diff --git a/src/mcp/server/lowlevel/server.py b/src/mcp/server/lowlevel/server.py
index 59de0ace4..df212cbdc 100644
--- a/src/mcp/server/lowlevel/server.py
+++ b/src/mcp/server/lowlevel/server.py
@@ -567,6 +567,7 @@ def streamable_http_app(
         stateless_http: bool = False,
         event_store: EventStore | None = None,
         retry_interval: int | None = None,
+        session_idle_timeout: float | None = None,
         transport_security: TransportSecuritySettings | None = None,
         host: str = "127.0.0.1",
         auth: AuthSettings | None = None,
@@ -591,6 +592,7 @@ def streamable_http_app(
             json_response=json_response,
             stateless=stateless_http,
             security_settings=transport_security,
+            session_idle_timeout=session_idle_timeout,
         )
         self._session_manager = session_manager
 
diff --git a/src/mcp/server/mcpserver/server.py b/src/mcp/server/mcpserver/server.py
index be77705da..3c21e7aed 100644
--- a/src/mcp/server/mcpserver/server.py
+++ b/src/mcp/server/mcpserver/server.py
@@ -1050,6 +1050,7 @@ def streamable_http_app(
         stateless_http: bool = False,
         event_store: EventStore | None = None,
         retry_interval: int | None = None,
+        session_idle_timeout: float | None = None,
         transport_security: TransportSecuritySettings | None = None,
         host: str = "127.0.0.1",
     ) -> Starlette:
@@ -1060,6 +1061,7 @@ def streamable_http_app(
             stateless_http=stateless_http,
             event_store=event_store,
             retry_interval=retry_interval,
+            session_idle_timeout=session_idle_timeout,
             transport_security=transport_security,
             host=host,
             auth=self.settings.auth,
diff --git a/src/mcp/server/streamable_http.py b/src/mcp/server/streamable_http.py
index f14201857..52f760db0 100644
--- a/src/mcp/server/streamable_http.py
+++ b/src/mcp/server/streamable_http.py
@@ -173,6 +173,10 @@ def __init__(
         self._terminated = False
         # Idle timeout cancel scope; managed by the session manager.
         self.idle_scope: anyio.CancelScope | None = None
+        # Number of requests currently in flight on this session. While > 0,
+        # the session manager suspends the idle deadline so that a long-running
+        # request cannot be reaped mid-flight.
+        self.idle_active_requests: int = 0
 
     @property
     def is_terminated(self) -> bool:
diff --git a/src/mcp/server/streamable_http_manager.py b/src/mcp/server/streamable_http_manager.py
index c25314eab..6e357a7f0 100644
--- a/src/mcp/server/streamable_http_manager.py
+++ b/src/mcp/server/streamable_http_manager.py
@@ -4,6 +4,7 @@
 
 import contextlib
 import logging
+import math
 from collections.abc import AsyncIterator
 from http import HTTPStatus
 from typing import TYPE_CHECKING, Any
@@ -136,6 +137,32 @@ async def lifespan(app: Starlette) -> AsyncIterator[None]:
                 # Clear any remaining server instances
                 self._server_instances.clear()
 
+    @contextlib.asynccontextmanager
+    async def _suspend_idle_timeout(self, transport: StreamableHTTPServerTransport) -> AsyncIterator[None]:
+        """Suspend the idle-timeout deadline while a request is in flight.
+
+        The idle timeout exists to reap sessions that receive no HTTP requests
+        for ``session_idle_timeout`` seconds; a request that is currently being
+        processed should not count as an idle session. While at least one
+        request is in flight we set the deadline to ``math.inf``; once the
+        last concurrent request completes we restore ``now + timeout``.
+        """
+        active = transport.idle_scope is not None and self.session_idle_timeout is not None
+        if active:
+            transport.idle_active_requests += 1
+            if transport.idle_active_requests == 1:
+                assert transport.idle_scope is not None
+                transport.idle_scope.deadline = math.inf
+        try:
+            yield
+        finally:
+            if active:
+                transport.idle_active_requests -= 1
+                if transport.idle_active_requests == 0:
+                    assert transport.idle_scope is not None
+                    assert self.session_idle_timeout is not None
+                    transport.idle_scope.deadline = anyio.current_time() + self.session_idle_timeout
+
     async def handle_request(self, scope: Scope, receive: Receive, send: Send) -> None:
         """Process ASGI request with proper session handling and transport setup.
 
