Ensure MCP works when inputSchema.properties is missing (#454)

Resolves #449 - TLDR, [OpenAI's
API](https://platform.openai.com/docs/api-reference/responses/create)
expects the properties field to be present, whereas the MCP schema
explicitly allows omitting the properties field. [MCP
Spec](https://github.com/modelcontextprotocol/specification/blob/main/schema/2025-03-26/schema.json)

src/agents/mcp/util.py

@@ -59,6 +59,11 @@ class MCPUtil:
         """Convert an MCP tool to an Agents SDK function tool."""
         invoke_func = functools.partial(cls.invoke_mcp_tool, server, tool)
         schema, is_strict = tool.inputSchema, False
+
+        # MCP spec doesn't require the inputSchema to have `properties`, but OpenAI spec does.
+        if "properties" not in schema:
+            schema["properties"] = {}
+
         if convert_schemas_to_strict:
             try:
                 schema = ensure_strict_json_schema(schema)

tests/mcp/test_mcp_util.py

@@ -63,7 +63,10 @@ async def test_get_all_function_tools():
 
     for idx, tool in enumerate(tools):
         assert isinstance(tool, FunctionTool)
-        assert tool.params_json_schema == schemas[idx]
+        if schemas[idx] == {}:
+            assert tool.params_json_schema == snapshot({"properties": {}})
+        else:
+            assert tool.params_json_schema == schemas[idx]
         assert tool.name == names[idx]
 
     # Also make sure it works with strict schemas
@@ -167,10 +170,7 @@ async def test_agent_convert_schemas_true():
 
     # Checks that additionalProperties is set to False
     assert bar_tool.params_json_schema == snapshot(
-        {
-            "type": "object",
-            "additionalProperties": {"type": "string"},
-        }
+        {"type": "object", "additionalProperties": {"type": "string"}, "properties": {}}
     )
     assert bar_tool.strict_json_schema is False, "bar_tool should not be strict"
 
@@ -220,7 +220,9 @@ async def test_agent_convert_schemas_false():
     assert foo_tool.params_json_schema == strict_schema
     assert foo_tool.strict_json_schema is False, "Shouldn't be converted unless specified"
 
-    assert bar_tool.params_json_schema == non_strict_schema
+    assert bar_tool.params_json_schema == snapshot(
+        {"type": "object", "additionalProperties": {"type": "string"}, "properties": {}}
+    )
     assert bar_tool.strict_json_schema is False
 
     assert baz_tool.params_json_schema == possible_to_convert_schema
@@ -255,8 +257,35 @@ async def test_agent_convert_schemas_unset():
     assert foo_tool.params_json_schema == strict_schema
     assert foo_tool.strict_json_schema is False, "Shouldn't be converted unless specified"
 
-    assert bar_tool.params_json_schema == non_strict_schema
+    assert bar_tool.params_json_schema == snapshot(
+        {"type": "object", "additionalProperties": {"type": "string"}, "properties": {}}
+    )
     assert bar_tool.strict_json_schema is False
 
     assert baz_tool.params_json_schema == possible_to_convert_schema
     assert baz_tool.strict_json_schema is False, "Shouldn't be converted unless specified"
+
+
+@pytest.mark.asyncio
+async def test_util_adds_properties():
+    """The MCP spec doesn't require the inputSchema to have `properties`, so we need to add it
+    if it's missing.
+    """
+    schema = {
+        "type": "object",
+        "description": "Test tool",
+    }
+
+    server = FakeMCPServer()
+    server.add_tool("test_tool", schema)
+
+    tools = await MCPUtil.get_all_function_tools([server], convert_schemas_to_strict=False)
+    tool = next(tool for tool in tools if tool.name == "test_tool")
+
+    assert isinstance(tool, FunctionTool)
+    assert "properties" in tool.params_json_schema
+    assert tool.params_json_schema["properties"] == {}
+
+    assert tool.params_json_schema == snapshot(
+        {"type": "object", "description": "Test tool", "properties": {}}
+    )