Building Claude Code with Boris Cherny
What happens when your first pull request at a top AI lab gets rejected — not because the code was bad, but because you wrote it by hand? Boris Cherny, creator and engineering lead of Claude Code, faced exactly that when he joined Anthropic. Within months, he went from handwriting code to shipping 20–30 pull requests a day, with the model writing 100% of every line. Yet the path from side project to one of the fastest-growing developer tools was anything but certain: Anthropic debated whether to keep Claude Code internal, fearing it would make them too productive. How did a research tool built to study safety in the wild become indispensable to engineers and non-engineers alike — and what does that mean for the future of software development?
Key Takeaways
Claude Code now writes approximately 80% of all code at Anthropic on average; Boris personally writes zero lines by hand, shipping 20–30 pull requests daily with Opus 4.5 writing every line.
Anthropic debated keeping Claude Code internal because of how productive it made engineers, but released it as a research preview to study safety in the wild — a decision that enabled rapid iteration and user feedback.
The fastest-growing use case for Claude Code is non-engineers: finance teams, sales teams, and data scientists now write code daily, signaling a shift toward multi-disciplinary generalists.
Code review has been restructured: Claude Code reviews every pull request in CI, catching ~80% of bugs; human review remains mandatory for safety and security, but the bottleneck has shifted from writing to verification.
The printing press analogy: just as scribes became writers when literacy democratized, software engineers will not disappear but transform — though Boris admits the transition is disorienting and requires letting go of attachment to handwriting code.
In a Nutshell
Claude Code is not just a productivity tool — it is a window into a future where the boundary between engineer and non-engineer dissolves, where prototyping replaces PRDs, and where the skill that matters most is no longer writing code but deciding what to build and how to verify it.
The First Pull Request That Got Rejected
Boris's handwritten code was turned down in favor of AI-generated output.
“Adam rejected my PR and he was like, 'Actually, you should use this Clyde thing for it instead.' And I was like, 'Okay, cool.' It took me like half a day to figure out how to use this tool because you have to like pass in a bunch of flags and like use it correctly. But then it spit out a working PR. It just one-shotted it. And I think for me, this was my first fuel hi moment at Anthropic cuz I I was just, oh my god, like I didn't know the model could do this.”
From Practical Hacker to Code Quality Lead
Boris learned to code selling Pokémon cards and cheating on math tests.
Boris's path into tech was anything but traditional. At 13, he taught himself HTML to make his eBay Pokémon card listings stand out — discovering that a blinking tag could double the sale price. In middle school, he programmed answers into his TI-83 graphing calculator, then built solvers when tests got harder, eventually dropping down to assembly to make programs run faster. Coding was never an end in itself; it was always a means to solve practical problems.
This pragmatic mindset carried through multiple startups, including a weed review site and a YC-backed medical software company where he shadowed doctors to understand why they weren't using the product. He learned that doctors had no time to boot up legacy systems, leading to pivots from web to Android to targeting different user personas. The lesson: hypothesis-driven iteration matters more than the technology stack.
At Meta, Boris spent seven years leading code quality across Instagram, Facebook, WhatsApp, and Messenger. He became one of the most prolific code authors and reviewers, automating code review by tracking repeated issues in a spreadsheet and writing lint rules whenever a pattern appeared more than three times. His work on the «Better Engineering» program — which mandated 20% of engineering time go to fixing tech debt — proved that code quality has a double-digit impact on productivity.
The Internal Debate: Keep It or Ship It?
Anthropic questioned whether releasing Claude Code would give away a productivity advantage.
The Internal Debate: Keep It or Ship It?
When Claude Code's internal adoption went vertical — reaching nearly 100% of technical employees at Anthropic — leadership debated whether to release it publicly. The concern was simple: if it made engineers this productive, why give competitors the same advantage? The decision to ship came down to Anthropic's core mission: studying safety in the wild. Synthetic evals in a petri dish could never capture real-world risks like prompt injection, tool misuse, or adversarial prompting. By releasing Claude Code, Anthropic could observe how users actually interacted with agentic AI, iterate on safety mechanisms, and build the permission and sandboxing systems that would later become critical infrastructure.
How Claude Code Actually Works
Boris's Zero-Handwritten-Code Workflow
Five parallel agents, plan mode, and notifications — or just the phone.
Start Five Parallel Checkouts Boris maintains five terminal tabs, each with a separate Git checkout. He cycles through them, starting Claude in plan mode (Shift + Tab twice) in each.
Iterate on the Plan The most critical step is refining the plan. With Opus 4.5 and 4.6, once the plan is solid, implementation is almost always a one-shot success.
Use Desktop or iOS for Overflow When terminal tabs fill up, Boris shifts to the Claude desktop app (with built-in Git worktree support) or even the iOS app, where he starts agents each morning while still in bed.
Tag Claude on GitHub PRs Boris uses the GitHub app integration daily. When reviewing a coworker's PR, he tags @Claude to request a lint rule, which Claude writes directly in the PR thread.
Ship Without Editing a Line During his December coding vacation, Boris shipped 10–20 PRs daily. Opus 4.5 wrote 100% of the code. The model introduced ~2 bugs; Boris estimates he would have introduced ~20 handwriting the same volume.
Key Numbers Behind Claude Code
Adoption, productivity, and the speed of building co-work.
Agent Teams and Uncorrelated Context Windows
Swarms enable complex builds by delegating across fresh context windows.
The Printing Press Moment
Scribes became writers; engineers will become something new.
“One metaphor I have for this moment in time is the printing press in the 1400s because there was a group of scribes that knew how to write. Some of the kings were illiterate who are employing the scribes. And if you think about what happened to the scribes, they ceased to become scribes, but now there's a category of writers and authors. These people now exist. And the reason they exist is because the market for literature just expanded a ton.”
What Still Matters and What Doesn't
Securities Mentioned
People
Glossary
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