Add BLE GATT server and ELM327 OBD-II emulator

Implement peripheral/GATT server support (14 tools) using BlueZ's
GattManager1 D-Bus API, enabling mcbluetooth to act as a BLE device
that advertises, accepts connections, and handles reads/writes/notifications.

Add ELM327 emulator (7 tools) that builds on the GATT server to emulate
a BLE OBD-II adapter over Nordic UART Service. Handles AT commands,
Mode 01/03/04/09 queries, configurable PIDs, DTCs, and VIN with
auto-response — no LLM round-trip needed for protocol handling.

New files:
- gatt_server.py: D-Bus ServiceInterface hierarchy + GattServerManager
- tools/gatt_server.py: MCP tool wrappers for server lifecycle
- tools/bt_elm327_emu.py: ELM327 protocol handler + NUS setup

Also adds 2 MCP resources (bluetooth://gatt/server, .../writes).

pyproject.toml

@@ -69,6 +69,7 @@ ignore = ["E501"]
 [tool.ruff.lint.per-file-ignores]
 # dbus-fast uses D-Bus type signatures ("o", "h", "a{sv}") as annotations
 "src/mcbluetooth/hfp_ag.py" = ["F821", "F722"]
+"src/mcbluetooth/gatt_server.py" = ["F821", "F722"]
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"

