# Design Decisions

## Stack
- **Language:** Rust
- **TUI Framework:** Ratatui + Crossterm
- **Async Runtime:** Tokio

## Architecture
- Channel boundary between TUI and core (fully decoupled)
- Module decomposition: `app`, `tui`, `core`, `provider`, `tools`, `sandbox`, `session`
- Headless mode: core without TUI, driven by script (enables benchmarking and CI)

## Model Integration
- Claude-first, multi-model via `ModelProvider` trait
- Common `StreamEvent` internal representation across providers
- Prompt caching-aware message construction

## UI
- **Agent view:** Tree-based hierarchy (not flat tabs) for sub-agent inspection
- **Modes:** Normal, Insert, Command (`:` prefix from Normal mode)
- **Activity modes:** Plan and Execute are visually distinct activities in the TUI
- **Streaming:** Barebones styled text initially, full markdown rendering deferred
- **Token usage:** Per-turn display (between user inputs), cumulative in status bar
- **Status bar:** Mode indicator, current activity (Plan/Execute), token totals, network policy state

## Planning Mode
- Distinct activity from execution — planner agent produces a plan file, does not execute
- Plan file is structured markdown: steps with descriptions, files involved, acceptance criteria
- Plan is reviewable and editable before execution (`:edit-plan` opens `$EDITOR`)
- User explicitly approves plan before execution begins
- Executor agent receives the plan file + project context, not the planning conversation
- Plan-step progress tracked during execution (complete/in-progress/failed)

## Sub-Agents
- Independent context windows with summary passed back to parent
- Fully autonomous once spawned
- Hard deny on unpermitted actions
- Plan executor is a specialized sub-agent where the plan replaces the summary
- Direct user interaction with sub-agents deferred

## Tool System
- Built-in tool system with `Tool` trait
- Core tools: `read_file`, `write_file`, `edit_file`, `shell_exec`, `list_directory`, `search_files`
- Approval gates by risk level: auto-approve (reads), confirm (writes/shell), deny
- MCP not implemented but interface designed to allow future adapter

## Sandboxing
- **Landlock** (Linux kernel-level):
  - Read-only: system-wide (`/`)
  - Read-write: project directory, temp directory
  - Network: blocked by default, toggleable via `:net on/off`
- Graceful degradation on older kernels
- All tool execution goes through `Sandbox` — tools never touch filesystem directly

## Session Logging
- JSONL format, one event per line
- Events: user message, assistant message, tool call, tool result, sub-agent spawn/result, plan created, plan step status
- Tree-addressable via parent IDs (enables conversation branching later)
- Token usage stored per event
- Linear UX for now, branching deferred

## Testing Strategy

### Unit Tests
- **`provider`:** SSE stream parsing from byte fixtures, message/tool serialization, `StreamEvent` variant correctness
- **`tools`:** Path canonicalization, traversal prevention, risk level classification, registry dispatch
- **`sandbox`:** Landlock policy construction, path validation logic (without applying kernel rules)
- **`core`:** Conversation tree operations (insert, query by parent, turn computation, token totals), orchestrator state machine transitions against mock `StreamEvent` sequences
- **`session`:** JSONL serialization roundtrips, parent ID chain reconstruction
- **`tui`:** Widget rendering via Ratatui `TestBackend`

### Integration Tests — Component Boundaries
- **Core ↔ Provider:** Mock `ModelProvider` replaying recorded API sessions (full SSE streams with tool use). Tests the complete orchestration loop deterministically without network.
- **Core ↔ TUI (channel boundary):** Orchestrator with mock provider connected to channels. Assert correct `UIEvent` sequence, inject `UserAction` messages, verify approval/denial flow.
- **Tools ↔ Sandbox:** Real file operations and shell commands in temp directories. Verify write confinement, path traversal rejection, network blocking. Skip Landlock-specific tests on older kernels in CI.

### Integration Tests — End to End
- **Recorded session replay:** Capture real Claude API HTTP request/response pairs, replay deterministically. Exercises full stack (core + channel + mock TUI) without cost or network dependency. Primary E2E test strategy.
- **Live API tests:** Small suite behind feature flag / env var. Verifies real API integration. Run manually before releases, not in CI.

### Benchmarking — SWE-bench
- **Target:** SWE-bench Verified (500 curated problems) as primary benchmark
- **Secondary:** SWE-bench Pro for testing planning mode on longer-horizon tasks
- **Approach:** Headless mode (core without TUI) produces unified diff patches, evaluated via SWE-bench Docker harness
- **Baseline:** mini-swe-agent (~100 lines Python, >74% on Verified) as calibration — if we score significantly below with same model, the issue is scaffolding
- **Cadence:** Milestone checks, not continuous CI (too expensive/slow)
- **Requirements:** x86_64, 120GB+ storage, 16GB RAM, 8 CPU cores

### Test Sequencing
- Phase 1: Unit tests for SSE parser, event types, message serialization
- Phase 4: Recorded session replay infrastructure (core loop complex enough to warrant it)
- Phase 6-7: Headless mode + first SWE-bench Verified run

## Configuration (Deferred)
- Single-user, hardcoded defaults for now
- Designed for later: global config, per-project `.agent.toml`, configurable keybindings

## Deferred Features
- Conversation branching (tree structure in log, linear UX for now)
- Direct sub-agent interaction
- MCP adapter
- Full markdown/syntax-highlighted rendering
- Session log viewer
- Per-project configuration
- Structured plan editor in TUI (use `$EDITOR` for now)