diff --git a/src/util/binary_envelope.py b/src/util/binary_envelope.py
new file mode 100644
index 0000000..9e2cc7d
--- /dev/null
+++ b/src/util/binary_envelope.py
@@ -0,0 +1,508 @@
+"""
+Compact binary controller → led-driver messages (ESP-NOW friendly).
+
+Header (5 bytes), same for v1 (legacy) and v2 (native binary):
+
+  0: version — 1 = legacy (JSON text blobs); 2 = native binary blobs
+  1: brightness — 0–127 scales to device 0–255; 128–255 = leave unchanged
+  2: byte length of presets section (0–255)
+  3: byte length of select section
+  4: byte length of default section
+
+v2 presets blob (no JSON):
+  u8 preset_count
+  each preset:
+    u8 name_len; name utf-8
+    u8 pattern_len; pattern utf-8 (``p``)
+    u8 color_count; color_count × (u8 r, u8 g, u8 b)
+    u16 delay_le (``d``)
+    u8 preset_brightness (``b``)
+    u8 auto (0/1) (``a``)
+    i16 n1..n6 little-endian (``n1``–``n6``)
+
+v2 select blob:
+  u8 entry_count
+  each:
+    u8 device_len; device utf-8
+    u8 preset_name_len; preset name utf-8
+    u8 has_step (0/1); optional u16 step_le
+
+v2 default blob:
+  u8 default_name_len; name utf-8
+  u8 target_count
+  each: u8 len; target name utf-8
+
+Legacy v1: sections are UTF-8 JSON text (see ``parse_binary_envelope_v1``).
+
+Keep ``5 + lp + ls + ld`` ≤ 245 for a single ESP-NOW frame body.
+"""
+
+from __future__ import annotations
+
+import json
+import struct
+from typing import Any, Dict, List, Optional, Tuple
+
+BINARY_ENVELOPE_VERSION_1 = 1
+BINARY_ENVELOPE_VERSION_2 = 2
+HEADER_LEN = 5
+
+
+def brightness_wire_from_0_255(value: int) -> int:
+    """Map device brightness 0–255 to wire 0–127."""
+    v = max(0, min(255, int(value)))
+    return (v * 127 + 127) // 255
+
+
+def brightness_0_255_from_wire(wire: int) -> int:
+    """Map wire 0–127 to device brightness 0–255."""
+    w = max(0, min(127, int(wire)))
+    return min(255, (w * 255) // 127)
+
+
+def _clamp_i16(x: int) -> int:
+    x = int(x)
+    return max(-32768, min(32767, x))
+
+
+def _colors_to_rgb_list(colors: Any) -> List[Tuple[int, int, int]]:
+    out: List[Tuple[int, int, int]] = []
+    if not colors:
+        return out
+    for c in colors:
+        if isinstance(c, str):
+            h = c.strip().lstrip("#")
+            if len(h) >= 6:
+                out.append(
+                    (int(h[0:2], 16), int(h[2:4], 16), int(h[4:6], 16))
+                )
+        elif isinstance(c, (list, tuple)) and len(c) >= 3:
+            out.append((int(c[0]), int(c[1]), int(c[2])))
+    return out
+
+
+def _pack_preset_dict(name: str, preset: Dict[str, Any]) -> bytes:
+    pname = name.encode("utf-8")
+    if len(pname) > 250:
+        raise ValueError("preset name too long")
+    pattern = str(preset.get("p") or preset.get("pattern", "off")).encode("utf-8")
+    if len(pattern) > 250:
+        raise ValueError("pattern string too long")
+    rgbs = _colors_to_rgb_list(preset.get("c") or preset.get("colors") or [])
+    if len(rgbs) > 255:
+        raise ValueError("too many colours")
+    delay = max(0, min(65535, int(preset.get("d") or preset.get("delay", 100))))
+    br = max(0, min(255, int(preset.get("b") or preset.get("brightness", 127))))
+    auto = 1 if preset.get("a", preset.get("auto", True)) else 0
+    parts = [
+        bytes([len(pname)]),
+        pname,
+        bytes([len(pattern)]),
+        pattern,
+        bytes([len(rgbs)]),
+    ]
+    for r, g, b in rgbs:
+        parts.append(bytes([r & 255, g & 255, b & 255]))
+    n1 = _clamp_i16(preset.get("n1", 0))
+    n2 = _clamp_i16(preset.get("n2", 0))
+    n3 = _clamp_i16(preset.get("n3", 0))
+    n4 = _clamp_i16(preset.get("n4", 0))
+    n5 = _clamp_i16(preset.get("n5", 0))
+    n6 = _clamp_i16(preset.get("n6", 0))
+    parts.append(
+        struct.pack(
+            "<HBBhhhhhh",
