fix: fall back to strict regex when path contains {#...} or literal #

_split_query_tail enabled lenient matching for page{#section}{?q},
but lenient matching's partition('#') stripped the fragment before
the path regex (which expects #section) could see it, causing
fullmatch to always fail.

Extended the path-portion fallback check to also bail on {#...}
expressions and literal # characters, mirroring the existing ? check.
Such templates are semantically unusual (query-after-fragment is not
valid URI structure) but now round-trip correctly via strict regex.

Author: maxisbey

src/mcp/shared/uri_template.py

@@ -579,14 +579,15 @@ def _split_query_tail(parts: list[_Part]) -> tuple[list[_Part], list[Variable]]:
     if first.operator != "?":
         return parts, []
 
-    # If the path portion contains a literal ? or a {?...} expression,
-    # the URI's ? split won't align with our template boundary. Fall
-    # back to strict regex.
+    # If the path portion contains a literal ?/# or a {?...}/{#...}
+    # expression, lenient matching's partition("#") then partition("?")
+    # would strip content the path regex expects to see. Fall back to
+    # strict regex.
     for part in parts[:split]:
         if isinstance(part, str):
-            if "?" in part:
+            if "?" in part or "#" in part:
                 return parts, []
-        elif part.operator == "?":
+        elif part.operator in ("?", "#"):
             return parts, []
 
     query_vars: list[Variable] = []

tests/shared/test_uri_template.py

@@ -452,6 +452,10 @@ def test_expand_rejects_invalid_value_types(value: object):
         ("api?x{?page}", "api?x?page=2", {"page": "2"}),
         # {?...} expression in path portion also falls through
         ("api{?q}x{?page}", "api?q=1x?page=2", {"q": "1", "page": "2"}),
+        # {#...} or literal # in path portion falls through: lenient
+        # matching would strip the fragment before the path regex sees it
+        ("page{#section}{?q}", "page#intro?q=x", {"section": "intro", "q": "x"}),
+        ("page#lit{?q}", "page#lit?q=x", {"q": "x"}),
         # Empty & segments in query are skipped
         ("search{?q}", "search?&q=hello&", {"q": "hello"}),
         # Duplicate query keys keep first value