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.claude/rules/coding-style.md

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+# Coding Style
+
+## Immutability (critical)
+
+Always create new objects, never mutate existing ones:
+
+- Wrong: modify original object in-place
+- Correct: return a new copy with the change applied
+
+Rationale: prevents hidden side effects, makes debugging easier,
+enables safe concurrency.
+
+## Core principles
+
+KISS — prefer the simplest solution that works. Optimize for clarity
+over cleverness.
+
+DRY — extract repeated logic into shared functions. Introduce
+abstractions when repetition is real, not speculative.
+
+YAGNI — do not build features before they are needed. Start simple,
+refactor when the pressure is real.
+
+Scope discipline: implement exactly what the task asks, no more. An
+adjacent problem you notice (a nearby bug, a tempting refactor, missing
+tests elsewhere) is REPORTED, never fixed in the same change. Never expand
+scope silently.
+
+## File organization
+
+Many small files over few large files:
+- 200-400 lines typical, 800 max
+- High cohesion, low coupling
+- Organize by feature/domain, not by type
+- Extract utilities from large modules
+- Grow the file *count*, not the file: when an edit would push a file past
+  the ~400-line typical ceiling, extract a new focused module instead of
+  appending to the existing one
+
+## Error handling
+
+- Handle errors explicitly at every level
+- User-facing code: provide friendly error messages
+- Server-side: log detailed error context
+- Never silently swallow errors
+
+## Input validation
+
+- Validate all user input before processing
+- Use schema-based validation where available
+- Fail fast with clear error messages
+- Never trust external data (API responses, user input, file content)
+
+## Naming conventions
+
+- Variables and functions: camelCase
+- Booleans: is, has, should, or can prefix
+- Interfaces, types, components: PascalCase
+- Constants: UPPER_SNAKE_CASE
+- Custom hooks: camelCase with use prefix
+
+## Code smells to avoid
+
+- Deep nesting (>4 levels) — use early returns
+- Magic numbers — use named constants
+- Long functions (>50 lines) — split into focused pieces
+- Large files (>800 lines) — extract modules