diff --git a/lib/logitech_receiver/rgb_effects_probe.py b/lib/logitech_receiver/rgb_effects_probe.py
new file mode 100644
index 00000000..fa4d642f
--- /dev/null
+++ b/lib/logitech_receiver/rgb_effects_probe.py
@@ -0,0 +1,124 @@
+"""Read-only corpus probe for HEADSET_RGB_ONBOARD_EFFECTS (0x0621) and
+HEADSET_RGB_SIGNATURE_EFFECTS (0x0622).
+
+Logs raw response bytes and lengths at INFO so field testers without
+``-dd`` can still capture the data. All calls are strictly read-side —
+no setters are invoked. If either feature isn't present the probe
+short-circuits cleanly.
+"""
+
+from __future__ import annotations
+
+import logging
+
+from . import exceptions
+from .hidpp20_constants import SupportedFeature
+
+logger = logging.getLogger(__name__)
+
+
+def _hex_or_none(data) -> str | None:
+    return data.hex() if data else None
+
+
+def _log_feature_table(device) -> None:
+    if not device.features:
+        return
+    try:
+        pairs = []
+        for idx in range(len(device.features)):
+            feat = device.features[idx]
+            pairs.append(f"{idx}:0x{int(feat):04X}" if feat is not None else f"{idx}:?")
+        logger.info("RGB probe: feature table for %s: %s", device, ", ".join(pairs))
+    except Exception as e:
+        logger.info("RGB probe: feature-table dump failed: %s", e)
+
+
+def _call(device, feature: SupportedFeature, fn: int, *params):
+    """Wrap feature_request with uniform INFO logging.
+
+    Returns the raw bytes on success, None on transport/no-feature, and
+    doesn't raise — FeatureCallError is caught and logged as an error code.
+    """
+    label = f"0x{int(feature):04X}.fn{fn:02X}"
+    if params:
+        label += "(" + ",".join(f"{b:02X}" for b in params) + ")"
+    try:
+        resp = device.feature_request(feature, fn, *params)
+    except exceptions.FeatureCallError as e:
+        logger.info("RGB probe: %s err=0x%02X", label, getattr(e, "error", 0) & 0xFF)
+        return None
+    except Exception as e:
+        logger.info("RGB probe: %s raised %r", label, e)
+        return None
+    if resp is None:
+        logger.info("RGB probe: %s no reply (feature unsupported or transport failure)", label)
+        return None
+    logger.info("RGB probe: %s resp=%s len=%d", label, _hex_or_none(resp), len(resp))
+    return resp
+
+
+def probe_onboard_effects(device) -> None:
+    """Probe 0x0621 RGBOnboardEffects read-side functions."""
+    feature = SupportedFeature.HEADSET_RGB_ONBOARD_EFFECTS
+    if not device.features or feature not in device.features:
+        return
+    logger.info("RGB probe: 0x0621 HEADSET_RGB_ONBOARD_EFFECTS present on %s", device)
+
+    # fn 0x00 getInfo — empty payload
+    _call(device, feature, 0x00)
+
+    # fn 0x10 getRGBClusterInfo — iterate cluster indexes 0..7, stop on error.
+    for cluster_idx in range(8):
+        resp = _call(device, feature, 0x10, cluster_idx)
+        if resp is None:
+            break
+
+    # fn 0x20 getRGBClusterEffect — current state per cluster.
+    for cluster_idx in range(8):
+        resp = _call(device, feature, 0x20, cluster_idx)
+        if resp is None:
+            break
+
+    # fn 0x40 getRGBCustomEffectName — single call, documented.
+    _call(device, feature, 0x40)
+
+
+def probe_signature_effects(device) -> None:
+    """Probe 0x0622 RGBSignatureEffects read-side functions."""
+    feature = SupportedFeature.HEADSET_RGB_SIGNATURE_EFFECTS
+    if not device.features or feature not in device.features:
+        return
+    logger.info("RGB probe: 0x0622 HEADSET_RGB_SIGNATURE_EFFECTS present on %s", device)
+
+    # fn 0x00 getSignatureEffectsInfo.
+    _call(device, feature, 0x00)
+
+    # fn 0x10 getSignatureEffectParams — iterate effectId 0..2 (Startup/Shutdown/Passive).
+    # effectId is u16 BE.
+    for eid in range(3):
+        _call(device, feature, 0x10, (eid >> 8) & 0xFF, eid & 0xFF)
+
+    # fn 0x30 getSignatureEffectState — same effectId range.
+    for eid in range(3):
+        _call(device, feature, 0x30, (eid >> 8) & 0xFF, eid & 0xFF)
+
+
+def probe(device) -> None:
+    """Run both read-only RGB-effects probes once per device.
+
+    Gated via ``_rgb_effects_probed`` so re-entry on reconnect / setting
+    rebuild doesn't spam the log with duplicate corpus dumps.
+    """
+    if getattr(device, "_rgb_effects_probed", False):
+        return
+    device._rgb_effects_probed = True
+    _log_feature_table(device)
+    try:
+        probe_onboard_effects(device)
+    except Exception as e:
+        logger.info("RGB probe: onboard-effects probe raised %r", e)
+    try:
+        probe_signature_effects(device)
+    except Exception as e:
+        logger.info("RGB probe: signature-effects probe raised %r", e)
diff --git a/lib/logitech_receiver/settings_templates.py b/lib/logitech_receiver/settings_templates.py
index 1087b901..8e7a6ee0 100644
--- a/lib/logitech_receiver/settings_templates.py
+++ b/lib/logitech_receiver/settings_templates.py
@@ -37,6 +37,7 @@ from . import headset_rgb
 from . import hidpp20
 from . import hidpp20_constants
 from . import logivoice
+from . import rgb_effects_probe
 from . import settings
 from . import settings_new
 from . import settings_validator
@@ -2051,6 +2052,17 @@ class HeadsetLEDControl(settings.Setting):
     validator_class = settings_validator.ChoicesValidator
     validator_options = {"choices": choices_universe}
 
+    @classmethod
+    def build(cls, device):
+        # One-shot read-only probe of 0x0621 / 0x0622 — logs the data the RE
+        # pass needs to pin down RGB onboard/signature effect structures.
+        # Skip cleanly if neither feature is exposed.
+        try:
+            rgb_effects_probe.probe(device)
+        except Exception as e:
+            logger.info("RGB effects probe raised %r", e)
+        return super().build(device)
+
     def write(self, value, save=True):
         # After switching to Solaar control, the firmware drops whatever
         # colors we'd programmed — so reassert the saved Primary + per-zone