+    type(mock_client).is_running = property(lambda _self: is_running)
+    mock_client.start.return_value = "mock-session-id"
+    mock_event = _make_mock_event()
+    mock_client.send_message.return_value = iter([mock_event])
+    mock_client.stop.return_value = None
+    return mock_client
+
+
+def _drain_generator(gen: Generator[Any, None, None]) -> list[Any]:
+    """Consume a generator and return all yielded values as a list."""
+    return list(gen)
+
+
+# ---------------------------------------------------------------------------
+# Fixture: fresh KubernetesSandboxManager instance
+# ---------------------------------------------------------------------------
+
+
+@pytest.fixture()
+def manager() -> Generator[Any, None, None]:
+    """Create a fresh KubernetesSandboxManager instance with all externals mocked.
+
+    This fixture:
+    1. Resets the singleton _instance so each test gets a fresh manager
+    2. Mocks kubernetes.config and kubernetes.client to prevent real K8s calls
+    3. Mocks get_packet_logger to prevent logging side effects
+    """
+    # Import here so patches are in effect when the class loads
+    with (
+        patch(f"{_K8S_MODULE}.config") as _mock_config,
+        patch(f"{_K8S_MODULE}.client") as _mock_k8s_client,
+        patch(f"{_K8S_MODULE}.k8s_stream"),
+        patch(_GET_PACKET_LOGGER) as mock_get_logger,
+    ):
+        # Set up the mock packet logger
+        mock_packet_logger = MagicMock()
+        mock_get_logger.return_value = mock_packet_logger
+
+        # Make config.load_incluster_config succeed (no-op)
+        _mock_config.load_incluster_config.return_value = None
+        _mock_config.ConfigException = Exception
+
+        # Make client constructors return mocks
+        _mock_k8s_client.ApiClient.return_value = MagicMock()
+        _mock_k8s_client.CoreV1Api.return_value = MagicMock()
+        _mock_k8s_client.BatchV1Api.return_value = MagicMock()
+        _mock_k8s_client.NetworkingV1Api.return_value = MagicMock()
+
+        # Reset singleton before importing
+        from onyx.server.features.build.sandbox.kubernetes.kubernetes_sandbox_manager import (
+            KubernetesSandboxManager,
+        )
+
+        KubernetesSandboxManager._instance = None
+
+        mgr = KubernetesSandboxManager()
+
+        # Ensure the _acp_clients dict exists (it should be initialized by
+        # the caching implementation)
+        if not hasattr(mgr, "_acp_clients"):
+            mgr._acp_clients = {}
+
+        yield mgr
+
+        # Reset singleton after test
+        KubernetesSandboxManager._instance = None
+
+
+# ---------------------------------------------------------------------------
+# Tests
+# ---------------------------------------------------------------------------
+
+
+def test_send_message_creates_client_on_first_call(manager: Any) -> None:
+    """First call to send_message() should create a new ACPExecClient,
+    call start(), cache it, and yield events from send_message()."""
+    sandbox_id: UUID = uuid4()
+    session_id: UUID = uuid4()
+    message = "hello world"
+
+    mock_event = _make_mock_event()
+    mock_client = _make_mock_client(is_running=True)
+    mock_client.send_message.return_value = iter([mock_event])
+
+    with patch(_ACP_CLIENT_CLASS, return_value=mock_client) as MockClass:
+        events = _drain_generator(manager.send_message(sandbox_id, session_id, message))
+
+    # Verify client was constructed
+    MockClass.assert_called_once()
+
+    # Verify start() was called with the correct session path
+    expected_cwd = f"/workspace/sessions/{session_id}"
+    mock_client.start.assert_called_once_with(cwd=expected_cwd)
+
+    # Verify send_message was called on the client
+    mock_client.send_message.assert_called_once_with(message)
+
+    # Verify we got the event
+    assert len(events) >= 1
+    # Find our mock event (filter out any SSEKeepalive or similar)
+    assert mock_event in events
+
+    # Verify client was cached
+    client_key = (sandbox_id, session_id)
+    assert client_key in manager._acp_clients
+    assert manager._acp_clients[client_key] is mock_client
+
+
+def test_send_message_reuses_cached_client(manager: Any) -> None:
+    """Second call with the same (sandbox_id, session_id) should NOT create
+    a new client. Should reuse the cached one."""
