diff --git a/tests/test_service_isolation.py b/tests/test_service_isolation.py
new file mode 100644
index 0000000..880b6e1
--- /dev/null
+++ b/tests/test_service_isolation.py
@@ -0,0 +1,483 @@
+"""
+Service isolation tests.
+
+Verifies that each background worker handles missing dependencies gracefully
+and that failures in one service do not cascade to others.
+
+Dependency graph under test:
+    Collector → (Docker SDK, state file, log file)
+    Ingester  → (Collector's JSON output, DB repo)
+    Attacker  → (DB repo)
+    Sniffer   → (MACVLAN interface, scapy, state file)
+    API       → (DB init, all workers)
+
+Each test disables or breaks one dependency and asserts the affected
+worker degrades gracefully while unrelated workers remain unaffected.
+"""
+
+import asyncio
+import json
+import os
+import time
+from pathlib import Path
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+
+# ─── Collector isolation ─────────────────────────────────────────────────────
+
+class TestCollectorIsolation:
+    """Collector depends on Docker SDK and state file."""
+
+    @pytest.mark.asyncio
+    async def test_collector_survives_docker_unavailable(self):
+        """Collector must not crash when Docker daemon is not running."""
+        import docker as docker_mod
+        from decnet.collector.worker import log_collector_worker
+
+        original_from_env = docker_mod.from_env
+        with patch.object(docker_mod, "from_env",
+                          side_effect=Exception("Cannot connect to Docker daemon")):
+            task = asyncio.create_task(log_collector_worker("/tmp/decnet-test-collector.log"))
+            await asyncio.sleep(0.1)
+            assert task.done()
+            exc = task.exception()
+            assert exc is None  # Should not propagate exceptions
+
+    @pytest.mark.asyncio
+    async def test_collector_survives_no_state_file(self):
+        """Collector must handle missing state file (no deckies deployed)."""
+        from decnet.collector.worker import _load_service_container_names
+
+        with patch("decnet.config.load_state", return_value=None):
+            result = _load_service_container_names()
+            assert result == set()
+
+    @pytest.mark.asyncio
+    async def test_collector_survives_empty_fleet(self):
+        """Collector runs but finds no matching containers when fleet is empty."""
+        import docker as docker_mod
+        from decnet.collector.worker import log_collector_worker
+
+        mock_client = MagicMock()
+        mock_client.containers.list.return_value = []
+        mock_client.events.side_effect = Exception("connection closed")
+
+        with patch.object(docker_mod, "from_env", return_value=mock_client):
+            with patch("decnet.config.load_state", return_value=None):
+                task = asyncio.create_task(log_collector_worker("/tmp/decnet-test-collector.log"))
+                await asyncio.sleep(0.1)
+                assert task.done()
+                assert task.exception() is None
+
+    def test_collector_container_filter_with_unknown_containers(self):
+        """is_service_container must reject containers not in state."""
+        from decnet.collector.worker import is_service_container
+
+        with patch("decnet.collector.worker._load_service_container_names",
+                    return_value={"decky-01-ssh", "decky-01-http"}):
+            assert is_service_container("decky-01-ssh") is True
+            assert is_service_container("random-container") is False
+            assert is_service_container("decky-99-ftp") is False
+
+
+# ─── Ingester isolation ──────────────────────────────────────────────────────
+
+class TestIngesterIsolation:
+    """Ingester depends on collector's JSON output and DB repo."""
+
+    @pytest.mark.asyncio
+    async def test_ingester_survives_missing_log_file(self):
+        """Ingester must wait patiently when JSON log file doesn't exist yet."""
+        from decnet.web.ingester import log_ingestion_worker
+
+        mock_repo = MagicMock()
+        iterations = 0
+
+        async def _controlled_sleep(seconds):
+            nonlocal iterations
+            iterations += 1
+            if iterations >= 3:
+                raise asyncio.CancelledError()
+
+        with patch.dict(os.environ, {"DECNET_INGEST_LOG_FILE": "/tmp/nonexistent-decnet-test.log"}):
+            with patch("decnet.web.ingester.asyncio.sleep", side_effect=_controlled_sleep):
+                task = asyncio.create_task(log_ingestion_worker(mock_repo))
+                with pytest.raises(asyncio.CancelledError):
+                    await task
+        # Should have waited at least 2 iterations without crashing
+        assert iterations >= 2
+        mock_repo.add_log.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_ingester_survives_no_log_file_env(self):
+        """Ingester must exit gracefully when DECNET_INGEST_LOG_FILE is unset."""