src/mcbluetooth/gatt_server.py

@@ -0,0 +1,717 @@
+"""BLE GATT Server implementation for BlueZ.
+
+Registers as a BLE peripheral via BlueZ's GattManager1 D-Bus API, allowing
+mcbluetooth to act as a GATT server. Remote BLE central devices can discover
+services, read/write characteristics, and subscribe to notifications.
+
+D-Bus hierarchy exported to BlueZ:
+    /mcbluetooth/gatt                           ObjectManager
+    /mcbluetooth/gatt/serviceN                  GattService1
+    /mcbluetooth/gatt/serviceN/charM            GattCharacteristic1
+    /mcbluetooth/gatt/serviceN/charM/descK      GattDescriptor1
+    /mcbluetooth/gatt/adv0                      LEAdvertisement1
+
+Reference patterns: agent.py (ServiceInterface, pending requests),
+hfp_ag.py (dedicated bus, module singleton, ProfileManager registration).
+"""
+
+import asyncio
+import logging
+from collections import deque
+from dataclasses import dataclass, field
+from datetime import UTC, datetime
+from typing import Any
+
+from dbus_fast import BusType, Message, MessageType, Variant
+from dbus_fast.aio import MessageBus
+from dbus_fast.service import ServiceInterface, dbus_property, method
+
+log = logging.getLogger(__name__)
+
+BLUEZ_SERVICE = "org.bluez"
+GATT_MANAGER_IFACE = "org.bluez.GattManager1"
+LE_ADV_MANAGER_IFACE = "org.bluez.LEAdvertisingManager1"
+APP_BASE_PATH = "/mcbluetooth/gatt"
+ADV_PATH = "/mcbluetooth/gatt/adv0"
+
+
+# ==================== Data Classes ====================
+
+
+@dataclass
+class WriteEvent:
+    """A write received from a remote BLE central device."""
+
+    index: int
+    timestamp: str
+    char_id: str
+    char_uuid: str
+    value: bytes
+    value_hex: str
+    value_string: str | None
+    device: str | None
+
+    def to_dict(self) -> dict[str, Any]:
+        d: dict[str, Any] = {
+            "index": self.index,
+            "timestamp": self.timestamp,
+            "char_id": self.char_id,
+            "char_uuid": self.char_uuid,
+            "value_hex": self.value_hex,
+            "length": len(self.value),
+        }
+        if self.value_string is not None:
+            d["value_string"] = self.value_string
+        if self.device:
+            d["device"] = self.device
+        return d
+
+
+@dataclass
+class ServerCharacteristic:
+    """Internal state for a server-side GATT characteristic."""
+
+    char_id: str
+    path: str
+    uuid: str
+    flags: list[str]
+    value: bytes = b""
+    notifying: bool = False
+    service_id: str = ""
+    desc_count: int = 0
+    dbus_obj: Any = field(default=None, repr=False)
+
+
+@dataclass
+class ServerDescriptor:
+    """Internal state for a server-side GATT descriptor."""
+
+    desc_id: str
+    path: str
+    uuid: str
+    flags: list[str]
+    value: bytes = b""
+    char_id: str = ""
+    dbus_obj: Any = field(default=None, repr=False)
+
+
+@dataclass
+class ServerService:
+    """Internal state for a server-side GATT service."""
+
+    service_id: str
+    path: str
+    uuid: str
+    primary: bool = True
+    char_count: int = 0
+    characteristics: dict[str, ServerCharacteristic] = field(default_factory=dict)
+    dbus_obj: Any = field(default=None, repr=False)
+
+
+# ==================== D-Bus Interfaces ====================
+
+
+class GattApplication(ServiceInterface):
+    """ObjectManager for the GATT application root.
+
+    BlueZ calls GetManagedObjects() on this path to discover all
+    services, characteristics, and descriptors we're hosting.
+    """
+
+    def __init__(self, manager: "GattServerManager"):
+        super().__init__("org.freedesktop.DBus.ObjectManager")
+        self._manager = manager
+
+    @method()
+    def GetManagedObjects(self) -> "a{oa{sa{sv}}}":  # noqa: F821
+        return self._manager._build_managed_objects()
+
+
+class GattServiceIface(ServiceInterface):
+    """org.bluez.GattService1 — exported at each service path.
+
+    No D-Bus methods; properties served via GetManagedObjects only.
+    """
+
+    def __init__(self):
+        super().__init__("org.bluez.GattService1")
+
+
+class GattCharacteristicIface(ServiceInterface):
+    """org.bluez.GattCharacteristic1 — handles reads/writes from remotes.
+
+    ReadValue returns the stored value. WriteValue stores the value AND
+    records a WriteEvent for LLM polling (like agent.py's pending_requests).
+    """
+
+    def __init__(self, char: ServerCharacteristic, manager: "GattServerManager"):
+        super().__init__("org.bluez.GattCharacteristic1")
+        self._char = char
+        self._manager = manager
+
+    @method()
+    def ReadValue(self, options: "a{sv}") -> "ay":  # noqa: F821
+        offset = 0
+        opt_offset = options.get("offset")
+        if opt_offset is not None:
+            offset = opt_offset.value if hasattr(opt_offset, "value") else int(opt_offset)
+        return list(self._char.value[offset:])
+
+    @method()
+    def WriteValue(self, value: "ay", options: "a{sv}") -> None:  # noqa: F821
+        new_bytes = bytes(value)
+        offset = 0
+        opt_offset = options.get("offset")
+        if opt_offset is not None:
+            offset = opt_offset.value if hasattr(opt_offset, "value") else int(opt_offset)
+
+        if offset > 0:
+            self._char.value = self._char.value[:offset] + new_bytes
+        else:
+            self._char.value = new_bytes
+
+        device = None
+        opt_device = options.get("device")
+        if opt_device is not None:
+            device = opt_device.value if hasattr(opt_device, "value") else str(opt_device)
+
+        self._manager._record_write(self._char.char_id, self._char.uuid, self._char.value, device)
+
+    @method()
+    def StartNotify(self) -> None:
+        self._char.notifying = True
+        log.info("GATT server: StartNotify on %s (%s)", self._char.char_id, self._char.uuid)
+
+    @method()
+    def StopNotify(self) -> None:
+        self._char.notifying = False
+        log.info("GATT server: StopNotify on %s (%s)", self._char.char_id, self._char.uuid)
+
+
+class GattDescriptorIface(ServiceInterface):
+    """org.bluez.GattDescriptor1 — handles reads/writes on descriptors."""
+
+    def __init__(self, desc: ServerDescriptor):
+        super().__init__("org.bluez.GattDescriptor1")
+        self._desc = desc
+
+    @method()
+    def ReadValue(self, options: "a{sv}") -> "ay":  # noqa: F821
+        offset = 0
+        opt_offset = options.get("offset")
+        if opt_offset is not None:
+            offset = opt_offset.value if hasattr(opt_offset, "value") else int(opt_offset)
+        return list(self._desc.value[offset:])
+
+    @method()
+    def WriteValue(self, value: "ay", options: "a{sv}") -> None:  # noqa: F821
+        self._desc.value = bytes(value)
+
+
+class LEAdvertisementIface(ServiceInterface):
+    """org.bluez.LEAdvertisement1 — BLE advertising data.
+
+    BlueZ reads advertisement properties via Properties.GetAll, so these
+    must be exposed as @dbus_property (unlike GATT objects which use
+    GetManagedObjects).
+    """
+
+    def __init__(self, adv_type: str, local_name: str, service_uuids: list[str]):
+        super().__init__("org.bluez.LEAdvertisement1")
+        self._type = adv_type
+        self._local_name = local_name
+        self._service_uuids = service_uuids
+
+    @method()
+    def Release(self) -> None:
+        log.info("GATT server: advertisement released by BlueZ")
+
+    @dbus_property()
+    def Type(self) -> "s":  # noqa: F821
+        return self._type
+
+    @dbus_property()
+    def LocalName(self) -> "s":  # noqa: F821
+        return self._local_name
+
+    @dbus_property()
+    def ServiceUUIDs(self) -> "as":  # noqa: F821
+        return self._service_uuids
+
+
+# ==================== GATT Server Manager ====================
+
+
+class GattServerManager:
+    """Manages the GATT server lifecycle and state.
+
+    Coordinates D-Bus object creation, BlueZ registration,
+    advertising, and write event buffering.
+    """
+
+    def __init__(self):
+        self._bus: MessageBus | None = None
+        self._app: GattApplication | None = None
+        self._adv: LEAdvertisementIface | None = None
+        self._services: dict[str, ServerService] = {}
+        self._characteristics: dict[str, ServerCharacteristic] = {}
+        self._descriptors: dict[str, ServerDescriptor] = {}
+        self._write_events: deque[WriteEvent] = deque(maxlen=200)
+        self._write_index: int = 0
+        self._service_count: int = 0
+        self._registered: bool = False
+        self._advertising: bool = False
+        self._write_callback: Any = None  # async callable(char_id, value, device)
+
+    # ---- Service construction ----
+
+    def add_service(self, uuid: str, primary: bool = True) -> str:
+        """Add a GATT service. Returns the service_id (e.g. 'service0')."""
+        service_id = f"service{self._service_count}"
+        self._service_count += 1
+        path = f"{APP_BASE_PATH}/{service_id}"
+
+        svc = ServerService(
+            service_id=service_id,
+            path=path,
+            uuid=uuid,
+            primary=primary,
+        )
+        svc.dbus_obj = GattServiceIface()
+        self._services[service_id] = svc
+
+        log.info("GATT server: added service %s (UUID=%s)", service_id, uuid)
+        return service_id
+
+    def add_characteristic(
+        self,
+        service_id: str,
+        uuid: str,
+        flags: list[str],
+        value: bytes = b"",
+    ) -> str:
+        """Add a characteristic to a service. Returns the char_id."""
+        svc = self._services.get(service_id)
+        if not svc:
+            raise ValueError(f"Service '{service_id}' not found")
+
+        char_idx = svc.char_count
+        svc.char_count += 1
+        char_id = f"{service_id}/char{char_idx}"
+        path = f"{APP_BASE_PATH}/{char_id}"
+
+        char = ServerCharacteristic(
+            char_id=char_id,
+            path=path,
+            uuid=uuid,
+            flags=flags,
+            value=value,
+            service_id=service_id,
+        )
+        char.dbus_obj = GattCharacteristicIface(char, self)
+        svc.characteristics[char_id] = char
+        self._characteristics[char_id] = char
+
+        log.info(
+            "GATT server: added characteristic %s (UUID=%s, flags=%s)",
+            char_id,
+            uuid,
+            flags,
+        )
+        return char_id
+
+    def add_descriptor(
+        self,
+        char_id: str,
+        uuid: str,
+        flags: list[str],
+        value: bytes = b"",
+    ) -> str:
+        """Add a descriptor to a characteristic. Returns the desc_id."""
+        char = self._characteristics.get(char_id)
+        if not char:
+            raise ValueError(f"Characteristic '{char_id}' not found")
+
+        desc_idx = char.desc_count
+        char.desc_count += 1
+        desc_id = f"{char_id}/desc{desc_idx}"
+        path = f"{APP_BASE_PATH}/{desc_id}"
+
+        desc = ServerDescriptor(
+            desc_id=desc_id,
+            path=path,
+            uuid=uuid,
+            flags=flags,
+            value=value,
+            char_id=char_id,
+        )
+        desc.dbus_obj = GattDescriptorIface(desc)
+        self._descriptors[desc_id] = desc
+
+        log.info("GATT server: added descriptor %s (UUID=%s)", desc_id, uuid)
+        return desc_id
+
+    def clear(self) -> None:
+        """Remove all services, characteristics, and descriptors."""
+        if self._registered:
+            raise RuntimeError("Cannot clear while registered — unregister first")
+
+        self._services.clear()
+        self._characteristics.clear()
+        self._descriptors.clear()
+        self._service_count = 0
+
+    # ---- Registration with BlueZ ----
+
+    async def register(self, adapter: str) -> None:
+        """Register the GATT application with BlueZ GattManager1."""
+        if self._registered:
+            return
+
+        if not self._services:
+            raise ValueError("No services defined — add at least one service first")
+
+        # Dedicated bus connection (like hfp_ag.py)
+        if not self._bus:
+            self._bus = await MessageBus(bus_type=BusType.SYSTEM).connect()
+
+        # Export ObjectManager at application root
+        self._app = GattApplication(self)
+        self._bus.export(APP_BASE_PATH, self._app)
+
+        # Export all service/characteristic/descriptor D-Bus objects
