Publish FastMCP 4 (alpha) docs to gofastmcp.com (#4624)

tests/tasks/server/test_sync_function_task_disabled.py

@@ -0,0 +1,99 @@
+"""
+Tests that synchronous functions cannot be used as background tasks.
+
+SEP-2663 tasks are tools-only. Docket requires async functions for background
+execution, so FastMCP raises ValueError when task=True is used with a sync tool
+function. These are registration-time checks and need no running server.
+"""
+
+import pytest
+
+from fastmcp import FastMCP
+from fastmcp.tools.function_tool import FunctionTool
+
+
+async def test_sync_tool_with_explicit_task_true_raises():
+    """Sync tool with task=True raises ValueError."""
+    mcp = FastMCP("test")
+
+    with pytest.raises(
+        ValueError, match="uses a sync function but has task execution enabled"
+    ):
+
+        @mcp.tool(task=True)
+        def sync_tool(x: int) -> int:
+            """A synchronous tool."""
+            return x * 2
+
+
+async def test_sync_tool_with_inherited_task_true_raises():
+    """Sync tool inheriting task=True from server raises ValueError."""
+    mcp = FastMCP("test", tasks=True)
+
+    with pytest.raises(
+        ValueError, match="uses a sync function but has task execution enabled"
+    ):
+
+        @mcp.tool()  # Inherits task=True from server
+        def sync_tool(x: int) -> int:
+            """A synchronous tool."""
+            return x * 2
+
+
+async def test_async_tool_with_task_true_remains_enabled():
+    """Async tools with task=True keep task support enabled."""
+    mcp = FastMCP("test")
+
+    @mcp.tool(task=True)
+    async def async_tool(x: int) -> int:
+        """An async tool."""
+        return x * 2
+
+    tool = await mcp.get_tool("async_tool")
+    assert isinstance(tool, FunctionTool)
+    assert tool.task_config.mode == "optional"
+
+
+async def test_sync_tool_with_task_false_works():
+    """Sync tool with explicit task=False works (no error)."""
+    mcp = FastMCP("test", tasks=True)
+
+    @mcp.tool(task=False)  # Explicitly disable
+    def sync_tool(x: int) -> int:
+        """A synchronous tool."""
+        return x * 2
+
+    tool = await mcp.get_tool("sync_tool")
+    assert isinstance(tool, FunctionTool)
+    assert tool.task_config.mode == "forbidden"
+
+
+# =============================================================================
+# Callable classes with async __call__
+# =============================================================================
+
+
+async def test_async_callable_class_tool_with_task_true_works():
+    """Callable class with async __call__ and task=True should work."""
+    from fastmcp.tools import Tool
+
+    class AsyncCallableTool:
+        async def __call__(self, x: int) -> int:
+            return x * 2
+
+    tool = Tool.from_function(AsyncCallableTool(), task=True)
+    assert tool.task_config.mode == "optional"
+
+
+async def test_sync_callable_class_tool_with_task_true_raises():
+    """Callable class with sync __call__ and task=True should raise."""
+    from fastmcp.tools import Tool
+
+    class SyncCallableTool:
+        def __call__(self, x: int) -> int:
+            return x * 2
+
+    with pytest.raises(
+        ValueError, match="uses a sync function but has task execution enabled"
+    ):
+        Tool.from_function(SyncCallableTool(), task=True)