diff --git a/headroom/telemetry/collector.py b/headroom/telemetry/collector.py
index 1439c1a26..34682c38d 100644
--- a/headroom/telemetry/collector.py
+++ b/headroom/telemetry/collector.py
@@ -500,7 +500,7 @@ class TelemetryCollector:
                 type_counts["string"] = type_counts.get("string", 0) + 1
             elif isinstance(v, bool):
                 type_counts["boolean"] = type_counts.get("boolean", 0) + 1
-            elif isinstance(v, (int, float)):
+            elif isinstance(v, int | float):
                 type_counts["numeric"] = type_counts.get("numeric", 0) + 1
             elif isinstance(v, list):
                 type_counts["array"] = type_counts.get("array", 0) + 1
@@ -532,7 +532,7 @@ class TelemetryCollector:
                 dist.looks_like_id = dist.unique_ratio > 0.9 and dist.avg_length > 5
 
         elif field_type == "numeric":
-            num_values = [v for v in values if isinstance(v, (int, float))]
+            num_values = [v for v in values if isinstance(v, int | float)]
             # Filter out infinity and NaN which can cause issues
             num_values = [
                 v
@@ -744,8 +744,19 @@ def get_telemetry_collector(
     if _telemetry_collector is None:
         with _collector_lock:
             if _telemetry_collector is None:
-                # Check environment for opt-out
-                if os.environ.get("HEADROOM_TELEMETRY_DISABLED", "").lower() in ("1", "true"):
+                # Honour HEADROOM_TELEMETRY (the documented opt-out var,
+                # also used by the Supabase beacon at telemetry/beacon.py).
+                # Pre-#390 this only checked HEADROOM_TELEMETRY_DISABLED,
+                # so users who set HEADROOM_TELEMETRY=off (the value in
+                # the docs) still saw /v1/telemetry report enabled=true.
+                # HEADROOM_TELEMETRY_DISABLED stays accepted for back-compat.
+                from headroom.telemetry.beacon import is_telemetry_enabled
+
+                disabled_legacy = os.environ.get("HEADROOM_TELEMETRY_DISABLED", "").lower() in (
+                    "1",
+                    "true",
+                )
+                if disabled_legacy or not is_telemetry_enabled():
                     config = config or TelemetryConfig()
                     config.enabled = False
 
diff --git a/tests/test_telemetry.py b/tests/test_telemetry.py
index 4ad51cf3c..b00c52d3f 100644
--- a/tests/test_telemetry.py
+++ b/tests/test_telemetry.py
@@ -576,6 +576,40 @@ class TestGlobalTelemetryCollector:
 
         assert collector._config.enabled is False
 
+    @pytest.mark.parametrize("off_value", ["off", "false", "0", "no", "disable", "disabled"])
+    def test_headroom_telemetry_off_disables_collector(self, monkeypatch, off_value):
+        """HEADROOM_TELEMETRY=off (and other documented opt-out values) disables
+        the collector — closes #390.
+
+        Pre-#390 the collector only honoured HEADROOM_TELEMETRY_DISABLED, which
+        is undocumented. Users following the docs set HEADROOM_TELEMETRY=off and
+        watched /v1/telemetry continue to report enabled=true. The collector now
+        consults `is_telemetry_enabled()` (the same predicate the Supabase beacon
+        uses), so both env vars take effect.
+        """
+        reset_telemetry_collector()
+        monkeypatch.delenv("HEADROOM_TELEMETRY_DISABLED", raising=False)
+        monkeypatch.setenv("HEADROOM_TELEMETRY", off_value)
+
+        collector = get_telemetry_collector()
+
+        assert collector._config.enabled is False, (
+            f"HEADROOM_TELEMETRY={off_value!r} must disable the collector — "
+            "this is the documented opt-out path. If this assertion fails the "
+            "collector is silently ignoring the user's opt-out and /v1/telemetry "
+            "will report enabled=true even when telemetry is supposed to be off."
+        )
+
+    def test_headroom_telemetry_on_keeps_collector_enabled(self, monkeypatch):
+        """Sanity check: the explicit on/unset path leaves the collector enabled."""
+        reset_telemetry_collector()
+        monkeypatch.delenv("HEADROOM_TELEMETRY_DISABLED", raising=False)
+        monkeypatch.setenv("HEADROOM_TELEMETRY", "on")
+
+        collector = get_telemetry_collector()
+
+        assert collector._config.enabled is True
+
 
 class TestRetrievalStatsModel:
     """Test RetrievalStats data model."""