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[SEP-1686] Raise ValueError when sync functions have task=True (#2554)
* Raise ValueError when sync functions have task=True Move validation from server.py decorators to the from_function() class methods on FunctionTool, FunctionPrompt, FunctionResource, and FunctionResourceTemplate. This ensures the check runs regardless of how the objects are created. * Fix async check for callable classes and staticmethods Move the task=True async validation to run AFTER callable classes and staticmethods are unwrapped, preventing false positives for async callable classes with sync-looking signatures.
This commit is contained in:
Jeremiah Lowin
GitHub
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commit
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8 changed files with 155 additions and 206 deletions
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@ -1,229 +1,86 @@
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"""
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Tests that synchronous functions cannot be used as background tasks.
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Docket requires async functions for background execution. FastMCP automatically
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disables task support for sync functions, with warnings for explicit task=True.
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Docket requires async functions for background execution. FastMCP raises
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ValueError when task=True is used with a sync function.
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"""
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from pytest import LogCaptureFixture
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import pytest
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from fastmcp import FastMCP
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from fastmcp.client import Client
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from fastmcp.utilities.tests import caplog_for_fastmcp
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async def test_sync_tool_with_explicit_task_true_warns_and_disables(
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caplog: LogCaptureFixture,
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):
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"""Sync tool with task=True logs warning and disables task support."""
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import logging
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async def test_sync_tool_with_explicit_task_true_raises():
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"""Sync tool with task=True raises ValueError."""
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mcp = FastMCP("test")
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with caplog_for_fastmcp(caplog):
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caplog.set_level(logging.INFO)
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mcp = FastMCP("test")
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with pytest.raises(ValueError, match="uses a sync function but has task=True"):
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@mcp.tool(task=True)
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def sync_tool(x: int) -> int:
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"""A synchronous tool."""
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logging.getLogger("fastmcp.myserver").info("I came from the tool!")
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return x * 2
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# Should have logged a warning during decoration
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assert "task=True but is synchronous" in caplog.text
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assert "Disabling task support" in caplog.text
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# Tool should have task=False after being disabled
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tool = await mcp.get_tool("sync_tool")
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assert tool.task is False
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async def test_sync_tool_with_inherited_task_true_raises():
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"""Sync tool inheriting task=True from server raises ValueError."""
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mcp = FastMCP("test", tasks=True)
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# Verify execution: even if client requests task=True, should execute immediately
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async with Client(mcp) as client:
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task = await client.call_tool("sync_tool", {"x": 5}, task=True)
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assert task.returned_immediately
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result = await task.result()
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assert result.data == 10
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# Should have seen the log from inside the function
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assert "I came from the tool!" in caplog.text
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async def test_sync_tool_with_inherited_task_true_quietly_disables(
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caplog: LogCaptureFixture,
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):
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"""Sync tool inheriting task=True from server disables quietly (no warning)."""
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import logging
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with caplog_for_fastmcp(caplog):
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caplog.set_level(logging.INFO)
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mcp = FastMCP("test", tasks=True)
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with pytest.raises(ValueError, match="uses a sync function but has task=True"):
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@mcp.tool() # Inherits task=True from server
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def sync_tool(x: int) -> int:
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"""A synchronous tool."""
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logging.getLogger("fastmcp.myserver").info("I came from the tool!")
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return x * 2
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# Should NOT have logged a warning (quietly disabled)
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assert "task=True but is synchronous" not in caplog.text
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# Tool should have task=False after being disabled
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tool = await mcp.get_tool("sync_tool")
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assert tool.task is False
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async def test_sync_prompt_with_explicit_task_true_raises():
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"""Sync prompt with task=True raises ValueError."""
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mcp = FastMCP("test")
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# Verify execution: should execute immediately
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async with Client(mcp) as client:
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task = await client.call_tool("sync_tool", {"x": 3}, task=True)
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assert task.returned_immediately
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result = await task.result()
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assert result.data == 6
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# Should have seen the log from inside the function
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assert "I came from the tool!" in caplog.text
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async def test_sync_prompt_with_explicit_task_true_warns_and_disables(
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caplog: LogCaptureFixture,
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):
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"""Sync prompt with task=True logs warning and disables task support."""
