Key query expansion off the template, not the value type

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BAKNJthovhc5VSi4MTiZUe

fastmcp_slim/fastmcp/resources/template.py

@@ -197,8 +197,12 @@ def expand_uri_template(uri_template: str, params: dict[str, Any]) -> str:
         names = [n.strip() for n in match.group(1).split(",")]
         parts = []
         for name in names:
-            # `{?tags*}` is the explode form; the value is a list, emitted as a
-            # repeated key so it round-trips back through match_uri_template.
+            # The template decides the serialization, not the runtime value:
+            # `{?tags*}` emits a repeated key, `{?tags}` stays a single value.
+            # Keying off the value type instead would expand a list under a
+            # plain `{?tags}`, which match_uri_template then reads back as just
+            # its first element.
+            exploded = name.endswith("*")
             name = name.removesuffix("*")
             underscored = name.replace("-", "_")
             if name in params:
@@ -207,8 +211,10 @@ def expand_uri_template(uri_template: str, params: dict[str, Any]) -> str:
                 value = params[underscored]
             else:
                 continue
-            values = value if isinstance(value, (list, tuple)) else [value]
-            parts.extend(f"{quote(name)}={quote(str(v))}" for v in values)
+            if exploded and isinstance(value, (list, tuple)):
+                parts.extend(f"{quote(name)}={quote(str(v))}" for v in value)
+            else:
+                parts.append(f"{quote(name)}={quote(str(value))}")
         if parts:
             return "?" + "&".join(parts)
         return ""

tests/resources/test_resource_template.py

@@ -1119,6 +1119,20 @@ class TestMatchExpandRoundTrip:
         assert params is not None
         assert expand_uri_template(template, params) == uri
 
+    def test_plain_query_param_does_not_repeat_a_sequence_value(self):
+        """The template decides repeatability, not the runtime value's type.
+
+        A list handed to a plain `{?tags}` must stay one value: expanding it as
+        a repeated key produced a URI that matched back to only its first
+        element, contradicting the scalar contract `{?tags}` documents.
+        """
+        uri = expand_uri_template("test://x{?tags}", {"tags": ["a", "b"]})
+        assert uri.count("tags=") == 1
+
+        params = match_uri_template(uri, "test://x{?tags}")
+        assert params is not None
+        assert expand_uri_template("test://x{?tags}", params) == uri
+
     @pytest.mark.parametrize(
         "template, params",
         [