diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3c7be33..7a9881a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,30 @@ Full module documentation: [hexdocs.pm/mob_dev](https://hexdocs.pm/mob_dev).
 
 ## [Unreleased]
 
+### Changed
+
+- **`mix mob.deploy` rejects unrecognised options instead of ignoring them.**
+  A run that passed an extra or misspelled flag used to carry on regardless;
+  it now fails and names the option. **If you have a wrapper script, CI step
+  or shell alias passing a flag this task does not accept, it will start
+  failing.** It also takes no positional arguments — `mix mob.deploy --native
+  ABC123` deployed to every device and said nothing, and now refuses.
+
+### Fixed
+
+- **`--beam-flags "-S 4:4 -A 4"` aborted the deploy.** `OptionParser` will not
+  consume a dash-prefixed argument as a string value, so the spelling this
+  repo prints in seven places — the moduledoc, five README recipes and both
+  battery-bench workflows — parsed as two unknown options. Under the previous
+  lenient parsing the value was silently dropped and the deploy carried on
+  with whatever `mob.exs` held; strict parsing turned that into a hard failure
+  whose message named `--beam-flags` itself as unknown. Both spellings work
+  now.
+- **The error said "Unknown option(s)" for options that are known.**
+  `--schedulers abc` reported `--schedulers` as unknown and discarded the
+  value, sending the reader to a help page that lists it. It now distinguishes
+  an unrecognised flag from a bad value and prints both.
+
 ### Added
 
 - **`mix mob.mutate`** — mutation testing for the lines a branch changed.
diff --git a/lib/mix/tasks/mob.deploy.ex b/lib/mix/tasks/mob.deploy.ex
index 1b79bc2..9d2e2f3 100644
--- a/lib/mix/tasks/mob.deploy.ex
+++ b/lib/mix/tasks/mob.deploy.ex
@@ -23,7 +23,7 @@ defmodule Mix.Tasks.Mob.Deploy do
 
     * `--native`              — build native binaries before pushing BEAMs
     * `--no-restart`          — push BEAMs but don't restart the app
-    * `--device <id>`         — target a specific device; use `mix mob.devices` to find IDs
+    * `-d`, `--device <id>`   — target a specific device; use `mix mob.devices` to find IDs
     * `--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
@@ -179,8 +179,10 @@ defmodule Mix.Tasks.Mob.Deploy do
 
   @impl Mix.Task
   def run(args) do
-    {opts, _argv, invalid} =
-      OptionParser.parse(args, strict: @switches, aliases: [d: :device])
+    {opts, argv, invalid} =
+      args
+      |> join_dashed_values()
+      |> OptionParser.parse(strict: @switches, aliases: [d: :device])
 
     # `switches:` silently discards anything it does not recognise, so
     # `mix mob.deploy -d <udid>` — `-d` was never aliased here, though
@@ -192,6 +194,17 @@ defmodule Mix.Tasks.Mob.Deploy do
       Mix.raise(invalid_options_message(invalid))
     end
 
+    # `mix mob.deploy --native ABC123` — a natural fumble of `--device` — parsed
+    # cleanly, deployed to every device, and said nothing. This task takes no
+    # positional arguments, so tolerating them is the same silent-ignore the
+    # strict parsing above was added to end.
+    unless argv == [] do
+      Mix.raise(
+        "mix mob.deploy takes no positional arguments, got: #{Enum.join(argv, ", ")}\n\n" <>
+          "Did you mean `--device #{hd(argv)}`?"
+      )
+    end
+
     # Under --json, stdout must carry ONE document and nothing else. Every
     # progress line in this task, the deployer and the native build is a plain
     # `IO.puts/1`, which resolves `:stdio` through the group leader — so
@@ -421,6 +434,47 @@ defmodule Mix.Tasks.Mob.Deploy do
     end
   end
 
