# Phase 5: Per-Project Instance Isolation - Context

**Gathered:** 2026-04-10 (assumptions mode)
**Status:** Ready for planning

<domain>
## Phase Boundary

Each project directory gets its own isolated Claude state so conversation history, todos, and settings do not bleed between projects. Launching from a git worktree shares state with the main worktree of the same repo. Two concurrent sessions in the same project do not corrupt each other. `claudebox --gc` removes instance directories whose project root no longer exists on disk.

Credentials (OAuth tokens, API keys) are intentionally shared across all instances — not isolated per project.

</domain>

<decisions>
## Implementation Decisions

### Instance Isolation Mechanism

- **D-01:** Use `CLAUDE_CONFIG_DIR` env var (via `--setenv CLAUDE_CONFIG_DIR "$HOME/.claudebox/instances/<hash>"`) to redirect Claude Code's data directory to a per-project path. The existing `~/.claudebox` bind mount stays intact — it is still needed for SANDBOX.md and CLAUDE.md management.
- **D-02:** Instance directories live at `~/.claudebox/instances/<hash>/` on the host. The hash is derived from the canonical project root (see D-03).
- **D-03:** `~/.claude.json` (top-level auth) is hardcoded by Claude Code to `$HOME` and is NOT redirected by `CLAUDE_CONFIG_DIR`. If it exists on the host at `$HOME/.claude.json`, bind-mount it read-write into the sandbox alongside the existing credential mounts. If absent, skip silently (same pattern as Phase 4 credentials).

### Project Root Identification

- **D-04:** Canonical project root = `git rev-parse --git-common-dir` resolved to absolute path, falling back to `$CWD` for non-git directories. This gives all worktrees of the same repo the same instance key, satisfying the worktree-sharing requirement.
- **D-05:** Hash the canonical root path with SHA-256 (or `sha256sum`, available via coreutils in the sandbox PATH) to produce the instance directory name. The hash is truncated to 16 hex chars for readability: `~/.claudebox/instances/a3f9c2b1d4e8f701/`.

### Credentials Stay Shared

- **D-06:** `~/.claudebox/.credentials.json` (OAuth tokens, Phase 4) and `~/.claude.json` are bind-mounted from the host top-level into every sandbox — not per-instance. OAuth token refreshes must write to a single file shared across all sessions; per-instance copies would cause divergence and re-auth failures.

### GC Mechanism

- **D-07:** On instance creation, write a `project-root` plaintext file inside the instance dir containing the canonical project root path. Example: `echo "$CANONICAL_ROOT" > "$INSTANCE_DIR/project-root"`.
- **D-08:** `claudebox --gc` iterates `~/.claudebox/instances/*/project-root`, reads each path, and removes any instance dir whose recorded path no longer exists on disk (`[[ ! -d "$root" ]]`). Print removed paths to stderr.
- **D-09:** Add `--gc` to the `case "$1" in` flag-parsing block (line 9 of claudebox.sh) alongside `--yes`, `--dry-run`, `--check`.

### Concurrent Session Safety

- **D-10:** No locking mechanism needed. `CLAUDE_CONFIG_DIR` gives each session its own directory tree; concurrent sessions in the same project share the same instance dir but Claude Code itself manages file-level concurrency within its own data dir (same as it does without claudebox). If Claude Code corrupts its own state under concurrency, that is Claude Code's problem, not claudebox's.

### Claude's Discretion

- Exact hash truncation length (16 chars recommended above, can adjust)
- Whether to print instance path at launch (could be a `--verbose` addition later)
- Handling of `~/.claudebox/instances/` directory creation (ensure it exists before writing)

</decisions>

<canonical_refs>
## Canonical References

**Downstream agents MUST read these before planning or implementing.**

### Codebase
- `claudebox.sh` — full script; flag parsing (lines 1-18), BWRAP_ARGS construction (lines 365-401), credential mount logic (lines 104-122), SANDBOX.md/CLAUDE.md management (lines 124-175)
- `flake.nix` — Nix packaging; runtimeInputs list (must include `coreutils` for `sha256sum`)

### Requirements
- `.planning/REQUIREMENTS.md` — v2 requirements section; INST-01 through INST-04 are NOT yet defined here (they need to be added during planning)
- `.planning/ROADMAP.md` — Phase 5 success criteria (lines 44-53)

### External (researched)
- Claude Code env var `CLAUDE_CONFIG_DIR`: official docs confirm it redirects sessions/settings/todos; `~/.claude.json` NOT redirected (hardcoded to `$HOME`)
- Claude Code does NOT auto-resume on launch; `--continue` / `--resume` are explicit opt-in flags

</canonical_refs>

<code_context>
## Existing Code Insights

### Reusable Assets
- Flag parsing block (lines 1-18 of claudebox.sh): add `--gc` case here using the existing pattern
- `BWRAP_ARGS` array with `+=` appends: add `--setenv CLAUDE_CONFIG_DIR "..."` using the same pattern as existing `--setenv` calls
- Credential detection pattern (lines 104-122): same `[[ -f "$FILE" ]]` pattern for `~/.claude.json` optional mount
- `coreutils` is already in `runtimeInputs` (flake.nix) — provides `sha256sum` for hashing

### Established Patterns
- All sandbox flags are built into `BWRAP_ARGS` array before `exec bwrap "${BWRAP_ARGS[@]}"` — new `--setenv CLAUDE_CONFIG_DIR` follows the same pattern
- Conditional mounts use `if [[ "$VAR" == true ]]; then BWRAP_ARGS+=(...)` — same pattern for `~/.claude.json`
- `--dry-run` block (lines 318-360) is a hardcoded parallel reproduction of `BWRAP_ARGS` — must also be updated to include `CLAUDE_CONFIG_DIR` and optional `~/.claude.json` mount
- `print_audit()` Mounts section must be updated to show the instance dir and `~/.claude.json` if mounted

### Integration Points
- Line 383: `--bind "$HOME/.claudebox" "$HOME/.claudebox"` — stays unchanged; `CLAUDE_CONFIG_DIR` is additive
- Line 384: `--symlink "$HOME/.claudebox" "$HOME/.claude"` — stays unchanged
- Lines 366-368: env var `--setenv` block in `BWRAP_ARGS` — insert `CLAUDE_CONFIG_DIR` here
- Lines 390-392: credential bind conditional — add parallel block for `~/.claude.json`
- Lines 333-360: dry-run echo block — add `CLAUDE_CONFIG_DIR` setenv echo + `~/.claude.json` conditional echo

</code_context>

<specifics>
## Specific Ideas

- Instance dirs at `~/.claudebox/instances/<16-char-hash>/` — human-debuggable length, not full 64-char SHA-256
- GC prints removed paths to stderr (consistent with claudebox's stderr-only output convention)
- `--gc` exits after running GC, does not launch Claude (same pattern as `--check`)

</specifics>

<deferred>
## Deferred Ideas

- Per-instance `settings.json` override (project-specific model selection) — Phase 7 (Named Profiles)
- `--gc --dry-run` to preview what would be removed — could be added but is Phase 5+ scope creep
- `claudebox --list-instances` to show all instances with their project roots — nice-to-have, not required

None — analysis stayed within phase scope.

</deferred>

---

*Phase: 05-per-project-instance-isolation*
*Context gathered: 2026-04-10*