# Architecture Decisions — Round 2

Date: 2026-05-26
Source: idea.md discussion, questions 19–30

## D-021: Project Model Version Migration

- **Decision**: Backup before migrate, user confirmation required.
- **Mechanism**: On opening a project with old-version `project.json`, auto-detect and prompt user. Backup to `~/.air/projects/<id>/backups/` before migration. Migration failure → rollback to backup, notify user.

## D-022: Scanner — Full Directory Tree, No Exclusions

- **Decision**: Scanner collects complete directory tree (file system metadata only — paths, extensions, file types), no recursion depth limit, no directory exclusions.
- **Why**: Directory tree traversal is fast (kernel inode walk), and the full tree provides useful structural information (e.g., detecting referenced components). File content is NOT read during scanning.
- **Timeout**: 15 seconds hard timeout as safety net. On timeout, return partial results with `incomplete` marker.
- **Update strategy**: First scan is full. Subsequent scans are incremental (mtime diff). On re-open, load `project.json` + quick check for top-level directory changes; trigger incremental scan only if changes detected.
- **Informed by**: Claude Code's zero-pre-scan approach (agentic search, no indexing), OpenCode's Tree-sitter AST + LLM hybrid.

## D-023: Project Rules Format — Markdown + Frontmatter

- **Decision**: Claude Code style — Markdown body with YAML frontmatter for structured metadata. Human-readable, git diff friendly, LLM natively understands.
- **Storage**: `~/.air/projects/<id>/project-rules.md` (project-authoritative). Export via `air memory export --project-rules AIR.md`.
- **Informed by**: Claude Code's CLAUDE.md format with frontmatter metadata.

## D-024: ExperienceMiner — Mid-Session Trigger + Curator + Self-Patch

- **Decision**:
  - **Triggers**: DebugRecord produced + session end + N-turn interval (Hermes-style Nudge Engine counter, ~10 turns or 10 tool calls). Background sub-agent, never blocks Main Agent.
  - **Scope**: Full extraction, no range limits. LLM decides what's worth saving.
  - **Validation**: Non-debug experiences use occurrence count (N=3) as threshold for promotion prompt. Debug experiences use evidence (build/test pass) as confidence; no numeric score needed.
  - **Deduplication**: Curator Daemon runs periodically (weekly), identifies overlapping skills, suggests merges, marks stale entries, archives unused ones.
  - **Self-patch**: When agent finds existing skill/rules wrong during execution, forwards to ExperienceMiner to patch.
  - **Data format**: SKILL.md (YAML frontmatter + Markdown body) under `~/.air/skills/`. Memories in `~/.air/projects/<id>/learned-memory.db`.
- **Informed by**: Hermes Agent's Nudge Engine, Curator Daemon, self-patching mechanism, and SKILL.md format.

## D-025: Context Compaction — Rule-File Driven, Copy-on-Write, Async

- **Decision**:
  - **Trigger**: 70% of context window capacity.
  - **Strategy**: Rule-file driven with three-tier inheritance (system default → user template at `~/.air/compaction-rules.yaml` → project override at `.air/compaction-rules.yaml`). System default template ensures projects without custom rules work normally. Editing user template affects all future new projects.
  - **Mechanism**: Copy-on-Write. On trigger, snapshot current messages 1-N. Compression runs asynchronously (independent Compactor sub-agent). New messages continue appending to original chain. On completion, a compaction marker is inserted between the compressed summary and post-snapshot messages. LLM sees: summary + marker + new messages. Original messages preserved for explicit LLM backtracking.
  - **Executor**: Independent Compactor sub-agent (not ContextAssembler, not Main Agent).

## D-026: Permission — Symlink Physical Resolution

- **Decision**: Symlinks resolved to physical path (follow `realpath`). This prevents project escape via symlink chains.

## D-027: Permission — .git/ Write Protection

- **Decision**: `.git/` directory write-protected by default (requires user confirmation). Can be overridden in `.air/permissions.yaml` project settings.

## D-028: Permission — Build Directory Unrestricted

- **Decision**: Build directories (`build/`, `out/`, etc.) have no special restrictions. C++ packaging workflows require agent ability to manually organize runtime libraries (.so/.dll deployment, rpath, resource file organization), so build directory access must be unrestricted.

## D-029: Permission — `~/.air/` System-Managed

- **Decision**: `~/.air/` and all contents are managed by AirCoding internally. PermissionEngine does not check AirCoding's own internal operations.

## D-030: Backup — Git-Based Repo

- **Decision**: Project-external file backup is a git repository at `~/.air/projects/<id>/backups/`. Each external file modification: `cp` to backups/ → `git add && git commit` (commit message: session_id, agent_type, reason). Version history, diff, compression all provided by git natively. `air restore --list` → `git log`; `air restore --session <id>` → `git log --grep <id>`. User manually deletes backups; no automatic cleanup.

## D-031: High-Risk Action Detection — Static Path Whitelist + LLM Escape Hatch

- **Decision**: Static path whitelist covers clearly-system paths (`/etc/fstab`, `/boot/`, `/etc/default/grub`, Windows registry system hives). `sudo` is NOT treated as high-risk (development machines need it for routine operations). LLM judgment only used as escape hatch when path matching can't decide.

## D-032: Restore Granularity

- **Decision**: Three levels — single file (most recent backup), specified time point, entire session. Git-backed storage makes all three straightforward.

## D-033: Testing Strategy

- **Decision**: Three layers:
  - **Unit tests** (`bun test`, CI per push, <30s, no LLM): Tool schema validation, PermissionEngine, ContextAssembler token math, EventBus routing, build system detection, config loading priority.
  - **Integration tests** (CI per push, <1min, recorded LLM fixture replay): Agent Loop state machine, Scheduler dependency resolution, sub-agent spawn → IPC → WorkerResult, session persist/restore, compaction Copy-on-Write, worktree lifecycle.
  - **E2E tests** (Release gate, real LLM): Full C++ project scenarios — add function + test, fix build error, multi-task dependency order, session resume. Must pass before every release.

## D-034: Distribution — Binary Tarball

- **Decision**: Binary distribution (Bun compile standalone executable, bundled Bun runtime). Distribution package = binary executable + Python scripts + default resource files (compaction rule templates, HUD presets, etc.). No public npm/channel until stable. Language-specific toolchains are first-class plugins, not bundled special treatment.

## D-035: Logging — Dual-File, 7-Day Retention

- **Decision**:
  - `air.log` — User-readable. Records startup failures, exceptions, environment configuration issues. Helps users self-diagnose.
  - `air.developer.log` — Full debug log, encrypted (may contain user privacy data, only decryptable by development team). Performance metrics written here. 7-day retention, auto-rotated.
  - Crash diagnostics: `air doctor` command collects version info, system environment, recent logs, crash stack trace into diagnostic package. Auto-detect abnormal exit on next startup, prompt user.

## D-036: Multi-Language Extension — Per-Language toolchain-* Packages

- **Decision**: Runtime (Agent Loop, Session, Scheduler, ToolRegistry, PermissionEngine) is language-agnostic. Each language is a `toolchain-<lang>` package + a `LanguageDetector` plugin implementing a common interface. Adding a new language = adding a `toolchain-python`/`toolchain-rust` etc. package, no core changes required. MVP ships with `toolchain-cpp`.

## D-037: Compaction Rules — Three-Tier Inheritance

- **Decision**: System default (built-in, never deleted, always fallback) → User template (`~/.air/compaction-rules.yaml`, editable, affects all future new projects) → Project rules (`.air/compaction-rules.yaml`, project-specific, highest priority).