fix(config): name the radios[] entry with a stray sync word

build_radio_stack pops _radio_id before building each radio, so the
warning never named the entry on a real node. A _radio_label survives the
pop, and the warning now points at radios[<id>].radio, saying the value is
inherited only when it is.

config.yaml.example documents sync_word.

config.yaml.example

@@ -599,6 +599,11 @@ radio:
   # Preamble length in symbols
   preamble_length: 32
 
+  # LoRa sync word, used by modem_tcp/modem_usb radios (the SX1262 driver
+  # ignores it). MeshCore uses 0x12; a modem on any other value hears nothing.
+  # radios[] entries inherit this value unless they set their own.
+  # sync_word: 0x12
+
   # Use implicit header mode
   implicit_header: false
 

repeater/config.py

@@ -517,19 +517,26 @@ def _checked_modem_sync_word(sync_word: int, board_config: dict, radio_type: str
     These are the drivers that actually push the value to the radio, and a
     modem on another sync word connects, answers pings and transmits, yet hears
     no MeshCore traffic -- a silent failure. A radios[] entry inherits the
-    top-level ``radio.sync_word``, which the SX1262 driver ignores, so a stray
-    top-level value is harmless until a modem entry picks it up.
+    top-level ``radio.sync_word``, which the SX1262 driver ignores (SX1262Radio
+    stores it but never calls setSyncWord), so a stray top-level value is
+    harmless until a modem entry picks it up.
     """
     if sync_word != MESHCORE_SYNC_WORD:
-        radio_id = board_config.get("_radio_id")
+        radio_id = board_config.get("_radio_label") or board_config.get("_radio_id")
+        if radio_id:
+            where = f"radios[{radio_id}].radio"
+            if board_config.get("_sync_word_inherited"):
+                where += " (it inherits the top-level radio.sync_word)"
+        else:
+            where = "its radio section"
         logger.warning(
             "%s sync_word is 0x%X, not MeshCore's 0x%02X: the modem will not hear "
-            "MeshCore traffic. Set sync_word: 0x%02X in its radio section%s.",
+            "MeshCore traffic. Set sync_word: 0x%02X in %s.",
             f"Radio {radio_id!r} ({radio_type})" if radio_id else f"{radio_type} radio",
             sync_word,
             MESHCORE_SYNC_WORD,
             MESHCORE_SYNC_WORD,
-            " (radios[] entries inherit the top-level radio.sync_word)" if radio_id else "",
+            where,
         )
     return sync_word
 
@@ -920,6 +927,10 @@ def _merge_radio_entry(global_config: dict, entry: dict) -> dict:
         raise ValueError(f"radios[] entry {radio_id!r} missing radio_type")
 
     merged["_radio_id"] = str(radio_id)
+    # build_radio_stack pops _radio_id; this survives it, to name the radio in logs.
+    merged["_radio_label"] = str(radio_id)
+    own_radio = entry.get("radio") if isinstance(entry.get("radio"), dict) else {}
+    merged["_sync_word_inherited"] = "sync_word" not in own_radio
     # Hint for CH341 path: do not install process-global SPI/GPIO defaults.
     merged["_ch341_per_instance"] = True
     return merged

tests/test_radio_config.py

@@ -364,7 +364,7 @@ def test_a_radios_entry_inheriting_a_stray_top_level_sync_word_is_named(monkeypa
 
     assert captured["sync_word"] == 0x3444
     assert "Radio 'narrow' (modem_tcp) sync_word is 0x3444" in caplog.text
-    assert "inherit the top-level radio.sync_word" in caplog.text
+    assert "inherits the top-level radio.sync_word" in caplog.text
 
 
 def test_a_radios_entry_overriding_the_stray_sync_word_is_not_warned_about(monkeypatch, caplog):
@@ -398,3 +398,67 @@ def test_an_explicit_null_sync_word_means_the_default(monkeypatch, caplog, radio
 
     assert captured["sync_word"] == 0x12
     assert "sync_word" not in caplog.text
+
+
+def test_build_radio_stack_names_the_radios_entry_with_the_stray_sync_word(monkeypatch, caplog):
+    """The real path: build_radio_stack pops _radio_id before building the
+    radio, so the warning must still name the entry."""
+    from repeater.config import build_radio_stack
+
+    captured = _capture_modem(monkeypatch, "modem_tcp")
+    config = {
+        "radio_type": "sx1262",
+        "radio": {**_modem_radio_cfg(), "sync_word": 13380},
+        "radios": [
+            {
+                "id": "narrow",
+                "radio_type": "modem_tcp",
+                "radio": {"frequency": 910525000},
+                "modem_tcp": {"host": "192.168.50.154"},
+            }
+        ],
+    }
+
+    with caplog.at_level("WARNING", logger="Config"):
+        build_radio_stack(config)
+
+    assert captured["sync_word"] == 0x3444
+    assert "Radio 'narrow' (modem_tcp) sync_word is 0x3444" in caplog.text
+    assert "inherits the top-level radio.sync_word" in caplog.text
+
+
+def test_each_modem_entry_gets_its_own_sync_word_and_warning(monkeypatch, caplog):
+    from repeater.config import build_radio_stack
+
+    pytest.importorskip("openhop_core.hardware.tcp_radio")
+    built = []
+
+    class _Dummy(_DummyRadio):
+        def __init__(self, **kwargs):
+            built.append(kwargs)
+
+    monkeypatch.setattr("openhop_core.hardware.tcp_radio.TCPLoRaRadio", _Dummy)
+    config = {
+        "radio_type": "sx1262",
+        "radio": {**_modem_radio_cfg(), "sync_word": 13380},
+        "radios": [
+            {"id": "local", "radio_type": "modem_tcp", "modem_tcp": {"host": "10.0.0.1"}},
+            {
+                "id": "narrow",
+                "radio_type": "modem_tcp",
+                "radio": {"sync_word": 0x34},
+                "modem_tcp": {"host": "10.0.0.2"},
+            },
+        ],
+        "fabric": {"default_radio": "local"},
+    }
+
+    with caplog.at_level("WARNING", logger="Config"):
+        build_radio_stack(config)
+
+    assert [b["sync_word"] for b in built] == [0x3444, 0x34]
+    lines = [r.getMessage() for r in caplog.records]
+    local = next(m for m in lines if "Radio 'local'" in m)
+    narrow = next(m for m in lines if "Radio 'narrow'" in m)
+    assert "radios[local].radio (it inherits" in local
+    assert "radios[narrow].radio." in narrow and "inherits" not in narrow