@@ -214,7 +214,7 @@ Additional parameters:
 - `opentelemetry_endpoint` - will be passed to `OTLPSpanExporter` as endpoint. Under `opentelemetry_exporter_protocol="http"` this is a full URL (e.g. `http://collector:4318/v1/traces`).
 - `opentelemetry_namespace` - will be passed to the `Resource`.
 - `opentelemetry_exporter_protocol` - OTLP exporter transport: `"grpc"` (default, needs the `otl` extra) or `"http"` (needs the `otl-http` extra, no `grpcio` - installable on free-threaded Python).
-- `opentelemetry_insecure` - whether the gRPC OTLP connection is insecure (default: `True`; gRPC only; for `http` the endpoint URL scheme carries security). While it is `True`, an endpoint pointing at a non-local host triggers a warning that telemetry is sent unencrypted.
+- `opentelemetry_insecure` - whether the gRPC OTLP connection is insecure (default: `None`; gRPC only; for `http` the endpoint URL scheme carries security). Left `None`, the exporter decides: `OTEL_EXPORTER_OTLP_TRACES_INSECURE` or `OTEL_EXPORTER_OTLP_METRICS_INSECURE` first, then `OTEL_EXPORTER_OTLP_INSECURE`, then the endpoint's scheme, where only `http://` connects in plaintext. Since 2.0 a schemeless endpoint such as `collector:4317` uses TLS, so write `http://collector:4317` for a plaintext collector, or set the field to `True`, which a `unix://` socket needs too. An `https://` endpoint uses TLS whatever the field says. When the connection is plaintext and the endpoint points at a non-local host, a warning says telemetry is sent unencrypted.
 - `opentelemetry_instrumentors` - a list of extra instrumentors.
 - `opentelemetry_log_traces` - traces will be logged to stdout.
 - `opentelemetry_sampler` - an `opentelemetry.sdk.trace.sampling.Sampler` deciding which traces are recorded. Unset, the SDK's own default applies: `parentbased_always_on`, which records every trace that is not the child of a non-recording remote parent, unless `OTEL_TRACES_SAMPLER` / `OTEL_TRACES_SAMPLER_ARG` say otherwise.
@@ -244,8 +244,8 @@ counted by both.
 ```python
 config = FastAPIConfig(
     service_name="my-service",
-    opentelemetry_endpoint="localhost:4317",
-    opentelemetry_metrics_endpoint="localhost:4317",
+    opentelemetry_endpoint="http://localhost:4317",
+    opentelemetry_metrics_endpoint="http://localhost:4317",
 )
 ```
 
 
@@ -74,7 +74,7 @@ class OpenTelemetryConfig(OpenTelemetryServiceFieldsConfig):
     )
     opentelemetry_endpoint: str | None = None
     opentelemetry_metrics_endpoint: str | None = None
-    opentelemetry_insecure: bool = True
+    opentelemetry_insecure: bool | None = None
     opentelemetry_exporter_protocol: typing.Literal["grpc", "http"] = "grpc"
     opentelemetry_instrumentors: list[typing.Union[InstrumentorWithParams, "BaseInstrumentor"]] = dataclasses.field(
         default_factory=list
@@ -96,32 +96,44 @@ def __post_init__(self) -> None:
 
     def _parse_remote_insecure_hosts(self) -> list[str]:
         """Return each distinct host an insecure, non-local endpoint points at, in declaration order."""
-        endpoints = (self.opentelemetry_endpoint, self.opentelemetry_metrics_endpoint)
-        hosts = (self._parse_remote_insecure_host(endpoint) for endpoint in endpoints)
+        endpoints = (
+            (self.opentelemetry_endpoint, "OTEL_EXPORTER_OTLP_TRACES_INSECURE"),
+            (self.opentelemetry_metrics_endpoint, "OTEL_EXPORTER_OTLP_METRICS_INSECURE"),
+        )
+        hosts = (self._parse_remote_insecure_host(endpoint, env_var) for endpoint, env_var in endpoints)
         return list(dict.fromkeys(host for host in hosts if host is not None))
 
-    def _parse_remote_insecure_host(self, endpoint: str | None) -> str | None:
+    def _parse_remote_insecure_host(self, endpoint: str | None, signal_insecure_env_var: str) -> str | None:
         """Return the host name if the endpoint is insecure AND non-local; else None."""
         if self.opentelemetry_exporter_protocol != "grpc":
             return None
-        if not endpoint or not self.opentelemetry_insecure:
-            return None
-        if endpoint.startswith("unix://"):
+        if not endpoint or endpoint.startswith("unix://"):
             return None
         # urlparse treats schemeless input as `path`, misparsing `host:port` forms.
         # Prepend `//` so urlparse always sees a network-location-style input.
         raw = endpoint
         if "://" not in raw:
             raw = f"//{raw}"
         parsed = urllib.parse.urlparse(raw)
-        # The gRPC exporter forces TLS for https whatever insecure says.
-        if parsed.scheme == "https":
+        if not self._is_grpc_plaintext(parsed.scheme, signal_insecure_env_var):
             return None
         host = (parsed.hostname or "").lower()
         if host in _LOCAL_HOSTS:
             return None
         return host
 
+    def _is_grpc_plaintext(self, scheme: str, signal_insecure_env_var: str) -> bool:
+        """Decide insecure as the gRPC exporter does: https, then the field, the variables, the scheme."""
+        if scheme == "https":
+            return False
+        if self.opentelemetry_insecure is not None:
+            return self.opentelemetry_insecure
+        for env_var in (signal_insecure_env_var, "OTEL_EXPORTER_OTLP_INSECURE"):
+            value = os.environ.get(env_var)
+            if value is not None:
+                return value.lower() == "true"
+        return scheme == "http"
+
 
