OpenAPIToolset drops additionalProperties dictionary request bodies #8037

Describe the Bug

An OpenAPI JSON request body described as a dictionary, for example type: object with additionalProperties: {type: string} and no named properties, produces an OpenAPIToolset tool with no body parameters. Calls silently send {}, discarding the supplied dictionary.

The declaration itself has properties: {}, so a model is not given a way to supply the dictionary. Passing it as a single body argument or as individual key/value arguments does not preserve it on current main.

Steps to Reproduce

Check out google/adk-python at 1cb926d5b8d702261ec82a26d3f1c4296e0db7d7, install the base dependencies, and run the script below with PYTHONPATH pointing to that checkout's src directory. It uses only httpx.MockTransport; no model, credentials, DNS lookup, or remote API is involved.

import asyncio
import json
import httpx
from google.adk.tools.openapi_tool.openapi_spec_parser.openapi_toolset import OpenAPIToolset

async def main():
    sent = []
    def handle(request):
        sent.append(json.loads(request.content))
        return httpx.Response(200, json={"ok": True})
    spec = {
        "openapi": "3.0.3",
        "info": {"title": "Labels", "version": "1"},
        "servers": [{"url": "https://example.invalid"}],
        "paths": {"/labels": {"post": {
            "operationId": "set_labels",
            "requestBody": {"required": True, "content": {
                "application/json": {"schema": {
                    "type": "object", "additionalProperties": {"type": "string"}
                }}
            }},
            "responses": {"200": {"description": "OK"}}
        }}}
    }
    toolset = OpenAPIToolset(
        spec_dict=spec,
        httpx_client_factory=lambda: httpx.AsyncClient(transport=httpx.MockTransport(handle)),
        allow_private_network_access=True,  # custom local transport; no DNS/network
    )
    tool = toolset.get_tool("set_labels")
    print("declaration:", tool._get_declaration().model_dump(exclude_none=True))
    payload = {"owner": "alice", "team": "red"}
    await tool.run_async(args={"body": payload}, tool_context=None)
    print("expected:", payload)
    print("sent:", sent)

asyncio.run(main())

Expected Behavior

The tool declaration exposes one dictionary body argument and the request contains {"owner":"alice","team":"red"}. Existing request bodies with named properties should retain their current flattened argument interface.

Observed Behavior

declaration parameters_json_schema.properties: {}
expected: {'owner': 'alice', 'team': 'red'}
sent: [{}]

Environment

• ADK source: 2.14.0 at 1cb926d5b8d702261ec82a26d3f1c4296e0db7d7 (main checked 2026-11-08).
• Python 3.12.13; macOS; httpx 0.28.1; FastAPI 0.139.2; Pydantic 2.13.4.
• LiteLLM: no. Model: N/A; this is deterministic tool construction/request serialization.
• Frequency: every call using this dictionary schema. Regression introduction: not bisected.

Source Trace and Proposed Contribution

OpenAPIToolset → OperationParser._process_request_body expands only schema.properties for every object body. For a dictionary that list is empty. RestApiTool._prepare_request_params then reconstructs an empty object from the absent parameters.

This is distinct from #6991/#6992: the failing schema explicitly has type: object and no composition keywords. It is also distinct from #5364's FunctionDeclaration handling and #7838's missing-schema crash; the media type here has a concrete schema.

I would like to contribute a focused fix that exposes pure dictionary bodies as one body parameter and passes their entries through, while preserving the existing interface for named-property objects. Please let me know if this is already being worked on or if a different representation is preferred. Regression coverage would include typed/free-form dictionaries, empty payloads, parameter-name collisions, and a named-property control.