diff --git a/README.md b/README.md
index 5a4c6327..ec035803 100644
--- a/README.md
+++ b/README.md
@@ -104,6 +104,9 @@ codealmanac setup --yes --garden-off
 codealmanac setup --yes --no-auto-update
 ```
 
+Setup exits before writing configuration or agent instructions on non-macOS
+systems because the production scheduler currently depends on `launchd`.
+
 To uninstall CodeAlmanac-owned local artifacts:
 
 ```bash
diff --git a/src/codealmanac/app.py b/src/codealmanac/app.py
index ca69e029..0002da70 100644
--- a/src/codealmanac/app.py
+++ b/src/codealmanac/app.py
@@ -1,3 +1,4 @@
+import platform
 from collections.abc import Sequence
 from dataclasses import dataclass
 
@@ -199,6 +200,9 @@ def create_services(
     adapters: AppAdapters,
 ) -> Services:
     repositories = RepositoriesService(RepositoryStore(local_state.database_path))
+    automation_unavailable_reason = default_scheduler_unavailable_reason(
+        adapters.scheduler
+    )
     automation = AutomationService(adapters.scheduler or LaunchdSchedulerAdapter())
     config_service = ConfigService(
         ConfigStore(),
@@ -248,6 +252,7 @@ def create_services(
         or PackageToolUninstaller(package_metadata, package_runner),
         config_service,
         runner_probe=harnesses,
+        automation_unavailable_reason=automation_unavailable_reason,
     )
     runs = RunsService(
         repositories,
@@ -287,6 +292,23 @@ def create_services(
     )
 
 
+def default_scheduler_unavailable_reason(
+    scheduler: SchedulerAdapter | None,
+    system_name: str | None = None,
+) -> str | None:
+    """Describe why the production scheduler cannot run on this platform."""
+    if scheduler is not None:
+        return None
+    detected_system = system_name or platform.system()
+    if detected_system == "Darwin":
+        return None
+    return (
+        "setup currently supports macOS only because scheduled automation uses "
+        f"launchd; detected {detected_system or 'an unknown platform'}. "
+        "No setup changes were made."
+    )
+
+
 def create_operation(services: Services) -> OperationRunner:
     return OperationRunner(
         services.repositories,
diff --git a/src/codealmanac/services/setup/service.py b/src/codealmanac/services/setup/service.py
index d5e7a772..9207ee5e 100644
--- a/src/codealmanac/services/setup/service.py
+++ b/src/codealmanac/services/setup/service.py
@@ -1,3 +1,4 @@
+from codealmanac.core.errors import ValidationFailed
 from codealmanac.services.automation.requests import RemoveAllAutomationRequest
 from codealmanac.services.config.models import (
     AutomationConfig,
@@ -36,6 +37,7 @@ def __init__(
         package_uninstaller: PackageUninstaller,
         config: ConfigService,
         runner_probe: RunnerReadinessProbe | None = None,
+        automation_unavailable_reason: str | None = None,
     ):
         self._instructions = instructions
         self._automation_remover = automation_remover
@@ -43,8 +45,11 @@ def __init__(
         self._package_uninstaller = package_uninstaller
         self._config = config
         self._runner_probe = runner_probe
+        self._automation_unavailable_reason = automation_unavailable_reason
 
     def run(self, request: RunSetupRequest) -> SetupResult:
+        if self._automation_unavailable_reason is not None:
+            raise ValidationFailed(self._automation_unavailable_reason)
         readiness = require_runner(self._runner_probe, request)
         config_update = self.set_config(request)
         changes = ()
diff --git a/tests/test_setup_service.py b/tests/test_setup_service.py
index e5228835..8af0842c 100644
--- a/tests/test_setup_service.py
+++ b/tests/test_setup_service.py
@@ -4,7 +4,8 @@
 import pytest
 from pydantic import ValidationError
 
-from codealmanac.core.errors import ExecutionFailed
+from codealmanac.app import default_scheduler_unavailable_reason
+from codealmanac.core.errors import ExecutionFailed, ValidationFailed
 from codealmanac.integrations.setup.instructions import (
     CLAUDE_IMPORT_LINE,
     CODEALMANAC_END,
@@ -380,6 +381,30 @@ def test_setup_without_probe_reports_no_readiness(home: Path):
     assert result.runner_readiness is None
 
 
+def test_setup_rejects_unsupported_platform_before_writing(home: Path):
+    automation = FakeSetupAutomationManager(home)
+    service = setup_service(
+        home,
+        automation=automation,
+        automation_unavailable_reason=default_scheduler_unavailable_reason(
+            None, "Linux"
+        ),
+    )
+
+    with pytest.raises(ValidationFailed, match="detected Linux"):
+        service.run(RunSetupRequest(targets=(SetupTarget.CODEX,)))
+
+    assert automation.applied == []
+    assert not (home / ".codealmanac/config.toml").exists()
+    assert not (home / ".codex/AGENTS.md").exists()
+
+
+def test_custom_scheduler_remains_available_on_non_macos():
+    scheduler = object()
+
+    assert default_scheduler_unavailable_reason(scheduler, "Linux") is None
+
+
 @pytest.fixture
 def home(tmp_path: Path) -> Path:
     return tmp_path / "home"
@@ -390,6 +415,7 @@ def setup_service(
     automation: "FakeSetupAutomationManager | None" = None,
     package_uninstaller: "FakePackageUninstaller | None" = None,
     runner_probe: "FakeRunnerProbe | None" = None,
+    automation_unavailable_reason: str | None = None,
 ) -> SetupService:
     automation_manager = automation or FakeSetupAutomationManager(home)
     config = ConfigService(
@@ -404,6 +430,7 @@ def setup_service(
         package_uninstaller or FakePackageUninstaller(skipped_package_result()),
         config=config,
         runner_probe=runner_probe,
+        automation_unavailable_reason=automation_unavailable_reason,
     )