A Claude Code plugin that keeps an idle session's prompt cache warm for a few cache lifetimes, then runs /compact on it.

When you leave a session idle past its cache TTL (1 hour on a Claude subscription, 5 minutes on an API key), your next message rewrites the whole context into the cache. On a long session that rewrite is the most expensive turn you send. This plugin sends a short keep-alive prompt just before the cache expires, so the next turn reads the cache instead of rewriting it. After a few pings it compacts the session and stops, so a session you never come back to leaves a small summary to rewrite instead of the full context.

Requires cmux. A hook can't submit a prompt to Claude Code, so the plugin types into the session's own cmux pane. tmux, iTerm2, and other terminals aren't supported yet. PRs welcome.

you stop typing
      │
Stop hook ──▶ timer: wait 2 s, read the cache TTL from the transcript
      │
      ▼  sleep TTL - 5 min (55 min for 1h, 4 min for 5m)
guards: session idle? input box empty? screen readable?
      │
      ├─ pings < N ─▶ type the keep-alive prompt ─▶ reply ends a turn ─▶ Stop again
      ├─ pings = N ─▶ type /compact
      └─ pings > N ─▶ nothing, until you send a message

Any message you type cancels the pending timer and resets the count. The plugin compares each prompt with the text it last sent, so its own pings don't reset the count.

cmux posts a notification every time a turn ends, pings included. After each ping's turn, the plugin dismisses the notifications on its pane created since it sent the ping, so pings don't leave unread markers. Notifications you had not read yet stay. A banner or phone push cmux already delivered can't be recalled.

Measured on a 70k-token session: the keep-alive turn read 70,471 tokens from the cache, wrote 133, and produced 4 output tokens.

A keep-alive costs one cache read. The turn it saves costs a full 1h cache write. Prices below are Anthropic's API rates for a 180k-token context (close to the 181,533 tokens a live ping read in the session that tested this).

What that means per idle gap on Opus 5.5:

you come back after...     without plugin   with plugin (3 pings, then compact)
under 1 h                  $0.04            $0.04    cache still warm, no ping sent
1 h to ~3 h 40 min         $1.44            ≤ $0.17  1-3 pings + warm resume
never                      $0               ~$0.12   3 pings, plus one /compact turn
after the compact          $1.44            summary-sized cold write

On Opus 5.5 a full warm-and-come-back cycle costs about a tenth of one cold resume, so the plugin pays for itself if you return to at least 1 idle session in 12 inside the warm window. On Sonnet 5 a read is a larger share of a write: the cycle costs about a fifth of a cold resume, and the bar is 1 in 6. On a subscription you pay in usage limits rather than dollars. The dollar figures show the ratios at API prices.

oh-my-pi (omp) has both halves of this built into the agent loop, which is where the idea came from. Claude Code exposes no hook that can send a request, so this plugin drives the terminal instead.

Sources: packages/coding-agent/CHANGELOG.md (18.3.5) and docs/compaction.md in the oh-my-pi repo.

claude plugin marketplace add STRML/cc-cache-warmer
claude plugin install cc-cache-warmer@cc-cache-warmer

Run Claude Code inside a cmux pane. Outside cmux, the plugin logs a skip and does nothing.

Set these as environment variables before you start claude.

Each skip is logged to warmer.log in the plugin's data directory (~/.claude/plugins/data/<plugin-id>/).

./tests/e2e.sh

Each scenario matches one row of the failure matrix in docs/PLAN.md. A fake cmux on PATH records every call. The results land in tests/out/results.txt. CI runs shellcheck and the suite on Linux and macOS on pushes to main and on every pull request.

MIT