ChatGPT
Checked on 2026-11-06 against developers.openai.com: connect to ChatGPT and build an MCP server. ChatGPT connects custom MCP servers with OAuth or with no authentication, not with an API key header, so it signs in to Crossmart with OAuth.
Sign in with OAuth
Section titled “Sign in with OAuth”Custom MCP servers are added in developer mode, on plans that allow it (full read and write is for Business and Enterprise/Edu workspaces; check OpenAI’s help article for your plan):
- Open chatgpt.com/plugins and click +.
- Choose Add custom MCP server.
- Name:
Crossmart. URL:https://mcp.crossmart.io/mcp. - Authentication: OAuth. Sign in to Crossmart, pick a workspace and approve the access.
- Create as a plugin.
Don’t choose “No authentication”: the Crossmart server refuses requests without a key or token.
Without OAuth
Section titled “Without OAuth”Use an agent that sends headers, such as Codex CLI from OpenAI, or call the REST API from your own code.
Skills
Section titled “Skills”ChatGPT has no documented SKILL.md support. The tool descriptions on the Crossmart server carry
the essential guidance (listing states, next steps) on their own.