fix: round-trip dotted URI template values

Author: 2Zest

src/mcp/shared/uri_template.py

@@ -997,6 +997,12 @@ def _scan_suffix(
         stops = _STOP_CHARS[var.operator]
         prev = atoms[i - 1] if i > 0 else None
 
+        # A dotted variable immediately following a literal has an unambiguous
+        # boundary: the operator's dot is the separator. Keep additional dots in
+        # the value so expansions such as `file{.ext}` with `ext="tar.gz"` round-trip.
+        if var.operator == "." and isinstance(prev, _Lit) and (i < 2 or isinstance(atoms[i - 2], _Lit)):
+            stops = "/?#"
+
         if atom.ifemp:
             # ;name or ;name=value. The preceding _Lit is ";name".
             # Try empty first: if the lit ends at pos the value is

tests/shared/test_uri_template.py

@@ -838,6 +838,7 @@ def test_match_explode_encoded_separator_in_segment():
         ("file://docs/{+path}", {"path": "src/main.py"}),
         ("search{?q,lang}", {"q": "mcp", "lang": "en"}),
         ("file{.ext}", {"ext": "txt"}),
+        ("file{.ext}", {"ext": "tar.gz"}),
         ("/files{/path*}", {"path": ["a", "b", "c"]}),
         ("{var}", {"var": "hello world"}),
         ("item{;id}", {"id": "42"}),