TandemKit

How It Works · Installation · Mission Lifecycle · FAQ

TandemKit

Describe your goal, approve the spec, then step away — Claude and Codex loop together until it's right.

TandemKit is a Claude Code plugin that runs three sessions — Planner, Generator, and Evaluator — with two of them pairing Claude and Codex as independent reviewers. You are only needed at two points: during planning (questions and spec approval) and at review (when evaluation passes and you give feedback or call it done). Between those two points, the Generator implements and the Evaluator verifies in a tight loop, with no manual review or copy-pasting from you. In both the Planner and Evaluator sessions, Claude automatically launches Codex as a background task using the official Codex plugin, so two different models independently investigate and converge on a result — everything inside Claude Code.

Why TandemKit?

Who Is It For?

You have a Claude Max subscription (which includes Claude Code) and a ChatGPT subscription (which includes Codex). You work on tasks complex enough to warrant the extra cost — TandemKit is not recommended for simple, small, or mechanical tasks, since the multi-session loop uses more tokens than a regular Claude session.

The Reasoning

Anthropic's Harness article (March 2026) identified the core problem with agentic sessions: Claude stops too early. A single session anchors on its own work, declares "looks good!" prematurely, and misses real bugs. The fix is a separate evaluator session that verifies independently rather than rubber-stamping its own output.

The article puts it well: "tuning a standalone evaluator to be skeptical turns out to be far more tractable than making a generator critical of its own work." That separation is TandemKit's foundation. On top of that,…