diff --git a/backend/fluksio/cli.py b/backend/fluksio/cli.py
index b476ea1..237ca09 100644
--- a/backend/fluksio/cli.py
+++ b/backend/fluksio/cli.py
@@ -21,6 +21,7 @@ import os
 import secrets
 import socket
 import sys
+from collections.abc import Callable
 from pathlib import Path
 from typing import Any
 
@@ -228,9 +229,26 @@ CONCURRENCY_FLAGS = {
     "max_workers": "FLOW_MAX_WORKERS",
     "max_cascades": "FLOW_MAX_CASCADES",
     "max_runs": "FLOW_MAX_RUNS",
+    "gpus": "FLOW_GPUS",
 }
 
 
+def _at_least(minimum: int) -> Callable[[str], int]:
+    """A flag's value, checked here rather than by the settings.
+
+    These are written into the environment before the settings are built, so a
+    number they refuse dies inside a pydantic import with no flag named in it.
+    """
+
+    def parse(text: str) -> int:
+        value = int(text)
+        if value < minimum:
+            raise argparse.ArgumentTypeError(f"is {value}, needs at least {minimum}")
+        return value
+
+    return parse
+
+
 #: What `serve` listens on when nobody says. Taken often enough — another
 #: engine, another framework's dev server — that dying on it is the first
 #: thing a zero-config start would hit.
@@ -426,25 +444,32 @@ def _parser() -> argparse.ArgumentParser:
     )
     serve.add_argument(
         "--max-runs",
-        type=int,
+        type=_at_least(1),
         default=None,
         metavar="N",
         help="batch runs driven at once (default 4, FLOW_MAX_RUNS)",
     )
     serve.add_argument(
         "--max-cascades",
-        type=int,
+        type=_at_least(1),
         default=None,
         metavar="N",
         help="cascades in flight at once (default 4, FLOW_MAX_CASCADES)",
     )
     serve.add_argument(
         "--max-workers",
-        type=int,
+        type=_at_least(1),
         default=None,
         metavar="N",
         help="python worker processes (default 4, FLOW_MAX_WORKERS)",
     )
+    serve.add_argument(
+        "--gpus",
+        type=_at_least(0),
+        default=None,
+        metavar="N",
+        help="GPUs on this machine a node may be given (default 0, FLOW_GPUS)",
+    )
     serve.set_defaults(func=cmd_serve)
 
     enroll = subparsers.add_parser(
diff --git a/backend/fluksio/flow/placement.py b/backend/fluksio/flow/placement.py
index 9b70dc6..da1111a 100644
--- a/backend/fluksio/flow/placement.py
+++ b/backend/fluksio/flow/placement.py
@@ -215,9 +215,18 @@ class Placer:
         said = [shape[2] for shape in capable if shape[2]]
         ram = min(ram, max(said)) if said and ram else ram
         if (cpus, gpus, ram) != (wanted.cpus, wanted.gpus, wanted.ram or 0):
+            # Cards are not detected, so a machine that has one still reports
+            # none until it is told — which reads as "no GPU here" to a node
+            # that then runs unserialised beside every other one.
+            hint = (
+                "; no machine here declares a GPU — `fluksio serve --gpus N` "
+                "(or FLOW_GPUS) says how many this one has"
+                if wanted.gpus and not gpus
+                else ""
+            )
             logger.warning(
                 "%s asked for %d cpu(s), %d gpu(s) and %s MB; "
-                "the largest machine here can give %d, %d and %s",
+                "the largest machine here can give %d, %d and %s%s",
                 node or "a node",
                 wanted.cpus,
                 wanted.gpus,
@@ -225,6 +234,7 @@ class Placer:
                 cpus,
                 gpus,
                 ram or "no stated",
+                hint,
             )
         return cpus, gpus, ram
 
diff --git a/backend/tests/flow/test_placement.py b/backend/tests/flow/test_placement.py
index d5edeca..73dabf5 100644
--- a/backend/tests/flow/test_placement.py
+++ b/backend/tests/flow/test_placement.py
@@ -129,7 +129,7 @@ def test_a_preferred_label_falls_back_here_and_is_still_accounted(loop):
         assert placer.local.snapshot()["cpus"]["free"] == 1
 
 
-def test_asking_for_more_than_anything_has_gets_what_there_is(loop):
+def test_asking_for_more_than_anything_has_gets_what_there_is(loop, caplog):
     """A flow written on a cluster still has to run on a laptop."""
     placer = placer_over(cpus=2)
 
@@ -138,6 +138,10 @@ def test_asking_for_more_than_anything_has_gets_what_there_is(loop):
         assert allocation.cpus == 2
         assert allocation.gpus == ()
 
+    # Cards are declared, not detected, so a machine that has one reads as
+    # having none until it is told — and the warning is where that is noticed.
+    assert "fluksio serve --gpus" in caplog.text
+
 
 def test_what_is_clamped_to_is_a_machine_that_exists(loop):
     """Each dimension taken separately can describe a machine nobody has.
diff --git a/backend/tests/test_cli.py b/backend/tests/test_cli.py
index 8ef1eaa..802ce09 100644
--- a/backend/tests/test_cli.py
+++ b/backend/tests/test_cli.py
@@ -520,6 +520,24 @@ def test_two_files_of_one_name_are_refused(tmp_path) -> None:
         _import(*_module_of(second), second)
 
 
+def test_a_serve_limit_is_refused_as_a_flag_not_as_a_traceback(capsys) -> None:
+    """These are written into the environment before the settings are built."""
+    import pytest
+
+    from fluksio.cli import _parser
+
+    parser = _parser()
+    assert parser.parse_args(["serve", "--max-workers", "2"]).max_workers == 2
+    # A machine may genuinely have no card, so zero is a number here.
+    assert parser.parse_args(["serve", "--gpus", "0"]).gpus == 0
+    assert parser.parse_args(["serve"]).gpus is None
+
+    for flag, value in (("--max-workers", "0"), ("--gpus", "-1")):
+        with pytest.raises(SystemExit):
+            parser.parse_args(["serve", flag, value])
+    assert "at least" in capsys.readouterr().err
+
+
 def test_run_and_sweep_take_what_to_sync() -> None:
     from fluksio.cli import _parser