Type checking belongs in CI but cannot replace behavioral tests

A type checker only prevents regressions when every developer and automation job applies consistent rules.

Mypy recommends pinning versions, sharing configuration, checking identical files, and running analysis in CI.

Gradual adoption works when teams establish a passing baseline, then tighten coverage without flooding reviews.

Permissive settings, ignored modules, missing dependency types, and broad escape hatches weaken the resulting signal.

If CI gates every change early enforces analysis, Integration tests catch contract mismatch still exercises real component connections.

E2E tests stay small and rare then samples critical journeys that static models cannot execute.

Gate type errors early, but retain layered tests for behavior, integration, infrastructure, and user outcomes.