Fix outputSchema to conform to MCP spec's object type requirement (#799)

When a stdio server had a tool that didn't return a dict, then:
```
  {
    "code": "invalid_value",
    "values": [
      "object"
    ],
    "path": [
      "tools",
      2,
      "outputSchema",
      "type"
    ],
    "message": "Invalid input: expected \"object\""
  }
```

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes the generated `outputSchema` shape for all non-`json` return
types by wrapping them under a `result` property, which may affect
clients/tests expecting primitive/array schemas despite being
spec-correct.
> 
> **Overview**
> Adjusts MCP tool `outputSchema` generation to **always** emit an
object schema, per the MCP spec that `structuredContent` must be a JSON
object.
> 
> `json` outputs remain a direct object schema, while primitive/array
outputs are now wrapped as `{ "type": "object", "properties": {
"result": <inner> } }` (preserving `enum`/`items`), and tests are
expanded to cover these cases. Bumps `arcade-mcp-server` version to
`1.18.0`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
7dd13bd33d6fdf6ebb778e1a3d9167ca89806f55. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

libs/arcade-mcp-server/arcade_mcp_server/convert.py

@@ -229,25 +229,42 @@ def build_input_schema_from_definition(definition: ToolDefinition) -> dict[str,
 
 
 def _build_value_schema_json(value_schema: Any) -> dict[str, Any]:
-    """Map a ValueSchema to a JSON schema fragment for outputSchema."""
-    schema: dict[str, Any] = {
-        "type": _map_type_to_json_schema_type(getattr(value_schema, "val_type", None)),
+    """Map a ValueSchema to a JSON Schema ``outputSchema``.
+
+    Per the MCP specification, ``outputSchema.type`` MUST be ``"object"``
+    because ``structuredContent`` is always a JSON object.
+
+    * **object** return types (``val_type == "json"``) are emitted directly
+      as ``{"type": "object", "properties": {…}}``.
+    * All other return types (primitives, arrays) are wrapped in
+      ``{"type": "object", "properties": {"result": <inner>}}`` to mirror
+      the wrapping performed at runtime by
+      :func:`convert_content_to_structured_content`.
+    """
+    val_type = getattr(value_schema, "val_type", None)
+
+    if val_type == "json":
+        schema: dict[str, Any] = {"type": "object"}
+        if getattr(value_schema, "properties", None):
+            schema["properties"] = {}
+            for prop_name, prop_schema in value_schema.properties.items():
+                schema["properties"][prop_name] = {
+                    "type": _map_type_to_json_schema_type(getattr(prop_schema, "val_type", None))
+                }
+                if getattr(prop_schema, "description", None):
+                    schema["properties"][prop_name]["description"] = prop_schema.description
+        return schema
+
+    inner_schema: dict[str, Any] = {
+        "type": _map_type_to_json_schema_type(val_type),
     }
     if getattr(value_schema, "enum", None):
-        schema["enum"] = list(value_schema.enum)
-    if getattr(value_schema, "val_type", None) == "array" and getattr(
-        value_schema, "inner_val_type", None
-    ):
-        schema["items"] = {"type": _map_type_to_json_schema_type(value_schema.inner_val_type)}
-    if getattr(value_schema, "val_type", None) == "json" and getattr(
-        value_schema, "properties", None
-    ):
-        schema["type"] = "object"
-        schema["properties"] = {}
-        for prop_name, prop_schema in value_schema.properties.items():
-            schema["properties"][prop_name] = {
-                "type": _map_type_to_json_schema_type(getattr(prop_schema, "val_type", None))
-            }
-            if getattr(prop_schema, "description", None):
-                schema["properties"][prop_name]["description"] = prop_schema.description
-    return schema
+        inner_schema["enum"] = list(value_schema.enum)
+    if val_type == "array" and getattr(value_schema, "inner_val_type", None):
+        inner_schema["items"] = {"type": _map_type_to_json_schema_type(value_schema.inner_val_type)}
+    return {
+        "type": "object",
+        "properties": {
+            "result": inner_schema,
+        },
+    }

libs/arcade-mcp-server/pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "arcade-mcp-server"
-version = "1.17.5"
+version = "1.18.0"
 description = "Model Context Protocol (MCP) server framework for Arcade.dev"
 readme = "README.md"
 authors = [{ name = "Arcade.dev" }]

libs/tests/arcade_mcp_server/test_convert.py

@@ -375,6 +375,111 @@ class TestCreateMCPTool:
         """Test that output schema is included when definition has one."""
         mcp_tool = create_mcp_tool(materialized_tool)
 
