fix: validate whole strings with re.fullmatch

With re.match, a $-anchored pattern also matches just before a single
trailing newline, so tool-name validation accepted "name\n". Switch the
tool-name and URI-template varname checks to re.fullmatch, which puts
the whole-string requirement at the call site, and fold the regression
cases into the existing invalid-character parametrizations.

Author: maxisbey

src/mcp/shared/tool_name_validation.py

@@ -17,10 +17,8 @@
 
 logger = logging.getLogger(__name__)
 
-# Regular expression for valid tool names according to SEP-986 specification.
-# End-anchored with \Z rather than $: in Python's default mode $ also matches
-# just before a single trailing newline, which would let "name\n" validate.
-TOOL_NAME_REGEX = re.compile(r"^[A-Za-z0-9._-]{1,128}\Z")
+# Regular expression for valid tool names according to SEP-986 specification
+TOOL_NAME_REGEX = re.compile(r"^[A-Za-z0-9._-]{1,128}$")
 
 # SEP reference URL for warning messages
 SEP_986_URL = "https://modelcontextprotocol.io/specification/2025-11-25/server/tools#tool-names"
@@ -79,7 +77,7 @@ def validate_tool_name(name: str) -> ToolNameValidationResult:
         warnings.append("Tool name starts or ends with a dot, which may cause parsing issues in some contexts")
 
     # Check for invalid characters
-    if not TOOL_NAME_REGEX.match(name):
+    if not TOOL_NAME_REGEX.fullmatch(name):
         # Find all invalid characters (unique, preserving order)
         invalid_chars: list[str] = []
         seen: set[str] = set()

src/mcp/shared/uri_template.py

@@ -813,7 +813,7 @@ def _parse_expression(template: str, body: str, pos: int) -> _Expression:
         explode = spec.endswith("*")
         name = spec[:-1] if explode else spec
 
-        if not _VARNAME_RE.match(name):
+        if not _VARNAME_RE.fullmatch(name):
             raise InvalidUriTemplate(
                 f"Invalid variable name {name!r} at position {pos}",
                 template=template,

tests/shared/test_tool_name_validation.py

@@ -65,12 +65,17 @@ def test_validate_tool_name_rejects_name_exceeding_max_length() -> None:
         ("get,user,profile", "','"),
         ("user/profile/update", "'/'"),
         ("user@domain.com", "'@'"),
+        # a single trailing newline slipped past `$` with re.match
+        ("valid_name\n", "'\\n'"),
+        ("a" * 127 + "\n", "'\\n'"),
     ],
     ids=[
         "with_spaces",
         "with_commas",
         "with_slashes",
         "with_at_symbol",
+        "with_trailing_newline",
+        "max_length_with_trailing_newline",
     ],
 )
 def test_validate_tool_name_rejects_invalid_characters(tool_name: str, expected_char: str) -> None:
@@ -80,22 +85,6 @@ def test_validate_tool_name_rejects_invalid_characters(tool_name: str, expected_
     assert any("invalid characters" in w and expected_char in w for w in result.warnings)
 
 
-@pytest.mark.parametrize(
-    "tool_name",
-    ["valid_name\n", "a" * 127 + "\n"],
-    ids=["trailing_newline", "max_length_plus_newline"],
-)
-def test_validate_tool_name_rejects_trailing_newline(tool_name: str) -> None:
-    """A trailing newline is not an allowed character and must be rejected.
-
-    Regression test: Python's ``$`` (unlike ``\\Z``) also matches just before a
-    single trailing newline, so a name like ``"valid_name\\n"`` slipped through.
-    """
-    result = validate_tool_name(tool_name)
-    assert result.is_valid is False
-    assert any("invalid characters" in w for w in result.warnings)
-
-
 def test_validate_tool_name_rejects_multiple_invalid_chars() -> None:
     """Names with multiple invalid chars should list all of them."""
     result = validate_tool_name("user name@domain,com")

tests/shared/test_uri_template.py

@@ -144,6 +144,8 @@ def test_parse_rejects_operator_without_variable():
         # RFC §2.3: dots only between varchars, not consecutive or trailing
         "foo..bar",
         "foo.",
+        # a single trailing newline slipped past `$` with re.match
+        "foo\n",
     ],
 )
 def test_parse_rejects_invalid_varname(name: str):