UriTemplate.match() cannot read back prefix{.ext} when ext holds a dot, though nothing else can claim it #3693

Initial Checks

• I confirm that I'm using the newest release of my line (the latest 2.x, or the latest 1.x if I'm still on v1)
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Release line

2.x (current stable)

Description

A resource registered as backup://nightly{.ext} cannot be read when the extension holds a dot. UriTemplate.expand({"ext": "tar.gz"}) gives backup://nightly.tar.gz, because . is unreserved. match() on that same string returns None, and resources/read answers "Unknown resource" for a URI the template itself produced.

The match() docstring lists this exact case as an inherent RFC 6570 §1.4 limitation. I think that overstates it. The ambiguity needs a second capture that could claim the dot, as in {name}{.ext}. When {.ext} is the first capture after a literal, the anchored branch of _scan_suffix knows where the value starts. Nothing else can take that dot. A client that reads the template from resources/templates/list also cannot know it must send %2E.

The fix bounds a non-explode {.var} in that branch by /, ? and # alone. The docstring sentence would then cover only the case with a capture on the left, which stays unmatched. With the change, tests/shared/test_uri_template.py plus tests/server/mcpserver go from 949 passed on c9356a2 to 950. The new case is ("file{.ext}", {"ext": "tar.gz"}) in test_roundtrip_expand_then_match, and it fails without the change.

Example Code

import anyio
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}"

async def main() -> None:
    template = mcp._resource_manager.list_templates()[0].parsed_template
    uri = template.expand({"ext": "tar.gz"})
    print("expand:", uri)
    async with Client(mcp) as client:
        for u in ("backup://nightly.zip", uri):
            try:
                r = await client.read_resource(u)
                print(u, "->", r.contents[0].text)
            except Exception as e:
                print(u, "->", type(e).__name__, e)

anyio.run(main)

$ .venv/bin/python -I repro_dot.py
expand: backup://nightly.tar.gz
backup://nightly.zip -> archive in zip
backup://nightly.tar.gz -> MCPError Unknown resource: backup://nightly.tar.gz
# not run: the full test suite, only the two test paths named above

Python & MCP Python SDK

Python 3.14.7
mcp at main c9356a2; uri_template.py is unchanged since v2.3.0

@maxisbey, this is the template module from #2356. As the reporter I would like to fix it, and the change is ready. Would you narrow the limitation to the anchored case?