fix(mcp): reject undeclared tool arguments instead of silently dropping them (#2709)

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>

tests/test_mcp_strict_args.py

@@ -0,0 +1,75 @@
+"""Unknown tool arguments are rejected, never silently dropped (#2709).
+
+The failure this pins: 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. The call REPORTED SUCCESS. Two real notes were persisted body-less
+before the pattern was noticed.
+
+The fix is at the dispatch seam, not per-tool: StrictArgsFastMCP rejects any
+call carrying arguments the tool does not declare, with a did-you-mean when
+one is close. An error the caller sees once beats data half-written forever.
+"""
+import pytest
+
+from scribe.mcp.server import StrictArgsFastMCP
+
+
+def _echo_server() -> StrictArgsFastMCP:
+    mcp = StrictArgsFastMCP("strict-test")
+
+    @mcp.tool()
+    def echo(text: str = "") -> str:
+        return text
+
+    return mcp
+
+
+async def test_declared_arguments_still_dispatch():
+    mcp = _echo_server()
+    result = await mcp.call_tool("echo", {"text": "hi"})
+    assert "hi" in str(result)
+
+
+async def test_unknown_argument_is_an_error_not_a_silent_drop():
+    """The load-bearing property: the call must FAIL, because succeeding is
+    what turned a typo into data loss."""
+    mcp = _echo_server()
+    with pytest.raises(ValueError) as exc:
+        await mcp.call_tool("echo", {"txt": "hi"})
+    msg = str(exc.value)
+    assert "'txt'" in msg
+    assert "did you mean 'text'?" in msg          # difflib near-miss hint
+    assert "Nothing was created or changed" in msg
+
+
+async def test_empty_arguments_pass():
+    mcp = _echo_server()
+    assert await mcp.call_tool("echo", {}) is not None
+
+
+async def test_unknown_tool_keeps_the_upstream_error():
+    """The gate must not swallow or reshape 'no such tool' — that error path
+    belongs to FastMCP and clients already understand it."""
+    mcp = _echo_server()
+    with pytest.raises(Exception) as exc:
+        await mcp.call_tool("no_such_tool", {"text": "hi"})
+    assert "no_such_tool" in str(exc.value)
+
+
+async def test_the_original_regression_create_note_with_content():
+    """Pin #2709 itself against the REAL server: `content=` on create_note
+    must raise before dispatch — naming the bad argument and listing `body`
+    among the accepted ones — instead of creating a body-less note."""
+    from scribe.mcp.server import build_mcp_server
+
+    mcp = build_mcp_server()
+    assert isinstance(mcp, StrictArgsFastMCP)   # the guard is actually mounted
+    with pytest.raises(ValueError) as exc:
+        await mcp.call_tool(
+            "create_note", {"title": "t", "content": "the body text"}
+        )
+    msg = str(exc.value)
+    assert "'content'" in msg
+    assert "body" in msg