bvandeusenandClaude Fable 5
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FastMCP validates tool arguments with a pydantic model whose extra-field policy is 'ignore', so create_note(content=...) — a plausible near-miss for body=, primed by add_task_log's content — ran successfully, stored body: '', and left a record embedding/search cannot see. Two real notes were persisted body-less before the pattern was noticed; create_task only 'worked' because those calls happened to use the right name. StrictArgsFastMCP rejects any tool call carrying arguments the tool does not declare, before dispatch, with a did-you-mean hint when one is close and the declared list when none is. Applied at the dispatch seam so every tool gets the guarantee — an error the caller sees once beats data half-written forever. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
76 lines
2.7 KiB
Python
76 lines
2.7 KiB
Python
"""Unknown tool arguments are rejected, never silently dropped (#2709).
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The failure this pins: FastMCP validates tool arguments with a pydantic model
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whose extra-field policy is "ignore", so `create_note(content=...)` — a
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plausible near-miss for `body=`, primed by add_task_log's `content` — ran
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successfully, stored `body: ""`, and left a record embedding/search cannot
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see. The call REPORTED SUCCESS. Two real notes were persisted body-less
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before the pattern was noticed.
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The fix is at the dispatch seam, not per-tool: StrictArgsFastMCP rejects any
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call carrying arguments the tool does not declare, with a did-you-mean when
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one is close. An error the caller sees once beats data half-written forever.
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"""
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import pytest
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from scribe.mcp.server import StrictArgsFastMCP
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def _echo_server() -> StrictArgsFastMCP:
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mcp = StrictArgsFastMCP("strict-test")
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@mcp.tool()
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def echo(text: str = "") -> str:
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return text
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return mcp
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async def test_declared_arguments_still_dispatch():
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mcp = _echo_server()
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result = await mcp.call_tool("echo", {"text": "hi"})
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assert "hi" in str(result)
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async def test_unknown_argument_is_an_error_not_a_silent_drop():
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"""The load-bearing property: the call must FAIL, because succeeding is
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what turned a typo into data loss."""
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mcp = _echo_server()
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with pytest.raises(ValueError) as exc:
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await mcp.call_tool("echo", {"txt": "hi"})
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msg = str(exc.value)
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assert "'txt'" in msg
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assert "did you mean 'text'?" in msg # difflib near-miss hint
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assert "Nothing was created or changed" in msg
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async def test_empty_arguments_pass():
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mcp = _echo_server()
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assert await mcp.call_tool("echo", {}) is not None
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async def test_unknown_tool_keeps_the_upstream_error():
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"""The gate must not swallow or reshape 'no such tool' — that error path
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belongs to FastMCP and clients already understand it."""
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mcp = _echo_server()
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with pytest.raises(Exception) as exc:
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await mcp.call_tool("no_such_tool", {"text": "hi"})
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assert "no_such_tool" in str(exc.value)
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async def test_the_original_regression_create_note_with_content():
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"""Pin #2709 itself against the REAL server: `content=` on create_note
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must raise before dispatch — naming the bad argument and listing `body`
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among the accepted ones — instead of creating a body-less note."""
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from scribe.mcp.server import build_mcp_server
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mcp = build_mcp_server()
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assert isinstance(mcp, StrictArgsFastMCP) # the guard is actually mounted
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with pytest.raises(ValueError) as exc:
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await mcp.call_tool(
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"create_note", {"title": "t", "content": "the body text"}
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)
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msg = str(exc.value)
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assert "'content'" in msg
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assert "body" in msg
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