@@ -196,10 +223,8 @@ async def _handle_stateful_request(self, scope: Scope, receive: Receive, send: S
         if request_mcp_session_id is not None and request_mcp_session_id in self._server_instances:
             transport = self._server_instances[request_mcp_session_id]
             logger.debug("Session already exists, handling request directly")
-            # Push back idle deadline on activity
-            if transport.idle_scope is not None and self.session_idle_timeout is not None:
-                transport.idle_scope.deadline = anyio.current_time() + self.session_idle_timeout  # pragma: no cover
-            await transport.handle_request(scope, receive, send)
+            async with self._suspend_idle_timeout(transport):
+                await transport.handle_request(scope, receive, send)
             return
 
         if request_mcp_session_id is None:
@@ -267,7 +292,8 @@ async def run_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORE
                 await self._task_group.start(run_server)
 
                 # Handle the HTTP request and return the response
-                await http_transport.handle_request(scope, receive, send)
+                async with self._suspend_idle_timeout(http_transport):
+                    await http_transport.handle_request(scope, receive, send)
         else:
             # Unknown or expired session ID - return 404 per MCP spec
             # TODO: Align error code once spec clarifies
diff --git a/tests/server/test_streamable_http_manager.py b/tests/server/test_streamable_http_manager.py
index 47cfbf14a..a0e217015 100644
--- a/tests/server/test_streamable_http_manager.py
+++ b/tests/server/test_streamable_http_manager.py
@@ -413,3 +413,184 @@ def test_session_idle_timeout_rejects_non_positive():
 def test_session_idle_timeout_rejects_stateless():
     with pytest.raises(RuntimeError, match="not supported in stateless"):
         StreamableHTTPSessionManager(app=Server("test"), session_idle_timeout=30, stateless=True)
+
+
+def test_session_idle_timeout_passthrough_lowlevel():
+    """Server.streamable_http_app() exposes session_idle_timeout to its manager."""
+    app = Server("test-passthrough")
+    app.streamable_http_app(session_idle_timeout=42)
+    assert app._session_manager is not None
+    assert app._session_manager.session_idle_timeout == 42
+
+
+def test_session_idle_timeout_passthrough_mcpserver():
+    """MCPServer.streamable_http_app() forwards session_idle_timeout to the low-level wrapper."""
+    from mcp.server.mcpserver.server import MCPServer
+
+    server = MCPServer("test-passthrough-mcp")
+    server.streamable_http_app(session_idle_timeout=17)
+    assert server._lowlevel_server._session_manager is not None
+    assert server._lowlevel_server._session_manager.session_idle_timeout == 17
+
+
+@pytest.mark.anyio
+async def test_suspend_idle_timeout_sets_deadline_inf_then_restores():
+    """The helper must set deadline=inf for the duration of a request, then restore now+timeout."""
+    import math
+
+    app = Server("test-suspend")
+    manager = StreamableHTTPSessionManager(app=app, session_idle_timeout=30)
+    transport = StreamableHTTPServerTransport(mcp_session_id="s1")
+    transport.idle_scope = anyio.CancelScope()
+    transport.idle_scope.deadline = anyio.current_time() + 30
+
+    async with manager._suspend_idle_timeout(transport):
+        assert transport.idle_active_requests == 1
+        assert transport.idle_scope.deadline == math.inf
+
+    assert transport.idle_active_requests == 0
+    # Deadline restored to a finite value approximately now + timeout
+    assert transport.idle_scope.deadline != math.inf
+    assert transport.idle_scope.deadline > anyio.current_time()
+
+
+@pytest.mark.anyio
+async def test_suspend_idle_timeout_only_restores_after_last_concurrent_request():
+    """With nested suspensions the deadline stays suspended until the outermost exit."""
+    import math
+
+    app = Server("test-suspend-nested")
+    manager = StreamableHTTPSessionManager(app=app, session_idle_timeout=30)
+    transport = StreamableHTTPServerTransport(mcp_session_id="s1")
+    transport.idle_scope = anyio.CancelScope()
+    transport.idle_scope.deadline = anyio.current_time() + 30
+
+    async with manager._suspend_idle_timeout(transport):
+        async with manager._suspend_idle_timeout(transport):
+            assert transport.idle_active_requests == 2
+            assert transport.idle_scope.deadline == math.inf
+        # After the inner exit, one request still in flight — deadline must remain suspended.
+        assert transport.idle_active_requests == 1
+        assert transport.idle_scope.deadline == math.inf
+
+    assert transport.idle_active_requests == 0
