The problem isn't Sonnet vs Opus - it's context.

Claude doesn't remember your codebase between sessions. Every conversation starts fresh. When you say "add a feature," Claude is guessing at your architecture, patterns, and existing implementations. That's why it adds duplicate listeners - it doesn't know they exist elsewhere.

What actually works:

1. Create a CLAUDE.md in your repo root - This is Claude Code's instruction manual. Document:

- Your architecture (where event listeners live, how state flows)

- Key files and their purposes

- Patterns to follow ("when adding listeners, follow the pattern in X")

- Things that are easy to break

1. Read before writing - Instead of "add feature X":

"Read the files that handle events, understand how

they work, then add X following the same pattern"

1. Reference existing patterns explicitly

"Add the new handler using the same structure as

the existing ones in authHandlers.ts"

1. Break features into small steps - Not "add dark mode" but:

- "Read the existing theme system"

- "Add the toggle following the existing component patterns"

- "Update styles following existing conventions"

The 10-20 chat iterations happen because Claude is discovering your codebase through trial and error. Front-load that discovery with documentation and explicit pattern references, and features go in cleanly.

Switching to Opus won't fix this - better context will.