+        from decnet.web.ingester import log_ingestion_worker
+
+        mock_repo = MagicMock()
+        with patch.dict(os.environ, {}, clear=False):
+            # Remove the env var if it exists
+            os.environ.pop("DECNET_INGEST_LOG_FILE", None)
+            await log_ingestion_worker(mock_repo)
+        # Should return immediately without error
+        mock_repo.add_log.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_ingester_survives_malformed_json(self, tmp_path):
+        """Ingester must skip malformed JSON lines without crashing."""
+        from decnet.web.ingester import log_ingestion_worker
+
+        json_file = tmp_path / "test.json"
+        json_file.write_text("not valid json\n{also broken\n")
+
+        mock_repo = MagicMock()
+        mock_repo.add_log = AsyncMock()
+        iterations = 0
+
+        async def _controlled_sleep(seconds):
+            nonlocal iterations
+            iterations += 1
+            if iterations >= 3:
+                raise asyncio.CancelledError()
+
+        with patch.dict(os.environ, {"DECNET_INGEST_LOG_FILE": str(tmp_path / "test.log")}):
+            with patch("decnet.web.ingester.asyncio.sleep", side_effect=_controlled_sleep):
+                task = asyncio.create_task(log_ingestion_worker(mock_repo))
+                with pytest.raises(asyncio.CancelledError):
+                    await task
+        mock_repo.add_log.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_ingester_exits_on_db_fatal_error(self, tmp_path):
+        """Ingester must exit cleanly on fatal DB errors (table missing, connection closed)."""
+        from decnet.web.ingester import log_ingestion_worker
+
+        json_file = tmp_path / "test.json"
+        valid_record = {
+            "timestamp": "2026-01-01 00:00:00",
+            "decky": "decky-01",
+            "service": "ssh",
+            "event_type": "login_attempt",
+            "attacker_ip": "10.0.0.1",
+            "fields": {},
+            "msg": "",
+            "raw_line": "<134>1 2026-01-01T00:00:00Z decky-01 ssh - login_attempt -",
+        }
+        json_file.write_text(json.dumps(valid_record) + "\n")
+
+        mock_repo = MagicMock()
+        mock_repo.add_log = AsyncMock(side_effect=Exception("no such table: logs"))
+
+        with patch.dict(os.environ, {"DECNET_INGEST_LOG_FILE": str(tmp_path / "test.log")}):
+            # Worker should exit the loop on fatal DB error
+            await log_ingestion_worker(mock_repo)
+        # Should have attempted to add the log before dying
+        mock_repo.add_log.assert_awaited_once()
+
+
+# ─── Attacker worker isolation ───────────────────────────────────────────────
+
+class TestAttackerWorkerIsolation:
+    """Attacker worker depends on DB repo."""
+
+    @pytest.mark.asyncio
+    async def test_attacker_worker_survives_db_error(self):
+        """Attacker worker must catch DB errors and continue looping."""
+        from decnet.web.attacker_worker import attacker_profile_worker
+
+        mock_repo = MagicMock()
+        mock_repo.get_all_logs_raw = AsyncMock(side_effect=Exception("DB is locked"))
+        mock_repo.get_max_log_id = AsyncMock(return_value=0)
+        mock_repo.set_state = AsyncMock()
+
+        iterations = 0
+
+        async def _controlled_sleep(seconds):
+            nonlocal iterations
+            iterations += 1
+            if iterations >= 3:
+                raise asyncio.CancelledError()
+
+        with patch("decnet.web.attacker_worker.asyncio.sleep", side_effect=_controlled_sleep):
+            task = asyncio.create_task(attacker_profile_worker(mock_repo))
+            with pytest.raises(asyncio.CancelledError):
+                await task
+        # Worker should have retried at least twice before we cancelled
+        assert iterations >= 2
+
+    @pytest.mark.asyncio
+    async def test_attacker_worker_survives_empty_db(self):
+        """Attacker worker must handle an empty database gracefully."""