+            delay,
+            br,
+            auto,
+            n1,
+            n2,
+            n3,
+            n4,
+            n5,
+            n6,
+        )
+    )
+    return b"".join(parts)
+
+
+def _pack_presets_blob(presets: Dict[str, Any]) -> bytes:
+    items = [(k, v) for k, v in presets.items() if isinstance(v, dict)]
+    out = [bytes([len(items)])]
+    for name, pdata in items:
+        out.append(_pack_preset_dict(str(name), pdata))
+    return b"".join(out)
+
+
+def _pack_select_blob(select: Dict[str, Any]) -> bytes:
+    out = [bytes([len(select)])]
+    for device, sel in select.items():
+        dev_b = str(device).encode("utf-8")
+        if len(dev_b) > 250:
+            raise ValueError("device name too long")
+        if isinstance(sel, (list, tuple)) and sel:
+            pn = str(sel[0]).encode("utf-8")
+            step = sel[1] if len(sel) > 1 else None
+        else:
+            pn = str(sel).encode("utf-8")
+            step = None
+        if len(pn) > 250:
+            raise ValueError("preset name too long")
+        if step is None:
+            out.append(
+                bytes([len(dev_b)])
+                + dev_b
+                + bytes([len(pn)])
+                + pn
+                + bytes([0])
+            )
+        else:
+            s = int(step)
+            if s < 0 or s > 65535:
+                raise ValueError("step out of range")
+            out.append(
+                bytes([len(dev_b)])
+                + dev_b
+                + bytes([len(pn)])
+                + pn
+                + bytes([1])
+                + struct.pack("<H", s)
+            )
+    return b"".join(out)
+
+
+def _pack_default_blob(default: str, targets: Optional[list]) -> bytes:
+    name_b = str(default).encode("utf-8")
+    if len(name_b) > 250:
+        raise ValueError("default name too long")
+    tlist = list(targets) if targets else []
+    if len(tlist) > 255:
+        raise ValueError("too many targets")
+    out = [bytes([len(name_b)]), name_b, bytes([len(tlist)])]
+    for t in tlist:
+        tb = str(t).encode("utf-8")
+        if len(tb) > 250:
+            raise ValueError("target name too long")
+        out.append(bytes([len(tb)]))
+        out.append(tb)
+    return b"".join(out)
+
+
+def pack_binary_envelope_v2(
+    *,
+    presets: Optional[Dict[str, Any]] = None,
+    select: Optional[Dict[str, Any]] = None,
+    default: Optional[str] = None,
+    default_targets: Optional[list] = None,
+    brightness_0_255: Optional[int] = None,
+) -> bytes:
+    """Build a v2 envelope (native binary sections, no JSON)."""
+    presets_bytes = (
+        _pack_presets_blob(presets) if presets is not None and presets else b""
+    )
+    select_bytes = (
+        _pack_select_blob(select) if select is not None and select else b""
+    )
+    default_bytes = (
+        _pack_default_blob(default, default_targets)
+        if default is not None
+        else b""
+    )
+
+    lp = len(presets_bytes)
+    ls = len(select_bytes)
+    ld = len(default_bytes)
+    if lp > 255 or ls > 255 or ld > 255:
+        raise ValueError("binary envelope section exceeds 255 bytes")
+
+    br_wire = (
+        255
+        if brightness_0_255 is None
+        else brightness_wire_from_0_255(brightness_0_255)
+    )
+    header = bytes([BINARY_ENVELOPE_VERSION_2, br_wire, lp, ls, ld])
+    return header + presets_bytes + select_bytes + default_bytes
+
+
+def pack_binary_envelope_v1(
+    *,
+    presets: Optional[Dict[str, Any]] = None,
+    select: Optional[Dict[str, Any]] = None,
+    default: Optional[str] = None,
+    default_targets: Optional[list] = None,
+    brightness_0_255: Optional[int] = None,
+) -> bytes:
+    """Legacy: JSON UTF-8 fragments (version byte 1). Prefer ``pack_binary_envelope_v2``."""
