Legacy Modernization Pattern — Subagents

Pattern: Legacy Modernization
Component: subagents.md
Version: 1.1 | Updated: 2026-07-16

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Subagent	Role	When invoked	Output
Legacy Behavior Mapper	Extracts golden behaviors from code/jobs/SME	Start of slice	Behavior catalog + fixtures
Parity Harness Builder	Compares legacy vs new outputs	Before / during shadow	Pass/fail report
Traffic Shift Controller	Defines ramp and abort	After parity green	Shift runbook

Behavior Mapper contract

• Prefer executable oracles (batch outputs, API recordings) over anecdotes
• Include edge cases: nulls, leap days, max cart, denied auth
• Mark unknown rules explicitly — do not invent

Parity Harness contract

• Same inputs to legacy and new whenever shadow is possible
• Tolerances explicit (e.g. money rounding 0.01)
• Failures block traffic increase

Traffic Shift Controller contract

• Steps include abort KPIs (error rate, conversion, latency)
• Always include immediate rollback to legacy path
• 100% requires business + eng acknowledgment