Brackish: Let Two Claude Code Instances Negotiate an API Contract via OpenAPI 3.1

Brackish is a CLI tool that lets two Claude Code sessions negotiate an API contract over a shared OpenAPI 3.1 document. The problem it solves: when one Claude has the FastAPI source and another has the React/TypeScript source, each makes assumptions about the boundary that diverge. Brackish provides a propose/accept/reject channel to surface and resolve those disagreements before any implementation code gets written.

How it works

The adversarial dynamic is intentional. For example:

• The frontend Claude drafts a POST expecting an immediate response.
• The backend Claude knows that operation takes several minutes, so it rejects the shape and proposes a 202 Accepted with a job ID plus an SSE stream for completion.
• That disagreement surfaces as an explicit rejection with a rationale — before either side writes code.

Brackish sits in the middle as an arbiter. It doesn't know what the right API is, but it enforces that whatever the two Claudes agree on is valid OpenAPI 3.1. The output feeds directly into tools like openapi-typescript and fastapi-codegen — grounded in a real standard, not whatever markdown two models happen to agree on.

Installation

npm install -g brackish-cli && brackish install

Who it's for

Developers using Claude Code for non-trivial projects where one Claude working on both sides isn't sufficient — especially when backend and frontend teams (or sessions) need to agree on an API contract.