list_accessible_customers returns concatenated string instead of proper array

[PhilipJohnBasile]

Description

The list_accessible_customers tool returns a List[str] which the FastMCP framework converts into multiple separate content items. When Claude processes this, it
concatenates them into a single string without delimiters.

Expected behavior

Response should be parseable as separate customer IDs:

["1234567890", "2345678901", "3456789012"]

Actual behavior

Claude sees:

123456789023456789013456789012

Root cause

Looking at the MCP server logs, the response is:

{
  "content": [
    {"type":"text","text":"1234567890"},
    {"type":"text","text":"2345678901"},
    {"type":"text","text":"3456789012"}
  ],
  "structuredContent": {
    "result": ["1234567890","2345678901","3456789012"]
  }
}

The structuredContent.result is correct, but the content array has multiple text items which get concatenated.

Suggested fix

In ads_mcp/tools/core.py, return a JSON-formatted string instead of a list:

@mcp.tool()
def list_accessible_customers() -> str:
    """Returns ids of customers directly accessible by the user authenticating the call."""
    ga_service = utils.get_googleads_service("CustomerService")
    accessible_customers = ga_service.list_accessible_customers()
    customer_ids = [
        cust_rn.removeprefix("customers/")
        for cust_rn in accessible_customers.resource_names
    ]
    import json
    return json.dumps({"customer_ids": customer_ids, "count": len(customer_ids)}, indent=2)

Environment

• google-ads-mcp version: 0.0.1
• Claude Desktop on Windows