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import logging
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with caplog_for_fastmcp(caplog):
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caplog.set_level(logging.INFO)
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mcp = FastMCP("test")
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with pytest.raises(ValueError, match="uses a sync function but has task=True"):
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@mcp.prompt(task=True)
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def sync_prompt() -> str:
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"""A synchronous prompt."""
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logging.getLogger("fastmcp.myserver").info("I came from the prompt!")
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return "Hello"
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# Should have logged a warning during decoration
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assert "task=True but is synchronous" in caplog.text
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assert "Disabling task support" in caplog.text
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# Prompt should have task=False
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prompt = await mcp.get_prompt("sync_prompt")
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assert prompt.task is False
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async def test_sync_prompt_with_inherited_task_true_raises():
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"""Sync prompt inheriting task=True from server raises ValueError."""
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mcp = FastMCP("test", tasks=True)
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# Verify execution: should execute immediately
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async with Client(mcp) as client:
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task = await client.get_prompt("sync_prompt", task=True)
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assert task.returned_immediately
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result = await task.result()
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assert "Hello" in str(result)
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# Should have seen the log from inside the function
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assert "I came from the prompt!" in caplog.text
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async def test_sync_prompt_with_inherited_task_true_quietly_disables(
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caplog: LogCaptureFixture,
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):
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"""Sync prompt inheriting task=True disables quietly."""
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import logging
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with caplog_for_fastmcp(caplog):
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caplog.set_level(logging.INFO)
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mcp = FastMCP("test", tasks=True)
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with pytest.raises(ValueError, match="uses a sync function but has task=True"):
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@mcp.prompt() # Inherits task=True from server
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def sync_prompt() -> str:
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"""A synchronous prompt."""
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logging.getLogger("fastmcp.myserver").info("I came from the prompt!")
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return "Hello"
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# Should NOT have logged a warning (quietly disabled)
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assert "task=True but is synchronous" not in caplog.text
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# Prompt should have task=False
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prompt = await mcp.get_prompt("sync_prompt")
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assert prompt.task is False
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async def test_sync_resource_with_explicit_task_true_raises():
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"""Sync resource with task=True raises ValueError."""
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mcp = FastMCP("test")
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# Verify execution: should execute immediately
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async with Client(mcp) as client:
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task = await client.get_prompt("sync_prompt", task=True)
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assert task.returned_immediately
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result = await task.result()
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assert "Hello" in str(result)
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# Should have seen the log from inside the function
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assert "I came from the prompt!" in caplog.text
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async def test_sync_resource_with_explicit_task_true_warns_and_disables(
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caplog: LogCaptureFixture,
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):
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"""Sync resource with task=True logs warning and disables task support."""
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import logging
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with caplog_for_fastmcp(caplog):
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caplog.set_level(logging.INFO)
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mcp = FastMCP("test")
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with pytest.raises(ValueError, match="uses a sync function but has task=True"):
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@mcp.resource("test://sync", task=True)
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def sync_resource() -> str:
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"""A synchronous resource."""
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logging.getLogger("fastmcp.myserver").info("I came from the resource!")
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return "data"
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# Should have logged a warning during decoration
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assert "task=True but is synchronous" in caplog.text
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assert "Disabling task support" in caplog.text
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# Resource should have task=False
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resource = await mcp._resource_manager.get_resource("test://sync")
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assert resource.task is False
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async def test_sync_resource_with_inherited_task_true_raises():
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"""Sync resource inheriting task=True from server raises ValueError."""
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mcp = FastMCP("test", tasks=True)
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# Verify execution: should execute immediately
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async with Client(mcp) as client:
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task = await client.read_resource("test://sync", task=True)
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assert task.returned_immediately
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result = await task.result()
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assert "data" in str(result)
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# Should have seen the log from inside the function
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assert "I came from the resource!" in caplog.text
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async def test_sync_resource_with_inherited_task_true_quietly_disables(
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caplog: LogCaptureFixture,
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):
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"""Sync resource inheriting task=True disables quietly."""
