[v1.x] fix: reject trailing newline in tool-name validation (#3086)

Co-authored-by: otiscuilei <otiscui@icloud.com>

Author: maxisbey

src/mcp/server/fastmcp/resources/templates.py

@@ -86,7 +86,7 @@ def matches(self, uri: str) -> dict[str, Any] | None:
         """Check if URI matches template and extract parameters."""
         # Convert template to regex pattern
         pattern = self.uri_template.replace("{", "(?P<").replace("}", ">[^/]+)")
-        match = re.match(f"^{pattern}$", uri)
+        match = re.fullmatch(pattern, uri)
         if match:
             return match.groupdict()
         return None

src/mcp/shared/tool_name_validation.py

@@ -77,7 +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()

tests/server/fastmcp/resources/test_resource_template.py

@@ -48,6 +48,21 @@ def my_func(key: str, value: int) -> dict[str, Any]:  # pragma: no cover
         assert template.matches("test://foo") is None
         assert template.matches("other://foo/123") is None
 
+    def test_template_matches_rejects_trailing_newline_after_literal(self):
+        """A trailing newline after a literal segment slipped past `$` with re.match."""
+
+        def my_func(key: str) -> str:  # pragma: no cover
+            return key
+
+        template = ResourceTemplate.from_function(
+            fn=my_func,
+            uri_template="test://{key}/data",
+            name="test",
+        )
+
+        assert template.matches("test://foo/data") == {"key": "foo"}
+        assert template.matches("test://foo/data\n") is None
+
     @pytest.mark.anyio
     async def test_create_resource(self):
         """Test creating a resource from a template."""

tests/shared/test_tool_name_validation.py

@@ -66,12 +66,17 @@ def test_rejects_name_exceeding_max_length(self) -> 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_rejects_invalid_characters(self, tool_name: str, expected_char: str) -> None: