diff --git a/python/packages/core/agent_framework/_mcp.py b/python/packages/core/agent_framework/_mcp.py
index a7d32007846..d9f7899898b 100644
--- a/python/packages/core/agent_framework/_mcp.py
+++ b/python/packages/core/agent_framework/_mcp.py
@@ -1138,6 +1138,8 @@ def __init__(
             )
         if max_host_payload_size_bytes is not None and max_host_payload_size_bytes <= 0:
             raise ValueError("max_host_payload_size_bytes must be positive or None.")
+        if request_timeout is not None and request_timeout <= 0:
+            raise ValueError("request_timeout must be positive or None.")
         allowed_modes = {
             "structured_first",
             "content_first",
@@ -2351,6 +2353,7 @@ async def _connect_on_owner(
             reset_discovery: If True, removes state derived from the previous session after its transport closes.
 
         Raises:
+            ValueError: If ``request_timeout`` was set to a non-positive value after construction.
             ToolException: If connection or session initialization fails.
         """
         log_level = _to_mcp_logging_level(logger.level)
@@ -2369,6 +2372,8 @@ async def _connect_on_owner(
             self._exit_stack = AsyncExitStack()
         connection = self._connection
         if connection is None:
+            if self.request_timeout is not None and self.request_timeout <= 0:
+                raise ValueError("request_timeout must be positive or None.")
             try:
                 try:
                     from mcp import Client, types
diff --git a/python/packages/core/tests/core/test_mcp.py b/python/packages/core/tests/core/test_mcp.py
index 3128023fffc..20d111c836b 100644
--- a/python/packages/core/tests/core/test_mcp.py
+++ b/python/packages/core/tests/core/test_mcp.py
@@ -1426,6 +1426,17 @@ def test_mcp_host_payload_size_limit_must_be_positive_or_none() -> None:
     assert unlimited.max_host_payload_size_bytes is None
 
 
+def test_mcp_request_timeout_must_be_positive_or_none() -> None:
+    with pytest.raises(ValueError, match="positive or None"):
+        MCPTool(name="invalid", request_timeout=0)  # type: ignore[abstract]  # ty: ignore[call-non-callable]
+
+    with pytest.raises(ValueError, match="positive or None"):
+        MCPStdioTool(name="invalid", command="test-command", request_timeout=-1)
+
+    unlimited = MCPTool(name="unlimited", request_timeout=None)  # type: ignore[abstract]  # ty: ignore[call-non-callable]
+    assert unlimited.request_timeout is None
+
+
 def test_mcp_host_payload_size_boundary_uri_serialization_and_early_abort(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None:
@@ -5421,6 +5432,25 @@ async def test_connect_preserves_request_timeout_over_24_hours() -> None:
             assert mock_client_class.call_args.kwargs["read_timeout_seconds"] == 90_000
 
 
+async def test_connect_revalidates_request_timeout_cleared_after_construction() -> None:
+    tool = MCPStdioTool(
+        name="test",
+        command="test-command",
+        load_tools=False,
+        load_prompts=False,
+        request_timeout=60,
+    )
+    tool.request_timeout = 0
+
+    with patch("mcp.Client") as mock_client_class:
+        mock_client_class.return_value = _mock_sdk_client(protocol_version="2026-07-28")
+
+        with pytest.raises(ValueError, match="request_timeout must be positive or None."):
+            await tool.connect()
+
+        assert mock_client_class.call_count == 0
+
+
 async def test_connect_retains_and_close_clears_sdk_client() -> None:
     """Test a framework-owned Client and its session share one lifecycle."""
     tool = MCPStdioTool(name="test", command="test-command", load_tools=False, load_prompts=False)