+        for svc in self._services.values():
+            self._bus.export(svc.path, svc.dbus_obj)
+            for char in svc.characteristics.values():
+                self._bus.export(char.path, char.dbus_obj)
+        for desc in self._descriptors.values():
+            self._bus.export(desc.path, desc.dbus_obj)
+
+        # Register with BlueZ
+        adapter_path = f"/org/bluez/{adapter}"
+        introspection = await self._bus.introspect(BLUEZ_SERVICE, adapter_path)
+        proxy = self._bus.get_proxy_object(BLUEZ_SERVICE, adapter_path, introspection)
+        gatt_mgr = proxy.get_interface(GATT_MANAGER_IFACE)
+
+        try:
+            await gatt_mgr.call_register_application(APP_BASE_PATH, {})
+            self._registered = True
+            log.info("GATT server: registered with BlueZ (%s)", adapter)
+        except Exception as e:
+            if "Already Exists" in str(e):
+                log.info("GATT server: stale registration — re-registering")
+                try:
+                    await gatt_mgr.call_unregister_application(APP_BASE_PATH)
+                except Exception:
+                    pass
+                await gatt_mgr.call_register_application(APP_BASE_PATH, {})
+                self._registered = True
+                log.info("GATT server: re-registered with BlueZ (%s)", adapter)
+            else:
+                raise
+
+    async def unregister(self, adapter: str) -> None:
+        """Unregister the GATT application from BlueZ."""
+        if self._advertising:
+            await self.set_advertising(adapter, False)
+
+        if self._bus and self._registered:
+            try:
+                adapter_path = f"/org/bluez/{adapter}"
+                introspection = await self._bus.introspect(BLUEZ_SERVICE, adapter_path)
+                proxy = self._bus.get_proxy_object(BLUEZ_SERVICE, adapter_path, introspection)
+                gatt_mgr = proxy.get_interface(GATT_MANAGER_IFACE)
+                await gatt_mgr.call_unregister_application(APP_BASE_PATH)
+            except Exception:
+                pass
+            self._registered = False
+
+        # Disconnect bus (cleans up all exports)
+        if self._bus:
+            self._bus.disconnect()
+            self._bus = None
+
+        self._app = None
+        self._adv = None
+        log.info("GATT server: unregistered")
+
+    # ---- Advertising ----
+
+    async def set_advertising(
+        self,
+        adapter: str,
+        enable: bool,
+        name: str = "mcbluetooth",
+        service_uuids: list[str] | None = None,
+    ) -> None:
+        """Start or stop BLE advertising."""
+        if not self._bus:
+            raise RuntimeError("Bus not connected — register first")
+
+        adapter_path = f"/org/bluez/{adapter}"
+        introspection = await self._bus.introspect(BLUEZ_SERVICE, adapter_path)
+        proxy = self._bus.get_proxy_object(BLUEZ_SERVICE, adapter_path, introspection)
+        adv_mgr = proxy.get_interface(LE_ADV_MANAGER_IFACE)
+
+        if enable:
+            if self._advertising:
+                return
+
+            uuids = service_uuids or [svc.uuid for svc in self._services.values()]
+            self._adv = LEAdvertisementIface("peripheral", name, uuids)
+            self._bus.export(ADV_PATH, self._adv)
+
+            try:
+                await adv_mgr.call_register_advertisement(ADV_PATH, {})
+                self._advertising = True
+                log.info("GATT server: advertising started (name=%s)", name)
+            except Exception as e:
+                if "Already Exists" in str(e):
+                    try:
+                        await adv_mgr.call_unregister_advertisement(ADV_PATH)
+                    except Exception:
+                        pass
+                    await adv_mgr.call_register_advertisement(ADV_PATH, {})
+                    self._advertising = True
+                    log.info("GATT server: advertising restarted (name=%s)", name)
+                else:
+                    raise
+        else:
+            if not self._advertising:
+                return
+            try:
+                await adv_mgr.call_unregister_advertisement(ADV_PATH)
+            except Exception:
+                pass
+            self._advertising = False
+            self._adv = None
+            log.info("GATT server: advertising stopped")
+
+    # ---- Value management ----
+
+    def set_value(self, char_id: str, value: bytes) -> None:
+        """Set characteristic value and auto-notify if subscribed."""
+        char = self._characteristics.get(char_id)
+        if not char:
+            raise ValueError(f"Characteristic '{char_id}' not found")
+
+        char.value = value
+        if char.notifying:
+            self._emit_notification(char)
+
+    def get_value(self, char_id: str) -> bytes:
+        """Get current characteristic value."""
+        char = self._characteristics.get(char_id)
+        if not char:
+            raise ValueError(f"Characteristic '{char_id}' not found")
+        return char.value
+
+    def notify(self, char_id: str) -> bool:
+        """Explicitly send notification for current value."""
+        char = self._characteristics.get(char_id)
+        if not char:
+            raise ValueError(f"Characteristic '{char_id}' not found")
+        if not char.notifying:
+            return False
+        self._emit_notification(char)
+        return True
+
+    def _emit_notification(self, char: ServerCharacteristic) -> None:
+        """Emit PropertiesChanged signal to trigger BLE notification."""
+        if not self._bus:
+            return
+
+        msg = Message(
+            message_type=MessageType.SIGNAL,
+            path=char.path,
+            interface="org.freedesktop.DBus.Properties",
+            member="PropertiesChanged",
+            signature="sa{sv}as",
+            body=[
+                "org.bluez.GattCharacteristic1",
+                {"Value": Variant("ay", list(char.value))},
+                [],
+            ],
+        )
+        self._bus.send_message(msg)
+        log.debug("GATT server: notification sent on %s (%d bytes)", char.char_id, len(char.value))
+
+    # ---- Write event monitoring ----
+
+    def _record_write(self, char_id: str, char_uuid: str, value: bytes, device: str | None) -> None:
+        """Record a write event and fire callback if registered."""
+        event = WriteEvent(
+            index=self._write_index,
+            timestamp=datetime.now(UTC).isoformat(),
+            char_id=char_id,
+            char_uuid=char_uuid,
+            value=value,
+            value_hex=value.hex(),
+            value_string=_try_decode(value),
+            device=device,
+        )
+        self._write_events.append(event)
+        self._write_index += 1
+
+        log.debug(
+            "GATT server: write on %s from %s (%d bytes)",
+            char_id,
+            device or "unknown",
+            len(value),
+        )
+
+        # Fire async callback (e.g. ELM327 emulator auto-responder)
+        if self._write_callback:
+            try:
+                loop = asyncio.get_running_loop()
+                loop.create_task(self._write_callback(char_id, value, device))
+            except RuntimeError:
+                pass
+
+    def get_write_events(
+        self,
+        since_index: int = 0,
+        char_id: str | None = None,
+        limit: int = 50,
+    ) -> list[dict[str, Any]]:
+        """Get write events, optionally filtered."""
+        events = [
+            e
+            for e in self._write_events
+            if e.index >= since_index and (char_id is None or e.char_id == char_id)
+        ]
+        return [e.to_dict() for e in events[:limit]]
+
+    def clear_write_events(self) -> int:
+        """Clear all write events. Returns count cleared."""
+        count = len(self._write_events)
+        self._write_events.clear()
+        return count
+
+    # ---- GetManagedObjects builder ----
+
+    def _build_managed_objects(self) -> dict[str, dict[str, dict[str, Variant]]]:
+        """Build the response for ObjectManager.GetManagedObjects().
+
+        Returns the complete D-Bus object hierarchy that BlueZ uses
+        to discover our GATT services, characteristics, and descriptors.
+        """
+        objects: dict[str, dict[str, dict[str, Variant]]] = {}
+
+        for svc in self._services.values():
+            char_paths = [c.path for c in svc.characteristics.values()]
+            objects[svc.path] = {
+                "org.bluez.GattService1": {
+                    "UUID": Variant("s", svc.uuid),
+                    "Primary": Variant("b", svc.primary),
+                    "Characteristics": Variant("ao", char_paths),
+                }
+            }
+
+            for char in svc.characteristics.values():
+                desc_paths = [
+                    d.path for d in self._descriptors.values() if d.char_id == char.char_id
+                ]
+                objects[char.path] = {
+                    "org.bluez.GattCharacteristic1": {
+                        "UUID": Variant("s", char.uuid),
+                        "Service": Variant("o", svc.path),
+                        "Flags": Variant("as", char.flags),
+                        "Descriptors": Variant("ao", desc_paths),
+                    }
+                }
+
+        for desc in self._descriptors.values():
+            char = self._characteristics.get(desc.char_id)
+            if char:
+                objects[desc.path] = {
+                    "org.bluez.GattDescriptor1": {
+                        "UUID": Variant("s", desc.uuid),
+                        "Characteristic": Variant("o", char.path),
+                        "Flags": Variant("as", desc.flags),
+                    }
+                }
+
+        return objects
+
+    # ---- Status ----
+
+    def get_status(self) -> dict[str, Any]:
+        """Get overall GATT server status."""
+        services = []
+        for svc in self._services.values():
+            chars = []
+            for char in svc.characteristics.values():
+                descs = [
+                    {
+                        "desc_id": d.desc_id,
+                        "uuid": d.uuid,
+                        "flags": d.flags,
+                    }
+                    for d in self._descriptors.values()
+                    if d.char_id == char.char_id
+                ]
+                chars.append(
+                    {
+                        "char_id": char.char_id,
+                        "uuid": char.uuid,
+                        "flags": char.flags,
+                        "value_length": len(char.value),
+                        "notifying": char.notifying,
+                        "descriptors": descs,
+                    }
+                )
+            services.append(
+                {
+                    "service_id": svc.service_id,
+                    "uuid": svc.uuid,
+                    "primary": svc.primary,
+                    "characteristics": chars,
+                }
+            )
+
+        return {
+            "registered": self._registered,
+            "advertising": self._advertising,
+            "services": services,
+            "write_event_count": len(self._write_events),
+            "write_event_total": self._write_index,
+        }
+
+
+# ==================== Module-level lifecycle ====================
+
+_manager: GattServerManager | None = None
+
+
+def get_gatt_server() -> GattServerManager:
+    """Get or create the global GATT server manager (singleton)."""
+    global _manager
+    if _manager is None:
+        _manager = GattServerManager()
+    return _manager
+
+
+async def shutdown_gatt_server(adapter: str) -> None:
+    """Shut down the GATT server completely."""
+    global _manager
+    if _manager:
+        try:
+            await _manager.unregister(adapter)
+        except Exception:
+            pass
+        _manager = None
+
+
+# ==================== Helpers ====================
+
+
+def _try_decode(value: bytes) -> str | None:
+    """Try to decode bytes as printable UTF-8."""
+    try:
+        decoded = value.decode("utf-8")
+        if all(c.isprintable() or c in "\r\n\t" for c in decoded):
+            return decoded
+        return None
+    except (UnicodeDecodeError, ValueError):
+        return None

