llmx/codex-rs/docs/codex_mcp_interface.md

Codex MCP Server Interface [experimental]

This document describes Codex’s experimental MCP server interface: a JSON‑RPC API that runs over the Model Context Protocol (MCP) transport to control a local Codex engine.

• Status: experimental and subject to change without notice
• Server binary: codex mcp-server (or codex-mcp-server)
• Transport: standard MCP over stdio (JSON‑RPC 2.0, line‑delimited)

Overview

Codex exposes a small set of MCP‑compatible methods to create and manage conversations, send user input, receive live events, and handle approval prompts. The types are defined in protocol/src/mcp_protocol.rs and re‑used by the MCP server implementation in mcp-server/.

At a glance:

• Conversations
  • newConversation → start a Codex session
  • sendUserMessage / sendUserTurn → send user input into a conversation
  • interruptConversation → stop the current turn
  • listConversations, resumeConversation, archiveConversation
• Configuration and info
  • getUserSavedConfig, setDefaultModel, getUserAgent, userInfo
  • model/list → enumerate available models and reasoning options
• Auth
  • loginApiKey, loginChatGpt, cancelLoginChatGpt, logoutChatGpt, getAuthStatus
• Utilities
  • gitDiffToRemote, execOneOffCommand
• Approvals (server → client requests)
  • applyPatchApproval, execCommandApproval
• Notifications (server → client)
  • loginChatGptComplete, authStatusChange
  • codex/event stream with agent events

See code for full type definitions and exact shapes: protocol/src/mcp_protocol.rs.

Starting the server

Run Codex as an MCP server and connect an MCP client:

codex mcp-server | your_mcp_client

For a simple inspection UI, you can also try:

npx @modelcontextprotocol/inspector codex mcp-server

Use the separate codex mcp subcommand to manage configured MCP server launchers in config.toml.

Conversations

Start a new session with optional overrides:

Request newConversation params (subset):

• model: string model id (e.g. "o3", "gpt-5", "gpt-5-codex")
• profile: optional named profile
• cwd: optional working directory
• approvalPolicy: untrusted | on-request | on-failure | never
• sandbox: read-only | workspace-write | danger-full-access
• config: map of additional config overrides
• baseInstructions: optional instruction override
• compactPrompt: optional replacement for the default compaction prompt
• includePlanTool / includeApplyPatchTool: booleans

Response: { conversationId, model, reasoningEffort?, rolloutPath }

Send input to the active turn:

• sendUserMessage → enqueue items to the conversation
• sendUserTurn → structured turn with explicit cwd, approvalPolicy, sandboxPolicy, model, optional effort, and summary

Interrupt a running turn: interruptConversation.

List/resume/archive: listConversations, resumeConversation, archiveConversation.

Models

Fetch the catalog of models available in the current Codex build with model/list. The request accepts optional pagination inputs:

• pageSize – number of models to return (defaults to a server-selected value)
• cursor – opaque string from the previous response’s nextCursor

Each response yields:

• items – ordered list of models. A model includes:
  • id, model, displayName, description
  • supportedReasoningEfforts – array of objects with:
    • reasoningEffort – one of minimal|low|medium|high
    • description – human-friendly label for the effort
  • defaultReasoningEffort – suggested effort for the UI
  • isDefault – whether the model is recommended for most users
• nextCursor – pass into the next request to continue paging (optional)

Event stream

While a conversation runs, the server sends notifications:

• codex/event with the serialized Codex event payload. The shape matches core/src/protocol.rs’s Event and EventMsg types. Some notifications include a _meta.requestId to correlate with the originating request.
• Auth notifications via method names loginChatGptComplete and authStatusChange.

Clients should render events and, when present, surface approval requests (see next section).

Approvals (server → client)

When Codex needs approval to apply changes or run commands, the server issues JSON‑RPC requests to the client:

• applyPatchApproval { conversationId, callId, fileChanges, reason?, grantRoot? }
• execCommandApproval { conversationId, callId, command, cwd, reason? }

The client must reply with { decision: "allow" | "deny" } for each request.

Auth helpers

For ChatGPT or API‑key based auth flows, the server exposes helpers:

• loginApiKey { apiKey }
• loginChatGpt → returns { loginId, authUrl }; browser completes flow; then loginChatGptComplete notification follows
• cancelLoginChatGpt { loginId }, logoutChatGpt, getAuthStatus { includeToken?, refreshToken? }

Example: start and send a message

{ "jsonrpc": "2.0", "id": 1, "method": "newConversation", "params": { "model": "gpt-5", "approvalPolicy": "on-request" } }

Server responds:

{ "jsonrpc": "2.0", "id": 1, "result": { "conversationId": "c7b0…", "model": "gpt-5", "rolloutPath": "/path/to/rollout.jsonl" } }

Then send input:

{ "jsonrpc": "2.0", "id": 2, "method": "sendUserMessage", "params": { "conversationId": "c7b0…", "items": [{ "type": "text", "text": "Hello Codex" }] } }

While processing, the server emits codex/event notifications containing agent output, approvals, and status updates.

Compatibility and stability

This interface is experimental. Method names, fields, and event shapes may evolve. For the authoritative schema, consult protocol/src/mcp_protocol.rs and the corresponding server wiring in mcp-server/.