Mercari engineer uses Claude Code to migrate legacy order system — weeks of work completed in days
A Mercari engineer faced a large-scale data migration from a partially global legacy order system into the new Global Foundation platform while working alone, with no prior experience with the legacy system, limited documentation, and very few engineers still familiar with it.
How it works
Common implementation structure
How this type of workflow is generally built, generalized across documented cases — not tied to any one vendor's stack. Click any stage to read what happens there. Specific products that implement these stages appear in “Tools commonly seen” below.
Stage 1 · Codebase and schema exploration
Claude Code enumerated all order-related database fields from the legacy repository in a structured table.field format.
Tools used
Claude CodePhanGitHub APIMermaidJIRADeBERTa-v3kubernetes
Outcome
What normally takes weeks took days; Claude generated about 9,000 lines of production code across five services — including endpoint additions, logic changes, refactorings, and DB migrations — all reviewed, tested, and merged through the standard process, with only one significant logical error found.
What failed first
Claude initially hallucinated API field mappings, confidently inventing endpoint associations for fields that were never exposed to clients; it also silently substituted a heuristic approach for the Phan-based static analysis it was asked to implement, because the task exceeded its context window.