--- title: "Building Claude Code with Boris Cherny" section: "raw" type: "source" created: "2026-08-27" updated: "2026-08-27" canonical: "https://pyweb.dev/wiki/raw/articles/boris-cherny-gergely-orosz-building-claude-code-2026" --- # Building Claude Code with Boris Cherny By Gergely Orosz with Boris Cherny (March 4, 2026). The Pragmatic Engineer Podcast & Deepdive. An in-depth technical discussion with Boris Cherny (creator and Head of Claude Code at Anthropic, former Principal Engineer at Meta, author of *Programming TypeScript*) on architecture, parallel agent workflows, deterministic review automation, and agentic search. ## Core Extraction Summary ### 1. Named Frameworks & Patterns (Author's Exact Words) - **"5 Parallel Claude Instances"**: Running 5 concurrent Claude Code terminal sessions across 5 isolated git checkouts or worktrees. - **"Plan mode -> One-shot implementation"**: Starting sessions in Plan mode, iterating to establish a rigid plan, then switching to auto-accept where Claude one-shots the implementation. - **"Agentic Search via Glob and Grep"**: Claude Code's retrieval mechanism that outperformed vector databases and recursive RAG by letting the model drive simple filesystem tools. - **"Automate yourself out of code review"** (3-4 occurrences rule): Boris's Meta pattern of logging repeated code review comments in a spreadsheet and writing a linter rule once a pattern hits 3-4 occurrences. - **"Prototypes replace PRDs"**: Replacing static product requirement documents with rapid functional prototypes built via agents. - **"Compounding Engineering / CLAUDE.md memory ratchet"**: Committing learnings, negative constraints, and architectural invariants to `CLAUDE.md` whenever an agent makes a mistake. - **"Medieval Scribe Analogy"**: Framing software engineers as modern scribes whose role expands from writing text to designing systems as coding becomes democratized. ### 2. Decision Rules - **When executing complex features**, start in Plan mode and refine until solid before writing code, **because** once a comprehensive plan exists, the model one-shots the implementation reliably. - **When searching large codebases in agent harnesses**, use model-directed glob and grep rather than vector RAG or pre-computed embeddings, **because** vector search suffers from stale indexes and permission boundaries. - **When an agent makes an incorrect change**, immediately commit the constraint to `CLAUDE.md`, **because** this ensures the entire engineering team benefits from the correction. - **When developing features**, invest in domain-specific verification tools (e.g. browser extensions for UI, test suites for backend), **because** closing the verification loop improves output quality by 2–3x. ### 3. Anti-Patterns & Failure Mechanisms - **"Incomplete Code Migrations"**: Leaving a codebase with multiple competing frameworks (e.g. half Python 2, half Python 3, or mixed REST/GraphQL), which degrades reasoning for both humans and LLMs. - **"Vector RAG for Codebase Search"**: Relying on semantic embeddings that fail on exact symbol references and return outdated file chunks. - **"Static Mocks & PRDs"**: Spending weeks on non-executable design specs instead of testing interactive prototypes. ### 4. Quantitative Claims & Qualifiers - Boris ships **20–30 PRs per day** by running **5 parallel Claude Code instances**. - Causal analysis at Meta proved that high codebase quality has a measurable, **double-digit-percent positive impact** on developer productivity. - Claude Cowork was built from scratch in approximately **10 days**. ### 5. What the Source Does NOT Claim - Does **NOT** claim that engineers no longer need deep systems understanding; explicitly emphasizes that fixing dev infrastructure and understanding type systems remain the highest-leverage engineering tasks. --- ## 10 Key Observations from Boris Cherny 1. **Boris ships 20-30 PRs a day by running 5 parallel Claude instances**: Works across 5 terminal tabs (each a separate checkout), starting Claude in plan mode, refining the plan, then auto-accepting edits. 2. **Code quality boosts productivity by double-digit percentages**: Meta causal analysis proved clean codebases boost output. Partially-migrated codebases confuse models and humans alike. 3. **Claude Code's "agentic search" is just glob and grep**: Plain model-driven glob and grep beat vector databases and recursive model indexing in empirical tests. 4. **Automating out of code review**: Logging repeated PR comments and writing lint rules once an issue occurred 3-4 times. 5. **Universal title ("Member of Technical Staff")**: Flattens organizational silos so engineers do product, design, and infrastructure. 6. **Claude Cowork built in 10 days**: Rapid response to latent demand from non-engineers, focusing engineering effort on sandboxing and OS-level safety. 7. **Prototypes replace PRDs**: Dozens of working prototypes built before shipping features instead of static Figma mocks. 8. **The year of the generalist**: Transition from single-threaded deep focus to managing parallel agents with rapid context-switching. 9. **Fix infrastructure before product**: High-leverage dev infra investments (migrations, type-checkers) unblock all downstream engineering. 10. **Medieval Scribe analogy**: Coding democratization expands the reach and demand for software engineers rather than eliminating them. --- ## Agent Navigation ### Machine endpoints - Knowledge graph: https://pyweb.dev/api/graph.json - Graph analysis: https://pyweb.dev/api/graph-analysis.json - Context index: https://pyweb.dev/llms.txt