diff --git a/src/nexus_sync/client/__main__.py b/src/nexus_sync/client/__main__.py
index 16f5ba4..5fa4d9b 100644
--- a/src/nexus_sync/client/__main__.py
+++ b/src/nexus_sync/client/__main__.py
@@ -1,6 +1,6 @@
-def main() -> None:
-    print("nexus-sync client")
+import sys
 
+from nexus_sync.client.runtime import main
 
 if __name__ == "__main__":
-    main()
+    raise SystemExit(main(sys.argv[1:]))
diff --git a/src/nexus_sync/client/runtime.py b/src/nexus_sync/client/runtime.py
new file mode 100644
index 0000000..3f72833
--- /dev/null
+++ b/src/nexus_sync/client/runtime.py
@@ -0,0 +1,169 @@
+import json
+import os
+import platform
+import socket
+import sys
+import urllib.error
+import urllib.request
+from dataclasses import dataclass
+from datetime import UTC, datetime
+from typing import Callable, Mapping
+
+from pydantic import ValidationError
+
+from nexus_sync.client.config import load_command_access_policy
+from nexus_sync.client.execute import CommandAccessPolicy, execute_command
+from nexus_sync.common import (
+    ClientInfo,
+    ClientPlatform,
+    ClientState,
+    CommandResult,
+    HeartbeatRequest,
+    HeartbeatResponse,
+)
+
+SERVER_URL_ENV = "NEXUS_SYNC_SERVER_URL"
+CLIENT_ID_ENV = "NEXUS_SYNC_CLIENT_ID"
+CLIENT_TOKEN_ENV = "NEXUS_SYNC_CLIENT_TOKEN"
+CLIENT_VERSION = "0.1.0"
+HEARTBEAT_PATH = "/api/v1/client/heartbeat"
+
+
+class ClientConfigError(ValueError):
+    pass
+
+
+class HeartbeatError(RuntimeError):
+    pass
+
+
+@dataclass(frozen=True)
+class ClientConfig:
+    server_url: str
+    client_id: str
+    token: str
+    command_access_policy: CommandAccessPolicy
+
+
+def load_client_config(env: Mapping[str, str] = os.environ) -> ClientConfig:
+    server_url = _required_env(env, SERVER_URL_ENV).rstrip("/")
+    client_id = _required_env(env, CLIENT_ID_ENV)
+    token = _required_env(env, CLIENT_TOKEN_ENV)
+    return ClientConfig(
+        server_url=server_url,
+        client_id=client_id,
+        token=token,
+        command_access_policy=load_command_access_policy(env),
+    )
+
+
+def build_heartbeat_request(config: ClientConfig) -> HeartbeatRequest:
+    now = datetime.now(UTC)
+    return HeartbeatRequest(
+        client_id=config.client_id,
+        observed_at=now,
+        client=ClientInfo(
+            hostname=socket.gethostname(),
+            platform=_current_platform(),
+            version=CLIENT_VERSION,
+        ),
+        state=ClientState(
+            local_time=datetime.now().astimezone(),
+            uptime_seconds=None,
+        ),
+        last_command_result=None,
+    )
+
+
+def send_heartbeat(
+    config: ClientConfig,
+    heartbeat: HeartbeatRequest,
+    *,
+    opener: Callable[[urllib.request.Request], object] = urllib.request.urlopen,
+) -> HeartbeatResponse:
+    payload = heartbeat.model_dump_json().encode()
+    request = urllib.request.Request(
+        f"{config.server_url}{HEARTBEAT_PATH}",
+        data=payload,
+        headers={
+            "Content-Type": "application/json",
+            "Authorization": f"Bearer {config.token}",
+        },
+        method="POST",
+    )
+
+    try:
+        with opener(request) as response:
+            body = response.read()
+    except urllib.error.HTTPError as error:
+        detail = error.read().decode(errors="replace")
+        raise HeartbeatError(f"heartbeat failed with HTTP {error.code}: {detail}") from error
+    except urllib.error.URLError as error:
+        raise HeartbeatError(f"heartbeat request failed: {error.reason}") from error
+    except OSError as error:
+        raise HeartbeatError(f"heartbeat request failed: {error}") from error
+
+    try:
+        return HeartbeatResponse.model_validate_json(body)
+    except ValidationError as error:
+        raise HeartbeatError(f"heartbeat response is invalid: {error}") from error
+
+
+def run_once(
+    config: ClientConfig,
+    *,
+    heartbeat_sender: Callable[
+        [ClientConfig, HeartbeatRequest],
+        HeartbeatResponse,
+    ] = send_heartbeat,
+    executor: Callable[..., CommandResult] = execute_command,
+) -> CommandResult | None:
+    response = heartbeat_sender(config, build_heartbeat_request(config))
+    if response.command is None:
+        return None
+
+    return executor(
+        response.command,
+        access_policy=config.command_access_policy,
+    )
+
+
+def main(argv: list[str] | None = None) -> int:
+    _ = argv
+    try:
+        config = load_client_config()
+        result = run_once(config)
+    except (ClientConfigError, HeartbeatError, ValueError) as error:
