Track which part of the aircraft reported a value, and resolve the battery

The drone sends BatteryStateChanged from two places with different values: the
true figure on buffer 126, corroborated by its own log and tracking a smooth
discharge, and a constant 0 on buffer 127 that appears in no log line. Keeping
the newest frame meant a healthy aircraft read as flat and preflight refused.

Events now carry their buffer and the store keeps the last value per source, so
a disagreement is visible instead of silently resolved. get_state reports the
sources whenever they differ, rather than handing over a winner.

preflight breaks the tie on physical grounds, not preference: an aircraft that
is powered and holding a link is not at 0%, so a zero from a live drone is not
credible while a non-zero one is. It says the number was contested either way,
and a genuine zero from every source still reports zero and still blocks.

src/mcbebop/arsdk/session.py

@@ -23,7 +23,7 @@ import logging
 import threading
 import time
 from collections.abc import Callable, Iterable
-from dataclasses import dataclass, field
+from dataclasses import dataclass, field, replace
 from functools import cache
 from typing import Any
 
@@ -70,15 +70,27 @@ class Timeouts:
 
 @dataclass
 class Reading:
-    """One telemetry key: the value, and when the drone last said so."""
+    """One telemetry key: the value, and when the drone last said so.
+
+    `sources` holds the last value seen from each buffer. It exists because
+    this aircraft sends `BatteryStateChanged` from two places with different
+    values, so "the newest frame" and "the truth" are not the same thing, and
+    a consumer needs to be able to see that rather than be handed a winner.
+    """
 
     value: Any
     at: float  # time.monotonic()
     count: int = 1
+    sources: dict[int, Any] = field(default_factory=dict)
 
     def age(self, now: float | None = None) -> float:
         return (now if now is not None else time.monotonic()) - self.at
 
+    @property
+    def conflicting(self) -> bool:
+        """True when two buffers are currently reporting different values."""
+        return len(set(map(repr, self.sources.values()))) > 1
+
 
 @dataclass
 class _Waiter:
@@ -119,7 +131,7 @@ def _name_for(ids: tuple[int, int, int]) -> str:
     return spec.name if spec is not None else f"cmd_{ids[0]}_{ids[1]}_{ids[2]}"
 
 
-def _as_event(decoded: Any, payload: bytes) -> Event | None:
+def _as_event(decoded: Any, payload: bytes, buffer_id: int = 0) -> Event | None:
     """Normalise whatever the codec returned into an `Event`.
 
     `decode_event` returns `(ids, values)`, which is the second branch. The
@@ -131,7 +143,7 @@ def _as_event(decoded: Any, payload: bytes) -> Event | None:
     if decoded is None:
         return None
     if isinstance(decoded, Event):
-        return decoded
+        return decoded if decoded.buffer_id else replace(decoded, buffer_id=buffer_id)
     ids = getattr(decoded, "ids", None)
     name = getattr(decoded, "name", None)
     values = getattr(decoded, "values", None)
@@ -151,6 +163,7 @@ def _as_event(decoded: Any, payload: bytes) -> Event | None:
         name=str(name) if name else _name_for(ids),  # type: ignore[arg-type]
         values=dict(values or {}),
         at=getattr(decoded, "at", None) or time.monotonic(),
+        buffer_id=buffer_id,
     )
 
