diff --git a/custom_components/localthings/diagnostics.py b/custom_components/localthings/diagnostics.py
index 7ed5870..8d75794 100644
--- a/custom_components/localthings/diagnostics.py
+++ b/custom_components/localthings/diagnostics.py
@@ -33,4 +33,12 @@ async def async_get_config_entry_diagnostics(
         "resources": redact_resources(coordinator.last_resources),
         "integration_version": integration.version,
         "smartthings_local_version": pkg_version("smartthings-local"),
+        "observe_mode": coordinator.observe_mode,
+        "observe_subscribed_hrefs": sorted(coordinator._observe.subscribed_hrefs),
+        "observe_fallback_hrefs": sorted(coordinator._observe.fallback_hrefs),
+        "observe_last_mode_change": coordinator._observe.last_mode_change_wall,
+        "observe_href_freshness_s": {
+            href: coordinator._cache.freshness_s(href)
+            for href in coordinator._observe.subscribed_hrefs
+        },
     }
diff --git a/tests/localthings/test_diagnostics.py b/tests/localthings/test_diagnostics.py
index 0c010af..e842125 100644
--- a/tests/localthings/test_diagnostics.py
+++ b/tests/localthings/test_diagnostics.py
@@ -27,3 +27,18 @@ async def test_diagnostics_shape_and_redaction(
     assert resources['/wirelessinfo/vs/0']['macaddressWiFi'] == REDACTED
     # Ordinary state survives.
     assert resources['/status/lock/vs/0']['x.com.samsung.da.ado.devicecontrol'] == 'On'
+
+
+async def test_diagnostics_include_observe_mode_fields(
+    hass: HomeAssistant, mock_entry, mock_coordinator_session
+) -> None:
+    await hass.config_entries.async_setup(mock_entry.entry_id)
+    await hass.async_block_till_done()
+
+    diag = await async_get_config_entry_diagnostics(hass, mock_entry)
+
+    assert diag['observe_mode'] == 'poll'
+    assert diag['observe_subscribed_hrefs'] == []
+    assert diag['observe_fallback_hrefs'] == []
+    assert 'observe_last_mode_change' in diag
+    assert diag['observe_href_freshness_s'] == {}