diff --git a/app/services/community/mdns.py b/app/services/community/mdns.py
new file mode 100644
index 0000000..efc3ca9
--- /dev/null
+++ b/app/services/community/mdns.py
@@ -0,0 +1,111 @@
+# app/services/community/mdns.py
+# MIT License
+# mDNS advertisement for Kiwi instances on the local network.
+# Advertises _kiwi._tcp.local so other Kiwi instances (and discovery apps)
+# can find this one without manual configuration.
+#
+# Opt-in only: enabled=False by default. Users are prompted on first community
+# tab access. Never advertised without explicit consent (a11y requirement).
+
+from __future__ import annotations
+
+import logging
+import socket
+from typing import Any
+
+logger = logging.getLogger(__name__)
+
+# Deferred import — avoid hard failure when zeroconf is not installed.
+try:
+    from zeroconf import ServiceInfo, Zeroconf
+    _ZEROCONF_AVAILABLE = True
+except ImportError:  # pragma: no cover
+    _ZEROCONF_AVAILABLE = False
+
+
+class KiwiMDNS:
+    """Context manager that advertises this Kiwi instance via mDNS (_kiwi._tcp.local).
+
+    Defaults to disabled. User must explicitly opt in via Settings.
+    feed_url is broadcast in the TXT record so peer instances know where to fetch posts.
+
+    Usage:
+        mdns = KiwiMDNS(
+            enabled=settings.MDNS_ENABLED,
+            port=8512,
+            feed_url="http://10.0.0.5:8512/api/v1/community/local-feed",
+        )
+        mdns.start()   # in lifespan startup
+        mdns.stop()    # in lifespan shutdown
+    """
+
+    SERVICE_TYPE = "_kiwi._tcp.local."
+
+    def __init__(
+        self,
+        port: int = 8512,
+        name: str | None = None,
+        feed_url: str = "",
+        enabled: bool = False,
+    ) -> None:
+        self._port = port
+        self._name = name or f"kiwi-{socket.gethostname()}"
+        self._feed_url = feed_url
+        self._enabled = enabled
+        self._zc: Any = None
+        self._info: Any = None
+
+    def start(self) -> None:
+        if not self._enabled:
+            logger.info("mDNS advertisement disabled (user opt-in required)")
+            return
+        try:
+            local_ip = _get_local_ip()
+            props = {b"product": b"kiwi", b"version": b"1"}
+            if self._feed_url:
+                props[b"feed"] = self._feed_url.encode()
+
+            self._info = ServiceInfo(
+                type_=self.SERVICE_TYPE,
+                name=f"{self._name}.{self.SERVICE_TYPE}",
+                addresses=[socket.inet_aton(local_ip)],
+                port=self._port,
+                properties=props,
+                server=f"{socket.gethostname()}.local.",
+            )
+            self._zc = Zeroconf()
+            self._zc.register_service(self._info)
+            logger.info("mDNS: advertising %s on %s:%d", self._name, local_ip, self._port)
+        except Exception as exc:
+            logger.warning("mDNS advertisement failed (non-fatal): %s", exc)
+            self._zc = None
+            self._info = None
+
+    def stop(self) -> None:
+        if self._zc and self._info:
+            try:
+                self._zc.unregister_service(self._info)
+                self._zc.close()
+                logger.info("mDNS: unregistered %s", self._name)
+            except Exception as exc:
+                logger.warning("mDNS unregister failed (non-fatal): %s", exc)
+            finally:
+                self._zc = None
+                self._info = None
+
+    def __enter__(self) -> "KiwiMDNS":
+        self.start()
+        return self
+
+    def __exit__(self, *_: object) -> None:
+        self.stop()
+
+
+def _get_local_ip() -> str:
+    """Return the primary non-loopback IPv4 address of this host."""
+    try:
+        with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as s:
+            s.connect(("8.8.8.8", 80))
+            return s.getsockname()[0]
+    except OSError:
+        return "127.0.0.1"
diff --git a/pyproject.toml b/pyproject.toml
index 1928abb..c454b95 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -24,6 +24,8 @@ dependencies = [
     "requests>=2.31",
     # CircuitForge shared scaffold
     "circuitforge-core>=0.8.0",
+    # mDNS advertisement (opt-in: _kiwi._tcp.local discovery)
+    "zeroconf>=0.131",
 ]
 
 [tool.setuptools.packages.find]
diff --git a/tests/services/community/test_mdns.py b/tests/services/community/test_mdns.py
new file mode 100644
index 0000000..8cfd4d3
--- /dev/null
+++ b/tests/services/community/test_mdns.py
@@ -0,0 +1,39 @@
+# tests/services/community/test_mdns.py
+import pytest
+from unittest.mock import MagicMock, patch
+from app.services.community.mdns import KiwiMDNS
+
+
+def test_mdns_does_not_advertise_when_disabled():
+    """When enabled=False, KiwiMDNS does not register any zeroconf service."""
+    with patch("app.services.community.mdns.Zeroconf") as mock_zc:
+        mdns = KiwiMDNS(enabled=False, port=8512, feed_url="http://localhost:8512/api/v1/community/local-feed")
+        mdns.start()
+        mock_zc.assert_not_called()
+
+
+def test_mdns_advertises_when_enabled():
+    with patch("app.services.community.mdns.Zeroconf") as mock_zc_cls:
+        with patch("app.services.community.mdns.ServiceInfo") as mock_si:
+            mock_zc = MagicMock()
+            mock_zc_cls.return_value = mock_zc
+            mdns = KiwiMDNS(enabled=True, port=8512, feed_url="http://localhost:8512/api/v1/community/local-feed")
+            mdns.start()
+            mock_zc.register_service.assert_called_once()
+
+
+def test_mdns_stop_unregisters_when_enabled():
+    with patch("app.services.community.mdns.Zeroconf") as mock_zc_cls:
+        with patch("app.services.community.mdns.ServiceInfo"):
+            mock_zc = MagicMock()
+            mock_zc_cls.return_value = mock_zc
+            mdns = KiwiMDNS(enabled=True, port=8512, feed_url="http://localhost:8512/api/v1/community/local-feed")
+            mdns.start()
+            mdns.stop()
+            mock_zc.unregister_service.assert_called_once()
+            mock_zc.close.assert_called_once()
+
+
+def test_mdns_stop_is_noop_when_not_started():
+    mdns = KiwiMDNS(enabled=False, port=8512, feed_url="http://localhost/feed")
+    mdns.stop()  # must not raise