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CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Build & Development Commands
+
+```bash
+cargo run --release      # Build and run
+cargo build --release    # Build for production
+cargo test               # Run all tests
+cargo test <name>        # Run tests matching name
+cargo doc --open         # Generate documentation
+```
+
+## Architecture
+
+TSHTS is a terminal spreadsheet following clean architecture with four layers:
+
+```
+src/
+├── domain/           # Core business logic (no external dependencies)
+│   ├── models.rs     # Spreadsheet, CellData
+│   ├── services.rs   # FormulaEvaluator, CsvExporter
+│   └── parser.rs     # Recursive descent formula parser
+├── application/      # Application orchestration
+│   └── state.rs      # App state, AppMode enum, undo/redo
+├── infrastructure/   # External integrations
+│   └── persistence.rs # File I/O, .tshts JSON serialization
+├── presentation/     # User interface
+│   ├── ui.rs         # Terminal rendering (ratatui)
+│   └── input.rs      # Keyboard handling (InputHandler)
+└── lib.rs, main.rs   # Entry points
+```
+
+## Architectural Invariants
+
+**Layer Dependencies**: Domain has no external dependencies. Application depends on domain. Infrastructure and presentation depend on both.
+
+**Formula Functions**: Registered in `FunctionRegistry` in `parser.rs`. All take `&[Value]` and return `Result<Value, String>`. The `Value` enum supports dual number/string types with `.to_number()`, `.to_string()`, and `.is_truthy()` conversions.
+
+**Cell Dependencies**: `Spreadsheet` tracks cell dependencies bidirectionally. When a cell changes via `set_cell()`, dependent cells automatically recalculate. Dependencies are not serialized - call `rebuild_dependencies()` after loading.
+
+**App Modes**: `AppMode` enum drives UI state. Each mode has a corresponding handler in `InputHandler` and rendering logic in `ui.rs`. State transitions go through methods on `App`.
+
+**Undo/Redo**: Cell modifications should use `App::set_cell_with_undo()` and `App::clear_cell_with_undo()` to enable undo. Direct `spreadsheet.set_cell()` bypasses undo tracking.
+
+**Formula Parser**: Recursive descent with operator precedence (low to high): equality, comparison, addition, concatenation, multiplication, power, unary, primary. Logical ops (AND, OR, NOT) are functions, not operators.
+
+## Dependencies
+
+- **ratatui/crossterm** - Terminal UI
+- **serde/serde_json** - .tshts file serialization
+- **csv** - CSV import/export
+- **reqwest** (blocking) - GET() formula function