+    sandbox_id: UUID = uuid4()
+    session_id: UUID = uuid4()
+    message_1 = "first message"
+    message_2 = "second message"
+
+    mock_event_1 = _make_mock_event()
+    mock_event_2 = _make_mock_event()
+    mock_client = _make_mock_client(is_running=True)
+
+    # send_message returns different events for each call
+    mock_client.send_message.side_effect = [
+        iter([mock_event_1]),
+        iter([mock_event_2]),
+    ]
+
+    with patch(_ACP_CLIENT_CLASS, return_value=mock_client) as MockClass:
+        events_1 = _drain_generator(
+            manager.send_message(sandbox_id, session_id, message_1)
+        )
+        events_2 = _drain_generator(
+            manager.send_message(sandbox_id, session_id, message_2)
+        )
+
+    # Constructor called only ONCE (on first send_message)
+    MockClass.assert_called_once()
+
+    # start() called only once
+    mock_client.start.assert_called_once()
+
+    # send_message called twice with different messages
+    assert mock_client.send_message.call_count == 2
+    mock_client.send_message.assert_any_call(message_1)
+    mock_client.send_message.assert_any_call(message_2)
+
+    # Both calls yielded events
+    assert mock_event_1 in events_1
+    assert mock_event_2 in events_2
+
+
+def test_send_message_replaces_dead_client(manager: Any) -> None:
+    """If cached client has is_running == False, should create a new one,
+    start it, and cache the replacement."""
+    sandbox_id: UUID = uuid4()
+    session_id: UUID = uuid4()
+    message = "test message"
+
+    # Create a dead client (is_running = False) and place it in the cache
+    dead_client = _make_mock_client(is_running=False)
+    client_key = (sandbox_id, session_id)
+    manager._acp_clients[client_key] = dead_client
+
+    # Create the replacement client
+    new_event = _make_mock_event()
+    new_client = _make_mock_client(is_running=True)
+    new_client.send_message.return_value = iter([new_event])
+
+    with patch(_ACP_CLIENT_CLASS, return_value=new_client) as MockClass:
+        events = _drain_generator(manager.send_message(sandbox_id, session_id, message))
+
+    # A new client was constructed (the dead one was replaced)
+    MockClass.assert_called_once()
+
+    # New client was started and used
+    new_client.start.assert_called_once()
+    new_client.send_message.assert_called_once_with(message)
+
+    # Cache now holds the new client
+    assert manager._acp_clients[client_key] is new_client
+
+    # Events from new client were yielded
+    assert new_event in events
+
+
+def test_send_message_different_sessions_get_different_clients(
+    manager: Any,
+) -> None:
+    """Two calls with different session_id values should create two
+    separate clients, each cached under its own key."""
+    sandbox_id: UUID = uuid4()
+    session_id_a: UUID = uuid4()
+    session_id_b: UUID = uuid4()
+    message = "test"
+
+    mock_client_a = _make_mock_client(is_running=True)
+    mock_client_b = _make_mock_client(is_running=True)
+
+    mock_event_a = _make_mock_event()
+    mock_event_b = _make_mock_event()
+    mock_client_a.send_message.return_value = iter([mock_event_a])
+    mock_client_b.send_message.return_value = iter([mock_event_b])
+
+    with patch(
+        _ACP_CLIENT_CLASS, side_effect=[mock_client_a, mock_client_b]
+    ) as MockClass:
+        events_a = _drain_generator(
+            manager.send_message(sandbox_id, session_id_a, message)
+        )
+        events_b = _drain_generator(
+            manager.send_message(sandbox_id, session_id_b, message)
+        )
+
+    # Two separate clients were constructed
+    assert MockClass.call_count == 2
+
+    # Both were started
+    mock_client_a.start.assert_called_once()
+    mock_client_b.start.assert_called_once()
+
+    # Each is cached under a different key
+    assert manager._acp_clients[(sandbox_id, session_id_a)] is mock_client_a
+    assert manager._acp_clients[(sandbox_id, session_id_b)] is mock_client_b
+
+    # Events from each client are correct
+    assert mock_event_a in events_a
+    assert mock_event_b in events_b
+
+
+def test_terminate_stops_all_sandbox_clients(manager: Any) -> None:
+    """terminate(sandbox_id) should stop all cached clients for that
+    sandbox and remove them from the cache."""