+  @doc false
+  @spec switches() :: keyword()
+  def switches, do: @switches
+
+  @doc """
+  Rewrite `--flag value` to `--flag=value` when the value starts with a dash.
+
+  `OptionParser` will not consume a dash-prefixed argument as a `:string`
+  value, so `--beam-flags "-S 4:4 -A 4"` — the spelling this repo prints in
+  seven places, including the README and both battery-bench workflows — parsed
+  as two unknown options. Under the old lenient parsing the value was silently
+  dropped and the deploy carried on with whatever `mob.exs` held; under strict
+  parsing it became a hard failure that named a valid option as unknown.
+
+  BEAM flags essentially all start with a dash, so this is not an edge case:
+  it is the documented invocation.
+  """
+  @spec join_dashed_values([String.t()]) :: [String.t()]
+  def join_dashed_values(args), do: join_dashed(args, [])
+
+  defp join_dashed([], acc), do: Enum.reverse(acc)
+
+  defp join_dashed(["--" | rest], acc), do: Enum.reverse(acc) ++ ["--" | rest]
+
+  defp join_dashed([flag, value | rest], acc) do
+    if string_switch?(flag) and String.starts_with?(value, "-") do
+      join_dashed(rest, ["#{flag}=#{value}" | acc])
+    else
+      join_dashed([value | rest], [flag | acc])
+    end
+  end
+
+  defp join_dashed([last], acc), do: Enum.reverse([last | acc])
+
+  # Only the switches whose value is free text can legitimately begin with a
+  # dash. Doing this for every switch would swallow `--android --ios`.
+  defp string_switch?("--" <> name),
+    do: Keyword.get(@switches, String.to_atom(String.replace(name, "-", "_"))) == :string
+
+  defp string_switch?(_), do: false
+
   @doc """
   The error for options the task does not accept.
 
@@ -429,11 +483,20 @@ defmodule Mix.Tasks.Mob.Deploy do
   """
   @spec invalid_options_message([{String.t(), String.t() | nil}]) :: String.t()
   def invalid_options_message(invalid) do
-    names = invalid |> Enum.map(&elem(&1, 0)) |> Enum.join(", ")
+    # `OptionParser`'s `invalid` list conflates "unrecognised flag" with
+    # "recognised flag, unparseable value". Reporting both as "unknown" sends
+    # someone who typed `--schedulers abc` to a help page that lists
+    # `--schedulers`, with the offending value discarded and nothing to go on.
+    # `mob.new_plugin` already got this right; this now matches it.
+    names =
+      Enum.map_join(invalid, ", ", fn
+        {flag, nil} -> flag
+        {flag, value} -> "#{flag} #{value}"
+      end)
 
-    "Unknown option(s): #{names}\n\n" <>
-      "Run `mix help mob.deploy` for the accepted options. " <>
-      "Short forms are not aliases except `-d` for `--device`."
+    "Unrecognized or invalid option(s): #{names}\n\n" <>
+      "Run `mix help mob.deploy` for the accepted options.\n" <>
+      "A value beginning with `-` needs the equals form: --beam-flags=\"-S 4:4\"."
   end
 
   @doc """
diff --git a/test/mix/tasks/mob_deploy_beam_flags_test.exs b/test/mix/tasks/mob_deploy_beam_flags_test.exs
index be032e5..8dcb8a6 100644
--- a/test/mix/tasks/mob_deploy_beam_flags_test.exs
+++ b/test/mix/tasks/mob_deploy_beam_flags_test.exs
@@ -475,7 +475,7 @@ defmodule Mix.Tasks.Mob.DeployBeamFlagsTest do
 
