Choose an agent runtime

Claude Code is the stable default. The fleet agents have run on Claude Code in production for a long time; it is what a new installation gets unless you ask for something else. pi and codex are experimental — pi works end to end for triage, prioritize, code, fix and review/retro; codex runs OpenAI models through the same secretless credential path but has no sub-agent roster yet, so review/retro are best left on Claude Code there. Neither runtime has completed a fleet pilot. Unless you are taking part in one, keep the default.

This page explains what the choice means and where it is made. You do not select anything on this page — the selection happens in the next step, Configuring GitHub, when fullsend github setup prompts for the runtime (press Enter for claude) or when you pass --runtime.

Fullsend supports multiple agent runtimes. A runtime is the program that runs inside the sandbox and drives the model — it owns the tool-use loop, hook wiring, and transcript format. The runner (fullsend) owns everything outside: sandbox lifecycle, credentials, metrics, and the verdict.

Available runtimes

Runtime	Status	Description	When to use
claude	Stable (default)	Claude Code on Vertex AI	Every production deployment — mature, full sub-agent support for review/retro
pi	Experimental (enablement phase)	Pi — Claude on Vertex by default; any provider pi supports by model name (e.g. Gemini on Vertex with the same credentials)	Opt-in pilots only; see Runtimes for known constraints
codex	Experimental	Codex — OpenAI models only, through the same secretless credential path	Opt-in pilots only, when you want GPT specifically. No sub-agent roster, so keeping review/retro on Claude Code is recommended; needs an OpenAI model named (FULLSEND_CODEX_MODEL) because the fleet harnesses ask for opus; see Codex

When and how the runtime is selected

1. Next step — Configuring GitHub. fullsend github setup <owner/repo> asks which runtime to use when run from a terminal; press Enter to keep claude. Passing --runtime skips the prompt. The setup PR it opens records the choice in .fullsend/config.yaml and describes how to change it. Nothing runs on this page — continue with Configuring GitHub.
2. Later — changing it. Edit runtime: in the repo's .fullsend/config.yaml (the setup PR shows the key), or re-run fullsend github setup <owner/repo> --runtime <claude|pi|codex>. To put one agent on a different runtime or model than the rest — say code on Claude Code while triage runs Grok on pi — set runtime: on that agent's agents: entry in the same file (fullsend agent set code --runtime claude); see Runtimes — per-agent settings. Fleets managed through repos.yaml set defaults.runtime (or a per-entry runtime) — fullsend repos set-default defaults.runtime pi — and run fullsend repos install; see fullsend repos.
3. Per run — trying without changing the repo. fullsend run --runtime pi --model google-vertex/gemini-2.5-flash, or the FULLSEND_RUNTIME / FULLSEND_MODEL / FULLSEND_EFFORT environment variables (flag beats environment beats the agent's agents: entry beats repo-wide config). In CI the same names work as repository variables. Reference: fullsend run and Runtimes — selecting and overriding.

Where to see what ran

After a run completes, the selected runtime and model appear in several places:

• Run plan block — Runtime: <name> (from <source>) printed at the start of every fullsend run
• Status comment — the terminal status comment on the issue/PR includes a footer with runtime, model, effort, and cost (see Cost data contract for how cost is sourced and aggregated)
• metrics.json — runtime, requested_runtime, runtime_source, requested_model, and override_source fields record what was selected and why
• stderr — runtime: selected "<name>" from <source> for script consumers

Next steps

• Configuring GitHub to set up your repo
• Runtimes for the full runtime reference, including model override precedence and the capability table