diff --git a/README.md b/README.md
index 1325297..b4e176d 100644
--- a/README.md
+++ b/README.md
@@ -36,6 +36,8 @@ TOML 解析检查。若还要核对外置 OmniSocketGo 版本，可设置任务
 ## 当前按键
 
 - 左 Z + 右 C 连续 3 秒：开始遥操；再次连续 3 秒：结束并限速回 Home。
+  若 Hub 尚未运行，本次长按只检测一次并立即结束，不发送控制数据；服务器启动后须先
+  稳定松开，再重新长按 3 秒。pending 和活动会话都不会自动重试或自动恢复。
 - 右 C + 左摇杆上下：HBWALK 前进/后退。
 - 左 Z + 右摇杆左右：HBWALK 原地转向。
 - 右 B 连续 3 秒：右手进入厂商“单食指”姿态；松开至少 0.5 秒后再次连续
diff --git a/docs/天工3.0本地同构臂遥操迁移部署指南.md b/docs/天工3.0本地同构臂遥操迁移部署指南.md
index cd53ce1..e90a7d9 100644
--- a/docs/天工3.0本地同构臂遥操迁移部署指南.md
+++ b/docs/天工3.0本地同构臂遥操迁移部署指南.md
@@ -427,6 +427,15 @@ omnisocket_server = "203.0.113.10:15000"
 
 不需要。`127.0.0.1` 永远表示 EAI 本机，和网卡地址无关。
 
+### Hub 没启动时已经长按了 Z+C，服务器恢复后怎么办？
+
+本次长按只做一次后台连接检测。Hub 未启动、机器人 Peer 不存在或首帧发送失败时，
+`teleop_start_pending` 会回到 `false`；Hub 离线时没有业务帧发出，Peer/发送错误时机器人
+不会收到控制数据。旧请求不会在服务器稍后恢复
+时自动重试。稳定松开 Z+C 至少 0.5 秒，待 Hub 与机器人接收端就绪后重新长按 3 秒即可。
+后台检测期间 sender 仍持续读取 5003，因此松键不会因连接等待而漏掉。活动会话断网同样
+按故障关闭处理，必须稳定松开后重新长按。
+
 ### 只改机器人 Peer，不改 EAI 可以吗？
 
 不可以。EAI 的 `--target-peer` 必须等于机器人的 `omnisocket_peer_id`，机器人
diff --git a/tg3_omnisocket_transport/README.md b/tg3_omnisocket_transport/README.md
index 0bf8095..a63ef8c 100644
--- a/tg3_omnisocket_transport/README.md
+++ b/tg3_omnisocket_transport/README.md
@@ -36,10 +36,19 @@ xTELE 已处理的标量或六维 BrainCoRevo2 目标。5002 的界面事件和
 xTELE 帧；连续长按左 Z + 右 C 3 秒后才生成新的 128-bit `session_id` 并开始发送。
 再次长按 3 秒时发送最后一帧 `stop`，等待有界 KCP 刷新后关闭 OmniSocket Session；
 之后 `frames_sent/bytes_sent` 不再增长，待机既没有 xTELE 业务帧，也没有该 sender 的
-底层注册/心跳。即使 Hub 离线，sender 仍持续读取本机 xTELE 并保持会话关闭；启动连接
-失败会使本次会话失效，必须松开组合键后重新长按，不会在 Hub 恢复时续发旧动作。
+底层注册/心跳。即使 Hub 离线，sender 仍持续读取本机 xTELE 并保持会话关闭。Z+C 满
+3 秒后先进入 `START_PENDING`，后台连接不会阻塞本机按键采样；本次长按只尝试连接一次。
+若 Hub 未启动，本次连接失败且不会发出业务帧；若机器人 Peer 不存在或首帧发送失败，
+机器人不会收到控制数据且 sender 在收到错误后立即关闭。这次请求都必须稳定松开后再做
+一次新的 3 秒长按；Hub 稍后恢复不会让旧请求自行启动。
+任一键松开、按键数据畸形或 5003 失联也会立即取消 pending。连接成功后还必须看到两帧
+时间戳最终递增且仍按住 Z+C 的新鲜 5003 数据，才发送第一帧 START，连接前排队的旧
+动作不会被续发。
 活动期间若 KCP 反馈超过 `500 ms` 未更新，或 `snd_queue+snd_buffer` 超过 100 帧，sender
 立即作废会话并关闭 Session，从源头丢弃待发队列，防止网络恢复后回放旧动作。
+5003 的源时间戳在活动期间也必须持续推进；允许相邻毫秒值短暂重复，但超过 `250 ms`
+不再推进、时间戳倒退、持续畸形 JSON 或超长帧都会立即作废会话。操作员 STOP 无论最终
+业务帧能否编码或发送，EAI 都会在本地关闭 Session，机器人端由输入超时保护停止并回 Home。
 
 每个业务 JSON 的 `tg3_transport` 由 sender 强制覆盖，不能由 5003 输入伪造：
 
@@ -55,7 +64,8 @@ xTELE 帧；连续长按左 Z + 右 C 3 秒后才生成新的 128-bit `session_i
 
 当前用户服务将 `start` 连续发送 500 个源帧，避免接收端只取最新帧时错过唯一启动事件；机器人对同一
 `session_id` 只做一次启动安全检查。`stop` 是该会话最后一个业务帧。服务重启、源数据
-失联或网络错误都会使会话失效，恢复后必须先松开组合键，再重新长按 3 秒。
+失联或网络错误都会使会话失效，恢复后必须先松开组合键，再重新长按 3 秒；pending 和
+活动会话都不会自动重试或自动恢复。
 `tg3_local_teleop` 内部直接拒绝非预期发送端、
 乱序、格式错误以及相对本次连接最低包龄额外排队超过 300 ms 的数据；相对包龄会消除
 两端系统时钟的固定偏差。遥操桥仍有 250 ms 输入失联保护。
diff --git a/tg3_omnisocket_transport/omnisocket_xtele_sender.py b/tg3_omnisocket_transport/omnisocket_xtele_sender.py
index b81ab53..586abfb 100755
--- a/tg3_omnisocket_transport/omnisocket_xtele_sender.py
+++ b/tg3_omnisocket_transport/omnisocket_xtele_sender.py
@@ -20,6 +20,7 @@ import os
 from pathlib import Path
 import signal
 import struct
+import threading
 import time
 from typing import Any
 import uuid
@@ -56,6 +57,8 @@ class XteleSender:
         self.last_status_at = 0.0
         self.last_frame_at = 0.0
         self.last_source_frame_at = 0.0
+        self.last_source_timestamp: float | None = None
+        self.last_source_timestamp_advanced_at = 0.0
         self.last_command_at = 0.0
         self.last_error = ""
         self.teleop_active = False
@@ -67,6 +70,21 @@ class XteleSender:
         self.combo_release_started_at: float | None = None
         # A service restart must never turn an already-held combo into a start.
         self.require_combo_release = True
+        # START is a two-phase transaction.  A physical 3-second hold creates
+        # a pending request; only a successful Hub connection followed by a
+        # newer raw frame that still shows Z+C pressed may send START.
+        self.start_pending = False
+        self.start_wait_fresh_after_connect = False
+        self.start_fresh_barrier_timestamp: float | None = None
+        self.latest_combo_state: bool | None = None
+        self.last_start_failure = ""
+        # Session.connect() may wait for the Hub registration timeout.  Run it
+        # outside the input loop so a release or stale xTELE source is still
+        # observed immediately while an attempt is in flight.
+        self.connect_attempt_generation = 0
+        self.connect_thread: threading.Thread | None = None
+        self.connect_result_lock = threading.Lock()
+        self.connect_result: tuple[int, Session | None, str] | None = None
         # Keep xTELE's processed right-hand stream isolated for the complete
         # right-B press and stable-release transaction.  The robot runs the
         # authoritative three-second gesture toggle from the raw B state.
@@ -92,7 +110,15 @@ class XteleSender:
             "frames_suppressed_inactive": 0,
             "teleop_starts": 0,
             "teleop_stops": 0,
+            "teleop_stop_send_failures": 0,
             "teleop_aborts": 0,
+            "teleop_start_requests": 0,
+            "start_connect_attempts": 0,
+            "start_connect_failures": 0,
+            "start_send_failures": 0,
+            "start_pending_cancels": 0,
+            "source_timestamp_errors": 0,
+            "source_timestamp_resets": 0,
         }
         signal.signal(signal.SIGINT, self._request_stop)
         signal.signal(signal.SIGTERM, self._request_stop)
@@ -100,28 +126,6 @@ class XteleSender:
     def _request_stop(self, _signum: int, _frame: Any) -> None:
         self.stop = True
 
-    def connect(self) -> bool:
-        self.close_session()
-        session = Session()
-        try:
-            session.connect(
-                server_addr=self.args.server,
-                peer_id=self.args.peer_id,
-                **CONTROL_DEFAULTS,
-            )
-        except OSError as exc:
-            self.last_error = f"OmniSocket connect failed: {exc}"
-            session.close()
-            self.write_status(force=True)
-            return False
-        self.session = session
-        self.session_connected_at = time.monotonic()
-        self.counters["connected"] = 1
-        self.counters["reconnects"] += 1
-        self.last_error = ""
-        self.write_status(force=True)
-        return True
-
     def close_session(self) -> None:
         if self.session is not None:
             try:
@@ -161,6 +165,12 @@ class XteleSender:
             "last_source_frame_age_s": None
             if self.last_source_frame_at == 0.0
             else round(time.monotonic() - self.last_source_frame_at, 4),
+            "last_source_timestamp": self.last_source_timestamp,
+            "source_timestamp_advance_age_s": None
+            if self.last_source_timestamp_advanced_at == 0.0
+            else round(
+                time.monotonic() - self.last_source_timestamp_advanced_at, 4
+            ),
             "teleop_active": self.teleop_active,
             "teleop_session_id": self.teleop_session_id,
             "teleop_session_seq": self.teleop_session_seq,
@@ -171,6 +181,13 @@ class XteleSender:
             "teleop_combo_release_hold_s": 0.0
             if self.combo_release_started_at is None
             else round(time.monotonic() - self.combo_release_started_at, 2),
+            "teleop_start_pending": self.start_pending,
+            "teleop_start_waiting_fresh_frame": (
+                self.start_wait_fresh_after_connect
+            ),
+            "teleop_start_connect_inflight": self._connect_inflight(),
+            "teleop_latest_combo_state": self.latest_combo_state,
+            "last_start_failure": self.last_start_failure,
             "right_b_processed_merge_suppressed": self.right_b_merge_suppressed,
             "right_b_merge_release_hold_s": 0.0
             if self.right_b_release_started_at is None
@@ -197,6 +214,106 @@ class XteleSender:
             pass
         self.last_status_at = now
 
+    def _connect_inflight(self) -> bool:
+        thread = self.connect_thread
+        return thread is not None and thread.is_alive()
+
+    def _connect_worker(self, generation: int) -> None:
+        session: Session | None = None
+        failure = ""
+        try:
+            session = Session()
+            session.connect(
+                server_addr=self.args.server,
+                peer_id=self.args.peer_id,
+                **CONTROL_DEFAULTS,
+            )
+        except OSError as exc:
+            failure = f"OmniSocket connect failed: {exc}"
+            if session is not None:
+                try:
+                    session.close()
+                except OSError:
+                    pass
+            session = None
+        except Exception as exc:
+            failure = f"unexpected OmniSocket connect failure: {exc}"
+            if session is not None:
+                try:
+                    session.close()
+                except OSError:
+                    pass
+            session = None
+        with self.connect_result_lock:
+            self.connect_result = (generation, session, failure)
+
+    def _start_connect_attempt(self) -> bool:
+        if self._connect_inflight():
+            return False
+        with self.connect_result_lock:
+            # A worker can publish its result just after the main loop polls
+            # it.  Let the next poll adopt/discard that result instead of
+            # overwriting (and leaking) a connected Session.
+            if self.connect_result is not None:
+                return False
+            self.connect_attempt_generation += 1
+            generation = self.connect_attempt_generation
+        thread = threading.Thread(
+            target=self._connect_worker,
+            args=(generation,),
+            name="tg3-omnisocket-start-connect",
+            daemon=True,
+        )
+        self.connect_thread = thread
+        thread.start()
+        return True
+
+    def _invalidate_connect_attempt(self) -> None:
+        # The SDK connect call is not cancellable.  Changing the generation
+        # makes its eventual result unusable; the main loop will close it.
+        self.connect_attempt_generation += 1
+
+    def _poll_connect_result(self, now: float) -> str | None:
+        with self.connect_result_lock:
+            result = self.connect_result
+            self.connect_result = None
+        if result is None:
+            return None
+        generation, session, failure = result
+        thread = self.connect_thread
+        if thread is not None:
+            thread.join(timeout=0.0)
+        self.connect_thread = None
+        if generation != self.connect_attempt_generation or not self.start_pending:
+            if session is not None:
+                try:
+                    session.close()
+                except OSError:
+                    pass
+            return "discarded"
+        if session is None:
+            self.counters["start_connect_failures"] += 1
+            self.last_error = failure or "OmniSocket Hub connection failed"
+            # One physical hold makes exactly one connection attempt.  A Hub
+            # failure consumes no robot command, but it does require a stable
+            # release before the operator may make a new three-second hold.
+            self._cancel_pending_start(self.last_error)
+            self.write_status(force=True)
+            return "failed"
+
+        self.session = session
+        self.session_connected_at = now
+        self.counters["connected"] = 1
+        self.counters["reconnects"] += 1
+        self.last_error = ""
+        # Do not trust anything queued while registration was in progress.
+        # Two advancing raw timestamps after adoption prove the source is
+        # still live before START can leave EAI.
+        self.start_wait_fresh_after_connect = True
+        self.start_fresh_barrier_timestamp = None
+        self.write_status(force=True)
+        return "connected"
+
     def _drain_responses(self) -> None:
         if self.session is None:
             return
@@ -478,24 +595,38 @@ class XteleSender:
         return encoded, bool(merged_sides)
 
     @staticmethod
-    def _start_stop_pressed(data: dict[str, object]) -> bool:
+    def _start_stop_pressed(data: dict[str, object]) -> bool | None:
+        """Return explicit Z+C state, or ``None`` for malformed buttons."""
+
         try:
             buttons = data["button"]
             left = buttons["left"]  # type: ignore[index]
             right = buttons["right"]  # type: ignore[index]
-            return (
-                len(left) >= 3  # type: ignore[arg-type]
-                and len(right) >= 3  # type: ignore[arg-type]
-                and bool(left[2])  # type: ignore[index]
-                and bool(right[2])  # type: ignore[index]
-            )
-        except (KeyError, TypeError):
-            return False
+            if len(left) < 3 or len(right) < 3:  # type: ignore[arg-type]
+                return None
+            left_z = left[2]  # type: ignore[index]
+            right_c = right[2]  # type: ignore[index]
+        except (IndexError, KeyError, TypeError):
+            return None
+        values: list[bool] = []
+        for value in (left_z, right_c):
+            if isinstance(value, bool):
+                values.append(value)
+            elif isinstance(value, int) and value in (0, 1):
+                values.append(bool(value))
+            else:
+                return None
+        return all(values)
 
     def _update_teleop_gate(
         self, now: float, data: dict[str, object]
     ) -> str | None:
         pressed = self._start_stop_pressed(data)
+        self.latest_combo_state = pressed
+        if pressed is None:
+            self.combo_started_at = None
+            self.combo_release_started_at = None
+            return None
         if self.require_combo_release:
             self.combo_started_at = None
             if pressed:
@@ -528,33 +659,149 @@ class XteleSender:
             self.counters["teleop_stops"] += 1
             return "stop"
 
