Measure savings

context-guru reports what it actually saved through the proxy's GET /stats endpoint, backed by an in-process metrics aggregator. Savings are measured on message content text — what the model reads — not the JSON envelope, so a control directive like cacheinject never looks "worse".

GET /stats

The proxy exposes GET /stats with in-process savings rollups. Savings are token-weighted (Σ saved / Σ before) — the honest aggregate, not a mean of per-request percentages. It also reports:

Field	Meaning
token-weighted savings	Σ saved / Σ before across all requests
wasted_tokens	content offloaded then re-served via expand (a premature offload)
bounces	how many offloads were re-served (the count behind wasted_tokens)
adjusted_saved	saved − wasted — bounce-adjusted, may be negative
top_passthrough	components that ran but never changed a request: dead weight to drop

Reading top_passthrough

A component in top_passthrough isn't necessarily broken. cacheinject always lands there — its savings are provider-side KV-cache hits, invisible to content-token counts. But a content-offloader that never fires is a candidate to drop from your pipeline.

The Emitter interface

The pipeline depends only on the Emitter interface (Component(Report) + Run(RunReport)), so it carries no telemetry-backend dependency. Swap implementations to route metrics where you need:

Emitter	Role
Slog	logs in the context_engineering.* vocabulary
Aggregator	in-process rollups behind /stats
Tee	fan-out to several emitters
NopEmitter	discard

A real session: scripts/cc-demo.sh

scripts/cc-demo.sh routes a real claude CLI session through the proxy and reads /stats before and after. It builds a tiny repo, starts the proxy with --preset balanced, points Claude Code's ANTHROPIC_BASE_URL at it, runs one claude -p task, and prints the stats delta:

export ANTHROPIC_BASE_URL=...            # upstream Anthropic-compatible endpoint
export ANTHROPIC_AUTH_TOKEN=...
scripts/cc-demo.sh
# == stats before ==  {...}
# (claude reads main.go + README.md through the proxy)
# == stats after ==   {...}   ← token-weighted savings for the session

It's the shortest way to see real savings on your own model without a full benchmark harness.

Benchmarks

For the full per-component SWE-bench evaluation — where mask delivers ~27% content-token savings with no reward loss, and how the /stats within-run metric is derived — see Benchmarks.