+    if presets is None:
+        presets_bytes = b""
+    else:
+        presets_bytes = json.dumps(presets, separators=(",", ":")).encode("utf-8")
+
+    if select is None:
+        select_bytes = b""
+    else:
+        select_bytes = json.dumps(select, separators=(",", ":")).encode("utf-8")
+
+    default_obj: Optional[Dict[str, Any]] = None
+    if default is not None:
+        default_obj = {
+            "default": default,
+            "targets": list(default_targets) if default_targets else [],
+        }
+    default_bytes = (
+        json.dumps(default_obj, separators=(",", ":")).encode("utf-8")
+        if default_obj is not None
+        else b""
+    )
+
+    lp = len(presets_bytes)
+    ls = len(select_bytes)
+    ld = len(default_bytes)
+    if lp > 255 or ls > 255 or ld > 255:
+        raise ValueError("binary envelope fragment exceeds 255 bytes")
+
+    br_wire = (
+        255
+        if brightness_0_255 is None
+        else brightness_wire_from_0_255(brightness_0_255)
+    )
+    header = bytes([BINARY_ENVELOPE_VERSION_1, br_wire, lp, ls, ld])
+    return header + presets_bytes + select_bytes + default_bytes
+
+
+def _decode_preset_record(
+    buf: bytes, off: int
+) -> Tuple[str, Dict[str, Any], int]:
+    if off + 1 > len(buf):
+        raise ValueError("truncated")
+    nl = buf[off]
+    off += 1
+    if off + nl > len(buf):
+        raise ValueError("truncated")
+    name = buf[off : off + nl].decode("utf-8")
+    off += nl
+    if off + 1 > len(buf):
+        raise ValueError("truncated")
+    pl = buf[off]
+    off += 1
+    if off + pl > len(buf):
+        raise ValueError("truncated")
+    pattern = buf[off : off + pl].decode("utf-8")
+    off += pl
+    if off + 1 > len(buf):
+        raise ValueError("truncated")
+    nc = buf[off]
+    off += 1
+    if off + nc * 3 > len(buf):
+        raise ValueError("truncated")
+    colors: List[str] = []
+    for _ in range(nc):
+        r, g, b = buf[off], buf[off + 1], buf[off + 2]
+        off += 3
+        colors.append(f"#{r:02x}{g:02x}{b:02x}")
+    if off + 16 > len(buf):
+        raise ValueError("truncated")
+    delay, br, auto, n1, n2, n3, n4, n5, n6 = struct.unpack_from(
+        "<HBBhhhhhh", buf, off
+    )
+    off += 16
+    preset = {
+        "p": pattern,
+        "c": colors,
+        "d": delay,
+        "b": br,
+        "a": bool(auto),
+        "n1": n1,
+        "n2": n2,
+        "n3": n3,
+        "n4": n4,
+        "n5": n5,
+        "n6": n6,
+    }
+    return name, preset, off
+
+
+def _decode_presets_blob(chunk: bytes) -> Dict[str, Any]:
+    if not chunk:
+        return {}
+    off = 0
+    if off + 1 > len(chunk):
+        raise ValueError("truncated")
+    count = chunk[off]
+    off += 1
+    out: Dict[str, Any] = {}
+    for _ in range(count):
+        name, preset, off = _decode_preset_record(chunk, off)
+        out[name] = preset
+    if off != len(chunk):
+        raise ValueError("presets blob length mismatch")
+    return out
+
+
+def _decode_select_blob(chunk: bytes) -> Dict[str, Any]:
+    if not chunk:
+        return {}
+    off = 0
+    if off + 1 > len(chunk):
+        raise ValueError("truncated")
+    count = chunk[off]
+    off += 1
+    out: Dict[str, Any] = {}
+    for _ in range(count):
+        if off + 1 > len(chunk):
+            raise ValueError("truncated")
+        dl = chunk[off]
+        off += 1
+        if off + dl > len(chunk):
+            raise ValueError("truncated")
+        device = chunk[off : off + dl].decode("utf-8")
+        off += dl
+        if off + 1 > len(chunk):
+            raise ValueError("truncated")
+        pl = chunk[off]
+        off += 1
+        if off + pl > len(chunk):
+            raise ValueError("truncated")
+        pname = chunk[off : off + pl].decode("utf-8")
+        off += pl
+        if off + 1 > len(chunk):
+            raise ValueError("truncated")
+        has_step = chunk[off]
+        off += 1
+        if has_step:
+            if off + 2 > len(chunk):
+                raise ValueError("truncated")
+            step = struct.unpack_from("<H", chunk, off)[0]
+            off += 2
+            out[device] = [pname, step]
+        else:
+            out[device] = [pname]
+    if off != len(chunk):
+        raise ValueError("select blob length mismatch")
+    return out
+
+
+def _decode_default_blob(chunk: bytes) -> Tuple[Optional[str], list]:
+    if not chunk:
+        return None, []
+    off = 0
+    if off + 1 > len(chunk):
+        raise ValueError("truncated")
+    nl = chunk[off]
+    off += 1
+    if off + nl > len(chunk):
+        raise ValueError("truncated")
+    default_name = chunk[off : off + nl].decode("utf-8") if nl else ""
+    off += nl
+    if off + 1 > len(chunk):
+        raise ValueError("truncated")
+    nt = chunk[off]
+    off += 1
+    targets: List[str] = []
+    for _ in range(nt):
+        if off + 1 > len(chunk):
+            raise ValueError("truncated")
+        tl = chunk[off]
+        off += 1
+        if off + tl > len(chunk):
+            raise ValueError("truncated")
+        targets.append(chunk[off : off + tl].decode("utf-8"))
+        off += tl
+    if off != len(chunk):
+        raise ValueError("default blob length mismatch")
+    return default_name, targets
+
+
+def parse_binary_envelope_v2(buf: bytes) -> Optional[Dict[str, Any]]:
+    """Decode native-binary v2 envelope into the v1 API dict shape."""
