diff --git a/agentlightning/controller/local_reconciler.py b/agentlightning/controller/local_reconciler.py
index 7971c829a..847bfe736 100644
--- a/agentlightning/controller/local_reconciler.py
+++ b/agentlightning/controller/local_reconciler.py
@@ -28,6 +28,10 @@
 _SHUTDOWN_WAIT_TIMEOUT = 5.0
 
 
+def _is_native_windows() -> bool:
+    return os.name == "nt"
+
+
 def _run_local_reconciler_worker(agent_class_path: str) -> int:
     try:
         if ":" in agent_class_path:
@@ -105,6 +109,10 @@ def __init__(
         self._stop = asyncio.Event()
 
     async def run(self) -> None:
+        if _is_native_windows():
+            raise RuntimeError(
+                "runner_type=local is not supported on native Windows; use Linux (for example, WSL) instead."
+            )
         log.info("LocalReconciler starting", pool_size=self._pool_size, tick=self._tick_interval)
         try:
             await self._reconcile_loop()
diff --git a/docs/01-quick-start.md b/docs/01-quick-start.md
index 95d094aa6..05ade8c4e 100644
--- a/docs/01-quick-start.md
+++ b/docs/01-quick-start.md
@@ -7,6 +7,8 @@ This quick start requires only one machine with one A100 GPU. It runs Agent Ligh
 Complete [Installation](00-installation.md), including the `verl` GPU stack.
 
 > AGL v1.0 itself is lightweight, but policy inference and GRPO updates still require the GPU stack used by `verl` and vLLM.
+>
+> This quick start uses `runner_type=local`, which is not supported on native Windows. On Windows, run it from a Linux environment such as WSL. See [Local runner limits](30-controller-configuration.md#local-runner-limits) for details.
 
 ## 1. Prepare the example
 
diff --git a/docs/30-controller-configuration.md b/docs/30-controller-configuration.md
index 2af5f2093..6ed8de93a 100644
--- a/docs/30-controller-configuration.md
+++ b/docs/30-controller-configuration.md
@@ -86,6 +86,8 @@ The local runner limits concurrent processes and periodically synchronizes their
 
 When the process pool reaches `maximum_size`, queued rollouts wait until capacity becomes available.
 
+**Platform support:** The local runner requires POSIX process-group signals, so `runner_type=local` is not supported on native Windows. The Controller exits at startup, before it queries or dispatches any rollouts. Run the local controller from Linux (for example, WSL) instead; `runner_type=k8s` is unaffected.
+
 ## How agents are launched
 
 In `k8s` mode, the Controller reads the Jinja Job template stored in each rollout. The template originates from `agentlightning.k8s.job_template_path` in the Trainer configuration. The Controller renders the template with the rollout's `input`, applies the Controller settings such as namespace, timeout, and cleanup TTL, and submits the resulting Kubernetes Job. It also injects rollout-specific `AGL_OPENAI_BASE_URL`, `AGL_EVENT_URL`, and `AGL_KEY` values into every container. See [Trainer Configuration](20-trainer-configuration.md#rollout-execution) for the template configuration and Jinja examples, and `agentlightning/controller/k8s_reconciler.py` for the implementation.
diff --git a/tests/controller/test_local_reconciler_platform.py b/tests/controller/test_local_reconciler_platform.py
new file mode 100644
index 000000000..18631982c
--- /dev/null
+++ b/tests/controller/test_local_reconciler_platform.py
@@ -0,0 +1,61 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+"""Platform support tests for the local reconciler."""
+
+from unittest.mock import AsyncMock
+
+import pytest
+from omegaconf import OmegaConf
+
+from agentlightning.client import AgentLightningAsyncClient
+from agentlightning.controller import local_reconciler
+from agentlightning.controller.local_reconciler import LocalReconciler
+
+
+@pytest.mark.asyncio
+async def test_run_fails_before_reconciling_or_spawning_on_native_windows(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    api = AsyncMock(spec=AgentLightningAsyncClient)
+    config = OmegaConf.create(
+        {
+            "runner_type": "local",
+            "local_runner": {"maximum_size": 1, "poll_interval": 0.01},
+        }
+    )
+    reconciler = LocalReconciler(api, config)
+    reconcile_loop = AsyncMock()
+    shutdown = AsyncMock()
+    monkeypatch.setattr(reconciler, "_reconcile_loop", reconcile_loop)
+    monkeypatch.setattr(reconciler, "_shutdown", shutdown)
+    monkeypatch.setattr(local_reconciler, "_is_native_windows", lambda: True)
+
+    with pytest.raises(RuntimeError, match="runner_type=local is not supported on native Windows"):
+        await reconciler.run()
+
+    reconcile_loop.assert_not_awaited()
+    shutdown.assert_not_awaited()
+    api.get.assert_not_awaited()
+    api.patch.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_run_reconciles_and_shuts_down_on_supported_platform(monkeypatch: pytest.MonkeyPatch) -> None:
+    api = AsyncMock(spec=AgentLightningAsyncClient)
+    config = OmegaConf.create(
+        {
+            "runner_type": "local",
+            "local_runner": {"maximum_size": 1, "poll_interval": 0.01},
+        }
+    )
+    reconciler = LocalReconciler(api, config)
+    reconcile_loop = AsyncMock()
+    shutdown = AsyncMock()
+    monkeypatch.setattr(reconciler, "_reconcile_loop", reconcile_loop)
+    monkeypatch.setattr(reconciler, "_shutdown", shutdown)
+    monkeypatch.setattr(local_reconciler, "_is_native_windows", lambda: False)
+
+    await reconciler.run()
+
+    reconcile_loop.assert_awaited_once_with()
+    shutdown.assert_awaited_once_with()