diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0e6f787..ebf3415 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- `_LazyStore` thread safety — added double-checked locking to prevent duplicate `PostgresStore`/connection pool creation under concurrent access from embedding workers, cleanup thread, or parallel requests ([#164](https://github.com/cmeans/mcp-awareness/issues/164))
+
 ## [0.16.1] - 2026-04-09
 
 ### Fixed
diff --git a/README.md b/README.md
index d1b5a7c..3937fb5 100644
--- a/README.md
+++ b/README.md
@@ -14,7 +14,7 @@
 > **Your AI's memory shouldn't be locked to one app. It should follow you everywhere.**
 
 > [!NOTE]
-> Early-stage but actively deployed — 725 tests, 16 releases, in daily use across Claude.ai, Claude Code, and Claude Desktop. See [Current status](#current-status) for what's working and what's planned.
+> Early-stage but actively deployed — 729 tests, 16 releases, in daily use across Claude.ai, Claude Code, and Claude Desktop. See [Current status](#current-status) for what's working and what's planned.
 
 ## What this is
 
@@ -396,7 +396,7 @@ For single-user deployments, secret path + WAF is sufficient. For multi-user, en
 - Secret path auth + Cloudflare WAF for edge-level access control
 - Docker Compose with Postgres, optional Ollama, named Cloudflare Tunnel, or ephemeral quick tunnel
 - Request timing instrumentation and `/health` endpoint
-- 725 tests (all against real Postgres + Ollama in CI), strict type checking, CI pipeline with coverage, QA gate
+- 729 tests (all against real Postgres + Ollama in CI), strict type checking, CI pipeline with coverage, QA gate
 
 ### Not yet implemented
 - Layer 2 (baseline) detection — rolling averages and deviation calculation
diff --git a/src/mcp_awareness/server.py b/src/mcp_awareness/server.py
index e05bace..d2ddcf9 100644
--- a/src/mcp_awareness/server.py
+++ b/src/mcp_awareness/server.py
@@ -32,6 +32,7 @@
 import os
 import pathlib
 import re
+import threading
 import time
 from datetime import datetime, timezone
 from typing import Any, Literal
@@ -174,10 +175,16 @@ class _LazyStore:
     """
 
     _instance: Store | None = None
+    _lock: threading.Lock = threading.Lock()
 
     def __getattr__(self, name: str) -> Any:
+        # Double-checked locking: safe in CPython — GIL ensures atomic
+        # reference assignment, so the outer check never sees a
+        # partially-constructed object.
         if _LazyStore._instance is None:
-            _LazyStore._instance = _create_store()
+            with _LazyStore._lock:
+                if _LazyStore._instance is None:
+                    _LazyStore._instance = _create_store()
         return getattr(_LazyStore._instance, name)
 
 
diff --git a/tests/test_server.py b/tests/test_server.py
index 8521909..f4fd2a3 100644
--- a/tests/test_server.py
+++ b/tests/test_server.py
@@ -20,6 +20,7 @@
 
 import json
 import os
+import threading
 
 import pytest
 from mcp.server.fastmcp.exceptions import ToolError
@@ -81,6 +82,195 @@ def test_raises_without_database_url(self, monkeypatch: pytest.MonkeyPatch) -> N
             server_mod._create_store()
 