 
@@ -265,7 +278,7 @@ class DroneSession:
         if len(frame.payload) < COMMAND_HEADER.size:
             return
         try:
-            event = _as_event(self._decode(frame.payload), frame.payload)
+            event = _as_event(self._decode(frame.payload), frame.payload, frame.buffer_id)
         except Exception:
             # An event we cannot decode is a gap in coverage, not a reason to
             # drop the link: the next frame may be the one that matters.
@@ -282,7 +295,9 @@ class DroneSession:
         with self._lock:
             for key, value in values.items():
                 prior = self._store.get(key)
-                self._store[key] = Reading(value, event.at, (prior.count + 1) if prior else 1)
+                sources = dict(prior.sources) if prior else {}
+                sources[event.buffer_id] = value
+                self._store[key] = Reading(value, event.at, (prior.count + 1) if prior else 1, sources)
             hit = [w for w in self._waiters if event.ids in w.ids and w.predicate(event)]
             for waiter in hit:
                 waiter.hit = event
@@ -309,7 +324,16 @@ class DroneSession:
         if keys is not None:
             wanted = list(keys)
             items = {k: v for k, v in items.items() if k in wanted or any(k.startswith(w) for w in wanted)}
-        return {k: {"value": v.value, "age": round(v.age(now), 3)} for k, v in sorted(items.items())}
+        out: dict[str, Any] = {}
+        for key, reading in sorted(items.items()):
+            entry: dict[str, Any] = {"value": reading.value, "age": round(reading.age(now), 3)}
+            if reading.conflicting:
+                # Say so rather than hand over a winner. The newest frame is not
+                # necessarily the true one when two parts of the aircraft report
+                # the same thing differently.
+                entry["sources"] = dict(reading.sources)
+            out[key] = entry
+        return out
 
     def values(self, keys: Iterable[str] | None = None) -> dict[str, Any]:
         """The same thing without the staleness wrapper, for internal checks."""

src/mcbebop/arsdk/types.py

@@ -90,6 +90,14 @@ class Event:
     name: str  # the command name, e.g. "BatteryStateChanged"
     values: dict[str, Any]  # keyed `<Command>_<arg>`, as our captures are
     at: float  # time.monotonic()
+    buffer_id: int = 0
+    """Which buffer carried it.
+
+    Kept because this aircraft sends the same command from more than one place
+    with different values: `BatteryStateChanged` arrives as the true figure on
+    one buffer and as a constant 0 on another. Without knowing the source, a
+    store that keeps the newest value silently reports whichever spoke last.
+    """
 
 
 class HandshakeError(RuntimeError):

src/mcbebop/models.py

@@ -64,6 +64,14 @@ class SendResult(BaseModel):
 class StateValue(BaseModel):
     value: object
     age: float = Field(description="Seconds since the drone last reported this. Old values may be stale.")
+    sources: dict[int, object] | None = Field(
+        default=None,
+        description=(
+            "Present only when parts of the aircraft disagree about this value, mapping the link "
+            "buffer each report arrived on to what it said. `value` is then merely the most recent "
+            "of them, not a decision about which is right."
+        ),
+    )
 
 
 class Check(BaseModel):

src/mcbebop/tools/state.py

@@ -28,6 +28,40 @@ def _sensor_results(values: dict) -> dict[str, bool]:
     return out
 
 
+def _battery(session) -> tuple[int | None, str]:
+    """The battery percentage, and how much to trust it.
+
+    This aircraft sends `BatteryStateChanged` from two places with different
+    values: the real figure, which its own log corroborates and which tracks a
+    smooth discharge, and a constant 0 that appears in no log line. Taking the
+    newest frame therefore reports a healthy drone as flat.
+
+    The tie is broken on physical grounds rather than preference: an aircraft
+    that is powered, holding a link and streaming telemetry is not at 0%, so a
+    zero from a live drone is not a credible reading while a non-zero one is.
+    The disagreement is reported either way, because a caller should know the
+    number was contested.
+    """
+    raw = session.state(["BatteryStateChanged_percent"]).get("BatteryStateChanged_percent")
+    if raw is None:
+        return None, "not reported"
+
+    sources = raw.get("sources") or {}
+    distinct = {v for v in sources.values()}
+    if len(distinct) < 2:
+        return raw["value"], f"{raw['value']}%"
+
+    credible = sorted(v for v in distinct if isinstance(v, int) and v > 0)
+    if not credible:
+        return None, f"sources disagree but none is credible: {sources}"
+    best = credible[-1]
+    others = sorted(distinct - {best})
+    return best, (
+        f"{best}%, contested: the aircraft also reports {others} on another channel. "
+        "A drone that is powered and holding a link is not at 0%, so that reading is disregarded."
+    )
+
+
 def register(mcp: FastMCP, settings: Settings) -> None:
     @mcp.tool(annotations={"readOnlyHint": True, "openWorldHint": False})
     async def get_state(
@@ -49,7 +83,10 @@ def register(mcp: FastMCP, settings: Settings) -> None:
         """
         session = require_session()
         raw = session.state(keys or None)
-        return {k: StateValue(value=v["value"], age=round(v["age"], 2)) for k, v in raw.items()}
+        return {
+            k: StateValue(value=v["value"], age=round(v["age"], 2), sources=v.get("sources"))
+            for k, v in raw.items()
+        }
 
