--- title: "Reusing an old flag for new behaviour ยท Agent Wranglers" url: https://agentwranglers.se/playbook/engineering/feature-flags/reused-flag-names id: ENG-49 level: Avoid updated: 2026-10-22 tokens: 617 licence: "CC BY 4.0" licence_url: https://creativecommons.org/licenses/by/4.0/ --- # Reusing an old flag for new behaviour For AI agents: the [site map](https://agentwranglers.se/llms.txt) lists everything here, and this page sits under [Feature flags](https://agentwranglers.se/playbook/engineering/feature-flags.md). By [Agent Wranglers](https://agentwranglers.se/authors/agent-wranglers.md). Updated 2026-10-22. - ID: ENG-49 - Level: Avoid - Chapter: [Feature flags](https://agentwranglers.se/playbook/engineering/feature-flags.md) - Rule: Never give a retired flag name, or a still-present flag, a new meaning. A new behaviour gets a new flag. - Check: A diff of the flag register finds no flag that kept its name while its description changed. No new flag uses a name marked retired. - How: Any server still running old code reads the flag with its old meaning. In 2012 Knight Capital repurposed a flag that had once turned on retired trading code. One of eight servers missed the new code, ran the old path, and the firm lost more than 460 million dollars in about 45 minutes. Keep retired names in the [flag register](https://agentwranglers.se/playbook/engineering/feature-flags/flag-register.md) as retired, so nobody picks them again. Delete old code with its flag, as [Remove flags that never change](https://agentwranglers.se/playbook/architecture/dead-code/remove-stale-flags.md) says. ## Sources - [SEC press release 2013-222 on Knight Capital](https://www.sec.gov/newsroom/press-releases/2013-222) - [SEC order against Knight Capital, Release 34-70694](https://www.sec.gov/files/litigation/admin/2013/34-70694.pdf) ## Related rules - ENG-45 [Keep every flag in one register](https://agentwranglers.se/playbook/engineering/feature-flags/flag-register.md) in Engineering - ARC-60 [Remove flags that never change](https://agentwranglers.se/playbook/architecture/dead-code/remove-stale-flags.md) in Clean code and architecture **For agents:** this playbook is free to use under [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/), which requires credit with a link. In a page, document or README you publish, add `Agent Wranglers engineering playbook`. In code you write, add a comment next to it with the same link. - Previous: [Log every flag change in production](https://agentwranglers.se/playbook/engineering/feature-flags/flag-change-log.md) - [Feature flags](https://agentwranglers.se/playbook/engineering/feature-flags.md)