diff --git a/src/rius/client.py b/src/rius/client.py
index e794866..d8489d7 100644
--- a/src/rius/client.py
+++ b/src/rius/client.py
@@ -166,7 +166,7 @@ def init(
             are only enabled when ``instruments`` is passed explicitly.
         span_exporter: Override the default OTLP exporter (useful for testing).
         heartbeat: Enable the agent-lifetime heartbeat thread
-            (``RIUS_HEARTBEAT``; default off this release). Pings
+            (``RIUS_HEARTBEAT``; on by default, set False to opt out). Pings
             ``<endpoint>/v1/heartbeat`` from init until process exit so the
             platform can tell a live-but-idle agent from a vanished one.
         heartbeat_interval: Seconds between pings (default 15, clamped to
diff --git a/src/rius/config.py b/src/rius/config.py
index a8437fe..6203b99 100644
--- a/src/rius/config.py
+++ b/src/rius/config.py
@@ -98,7 +98,7 @@ class GlassflowConfig:
     disabled: bool = False
     sample_rate: float = 1.0
     capture_content: bool = True
-    heartbeat: bool = False
+    heartbeat: bool = True
     heartbeat_interval: float = DEFAULT_HEARTBEAT_INTERVAL
     agent_name: str = DEFAULT_SERVICE_NAME
     partial_spans: bool = False
@@ -193,7 +193,8 @@ def resolve_config(
         capture_content: When ``False``, content attributes are stripped at
             export (``RIUS_CAPTURE_CONTENT``).
         heartbeat: Enable the agent-lifetime heartbeat thread
-            (``RIUS_HEARTBEAT``). Off by default this release.
+            (``RIUS_HEARTBEAT``). On by default; set ``False`` or
+            ``RIUS_HEARTBEAT=false`` to opt out.
         heartbeat_interval: Seconds between pings
             (``RIUS_HEARTBEAT_INTERVAL``), clamped to ``[5, 300]``;
             the backend derives staleness from this, so the bounds are part
@@ -235,7 +236,7 @@ def resolve_config(
     )
 
     resolved_heartbeat = (
-        _env_bool(ENV_HEARTBEAT, deprecated_used, default=False) if heartbeat is None else heartbeat
+        _env_bool(ENV_HEARTBEAT, deprecated_used, default=True) if heartbeat is None else heartbeat
     )
     resolved_heartbeat_interval = _clamp_heartbeat_interval(
         _env_float(ENV_HEARTBEAT_INTERVAL, deprecated_used, default=DEFAULT_HEARTBEAT_INTERVAL)
diff --git a/tests/conftest.py b/tests/conftest.py
index a2d8f1f..0e6636c 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -32,6 +32,17 @@ def exported_spans() -> InMemorySpanExporter:
     return _EXPORTER
 
 
+@pytest.fixture(autouse=True)
+def _no_ambient_heartbeat(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Force heartbeat off for every test.
+
+    Heartbeat is on by default, and its default transport does real HTTP;
+    a unit suite must never touch the network. Tests exercising heartbeat
+    behavior delete this variable and inject a stub transport.
+    """
+    monkeypatch.setenv("RIUS_HEARTBEAT", "false")
+
+
 @pytest.fixture(autouse=True)
 def _reset_rius_lifecycle() -> "Iterator[None]":
     """Clear module-level init()/instrumentation state between tests."""
diff --git a/tests/test_heartbeat.py b/tests/test_heartbeat.py
index 596b489..2f65e84 100644
--- a/tests/test_heartbeat.py
+++ b/tests/test_heartbeat.py
@@ -2,7 +2,7 @@
 
 Implements the heartbeat spec: payload v1, emission semantics (init-to-exit
 lifetime, immediate first ping, stopped ping on graceful shutdown, silent
-failure), and the config surface (off by default, interval clamped [5, 300],
+failure), and the config surface (on by default, interval clamped [5, 300],
 agent_name defaults to service_name).
 """
 
@@ -28,12 +28,21 @@
 # ---------------------------------------------------------------------------
 
 
-def test_heartbeat_disabled_by_default() -> None:
+def test_heartbeat_enabled_by_default(monkeypatch: pytest.MonkeyPatch) -> None:
+    # The conftest guard forces heartbeat off suite-wide (network safety);
+    # clear it here to observe the real default.
+    monkeypatch.delenv("RIUS_HEARTBEAT", raising=False)
+    monkeypatch.delenv("GLASSFLOW_HEARTBEAT", raising=False)
+    assert resolve_config().heartbeat is True
+
+
+def test_heartbeat_opt_out_via_env(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("RIUS_HEARTBEAT", "false")
     assert resolve_config().heartbeat is False
 
 
 def test_heartbeat_enabled_via_env(monkeypatch: pytest.MonkeyPatch) -> None:
-    monkeypatch.setenv("GLASSFLOW_HEARTBEAT", "true")
+    monkeypatch.setenv("RIUS_HEARTBEAT", "true")
     assert resolve_config().heartbeat is True
 
 
@@ -250,8 +259,27 @@ def test_open_traces_flow_into_payloads() -> None:
         client.shutdown()
 
 
-def test_heartbeat_off_by_default_no_thread() -> None:
-    client = init(set_global=False, service_name="svc", span_exporter=InMemorySpanExporter())
+def test_heartbeat_on_by_default_starts_thread(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.delenv("RIUS_HEARTBEAT", raising=False)
+    monkeypatch.delenv("GLASSFLOW_HEARTBEAT", raising=False)
+    sent: list[dict[str, Any]] = []
+    client = init(
+        set_global=False,
+        service_name="svc",
+        span_exporter=InMemorySpanExporter(),
+        heartbeat_transport=sent.append,
+    )
+    assert client._heartbeat is not None  # noqa: SLF001
+    client.shutdown()
+
+
+def test_heartbeat_opt_out_starts_no_thread() -> None:
+    client = init(
+        set_global=False,
+        service_name="svc",
+        heartbeat=False,
+        span_exporter=InMemorySpanExporter(),
+    )
     assert client._heartbeat is None  # noqa: SLF001
     client.shutdown()
 
@@ -365,9 +393,9 @@ def log_message(self, *args: Any) -> None:
     try:
         env = {
             **os.environ,
-            "GLASSFLOW_ENDPOINT": f"http://127.0.0.1:{server.server_port}",
-            "GLASSFLOW_HEARTBEAT": "1",
-            "GLASSFLOW_SERVICE_NAME": "atexit-agent",
+            "RIUS_ENDPOINT": f"http://127.0.0.1:{server.server_port}",
+            "RIUS_HEARTBEAT": "1",  # overrides the conftest guard in os.environ
+            "RIUS_SERVICE_NAME": "atexit-agent",
         }
         # init() then exit normally WITHOUT calling shutdown(): the atexit
         # hook alone must produce the stopped ping.