Tool calls fail with 400 Bad Request due to strict JSON Schema violations (OpenAI)

The ADK framework's ChatCompletionsRequest serialization generates JSON payloads that violate strict OpenAI/Groq requirements. Specifically, it maps empty arguments maps and empty function response objects to null or omits them entirely, which causes the downstream API to drop the context and hallucinate.

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🔴 Required Information

Describe the Bug:
When using third-party OpenAI-compatible providers like Groq, multi-turn tool calling fails. If an agent executes a tool that requires 0 arguments (like getCurrentTime), the ADK serializes the assistant's message with arguments: null or omits the arguments field. Furthermore, during tool declaration, it omits the parameters schema object entirely for zero-argument functions. Strict parsers (like Groq) reject or silently drop this malformed tool history, leading the model to lose context. Consequently, the model hallucinates raw XML-like tool calls, causing a fatal 400 Bad Request error. Additionally, JSON Schema enum types for tools are incorrectly serialized in uppercase (e.g., "STRING" instead of "string").

Steps to Reproduce:

1. Configure an ADK agent with multiple tools (e.g., getCurrentTime, getWeather).
2. Set the model provider to groq/llama-3.3-70b-versatile.
3. Send a prompt that requires executing a tool with no arguments.
4. The first tool executes, but the subsequent request to the model crashes with a 400 Bad Request.

Expected Behavior:
The ADK ChatCompletionsRequest should strictly comply with the OpenAI JSON specifications:

• arguments must be a non-null JSON string (e.g., "{}" if empty).
• content for tool roles must be a non-null string (e.g., "{}" if empty).
• parameters schema objects must exist in function declarations, even for zero-argument functions (e.g., {"type":"object", "properties":{}}).
• JSON Schema Type enums must be serialized in lowercase.

Observed Behavior:
The framework throws a 400 Bad Request exception due to model hallucinations caused by dropped context:

Exception in thread "main" java.lang.RuntimeException: java.io.IOException: HTTP request failed with status: Response{protocol=h2, code=400, message=, url=https://api.groq.com/openai/v1/chat/completions} - body: {"error":{"message":"tool call validation failed: attempted to call tool 'brave_search' which was not in request.tools","type":"invalid_request_error","code":"tool_use_failed","failed_generation":"<function=brave_search>{\"query\":\"HACK stock price\"}</function>"}}

Environment Details:

• ADK Library Version: 1.4.1-SNAPSHOT
• OS: Windows (also reproducible on macOS/Linux)
• TS Version: N/A

Model Information:

• Which model is being used: groq/llama-3.3-70b-versatile

🟡 Optional Information

How often has this issue occurred?:

• Always (100%)