diff --git a/src/ria_toolkit_oss/agent/streamer.py b/src/ria_toolkit_oss/agent/streamer.py
index 1e82a5f..6146a5a 100644
--- a/src/ria_toolkit_oss/agent/streamer.py
+++ b/src/ria_toolkit_oss/agent/streamer.py
@@ -249,9 +249,16 @@ class Streamer:
             await self._send_error(app_id, "start missing radio_config.device")
             return
 
+        # Open the SDR in a thread, never inline. The open is blocking and can be
+        # slow — a USRP shells out to uhd_find_devices and loads its FPGA, which
+        # takes seconds — and doing it on the event loop freezes the WebSocket
+        # keepalive long enough that the hub drops the agent and stops the app.
+        # (A Pluto opens fast enough to slip under the timeout, which is why it
+        # worked where a USRP hung.)
+        loop = asyncio.get_running_loop()
         try:
-            sdr, device_key = self._registry.acquire(device, identifier)
-            _apply_sdr_config(sdr, radio_config)
+            sdr, device_key = await loop.run_in_executor(None, self._registry.acquire, device, identifier)
+            await loop.run_in_executor(None, _apply_sdr_config, sdr, radio_config)
         except Exception as exc:
             logger.exception("Failed to open SDR %r", device)
             await self._send_error(app_id, f"SDR init failed: {_friendly_sdr_error(device, exc)}")
@@ -385,42 +392,51 @@ class Streamer:
             await self._send_tx_status(app_id, "error", "tx_start missing radio_config.device")
             return
 
-        device_key: tuple[str, str | None] | None = None
-        sdr: Any = None
-        try:
-            sdr, device_key = self._registry.acquire(device, identifier)
-            _apply_sdr_config(sdr, radio_config)
-            # init_tx is mandatory for any driver that exposes it: drivers
-            # that gate _stream_tx on _tx_initialized (Pluto, HackRF, USRP,
-            # …) crash with a confusing "TX was not initialized" error 2 s
-            # later in the executor thread if we skip it. Treat the three
-            # required keys as a hard contract — a missing one is a hub-side
-            # manifest bug and we want it surfaced immediately, not papered
-            # over with stale radio state.
-            if hasattr(sdr, "init_tx"):
-                init_args = {k: radio_config.get(f"tx_{k}") for k in ("sample_rate", "center_frequency", "gain")}
-                missing = [f"tx_{k}" for k, v in init_args.items() if v is None]
-                if missing:
-                    raise ValueError(f"tx_start missing required radio_config keys: {missing}")
-                sdr.init_tx(
-                    sample_rate=init_args["sample_rate"],
-                    center_frequency=init_args["center_frequency"],
-                    gain=init_args["gain"],
-                    channel=radio_config.get("tx_channel", 0),
-                    gain_mode=radio_config.get("tx_gain_mode", "manual"),
-                )
-        except Exception as exc:
-            if device_key is not None:
-                if self._registry.release(device_key):
+        # Open + init the SDR in a thread, never inline — the open is blocking and
+        # slow on a USRP (uhd_find_devices + FPGA load), and freezing the event
+        # loop stalls the WebSocket keepalive until the hub drops us. Cleanup on
+        # failure (release/close) stays inside the thread so a partial open never
+        # leaks a device handle.
+        def _open_and_init_tx() -> tuple[Any, tuple[str, str | None]]:
+            sdr_local, key_local = self._registry.acquire(device, identifier)
+            try:
+                _apply_sdr_config(sdr_local, radio_config)
+                # init_tx is mandatory for any driver that exposes it: drivers
+                # that gate _stream_tx on _tx_initialized (Pluto, HackRF, USRP,
+                # …) crash with a confusing "TX was not initialized" error 2 s
+                # later in the executor thread if we skip it. Treat the three
+                # required keys as a hard contract — a missing one is a hub-side
+                # manifest bug and we want it surfaced immediately, not papered
+                # over with stale radio state.
+                if hasattr(sdr_local, "init_tx"):
+                    init_args = {k: radio_config.get(f"tx_{k}") for k in ("sample_rate", "center_frequency", "gain")}
+                    missing = [f"tx_{k}" for k, v in init_args.items() if v is None]
+                    if missing:
+                        raise ValueError(f"tx_start missing required radio_config keys: {missing}")
+                    sdr_local.init_tx(
+                        sample_rate=init_args["sample_rate"],
+                        center_frequency=init_args["center_frequency"],
+                        gain=init_args["gain"],
+                        channel=radio_config.get("tx_channel", 0),
+                        gain_mode=radio_config.get("tx_gain_mode", "manual"),
+                    )
+            except Exception:
+                if self._registry.release(key_local):
                     try:
-                        sdr.close()
+                        sdr_local.close()
                     except Exception:
                         pass
+                raise
+            return sdr_local, key_local
+
+        self._loop = asyncio.get_running_loop()
+        try:
+            sdr, device_key = await self._loop.run_in_executor(None, _open_and_init_tx)
+        except Exception as exc:
             logger.exception("Failed to init TX on %r", device)
             await self._send_tx_status(app_id, "error", f"tx init failed: {_friendly_sdr_error(device, exc)}")
             return
 
-        self._loop = asyncio.get_running_loop()
         session = TxSession(
             app_id=app_id,
             sdr=sdr,
diff --git a/src/ria_toolkit_oss/agent/ws_client.py b/src/ria_toolkit_oss/agent/ws_client.py
index a33991d..ab5bd98 100644
--- a/src/ria_toolkit_oss/agent/ws_client.py
+++ b/src/ria_toolkit_oss/agent/ws_client.py
@@ -119,9 +119,15 @@ class WsClient:
             await asyncio.sleep(self.reconnect_pause)
 
     async def _heartbeat_loop(self, heartbeat: HeartbeatBuilder) -> None:
+        loop = asyncio.get_running_loop()
         while True:
             try:
-                await self.send_json(heartbeat())
+                # Build off the event loop: a heartbeat can probe SDR hardware
+                # (e.g. uhd_find_devices on a USRP), which blocks for seconds and
+                # would otherwise freeze the WebSocket keepalive long enough for
+                # the hub to drop the agent.
+                payload = await loop.run_in_executor(None, heartbeat)
+                await self.send_json(payload)
             except Exception as exc:
                 logger.debug("Heartbeat send failed: %s", exc)
                 return