+        from decnet.web.attacker_worker import _WorkerState, _incremental_update
+
+        mock_repo = MagicMock()
+        mock_repo.get_all_logs_raw = AsyncMock(return_value=[])
+        mock_repo.get_max_log_id = AsyncMock(return_value=0)
+        mock_repo.set_state = AsyncMock()
+
+        state = _WorkerState()
+        await _incremental_update(mock_repo, state)
+        assert state.initialized is True
+        assert state.last_log_id == 0
+
+
+# ─── Sniffer isolation ───────────────────────────────────────────────────────
+
+class TestSnifferIsolation:
+    """Sniffer depends on MACVLAN interface, scapy, and state file."""
+
+    @pytest.mark.asyncio
+    async def test_sniffer_survives_missing_interface(self):
+        """Sniffer must exit gracefully when MACVLAN interface doesn't exist."""
+        from decnet.sniffer.worker import sniffer_worker
+
+        with patch("decnet.sniffer.worker._interface_exists", return_value=False):
+            await sniffer_worker("/tmp/decnet-test-sniffer.log")
+        # Should return without error
+
+    @pytest.mark.asyncio
+    async def test_sniffer_survives_no_state(self):
+        """Sniffer must exit gracefully when no deckies are deployed."""
+        from decnet.sniffer.worker import sniffer_worker
+
+        with patch("decnet.sniffer.worker._interface_exists", return_value=True):
+            with patch("decnet.config.load_state", return_value=None):
+                await sniffer_worker("/tmp/decnet-test-sniffer.log")
+        # Should return without error
+
+    @pytest.mark.asyncio
+    async def test_sniffer_survives_scapy_import_error(self):
+        """Sniffer must handle missing scapy library gracefully."""
+        from decnet.sniffer.worker import _sniff_loop
+
+        import threading
+        stop = threading.Event()
+
+        with patch("decnet.config.load_state", return_value=None):
+            with patch.dict("sys.modules", {"scapy": None, "scapy.sendrecv": None}):
+                # Should exit gracefully (no deckies = early return before scapy import)
+                _sniff_loop("fake0", Path("/tmp/test.log"), Path("/tmp/test.json"), stop)
+
+    @pytest.mark.asyncio
+    async def test_sniffer_survives_scapy_crash(self):
+        """Sniffer must handle scapy runtime errors without crashing the API."""
+        from decnet.sniffer.worker import sniffer_worker
+
+        mock_state = MagicMock()
+        mock_config = MagicMock()
+        mock_config.deckies = [MagicMock(ip="192.168.1.10", name="decky-01")]
+
+        with patch("decnet.sniffer.worker._interface_exists", return_value=True):
+            with patch("decnet.config.load_state", return_value=(mock_config, Path("/tmp"))):
+                with patch("decnet.sniffer.worker.asyncio.to_thread",
+                           side_effect=Exception("scapy segfault")):
+                    # Should catch and log, not raise
+                    await sniffer_worker("/tmp/decnet-test-sniffer.log")
+
+    def test_sniffer_engine_ignores_non_decky_traffic(self):
+        """Engine must silently skip packets not involving any known decky."""
+        from decnet.sniffer.fingerprint import SnifferEngine
+
+        written: list[str] = []
+        engine = SnifferEngine(
+            ip_to_decky={"192.168.1.10": "decky-01"},
+            write_fn=written.append,
+        )
+        # Simulate a packet between two unknown IPs
+        pkt = MagicMock()
+        pkt.haslayer.return_value = True
+        ip_layer = MagicMock()
+        ip_layer.src = "10.0.0.1"
+        ip_layer.dst = "10.0.0.2"
+        tcp_layer = MagicMock()
+        tcp_layer.sport = 12345
+        tcp_layer.dport = 443
+        tcp_layer.flags = MagicMock(value=0x10)
+        tcp_layer.payload = b""
+        pkt.__getitem__ = lambda self, cls: ip_layer if cls.__name__ == "IP" else tcp_layer
+        # Import layers for haslayer check
+        from scapy.layers.inet import IP, TCP
+        pkt.haslayer.side_effect = lambda layer: True
+
+        engine.on_packet(pkt)
+        assert written == []  # Nothing written for non-decky traffic
+
+
+# ─── API lifespan isolation ──────────────────────────────────────────────────
+
+class TestApiLifespanIsolation:
+    """API lifespan must survive individual worker failures."""
