diff --git a/decisions/2026-09-11-mob-deploy-refuses-ambiguous-multi-device.md b/decisions/2026-09-11-mob-deploy-refuses-ambiguous-multi-device.md
new file mode 100644
index 0000000..4ed04df
--- /dev/null
+++ b/decisions/2026-09-11-mob-deploy-refuses-ambiguous-multi-device.md
@@ -0,0 +1,64 @@
+# mob.deploy refuses an ambiguous multi-device fan-out
+
+- Date: 2026-09-11
+- Status: accepted
+- Linear: MOB-182
+- GitHub: mob_dev#53
+
+## Context
+
+`mix mob.deploy` without `--device` used to target every reachable device.
+When only one device was around, this was ergonomic: `mix mob.deploy`
+deployed to it. When more than one was around — a laptop's own simulator
+plus a teammate's paired phone, a spare emulator left running by another
+worktree, a Bluetooth-linked device the author forgot about — the same
+command silently fanned out to all of them. The behavior was documented
+but the failure mode was invisible: the author saw their expected device
+succeed and had no reason to check whether *others* also received a build.
+
+mob_dev already fixed the adjacent case in this class — `mix mob.deploy -d
+X` where `-d` was never aliased silently deployed to every device, and
+`mix mob.deploy --native ABC123` (positional-arg fumble of `--device`)
+did the same. That's the same discipline as MOB-150 (deploy exits
+honestly): the failure mode a caller can't see is worse than an error at
+the tin. Refusing the ambiguous case moves this class of bug from
+"invisible until the teammate notices" to "loud at run start."
+
+## Decision
+
+`mix mob.deploy` refuses to proceed when **two or more** devices are
+reachable (after `--android` / `--ios` platform narrowing) unless the run
+narrows explicitly:
+
+- `--device <id>` or `--ios-device <id>` — target a specific device.
+  Either flag disables the gate.
+- `--all` — new flag. Explicit opt-in to fan out to every reachable
+  device. Preserves the prior default behavior for callers who wanted it.
+
+Single-device runs proceed exactly as before. The gate lives in a pure
+predicate (`MobDev.Deployer.check_fanout_gate/2`) so the matrix is unit-
+testable without adb/simctl. The mix task does one lightweight discovery
+pass ahead of compile / native-build so an ambiguous run is refused before
+those costs.
+
+## Consequences
+
+- Multi-device CI configurations must add `--all` (or `--device X` if they
+  really wanted one target). Documented in the `mix mob.deploy` help.
+- Single-device dev flow (one Android emulator OR one iOS sim) is
+  unchanged.
+- Mixed dev flow (one Android emulator AND one iOS sim, the common
+  parallel-platform case) now requires `--all`. This is a real ergonomic
+  cost — kept small by the flag being a single character (`-a` is left
+  free; not aliased in this change to keep the surface obvious). If daily
+  friction is high we can revisit an "authenticated devices" concept
+  (mob.exs pins the two devices you always deploy to; anything else is a
+  guest and requires --all).
+- `mob.push` still fans out silently; the issue thread (mob_dev#53) called
+  it "the first fan-out hazard." That's a separate follow-up; MOB-182 is
+  scoped to `mob.deploy` because that's the flow that also builds and
+  installs, which is the more expensive mistake.
+- The gate does not fire when `deploy_all` runs headlessly (some scripts
+  do that); those callers pass `all: true` to `deploy_all`'s underlying
+  discovery, which the check consults through the task's option-parsing
+  layer.
diff --git a/lib/mix/tasks/mob.deploy.ex b/lib/mix/tasks/mob.deploy.ex
index 7158f9b..9e116fb 100644
--- a/lib/mix/tasks/mob.deploy.ex
+++ b/lib/mix/tasks/mob.deploy.ex
@@ -24,6 +24,10 @@ defmodule Mix.Tasks.Mob.Deploy do
     * `--native`              — build native binaries before pushing BEAMs
     * `--no-restart`          — push BEAMs but don't restart the app
     * `-d`, `--device <id>`   — target a specific device; use `mix mob.devices` to find IDs
+    * `--all`                 — allow deploying to every reachable device. Required when
+                              two or more devices match the resolved platform set and no
+                              `--device` / `--ios-device` filter narrows the target. Single-
+                              device runs proceed without it. See MOB-182.
     * `--dist-port <N>`       — pin the BEAM dist listen port (default: auto-allocated per
                               device, `9100 + index`). Use to resolve EPMD collisions when
                               multiple sims/emulators are running the same app concurrently
@@ -139,12 +143,17 @@ defmodule Mix.Tasks.Mob.Deploy do
     * `mix mob.deploy --device NOPE` matching no device — exit 1.
     * `mix mob.deploy --android --native` that built the APK with no device
       attached — exit 0. The artifact is what was asked for.
+    * `mix mob.deploy` with two or more devices reachable and no `--device` /
+      `--ios-device` / `--all` — exit 1 before build (MOB-182). Refusing an
+      ambiguous fan-out is the safer default than silently deploying to a
+      teammate's phone; add `--all` when the fan-out is intended.
 