 @dataclasses.dataclass(kw_only=True, frozen=True)
 class OpenTelemetryAsgiConfig(OpenTelemetryConfig):
 
@@ -293,6 +293,73 @@ def test_opentelemetry_config_no_warning_for_unix_socket_endpoint() -> None:
         )
 
 
+_INSECURE_ENV_VARS: typing.Final = (
+    "OTEL_EXPORTER_OTLP_INSECURE",
+    "OTEL_EXPORTER_OTLP_TRACES_INSECURE",
+    "OTEL_EXPORTER_OTLP_METRICS_INSECURE",
+)
+_SIGNAL_INSECURE_ENV_VAR: typing.Final = "OTEL_EXPORTER_OTLP_{signal}_INSECURE"
+_REMOTE_HTTP: typing.Final = "http://collector.example.com:4317"
+_REMOTE_SCHEMELESS: typing.Final = "collector.example.com:4317"
+
+
+@pytest.fixture
+def without_insecure_env(monkeypatch: pytest.MonkeyPatch) -> None:
+    for env_var in _INSECURE_ENV_VARS:
+        monkeypatch.delenv(env_var, raising=False)
+
+
+@pytest.mark.usefixtures("without_insecure_env")
+@pytest.mark.parametrize("signal", ["traces", "metrics"])
+@pytest.mark.parametrize(
+    ("endpoint", "insecure", "env", "expected_plaintext"),
+    [
+        pytest.param(_REMOTE_HTTP, None, {}, True, id="http-scheme"),
+        pytest.param(_REMOTE_SCHEMELESS, None, {}, False, id="schemeless"),
+        pytest.param("https://collector.example.com:4317", True, {}, False, id="https-forces-tls"),
+        pytest.param(_REMOTE_SCHEMELESS, True, {}, True, id="field-on"),
+        pytest.param(_REMOTE_HTTP, False, {}, False, id="field-off"),
+        pytest.param(_REMOTE_HTTP, None, {"OTEL_EXPORTER_OTLP_INSECURE": "false"}, False, id="generic-env-off"),
+        pytest.param(_REMOTE_SCHEMELESS, None, {"OTEL_EXPORTER_OTLP_INSECURE": "true"}, True, id="generic-env-on"),
+        pytest.param(_REMOTE_HTTP, None, {_SIGNAL_INSECURE_ENV_VAR: "false"}, False, id="signal-env-off"),
+        pytest.param(_REMOTE_SCHEMELESS, None, {_SIGNAL_INSECURE_ENV_VAR: "true"}, True, id="signal-env-on"),
+        pytest.param(
+            _REMOTE_SCHEMELESS,
+            None,
+            {_SIGNAL_INSECURE_ENV_VAR: "true", "OTEL_EXPORTER_OTLP_INSECURE": "false"},
+            True,
+            id="signal-env-wins",
+        ),
+        pytest.param(_REMOTE_HTTP, True, {_SIGNAL_INSECURE_ENV_VAR: "false"}, True, id="field-wins"),
+    ],
+)
+def test_insecure_warning_fires_exactly_when_the_grpc_exporter_sends_plaintext(
+    signal: typing.Literal["traces", "metrics"],
+    endpoint: str,
+    insecure: bool | None,
+    env: dict[str, str],
+    expected_plaintext: bool,
+) -> None:
+    signal_env = {env_var.format(signal=signal.upper()): value for env_var, value in env.items()}
+    endpoint_field = "opentelemetry_endpoint" if signal == "traces" else "opentelemetry_metrics_endpoint"
+    fields: dict[str, typing.Any] = {endpoint_field: endpoint}
+    if insecure is not None:
+        fields["opentelemetry_insecure"] = insecure
+
+    with patch.dict(os.environ, signal_env):
+        with warnings.catch_warnings(record=True) as caught:
+            warnings.simplefilter("always")
+            config = OpenTelemetryConfig(**fields)
+        instrument = OpenTelemetryInstrument(bootstrap_config=config)
+        exporter = instrument._build_span_exporter() if signal == "traces" else instrument._build_metric_exporter()  # noqa: SLF001
+    assert exporter is not None
+    try:
+        assert exporter._insecure is expected_plaintext  # noqa: SLF001  # ty: ignore[unresolved-attribute]
+        assert any("unencrypted" in str(w.message) for w in caught) is expected_plaintext
+    finally:
+        exporter.shutdown()
+
+
 def test_opentelemetry_teardown_aggregates_instrumentor_and_shutdown_errors() -> None:
     class BoomInstrumentor(BaseInstrumentor):
         def instrumentation_dependencies(self) -> typing.Collection[str]:
@@ -401,7 +468,7 @@ def test_bootstrap_grpc_protocol_uses_grpc_exporter() -> None:
     try:
         with patch.object(otel_module, "OTLPGrpcSpanExporter") as mock_grpc:
             instrument.bootstrap()
-        mock_grpc.assert_called_once_with(endpoint="localhost:4317", insecure=True)
+        mock_grpc.assert_called_once_with(endpoint="localhost:4317", insecure=None)
     finally:
         instrument.teardown()
 
 
@@ -15,7 +15,7 @@
 from lite_bootstrap.instruments.opentelemetry_instrument import OpenTelemetryConfig
 
 
-_REMOTE_INSECURE_ENDPOINT: typing.Final = "collector.example.com:4317"
+_REMOTE_INSECURE_ENDPOINT: typing.Final = "http://collector.example.com:4317"
 
 
 @pytest.mark.parametrize(