diff --git a/aiofmp/base.py b/aiofmp/base.py
index 68c470a..b67c455 100644
--- a/aiofmp/base.py
+++ b/aiofmp/base.py
@@ -140,6 +140,15 @@ def __init__(
         # Session management
         self._session: aiohttp.ClientSession | None = None
         self._session_owner = True
+        # Reference count of active start()/`async with` scopes. A shared
+        # client (e.g. the MCP server's global singleton) is entered
+        # concurrently by many in-flight requests, each wrapping its work in
+        # `async with client:`. Closing the session on the FIRST scope exit
+        # would tear it out from under the others ("Connector is closed",
+        # then a None session -> "'NoneType' object has no attribute 'get'").
+        # Instead we only close once the LAST concurrent scope exits.
+        self._session_refcount = 0
+        self._session_lock = asyncio.Lock()
 
         # Rate limiting
         self._request_semaphore = asyncio.Semaphore(max_concurrent_requests)
@@ -171,19 +180,40 @@ async def __aexit__(self, exc_type, exc_val, exc_tb):
         await self.close()
 
     async def start(self):
-        """Start the client session if not already started"""
-        if self._session is None:
-            timeout = aiohttp.ClientTimeout(total=self.timeout)
-            self._session = aiohttp.ClientSession(timeout=timeout)
-            self._session_owner = True
-            logger.debug("FMP client session started")
+        """Start the client session if not already started.
+
+        Reference-counted: concurrent ``async with client:`` scopes on a shared
+        client all share a single session. See :meth:`close`.
+        """
+        async with self._session_lock:
+            if self._session is None:
+                timeout = aiohttp.ClientTimeout(total=self.timeout)
+                self._session = aiohttp.ClientSession(timeout=timeout)
+                self._session_owner = True
+                logger.debug("FMP client session started")
+            # Count the scope only after a session is guaranteed to exist, so a
+            # failed session creation can't leak a reference (a raising
+            # __aenter__ means __aexit__/close() never runs to balance it).
+            self._session_refcount += 1
 
     async def close(self):
-        """Close the client session"""
-        if self._session_owner and self._session:
-            await self._session.close()
-            self._session = None
-            logger.debug("FMP client session closed")
+        """Close the client session once the last active scope exits.
+
+        Decrements the scope reference count and only tears down the
+        underlying session when it reaches zero, so overlapping requests on a
+        shared client keep a live session for the duration of their own scope.
+        """
+        async with self._session_lock:
+            if self._session_refcount > 0:
+                self._session_refcount -= 1
+            if (
+                self._session_refcount == 0
+                and self._session_owner
+                and self._session is not None
+            ):
+                await self._session.close()
+                self._session = None
+                logger.debug("FMP client session closed")
 
     async def _make_request(
         self, endpoint: str, params: dict[str, Any] | None = None, method: str = "GET"
diff --git a/tests/test_base.py b/tests/test_base.py
new file mode 100644
index 0000000..d49e910
--- /dev/null
+++ b/tests/test_base.py
@@ -0,0 +1,100 @@
+"""
+Unit tests for FMPBaseClient session lifecycle.
+
+Regression coverage for the shared-session concurrency race: the MCP server
+reuses a single global client that many in-flight requests enter concurrently
+via ``async with client:``. The first scope to exit must NOT close the session
+out from under the others (which produced "Connector is closed" and
+"'NoneType' object has no attribute 'get'" in production).
+"""
+
+import asyncio
+from unittest.mock import patch
+
+import pytest
+
+from aiofmp.base import FMPBaseClient
+
+
+class _FakeSession:
+    """Minimal stand-in for aiohttp.ClientSession that tracks close()."""
+
+    instances: list["_FakeSession"] = []
+
+    def __init__(self, *args, **kwargs):
+        self.closed = False
+        _FakeSession.instances.append(self)
+
+    async def close(self):
+        self.closed = True
+
+
+@pytest.fixture(autouse=True)
+def _reset_fake_sessions():
+    _FakeSession.instances.clear()
+    yield
+    _FakeSession.instances.clear()
+
+
+class TestSessionLifecycle:
+    """Reference-counted start()/close() for safe concurrent use."""
+
+    @pytest.mark.asyncio
+    async def test_overlapping_scopes_share_one_session(self):
+        client = FMPBaseClient(api_key="test")
+        with patch("aiofmp.base.aiohttp.ClientSession", _FakeSession):
+            await client.start()  # scope A enters
+            session = client._session
+            assert session is not None
+
+            await client.start()  # scope B enters (concurrent)
+            assert client._session is session  # reuse, not a second session
+            assert len(_FakeSession.instances) == 1
+
+            # Scope A exits — session MUST survive for still-active scope B.
+            await client.close()
+            assert client._session is session
+            assert session.closed is False
+
+            # Scope B exits — last user gone, now it closes.
+            await client.close()
+            assert client._session is None
+            assert session.closed is True
+
+    @pytest.mark.asyncio
+    async def test_new_session_created_after_full_drain(self):
+        client = FMPBaseClient(api_key="test")
+        with patch("aiofmp.base.aiohttp.ClientSession", _FakeSession):
+            await client.start()
+            first = client._session
+            await client.close()
+            assert client._session is None
+
+            # A later request re-opens a fresh session.
+            await client.start()
+            assert client._session is not None
+            assert client._session is not first
+            await client.close()
+
+    @pytest.mark.asyncio
+    async def test_concurrent_enter_exit_keeps_session_alive_mid_flight(self):
+        """Mimics repro: many concurrent `async with` scopes, one shared client."""
+        client = FMPBaseClient(api_key="test")
+        observed_closed_mid_request = []
+
+        with patch("aiofmp.base.aiohttp.ClientSession", _FakeSession):
+
+            async def one_request():
+                async with client:  # start() / close()
+                    # Yield control so other scopes interleave enter/exit here.
+                    await asyncio.sleep(0)
+                    # The session we're about to "use" must be open.
+                    observed_closed_mid_request.append(client._session is None)
+                    await asyncio.sleep(0)
+
+            await asyncio.gather(*[one_request() for _ in range(20)])
+
+        # No request ever saw a torn-down session while inside its own scope.
+        assert not any(observed_closed_mid_request)
+        # And everything is cleaned up after the last scope exits.
+        assert client._session is None