-        self.teleop_active = True
+        self.start_pending = True
         self.teleop_session_id = uuid.uuid4().hex
         self.teleop_session_seq = 0
         self.start_markers_remaining = self.args.start_marker_frames
-        self.counters["teleop_starts"] += 1
-        return "start"
+        self.start_wait_fresh_after_connect = False
+        self.last_start_failure = ""
+        self.last_error = ""
+        self.counters["teleop_start_requests"] += 1
+        return "start_pending"
 
-    def _abort_teleop(self, reason: str) -> None:
-        if self.teleop_active:
-            self.counters["teleop_aborts"] += 1
+    def _clear_pending_start_state(self) -> None:
+        self.start_pending = False
+        self.start_wait_fresh_after_connect = False
+        self.start_fresh_barrier_timestamp = None
+
+    def _cancel_pending_start(self, reason: str) -> None:
+        if not self.start_pending:
+            return
+        self._invalidate_connect_attempt()
+        self.close_session()
+        self._clear_pending_start_state()
         self.teleop_active = False
         self.teleop_session_id = None
         self.teleop_session_seq = 0
         self.start_markers_remaining = 0
         self.combo_started_at = None
         self.require_combo_release = True
-        self.last_error = reason
+        self.last_start_failure = reason
+        self.counters["start_pending_cancels"] += 1
 
-    def _send_payload(self, payload: bytes) -> bool:
-        if self.session is None:
-            self._abort_teleop("OmniSocket session is unavailable")
+    def _pending_connect_ready(self, _now: float) -> bool:
+        if not self.start_pending or self.latest_combo_state is not True:
             return False
+        if self.session is not None:
+            return False
+        if self._connect_inflight():
+            return False
+        return True
+
+    def _commit_pending_start_sent(self, _now: float) -> None:
+        self.counters["teleop_starts"] += 1
+        self.start_pending = False
+        self.start_wait_fresh_after_connect = False
+        self.start_fresh_barrier_timestamp = None
+        self.teleop_active = True
+        self.last_start_failure = ""
+
+    def _abort_teleop(self, reason: str) -> None:
+        if self.teleop_active or self.start_pending:
+            self.counters["teleop_aborts"] += 1
+        self._invalidate_connect_attempt()
+        self.teleop_active = False
+        self._clear_pending_start_state()
+        self.teleop_session_id = None
+        self.teleop_session_seq = 0
+        self.start_markers_remaining = 0
+        self.combo_started_at = None
+        self.require_combo_release = True
+        self.last_error = reason
+        self.last_start_failure = reason
+
+    @staticmethod
+    def _raw_source_timestamp(data: dict[str, object]) -> float | None:
+        value = data.get("timestamp")
+        if isinstance(value, bool) or not isinstance(value, (int, float)):
+            return None
+        timestamp = float(value)
+        return timestamp if math.isfinite(timestamp) else None
+
+    def _check_source_clock(
+        self, now: float, data: dict[str, object]
+    ) -> tuple[bool, str]:
+        """Reject a missing, regressing or persistently frozen xTELE clock."""
+
+        timestamp = self._raw_source_timestamp(data)
+        if timestamp is None:
+            return False, "local xTELE timestamp is missing or malformed"
+        previous = self.last_source_timestamp
+        if previous is None or timestamp > previous:
+            self.last_source_timestamp = timestamp
+            self.last_source_timestamp_advanced_at = now
+            return True, ""
+        if timestamp == previous:
+            if self.last_source_timestamp_advanced_at == 0.0:
+                self.last_source_timestamp_advanced_at = now
+                return True, ""
+            if (
+                now - self.last_source_timestamp_advanced_at
+                <= self.args.source_timeout_s
+            ):
+                return True, ""
+            return False, "local xTELE timestamp stopped advancing"
+
+        if not self.teleop_active and not self.start_pending:
+            # A producer restart while idle is harmless only after a fresh
+            # release.  Reset the clock baseline but never inherit a held
+            # Z+C combination across that restart.
+            self.last_source_timestamp = timestamp
+            self.last_source_timestamp_advanced_at = now
+            self.combo_started_at = None
+            self.combo_release_started_at = None
+            self.require_combo_release = True
+            self.latest_combo_state = None
+            self.counters["source_timestamp_resets"] += 1
+            return True, ""
+        return False, "local xTELE timestamp moved backwards"
+
+    def _check_pending_start_freshness(
+        self, data: dict[str, object]
+    ) -> tuple[str, str]:
+        """Gate START on two advancing frames received after connect adoption."""
+
+        if not self.start_wait_fresh_after_connect:
+            return "ready", ""
+        source_timestamp = self._raw_source_timestamp(data)
+        if source_timestamp is None:
+            return (
+                "cancel",
+                "pending START cannot prove a fresh xTELE timestamp",
+            )
+        if self.start_fresh_barrier_timestamp is None:
+            self.start_fresh_barrier_timestamp = source_timestamp
+            return "wait", ""
+        if source_timestamp < self.start_fresh_barrier_timestamp:
+            return (
+                "cancel",
+                "pending START xTELE timestamp moved backwards",
+            )
+        if source_timestamp == self.start_fresh_barrier_timestamp:
+            # xTELE runs near 100 Hz while its millisecond timestamp can repeat
+            # for adjacent frames.  Keep waiting; never treat a duplicate as
+            # proof of freshness and never force a new physical button cycle.
+            return "wait", ""
+        self.start_wait_fresh_after_connect = False
+        self.start_fresh_barrier_timestamp = None
+        return "ready", ""
+
+    def _try_send_payload(self, payload: bytes) -> tuple[bool, str]:
+        if self.session is None:
+            return False, "OmniSocket session is unavailable"
         unhealthy = self._session_unhealthy_reason()
         if unhealthy:
-            self._abort_teleop(unhealthy)
-            self.close_session()
-            return False
+            return False, unhealthy
         self.packet_sequence += 1
         packet = HEADER.pack(
             MAGIC, self.packet_sequence, time.time_ns(), len(payload)
@@ -562,14 +809,20 @@ class XteleSender:
         try:
             self.session.send(to=self.args.target_peer, data=packet)
         except OSError as exc:
-            self._abort_teleop(f"OmniSocket send failed: {exc}")
-            self.close_session()
-            return False
+            return False, f"OmniSocket send failed: {exc}"
         self.counters["frames_sent"] += 1
         self.counters["bytes_sent"] += len(payload)
         self.last_frame_at = time.monotonic()
         self.last_error = ""
-        return True
+        return True, ""
+
+    def _send_payload(self, payload: bytes) -> bool:
+        sent, reason = self._try_send_payload(payload)
+        if sent:
+            return True
+        self._abort_teleop(reason)
+        self.close_session()
+        return False
 
     def _session_unhealthy_reason(self) -> str | None:
         if self.session is None:
@@ -620,6 +873,16 @@ class XteleSender:
             self._drain_responses()
             time.sleep(0.01)
 