   `--native` fails the run when a platform you named built nothing at all, which
   is what a missing `sdk.dir` in `android/local.properties` produces.
   """
 
   alias MobDev.Device
+  alias MobDev.Discovery.{Android, IOS}
 
   @switches [
     native: :boolean,
@@ -157,6 +166,11 @@ defmodule Mix.Tasks.Mob.Deploy do
     android: :boolean,
     ios: :boolean,
     device: :string,
+    # `--all` opts into fanning out to every reachable device when the run
+    # would otherwise touch two or more of them. Without it, an ambiguous
+    # multi-device run is refused up front (MOB-182). Single-device runs
+    # ignore the flag.
+    all: :boolean,
     schedulers: :integer,
     beam_flags: :string,
     # Manual overrides for the BEAM-distribution surface — useful when
@@ -248,6 +262,32 @@ defmodule Mix.Tasks.Mob.Deploy do
         if native and :ios in platforms,
           do: MobDev.NativeBuild.detect_physical_ios()
 
+    # MOB-182 safety gate: refuse a fan-out to multiple devices unless the
+    # caller opted in with `--all` or narrowed with `--device` / `--ios-device`.
+    # The gate itself lives inside `MobDev.Deployer.deploy_all/1` so a device
+    # plugged in between here and the deploy loop can't sneak past it. That's
+    # the authoritative check.
+    #
+    # We ALSO run it here on a cheap pre-scan so the refusal lands before the
+    # compile / native-build tax rather than after. The task's `deploy_all`
+    # call re-verifies against fresh discovery — a device state that changes
+    # between the pre-scan and the deploy is caught there.
+    reachable_preview = discover_targeted_devices(platforms, device_id, effective_device_id)
+
+    case MobDev.Deployer.check_fanout_gate(reachable_preview,
+           device: device_id,
+           ios_device: effective_device_id,
+           all: opts[:all] == true
+         ) do
+      :ok ->
+        :ok
+
+      {:refuse, lines} ->
+        Enum.each(lines, &IO.puts/1)
+        emit_json(opts, [], [], [], "Refused: multiple devices without --device / --all")
+        Mix.raise("Refused ambiguous multi-device deploy")
+    end
+
     # Validate every targeted device against the project's enabled
     # features (Pythonx, etc.) BEFORE we waste time on a multi-minute
     # native build that the device couldn't have run anyway. See
@@ -321,7 +361,11 @@ defmodule Mix.Tasks.Mob.Deploy do
         # nil → auto-allocation (per-device port + auto-derived suffix).
         # Set → all targeted devices use these values verbatim.
         dist_port: opts[:dist_port],
-        node_suffix: opts[:node_suffix]
+        node_suffix: opts[:node_suffix],
+        # MOB-182: opt-in to the multi-device fan-out. `deploy_all` runs the
+        # authoritative refuse-if-ambiguous check on its own discovery — the
+        # pre-scan above is only an ergonomic early-refuse.
+        all: opts[:all] == true
       )
 
     Enum.each(format_summary(deployed, failed, skipped, restart: restart), &IO.puts/1)
@@ -433,6 +477,38 @@ defmodule Mix.Tasks.Mob.Deploy do
 
   defp names(platforms), do: platforms |> Enum.map(&"--#{&1}") |> Enum.join(", ")
 
+  # MOB-182: shape the discovered device list the same way
+  # `MobDev.Deployer.deploy_all/1` does (drop unauthorized Android, apply
+  # any `--device` / `--ios-device` filter). Returning the exact set that
+  # `deploy_all` would target keeps the fan-out gate honest — a mismatch
+  # here would either let a genuinely-ambiguous run through or refuse one
+  # that would have narrowed to a single device inside the deployer.
+  defp discover_targeted_devices(platforms, device_id, ios_device_id) do
+    android =
+      if :android in platforms do
+        Android.list_devices()
+        |> Enum.reject(&(&1.status == :unauthorized))
+        |> filter_by_id(device_id)
+      else
+        []
+      end
+
+    ios =
+      if :ios in platforms do
+        IOS.list_devices() |> filter_by_id(ios_device_id || device_id)
+      else
+        []
+      end
+
+    android ++ ios
+  end
+
+  defp filter_by_id(devices, nil), do: devices
+
+  defp filter_by_id(devices, id) do
+    Enum.filter(devices, &Device.match_id?(&1, id))
+  end
+
   # Written to the REAL stdout, not the group leader — which under --json now
   # points at stderr so the progress prose gets out of the document's way.
   defp emit_json(opts, deployed, failed, skipped, message) do
diff --git a/lib/mob_dev/deployer.ex b/lib/mob_dev/deployer.ex
index 2345d3e..cde00ec 100644
--- a/lib/mob_dev/deployer.ex
+++ b/lib/mob_dev/deployer.ex
@@ -84,6 +84,24 @@ defmodule MobDev.Deployer do
 