+    if not isinstance(buf, (bytes, bytearray)) or len(buf) < HEADER_LEN:
+        return None
+    if buf[0] != BINARY_ENVELOPE_VERSION_2:
+        return None
+    lp, ls, ld = buf[2], buf[3], buf[4]
+    need = HEADER_LEN + lp + ls + ld
+    if len(buf) != need:
+        return None
+
+    off = HEADER_LEN
+    presets_chunk = buf[off : off + lp]
+    off += lp
+    select_chunk = buf[off : off + ls]
+    off += ls
+    default_chunk = buf[off : off + ld]
+
+    data: Dict[str, Any] = {"v": "1"}
+    br = buf[1]
+    if br < 128:
+        data["b"] = brightness_0_255_from_wire(br)
+
+    try:
+        if lp:
+            data["presets"] = _decode_presets_blob(bytes(presets_chunk))
+        if ls:
+            data["select"] = _decode_select_blob(bytes(select_chunk))
+        if ld:
+            dname, targets = _decode_default_blob(bytes(default_chunk))
+            data["default"] = dname
+            data["targets"] = targets
+    except (ValueError, UnicodeError):
+        return None
+
+    return data
+
+
+def parse_binary_envelope_v1(buf: bytes) -> Optional[Dict[str, Any]]:
+    """
+    Decode legacy v1 bytes (JSON text blobs) into a v1 API dict.
+    Returns None if ``buf`` is not a valid v1 envelope.
+    """
+    if not isinstance(buf, (bytes, bytearray)) or len(buf) < HEADER_LEN:
+        return None
+    if buf[0] != BINARY_ENVELOPE_VERSION_1:
+        return None
+    lp, ls, ld = buf[2], buf[3], buf[4]
+    need = HEADER_LEN + lp + ls + ld
+    if len(buf) != need:
+        return None
+
+    off = HEADER_LEN
+    presets_chunk = buf[off : off + lp]
+    off += lp
+    select_chunk = buf[off : off + ls]
+    off += ls
+    default_chunk = buf[off : off + ld]
+
+    data: Dict[str, Any] = {"v": "1"}
+
+    br = buf[1]
+    if br < 128:
+        data["b"] = brightness_0_255_from_wire(br)
+
+    if lp:
+        try:
+            data["presets"] = json.loads(presets_chunk.decode("utf-8"))
+        except (ValueError, UnicodeError):
+            return None
+    if ls:
+        try:
+            data["select"] = json.loads(select_chunk.decode("utf-8"))
+        except (ValueError, UnicodeError):
+            return None
+    if ld:
+        try:
+            extra = json.loads(default_chunk.decode("utf-8"))
+        except (ValueError, UnicodeError):
+            return None
+        if isinstance(extra, dict):
+            for k, v in extra.items():
+                data[k] = v
+
+    return data
+
+
+def parse_binary_envelope(buf: bytes) -> Optional[Dict[str, Any]]:
+    """Try v2 (native binary), then v1 (JSON fragments)."""
+    d = parse_binary_envelope_v2(buf)
+    if d is not None:
+        return d
+    return parse_binary_envelope_v1(buf)
\ No newline at end of file
diff --git a/src/util/message.py b/src/util/message.py
new file mode 100644
index 0000000..7df8c38
--- /dev/null
+++ b/src/util/message.py
@@ -0,0 +1,25 @@
+"""JSON wire representation for controller messages (binary packing can replace later)."""