+    def _finish_stop_session(self) -> None:
+        """Close an operator STOP locally even if its final frame failed."""
+
+        self.teleop_active = False
+        self.teleop_session_id = None
+        self.teleop_session_seq = 0
+        self.start_markers_remaining = 0
+        self._flush_session()
+        self.close_session()
+
     def run(self) -> int:
         context = zmq.Context()
         source = context.socket(zmq.SUB)
@@ -643,6 +906,10 @@ class XteleSender:
         try:
             while not self.stop:
                 events = dict(poller.poll(100))
+                # Connection attempts run in a worker so this loop can keep
+                # observing physical releases and source freshness.  Only the
+                # main thread ever adopts a completed Session.
+                self._poll_connect_result(time.monotonic())
                 if command_source is not None and command_source in events:
                     command_raw = command_source.recv()
                     self.counters["command_frames_received"] += 1
@@ -669,6 +936,10 @@ class XteleSender:
                     ):
                         self.combo_started_at = None
                         self.combo_release_started_at = None
+                        if self.start_pending:
+                            self._cancel_pending_start(
+                                "local xTELE source became stale during START"
+                            )
                         if self.teleop_active:
                             self._abort_teleop(
                                 "local xTELE source became stale; a new Z+C hold "
@@ -683,6 +954,17 @@ class XteleSender:
                 self.counters["frames_received"] += 1
                 self.counters["bytes_received"] += len(raw)
                 if not raw or len(raw) > MAX_PAYLOAD_BYTES:
+                    self.combo_started_at = None
+                    self.combo_release_started_at = None
+                    if self.start_pending:
+                        self._cancel_pending_start(
+                            "malformed local xTELE frame during START"
+                        )
+                    if self.teleop_active:
+                        self._abort_teleop(
+                            "malformed local xTELE frame during active session"
+                        )
+                        self.close_session()
                     self.counters["dropped_malformed"] += 1
                     continue
 
@@ -695,11 +977,33 @@ class XteleSender:
                     # physical three-second start/stop hold.
                     self.combo_started_at = None
                     self.combo_release_started_at = None
+                    if self.start_pending:
+                        self._cancel_pending_start(
+                            "malformed local xTELE JSON during START"
+                        )
+                    if self.teleop_active:
+                        self._abort_teleop(
+                            "malformed local xTELE JSON during active session"
+                        )
+                        self.close_session()
                     self.counters["dropped_malformed"] += 1
                     continue
 
                 now = time.monotonic()
                 self.last_source_frame_at = now
+                clock_ok, clock_reason = self._check_source_clock(now, data)
+                if not clock_ok:
+                    self.combo_started_at = None
+                    self.combo_release_started_at = None
+                    self.latest_combo_state = None
+                    self.counters["source_timestamp_errors"] += 1
+                    self.counters["dropped_malformed"] += 1
+                    if self.start_pending:
+                        self._cancel_pending_start(clock_reason)
+                    if self.teleop_active:
+                        self._abort_teleop(clock_reason)
+                        self.close_session()
+                    continue
                 transition = self._update_teleop_gate(now, data)
 
                 command = None
@@ -723,6 +1027,86 @@ class XteleSender:
                     now, data, processed_right
                 )
 
+                if self.start_pending and self.latest_combo_state is not True:
+                    if self.latest_combo_state is False:
+                        pending_cancel_reason = (
+                            "Z+C released before START reached the robot"
+                        )
+                    else:
+                        pending_cancel_reason = (
+                            "malformed Z+C state during pending START"
+                        )
+                    self._cancel_pending_start(pending_cancel_reason)
+
+                if self.start_pending:
+                    if self.session is None:
+                        if not self._pending_connect_ready(now):
+                            self.counters["frames_suppressed_inactive"] += 1
+                            self._drain_responses()
+                            self.write_status()
+                            continue
+                        if self._start_connect_attempt():
+                            self.counters["start_connect_attempts"] += 1
+                        self.counters["frames_suppressed_inactive"] += 1
+                        self.write_status(force=True)
+                        continue
+
+                    freshness, freshness_reason = (
+                        self._check_pending_start_freshness(data)
+                    )
+                    if freshness == "wait":
+                        self.counters["frames_suppressed_inactive"] += 1
+                        self.write_status()
+                        continue
+                    if freshness == "cancel":
+                        self._cancel_pending_start(freshness_reason)
+                        self.counters["frames_suppressed_inactive"] += 1
+                        continue
+
+                    session_id = self.teleop_session_id
+                    if not session_id:
+                        self._cancel_pending_start(
+                            "pending START session ID is unavailable"
+                        )
+                        self.counters["frames_suppressed_inactive"] += 1
+                        continue
+                    self.teleop_session_seq += 1
+                    try:
+                        payload, merged = self._build_payload(
+                            data,
+                            command,
+                            session_id,
+                            self.teleop_session_seq,
+                            "start",
+                            "",
+                            suppress_processed_right,
+                        )
+                    except ValueError:
+                        self._cancel_pending_start(
+                            "cannot encode a fresh pending START frame"
+                        )
+                        self.counters["dropped_malformed"] += 1
+                        continue
+                    if len(payload) > MAX_PAYLOAD_BYTES:
+                        self._cancel_pending_start(
+                            "pending START frame exceeds maximum size"
+                        )
+                        self.counters["dropped_malformed"] += 1
+                        continue
+                    sent, failure = self._try_send_payload(payload)
+                    if not sent:
+                        self.counters["start_send_failures"] += 1
+                        self._cancel_pending_start(failure)
+                        self.counters["frames_suppressed_inactive"] += 1
+                        continue
+                    if merged:
+                        self.counters["command_hand_merges"] += 1
+                    self.start_markers_remaining -= 1
+                    self._commit_pending_start_sent(now)
+                    self._drain_responses()
+                    self.write_status(force=True)
+                    continue
+
                 if not self.teleop_active and transition != "stop":
                     self.counters["frames_suppressed_inactive"] += 1
                     self._drain_responses()
@@ -735,10 +1119,10 @@ class XteleSender:
                     self.counters["frames_suppressed_inactive"] += 1
                     continue
 
-                if self.session is None and not self.connect():
+                if self.session is None:
                     self._abort_teleop(
-                        "cannot start teleoperation because OmniSocket Hub is "
-                        "unavailable; release Z+C before retrying"
+                        "active OmniSocket session is unavailable; a new Z+C "
+                        "cycle is required"
                     )
                     self.counters["frames_suppressed_inactive"] += 1
                     continue
@@ -753,44 +1137,58 @@ class XteleSender:
                 else:
                     session_state = "active"
                     stop_reason = ""
+                stop_requested = transition == "stop"
                 try:
-                    payload, merged = self._build_payload(
-                        data,
-                        command,
-                        session_id,
-                        self.teleop_session_seq,
-                        session_state,
-                        stop_reason,
-                        suppress_processed_right,
-                    )
-                except ValueError:
-                    self.counters["dropped_malformed"] += 1
-                    continue
-                if merged:
-                    self.counters["command_hand_merges"] += 1
-                if len(payload) > MAX_PAYLOAD_BYTES:
-                    self.counters["dropped_malformed"] += 1
-                    continue
+                    try:
+                        payload, merged = self._build_payload(
+                            data,
+                            command,
+                            session_id,
+                            self.teleop_session_seq,
+                            session_state,
+                            stop_reason,
+                            suppress_processed_right,
+                        )
+                    except ValueError:
+                        self.counters["dropped_malformed"] += 1
+                        if stop_requested:
+                            self.counters["teleop_stop_send_failures"] += 1
+                            self.last_error = "cannot encode operator STOP frame"
+                        continue
+                    if merged:
+                        self.counters["command_hand_merges"] += 1
+                    if len(payload) > MAX_PAYLOAD_BYTES:
+                        self.counters["dropped_malformed"] += 1
+                        if stop_requested:
+                            self.counters["teleop_stop_send_failures"] += 1
+                            self.last_error = (
+                                "operator STOP frame exceeds maximum size"
+                            )
+                        continue
 
