fix: reject trailing newline in resource template matching

The same $-with-re.match pattern: a URI with a single trailing newline
after a literal template segment matched as if clean. Switch to
re.fullmatch. Parameter segments matching raw newlines into values is a
separate pre-existing looseness, unchanged here.

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

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."""