secondmate

Your agent writes the code. A different model tries to break it.
Only what survives ships, and only when you say go. 🐾

---

secondmate is a Claude Code plugin that makes your AI coding agent check its own work with a second opinion.

When an agent writes code, the same agent usually reviews it, so it misses its own mistakes. secondmate splits the job in two: one agent writes the change (the maker), a different AI model reviews it (the checker), and nothing gets merged until the checker passes it, the change clears an automated safety gate, and you give the final OK.

Because the checker is a different model than the maker, it catches bugs the maker is blind to. secondmate also keeps long runs from going off the rails, remembers your decisions across restarts, and can hand hard analysis to a reasoning model on the side. Works with any coding-agent CLI.

⚡ Quick start

claude plugin marketplace add eshwarvijay/secondmate
claude plugin install secondmate@secondmate
# restart Claude Code, then let it set itself up — you just approve each step:
/secondmate-doctor

Then run any change through the loop:

/loop-task add a rate limiter to the API and prove it works

secondmate spawns the maker in an isolated worktree, runs a different model as an edit-locked checker, gates on its {verdict} plus a clean verify-gate, and asks you before anything merges. Need a quick read-only analysis? /secondmate-reason why does this test flake only in CI?

🔁 How it works

flowchart LR
    A[loop-task: goal] --> M[maker<br/>isolated worktree]
    M --> C[checker<br/>different model, read-only]
    C --> V{verdict}
    V -- fail --> M
    V -- pass --> G{verify-gate}
    G -- refuse --> M
    G -- pass --> H{{your approval}}
    H -- merge --> D[ship]

Loading

Deeper dive: docs/ARCHITECTURE.md — roles, the guarantee behind every stage, and the full component map.

💡 Best with herdr. Run secondmate inside herdr (a tmux-backed terminal multiplexer for coding agents) and the maker and the cross-model checker each get their own live pane you can watch and jump into: the whole loop, visible. It's how I run it, on herdr + tmux. Anywhere else it falls back to in-process sub-agents, same guards and all.

🧰 What's inside

Component	What it does
secondmate skill	The maker/checker SOP: triage → spawn → check → gate → hold → integrate
SessionStart hook	Surfaces durable open decisions each session so a restart never drops a pending gate
bin/hold.py	Durable human-gate decisions (hold / answer / open)
bin/verify-gate.sh	Pre-integration gate: clean tree, non-empty diff, exact-SHA match, tests
bin/launch-checker.sh	Edit-locked (--exclude-tools edit,write) cross-model checker + verdict-envelope contract
bin/verdict.py	Parse the checker's {verdict} → exit 0 pass / 1 fail / 2 error·refused
bin/loop-guard.sh	Stuck-loop abort + per-run round cap + global spawn cap
bin/run-round.sh	Wall-clock timeout + idle watchdog + paired audit record (even on kill)
bin/prune-output.sh	Model-free head/tail truncation of bulky logs
bin/new-worktree.sh	Isolated git worktree per maker (never the primary checkout)
bin/reason.sh	Read-only, tool-free reasoning one-shot on a reasoning model
bin/herdr-pane.sh	When in herdr: spawn/delegate the maker + checker into visible side-by-side panes (race-proof) you can watch

Commands: /secondmate-doctor · /secondmate-reason · /secondmate-verify · /loop-task

🔎 Specialized review lenses

The checker isn't one generic reviewer. Per task, secondmate loads only the specialized disciplines the diff actually needs — a router → sub-skill design (progressive disclosure), so the checker gets focused context instead of a generic dump. Lenses live in bin/lenses/<role>/; the supervisor reads each role's ROUTER.md, matches the diff, and injects just the matching sub-lenses (--lens redteam/injection --lens qa/coverage).

Role	Sub-lenses
redteam (security)	injection · access-control · server-side-requests · deserialization · secrets-supply-chain · llm
qa (tests / behavior)	test-reality · coverage · risk-flagging · behavioral-contracts
reverse-engineer (unfamiliar / third-party code)	dataflow · hidden-behavior · intent-vs-impl · unknown-code
research (groundedness)	groundedness · prior-art · assumption-audit

Add your own: drop bin/lenses/<role>/<name>.md (a lean, specialized discipline) and list it in that role's ROUTER.md.

📦 Requirements

git, gh, python3, and (recommended) a checker harness CLI (defaults to pi). Just run /secondmate-doctor: it detects and installs everything, asking only for your approval, and never hands you a manual checklist.

No harness? You still get a cross-model check. secondmate falls back to a second Claude model as the checker, in-session, so the maker is still not the checker. An external harness like pi gives a stronger cross-vendor check (a different vendor, not just a different Claude model). Either way, the models need credentials only you can supply.

Companions — installed by /secondmate-doctor

Companion	What it adds	Install
pi	default checker + reasoning harness (multi-model)	npm install -g @earendil-works/pi-coding-agent
herdr	visible multi-pane maker/checker orchestration	brew install herdr
ponytail	complexity / over-engineering lens	claude plugin install ponytail@ponytail
loop-task	maker/checker loop driver command	bundled — ships with secondmate
adhd	divergent ideation for open-ended triage	claude plugin marketplace add UditAkhourii/adhd && claude plugin install adhd@adhd

Configuration — env vars, all optional (sane defaults)

Var	Default	Purpose
SM_CHECKER_HARNESS	pi	checker CLI
SM_CHECKER_PROVIDER	amazon-bedrock	checker provider
SM_CHECKER_MODEL	global.openai.gpt-5.6-terra	checker model (a different family than the maker)
SM_CHECKER_THINKING	high	checker reasoning effort
SM_CHECKER_PROMPT	bin/checker-prompt.md	base checker discipline (point at your own to override)
SM_REASON_HARNESS	pi	reasoning CLI
SM_REASON_PROVIDER	amazon-bedrock	reasoning provider
SM_REASON_MODEL	r1	default reasoning model alias (r1 / gpt / sonnet / full id)
SM_HOLD_LEDGER	./decisions.jsonl	per-repo decision ledger
SM_LOOP_STATE	./.secondmate	loop-guard state dir
SM_WT_ROOT	~/.secondmate-worktrees	where maker worktrees are created

The default model IDs are Amazon Bedrock inference-profile IDs — override them for your provider.

Manual install & verify

# install from a local clone instead of GitHub
claude plugin marketplace add /path/to/secondmate
claude plugin install secondmate@secondmate

# verify everything
bin/verdict.py selfcheck && bin/loop-guard.sh selfcheck && bin/verify-gate.sh --selfcheck \
  && bin/prune-output.sh --selfcheck && bin/run-round.sh selfcheck && bin/reason.sh --selfcheck \
  && bin/doctor.sh --selfcheck && echo ALL_OK
claude plugin validate .

License

MIT © Eshwar Vijay