DoubleCheck MCP Server

DoubleCheck is a minimal MCP server that gives the Codex CLI two “second-opinion” tools: a Gemini plan check (gemini_plan_check, default model models/gemini-3-pro-preview) and a Claude Sonnet code review (sonnet_code_review, default model claude-sonnet-4-5-20250929).

Quick start

• Requirements: macOS/Linux shell, Python 3.11+, uv, outbound network access.

• From this directory, run ./setup.sh (or bash setup.sh if the file isn’t executable). It installs dependencies with uv, ensures .env exists, and writes a [mcp_servers.doublecheck] entry to ~/.codex/config.toml with absolute paths (creating the file if needed).

• Open .env and set your API keys (the file is already .gitignore’d):

  GOOGLE_API_KEY=your-google-api-key   # get from ai.google.dev
  ANTHROPIC_API_KEY=your-anthropic-key # get from console.anthropic.com

• Copy AGENTS.md into the repo where you’ll use Codex so it can load this MCP server (it’s the agent manifest), e.g. cp /path/to/doublecheck-mcp/AGENTS.md /path/to/your/repo/AGENTS.md.

• In that repo, run Codex pointing at the agents file, for example: codex --agents ./AGENTS.md "review this diff" (adjust the flag if your Codex CLI uses a different one). Codex will start the configured DoubleCheck server automatically—no need to run server.py manually.

Tools

• gemini_plan_check(plan_description, expectations=None, context=None) — plan critique sent to Google’s Gemini model (default models/gemini-3-pro-preview). Optional context is wrapped in <context> delimiters and truncated to ~6000 characters to keep the plan and metadata distinct.
• gemini_edit_plan_check(plan_description, file_contents, file_path=None, expectations=None, context=None) — like gemini_plan_check, but includes the target file content (truncated) and optional path in the prompt so edit plans can be reviewed with the live file context.
• sonnet_code_review(code_snippet, context=None) — code review sent to Anthropic’s Claude Sonnet model (default claude-sonnet-4-5-20250929).

Notes

• Defaults use models/gemini-3-pro-preview and claude-sonnet-4-5-20250929; override them with DOUBLECHECK_GEMINI_MODEL and DOUBLECHECK_SONNET_MODEL if your account uses different slugs.
• setup.sh installs via uv and wires Codex to this server; you don’t need to activate a virtualenv afterward.
• Setup script targets a Unix shell; on Windows use WSL or mirror the steps manually (uv sync, create .env, update ~/.codex/config.toml).
• Keep .env private (already ignored by git). Lock it down with chmod 600 .env.
• The server keeps no state on disk; it just forwards requests to the hosted models. Ensure you’re comfortable sending the provided code/plans to Google and Anthropic. You can copy AGENTS.md anywhere because it relies on the absolute MCP entry written to ~/.codex/config.toml.

Consider editing your ~/.codex/config.toml to add:

[features]
web_search_request = true