How Block orchestrates Claude Fable across thousands of pull requests

Block builds tools that help businesses and individuals participate in the economy, including Square for sellers and Cash App for consumers. Bradley Axen leads AI capabilities at Block, where his team builds end-to-end AI products, including Buzz, an open-source workspace for human-agent teams (opens in new tab) , and partners with the Square and Cash App teams to build customer-facing AI features. Brad spoke with Anthropic about how frontier models like Claude Opus 5 and Claude Fable 5 have transformed what it means to be an engineer, how Block applies agent orchestration patterns with Fable, and the role of safeguards when every employee has access to frontier models.

The biggest impact AI has had on our work at Block is how we build things, and you couldn't overstate how impactful that is. We saw the beginning of it in 2025 with tools like Claude Code, which at the time, required a lot more manual intervention. As models become more intelligent, design and engineering work has shifted toward designing the migrations themselves, reviewing agent behavior, and building more customer-facing features, which was always the goal.

The next phase is to bring these transformative technologies to our customers, through Block’s products. There's just as much value AI can bring to helping a small business handle the back-of-house operations it doesn't want to spend its time on, or to helping a Cash App customer manage their finances. With Claude, we have a whole new lever in technology that can change what people expect software to do for them. Imagine if AI could respond to customer needs and build new interfaces on the fly? Increasingly, I think our products will be built on demand with AI to solve specific problems.

The biggest thing that has changed with Claude’s newer frontier models is the scale at which we can build software. Writing code is still the core component, but now we'll do company-wide migrations with AI. We've all had that dream for a long time, and in 2025 it rarely panned out. Before Claude Fable 5, you had to have a particularly well-shaped problem for frontier models to really shine.

In 2026, AI is a major part of how we run all large-scale code migrations. For us, a migration might mean hundreds or thousands of individual pull requests across multiple repos and millions of lines of code. In the past, humans had to coordinate that work, and models could take on one or a few PRs at a time. With Fable, we're seeing it orchestrate the work across many sessions. A human engineer steers, and Fable focuses on the higher-level design work: data models, API specs, algorithms. It then directs up to dozens of smaller, more cost-effective models like Opus or Sonnet to do the individual tasks, such as making the actual file edits and running tests.

The economics work out well. Early results show the orchestrator using frontier tokens for complex, upfront planning before driving a higher percentage of tokens to smaller worker models, without a drop in quality. We run the whole setup on Buzz, our open source collaboration workspace where humans and AI agents work side by side in shared channels and threads. It gives us great visibility into how the team is progressing.