Monid is OpenRouter for agent tools, and it meters on the wire response.

monid-ai/monid, MIT-licensed at 214 stars since August 26, fronts 2,000+ tools across 72+ providers behind one base URL and API key, with per-call endpoint selection using live pricing and observed p50/p95 health data. Its distinguishing choice is billing on the actual API response rather than the call: a vendor error, an unmatched company or an unresolved person settles at zero. No absolute latency benchmarks published, only relative discovery data, so the routing-quality claim is unverified. The billing model is the part I'd copy.