diff --git a/README.md b/README.md
index 7a0e9ae..bdd7988 100644
--- a/README.md
+++ b/README.md
@@ -182,6 +182,24 @@ device page in HA.
 
 Requirements: Settings > Developer options > Network debugging enabled on the Shield.
 
+### Discovery (zeroconf)
+
+Home Assistant can discover a Shield through the `_androidtvremote2._tcp`
+service, and the discovered address is pre-filled on the setup form. The dedup
+rules are Home Assistant's own, and they differ with what the discovery carries:
+
+- **with** the device MAC, a duplicate aborts as "already configured", and an
+  existing entry's hosts are updated when the address changed;
+- **without** a MAC, the flow follows HA's stricter
+  `_async_handle_discovery_without_unique_id` contract: **any** existing entry
+  for this integration aborts the discovery, and only one discovery flow runs at
+  a time.
+
+So a second (or third) Shield that advertises no MAC has to be added **manually**
+— Settings > Devices & Services > Add Integration > "NVIDIA Shield" — rather than
+by discovery. That is a deliberate, tested contract (t37 D3), not a bug: the
+helper the flow calls is a coroutine and its dedup only runs when awaited.
+
 ### Options
 
 - **Jellyfin Media Player** — auto-detected or manually selected
diff --git a/tests/ha_api_drift.py b/tests/ha_api_drift.py
index c163d6b..e958b0a 100644
--- a/tests/ha_api_drift.py
+++ b/tests/ha_api_drift.py
@@ -47,6 +47,12 @@ interpreter floor Home Assistant itself declares lives here too
 (``HOMEASSISTANT_PYTHON_FLOOR``), where CI's guard reads it instead of repeating
 the number.
 
+Behaviour note for users (t37 D3): MAC-less zeroconf discovery now follows Home
+Assistant's stricter contract — the awaited
+``_async_handle_discovery_without_unique_id`` aborts ``already_configured`` as
+soon as ANY entry exists for this domain, so a second Shield that advertises no
+MAC is added manually. See README "Discovery (zeroconf)".
+
 Without homeassistant installed the sweep cannot resolve anything: ``scan()``
 returns a single ``no-homeassistant`` finding, the counters stay at zero (so the
 floors fail on purpose) and nothing is ever reported green. The pytest wrapper