+        print(f"nexus-sync client error: {error}", file=sys.stderr)
+        return 1
+
+    if result is None:
+        print("heartbeat accepted; no command")
+        return 0
+
+    print(
+        json.dumps(
+            result.model_dump(mode="json"),
+            ensure_ascii=False,
+            separators=(",", ":"),
+        )
+    )
+    return 0
+
+
+def _required_env(env: Mapping[str, str], name: str) -> str:
+    value = env.get(name)
+    if value is None or not value.strip():
+        raise ClientConfigError(f"{name} is required")
+    return value.strip()
+
+
+def _current_platform() -> ClientPlatform:
+    system = platform.system().lower()
+    if system == "linux":
+        return ClientPlatform.LINUX
+    if system == "darwin":
+        return ClientPlatform.DARWIN
+    if system == "windows":
+        return ClientPlatform.WINDOWS
+    return ClientPlatform.UNKNOWN
diff --git a/tests/test_client_runtime.py b/tests/test_client_runtime.py
new file mode 100644
index 0000000..f6af3e8
--- /dev/null
+++ b/tests/test_client_runtime.py
@@ -0,0 +1,237 @@
+import io
+import urllib.error
+import urllib.request
+from datetime import UTC, datetime
+
+import pytest
+
+from nexus_sync.client.execute import CommandAccessPolicy
+from nexus_sync.client.runtime import (
+    CLIENT_ID_ENV,
+    CLIENT_TOKEN_ENV,
+    SERVER_URL_ENV,
+    ClientConfig,
+    ClientConfigError,
+    HeartbeatError,
+    build_heartbeat_request,
+    load_client_config,
+    main,
+    run_once,
+    send_heartbeat,
+)
+from nexus_sync.common import (
+    Command,
+    CommandKind,
+    CommandResult,
+    CommandResultStatus,
+    HeartbeatRequest,
+    HeartbeatResponse,
+)
+
+
+def _config(policy: CommandAccessPolicy | None = None) -> ClientConfig:
+    return ClientConfig(
+        server_url="https://nexus.example.test",
+        client_id="macbook-pro-01",
+        token="client-token",
+        command_access_policy=policy or CommandAccessPolicy.deny_all(),
+    )
+
+
+def test_load_client_config_reads_required_env_and_normalizes_server_url() -> None:
+    config = load_client_config(
+        {
+            SERVER_URL_ENV: "https://nexus.example.test/",
+            CLIENT_ID_ENV: "macbook-pro-01",
+            CLIENT_TOKEN_ENV: "client-token",
+            "NEXUS_SYNC_ALLOWED_COMMANDS": "hostname",
+        }
+    )
+
+    assert config.server_url == "https://nexus.example.test"
+    assert config.client_id == "macbook-pro-01"
+    assert config.token == "client-token"
+    assert config.command_access_policy.allows("hostname")
+    assert not config.command_access_policy.allows("network_interfaces")
+
+
+@pytest.mark.parametrize("missing_name", [SERVER_URL_ENV, CLIENT_ID_ENV, CLIENT_TOKEN_ENV])
+def test_load_client_config_requires_env_values(missing_name: str) -> None:
+    env = {
+        SERVER_URL_ENV: "https://nexus.example.test",
+        CLIENT_ID_ENV: "macbook-pro-01",
+        CLIENT_TOKEN_ENV: "client-token",
+    }
+    del env[missing_name]
+
+    with pytest.raises(ClientConfigError, match=missing_name):
+        load_client_config(env)
+
+
+def test_build_heartbeat_request_contains_client_state(monkeypatch) -> None:
+    monkeypatch.setattr("socket.gethostname", lambda: "macbook-pro.local")
+    monkeypatch.setattr("platform.system", lambda: "Darwin")
+
+    heartbeat = build_heartbeat_request(_config())
+    serialized = heartbeat.model_dump(mode="json")
+
+    assert heartbeat.client_id == "macbook-pro-01"
+    assert heartbeat.client.hostname == "macbook-pro.local"
+    assert heartbeat.client.platform == "darwin"
+    assert heartbeat.client.version == "0.1.0"
+    assert heartbeat.last_command_result is None
+    assert serialized["client_id"] == "macbook-pro-01"
+    assert serialized["state"]["uptime_seconds"] is None
+
+
+def test_send_heartbeat_posts_json_with_bearer_token() -> None:
+    captured = {}
+
+    def fake_opener(request: urllib.request.Request):
+        captured["url"] = request.full_url
+        captured["authorization"] = request.get_header("Authorization")
+        captured["content_type"] = request.get_header("Content-type")
+        captured["data"] = request.data
+        return _Response(
+            HeartbeatResponse(
+                server_time=datetime(2026, 5, 24, 13, 20, 30, tzinfo=UTC),
+                next_poll_after_seconds=60,
+                command=None,
+            )
+            .model_dump_json()
+            .encode()
+        )
+
+    response = send_heartbeat(
+        _config(),