src/mcbluetooth/resources.py

@@ -17,6 +17,7 @@ from dataclasses import asdict
 from fastmcp import FastMCP
 
 from mcbluetooth.dbus_client import get_client, get_notify_manager
+from mcbluetooth.gatt_server import get_gatt_server
 
 
 def register_resources(mcp: FastMCP) -> None:
@@ -307,3 +308,43 @@ def register_resources(mcp: FastMCP) -> None:
             },
             indent=2,
         )
+
+    # ==================== GATT Server Resources ====================
+
+    @mcp.resource(
+        "bluetooth://gatt/server",
+        name="GATT Server State",
+        description=(
+            "Current state of the BLE GATT server (peripheral mode). "
+            "Shows registered services, characteristics, advertising status, "
+            "and write event statistics."
+        ),
+        mime_type="application/json",
+    )
+    async def resource_gatt_server() -> str:
+        """Get GATT server state."""
+        mgr = get_gatt_server()
+        return json.dumps(mgr.get_status(), indent=2)
+
+    @mcp.resource(
+        "bluetooth://gatt/server/writes",
+        name="GATT Server Write Events",
+        description=(
+            "Write events received from remote BLE central devices. "
+            "Shows the most recent writes to server characteristics "
+            "with timestamps, values, and source device info."
+        ),
+        mime_type="application/json",
+    )
+    async def resource_gatt_server_writes() -> str:
+        """Get recent write events from remote clients."""
+        mgr = get_gatt_server()
+        events = mgr.get_write_events(since_index=0, limit=50)
+        return json.dumps(
+            {
+                "count": len(events),
+                "total": mgr._write_index,
+                "events": events,
+            },
+            indent=2,
+        )

src/mcbluetooth/server.py

@@ -3,7 +3,17 @@
 from fastmcp import FastMCP
 
 from mcbluetooth import resources
-from mcbluetooth.tools import adapter, audio, ble, device, hfp, monitor, obex
+from mcbluetooth.tools import (
+    adapter,
+    audio,
+    ble,
+    bt_elm327_emu,
+    device,
+    gatt_server,
+    hfp,
+    monitor,
+    obex,
+)
 
 mcp = FastMCP(
     name="mcbluetooth",
@@ -36,6 +46,25 @@ To capture BLE notifications:
 3. Read notifications: Use the bluetooth://ble/{address}/{uuid}/notifications resource
 4. Subscribe to the resource for real-time updates (client-side)
 
+### GATT Server Resources (peripheral mode)
+- bluetooth://gatt/server - GATT server state (registered, services, advertising)
+- bluetooth://gatt/server/writes - Write events from remote clients
+
+To act as a BLE peripheral (GATT server):
+1. Add services: bt_gatt_server_add_service(uuid)
+2. Add characteristics: bt_gatt_server_add_characteristic(service_id, uuid, flags)
+3. Register: bt_gatt_server_register(adapter)
+4. Advertise: bt_gatt_server_advertise(adapter, enable=True, name="MyDevice")
+5. Monitor writes: bt_gatt_server_read_writes(since_index=0)
+6. Respond: bt_gatt_server_set_value(char_id, value) — auto-notifies subscribers
+
+### ELM327 OBD-II Emulator
+For quick OBD-II BLE adapter emulation (Nordic UART Service):
+1. bt_elm327_emu_start(adapter, name="OBDII") — sets up NUS, advertises
+2. bt_elm327_emu_set_pid(0x0C, "0BB8", "hex") — set RPM to 750
+3. bt_elm327_emu_read_commands() — see what the OBD app sent
+4. bt_elm327_emu_stop(adapter) — clean up
+
 ## Tools
 All tools require an explicit 'adapter' parameter (e.g., "hci0").
 Use bt_list_adapters() to discover available adapters.
@@ -56,6 +85,8 @@ device.register_tools(mcp)
 audio.register_tools(mcp)
 hfp.register_tools(mcp)
 ble.register_tools(mcp)
+gatt_server.register_tools(mcp)
+bt_elm327_emu.register_tools(mcp)
 monitor.register_tools(mcp)
 obex.register_tools(mcp)
 