     all = android ++ ios
 
+    # MOB-182: authoritative fan-out gate. `mix mob.deploy` runs the same
+    # check up front on a pre-scan so the refusal lands before the compile
+    # tax, but a device plugged in between that pre-scan and this
+    # discovery would sneak past it. This is the check that closes that
+    # window. See `check_fanout_gate/2` for the decision matrix.
+    case check_fanout_gate(all,
+           device: device_id,
+           ios_device: ios_device_id,
+           all: Keyword.get(opts, :all, false)
+         ) do
+      :ok ->
+        :ok
+
+      {:refuse, lines} ->
+        Enum.each(lines, &IO.puts/1)
+        Mix.raise("Refused ambiguous multi-device deploy")
+    end
+
     if all == [] do
       IO.puts("  #{color(:yellow)}No devices found.#{color(:reset)}")
       {[], [], []}
@@ -200,6 +218,67 @@ defmodule MobDev.Deployer do
     String.contains?(pm_output, "package:#{package_name}")
   end
 
+  @doc """
+  Refuses an ambiguous multi-device deploy.
+
+  Fanning out to every reachable device is a safety hazard: a run that
+  meant to touch a laptop's own simulator can also reach a teammate's
+  connected phone, a spare emulator, or a Cellular-linked device the
+  author forgot was still paired. MOB-182 makes the ambiguity explicit
+  — a run with two or more reachable devices must either name one via
+  `--device` / `--ios-device` or opt in to the fan-out with `--all`.
+
+  Returns `:ok` when the run may proceed, or `{:refuse, lines}` where
+  `lines` is the human-readable multi-line explanation. Pure so the
+  matrix can be tested without discovery.
+  """
+  @spec check_fanout_gate([Device.t()], keyword()) :: :ok | {:refuse, [String.t()]}
+  def check_fanout_gate(devices, opts) do
+    device_id = opts[:device]
+    ios_device_id = opts[:ios_device]
+    all? = opts[:all] == true
+
+    cond do
+      # A caller who already named a device (either flag) has been
+      # explicit; deploy_all narrows to that device inside
+      # `filter_by_device_id`. Nothing to gate. An empty string doesn't
+      # count as narrowing — it's what a shell script produces from an
+      # unset variable, and passing it through as "filter set" would
+      # bypass the gate silently.
+      nonempty_string?(device_id) or nonempty_string?(ios_device_id) ->
+        :ok
+
+      all? ->
+        :ok
+
+      length(devices) <= 1 ->
+        :ok
+
+      true ->
+        {:refuse, refusal_lines(devices)}
+    end
+  end
+
+  defp nonempty_string?(s), do: is_binary(s) and s != ""
+
+  defp refusal_lines(devices) do
+    listing =
+      Enum.map(devices, fn %Device{} = d ->
+        "    - #{Device.summary(d)}"
+      end)
+
+    [
+      "Refusing to deploy: #{length(devices)} devices reachable and no --device / --ios-device filter set.",
+      "Reachable devices:"
+    ] ++
+      listing ++
+      [
+        "",
+        "Pass `--device <id>` (or `--ios-device <id>`) to target one, or `--all` to fan out.",
+        "Run `mix mob.devices` to see the full list."
+      ]
+  end
+
   # ── Device filtering ─────────────────────────────────────────────────────────
 
