Codex MCP: Setup, Per-Repo Config, and Tool Approvals

Codex MCP enables OpenAI’s Codex CLI, IDE extension, and ChatGPT desktop app to connect to external Model Context Protocol servers through shared or repository-specific configuration, allowing agents to access tools and information outside a codebase. The guide emphasizes security controls because audits have found tool-poisoning and other vulnerabilities in open-source MCP servers, recommending trusted projects, narrowly scoped tool allow lists, environment-based secrets, and approval settings that distinguish read-only from destructive actions. It explains how to register local STDIO or remote HTTP servers, authenticate with OAuth or tokens, and use TestMu AI’s 88-tool hosted server as an example for test analysis and automation workflows. Although tools are generally loaded on demand and impose no initial context cost, forcing extensive tool definitions into prompts can substantially increase token use, making selective allow lists useful for both safety and efficiency. In noninteractive `codex exec` runs, calls requiring approval are blocked without changing the zero exit code, so CI pipelines should explicitly inspect JSON events for failed MCP calls. Hooks can invoke already connected MCP tools but bypass normal approvals, while Codex itself can no longer operate as an MCP server; integrations requiring Codex services must instead use its experimental JSON-RPC app server or invoke `codex exec`.

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