src/mcbluetooth/tools/bt_elm327_emu.py

@@ -0,0 +1,678 @@
+"""ELM327 OBD-II BLE Emulator.
+
+Emulates an ELM327 v1.5 adapter over BLE using Nordic UART Service (NUS).
+OBD-II apps (Torque, Car Scanner, etc.) connect via BLE, send AT commands
+and OBD-II PID queries, and receive simulated responses.
+
+The emulator handles the full ELM327 protocol:
+    - AT command set (ATZ, ATE0, ATH1, ATSP, etc.)
+    - Mode 01 (live data) with configurable PID values
+    - Mode 03 (stored DTCs)
+    - Mode 09 (VIN query)
+    - Supported PID bitmaps auto-generated from configured PIDs
+
+NUS UUIDs (same as real BLE OBD adapters):
+    Service: 6E400001-B5A3-F393-E0A9-E50E24DCCA9E
+    RX (write):  6E400002-... (client writes commands here)
+    TX (notify): 6E400003-... (server sends responses here)
+"""
+
+from __future__ import annotations
+
+import logging
+from collections import deque
+from dataclasses import dataclass
+from datetime import UTC, datetime
+from typing import Any
+
+from fastmcp import FastMCP
+
+from mcbluetooth.gatt_server import get_gatt_server
+
+log = logging.getLogger(__name__)
+
+# Nordic UART Service UUIDs
+NUS_SERVICE_UUID = "6e400001-b5a3-f393-e0a9-e50e24dcca9e"
+NUS_RX_CHAR_UUID = "6e400002-b5a3-f393-e0a9-e50e24dcca9e"  # Client writes here
+NUS_TX_CHAR_UUID = "6e400003-b5a3-f393-e0a9-e50e24dcca9e"  # Server notifies here
+
+# Default ECU header (primary ECU response address)
+ECU_HEADER = "7E8"
+
+# Common OBD-II PIDs with default values
+DEFAULT_PIDS: dict[int, tuple[bytes, str]] = {
+    0x05: (bytes([70]), "Coolant temp (°C, offset -40) → 30°C"),
+    0x0C: (bytes([0x0B, 0xB8]), "Engine RPM (× ¼) → 750 RPM"),
+    0x0D: (bytes([0]), "Vehicle speed (km/h) → 0"),
+    0x0F: (bytes([55]), "Intake air temp (°C, offset -40) → 15°C"),
+    0x11: (bytes([25]), "Throttle position (%) → ~10%"),
+    0x1F: (bytes([0x00, 0x00]), "Runtime since start (sec) → 0"),
+    0x2F: (bytes([51]), "Fuel tank level (%) → 20%"),
+    0x46: (bytes([60]), "Ambient air temp (°C, offset -40) → 20°C"),
+}
+
+
+@dataclass
+class CommandRecord:
+    """A command received from the OBD-II client."""
+
+    index: int
+    timestamp: str
+    raw: str
+    parsed: str
+    response: str
+
+
+class ELM327Emulator:
+    """ELM327 AT command and OBD-II protocol handler."""
+
+    def __init__(self):
+        self.device_id = "ELM327 v1.5"
+        self.protocol = "6"  # ISO 15765-4 CAN (11/500)
+        self.echo = False
+        self.headers = True
+        self.spaces = False
+        self.linefeed = False
+        self.pid_values: dict[int, bytes] = {}
+        self.dtc_codes: list[str] = []
+        self.vin = "1MCBT00TESTING12345"
+        self._commands: deque[CommandRecord] = deque(maxlen=200)
+        self._cmd_index = 0
+        self._started = False
+        self._rx_char_id: str | None = None
+        self._tx_char_id: str | None = None
+        self._adapter: str | None = None
+        self._buf = ""  # Accumulates partial writes
+
+    def set_defaults(self) -> None:
+        """Load default PID values for a realistic idle vehicle."""
+        for pid, (value, _desc) in DEFAULT_PIDS.items():
+            self.pid_values[pid] = value
+
+    def handle_data(self, data: bytes) -> str | None:
+        """Process incoming bytes, return response when a complete command is received.
+
+        ELM327 commands are terminated with CR (\\r). Accumulates partial
+        writes until a complete command is received.
+        """
+        try:
+            text = data.decode("utf-8", errors="replace")
+        except Exception:
+            return None
+
+        self._buf += text
+
+        # Check for command terminator
+        if "\r" not in self._buf:
+            return None
+
+        # Split on CR, process first complete command
+        line, self._buf = self._buf.split("\r", 1)
+        line = line.strip()
+        if not line:
+            return None
+
+        response = self._process_command(line)
+
+        # Record command
+        record = CommandRecord(
+            index=self._cmd_index,
+            timestamp=datetime.now(UTC).isoformat(),
+            raw=line,
+            parsed=line.upper().strip(),
+            response=response.replace("\r", "\\r").replace("\n", "\\n"),
+        )
+        self._commands.append(record)
+        self._cmd_index += 1
+
+        return response
+
+    def _process_command(self, line: str) -> str:
+        """Parse and handle an AT or OBD command."""
+        cmd = line.strip().upper()
+
+        if cmd.startswith("AT"):
+            return self._handle_at(cmd)
+
+        # OBD-II query
+        return self._handle_obd(cmd)
+
+    def _handle_at(self, cmd: str) -> str:
+        """Handle ELM327 AT commands."""
+        cr = "\r\n" if self.linefeed else "\r"
+
+        if cmd == "ATZ":
+            self.echo = False
+            self.headers = True
+            self.spaces = False
+            return f"{cr}{self.device_id}{cr}{cr}>"
+
+        if cmd == "ATI":
+            return f"{self.device_id}{cr}{cr}>"
+
+        if cmd == "AT@1":
+            return f"mcbluetooth OBD-II Emulator{cr}{cr}>"
+
+        if cmd == "ATE0":
+            self.echo = False
+            return f"OK{cr}{cr}>"
+
+        if cmd == "ATE1":
+            self.echo = True
+            return f"OK{cr}{cr}>"
+
+        if cmd == "ATH0":
+            self.headers = False
+            return f"OK{cr}{cr}>"
+
+        if cmd == "ATH1":
+            self.headers = True
+            return f"OK{cr}{cr}>"
+
+        if cmd == "ATS0":
+            self.spaces = False
+            return f"OK{cr}{cr}>"
+
+        if cmd == "ATS1":
+            self.spaces = True
+            return f"OK{cr}{cr}>"
+
+        if cmd.startswith("ATL"):
+            self.linefeed = cmd.endswith("1")
+            cr = "\r\n" if self.linefeed else "\r"
+            return f"OK{cr}{cr}>"
+
+        if cmd == "ATDPN":
+            return f"A{self.protocol}{cr}{cr}>"
+
+        if cmd == "ATDP":
+            protocols = {
+                "0": "AUTO",
+                "6": "ISO 15765-4 (CAN 11/500)",
+                "7": "ISO 15765-4 (CAN 29/500)",
+                "8": "ISO 15765-4 (CAN 11/250)",
+                "9": "ISO 15765-4 (CAN 29/250)",
+            }
+            name = protocols.get(self.protocol, "AUTO")
+            return f"{name}{cr}{cr}>"
+
+        if cmd == "ATRV":
+            return f"12.6V{cr}{cr}>"
+
+        # Commands that just get OK
+        ok_prefixes = (
+            "ATSP",
+            "ATST",
+            "ATAT",
+            "ATM",
+            "ATCAF",
+            "ATD",
+            "ATCE",
+            "ATCF",
+            "ATCM",
+            "ATSH",
+            "ATFC",
+            "ATWM",
+            "ATMA",
+            "ATAR",
+            "ATPC",
+        )
+        for prefix in ok_prefixes:
+            if cmd.startswith(prefix):
+                return f"OK{cr}{cr}>"
+
+        if cmd == "AT":
+            return f"OK{cr}{cr}>"
+
+        return f"?{cr}{cr}>"
+
+    def _handle_obd(self, cmd: str) -> str:
+        """Handle OBD-II service queries."""
+        cr = "\r\n" if self.linefeed else "\r"
+        cmd = cmd.replace(" ", "")
+
+        if len(cmd) < 4:
+            return f"?{cr}{cr}>"
+
+        try:
+            mode = int(cmd[:2], 16)
+            pid = int(cmd[2:4], 16)
+        except ValueError:
+            return f"?{cr}{cr}>"
+
+        if mode == 0x01:
+            return self._handle_mode01(pid) + f"{cr}>"
