No-arg MCP tool schemas omit `properties`, causing OpenAI/Codex rejection

[vish-dini]

Summary

help (and likely other zero-argument MCP tools) are exposing inputSchema as {"type":"object"} without an explicit properties field.

OpenAI/Codex rejects the converted function schema with:

Invalid schema for function 'inboxapi__help': In context=(), object schema missing properties.

Local validation

I checked the CLI proxy code path locally. tools/list responses are rewritten, but schemas that already look like a bare object are passed through without normalization. That means a no-arg MCP tool can still surface as:

{ "type": "object" }

which becomes an OpenAI function parameters object missing properties.

Expected

For no-argument tools, the schema should include explicit empty properties, for example:

{
  "type": "object",
  "properties": {},
  "required": [],
  "additionalProperties": false
}

Impact

This blocks adding InboxAPI as an MCP server in OpenClaw/Codex whenever a tool like help is converted into an OpenAI-style function tool.

Notes

It may be worth normalizing all no-argument MCP tools, not just help, so every inputSchema that is logically empty still emits an explicit empty properties object.

[sara-dini]

Validated in local env against live endpoint on 2026-05-06.

Repro:

curl -sS -X POST https://mcp.inboxapi.ai/mcp \
  -H 'content-type: application/json' \
  -H 'accept: application/json, text/event-stream' \
  --data '{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}'

Observed help schema is currently:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "EmptyObject",
  "description": "This is commonly used for representing empty objects in MCP messages.\n\nwithout returning any specific data.",
  "type": "object"
}

I parsed all current tools from that response:

• total tools: 25
• object schemas missing properties: 1
• affected tool: help

So this is reproducible and currently present in production MCP output.