+        HeartbeatRequest(
+            client_id="macbook-pro-01",
+            observed_at=datetime(2026, 5, 24, 13, 20, 30, tzinfo=UTC),
+            client={
+                "hostname": "macbook-pro.local",
+                "platform": "darwin",
+                "version": "0.1.0",
+            },
+            state={
+                "local_time": datetime(2026, 5, 24, 16, 20, 30, tzinfo=UTC),
+                "uptime_seconds": None,
+            },
+            last_command_result=None,
+        ),
+        opener=fake_opener,
+    )
+
+    assert response.command is None
+    assert captured["url"] == "https://nexus.example.test/api/v1/client/heartbeat"
+    assert captured["authorization"] == "Bearer client-token"
+    assert captured["content_type"] == "application/json"
+    assert b"macbook-pro-01" in captured["data"]
+
+
+def test_send_heartbeat_maps_http_error_to_runtime_error() -> None:
+    def fake_opener(_request: urllib.request.Request):
+        raise urllib.error.HTTPError(
+            url="https://nexus.example.test/api/v1/client/heartbeat",
+            code=401,
+            msg="Unauthorized",
+            hdrs={},
+            fp=io.BytesIO(b'{"detail":"invalid bearer token"}'),
+        )
+
+    with pytest.raises(HeartbeatError, match="HTTP 401"):
+        send_heartbeat(_config(), build_heartbeat_request(_config()), opener=fake_opener)
+
+
+def test_run_once_without_command_does_not_call_executor() -> None:
+    executor_called = False
+
+    def fake_sender(_config: ClientConfig, _heartbeat: HeartbeatRequest) -> HeartbeatResponse:
+        return HeartbeatResponse(
+            server_time=datetime(2026, 5, 24, 13, 20, 30, tzinfo=UTC),
+            next_poll_after_seconds=60,
+            command=None,
+        )
+
+    def fake_executor(*_args, **_kwargs):
+        nonlocal executor_called
+        executor_called = True
+        raise AssertionError("executor should not be called")
+
+    result = run_once(_config(), heartbeat_sender=fake_sender, executor=fake_executor)
+
+    assert result is None
+    assert executor_called is False
+
+
+def test_run_once_executes_command_with_configured_access_policy() -> None:
+    policy = CommandAccessPolicy.allow(["hostname"])
+    seen = {}
+
+    def fake_sender(_config: ClientConfig, _heartbeat: HeartbeatRequest) -> HeartbeatResponse:
+        return HeartbeatResponse(
+            server_time=datetime(2026, 5, 24, 13, 20, 30, tzinfo=UTC),
+            next_poll_after_seconds=10,
+            command=Command(
+                id="cmd_01JY3H8V8W8P3FXDR3S2BM7M6B",
+                kind=CommandKind.EXEC,
+                name="hostname",
+                args={},
+                timeout_seconds=30,
+            ),
+        )
+
+    def fake_executor(command: Command, **kwargs) -> CommandResult:
+        seen["command"] = command
+        seen["access_policy"] = kwargs["access_policy"]
+        return CommandResult(
+            command_id=command.id,
+            status=CommandResultStatus.SUCCEEDED,
+            return_code=0,
+            stdout="host\n",
+        )
+
+    result = run_once(_config(policy), heartbeat_sender=fake_sender, executor=fake_executor)
+
+    assert result is not None
+    assert result.status == CommandResultStatus.SUCCEEDED
+    assert seen["command"].name == "hostname"
+    assert seen["access_policy"] == policy
+
+
+def test_main_returns_non_zero_for_missing_config(monkeypatch, capsys) -> None:
+    monkeypatch.delenv(SERVER_URL_ENV, raising=False)
+    monkeypatch.delenv(CLIENT_ID_ENV, raising=False)
+    monkeypatch.delenv(CLIENT_TOKEN_ENV, raising=False)
+
+    exit_code = main([])
+
+    captured = capsys.readouterr()
+    assert exit_code == 1
+    assert SERVER_URL_ENV in captured.err
+
+
+def test_main_prints_success_without_command(monkeypatch, capsys) -> None:
+    monkeypatch.setenv(SERVER_URL_ENV, "https://nexus.example.test")
+    monkeypatch.setenv(CLIENT_ID_ENV, "macbook-pro-01")
+    monkeypatch.setenv(CLIENT_TOKEN_ENV, "client-token")
+    monkeypatch.setattr("nexus_sync.client.runtime.run_once", lambda _config: None)
+
+    exit_code = main([])
+
+    captured = capsys.readouterr()
+    assert exit_code == 0
+    assert captured.out == "heartbeat accepted; no command\n"
+
+
+class _Response:
+    def __init__(self, body: bytes) -> None:
+        self._body = body
+
+    def __enter__(self) -> "_Response":
+        return self
+
+    def __exit__(self, *_args) -> None:
+        return None
+
+    def read(self) -> bytes:
+        return self._body