Plan Mode and the Cost of a Wrong Assumption

An engineer asks Claude Code, in default (Manual) permission mode with no plan mode, to "convert the order-processing pipeline from a synchronous queue consumer to an async one." Fifteen minutes later, Claude has changed 9 files. The engineer then discovers that a LegacyOrderMetrics module — which nothing in the task description mentioned — depends on a synchronous side effect inside the queue consumer, and 3 of the 9 changed files now silently break metrics collection.

1. Explain, mechanistically, why this failure mode is more expensive to fix after the 9 files were changed than it would have been to catch before any file changed.
2. Would plan mode have definitely caught the LegacyOrderMetrics dependency? What does it depend on for that to happen, and what is a realistic reason it might not catch it?
3. Propose a better prompt or workflow for this task that reduces the chance of this exact failure recurring, using concepts from this subject.