Save Tokens

You are working on save-tokens — the feature that installs Claude Code hooks and commands to warn about token usage, auto-save handoffs before compaction, and support session rotation.

Domain purpose

aspens save-tokens installs three Claude Code hooks (statusline, prompt-guard, precompact) plus /save-handoff, /resume-handoff-latest, and /resume-handoff slash commands. The hooks observe Claude's own token telemetry and inject system messages telling Claude to rotate sessions before context blow-up. Handoff files persist enough state in .aspens/sessions/ to resume seamlessly in a fresh session.

Business rules / invariants

• Claude-only feature. Hooks and statusline only work with Claude Code; Codex has no save-tokens integration. Config lives in .aspens.json under saveTokens.
• Config thresholds default 175k warn / 200k compact. Number.MAX_SAFE_INTEGER is the disabled sentinel for either threshold; target.js#isValidSaveTokensConfig() validates shape, types, and threshold ordering — invalid config causes readConfig() to return null.
• writeConfig preserves feature config — saveTokens is preserved across writeConfig calls unless explicitly set to null.
• Sessions dir gitignored — .aspens/sessions/.gitignore excludes everything except .gitignore and README.md.
• Settings backup — first install creates .claude/settings.json.bak if settings exist and no backup exists yet.
• StatusLine pre-existence guard — canInstallSaveTokensStatusLine() refuses to overwrite an unrelated custom statusLine.command; when refused, applyStatusLineAvailability() forces both thresholds to MAX_SAFE_INTEGER and disables sessionRotation.

Non-obvious behaviors

• Three hook entry points dispatched by subcommand. Shell wrappers (*.sh) read stdin, resolve project dir, and call save-tokens.mjs with statusline, prompt-guard, or precompact. ASPENS_PROJECT_DIR env var overrides CLAUDE_PROJECT_DIR; both fall back to cwd.
• Statusline doubles as telemetry recorder. recordClaudeContextTelemetry() writes .aspens/sessions/claude-context.json summing input_tokens + cache_creation_input_tokens + cache_read_input_tokens + output_tokens from context_window.current_usage. Telemetry older than 5 minutes is ignored by the prompt guard.
• Prompt-guard speaks to Claude, not the user. Above compactAtTokens: saves a handoff (reason rotation-threshold if sessionRotation, else compact-threshold) and stdout-prints an "IMPORTANT — you must tell the user" block instructing fresh session + /resume-handoff-latest. Above warnAtTokens: suggests /save-handoff. When telemetry is missing, prints a one-time link to the aspens issues page.
• Precompact hook only fires when saveHandoff is enabled and claude.enabled !== false; reason is precompact.
• extractSessionFacts() parses the JSONL transcript to extract originalTask (first user message, 500 char cap), recentPrompts (last 3, 200 char cap each), filesModified (Edit/Write tool_use file_path), gitCommits (regex on Bash git commit -m "..."), and branch (from gitBranch on user records). Transcript path is validated to be inside projectDir; falls back to input.prompt when transcript is missing or outside the project.
• Transcript parsing is best-effort — all errors caught, invalid JSON lines silently skipped; returns empty facts on failure.
• pruneOldHandoffs() keeps newest 10 *-handoff.md files by lexicographic (timestamp) sort; older are unlinked. latestHandoff() uses the same sort.
• --recommended install is non-interactive — used standalone or invoked by doc init --recommended (which also installs agents and the doc-sync git hook).
• --remove cleans legacy artifacts too — removes both .sh and historic .mjs variants of each hook, plus the legacy save-tokens-resume.md command; strips statusLine if it points at save-tokens and filters any hook entry whose command contains save-tokens-; nulls saveTokens in .aspens.json.

Critical files (purpose, not inventory)

• src/commands/save-tokens.js — install/remove orchestration; interactive multiselect (warnings, handoffs), --recommended, --remove. Exports installSaveTokensRecommended() consumed by doc-init.js.
• src/lib/save-tokens.js — config + settings + template content builders (DEFAULT_SAVE_TOKENS_CONFIG, buildSaveTokensConfig, buildSaveTokensSettings, buildSaveTokensGitignore, buildSaveTokensReadme, buildSaveTokensRecommendations).
• src/templates/hooks/save-tokens.mjs — runtime library: runStatusline, runPromptGuard, runPrecompact, saveHandoff, extractSessionFacts, telemetry record/read, handoff pruning, main() subcommand dispatch.
• src/templates/hooks/save-tokens-{statusline,prompt-guard,precompact}.sh — shell wrappers; resolve PROJECT_DIR via cd "$SCRIPT_DIR/../.." && pwd and exec the .mjs with the matching subcommand.
• src/templates/commands/{save-handoff,resume-handoff-latest,resume-handoff}.md — slash command bodies installed under .claude/commands/.

Critical Rules

• Settings merge uses mergeSettings() from skill-writer.js — save-tokens hooks are treated as aspens-managed; do not hand-edit .claude/settings.json to add/remove save-tokens entries.
• Hooks write to stdout to inject into Claude's context — comments in runPromptGuard() flag this; keep those messages addressed to Claude ("Tell the user...") not to a human reader.
• Token sum includes cache tokens — do not switch to just input_tokens + output_tokens; that under-counts Claude's effective context use.

References

• Impact integration: src/lib/impact.js — evaluateSaveTokensHealth() validates installed state

Last Updated: 2026-05-11