-                sent = self._send_payload(payload)
-                if sent and session_state == "start":
-                    self.start_markers_remaining -= 1
-                if transition == "stop":
-                    # STOP is the final xTELE business frame.  The underlying
-                    # Session is flushed for bounded delivery and then closed;
-                    # the next physical START creates a fresh registration.
-                    self.teleop_session_id = None
-                    self.teleop_session_seq = 0
-                    self.start_markers_remaining = 0
-                    self._flush_session()
-                    self.close_session()
+                    sent = self._send_payload(payload)
+                    if not sent and stop_requested:
+                        self.counters["teleop_stop_send_failures"] += 1
+                    if sent and session_state == "start":
+                        self.start_markers_remaining -= 1
+                finally:
+                    if stop_requested:
+                        # STOP is final even when encode/send fails.  The robot
+                        # then falls back to its input timeout; EAI must never
+                        # retain a registered, locally inactive Session.
+                        self._finish_stop_session()
                 self._drain_responses()
                 self.write_status()
         finally:
+            self._invalidate_connect_attempt()
             source.close()
             if command_source is not None:
                 command_source.close()
             context.term()
+            thread = self.connect_thread
+            if thread is not None:
+                thread.join(timeout=3.5)
+                self._poll_connect_result(time.monotonic())
             self.close_session()
             self.write_status(force=True)
         return 0
diff --git a/tg3_omnisocket_transport/test_session_gate.py b/tg3_omnisocket_transport/test_session_gate.py
index 6a2ba2c..b50cf32 100644
--- a/tg3_omnisocket_transport/test_session_gate.py
+++ b/tg3_omnisocket_transport/test_session_gate.py
@@ -7,8 +7,10 @@ import importlib.util
 import json
 from pathlib import Path
 import sys
+from threading import Event
 from types import ModuleType
 import unittest
+from unittest import mock
 
 
 fake_omnisocket = ModuleType("omnisocket")
@@ -53,6 +55,71 @@ def frame(pressed: bool) -> dict[str, object]:
 
 
 class SessionGateTest(unittest.TestCase):
+    class FakeSession:
+        def __init__(
+            self,
+            responses: list[tuple[str, int, bytes]] | None = None,
+        ) -> None:
+            self.responses = list(responses or [])
+            self.sent: list[tuple[str, bytes]] = []
+            self.closed = False
+
+        @staticmethod
+        def stats() -> dict[str, int]:
+            return {"connected": 1, "registered": 1}
+
+        @staticmethod
+        def kcp_stats() -> dict[str, int]:
+            return {
+                "snd_queue": 0,
+                "snd_buffer": 0,
+                "last_feedback_age_ms": 0,
+            }
+
+        def send(self, *, to: str, data: bytes) -> None:
+            self.sent.append((to, data))
+
+        def recv(self, timeout_ms: int) -> tuple[str, int, bytes] | None:
+            self.assert_zero_timeout(timeout_ms)
+            if self.responses:
+                return self.responses.pop(0)
+            return None
+
+        @staticmethod
+        def assert_zero_timeout(timeout_ms: int) -> None:
+            if timeout_ms != 0:
+                raise AssertionError(f"unexpected receive timeout: {timeout_ms}")
+
+        def close(self) -> None:
+            self.closed = True
+
+    class BlockingConnectSession(FakeSession):
+        def __init__(
+            self,
+            entered: Event,
+            release: Event,
+            failure: OSError | None = None,
+        ) -> None:
+            super().__init__()
+            self.entered = entered
+            self.release = release
+            self.failure = failure
+            self.connect_args: tuple[str, str, dict[str, object]] | None = None
+
+        def connect(
+            self,
+            *,
+            server_addr: str,
+            peer_id: str,
+            **options: object,
+        ) -> None:
+            self.connect_args = (server_addr, peer_id, options)
+            self.entered.set()
+            if not self.release.wait(timeout=1.0):
+                raise RuntimeError("test did not release blocked connect")
+            if self.failure is not None:
+                raise self.failure
+
     def setUp(self) -> None:
         self.sender = sender_module.XteleSender(args())
 
@@ -67,7 +134,43 @@ class SessionGateTest(unittest.TestCase):
         self.assertFalse(self.sender.require_combo_release)
         return finished_at
 
-    def test_boot_requires_stable_release_before_start(self) -> None:
+    def create_pending_start(self, started_at: float = 0.0) -> float:
+        released_at = self.stable_release(started_at)
+        self.assertIsNone(
+            self.sender._update_teleop_gate(released_at + 0.01, frame(True))
+        )
+        pending_at = released_at + 3.02
+        self.assertEqual(
+            self.sender._update_teleop_gate(pending_at, frame(True)),
+            "start_pending",
+        )
+        self.assertTrue(self.sender.start_pending)
+        self.assertFalse(self.sender.teleop_active)
+        return pending_at
+
+    def cancel_pending_from_latest_frame(self) -> None:
+        if (
+            self.sender.start_pending
+            and self.sender.latest_combo_state is not True
+        ):
+            reason = (
+                "released"
+                if self.sender.latest_combo_state is False
+                else "malformed"
+            )
+            self.sender._cancel_pending_start(reason)
+
+    def attach_session(
+        self,
+        responses: list[tuple[str, int, bytes]] | None = None,
+    ) -> FakeSession:
+        session = self.FakeSession(responses)
+        self.sender.session = session
+        self.sender.session_connected_at = 100.0
+        self.sender.counters["connected"] = 1
+        return session
+
+    def test_boot_requires_stable_release_before_pending_start(self) -> None:
         self.assertIsNone(self.sender._update_teleop_gate(0.0, frame(True)))
         self.assertIsNone(self.sender._update_teleop_gate(4.0, frame(True)))
         released_at = self.stable_release(5.0)
@@ -76,44 +179,574 @@ class SessionGateTest(unittest.TestCase):
         )
         self.assertEqual(
             self.sender._update_teleop_gate(released_at + 3.02, frame(True)),
-            "start",
+            "start_pending",
         )
-        self.assertTrue(self.sender.teleop_active)
+        self.assertTrue(self.sender.start_pending)
+        self.assertFalse(self.sender.teleop_active)
+        self.assertEqual(self.sender.counters["teleop_start_requests"], 1)
+        self.assertEqual(self.sender.counters["teleop_starts"], 0)
+        self.assertIsNotNone(self.sender.teleop_session_id)
+        self.assertTrue(
+            self.sender._pending_connect_ready(released_at + 3.02)
+        )
+
+    def test_boot_malformed_buttons_cannot_clear_release_requirement(self) -> None:
+        malformed = frame(False)
+        malformed["button"] = {
+            "left": [0, 0, 0],
+            "right": [0, 0, "0"],
+        }
+
+        self.assertIsNone(self.sender._update_teleop_gate(0.0, frame(False)))
+        self.assertEqual(self.sender.combo_release_started_at, 0.0)
+        self.assertIsNone(self.sender._update_teleop_gate(0.49, malformed))
+        self.assertTrue(self.sender.require_combo_release)
+        self.assertIsNone(self.sender.combo_release_started_at)
+        self.assertIsNone(self.sender.latest_combo_state)
+
+        # The release timer must restart after malformed input; time before the
+        # malformed frame cannot be accumulated toward re-arming.
+        self.assertIsNone(self.sender._update_teleop_gate(0.50, frame(False)))
+        self.assertIsNone(self.sender._update_teleop_gate(0.99, frame(False)))
+        self.assertTrue(self.sender.require_combo_release)
+        self.assertIsNone(self.sender._update_teleop_gate(1.01, frame(False)))
+        self.assertFalse(self.sender.require_combo_release)
+
+    def test_start_stop_parser_accepts_only_explicit_boolean_buttons(self) -> None:
+        self.assertTrue(self.sender._start_stop_pressed(frame(True)))
+        self.assertFalse(self.sender._start_stop_pressed(frame(False)))
+        for invalid in (1.0, "1", None, 2, -1):
+            with self.subTest(value=invalid):
+                malformed = frame(True)
+                malformed["button"] = {
+                    "left": [0, 0, invalid],
+                    "right": [0, 0, 1],
+                }
+                self.assertIsNone(
+                    self.sender._start_stop_pressed(malformed)
+                )
 