       assert message =~ "-d"
       assert message =~ "--devcie"
-      assert message =~ "Unknown option"
+      assert message =~ "Unrecognized or invalid option"
     end
   end
 end
diff --git a/test/mix/tasks/mob_deploy_parsing_test.exs b/test/mix/tasks/mob_deploy_parsing_test.exs
new file mode 100644
index 0000000..ed11d26
--- /dev/null
+++ b/test/mix/tasks/mob_deploy_parsing_test.exs
@@ -0,0 +1,92 @@
+defmodule Mix.Tasks.Mob.DeployParsingTest do
+  @moduledoc """
+  Every invocation this repo prints at a user must actually parse.
+
+  Switching `mix mob.deploy` to strict parsing turned a silent drop into a hard
+  failure, which was the point — but it also broke
+  `--beam-flags "-S 4:4 -A 4"`, the spelling documented in seven places
+  including the README and both battery-bench workflows. The four tests that
+  shipped with that change were all source-text assertions and none of them
+  ran the parser, so none could have caught it.
+  """
+  use ExUnit.Case, async: true
+
+  alias Mix.Tasks.Mob.Deploy
+
+  defp parse(argv) do
+    argv
+    |> Deploy.join_dashed_values()
+    |> OptionParser.parse(strict: Deploy.switches(), aliases: [d: :device])
+  end
+
+  describe "the documented invocations" do
+    # Lifted from the moduledoc, README and the two battery-bench tasks. If a
+    # switch is renamed or dropped, this fails rather than the docs going stale
+    # in silence.
+    @documented [
+      ~w(--native),
+      ~w(--no-restart),
+      ~w(--slim),
+      ~w(--android),
+      ~w(--ios),
+      ~w(--native --android),
+      ~w(--device ABC123),
+      ~w(-d ABC123),
+      ~w(--dist-port 9200),
+      ~w(--node-suffix sim1),
+      ~w(--schedulers 4),
+      ~w(--json),
+      ["--beam-flags", ""],
+      ["--beam-flags", "-S 4:4 -A 4"],
+      ["--beam-flags", "+S 4:4"],
+      ["--beam-flags=-S 4:4 -A 4"],
+      ["--beam-flags", "-S 4:4 -A 8", "--android"]
+    ]
+
+    for argv <- @documented do
+      test "parses #{inspect(argv)}" do
+        assert {_opts, [], []} = parse(unquote(argv))
+      end
+    end
+
+    test "a dash-prefixed BEAM flag keeps its value intact" do
+      # The regression: OptionParser will not consume a dash-prefixed argument
+      # as a :string value, so this parsed as two unknown options and aborted a
+      # run the README tells you to make.
+      assert {[beam_flags: "-S 4:4 -A 4"], [], []} = parse(["--beam-flags", "-S 4:4 -A 4"])
+    end
+
+    test "joining only applies to switches whose value is free text" do
+      # Doing it for every switch would swallow the flag after a boolean.
+      assert {[android: true, ios: true], [], []} = parse(~w(--android --ios))
+      assert {[native: true, device: "X"], [], []} = parse(~w(--native --device X))
+    end
+
+    test "a `--` separator still ends option parsing" do
+      # OptionParser consumes the separator itself and returns the rest as
+      # positional. The task rejects positionals, so this is a loud failure
+      # rather than a silent deploy-to-everything — which is the point.
+      assert {[native: true], ["-S"], []} = parse(~w(--native -- -S))
+    end
+  end
+
+  describe "what it still refuses" do
+    test "an unrecognised flag" do
+      assert {_, _, [{"--devcie", _}]} = parse(~w(--devcie X))
+    end
+
+    test "a recognised flag with an unparseable value is named with its value" do
+      # Reporting `--schedulers abc` as merely "unknown" sends the user to a
+      # help page that lists --schedulers, with the bad value discarded.
+      assert {_, _, invalid} = parse(~w(--schedulers abc))
+
+      message = Deploy.invalid_options_message(invalid)
+      assert message =~ "--schedulers abc"
+      assert message =~ "Unrecognized or invalid"
+    end
+
+    test "the message points at the equals form for dashed values" do
+      assert Deploy.invalid_options_message([{"--beam-flags", nil}]) =~ "--beam-flags="
+    end
+  end
+end
diff --git a/test/mob_dev/wiring_test.exs b/test/mob_dev/wiring_test.exs
index 7aab467..b9e9f04 100644
--- a/test/mob_dev/wiring_test.exs
+++ b/test/mob_dev/wiring_test.exs
@@ -197,8 +197,11 @@ defmodule MobDev.WiringTest do
       # restores the silent-ignore.
       source = @deploy_task
 
-      assert source =~ "OptionParser.parse(args, strict: @switches, aliases: [d: :device])"
-      refute source =~ "OptionParser.parse(args, switches: @switches)"
+      # Fragments, not the whole line: this call grew a `join_dashed_values/1`
+      # step and `mix format` rewrapped it, which would have broken an
+      # exact-line assertion with no behaviour change.
+      assert source =~ "strict: @switches, aliases: [d: :device]"
+      refute source =~ "switches: @switches"
     end
 
     test "the run raises rather than proceeding" do