Development

A model codes uninterrupted for a week and writes a browser

development2026-01-15

Claim

Cursor's CEO reported building a complete browser with a custom Rust rendering engine by running GPT-5.2 Codex uninterrupted for one week, generating three million lines of code.

Reported metric: 1 week / 3M lines

Reporting date: 2026-01-15 (not necessarily the event date)

Sources

Attribution and reporting linked by this record; links do not imply independent verification.

Welcome to January 15, 2026 (theinnermostloop.substack.com)

METR’s published autonomy horizon was about five hours.

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About

GPT-5.2-Codex

Reported in

Welcome to January 15, 2026

Actor

Evidence for

Supersedes

METR puts Opus 4.5 at a 4h49m autonomy horizon

Referenced by

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Citation formatCitation text

A model codes uninterrupted for a week and writes a browser
Innermost Loop KB — secondary record
Reporting date: 2026-01-15
KB permalink: https://nicholsn.github.io/innermost-loop-kb/developments/2026-01-15-codex-runs-a-week-3m-lines
Claim: Cursor's CEO reported building a complete browser with a custom Rust rendering engine by running GPT-5.2 Codex uninterrupted for one week, generating three million lines of code.
Attributed sources:
- Welcome to January 15, 2026 — https://theinnermostloop.substack.com/p/welcome-to-january-15-2026
This citation does not imply independent verification of linked-source claims.

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