+    assert transport.idle_scope.deadline != math.inf
+
+
+@pytest.mark.anyio
+async def test_suspend_idle_timeout_no_op_without_timeout():
+    """When session_idle_timeout is None the helper must not touch any state."""
+    app = Server("test-suspend-noop")
+    manager = StreamableHTTPSessionManager(app=app)  # no timeout
+    transport = StreamableHTTPServerTransport(mcp_session_id="s1")
+
+    async with manager._suspend_idle_timeout(transport):
+        assert transport.idle_active_requests == 0
+        assert transport.idle_scope is None
+
+    assert transport.idle_active_requests == 0
+
+
+@pytest.mark.anyio
+async def test_long_running_request_outlives_idle_timeout():
+    """A handler running longer than session_idle_timeout must still complete successfully.
+
+    This is the regression for the second half of #2455: previously the idle scope
+    fired mid-request and cancelled the in-flight tool call.
+    """
+    from mcp.server import ServerRequestContext
+    from mcp.types import (
+        CallToolRequestParams,
+        CallToolResult,
+        ListToolsResult,
+        PaginatedRequestParams,
+        TextContent,
+        Tool,
+    )
+
+    host = "testserver-long-request"
+    timeout = 0.1
+    handler_delay = 0.4
+
+    async def handle_list_tools(ctx: ServerRequestContext, params: PaginatedRequestParams | None) -> ListToolsResult:
+        return ListToolsResult(tools=[Tool(name="slow", input_schema={"type": "object"})])
+
+    async def handle_call_tool(ctx: ServerRequestContext, params: CallToolRequestParams) -> CallToolResult:
+        await anyio.sleep(handler_delay)
+        return CallToolResult(content=[TextContent(type="text", text="done")])
+
+    app = Server("test-slow-handler", on_list_tools=handle_list_tools, on_call_tool=handle_call_tool)
+    mcp_app = app.streamable_http_app(host=host, session_idle_timeout=timeout)
+
+    async with (
+        mcp_app.router.lifespan_context(mcp_app),
+        httpx.ASGITransport(mcp_app) as transport,
+        httpx.AsyncClient(transport=transport) as http_client,
+        Client(streamable_http_client(f"http://{host}/mcp", http_client=http_client)) as client,
+    ):
+        result = await client.call_tool("slow", {})
+        assert not result.is_error
+        assert result.content[0].text == "done"  # type: ignore[union-attr]
+
+
+@pytest.mark.anyio
+async def test_idle_session_reaped_after_request_completes():
+    """After a request finishes the idle deadline resumes; the session is eventually reaped."""
+    app = Server("test-reap-after-request")
+    timeout = 0.05
+    manager = StreamableHTTPSessionManager(app=app, session_idle_timeout=timeout)
+
+    async with manager.run():
+        sent_messages: list[Message] = []
+
+        async def mock_send(message: Message):
+            sent_messages.append(message)
+
+        scope: dict[str, Any] = {
+            "type": "http",
+            "method": "POST",
+            "path": "/mcp",
+            "headers": [(b"content-type", b"application/json")],
+        }
+
+        async def mock_receive():  # pragma: no cover
+            return {"type": "http.request", "body": b"", "more_body": False}
+
+        await manager.handle_request(scope, mock_receive, mock_send)
+
+        session_id: str | None = None
+        for msg in sent_messages:  # pragma: no branch
+            if msg["type"] == "http.response.start":  # pragma: no branch
+                for header_name, header_value in msg.get("headers", []):  # pragma: no branch
+                    if header_name.decode().lower() == MCP_SESSION_ID_HEADER.lower():
+                        session_id = header_value.decode()
+                        break
+                if session_id:  # pragma: no branch
+                    break
+        assert session_id is not None
+
+        # Once the request completed the idle deadline should have resumed; wait it out.
+        await anyio.sleep(timeout * 4)
+
+        response_messages: list[Message] = []
+
+        async def capture_send(message: Message):
+            response_messages.append(message)
+
+        scope_with_session: dict[str, Any] = {
+            "type": "http",
+            "method": "POST",
+            "path": "/mcp",
+            "headers": [
+                (b"content-type", b"application/json"),
+                (b"mcp-session-id", session_id.encode()),
+            ],
+        }
+        await manager.handle_request(scope_with_session, mock_receive, capture_send)
+
+        response_start = next(
+            (msg for msg in response_messages if msg["type"] == "http.response.start"),
+            None,
+        )
+        assert response_start is not None
+        assert response_start["status"] == 404