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Code with Codex

Codex Responses includes stable prompt-cache keys and cache-preserving Astra effort changes.

Use the codex pack to start fast-agent with a coding agent, a Codex-optimised filesystem search sub-agent, Responses-family transport, and model-selected filesystem editing tools.

uvx fast-agent-mcp@latest --pack codex

This starts fast-agent pre-configured for a Codex-style coding workflow.

What the codex pack gives you

  • A dev coding agent for interactive software work
  • A bounded rg-first search helper backed by codexspark
  • WebSocket-capable transport for modern Codex/OpenAI models
  • Filesystem editing tools selected for the model: Astra defaults to write_text_file plus edit_file; patch-oriented models use apply_patch
  • Preconfigured MCP targets available from /mcp attach

The coding agent has a minimal system prompt plus tools for the shell, filesystem and fast-agent services. AGENTS.md is included automatically if present. Customise the agent by editing .fast-agent/agent-cards/dev.md.

Astra's writer/editor default applies to both Responses and Codex Responses, including the astra and codexplan aliases. To explicitly select the Codex-style patch interface instead:

shell_execution:
  write_text_file_mode: apply_patch

When apply_patch tool calls are previewed in the console, large patches are collapsed with a (+N more lines) tail. You can tune or disable that limit via logger.apply_patch_preview_max_lines in fast-agent.yaml, or interactively with fast-agent config display.

Use /skills to discover, add, remove and update skills. Use /connect to connect to MCP Servers.

Codex authentication

If you want to use the Codex OAuth models directly, authenticate once first:

fast-agent auth provider login codex

Login uses device auth by default: open the displayed URL in a browser and enter the one-time code. No local callback server is required. Only enter a code from a login you started yourself.

If device auth is unavailable for your account, use the existing browser callback flow instead:

fast-agent auth provider login codex --method browser

Device login failures do not automatically switch to browser login. Credentials are stored in your OS keyring, with a secure file fallback. After that you can use Codex OAuth model aliases such as:

  • codexplan — GPT-6-Astra with medium reasoning

If you prefer, you can also run model setup explicitly:

uvx fast-agent-mcp@latest model setup

Use fast-agent go --model 'astra?web_search=true' to enable automatic web_run on Codex Lite without shell access; use web_search=false to disable it. In a running conversation, /model web_search on and /model web_search off control the same feature. Sol and public Responses keep their hosted search route. See standalone web search for configuration, the internal endpoint caveat, and a runnable library example with caller-supplied authentication.

Installation

fast-agent requires Python 3.12 or above. Install with:

uv tool install -U fast-agent-mcp

Or a specific version of Python:

uv tool install --python 3.12 -U fast-agent-mcp

This installs the fast-agent executable.

Next steps

From the fast-agent prompt:

  • Use /skills to view and manage skills
  • Use /mcp attach <name> to attach preconfigured MCP servers; use /connect for ad-hoc targets
  • Ask the agent to create additional cards in .fast-agent/agent-cards/
  • Switch agents with @
  • Configure compaction, hooks or automation with the available skills