+
+    @pytest.mark.asyncio
+    async def test_api_survives_all_workers_failing(self):
+        """API must start and serve requests even if every worker fails to start."""
+        from decnet.web.api import lifespan
+
+        mock_app = MagicMock()
+        mock_repo = MagicMock()
+        mock_repo.initialize = AsyncMock()
+
+        with patch("decnet.web.api.repo", mock_repo):
+            with patch("decnet.web.api.log_ingestion_worker",
+                       side_effect=Exception("ingester exploded")):
+                with patch("decnet.web.api.log_collector_worker",
+                           side_effect=Exception("collector exploded")):
+                    with patch("decnet.web.api.attacker_profile_worker",
+                               side_effect=Exception("attacker exploded")):
+                        with patch("decnet.sniffer.sniffer_worker",
+                                   side_effect=Exception("sniffer exploded")):
+                            # API should still start
+                            async with lifespan(mock_app):
+                                mock_repo.initialize.assert_awaited_once()
+
+    @pytest.mark.asyncio
+    async def test_api_survives_db_init_failure(self):
+        """API must survive even if DB never initializes (5 failed attempts)."""
+        from decnet.web.api import lifespan
+
+        mock_app = MagicMock()
+        mock_repo = MagicMock()
+        mock_repo.initialize = AsyncMock(side_effect=Exception("DB locked"))
+
+        with patch("decnet.web.api.repo", mock_repo):
+            with patch("decnet.web.api.asyncio.sleep", new_callable=AsyncMock):
+                with patch("decnet.web.api.log_ingestion_worker", return_value=asyncio.sleep(0)):
+                    with patch("decnet.web.api.log_collector_worker", return_value=asyncio.sleep(0)):
+                        with patch("decnet.web.api.attacker_profile_worker", return_value=asyncio.sleep(0)):
+                            async with lifespan(mock_app):
+                                # DB init failed 5 times but API is running
+                                assert mock_repo.initialize.await_count == 5
+
+    @pytest.mark.asyncio
+    async def test_api_survives_sniffer_import_failure(self):
+        """API must start even if the sniffer module cannot be imported."""
+        from decnet.web.api import lifespan
+
+        mock_app = MagicMock()
+        mock_repo = MagicMock()
+        mock_repo.initialize = AsyncMock()
+
+        with patch("decnet.web.api.repo", mock_repo):
+            with patch("decnet.web.api.log_ingestion_worker", return_value=asyncio.sleep(0)):
+                with patch("decnet.web.api.log_collector_worker", return_value=asyncio.sleep(0)):
+                    with patch("decnet.web.api.attacker_profile_worker", return_value=asyncio.sleep(0)):
+                        # Simulate sniffer import failure
+                        import builtins
+                        real_import = builtins.__import__
+
+                        def _mock_import(name, *args, **kwargs):
+                            if name == "decnet.sniffer":
+                                raise ImportError("No module named 'scapy'")
+                            return real_import(name, *args, **kwargs)
+
+                        with patch("builtins.__import__", side_effect=_mock_import):
+                            async with lifespan(mock_app):
+                                mock_repo.initialize.assert_awaited_once()
+
+    @pytest.mark.asyncio
+    async def test_shutdown_handles_already_dead_tasks(self):
+        """Shutdown must not crash when tasks have already completed or failed."""
