diff --git a/docs/cli.md b/docs/cli.md
index afd2092..5c3c54e 100644
--- a/docs/cli.md
+++ b/docs/cli.md
@@ -55,7 +55,7 @@ With no command, MeshTerm launches the interactive menu instead.
 | `--ble ADDRESS` | Bluetooth address of a companion; selects the BLE transport. |
 | `--ble-pin PIN` | Pairing PIN, if the Bluetooth companion asks for one. |
 | `--tcp HOST[:PORT]` | Network address of a TCP companion; selects the TCP transport. Default port 5000. |
-| `--spi` | The LoRa radio on this machine's own SPI bus; selects the SPI transport. MeshTerm runs the node for the length of the command. Uses the first SPI profile's wiring, else the uConsole AIO's — see [Adding a radio on the SPI bus](configuration.md#adding-a-radio-on-the-spi-bus). Linux only. |
+| `--spi` | The LoRa radio on this machine's own SPI bus; selects the SPI transport. MeshTerm runs the node for the length of the command. With one radio attached, that's the one, profile or not; with none attached, the one SPI profile, else the uConsole AIO's wiring. With two, it refuses (exit `3`) and lists them — pick one with `-p`. See [Adding a radio on the SPI bus](configuration.md#adding-a-radio-on-the-spi-bus). Linux only. |
 | `--mock` | Use the built-in simulator instead of real hardware. Nothing transmits, and the run records to `meshterm-mock.db` rather than your real history. |
 | `--db PATH` | Use this SQLite database instead of `~/.meshterm/meshterm.db` (or `meshterm-mock.db`, under `--mock`). **It moves the database and nothing else** — see [Where MeshTerm keeps its state](#where-meshterm-keeps-its-state). |
 | `--json` | Print the answer as JSON instead of aligned text. |
diff --git a/meshterm/context.py b/meshterm/context.py
index 6af1a1c..3ea1217 100644
--- a/meshterm/context.py
+++ b/meshterm/context.py
@@ -661,7 +661,7 @@ class AppContext:
         """The wiring of the SPI radio to open, or ``None`` when another transport is meant.
 
         In priority order: an SPI profile, the radio picked on the startup splash, ``--spi``
-        (the first SPI profile's wiring, else the AIO's), then the remembered default when it
+        (the one radio attached — see :meth:`_spi_for_flag`), then the remembered default when it
         was an SPI radio and nothing else was named. Anything else named explicitly — a port,
         an address, a host, another kind of profile — means this session is not about the SPI
         radio.
@@ -672,10 +672,7 @@ class AppContext:
         if chosen is not None and chosen.is_spi:
             return chosen.spi or self.spi_wiring_for(chosen.port)  # type: ignore[return-value]
         if self.spi_override:
-            return next(
-                (p.spi or SpiWiring() for p in self.settings.profiles.values() if p.is_spi),
-                SpiWiring(),
-            )
+            return self._spi_for_flag()
         if self.port_override or self.ble_override or self.tcp_override or self.profile:
             return None
         remembered = self.device_store.load()
@@ -684,6 +681,43 @@ class AppContext:
         self.selected_device = None  # remembered, not freshly discovered this session
         return self.spi_wiring_for(remembered.target)
 
+    def _spi_for_flag(self) -> SpiWiring:
+        """The radio a bare ``--spi`` means: the one there is, and never a guess between two.
+
+        Nearly everyone with a radio on the SPI bus has exactly one, and for them ``--spi``
+        needs nothing more — attached radios are counted first, so a single one is used
+        whether or not a profile names it. Only when two are attached, or (with none
+        attached to count) two SPI profiles are configured, is the flag ambiguous; it used
+        to take whichever profile ``config.toml`` listed first, which is a coin toss that
+        transmits. With none attached and one profile or none, the choice is made from the
+        configuration, so the connect attempt can say what is missing.
+
+        Raises:
+            DeviceSelectionError: When more than one radio could be meant.
+        """
+        from .core.selection import DeviceSelectionError
+        from .core.spiradio import spi_radios
+
+        profiles = self.settings.profiles
+        attached = spi_radios(profiles)
+        if len(attached) == 1:
+            return attached[0].spi  # type: ignore[return-value]
+        configured = [p for p in profiles.values() if p.is_spi]
+        if not attached and len(configured) <= 1:
+            return (configured[0].spi or SpiWiring()) if configured else SpiWiring()
+        if attached:
+            rows = [f"  {d.name or '(no profile)':<16} {d.port}" for d in attached]
+            what = "Several radios are attached to the SPI bus"
+        else:
+            rows = [f"  {p.name:<16} {(p.spi or SpiWiring()).spidev}" for p in configured]
+            what = "Several SPI profiles are configured"
+        raise DeviceSelectionError(
+            f"{what}, and --spi doesn't say which.\n"
+            + "\n".join(rows)
+            + "\nChoose one with -p <PROFILE>. A radio with no profile needs one first — see "
+            "'Adding a radio on the SPI bus' in docs/configuration.md."
+        )
+
     def spi_wiring_for(self, spidev: str) -> SpiWiring:
         """The wiring for the radio on ``spidev``: a profile's that names it, else the AIO's.
 
diff --git a/meshterm/tools/devices.py b/meshterm/tools/devices.py
index f8ddc33..aa60103 100644
--- a/meshterm/tools/devices.py
+++ b/meshterm/tools/devices.py
@@ -25,6 +25,7 @@ from ..core.discovery import (
     discover_all,
     discover_devices,
 )
+from ..core.selection import DeviceSelectionError
 from ..core.spiradio import spi_radios
 from .base import Tool, ToolResult, register
 
@@ -81,11 +82,15 @@ class DevicesTool(Tool):
         known = ctx.device_store.load_all()
         remembered = ctx.device_store.load()
         active = ctx.selected_device
-        spi = (
-            ctx.resolve_spi()
-            if ctx.spi_override or (ctx.profile is not None and ctx.profile.is_spi)
-            else None
-        )
+        try:
+            spi = (
+                ctx.resolve_spi()
+                if ctx.spi_override or (ctx.profile is not None and ctx.profile.is_spi)
+                else None
+            )
+        except DeviceSelectionError:
+            spi = None  # an ambiguous --spi: the listing is how the reader finds out which
+
         active_target = (
             ctx.ble_override
             or ctx.port_override
diff --git a/tests/test_spiradio.py b/tests/test_spiradio.py
index db7f42e..f5042be 100644
--- a/tests/test_spiradio.py
+++ b/tests/test_spiradio.py
@@ -152,6 +152,53 @@ def test_spi_flag_takes_the_spi_profile_wiring(contexts) -> None:  # noqa: ANN00
     assert contexts(spi_override=True).active_transport == "spi"
 
 
+def test_spi_flag_takes_the_one_radio_attached(contexts, monkeypatch) -> None:  # noqa: ANN001
+    """The common case: one radio on the bus, and --spi is all it takes, profile or not."""
+    monkeypatch.setattr(spiradio, "spi_present", lambda w: w.spidev == "/dev/spidev1.0")
+    elsewhere = SpiWiring(bus_id=0, reset_pin=17)  # a profile for a radio not plugged in
+    profiles = {"hat": DeviceProfile(name="hat", transport="spi", spi=elsewhere)}
+    assert contexts(spi_override=True, profiles=profiles).resolve_spi() == SpiWiring()
+    assert contexts(spi_override=True).resolve_spi() == SpiWiring()
+
+
+def test_spi_flag_refuses_to_guess_between_two_attached_radios(
+    contexts,  # noqa: ANN001
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Two radios on the bus: --spi names neither, so it lists them and asks for -p."""
+    from meshterm.core.selection import DeviceSelectionError
+
+    monkeypatch.setattr(spiradio, "spi_present", lambda w: True)
+    hat = SpiWiring(bus_id=0, reset_pin=17)
+    profiles = {"hat": DeviceProfile(name="hat", transport="spi", spi=hat)}
+    with pytest.raises(DeviceSelectionError) as err:
+        contexts(spi_override=True, profiles=profiles).resolve_spi()
+    message = str(err.value)
+    assert "Several radios are attached to the SPI bus" in message
+    assert "hat" in message and "/dev/spidev0.0" in message
+    assert "(no profile)" in message and "/dev/spidev1.0" in message
+    assert "-p <PROFILE>" in message
+    # A profile still says exactly which, however many there are.
+    ctx = contexts(profile=profiles["hat"], profiles=profiles)
+    assert ctx.resolve_spi() == hat
+
+
+def test_spi_flag_refuses_two_profiles_with_nothing_attached(
+    contexts,  # noqa: ANN001
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """With no radio to count, two SPI profiles leave --spi as ambiguous as two radios."""
+    from meshterm.core.selection import DeviceSelectionError
+
+    monkeypatch.setattr(spiradio, "spi_present", lambda w: False)
+    profiles = {
+        "aio": DeviceProfile(name="aio", transport="spi"),
+        "hat": DeviceProfile(name="hat", transport="spi", spi=SpiWiring(bus_id=0)),
+    }
+    with pytest.raises(DeviceSelectionError, match="Several SPI profiles are configured"):
+        contexts(spi_override=True, profiles=profiles).resolve_spi()
+
+
 def test_another_named_radio_is_not_the_spi_radio(contexts) -> None:  # noqa: ANN001
     """A port, an address or a host named outright means the session is about that radio."""
     for override in (