MCP tool argument object serialized as string causing schema validation failure

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When invoking the WordPress MCP execute-ability tool, the parameters argument appears to be serialized as a JSON string instead of an object.

This causes schema validation to fail on the WordPress MCP server with:

Ability "mcp-adapter/execute-ability" has invalid input.
Reason: input[parameters] is not of type object.

The tool schema defines parameters as:

parameters:
type: object

However the server appears to receive:

parameters: "{\"title\":\"MCP TEST\",\"status\":\"draft\"}"

instead of:

parameters: { "title": "MCP TEST", "status": "draft" }

This results in validation rejecting the input.

Important detail: a direct MCP invocation using call_mcp_tool with the same payload succeeds, creating a draft WordPress post.

This suggests the object → string conversion is happening in the client/tool invocation layer rather than on the WordPress server.

What Should Happen?

The MCP client should send parameters as a JSON object when invoking tools.

For example:

{
"ability_name": "content/create-post",
"parameters": {
"title": "MCP TEST",
"status": "draft"
}
}

This payload works correctly when executed directly via MCP, so the expected behavior is that the same object structure reaches the server during normal tool invocation.

Error Messages/Logs

The MCP client should send `parameters` as a JSON object when invoking tools.

For example:

{
  "ability_name": "content/create-post",
  "parameters": {
    "title": "MCP TEST",
    "status": "draft"
  }
}

This payload works correctly when executed directly via MCP, so the expected behavior is that the same object structure reaches the server during normal tool invocation.

Steps to Reproduce

1. Configure WordPress MCP in Cursor using:

{
"wordpress": {
"command": "npx",
"args": ["-y", "@automattic/mcp-wordpress-remote@latest"]
}
}

1. Ensure WordPress has the following plugins active:

• MCP Adapter 0.4.1
• MCP Expose Abilities 3.0.25

1. Invoke the WordPress MCP tool mcp-adapter-execute-ability via the agent/tool interface.

1. Call the ability:

ability_name: "content/create-post"

1. Provide arguments:

{
"ability_name": "content/create-post",
"parameters": {
"title": "MCP TEST",
"status": "draft"
}
}

1. Observe the server response:

Ability "mcp-adapter/execute-ability" has invalid input.
Reason: input[parameters] is not of type object.

Control test:

Calling the same tool directly via MCP (call_mcp_tool) with the same payload succeeds and creates a draft post, indicating the server stack works correctly when the argument remains a true object.

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

 2.1.72 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_