Gemini streaming error: "Invalid character '{' after top-level value"

[djoreilly]

Get this error sometimes when using Gemini and streaming is enabled:

agent error: convert schema message failed: unmarshal tool call arguments failed: invalid character '{' after top-level value

Reproducer:

from typing import List, Dict, Any
from fastmcp import FastMCP, Context

mcp = FastMCP("fruit-store")


@mcp.tool()
def list_fruits(ctx: Context) -> List[str]:
    """
    Returns all fruits for sale in the store.
    """
    return ["apples", "oranges"]


@mcp.tool()
def fruit_details(ctx: Context, fruit_name: str) -> Dict[str, Any]:
    """Gets details of the specified fruit

    Args:
        fruit_name: the name of the fruit.
    Returns:
        An object that contains the following:
            - fruit_name: the name of the fruit
            - price: the price of the fruit
    """
    return {"fruit_name": fruit_name, "price": 42}

{
  "mcpServers": {
    "fruit-store": {
      "command": "uv",
      "args": ["run", "fastmcp", "run", "main.py:mcp"]
    }
  }
}

$ mcphost -m google:gemini-2.5-flash --config mcphost-fruit.json

  ┃  what fruit is the cheapest?                                                                              ┃
  ┃                                                                                         doreilly (10:28)  ┃
  ┃                                                                                                           ┃

  ┃                                               ┃
  ┃   Executing fruit-store__list_fruits (10:28)  ┃
  ┃                                               ┃

  ┃                                                                                                           ┃
  ┃  ["apples","oranges"]                                                                                     ┃
  ┃                                                                                                           ┃

  ┃                                                            ┃
  ┃  Arguments: "fruit_name":"apples"}{"fruit_name":"oranges"  ┃
  ┃   Executing fruit-store__fruit_details (10:28)             ┃
  ┃                                                            ┃

  ┃                                                                                                    ┃
  ┃  Error: Tool execution error: invalid JSON arguments: invalid character '{' after top-level value  ┃
  ┃                                                                                                    ┃

  ┃
  ┃
  ┃  agent error: convert schema message failed: unmarshal tool call arguments failed: invalid character '{' after top-level value  ┃
  ┃   Error (10:28)
  ┃
  ┃
  ┃

  ┃
  ┃
  ┃  agent error: convert schema message failed: unmarshal tool call arguments failed: invalid character '{' after top-level value  ┃
  ┃   Error (10:28)

The error does not happen every time, and it never happens if --stream=false is used.
I added lines to print the msgs that are coming from Gemini here. When the problem happens, the msg looks like this:

{
  "role": "assistant",
  "content": "",
  "tool_calls": [
    {
      "id": "fruit-store__fruit_details",
      "type": "",
      "function": {
        "name": "fruit-store__fruit_details",
        "arguments": "{\"fruit_name\":\"apples\"}"
      }
    },
    {
      "id": "fruit-store__fruit_details",
      "type": "",
      "function": {
        "name": "fruit-store__fruit_details",
        "arguments": "{\"fruit_name\":\"oranges\"}"
      }
    }
  ],
  "response_meta": {
    "finish_reason": "STOP",
    "usage": {
      "prompt_tokens": 202,
      "prompt_token_details": {
        "cached_tokens": 0
      },
      "completion_tokens": 43,
      "total_tokens": 245,
      "completion_token_details": {}
    }
  }
}

It seems Gemini decided to use parallel function calling in this case.
The problem for mcphost is because Gemini uses the same "id": "fruit-store__fruit_details" for both tool calls, and this is used as the key in the accumulatedToolCalls map.