Plan mode: regenerated plans don't show what changed

Problem

When in plan mode, suggesting an amendment to a plan causes Claude to regenerate the entire plan from scratch without any indication of what changed. This forces the user to mentally diff a large block of text to find the delta, which is a poor experience.

Expected behaviour

When a plan is amended, the response should clearly communicate what changed rather than dumping the full plan again. Some possible approaches:

• Show only the changed sections with surrounding context
• Highlight or annotate what was added/removed/modified
• Provide a brief summary of changes before the updated plan

Current workaround

Users can explicitly ask "only show me what changed" or "what did you change from the previous plan?" but this shouldn't be necessary — the default behaviour should surface the diff.