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legacy

Apply this opinionated workflow when modernizing legacy systems: strangler fig pattern, branch by abstraction, characterization tests, incremental monolith decomposition, framework upgrades, feature-flagged migration with rollback.

plugins/engineering/cortex-modernization/skills/legacy/SKILL.md

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# Legacy Modernization

Incremental migration of running systems. The constraint that shapes every decision here: the system is in production and must keep working while it changes.

## The sequence

Order matters more than the individual techniques — each step exists to make the next one safe.

1. **Characterize before changing.** Write tests that pin current behaviour, bugs included. You are not testing what the code _should_ do; you are recording what it _does_, so a behaviour diff becomes visible. When an assertion looks wrong, that is the point — write it down and say so:

   ```python
   def test_domestic_lightweight():
       order = {"weight": 5, "destination": "domestic", "priority": False}
       # Current behaviour: nothing charged under 10kg. Looks like a bug —