+        if mode == 0x03:
+            return self._handle_mode03() + f"{cr}>"
+        if mode == 0x04:
+            self.dtc_codes.clear()
+            return f"44{cr}{cr}>"
+        if mode == 0x09:
+            return self._handle_mode09(pid) + f"{cr}>"
+
+        return f"NO DATA{cr}{cr}>"
+
+    def _handle_mode01(self, pid: int) -> str:
+        """Handle Mode 01 (current data) queries."""
+        cr = "\r\n" if self.linefeed else "\r"
+
+        # Supported PID bitmaps
+        if pid in (0x00, 0x20, 0x40, 0x60, 0x80, 0xA0, 0xC0, 0xE0):
+            bitmap = self._build_pid_bitmap(pid)
+            return self._format_response(0x41, pid, bitmap)
+
+        # Configured PID value
+        if pid in self.pid_values:
+            return self._format_response(0x41, pid, self.pid_values[pid])
+
+        return f"NO DATA{cr}"
+
+    def _handle_mode03(self) -> str:
+        """Handle Mode 03 (stored DTCs) query."""
+        if not self.dtc_codes:
+            return self._format_response(0x43, 0x00, bytes([0x00, 0x00, 0x00, 0x00]))
+
+        # Pack DTCs into response (2 bytes each)
+        dtc_bytes = bytearray()
+        for code in self.dtc_codes[:3]:  # Max 3 per frame
+            dtc_bytes.extend(self._encode_dtc(code))
+
+        # Pad to even number of DTCs
+        while len(dtc_bytes) % 4 != 0:
+            dtc_bytes.extend(b"\x00\x00")
+
+        return self._format_response(0x43, len(self.dtc_codes), bytes(dtc_bytes))
+
+    def _handle_mode09(self, pid: int) -> str:
+        """Handle Mode 09 (vehicle info) queries."""
+        cr = "\r\n" if self.linefeed else "\r"
+
+        if pid == 0x00:
+            # Supported Mode 09 PIDs: 02 (VIN)
+            return self._format_response(0x49, 0x00, bytes([0x40, 0x00, 0x00, 0x00]))
+
+        if pid == 0x02:
+            # VIN (17 chars, multi-frame)
+            vin_bytes = self.vin.encode("ascii")[:17].ljust(17, b"0")
+            if self.headers:
+                lines = []
+                # First frame: 10 14 49 02 01 VIN[0:3]
+                lines.append(
+                    f"{ECU_HEADER} 10 14 49 02 01 " + " ".join(f"{b:02X}" for b in vin_bytes[:3])
+                )
+                # Consecutive frames
+                offset = 3
+                seq = 1
+                while offset < len(vin_bytes):
+                    chunk = vin_bytes[offset : offset + 7]
+                    frame = f"{ECU_HEADER} 2{seq:01X} " + " ".join(f"{b:02X}" for b in chunk)
+                    lines.append(frame)
+                    offset += 7
+                    seq = (seq + 1) & 0x0F
+                return cr.join(lines) + cr
+            else:
+                return f"4902{vin_bytes.hex().upper()}{cr}"
+
+        return f"NO DATA{cr}"
+
+    def _format_response(self, mode_resp: int, pid: int, data: bytes) -> str:
+        """Format an OBD-II response with optional headers."""
+        cr = "\r\n" if self.linefeed else "\r"
+        length = 2 + len(data)  # mode + pid + data
+
+        if self.headers:
+            sep = " " if self.spaces else ""
+            data_hex = sep.join(f"{b:02X}" for b in data)
+            return f"{ECU_HEADER}{sep}{length:02X}{sep}{mode_resp:02X}{sep}{pid:02X}{sep}{data_hex}{cr}"
+        else:
+            data_hex = "".join(f"{b:02X}" for b in data)
+            return f"{mode_resp:02X}{pid:02X}{data_hex}{cr}"
+
+    def _build_pid_bitmap(self, base_pid: int) -> bytes:
+        """Build a 4-byte bitmap of supported PIDs for a range."""
+        bitmap = 0
+        all_pids = set(self.pid_values.keys())
+
+        for i in range(32):
+            actual_pid = base_pid + i + 1
+            if actual_pid in all_pids:
+                bitmap |= 1 << (31 - i)
+
+        # Set bit 32 (= bit 0) if the next range has supported PIDs
+        next_base = base_pid + 0x20
+        if next_base <= 0xE0:
+            for p in all_pids:
+                if next_base < p <= next_base + 0x20:
+                    bitmap |= 1
+                    break
+
+        return bitmap.to_bytes(4, "big")
+
+    @staticmethod
+    def _encode_dtc(code: str) -> bytes:
+        """Encode a DTC string like 'P0301' into 2 bytes."""
+        prefixes = {"P": 0, "C": 1, "B": 2, "U": 3}
+        prefix = prefixes.get(code[0].upper(), 0)
+        number = int(code[1:], 16)
+        value = (prefix << 14) | number
+        return value.to_bytes(2, "big")
+
+    def get_commands(
+        self,
+        since_index: int = 0,
+        limit: int = 50,
+    ) -> list[dict[str, Any]]:
+        """Get recorded commands."""
+        records = [
+            {
+                "index": r.index,
+                "timestamp": r.timestamp,
+                "command": r.raw,
+                "response_preview": r.response[:80],
+            }
+            for r in self._commands
+            if r.index >= since_index
+        ]
+        return records[:limit]
+
+
+# ==================== Module-level emulator ====================
+
+_emulator: ELM327Emulator | None = None
+
+
+def _get_emulator() -> ELM327Emulator:
+    """Get the global emulator instance."""
+    global _emulator
+    if _emulator is None:
+        _emulator = ELM327Emulator()
+    return _emulator
+
+
+async def _on_write(char_id: str, value: bytes, device: str | None) -> None:
+    """Callback fired when a remote client writes to the RX characteristic."""
+    emu = _get_emulator()
+    if not emu._started or char_id != emu._rx_char_id:
+        return
+
+    response = emu.handle_data(value)
+    if response and emu._tx_char_id:
+        mgr = get_gatt_server()
+        mgr.set_value(emu._tx_char_id, response.encode("utf-8"))
+
+
+# ==================== Tool registration ====================
+
+
+def register_tools(mcp: FastMCP) -> None:
+    """Register ELM327 emulator tools with the MCP server."""
+
+    @mcp.tool()
+    async def bt_elm327_emu_start(
+        adapter: str,
+        name: str = "OBDII",
+        load_defaults: bool = True,
+    ) -> dict[str, Any]:
+        """Start the ELM327 OBD-II BLE emulator.
+
+        Sets up a Nordic UART Service (NUS) GATT server, registers with
+        BlueZ, and starts advertising. OBD-II apps can then discover and
+        connect to this emulated adapter.
+
+        The emulator auto-responds to AT commands and OBD-II PID queries.
+        Configure PID values with bt_elm327_emu_set_pid().
+
+        Args:
+            adapter: Bluetooth adapter (e.g., "hci0").
+            name: BLE device name shown to scanners (default "OBDII").
+            load_defaults: Pre-load default PID values for a realistic
+                           idle vehicle (RPM 750, coolant 30°C, etc.).
+
+        Returns:
+            Status with NUS characteristic IDs and default PID info.
+        """
+        emu = _get_emulator()
+        if emu._started:
+            return {"status": "error", "error": "Emulator already running"}
+
+        try:
+            if load_defaults:
+                emu.set_defaults()
+
+            mgr = get_gatt_server()
+
+            # Build NUS GATT hierarchy
+            svc_id = mgr.add_service(NUS_SERVICE_UUID)
+            rx_id = mgr.add_characteristic(
+                svc_id,
+                NUS_RX_CHAR_UUID,
+                flags=["write", "write-without-response"],
+            )
+            tx_id = mgr.add_characteristic(
+                svc_id,
+                NUS_TX_CHAR_UUID,
+                flags=["notify"],
