Session Watcher

LLM context economics, in your terminal.

Session Watcher treats your prompt cache as inventory — it uses EOQ theory to tell you whether the current context is still worth carrying, and when to restart. Works with any session-based coding agent.

What it does

Session Watcher monitors your agent's session transcript in real time and answers one question: should I restart this session?

• Real-time dashboard — Chart.js SPA with SSE streaming: token stock L, exit line L*, per-turn cost, bill progress, rate-lamp interrupt meter

• Statusline widget — one-line shell command for your terminal status bar: lamp · progress bar · turn counter · cost rate · L/b · model

• Zero context pollution — the MCP tools return only a URL. No metric number, no transcript content, no session state ever re-enters the model's context

• Fully local — all state lives in a sidecar JSON file; no cloud, no telemetry, no API calls

Related MCP server: OpenExp

How it works

Your coding agent (Claude Code, OpenCode, OpenClaw, etc.)
        │  writes session transcripts
        ▼
┌──────────────────────────────────────────┐
│  Session Watcher (local sidecar daemon)   │
│  ─────────────────────────────────────── │
│  fold.js     — tail JSONL, fold usage    │
│  baseline.js — detect cold-start L_base  │
│  metrics.js  — compute L* (EOQ-optimal)  │
│  rate-lamp.js — interrupt meter (burn)   │
│  server.js   — Express + SSE dashboard   │
│  statusline.js — one-line shell client   │
└──────────────────────────────────────────┘
        │  dashboard :31393  ·  statusline  ·  MCP
        ▼
   Your browser / terminal status bar

Core model: L = cache_read_input_tokens (your context stock, rent-free while cached). When L crosses the EOQ-optimal restart line L*, the dashboard and statusline signal restart. See the paper for the full derivation — EOQ inventory theory mapped to LLM prompt caching.

Install

Plugin (recommended)

# 1. Add the marketplace (one-time)
claude plugin marketplace add nomadop/session-watcher

# 2. Install the plugin
claude plugin install session-watcher@session-watcher

Or from within a Claude Code session:

/plugin marketplace add nomadop/session-watcher
/plugin install session-watcher@session-watcher
/reload-plugins

This registers:

• MCP tools (start_watcher, stop_watcher, watcher_status) — available in every session

• SessionStart hook — auto-launches the dashboard server on each session

• Stop hook — evaluates the restart gate on each stop boundary

If you installed or updated in an already-running session, run /reload-plugins to activate.

Manual MCP

{
  "mcpServers": {
    "session-watcher": { "command": "node", "args": ["/path/to/index.js"] }
  }
}

Add to your project's .mcp.json or ~/.claude/settings.json. Then call start_watcher — it launches the dashboard and returns its URL.

Auto-launch with every session

{
  "hooks": { "SessionStart": [{ "command": "<path>/hooks/session-start.js" }] }
}

Add to ~/.claude/settings.json. The watcher starts automatically, fire-and-forget.

Statusline

The plugin system does not yet support declaring a statusline. Add to your ~/.claude/settings.json:

{
  "statusLine": {
    "type": "command",
    "command": "<plugin-install-path>/dist/statusline.js"
  }
}

Find your plugin path with:

find ~/.claude/plugins/cache -path '*/session-watcher/*/dist/statusline.js' -print

Or check via claude plugin details session-watcher@session-watcher.

Note: the plugin cache path changes on version update. After updating, re-run the command above and update your statusline path.

One compact line:

Quick Start

git clone https://github.com/nomadop/session-watcher.git
cd session-watcher
npm install
node server.js --project ~/.claude/projects/<project> --open

Additional flags: --ratio <N> (override C_RATIO), --port <N> (fixed port), --lbase <N> (override baseline — only when transcript has no cold start).

Then open http://localhost:31393 in your browser. The dashboard updates in real time as your agent runs.

MCP tools

All three are read-only — they never return metric values or session content into the model's context.

Agent support

Session Watcher is agent-agnostic. The measurement pipeline (fold, baseline, L*, rate-lamp) only needs cache_read_input_tokens from each turn — it doesn't care which agent produced the transcript.

Adding a new agent requires implementing one interface: extract cache_read_input_tokens from the agent's session transcript. See lib/extract.js for the Claude Code reference driver. PRs welcome.

Paper

Context Is Inventory: A Rent-or-Buy Model for Prompt-Cached LLM Sessions Longju Cheng (2026) · DOI: 10.5281/zenodo.21236704

The paper derives the full theoretical specification: EOQ→LLM mapping, the 41.4% movable-cost bound, the ski-rental restart strategy, and measurements on 1,016 real session transcripts. See paper/paper.pdf.

The current release implements the full paper specification: the measurement pipeline (fold, baseline, L*, φ), the billing gauge (billProgress), the rate-lamp interrupt meter with wall-position scaling, model-tier-specific pricing ratios, deep-water latching, and the restart recommendation signal via dashboard and statusline.

Uninstall

claude plugin uninstall session-watcher@session-watcher
# Remove state directory (optional):
rm -rf ~/.session-watcher

Test

npm test              # unit + integration (node:test)
npx playwright test   # E2E (requires running server)

Citation

@unpublished{cheng2026context,
  author = {Longju Cheng},
  title  = {Context Is Inventory: A Rent-or-Buy Model for Prompt-Cached LLM Sessions},
  year   = 2026,
  doi    = {10.5281/zenodo.21236704},
  url    = {https://doi.org/10.5281/zenodo.21236704},
  note   = {Preprint}
}

License

MIT