# how-claude-code-is-built-pragmatic-engineer-2025-09-15

## Veille

Building Claude Code - AI-first Architecture - Product Engineering - Pragmatic Engineer

## Titre Article

HOW CLAUDE CODE IS BUILT

## Date

2025-09-15

## URL

https://newsletter.pragmaticengineer.com/p/how-claude-code-is-built

## Keywords

Claude Code, Anthropic, AI, software engineering, development, AI agents, terminal UX, rapid prototyping, tech stack, TypeScript, React, Ink, Yoga, Bun, security, permissions, productivity, future of development, Gergely Orosz, Boris Cherny, Sid Bidasaria, Cat Wu

## Authors

Gergely Orosz (auteur de l'article), Boris Cherny, Sid Bidasaria, Cat Wu (équipe fondatrice de Claude Code)

## Ton

**Profile:** Conversational-Professional | Interview-based journalism | Educational-Analytical | Intermediate-Expert

Orosz adopts a long-form tech journalism style combining narrative and in-depth technical exploration. The chronological structure genesis → architecture → impact traces the product's development journey. Multiple voices (Cherny, Bidasaria, Wu) create a richness of perspectives. Revealing details (90% self-written code, 50% adoption in 5 days) ground ambitious claims. Technical language assumed as known (TypeScript, React/Ink, Yoga, Bun) targets a developer audience. Candid admissions (permissions system challenges, extreme dogfooding) convey authenticity. Typical of The Pragmatic Engineer's long-form pieces combining privileged access and critical analysis, aimed at engineering leaders and practitioners.

## Pense-betes

- Claude Code generates $500M in annual revenue
- Initial prototype (Sept 2024): identifying the music being played
- Breakthrough: access to the file system and bash
- 50% of Anthropic engineers use it daily 5 days after the internal test
- About 90% of Claude Code's code is written by Claude Code itself
- "On-distribution" tech stack: TypeScript, React/Ink, Yoga, Bun
- 60-100 internal releases per day, 1 external release daily
- 20 UI prototypes in 2 days for the "to-do list" feature
- 67% increase in pull requests at Anthropic
- Granular permissions system for irreversible actions
- React frontend: most complex part with giant editable canvas
- Tool Use: allows the AI to use tools (read files, list directories)

## RésuméDe400mots

Gergely Orosz's article offers a rare look into the development of Claude Code, Anthropic's AI-based development tool that has rapidly generated over $500 million in annual revenue. Through interviews with founding engineers Boris Cherny and Sid Bidasaria, as well as product lead Cat Wu, the article explores the genesis, architecture, and "AI-first" engineering approach behind this groundbreaking tool.

Claude Code began in September 2024 as a simple prototype by Boris Cherny, initially capable only of identifying the music being played. The decisive breakthrough came from giving it access to the file system and bash commands, allowing it to explore codebases and answer questions by autonomously reading and following file imports. This discovery revealed a "product overhang": the model's capability already existed, but no product was fully harnessing it.

The prototype quickly gained popularity within Anthropic, with 50% of engineers using it daily just five days after an internal test release in November 2024. Despite internal debate over keeping the tool as a competitive advantage, Anthropic decided to launch it publicly to deepen its understanding of model safety and capabilities. The team, initially just Boris alone, grew to about a dozen engineers by July, working with a high degree of autonomy and a focus on rapid prototyping.

The usefulness of Claude Code extended beyond developers, with data scientists also adopting it for queries and visualizations. The tool contributed to a 67% increase in the number of pull requests at Anthropic, despite a doubling of engineering headcount, demonstrating a substantial productivity gain.

Claude Code's technology stack is "on distribution" for the Claude model, using technologies the model already masters: TypeScript, React with Ink for the terminal user interface, Yoga for layout, and Bun for building and packaging. Remarkably, about 90% of Claude Code's code is written by Claude Code itself, illustrating a "dogfooding" approach taken to the extreme.

The architecture favors simplicity, acting as a lightweight interface that exposes tools and UI hooks to the Claude model, which performs the bulk of the complex work. The team constantly refines the system, often removing code and simplifying prompts as new model versions are released.

A crucial aspect is the permissions system, designed to prevent the AI from making irreversible changes without user consent. It offers granular control, allowing users to grant permissions once, for future sessions, or to deny them, with multi-level configuration options.

The development process is characterized by extreme speed. The team ships 60 to 100 internal releases per day and one external release daily. Prototyping is exceptionally fast: Boris Cherny built about 20 UI prototypes for a "to-do list" feature in just two days, iterating rapidly based on prompts and feedback.

This rapid iteration, made possible by AI agents, considerably accelerates the design and implementation of new features, fundamentally changing the pace of prototyping. Claude Code also introduces innovative terminal user-experience features, leveraging the interactive nature of LLM-powered terminals.

## GrapheDeConnaissance

- Boris Cherny —a_créé→ Claude Code (TECHNOLOGIE, 0.99)
- Boris Cherny —travaille_chez→ Anthropic (ORGANISATION, 0.99)
- Sid Bidasaria —a_créé→ subagents (TECHNOLOGIE, 0.97)
- Cat Wu —fait_partie_de→ équipe fondatrice (CONCEPT, 0.97)
- Claude Code —utilise→ TypeScript (TECHNOLOGIE, 0.99)
- Claude Code —utilise→ Ink (TECHNOLOGIE, 0.98)
- Claude Code —utilise→ Bun (TECHNOLOGIE, 0.97)
- Claude Code —mesure→ 500M$ de revenus annuels (MESURE, 0.95)
- Claude Code —est_basé_sur→ modèle Claude (TECHNOLOGIE, 0.99)
- Anthropic —mesure→ 90% du code Claude Code écrit par Claude Code (MESURE, 0.95)
- Anthropic —mesure→ augmentation 67% des pull requests (MESURE, 0.93)
- genèse du produit —est_basé_sur→ product overhang (CONCEPT, 0.88)
- système de permissions —réduit→ risque d'actions irréversibles (CONCEPT, 0.95)
- Gergely Orosz —publie→ article Claude Code (DOCUMENT, 0.99)

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Canonical: https://www.thekb.eu/en/fiches/how-claude-code-is-built-pragmatic-engineer-2025-09-15/