+                value=f"{emu.device_id}\r\n>".encode(),
+            )
+
+            emu._rx_char_id = rx_id
+            emu._tx_char_id = tx_id
+            emu._adapter = adapter
+
+            # Register write callback for auto-response
+            mgr._write_callback = _on_write
+
+            # Register + advertise
+            await mgr.register(adapter)
+            await mgr.set_advertising(
+                adapter,
+                True,
+                name=name,
+                service_uuids=[NUS_SERVICE_UUID],
+            )
+
+            emu._started = True
+
+            pid_info = (
+                {pid: desc for pid, (_val, desc) in DEFAULT_PIDS.items()} if load_defaults else {}
+            )
+
+            return {
+                "status": "ok",
+                "emulator": "ELM327 v1.5",
+                "adapter": adapter,
+                "name": name,
+                "rx_char_id": rx_id,
+                "tx_char_id": tx_id,
+                "default_pids": pid_info,
+                "vin": emu.vin,
+            }
+        except Exception as exc:
+            return {"status": "error", "error": str(exc)}
+
+    @mcp.tool()
+    async def bt_elm327_emu_stop(adapter: str) -> dict[str, Any]:
+        """Stop the ELM327 OBD-II BLE emulator.
+
+        Stops advertising, unregisters GATT services, and cleans up.
+
+        Args:
+            adapter: Bluetooth adapter (e.g., "hci0").
+
+        Returns:
+            Status confirming shutdown.
+        """
+        global _emulator
+        emu = _get_emulator()
+
+        if not emu._started:
+            return {"status": "ok", "was_running": False}
+
+        try:
+            mgr = get_gatt_server()
+            mgr._write_callback = None
+            await mgr.unregister(adapter)
+            mgr.clear()
+        except Exception:
+            pass
+
+        emu._started = False
+        _emulator = None
+        return {"status": "ok", "was_running": True}
+
+    @mcp.tool()
+    async def bt_elm327_emu_status() -> dict[str, Any]:
+        """Get ELM327 emulator status.
+
+        Returns:
+            - running: Whether the emulator is active
+            - pids: Configured PID values with descriptions
+            - dtcs: Active diagnostic trouble codes
+            - command_count: Number of commands received
+            - settings: Current ELM327 settings (echo, headers, etc.)
+        """
+        emu = _get_emulator()
+
+        pids = {}
+        for pid, value in emu.pid_values.items():
+            desc = ""
+            if pid in DEFAULT_PIDS:
+                _, desc = DEFAULT_PIDS[pid]
+            pids[f"0x{pid:02X}"] = {
+                "value_hex": value.hex(),
+                "value_bytes": list(value),
+                "description": desc,
+            }
+
+        return {
+            "status": "ok",
+            "running": emu._started,
+            "device_id": emu.device_id,
+            "adapter": emu._adapter,
+            "protocol": emu.protocol,
+            "pids": pids,
+            "dtcs": emu.dtc_codes,
+            "vin": emu.vin,
+            "command_count": emu._cmd_index,
+            "settings": {
+                "echo": emu.echo,
+                "headers": emu.headers,
+                "spaces": emu.spaces,
+                "linefeed": emu.linefeed,
+            },
+        }
+
+    @mcp.tool()
+    async def bt_elm327_emu_set_pid(
+        pid: int,
+        value: str,
+        value_type: str = "hex",
+    ) -> dict[str, Any]:
+        """Set an OBD-II PID response value.
+
+        When an OBD app queries this PID (Mode 01), the emulator returns
+        the configured value. Values follow OBD-II encoding conventions.
+
+        Common PIDs:
+            0x05: Coolant temp (1 byte, value - 40 = °C)
+            0x0C: Engine RPM (2 bytes, value / 4 = RPM)
+            0x0D: Vehicle speed (1 byte = km/h)
+            0x0F: Intake air temp (1 byte, value - 40 = °C)
+            0x11: Throttle position (1 byte, value / 2.55 = %)
+            0x2F: Fuel tank level (1 byte, value / 2.55 = %)
+            0x46: Ambient air temp (1 byte, value - 40 = °C)
+
+        Args:
+            pid: PID number (e.g., 0x0C for RPM).
+            value: Encoded value. For RPM of 3000: "0BB8" (hex) or "3000" (int).
+            value_type: "hex" (raw bytes), "int" (little-endian integer),
+                        or "string" (UTF-8).
+
+        Returns:
+            Status with the configured PID value.
+        """
+        emu = _get_emulator()
+
+        try:
+            if value_type == "hex":
+                data = bytes.fromhex(value)
+            elif value_type == "int":
+                int_val = int(value)
+                byte_len = max(1, (int_val.bit_length() + 7) // 8)
+                data = int_val.to_bytes(byte_len, "big")  # OBD uses big-endian
+            elif value_type == "string":
+                data = value.encode("utf-8")
+            else:
+                return {"status": "error", "error": f"Unknown value_type: {value_type}"}
+
+            emu.pid_values[pid] = data
+            return {
+                "status": "ok",
+                "pid": f"0x{pid:02X}",
+                "value_hex": data.hex(),
+                "value_bytes": list(data),
+            }
+        except Exception as exc:
+            return {"status": "error", "error": str(exc)}
+
+    @mcp.tool()
+    async def bt_elm327_emu_set_dtcs(dtc_codes: list[str]) -> dict[str, Any]:
+        """Set diagnostic trouble codes for Mode 03 queries.
+
+        Args:
+            dtc_codes: List of DTC strings (e.g., ["P0301", "P0420", "C0035"]).
+                       P = Powertrain, C = Chassis, B = Body, U = Network.
+
+        Returns:
+            Status with configured DTCs.
+        """
+        emu = _get_emulator()
+        emu.dtc_codes = [c.upper() for c in dtc_codes]
+        return {
+            "status": "ok",
+            "dtcs": emu.dtc_codes,
+            "count": len(emu.dtc_codes),
+        }
+
+    @mcp.tool()
+    async def bt_elm327_emu_set_vin(vin: str) -> dict[str, Any]:
+        """Set the Vehicle Identification Number for Mode 09 queries.
+
+        Args:
+            vin: 17-character VIN string.
+
+        Returns:
+            Status with configured VIN.
+        """
+        emu = _get_emulator()
+        if len(vin) != 17:
+            return {"status": "error", "error": f"VIN must be 17 characters (got {len(vin)})"}
+        emu.vin = vin.upper()
+        return {"status": "ok", "vin": emu.vin}
+
+    @mcp.tool()
+    async def bt_elm327_emu_read_commands(
+        since_index: int = 0,
+        limit: int = 50,
+    ) -> dict[str, Any]:
+        """Read commands received from the OBD-II client.
+
+        Monitor what the connected app is requesting. Useful for debugging
+        protocol issues or understanding app behavior.
+
+        Args:
+            since_index: Only return commands with index >= this value.
+            limit: Maximum commands to return.
+
+        Returns:
+            List of received commands with timestamps and responses.
+        """
+        emu = _get_emulator()
+        cmds = emu.get_commands(since_index, limit)
+        next_idx = max((c["index"] for c in cmds), default=since_index) + 1 if cmds else since_index
+        return {
+            "status": "ok",
+            "commands": cmds,
+            "count": len(cmds),
+            "next_index": next_idx,
+        }