+    sandbox_id: UUID = uuid4()
+    session_id_1: UUID = uuid4()
+    session_id_2: UUID = uuid4()
+
+    client_1 = _make_mock_client(is_running=True)
+    client_2 = _make_mock_client(is_running=True)
+
+    manager._acp_clients[(sandbox_id, session_id_1)] = client_1
+    manager._acp_clients[(sandbox_id, session_id_2)] = client_2
+
+    # Mock _cleanup_kubernetes_resources to prevent actual K8s calls
+    with patch.object(manager, "_cleanup_kubernetes_resources"):
+        manager.terminate(sandbox_id)
+
+    # Both clients should have been stopped
+    client_1.stop.assert_called_once()
+    client_2.stop.assert_called_once()
+
+    # Both should be removed from cache
+    assert (sandbox_id, session_id_1) not in manager._acp_clients
+    assert (sandbox_id, session_id_2) not in manager._acp_clients
+
+
+def test_terminate_leaves_other_sandbox_clients(manager: Any) -> None:
+    """terminate(sandbox_id_A) should NOT affect clients cached for
+    sandbox_id_B."""
+    sandbox_id_a: UUID = uuid4()
+    sandbox_id_b: UUID = uuid4()
+    session_id_a: UUID = uuid4()
+    session_id_b: UUID = uuid4()
+
+    client_a = _make_mock_client(is_running=True)
+    client_b = _make_mock_client(is_running=True)
+
+    manager._acp_clients[(sandbox_id_a, session_id_a)] = client_a
+    manager._acp_clients[(sandbox_id_b, session_id_b)] = client_b
+
+    # Terminate only sandbox A
+    with patch.object(manager, "_cleanup_kubernetes_resources"):
+        manager.terminate(sandbox_id_a)
+
+    # Client A stopped and removed
+    client_a.stop.assert_called_once()
+    assert (sandbox_id_a, session_id_a) not in manager._acp_clients
+
+    # Client B untouched
+    client_b.stop.assert_not_called()
+    assert (sandbox_id_b, session_id_b) in manager._acp_clients
+    assert manager._acp_clients[(sandbox_id_b, session_id_b)] is client_b
+
+
+def test_cleanup_session_stops_session_client(manager: Any) -> None:
+    """cleanup_session_workspace(sandbox_id, session_id) should stop and
+    remove the specific session's client from the cache."""
+    sandbox_id: UUID = uuid4()
+    session_id: UUID = uuid4()
+
+    cached_client = _make_mock_client(is_running=True)
+    manager._acp_clients[(sandbox_id, session_id)] = cached_client
+
+    # Mock the k8s exec call that runs the cleanup script
+    with patch.object(manager, "_stream_core_api") as mock_stream_api:
+        mock_stream_api.connect_get_namespaced_pod_exec = MagicMock()
+        with patch(f"{_K8S_MODULE}.k8s_stream", return_value="cleanup ok"):
+            manager.cleanup_session_workspace(sandbox_id, session_id)
+
+    # Client should have been stopped
+    cached_client.stop.assert_called_once()
+
+    # Client should be removed from the cache
+    assert (sandbox_id, session_id) not in manager._acp_clients
+
+
+def test_cleanup_session_handles_no_cached_client(manager: Any) -> None:
+    """cleanup_session_workspace() should not error when there's no cached
+    client for that session."""
+    sandbox_id: UUID = uuid4()
+    session_id: UUID = uuid4()
+
+    # Ensure no client is cached for this pair
+    assert (sandbox_id, session_id) not in manager._acp_clients
+
+    # Mock the k8s exec call that runs the cleanup script
+    with patch.object(manager, "_stream_core_api") as mock_stream_api:
+        mock_stream_api.connect_get_namespaced_pod_exec = MagicMock()
+        with patch(f"{_K8S_MODULE}.k8s_stream", return_value="cleanup ok"):
+            # This should NOT raise
+            manager.cleanup_session_workspace(sandbox_id, session_id)
+
+    # Cache is still empty for this key
+    assert (sandbox_id, session_id) not in manager._acp_clients