Keep your cache warm.
Know the cost when it isn’t.

A mod for Claude Code’s prompt cache. Keep it warm while you take a break, or see the estimated rewrite cost before you send a cold prompt.

Install Cache Tax See it in a real session

Free and open source. Claude Code 2.1.287+. Warming needs a one-hour cache and Claude Code left running. Pings cost tokens.

Before your break

Run /keepwarm 90m for a 90-minute window. After 50 idle minutes, Cache Tax sends a model request over the session to refresh its cache.

Each ping checks its usage. No cache reads, or writes reaching 10% of reads, stop the loop. /keepwarm off stops it too. An already-cold session waits for your next turn before pinging.

When you return cold

For a context of at least 50,000 tokens, the guard stops your ordinary message once when its one-hour clock says cold. You see the estimated rewrite cost. Resend to continue.

After a detected cold write, it automatically arms at least three hours of warming. Run /cache-tax to see the window and this session’s cold-write tally.

Recorded in real sessions

One verified run on Fable 5.1: a ping at minute 50 read the cache, and the message sent at minute 62 read 156,886 tokens from cache. One run, one configuration.

Recorded status slot: last ping read 75k tokens, costing $0.02. Open full-size recording. — Sonnet 5, 16 September 2026: a 75k-token cache read at $0.02. This recording used the one-minute testing interval on 2.0.0. Its $0.45 rewrite estimate used an older price row; the current build prints $0.30 for the same context. A receipt for this session, not a universal ping price.

See the cold-send refusal

Recorded cold-send guard showing a $6.61 rewrite estimate. Open full-size recording. — Recorded on 2.1.1, before the 2.1.2 wording change. The estimate was $6.61; the subsequent reported cache write was $6.28. Current wording says “up to” the estimated token count. These are API-equivalent costs, not extra subscription charges.

Two commands to install.

Use Claude Code 2.1.287 or later. Mods are on by default; no early-access flag is needed.

claude plugin marketplace add karanb192/claude-code-mods
claude plugin install cache-tax@claude-code-mods

Restart Claude Code or run /reload-plugins in an open session. Then run /keepwarm 90m before your break. Plain /keepwarm arms six hours.

Check your cache lifetime. Warming uses a 50-minute timer. If your main cache defaults to five minutes, set promptCacheTtl to "1h" first.

Prefer a settings hook? The hook version warns by default, can refuse a cold send once, and provides a standalone status-line countdown. Warming is part of this mod.

Know what you are paying for.

A ping bills reads, writes, uncached input and output. Fork output is uncapped. The displayed dollars are API-equivalent estimates, not additional subscription charges. Quota savings have not been measured.
Prefix changes can break the cache before its timer expires. A warm read proves that ping hit the cache; it does not guarantee your next message will.
The mod makes model requests over your session and stores its settings. The README lists its hooks, engine calls and threat model.