Follow-up fix for Invalid schema in MCP tool (ref #343)

[pedro-inf-custodio]

Description

This PR follows up on #343, which was previously closed. After its closure, additional analysis and evidence were shared showing that the issue still persists (discussion in #343 ).

As some time has passed, I’m opening this new PR to continue the discussion in a clean state and propose a concrete fix based on that analysis.

Summary

• The issue is still reproducible under certain conditions.
• This PR addresses the remaining problem and includes supporting changes where needed.

Reproduction

from langchain_mcp_adapters.client import MultiServerMCPClient
from langchain.agents import create_agent
from langchain_openai import AzureChatOpenAI

mcp_config = {
    "awslabs.ecs-mcp-server": {
        "command": "uvx",
        "args": ["--from", "awslabs-ecs-mcp-server", "ecs-mcp-server"],
        "env": {
            "FASTMCP_LOG_LEVEL": "ERROR",
            "ALLOW_WRITE": "false",
            "ALLOW_SENSITIVE_DATA": "false",
        },
        "transport": "stdio",
    }
}
mcp_client = MultiServerMCPClient(connections=mcp_config)
mcp_tools = await mcp_client.get_tools()


agent = create_agent(
    model=AzureChatOpenAI(
        azure_deployment="gpt-4.1",
        azure_endpoint=...,
        api_key=...,
        api_version="2024-12-01-preview",
    ),
    tools=mcp_tools,
)


agent.invoke(
    {
        "messages": [
            {
                "role": "user",
                "content": "What are the all the AWS regions where ECS is available?",
            }
        ]
    }
)