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import logging
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with caplog_for_fastmcp(caplog):
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caplog.set_level(logging.INFO)
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mcp = FastMCP("test", tasks=True)
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with pytest.raises(ValueError, match="uses a sync function but has task=True"):
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@mcp.resource("test://sync") # Inherits task=True from server
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def sync_resource() -> str:
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"""A synchronous resource."""
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logging.getLogger("fastmcp.myserver").info("I came from the resource!")
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return "data"
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# Should NOT have logged a warning (quietly disabled)
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assert "task=True but is synchronous" not in caplog.text
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# Resource should have task=False
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resource = await mcp._resource_manager.get_resource("test://sync")
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assert resource.task is False
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# Verify execution: should execute immediately
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async with Client(mcp) as client:
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task = await client.read_resource("test://sync", task=True)
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assert task.returned_immediately
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result = await task.result()
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assert "data" in str(result)
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# Should have seen the log from inside the function
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assert "I came from the resource!" in caplog.text
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async def test_async_tool_with_task_true_remains_enabled():
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"""Async tools with task=True keep task support enabled."""
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@ -265,3 +122,85 @@ async def test_async_resource_with_task_true_remains_enabled():
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# Resource should have task=True
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resource = await mcp._resource_manager.get_resource("test://async")
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assert resource.task is True
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async def test_sync_tool_with_task_false_works():
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"""Sync tool with explicit task=False works (no error)."""
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mcp = FastMCP("test", tasks=True)
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@mcp.tool(task=False) # Explicitly disable
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def sync_tool(x: int) -> int:
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"""A synchronous tool."""
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return x * 2
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tool = await mcp.get_tool("sync_tool")
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assert tool.task is False
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async def test_sync_prompt_with_task_false_works():
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"""Sync prompt with explicit task=False works (no error)."""
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mcp = FastMCP("test", tasks=True)
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@mcp.prompt(task=False) # Explicitly disable
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def sync_prompt() -> str:
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"""A synchronous prompt."""
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return "Hello"
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prompt = await mcp.get_prompt("sync_prompt")
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assert prompt.task is False
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async def test_sync_resource_with_task_false_works():
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"""Sync resource with explicit task=False works (no error)."""
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mcp = FastMCP("test", tasks=True)
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@mcp.resource("test://sync", task=False) # Explicitly disable
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def sync_resource() -> str:
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"""A synchronous resource."""
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return "data"
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resource = await mcp._resource_manager.get_resource("test://sync")
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assert resource.task is False
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# =============================================================================
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# Callable classes and staticmethods with async __call__
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# =============================================================================
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async def test_async_callable_class_tool_with_task_true_works():
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"""Callable class with async __call__ and task=True should work."""
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from fastmcp.tools import Tool
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class AsyncCallableTool:
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async def __call__(self, x: int) -> int:
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return x * 2
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# Callable classes use Tool.from_function() directly
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tool = Tool.from_function(AsyncCallableTool(), task=True)
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assert tool.task is True
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async def test_async_callable_class_prompt_with_task_true_works():
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"""Callable class with async __call__ and task=True should work."""
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from fastmcp.prompts import Prompt
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class AsyncCallablePrompt:
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async def __call__(self) -> str:
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return "Hello"
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# Callable classes use Prompt.from_function() directly
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prompt = Prompt.from_function(AsyncCallablePrompt(), task=True)
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assert prompt.task is True
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async def test_sync_callable_class_tool_with_task_true_raises():
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"""Callable class with sync __call__ and task=True should raise."""
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from fastmcp.tools import Tool
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class SyncCallableTool:
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def __call__(self, x: int) -> int:
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return x * 2
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with pytest.raises(ValueError, match="uses a sync function but has task=True"):
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Tool.from_function(SyncCallableTool(), task=True)
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