 
+class TestLazyStoreThreadSafety:
+    """Verify _LazyStore only creates one store under concurrent access."""
+
+    def test_concurrent_access_creates_single_instance(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Multiple threads racing through __getattr__ must produce exactly one store."""
+        call_count = 0
+        lock = threading.Lock()
+        barrier = threading.Barrier(10)
+
+        class _FakeStore:
+            def ping(self) -> str:
+                return "ok"
+
+        def _counting_create_store() -> _FakeStore:
+            nonlocal call_count
+            with lock:
+                call_count += 1
+            return _FakeStore()
+
+        monkeypatch.setattr(server_mod, "_create_store", _counting_create_store)
+
+        lazy = server_mod._LazyStore()
+        server_mod._LazyStore._instance = None
+        errors: list[Exception] = []
+
+        def _access() -> None:
+            try:
+                barrier.wait()  # all threads launch simultaneously
+                lazy.ping()
+            except Exception as exc:
+                errors.append(exc)
+
+        threads = [threading.Thread(target=_access) for _ in range(10)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join(timeout=5)
+
+        # Restore to avoid leaking into other tests
+        server_mod._LazyStore._instance = None
+
+        assert not errors, f"Threads raised: {errors}"
+        assert call_count == 1, f"_create_store called {call_count} times, expected 1"
+
+    def test_cleanup_thread_and_request_handler_race(
+        self, store: Store, pg_dsn: str, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Cleanup daemon thread and request handler must share a single store."""
+        monkeypatch.setenv("AWARENESS_DATABASE_URL", pg_dsn)
+        call_count = 0
+        count_lock = threading.Lock()
+        go = threading.Event()
+
+        real_create = server_mod._create_store
+
+        def _tracking_create_store() -> Store:
+            nonlocal call_count
+            with count_lock:
+                call_count += 1
+            return real_create()
+
+        monkeypatch.setattr(server_mod, "_create_store", _tracking_create_store)
+        server_mod._LazyStore._instance = None
+        lazy = server_mod._LazyStore()
+        errors: list[Exception] = []
+
+        def _simulate_cleanup() -> None:
+            """Simulates a daemon thread (like cleanup) accessing the store."""
+            try:
+                go.wait()
+                _ = lazy.get_stats
+            except Exception as exc:
+                errors.append(exc)
+
+        def _simulate_request() -> None:
+            """Simulates a request thread accessing the store."""
+            try:
+                go.wait()
+                _ = lazy.add
+            except Exception as exc:
+                errors.append(exc)
+
+        t_cleanup = threading.Thread(target=_simulate_cleanup, daemon=True)
+        t_request = threading.Thread(target=_simulate_request)
+        t_cleanup.start()
+        t_request.start()
+        go.set()
+        t_cleanup.join(timeout=5)
+        t_request.join(timeout=5)
+
+        server_mod._LazyStore._instance = None
+
+        assert not errors, f"Threads raised: {errors}"
+        assert call_count == 1, f"_create_store called {call_count} times, expected 1"
+
+    def test_embedding_worker_and_request_handler_race(
+        self, store: Store, pg_dsn: str, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Embedding thread pool worker and request handler must share a single store."""
+        monkeypatch.setenv("AWARENESS_DATABASE_URL", pg_dsn)
+        call_count = 0
+        count_lock = threading.Lock()
+        go = threading.Event()
+
+        real_create = server_mod._create_store
+
+        def _tracking_create_store() -> Store:
+            nonlocal call_count
+            with count_lock:
+                call_count += 1
+            return real_create()
+
+        monkeypatch.setattr(server_mod, "_create_store", _tracking_create_store)
+        server_mod._LazyStore._instance = None
+        lazy = server_mod._LazyStore()
+        errors: list[Exception] = []
+
+        def _simulate_embedding() -> None:
+            """Simulates an embedding worker accessing store.upsert_embedding."""
+            try:
+                go.wait()
+                _ = lazy.upsert_embedding
+            except Exception as exc:
+                errors.append(exc)
+
+        def _simulate_request() -> None:
+            """Simulates a request thread accessing store.get_knowledge."""
+            try:
+                go.wait()
+                _ = lazy.get_knowledge
+            except Exception as exc:
+                errors.append(exc)
+
+        t_embed = threading.Thread(target=_simulate_embedding, name="embed-0")
+        t_request = threading.Thread(target=_simulate_request)
+        t_embed.start()
+        t_request.start()
+        go.set()
+        t_embed.join(timeout=5)
+        t_request.join(timeout=5)
+
+        server_mod._LazyStore._instance = None
+
+        assert not errors, f"Threads raised: {errors}"
+        assert call_count == 1, f"_create_store called {call_count} times, expected 1"
+
+    def test_concurrent_real_postgres_store_creation(
+        self, pg_dsn: str, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Integration: 10 threads racing to create a real PostgresStore + pool."""
+        monkeypatch.setenv("AWARENESS_DATABASE_URL", pg_dsn)
+        call_count = 0
+        count_lock = threading.Lock()
+        barrier = threading.Barrier(10)
+
+        real_create = server_mod._create_store
+
+        def _tracking_create_store() -> Store:
+            nonlocal call_count
+            with count_lock:
+                call_count += 1
+            return real_create()
+
+        monkeypatch.setattr(server_mod, "_create_store", _tracking_create_store)
+        server_mod._LazyStore._instance = None
+        lazy = server_mod._LazyStore()
+        errors: list[Exception] = []
+
+        def _access() -> None:
+            try:
+                barrier.wait()  # synchronize start, not factory entry
+                _ = lazy.add  # triggers __getattr__ → _create_store
+            except Exception as exc:
+                errors.append(exc)
+
+        threads = [threading.Thread(target=_access) for _ in range(10)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join(timeout=10)
+
+        server_mod._LazyStore._instance = None
+
+        assert not errors, f"Threads raised: {errors}"
+        assert call_count == 1, f"_create_store called {call_count} times, expected 1"
+
+
 # ---------------------------------------------------------------------------
 # Resource tests
 # ---------------------------------------------------------------------------