diff --git a/fastapi_mcp/transport/http.py b/fastapi_mcp/transport/http.py
index 47af6f06..a730ee48 100644
--- a/fastapi_mcp/transport/http.py
+++ b/fastapi_mcp/transport/http.py
@@ -29,6 +29,7 @@ def __init__(
         self._manager_task: asyncio.Task | None = None
         self._manager_started = False
         self._startup_lock = asyncio.Lock()
+        self._ready_event: asyncio.Event | None = None
 
     async def _ensure_session_manager_started(self) -> None:
         """
@@ -57,10 +58,16 @@ async def _ensure_session_manager_started(self) -> None:
                 security_settings=self.security_settings,
             )
 
-            # Start the session manager in a background task
+            # Start the session manager in a background task and wait until its
+            # task group is initialized. A fixed sleep is unreliable and can
+            # allow requests before the message router is ready (#259).
+            ready_event = asyncio.Event()
+            self._ready_event = ready_event
+
             async def run_session_manager():
                 try:
                     async with self._session_manager.run():
+                        ready_event.set()
                         logger.info("StreamableHTTP session manager is running")
                         # Keep running until cancelled
                         await asyncio.Event().wait()
@@ -68,15 +75,17 @@ async def run_session_manager():
                     logger.info("StreamableHTTP session manager is shutting down")
                     raise
                 except Exception:
+                    if not ready_event.is_set():
+                        ready_event.set()
                     logger.exception("Error in StreamableHTTP session manager")
                     raise
 
             self._manager_task = asyncio.create_task(run_session_manager())
+            await ready_event.wait()
+            if self._manager_task.done():
+                self._manager_task.result()
             self._manager_started = True
 
-            # Give the session manager a moment to initialize
-            await asyncio.sleep(0.1)
-
     async def handle_fastapi_request(self, request: Request) -> Response:
         """
         Handle a FastAPI request by delegating to the session manager.
@@ -134,3 +143,4 @@ async def shutdown(self) -> None:
             except asyncio.CancelledError:
                 pass
         self._manager_started = False
+        self._ready_event = None
diff --git a/tests/test_http_transport.py b/tests/test_http_transport.py
new file mode 100644
index 00000000..867e6ce0
--- /dev/null
+++ b/tests/test_http_transport.py
@@ -0,0 +1,19 @@
+import pytest
+from mcp.server.lowlevel.server import Server
+
+from fastapi_mcp.transport.http import FastApiHttpSessionManager
+
+
+@pytest.mark.anyio
+async def test_session_manager_startup_waits_for_task_group() -> None:
+    """Regression for #259: startup must initialize the session manager task group."""
+    mcp_server = Server("test-server")
+    manager = FastApiHttpSessionManager(mcp_server=mcp_server)
+
+    await manager._ensure_session_manager_started()
+
+    assert manager._manager_started is True
+    assert manager._session_manager is not None
+    assert manager._session_manager._task_group is not None
+
+    await manager.shutdown()