     def test_single_false_frame_cannot_rearm_stop(self) -> None:
-        released_at = self.stable_release(0.0)
-        self.sender._update_teleop_gate(released_at + 0.01, frame(True))
-        self.assertEqual(
-            self.sender._update_teleop_gate(released_at + 3.02, frame(True)),
-            "start",
-        )
+        pending_at = self.create_pending_start()
+        self.sender._commit_pending_start_sent(pending_at + 0.01)
 
         self.assertIsNone(
-            self.sender._update_teleop_gate(released_at + 3.03, frame(False))
+            self.sender._update_teleop_gate(pending_at + 0.02, frame(False))
         )
         self.assertIsNone(
-            self.sender._update_teleop_gate(released_at + 3.04, frame(True))
+            self.sender._update_teleop_gate(pending_at + 0.03, frame(True))
         )
         self.assertIsNone(
-            self.sender._update_teleop_gate(released_at + 7.00, frame(True))
+            self.sender._update_teleop_gate(pending_at + 4.00, frame(True))
         )
         self.assertTrue(self.sender.teleop_active)
         self.assertTrue(self.sender.require_combo_release)
 
     def test_stable_release_allows_separate_stop_hold(self) -> None:
-        released_at = self.stable_release(0.0)
-        self.sender._update_teleop_gate(released_at + 0.01, frame(True))
-        self.assertEqual(
-            self.sender._update_teleop_gate(released_at + 3.02, frame(True)),
-            "start",
-        )
-        second_release = self.stable_release(released_at + 3.03)
+        pending_at = self.create_pending_start()
+        self.sender._commit_pending_start_sent(pending_at + 0.01)
+        second_release = self.stable_release(pending_at + 0.02)
         self.sender._update_teleop_gate(second_release + 0.01, frame(True))
         self.assertEqual(
             self.sender._update_teleop_gate(second_release + 3.02, frame(True)),
             "stop",
         )
         self.assertFalse(self.sender.teleop_active)
+        self.assertEqual(self.sender.counters["teleop_starts"], 1)
+        self.assertEqual(self.sender.counters["teleop_stops"], 1)
+
+    def test_cancelled_start_requires_release_before_a_new_hold(self) -> None:
+        pending_at = self.create_pending_start()
+        first_session_id = self.sender.teleop_session_id
+        self.sender._cancel_pending_start("Hub offline")
+
+        self.assertFalse(self.sender.start_pending)
+        self.assertFalse(self.sender.teleop_active)
+        self.assertTrue(self.sender.require_combo_release)
+        self.assertEqual(self.sender.last_start_failure, "Hub offline")
+        self.assertIsNone(self.sender.teleop_session_id)
+
+        # A consumed hold cannot silently become another pending request while
+        # the operator keeps Z+C pressed, regardless of elapsed time.
+        self.assertIsNone(
+            self.sender._update_teleop_gate(
+                pending_at + 10.0, frame(True)
+            )
+        )
+        self.assertFalse(self.sender.start_pending)
+        self.assertFalse(self.sender._pending_connect_ready(pending_at + 10.0))
+
+        # A stable release followed by a new complete hold creates a distinct
+        # logical session which can make one new connection attempt.
+        released_at = self.stable_release(pending_at + 10.01)
+        self.assertIsNone(
+            self.sender._update_teleop_gate(
+                released_at + 0.01, frame(True)
+            )
+        )
+        self.assertEqual(
+            self.sender._update_teleop_gate(
+                released_at + 3.02, frame(True)
+            ),
+            "start_pending",
+        )
+        self.assertTrue(self.sender.start_pending)
+        self.assertIsNotNone(self.sender.teleop_session_id)
+        self.assertNotEqual(self.sender.teleop_session_id, first_session_id)
+        self.assertEqual(self.sender.counters["teleop_start_requests"], 2)
+        self.assertEqual(self.sender.counters["teleop_starts"], 0)
+
+    def test_pending_start_release_cancels_and_closes_session(self) -> None:
+        pending_at = self.create_pending_start()
+        session = self.attach_session()
+        self.sender.start_wait_fresh_after_connect = True
+
+        self.assertIsNone(
+            self.sender._update_teleop_gate(pending_at + 0.01, frame(False))
+        )
+        self.cancel_pending_from_latest_frame()
+
+        self.assertFalse(self.sender.start_pending)
+        self.assertFalse(self.sender.teleop_active)
+        self.assertIsNone(self.sender.teleop_session_id)
+        self.assertIsNone(self.sender.session)
+        self.assertTrue(session.closed)
+        self.assertEqual(session.sent, [])
+        self.assertEqual(self.sender.counters["start_pending_cancels"], 1)
+        self.assertTrue(self.sender.require_combo_release)
+        self.assertFalse(self.sender._pending_connect_ready(pending_at + 10.0))
+
+    def test_pending_start_malformed_button_state_cancels(self) -> None:
+        pending_at = self.create_pending_start()
+        malformed = frame(True)
+        malformed["button"] = {"left": [0, 0], "right": [0, 0, 1]}
+
+        self.assertIsNone(
+            self.sender._update_teleop_gate(pending_at + 0.01, malformed)
+        )
+        self.cancel_pending_from_latest_frame()
+
+        self.assertFalse(self.sender.start_pending)
+        self.assertIsNone(self.sender.latest_combo_state)
+        self.assertEqual(self.sender.last_start_failure, "malformed")
+        self.assertEqual(self.sender.counters["start_pending_cancels"], 1)
+
+    def test_pending_freshness_waits_then_accepts_advancing_timestamp(self) -> None:
+        self.create_pending_start()
+        self.sender.start_wait_fresh_after_connect = True
+        first = frame(True)
+        first["timestamp"] = 100.0
+        second = frame(True)
+        second["timestamp"] = 100.01
+
+        self.assertEqual(
+            self.sender._check_pending_start_freshness(first),
+            ("wait", ""),
+        )
+        self.assertEqual(self.sender.start_fresh_barrier_timestamp, 100.0)
+        self.assertTrue(self.sender.start_wait_fresh_after_connect)
+        self.assertFalse(self.sender.teleop_active)
+
+        self.assertEqual(
+            self.sender._check_pending_start_freshness(second),
+            ("ready", ""),
+        )
+        self.assertIsNone(self.sender.start_fresh_barrier_timestamp)
+        self.assertFalse(self.sender.start_wait_fresh_after_connect)
+        self.assertTrue(self.sender.start_pending)
+        self.assertEqual(self.sender.counters["teleop_starts"], 0)
+
+    def test_pending_freshness_missing_timestamp_cancels(self) -> None:
+        self.create_pending_start()
+        session = self.attach_session()
+        self.sender.start_wait_fresh_after_connect = True
+
+        outcome, reason = self.sender._check_pending_start_freshness(
+            frame(True)
+        )
+        self.assertEqual(outcome, "cancel")
+        self.assertIn("fresh xTELE timestamp", reason)
+        self.sender._cancel_pending_start(reason)
+
+        self.assertFalse(self.sender.start_pending)
+        self.assertFalse(self.sender.teleop_active)
+        self.assertTrue(session.closed)
+        self.assertEqual(session.sent, [])
+
+    def test_pending_freshness_repeated_timestamp_waits_for_later_advance(
+        self,
+    ) -> None:
+        self.create_pending_start()
+        self.sender.start_wait_fresh_after_connect = True
+        first = frame(True)
+        first["timestamp"] = 100.0
+        repeated = frame(True)
+        repeated["timestamp"] = 100.0
+        advanced = frame(True)
+        advanced["timestamp"] = 100.01
+
+        self.assertEqual(
+            self.sender._check_pending_start_freshness(first),
+            ("wait", ""),
+        )
+        self.assertEqual(