     @mcp.tool(annotations={"readOnlyHint": True, "openWorldHint": False})
     async def watch_state(
@@ -96,13 +133,8 @@ def register(mcp: FastMCP, settings: Settings) -> None:
                 # one that says nothing at all.
                 (blocking if blocks else advisory).append(f"{name}: not reported")
 
-        battery = v.get("BatteryStateChanged_percent")
-        add(
-            "battery",
-            None if battery is None else battery >= 30,
-            "not reported" if battery is None else f"{battery}%",
-            blocks=True,
-        )
+        battery, battery_detail = _battery(session)
+        add("battery", None if battery is None else battery >= 30, battery_detail, blocks=True)
 
         rssi = v.get("WifiSignalChanged_rssi")
         add("link", None if rssi is None else rssi > -75, "not reported" if rssi is None else f"{rssi} dBm")

tests/test_battery_conflict.py

@@ -0,0 +1,60 @@
+"""The aircraft reports its battery twice, with different values.
+
+Observed on the live drone 2026-10-02: `BatteryStateChanged` (0-5-1) arrives as
+`00 05 01 00 40` on buffer 126 and `00 05 01 00 00` on buffer 127, repeatedly.
+The 64 is corroborated by the drone's own log and tracks a smooth discharge; the
+zeros appear in no log line. A store that keeps the newest frame therefore
+reported a healthy aircraft as flat, and preflight refused on it.
+"""
+
+import pytest
+
+from mcbebop.arsdk.session import Reading
+from mcbebop.arsdk.types import Event
+from mcbebop.tools.state import _battery
+
+KEY = "BatteryStateChanged_percent"
+
+
+class FakeSession:
+    def __init__(self, entry):
+        self.entry = entry
+
+    def state(self, keys):
+        return {KEY: self.entry} if self.entry is not None else {}
+
+
+def test_reading_detects_disagreement():
+    assert Reading(0, 1.0, 4, {126: 64, 127: 0}).conflicting
+    assert not Reading(64, 1.0, 1, {126: 64}).conflicting
+    assert not Reading(64, 1.0, 2, {126: 64, 127: 64}).conflicting
+
+
+def test_event_carries_its_buffer():
+    """Without the source, the store cannot tell two reporters apart."""
+    assert Event(ids=(0, 5, 1), name="x", values={}, at=0.0).buffer_id == 0
+    assert Event(ids=(0, 5, 1), name="x", values={}, at=0.0, buffer_id=127).buffer_id == 127
+
+
+def test_the_real_conflict_resolves_to_the_credible_value():
+    pct, detail = _battery(FakeSession({"value": 0, "age": 0.1, "sources": {126: 64, 127: 0}}))
+    assert pct == 64, "the newest frame said 0; the credible one is 64"
+    assert "contested" in detail and "0" in detail, "the caller must learn the number was disputed"
+
+
+def test_a_lone_reading_is_not_described_as_contested():
+    pct, detail = _battery(FakeSession({"value": 64, "age": 0.1, "sources": {126: 64}}))
+    assert pct == 64 and detail == "64%"
+
+
+@pytest.mark.parametrize(
+    ("entry", "expected"),
+    [
+        ({"value": 58, "age": 0.1}, 58),
+        ({"value": 0, "age": 0.1, "sources": {126: 0, 127: 0}}, 0),
+        (None, None),
+    ],
+)
+def test_other_shapes(entry, expected):
+    """A genuine zero from every source still reports zero, and still blocks."""
+    assert _battery(FakeSession(entry))[0] == expected