src/mcbluetooth/tools/gatt_server.py

@@ -0,0 +1,392 @@
+"""GATT Server tools for Bluetooth MCP server.
+
+These tools let Linux act as a BLE peripheral (GATT server) that
+advertises services, accepts connections, and handles reads/writes
+from remote central devices. Use cases include: emulating sensors,
+building test harnesses, mocking BLE devices, protocol fuzzing.
+
+Typical flow:
+    1. bt_gatt_server_add_service(uuid)                    → service0
+    2. bt_gatt_server_add_characteristic(service0, ...)    → service0/char0
+    3. bt_gatt_server_register(adapter="hci0")             → registered
+    4. bt_gatt_server_advertise(adapter="hci0", enable=True, name="MyDevice")
+    5. Remote connects, writes → bt_gatt_server_read_writes()
+    6. Respond via bt_gatt_server_set_value()               → auto-notifies
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from fastmcp import FastMCP
+
+from mcbluetooth.gatt_server import get_gatt_server, shutdown_gatt_server
+
+
+def _parse_value(value: str, value_type: str) -> bytes:
+    """Convert a string value to bytes based on value_type."""
+    if value_type == "hex":
+        return bytes.fromhex(value)
+    elif value_type == "string":
+        return value.encode("utf-8")
+    elif value_type == "int":
+        int_val = int(value)
+        byte_len = max(1, (int_val.bit_length() + 7) // 8)
+        return int_val.to_bytes(byte_len, "little")
+    else:
+        raise ValueError(f"Unknown value_type: {value_type}")
+
+
+def register_tools(mcp: FastMCP) -> None:
+    """Register GATT server tools with the MCP server."""
+
+    # ---- Lifecycle ----
+
+    @mcp.tool()
+    async def bt_gatt_server_register(adapter: str) -> dict[str, Any]:
+        """Register the GATT application with BlueZ.
+
+        Exports all added services/characteristics to D-Bus and calls
+        GattManager1.RegisterApplication. Must add at least one service
+        before registering.
+
+        After registration, remote BLE centrals can discover and interact
+        with the hosted services (once advertising is started).
+
+        Args:
+            adapter: Adapter name (e.g., "hci0").
+
+        Returns:
+            Registration status with service count.
+        """
+        try:
+            mgr = get_gatt_server()
+            await mgr.register(adapter)
+            status = mgr.get_status()
+            return {
+                "status": "ok",
+                "registered": True,
+                "service_count": len(status["services"]),
+            }
+        except Exception as exc:
+            return {"status": "error", "error": str(exc)}
+
+    @mcp.tool()
+    async def bt_gatt_server_unregister(adapter: str) -> dict[str, Any]:
+        """Unregister the GATT application from BlueZ.
+
+        Stops advertising (if active) and removes the application
+        registration. Services remain in memory and can be re-registered.
+
+        Args:
+            adapter: Adapter name (e.g., "hci0").
+
+        Returns:
+            Status confirming unregistration.
+        """
+        try:
+            await shutdown_gatt_server(adapter)
+            return {"status": "ok", "registered": False}
+        except Exception as exc:
+            return {"status": "error", "error": str(exc)}
+
+    @mcp.tool()
+    async def bt_gatt_server_status() -> dict[str, Any]:
+        """Get GATT server status.
+
+        Returns:
+            - registered: Whether the application is registered with BlueZ
+            - advertising: Whether BLE advertising is active
+            - services: List of services with their characteristics
+            - write_event_count: Number of buffered write events
+        """
+        mgr = get_gatt_server()
+        return {"status": "ok", **mgr.get_status()}
+
+    # ---- Service construction ----
+
+    @mcp.tool()
+    async def bt_gatt_server_add_service(
+        uuid: str,
+        primary: bool = True,
+    ) -> dict[str, Any]:
+        """Add a GATT service to the server.
+
+        Services must be added BEFORE registering with BlueZ.
+        If already registered, unregister first, add services, then re-register.
+
+        Args:
+            uuid: Service UUID (e.g., "180F" for Battery Service, or full
+                  128-bit "6E400001-B5A3-F393-E0A9-E50E24DCCA9E" for custom).
+            primary: Whether this is a primary service (default True).
+
+        Returns:
+            The assigned service_id (e.g., "service0") for adding characteristics.
+        """
+        try:
+            mgr = get_gatt_server()
+            service_id = mgr.add_service(uuid, primary)
+            return {"status": "ok", "service_id": service_id, "uuid": uuid}
+        except Exception as exc:
+            return {"status": "error", "error": str(exc)}
+
+    @mcp.tool()
+    async def bt_gatt_server_add_characteristic(
+        service_id: str,
+        uuid: str,
+        flags: list[str],
+        value: str | None = None,
+        value_type: str = "hex",
+    ) -> dict[str, Any]:
+        """Add a characteristic to a GATT service.
+
+        Args:
+            service_id: Service to add to (e.g., "service0").
+            uuid: Characteristic UUID.
+            flags: List of flags/permissions. Common values:
+                   "read", "write", "write-without-response", "notify", "indicate",
+                   "encrypt-read", "encrypt-write".
+            value: Initial value (optional). Format depends on value_type.
+            value_type: How to interpret value — "hex", "string", or "int".
+
+        Returns:
+            The assigned char_id (e.g., "service0/char0") for setting values.
+        """
+        try:
+            mgr = get_gatt_server()
+            initial = _parse_value(value, value_type) if value else b""
+            char_id = mgr.add_characteristic(service_id, uuid, flags, initial)
+            return {
+                "status": "ok",
+                "char_id": char_id,
+                "uuid": uuid,
+                "flags": flags,
+            }
+        except Exception as exc:
+            return {"status": "error", "error": str(exc)}
+
+    @mcp.tool()
+    async def bt_gatt_server_add_descriptor(
+        char_id: str,
+        uuid: str,
+        flags: list[str],
+        value: str | None = None,
+        value_type: str = "hex",
+    ) -> dict[str, Any]:
+        """Add a descriptor to a GATT characteristic.
+
+        Note: CCCD (0x2902) for notify/indicate is auto-managed by BlueZ —
+        don't add it manually.
+
+        Args:
+            char_id: Characteristic to add to (e.g., "service0/char0").
+            uuid: Descriptor UUID (e.g., "2901" for Characteristic User Description).
+            flags: List of flags (e.g., ["read"], ["read", "write"]).
+            value: Initial value (optional).
+            value_type: How to interpret value — "hex", "string", or "int".
+
+        Returns:
+            The assigned desc_id.
+        """
+        try:
+            mgr = get_gatt_server()
+            initial = _parse_value(value, value_type) if value else b""
+            desc_id = mgr.add_descriptor(char_id, uuid, flags, initial)
+            return {"status": "ok", "desc_id": desc_id, "uuid": uuid}
+        except Exception as exc:
+            return {"status": "error", "error": str(exc)}
+
+    @mcp.tool()
+    async def bt_gatt_server_clear() -> dict[str, Any]:
+        """Remove all services, characteristics, and descriptors.
+
+        Must unregister first. Use this to rebuild the service hierarchy
+        from scratch.
+
+        Returns:
+            Status confirming services were cleared.
+        """
+        try:
+            mgr = get_gatt_server()
+            mgr.clear()
+            return {"status": "ok", "cleared": True}
+        except Exception as exc:
+            return {"status": "error", "error": str(exc)}
+
+    # ---- Value management ----
+
+    @mcp.tool()
+    async def bt_gatt_server_set_value(
+        char_id: str,
+        value: str,
+        value_type: str = "hex",
+    ) -> dict[str, Any]:
+        """Set a characteristic value on the GATT server.
+
+        If a remote client has subscribed to notifications on this
+        characteristic, the new value is automatically pushed via
+        BLE notification.
+
+        Args:
+            char_id: Characteristic ID (e.g., "service0/char0").
+            value: Value to set.
+            value_type: How to interpret value — "hex" (e.g., "0102ff"),
+                        "string" (UTF-8 text), or "int" (decimal number).
+
+        Returns:
+            Status with value details and whether notification was sent.
+        """
+        try:
+            mgr = get_gatt_server()
+            data = _parse_value(value, value_type)
+            char = mgr._characteristics.get(char_id)
+            was_notifying = char.notifying if char else False
+            mgr.set_value(char_id, data)
+            return {
+                "status": "ok",
+                "char_id": char_id,
+                "value_hex": data.hex(),
+                "length": len(data),
+                "notified": was_notifying,
+            }
+        except Exception as exc:
+            return {"status": "error", "error": str(exc)}
+
+    @mcp.tool()
+    async def bt_gatt_server_get_value(char_id: str) -> dict[str, Any]:
+        """Read a characteristic value from the server side.
+
+        This reads the value stored on OUR server, not from a remote device.
+        Use bt_ble_read() for reading from remote GATT servers.
+
+        Args:
+            char_id: Characteristic ID (e.g., "service0/char0").
+
+        Returns:
+            The current value as hex, bytes, and string (if decodable).
+        """
+        try:
+            mgr = get_gatt_server()
+            value = mgr.get_value(char_id)
+            result: dict[str, Any] = {
+                "status": "ok",
+                "char_id": char_id,
+                "value_hex": value.hex(),
+                "value_bytes": list(value),
+                "length": len(value),
+            }
+            try:
+                result["value_string"] = value.decode("utf-8")
+            except UnicodeDecodeError:
+                pass
+            return result
+        except Exception as exc:
+            return {"status": "error", "error": str(exc)}
+
+    @mcp.tool()
+    async def bt_gatt_server_notify(char_id: str) -> dict[str, Any]:
+        """Explicitly send a BLE notification for the current value.
+
+        Normally, bt_gatt_server_set_value auto-notifies. Use this to
+        re-send the current value without changing it.
+
+        Args:
+            char_id: Characteristic ID (e.g., "service0/char0").
+
+        Returns:
+            Status indicating whether notification was sent.
+        """
+        try:
+            mgr = get_gatt_server()
+            sent = mgr.notify(char_id)
+            if sent:
+                return {"status": "ok", "notified": True}
+            return {
+                "status": "ok",
+                "notified": False,
+                "reason": "No client subscribed to notifications",
+            }
+        except Exception as exc:
+            return {"status": "error", "error": str(exc)}
+
+    # ---- Advertising ----
+
+    @mcp.tool()
+    async def bt_gatt_server_advertise(
+        adapter: str,
+        enable: bool,
+        name: str = "mcbluetooth",
+        service_uuids: list[str] | None = None,
+    ) -> dict[str, Any]:
+        """Start or stop BLE advertising.
+
+        When advertising, the device becomes discoverable to BLE scanners
+        and can accept connections from central devices.
+
+        Must register the GATT application first.
+
+        Args:
+            adapter: Adapter name (e.g., "hci0").
+            enable: True to start advertising, False to stop.
+            name: Local name shown to scanners (default "mcbluetooth").
+            service_uuids: UUIDs to include in advertisement. If not specified,
+                           uses all registered service UUIDs.
+
+        Returns:
+            Advertising status.
+        """
+        try:
+            mgr = get_gatt_server()
+            await mgr.set_advertising(adapter, enable, name=name, service_uuids=service_uuids)
+            return {
+                "status": "ok",
+                "advertising": enable,
+                "name": name if enable else None,
+            }
+        except Exception as exc:
+            return {"status": "error", "error": str(exc)}
+
+    # ---- Write event monitoring ----
+
+    @mcp.tool()
+    async def bt_gatt_server_read_writes(
+        since_index: int = 0,
+        char_id: str | None = None,
+        limit: int = 50,
+    ) -> dict[str, Any]:
+        """Read write events from remote BLE clients.
+
+        When a remote central device writes to a server characteristic,
+        the write is buffered here. Poll this to see incoming commands/data.
+
+        Args:
+            since_index: Only return events with index >= this value.
+                         Start with 0, then use the highest returned index + 1.
+            char_id: Filter to writes on this characteristic (optional).
+            limit: Maximum events to return (default 50).
+
+        Returns:
+            List of write events with value, timestamp, and source device.
+        """
+        mgr = get_gatt_server()
+        events = mgr.get_write_events(since_index, char_id, limit)
+        next_index = (
+            max((e["index"] for e in events), default=since_index) + 1 if events else since_index
+        )
+        return {
+            "status": "ok",
+            "events": events,
+            "count": len(events),
+            "next_index": next_index,
+        }
+
+    @mcp.tool()
+    async def bt_gatt_server_clear_writes() -> dict[str, Any]:
+        """Clear the write event buffer.
+
+        Returns:
+            Number of events cleared.
+        """
+        mgr = get_gatt_server()
+        count = mgr.clear_write_events()
+        return {"status": "ok", "cleared_count": count}