+
+from __future__ import annotations
+
+import json
+from typing import Any, Dict, Union
+
+
+class Message:
+    """Round-trip API dicts as compact UTF-8 JSON."""
+
+    def encode(self, data: Dict[str, Any]) -> bytes:
+        """Encode a JSON-serialisable mapping (typically a v1 API dict) to bytes."""
+        return json.dumps(data, separators=(",", ":")).encode("utf-8")
+
+    def decode(self, payload: Union[str, bytes, bytearray]) -> Dict[str, Any]:
+        """Decode UTF-8 JSON bytes or string into a dict."""
+        if isinstance(payload, (bytes, bytearray)):
+            text = payload.decode("utf-8")
+        else:
+            text = payload
+        obj = json.loads(text)
+        if not isinstance(obj, dict):
+            raise TypeError("JSON root must be an object")
+        return obj
diff --git a/tests/test_binary_envelope.py b/tests/test_binary_envelope.py
new file mode 100644
index 0000000..aeaf7d3
--- /dev/null
+++ b/tests/test_binary_envelope.py
@@ -0,0 +1,93 @@
+"""Tests for compact binary controller envelopes (host util)."""
+
+import sys
+from pathlib import Path
+
+PROJECT_ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(PROJECT_ROOT / "src"))
+
+from util.binary_envelope import (  # noqa: E402
+    BINARY_ENVELOPE_VERSION_2,
+    brightness_wire_from_0_255,
+    brightness_0_255_from_wire,
+    pack_binary_envelope_v2,
+    parse_binary_envelope,
+    parse_binary_envelope_v2,
+    parse_binary_envelope_v1,
+)
+
+
+def test_brightness_round_trip_extremes():
+    assert brightness_0_255_from_wire(brightness_wire_from_0_255(0)) == 0
+    assert brightness_0_255_from_wire(brightness_wire_from_0_255(255)) == 255
+
+
+def test_pack_parse_v2_brightness_only():
+    raw = pack_binary_envelope_v2(brightness_0_255=128)
+    assert raw[0] == BINARY_ENVELOPE_VERSION_2
+    data = parse_binary_envelope_v2(raw)
+    assert data == {"v": "1", "b": 128}
+
+
+def test_pack_parse_v2_full():
+    raw = pack_binary_envelope_v2(
+        presets={
+            "a": {
+                "p": "on",
+                "c": ["#ffffff"],
+                "d": 10,
+                "b": 255,
+                "a": True,
+                "n1": 1,
+                "n2": -2,
+                "n3": 3,
+                "n4": 4,
+                "n5": 5,
+                "n6": 6,
+            }
+        },
+        select={"dev": ["a"]},
+        default="a",
+        default_targets=["dev"],
+        brightness_0_255=64,
+    )
+    assert len(raw) <= 250
+    data = parse_binary_envelope_v2(raw)
+    assert data["v"] == "1"
+    assert data["b"] == 64
+    assert data["presets"]["a"]["p"] == "on"
+    assert data["presets"]["a"]["n2"] == -2
+    assert data["select"]["dev"] == ["a"]
+    assert data["default"] == "a"
+    assert data["targets"] == ["dev"]
+
+    merged = parse_binary_envelope(raw)
+    assert merged == data
+
+
+def test_v2_wire_not_utf8_json():
+    raw = pack_binary_envelope_v2(
+        presets={"x": {"p": "blink", "c": ["#112233"]}},
+        brightness_0_255=None,
+    )
+    assert raw[0] == BINARY_ENVELOPE_VERSION_2
+    assert parse_binary_envelope_v1(raw) is None
+
+
+def test_dont_change_brightness_v2():
+    raw = pack_binary_envelope_v2(brightness_0_255=None)
+    data = parse_binary_envelope_v2(raw)
+    assert "b" not in data
+
+
+def test_json_wire_not_v2():
+    assert parse_binary_envelope_v2(b'{"v":"1"}') is None
+
+
+def test_legacy_v1_parse_via_dispatcher():
+    import json
+
+    inner = json.dumps({"x": {"p": "on"}}, separators=(",", ":")).encode()
+    raw = bytes([1, 255, len(inner), 0, 0]) + inner
+    d = parse_binary_envelope(raw)
+    assert d["presets"]["x"]["p"] == "on"