+        from decnet.web.api import lifespan
+
+        mock_app = MagicMock()
+        mock_repo = MagicMock()
+        mock_repo.initialize = AsyncMock()
+
+        # Workers that complete immediately
+        async def _instant_worker(*args):
+            return
+
+        with patch("decnet.web.api.repo", mock_repo):
+            with patch("decnet.web.api.log_ingestion_worker", side_effect=_instant_worker):
+                with patch("decnet.web.api.log_collector_worker", side_effect=_instant_worker):
+                    with patch("decnet.web.api.attacker_profile_worker", side_effect=_instant_worker):
+                        async with lifespan(mock_app):
+                            # Let tasks finish
+                            await asyncio.sleep(0.05)
+                        # Shutdown should handle already-done tasks gracefully
+
+
+# ─── Cross-service cascade tests ────────────────────────────────────────────
+
+class TestCascadeIsolation:
+    """Verify that failure in one service does not cascade to others."""
+
+    @pytest.mark.asyncio
+    async def test_collector_failure_does_not_kill_ingester(self, tmp_path):
+        """When collector dies, ingester must keep waiting (not crash)."""
+        from decnet.web.ingester import log_ingestion_worker
+
+        json_file = tmp_path / "cascade.json"
+        # File doesn't exist — simulates collector never writing
+
+        mock_repo = MagicMock()
+        mock_repo.add_log = AsyncMock()
+        iterations = 0
+
+        async def _controlled_sleep(seconds):
+            nonlocal iterations
+            iterations += 1
+            if iterations >= 5:
+                raise asyncio.CancelledError()
+
+        with patch.dict(os.environ, {"DECNET_INGEST_LOG_FILE": str(tmp_path / "cascade.log")}):
+            with patch("decnet.web.ingester.asyncio.sleep", side_effect=_controlled_sleep):
+                task = asyncio.create_task(log_ingestion_worker(mock_repo))
+                with pytest.raises(asyncio.CancelledError):
+                    await task
+        # Ingester should have been patiently waiting, not crashing
+        assert iterations >= 4
+        mock_repo.add_log.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_ingester_failure_does_not_kill_attacker(self):
+        """When ingester dies, attacker worker must keep running independently."""
+        from decnet.web.attacker_worker import attacker_profile_worker
+
+        mock_repo = MagicMock()
+        mock_repo.get_all_logs_raw = AsyncMock(return_value=[])
+        mock_repo.get_max_log_id = AsyncMock(return_value=0)
+        mock_repo.set_state = AsyncMock()
+        mock_repo.get_logs_after_id = AsyncMock(return_value=[])
+
+        iterations = 0
+
+        async def _controlled_sleep(seconds):
+            nonlocal iterations
+            iterations += 1
+            if iterations >= 3:
+                raise asyncio.CancelledError()
+
+        with patch("decnet.web.attacker_worker.asyncio.sleep", side_effect=_controlled_sleep):
+            task = asyncio.create_task(attacker_profile_worker(mock_repo))
+            with pytest.raises(asyncio.CancelledError):
+                await task
+        # Attacker should have run independently
+        assert iterations >= 2
+
+    @pytest.mark.asyncio
+    async def test_sniffer_crash_does_not_affect_collector(self):
+        """Sniffer crash must not affect collector operation."""
+        from decnet.collector.worker import is_service_container, is_service_event
+
+        # These should work regardless of sniffer state
+        with patch("decnet.collector.worker._load_service_container_names",
+                    return_value={"decky-01-ssh"}):
+            assert is_service_container("decky-01-ssh") is True
+            assert is_service_event({"name": "decky-01-ssh"}) is True
+
+    @pytest.mark.asyncio
+    async def test_db_failure_does_not_crash_sniffer(self):
+        """Sniffer has no DB dependency — must be completely unaffected by DB issues."""
+        from decnet.sniffer.fingerprint import SnifferEngine
+
+        written: list[str] = []
+        engine = SnifferEngine(
+            ip_to_decky={"192.168.1.10": "decky-01"},
+            write_fn=written.append,
+        )
+        # Engine should work with zero DB interaction
+        engine._log("decky-01", "tls_client_hello", src_ip="10.0.0.1", ja3="abc", ja4="def")
+        assert len(written) == 1
+        assert "decky-01" in written[0]