Cache Stats

A local-first GitHub Copilot canvas for understanding prompt-cache reuse, expiry, token composition, and AIU cost while a session is still active.

imageAn illustrative Cache Stats canvas showing live cache health and AIU exposure

Preview data is illustrative and does not contain session information.

The one-bit HyperCard stack has three focused views:

• Live Cache shows expiry state, reuse, and the cost exposed if the reusable prompt prefix must be rebuilt.
• Composition separates fresh input, cache reads, cache writes, output, and reasoning tokens.
• Turn History makes cold starts and rebuild costs visible turn by turn.

The canvas refreshes locally every two seconds. Opening and using it consumes no model tokens.

Install

In a GitHub Copilot session, ask:

Install this extension: https://github.com/arafattehsin/cache-stats-extension

Then ask:

Open the cache stats canvas

Before marketplace approval, install the public repository directly:

copilot plugin install arafattehsin/cache-stats-extension

After marketplace approval, install it from Awesome Copilot:

copilot plugin install cache-stats@awesome-copilot

Requirements

• A GitHub Copilot client with canvas-extension support
• A Node.js runtime with the built-in node:sqlite module
• A local Copilot session store containing assistant_usage_events

Privacy

Cache Stats reads ~/.copilot/session-store.db in read-only mode and serves the canvas from an ephemeral 127.0.0.1 port. It does not modify the session store, call a model, load remote assets, or send telemetry.

The local canvas API includes the active session ID and database path in its response for diagnostics. That endpoint is available only through the loopback server created for the open canvas instance.

How the numbers work

input_tokens is the complete prompt for one model call:

fresh input = input_tokens - cache_read_tokens - cache_write_tokens

Per-token billing rates come from each row's token_details_json, so baseline cost, actual cost, savings, and rebuild exposure follow the model that served the call instead of relying on hard-coded prices.

A turn starts with one user-initiated call and includes its agent, sub-agent, and compaction follow-ups. A turn is marked REBUILT when its entry call reads no cached prefix but writes a substantial one. Live expiry comes from Copilot's session.usage_checkpoint event.

Development

npm install
npm run check
copilot plugin install .

Reinstall the local plugin after changing source files because Copilot caches installed plugin content.

Package layout

Environment overrides

• COPILOT_CACHE_STATS_DB - explicit path to the session store
• COPILOT_HOME - alternate Copilot home directory

License

MIT