+            self.sender._check_pending_start_freshness(repeated),
+            ("wait", ""),
+        )
+        self.assertEqual(self.sender.start_fresh_barrier_timestamp, 100.0)
+        self.assertTrue(self.sender.start_wait_fresh_after_connect)
+        self.assertTrue(self.sender.start_pending)
+        self.assertEqual(
+            self.sender._check_pending_start_freshness(advanced),
+            ("ready", ""),
+        )
+        self.assertIsNone(self.sender.start_fresh_barrier_timestamp)
+        self.assertFalse(self.sender.start_wait_fresh_after_connect)
+        self.assertTrue(self.sender.start_pending)
+
+    def test_pending_freshness_backwards_timestamp_cancels(self) -> None:
+        self.create_pending_start()
+        session = self.attach_session()
+        self.sender.start_wait_fresh_after_connect = True
+        first = frame(True)
+        first["timestamp"] = 100.0
+        backwards = frame(True)
+        backwards["timestamp"] = 99.99
+
+        self.assertEqual(
+            self.sender._check_pending_start_freshness(first),
+            ("wait", ""),
+        )
+        outcome, reason = self.sender._check_pending_start_freshness(backwards)
+        self.assertEqual(outcome, "cancel")
+        self.assertIn("moved backwards", reason)
+        self.sender._cancel_pending_start(reason)
+
+        self.assertFalse(self.sender.start_pending)
+        self.assertTrue(session.closed)
+        self.assertEqual(session.sent, [])
+
+    def test_background_connect_success_is_adopted_as_pending(self) -> None:
+        self.create_pending_start()
+        entered = Event()
+        release = Event()
+        session = self.BlockingConnectSession(entered, release)
+
+        with (
+            mock.patch.object(sender_module, "Session", return_value=session),
+            mock.patch.object(self.sender, "write_status"),
+        ):
+            self.assertTrue(self.sender._start_connect_attempt())
+            self.assertTrue(entered.wait(timeout=1.0))
+            self.assertTrue(self.sender._connect_inflight())
+            thread = self.sender.connect_thread
+            self.assertIsNotNone(thread)
+            release.set()
+            thread.join(timeout=1.0)  # type: ignore[union-attr]
+            self.assertFalse(thread.is_alive())  # type: ignore[union-attr]
+            self.assertEqual(
+                self.sender._poll_connect_result(50.0), "connected"
+            )
+
+        self.assertIs(self.sender.session, session)
+        self.assertFalse(session.closed)
+        self.assertTrue(self.sender.start_pending)
+        self.assertFalse(self.sender.teleop_active)
+        self.assertTrue(self.sender.start_wait_fresh_after_connect)
+        self.assertIsNone(self.sender.start_fresh_barrier_timestamp)
+        self.assertEqual(self.sender.counters["connected"], 1)
+        self.assertEqual(self.sender.counters["reconnects"], 1)
+        self.assertEqual(
+            session.connect_args,
+            (self.sender.args.server, self.sender.args.peer_id, {}),
+        )
+        self.sender.close_session()
+
+    def test_release_during_connect_discards_and_closes_late_result(self) -> None:
+        pending_at = self.create_pending_start()
+        entered = Event()
+        release = Event()
+        session = self.BlockingConnectSession(entered, release)
+
+        with mock.patch.object(sender_module, "Session", return_value=session):
+            self.assertTrue(self.sender._start_connect_attempt())
+            self.assertTrue(entered.wait(timeout=1.0))
+            thread = self.sender.connect_thread
+            self.assertIsNotNone(thread)
+
+            self.assertIsNone(
+                self.sender._update_teleop_gate(
+                    pending_at + 0.01, frame(False)
+                )
+            )
+            self.cancel_pending_from_latest_frame()
+            self.assertFalse(self.sender.start_pending)
+
+            release.set()
+            thread.join(timeout=1.0)  # type: ignore[union-attr]
+            self.assertFalse(thread.is_alive())  # type: ignore[union-attr]
+            self.assertEqual(
+                self.sender._poll_connect_result(pending_at + 0.02),
+                "discarded",
+            )
+
+        self.assertTrue(session.closed)
+        self.assertIsNone(self.sender.session)
+        self.assertFalse(self.sender.teleop_active)
+        self.assertIsNone(self.sender.teleop_session_id)
+        self.assertEqual(self.sender.counters["connected"], 0)
+        self.assertEqual(self.sender.counters["teleop_starts"], 0)
+        self.assertEqual(self.sender.counters["start_pending_cancels"], 1)
+
+    def test_background_connect_failure_consumes_hold_without_retry(self) -> None:
+        pending_at = self.create_pending_start()
+        first_session_id = self.sender.teleop_session_id
+        entered = Event()
+        release = Event()
+        session = self.BlockingConnectSession(
+            entered,
+            release,
+            OSError("connection refused"),
+        )
+
+        with (
+            mock.patch.object(sender_module, "Session", return_value=session),
+            mock.patch.object(self.sender, "write_status"),
+        ):
+            # run() owns this counter; mirror its one increment around the
+            # lower-level worker call used by this unit test.
+            self.sender.counters["start_connect_attempts"] += 1
+            self.assertTrue(self.sender._start_connect_attempt())
+            self.assertTrue(entered.wait(timeout=1.0))
+            thread = self.sender.connect_thread
+            self.assertIsNotNone(thread)
+            release.set()
+            thread.join(timeout=1.0)  # type: ignore[union-attr]
+            self.assertFalse(thread.is_alive())  # type: ignore[union-attr]
+            self.assertEqual(self.sender._poll_connect_result(20.0), "failed")
+
+        self.assertTrue(session.closed)
+        self.assertEqual(session.sent, [])
+        self.assertIsNone(self.sender.session)
+        self.assertFalse(self.sender.start_pending)
+        self.assertFalse(self.sender.teleop_active)
+        self.assertIsNone(self.sender.teleop_session_id)
+        self.assertTrue(self.sender.require_combo_release)
+        self.assertEqual(self.sender.counters["start_connect_failures"], 1)
+        self.assertEqual(self.sender.counters["start_connect_attempts"], 1)
+        self.assertEqual(self.sender.counters["teleop_starts"], 0)
+        self.assertEqual(self.sender.counters["frames_sent"], 0)
+        self.assertIn("connection refused", self.sender.last_start_failure)
+        self.assertFalse(self.sender._pending_connect_ready(100.0))
+
+        # Keeping the failed Z+C hold pressed cannot create another request or
+        # connection attempt after the Hub later becomes available.
+        self.assertIsNone(
+            self.sender._update_teleop_gate(25.0, frame(True))
+        )
+        self.assertFalse(self.sender.start_pending)
+        self.assertEqual(self.sender.counters["teleop_start_requests"], 1)
+        self.assertEqual(self.sender.counters["start_connect_attempts"], 1)
+
+        released_at = self.stable_release(25.01)
+        self.assertIsNone(
+            self.sender._update_teleop_gate(
+                released_at + 0.01, frame(True)
+            )
+        )
+        self.assertEqual(
+            self.sender._update_teleop_gate(
+                released_at + 3.02, frame(True)
+            ),
+            "start_pending",
+        )
+        self.assertTrue(self.sender.start_pending)
+        self.assertNotEqual(self.sender.teleop_session_id, first_session_id)
+        self.assertEqual(self.sender.counters["teleop_start_requests"], 2)
+        self.assertTrue(self.sender._pending_connect_ready(released_at + 3.02))
+
+    def test_session_constructor_failure_consumes_hold_without_retry(self) -> None:
+        self.create_pending_start()
+        constructor_called = Event()
+
+        def broken_session_constructor() -> object:
+            constructor_called.set()
+            raise RuntimeError("Session constructor failed")
+
+        with (
+            mock.patch.object(
+                sender_module,
+                "Session",
+                side_effect=broken_session_constructor,
+            ),
+            mock.patch.object(self.sender, "write_status"),
+        ):
+            self.assertTrue(self.sender._start_connect_attempt())
+            self.assertTrue(constructor_called.wait(timeout=1.0))
+            thread = self.sender.connect_thread
+            self.assertIsNotNone(thread)
+            thread.join(timeout=1.0)  # type: ignore[union-attr]
+            self.assertFalse(thread.is_alive())  # type: ignore[union-attr]
+            self.assertEqual(self.sender._poll_connect_result(30.0), "failed")
+
+        self.assertIsNone(self.sender.session)
+        self.assertFalse(self.sender.start_pending)
+        self.assertFalse(self.sender.teleop_active)
+        self.assertIsNone(self.sender.teleop_session_id)
+        self.assertTrue(self.sender.require_combo_release)
+        self.assertEqual(self.sender.counters["start_connect_failures"], 1)
+        self.assertIn("Session constructor failed", self.sender.last_start_failure)
+        self.assertEqual(self.sender.counters["teleop_starts"], 0)
+        self.assertFalse(self.sender._pending_connect_ready(300.0))
+
+    def test_successful_start_commit_activates_once(self) -> None:
+        pending_at = self.create_pending_start()
+        self.sender._commit_pending_start_sent(pending_at + 0.01)
+
+        self.assertFalse(self.sender.start_pending)
+        self.assertTrue(self.sender.teleop_active)
+        self.assertEqual(self.sender.counters["teleop_starts"], 1)
+        self.assertEqual(self.sender.counters["teleop_start_requests"], 1)
+
+    def test_unknown_target_is_an_ordinary_remote_error_and_aborts(self) -> None:
+        pending_at = self.create_pending_start()
+        self.sender._commit_pending_start_sent(pending_at + 0.01)
+        session = self.attach_session(
+            [("hub", fake_omnisocket.MSG_TYPE_ERROR, b"unknown target: robot")]
+        )
+
+        self.sender._drain_responses()
+
+        self.assertTrue(session.closed)
+        self.assertIsNone(self.sender.session)
+        self.assertFalse(self.sender.start_pending)
+        self.assertFalse(self.sender.teleop_active)
+        self.assertIsNone(self.sender.teleop_session_id)
+        self.assertTrue(self.sender.require_combo_release)
+        self.assertEqual(self.sender.counters["teleop_aborts"], 1)
+        self.assertEqual(self.sender.counters["teleop_starts"], 1)
+        self.assertIn("unknown target: robot", self.sender.last_error)
+
+    def test_other_remote_error_also_aborts(self) -> None:
+        pending_at = self.create_pending_start()
+        self.sender._commit_pending_start_sent(pending_at + 0.01)
+        session = self.attach_session(
+            [("hub", fake_omnisocket.MSG_TYPE_ERROR, b"route unavailable")]
+        )
+
+        self.sender._drain_responses()
+
+        self.assertTrue(session.closed)
+        self.assertFalse(self.sender.start_pending)
+        self.assertFalse(self.sender.teleop_active)
+        self.assertEqual(self.sender.counters["teleop_aborts"], 1)
+        self.assertIn("route unavailable", self.sender.last_error)
+
+    def test_source_clock_allows_adjacent_repeat_under_timeout(self) -> None:
+        first = frame(False)
+        first["timestamp"] = 2507
+        repeated = frame(False)
+        repeated["timestamp"] = 2507
+
+        self.assertEqual(
+            self.sender._check_source_clock(10.0, first),
+            (True, ""),
+        )
+        self.assertEqual(
+            self.sender._check_source_clock(10.24, repeated),
+            (True, ""),
+        )
+        self.assertEqual(self.sender.last_source_timestamp, 2507.0)
+        self.assertEqual(self.sender.last_source_timestamp_advanced_at, 10.0)
+
+    def test_source_clock_rejects_persistent_freeze_over_timeout(self) -> None:
+        first = frame(False)
+        first["timestamp"] = 2517
+        repeated = frame(False)
+        repeated["timestamp"] = 2517
+
+        self.assertEqual(
+            self.sender._check_source_clock(10.0, first),
+            (True, ""),
+        )
+        self.assertEqual(
+            self.sender._check_source_clock(10.24, repeated),
+            (True, ""),
+        )
+        clock_ok, reason = self.sender._check_source_clock(10.251, repeated)
+        self.assertFalse(clock_ok)
+        self.assertIn("stopped advancing", reason)
+        self.assertEqual(self.sender.last_source_timestamp, 2517.0)
+
+    def test_source_clock_rejects_backwards_timestamp_while_active(self) -> None:
+        first = frame(False)
+        first["timestamp"] = 3000
+        backwards = frame(False)
+        backwards["timestamp"] = 2999
+        self.assertEqual(
+            self.sender._check_source_clock(10.0, first),
+            (True, ""),
+        )
+        self.sender.teleop_active = True
+
+        clock_ok, reason = self.sender._check_source_clock(10.01, backwards)
+
+        self.assertFalse(clock_ok)
+        self.assertIn("moved backwards", reason)
+        self.assertEqual(self.sender.last_source_timestamp, 3000.0)
+        self.assertEqual(self.sender.counters["source_timestamp_resets"], 0)
+
+    def test_source_clock_idle_backwards_resets_and_requires_release(self) -> None:
+        first = frame(False)
+        first["timestamp"] = 4000
+        restarted = frame(True)
+        restarted["timestamp"] = 10
+        self.assertEqual(
+            self.sender._check_source_clock(10.0, first),
+            (True, ""),
+        )
+        self.sender.require_combo_release = False
+        self.sender.combo_started_at = 10.01
+        self.sender.combo_release_started_at = 10.02
+        self.sender.latest_combo_state = True
+
+        self.assertEqual(
+            self.sender._check_source_clock(10.03, restarted),
+            (True, ""),
+        )
+
+        self.assertEqual(self.sender.last_source_timestamp, 10.0)
+        self.assertEqual(self.sender.last_source_timestamp_advanced_at, 10.03)
+        self.assertTrue(self.sender.require_combo_release)
+        self.assertIsNone(self.sender.combo_started_at)
+        self.assertIsNone(self.sender.combo_release_started_at)
+        self.assertIsNone(self.sender.latest_combo_state)
+        self.assertEqual(self.sender.counters["source_timestamp_resets"], 1)
+
+    def test_source_clock_rejects_missing_timestamp(self) -> None:
+        clock_ok, reason = self.sender._check_source_clock(10.0, frame(False))
+
+        self.assertFalse(clock_ok)
+        self.assertIn("missing or malformed", reason)
+        self.assertIsNone(self.sender.last_source_timestamp)
+
+    def test_finish_stop_session_clears_state_and_closes_session(self) -> None:
+        session = self.attach_session()
+        self.sender.teleop_active = True
+        self.sender.teleop_session_id = "active-session"
+        self.sender.teleop_session_seq = 42
+        self.sender.start_markers_remaining = 7
+
+        self.sender._finish_stop_session()
+
+        self.assertFalse(self.sender.teleop_active)
+        self.assertIsNone(self.sender.teleop_session_id)
+        self.assertEqual(self.sender.teleop_session_seq, 0)
+        self.assertEqual(self.sender.start_markers_remaining, 0)
+        self.assertIsNone(self.sender.session)
+        self.assertTrue(session.closed)
+        self.assertEqual(session.sent, [])
+        self.assertEqual(self.sender.counters["connected"], 0)
 
     def test_untrusted_transport_metadata_is_overwritten(self) -> None:
         data = frame(False)