-        # The fixture's output has value_schema=ValueSchema(val_type="number")
+        # The fixture's output has value_schema=ValueSchema(val_type="number").
+        # Per MCP spec, outputSchema.type must be "object"; non-object return
+        # types are wrapped in {"result": <inner>}.
         assert mcp_tool.outputSchema is not None
-        assert mcp_tool.outputSchema["type"] == "number"
+        assert mcp_tool.outputSchema["type"] == "object"
+        assert mcp_tool.outputSchema["properties"]["result"]["type"] == "number"
+
+    def _make_tool_with_output(self, value_schema: ValueSchema):
+        """Helper to create a materialized tool with a given output ValueSchema."""
+        tool_def = ToolDefinition(
+            name="test",
+            fully_qualified_name="Test.test",
+            description="Test",
+            toolkit=ToolkitDefinition(name="Test"),
+            input=ToolInput(parameters=[]),
+            output=ToolOutput(
+                description="Test output",
+                value_schema=value_schema,
+            ),
+            requirements=ToolRequirements(),
+        )
+
+        @tool
+        def f() -> Annotated[str, "result"]:
+            return "result"
+
+        input_model, output_model = create_func_models(f)
+        meta = ToolMeta(module=f.__module__, toolkit=tool_def.toolkit.name)
+        mat_tool = MaterializedTool(
+            tool=f,
+            definition=tool_def,
+            meta=meta,
+            input_model=input_model,
+            output_model=output_model,
+        )
+        return create_mcp_tool(mat_tool)
+
+    @pytest.mark.parametrize(
+        "val_type",
+        ["string", "integer", "number", "boolean"],
+    )
+    def test_output_schema_primitive_types_wrapped_as_object(self, val_type):
+        """Primitive output types must be wrapped so outputSchema.type == 'object'."""
+        mcp_tool = self._make_tool_with_output(ValueSchema(val_type=val_type))
+        schema = mcp_tool.outputSchema
+
+        assert schema is not None
+        assert schema["type"] == "object"
+        expected_json_type = {
+            "string": "string",
+            "integer": "integer",
+            "number": "number",
+            "boolean": "boolean",
+        }[val_type]
+        assert schema["properties"]["result"]["type"] == expected_json_type
+
+    def test_output_schema_array_type_wrapped_as_object(self):
+        """Array output type must be wrapped so outputSchema.type == 'object'."""
+        mcp_tool = self._make_tool_with_output(
+            ValueSchema(val_type="array", inner_val_type="string")
+        )
+        schema = mcp_tool.outputSchema
+
+        assert schema is not None
+        assert schema["type"] == "object"
+        result_prop = schema["properties"]["result"]
+        assert result_prop["type"] == "array"
+        assert result_prop["items"]["type"] == "string"
+
+    def test_output_schema_enum_preserved_in_wrapper(self):
+        """Enum values must be preserved inside the wrapped result property."""
+        mcp_tool = self._make_tool_with_output(
+            ValueSchema(val_type="string", enum=["a", "b", "c"])
+        )
+        schema = mcp_tool.outputSchema
+
+        assert schema is not None
+        assert schema["type"] == "object"
+        assert schema["properties"]["result"]["enum"] == ["a", "b", "c"]
+
+    def test_output_schema_json_type_not_wrapped(self):
+        """Object (json) output types are already type 'object', not wrapped."""
+        mcp_tool = self._make_tool_with_output(
+            ValueSchema(
+                val_type="json",
+                properties={
+                    "name": ValueSchema(val_type="string", description="A name"),
+                    "count": ValueSchema(val_type="integer"),
+                },
+            )
+        )
+        schema = mcp_tool.outputSchema
+
+        assert schema is not None
+        assert schema["type"] == "object"
+        assert "result" not in schema.get("properties", {})
+        assert schema["properties"]["name"]["type"] == "string"
+        assert schema["properties"]["name"]["description"] == "A name"
+        assert schema["properties"]["count"]["type"] == "integer"
+
+    def test_output_schema_json_type_without_properties(self):
+        """Object (json) output type with no properties is a bare object schema."""
+        mcp_tool = self._make_tool_with_output(ValueSchema(val_type="json"))
+        schema = mcp_tool.outputSchema
+
+        assert schema is not None
+        assert schema["type"] == "object"
+        assert "properties" not in schema