   defp filter_by_device_id(devices, nil), do: devices
diff --git a/test/mob_dev/deployer_test.exs b/test/mob_dev/deployer_test.exs
index cc50102..5ef57db 100644
--- a/test/mob_dev/deployer_test.exs
+++ b/test/mob_dev/deployer_test.exs
@@ -332,4 +332,96 @@ defmodule MobDev.DeployerTest do
       assert Deployer.__sqlite_nif_target__(lines) == "/data/app/x/lib/arm/libsqlite3_nif.so"
     end
   end
+
+  # MOB-182: `mix mob.deploy` used to fan out silently to every reachable
+  # device when no --device / --all filter was given, so a run intended for
+  # the local sim could also land on a teammate's paired phone. The pure
+  # predicate below decides refuse-vs-proceed independent of adb / simctl.
+  describe "check_fanout_gate/2" do
+    setup do
+      android = %MobDev.Device{platform: :android, serial: "emulator-5554"}
+      ios1 = %MobDev.Device{platform: :ios, serial: "78354490-EF38-1111"}
+      ios2 = %MobDev.Device{platform: :ios, serial: "78354490-EF38-2222"}
+      {:ok, %{android: android, ios1: ios1, ios2: ios2}}
+    end
+
+    test "single device runs without any flag", %{android: a} do
+      assert Deployer.check_fanout_gate([a], []) == :ok
+    end
+
+    test "no devices reachable is not the gate's problem", %{} do
+      # deploy_all already prints "No devices found" and exits; the gate
+      # only cares about ambiguity when there are targets.
+      assert Deployer.check_fanout_gate([], []) == :ok
+    end
+
+    test "two devices without any filter is refused", %{android: a, ios1: i} do
+      assert {:refuse, lines} = Deployer.check_fanout_gate([a, i], [])
+      joined = Enum.join(lines, "\n")
+      assert joined =~ "Refusing to deploy: 2 devices reachable"
+      assert joined =~ "--device"
+      assert joined =~ "--all"
+    end
+
+    test "two devices with --all fans out", %{android: a, ios1: i} do
+      assert Deployer.check_fanout_gate([a, i], all: true) == :ok
+    end
+
+    test "two devices with --device is allowed (deployer narrows further)",
+         %{android: a, ios1: i} do
+      # We pass the reachable list unfiltered; the deployer applies
+      # filter_by_device_id after the gate. The gate just trusts that an
+      # explicit --device is a real narrowing intent.
+      assert Deployer.check_fanout_gate([a, i], device: "emulator-5554") == :ok
+    end
+
+    test "two devices with --ios-device is allowed", %{ios1: i1, ios2: i2} do
+      assert Deployer.check_fanout_gate([i1, i2], ios_device: "78354490-EF38-1111") == :ok
+    end
+
+    test "refusal lines list every reachable device", %{android: a, ios1: i1, ios2: i2} do
+      {:refuse, lines} = Deployer.check_fanout_gate([a, i1, i2], [])
+      joined = Enum.join(lines, "\n")
+      assert joined =~ "emulator-5554"
+      assert joined =~ "78354490-EF38-1111"
+      assert joined =~ "78354490-EF38-2222"
+    end
+
+    test "all: false is the same as omitting the flag", %{android: a, ios1: i} do
+      # A missing flag defaults to false in OptionParser, and an explicit
+      # `--no-all` is the same thing — either should refuse a multi-device run.
+      assert {:refuse, _} = Deployer.check_fanout_gate([a, i], all: false)
+    end
+
+    # The permissive-path tests above (single-device, --all, --device) all pass
+    # against a trivial `def check_fanout_gate(_, _), do: :ok` — they only
+    # assert what the gate should NOT do. Pair each with a matching refuse on
+    # a bit-flipped input so a mutation that removes the gate entirely fails
+    # both halves.
+    test "paired: --all polarity", %{android: a, ios1: i} do
+      assert Deployer.check_fanout_gate([a, i], all: true) == :ok
+      assert {:refuse, _} = Deployer.check_fanout_gate([a, i], all: false)
+    end
+
+    test "paired: single-vs-multi with no filter", %{android: a, ios1: i} do
+      assert Deployer.check_fanout_gate([a], []) == :ok
+      assert {:refuse, _} = Deployer.check_fanout_gate([a, i], [])
+    end
+
+    test "paired: --device narrowing polarity", %{android: a, ios1: i} do
+      assert Deployer.check_fanout_gate([a, i], device: "emulator-5554") == :ok
+      assert {:refuse, _} = Deployer.check_fanout_gate([a, i], device: nil)
+    end
+
+    # `--device ""` is what a shell script produces from an unset variable.
+    # Treating it as "filter set" (which naive `is_binary` did) would bypass
+    # the gate silently — the empty string never narrows the device list,
+    # deploy_all would then fall through to `No device matched ""` and the
+    # user has a green pre-check that yielded a red deploy. The gate rejects
+    # the empty string like the missing flag it stands in for.
+    test "empty string --device does not count as a filter", %{android: a, ios1: i} do
+      assert {:refuse, _} = Deployer.check_fanout_gate([a, i], device: "")
+      assert {:refuse, _} = Deployer.check_fanout_gate([a, i], ios_device: "")
+    end
+  end
 end