MCP Usage Metering + agent coordination (Dirtsmith)

Free first call, one step, no key, no signup: curl -s https://gap-functions.bfurrow26.workers.dev/try returns a tiny packet, the solve rule, and a one-liner. POST the answers back to /try and you get the real tool result. ?tool=agent_lease_lock (or any tool below) for the others.

Search words: usage metering, metered billing, idempotent, reserve before pay, spend cap, budget, distributed lock / lease lock, fencing token, semaphore, concurrency limit, heartbeat, dead-man switch, free first call.

Hosted tool that records one billable usage event per MCP tool call, idempotently, and returns an invoice preview. Built for MCP server authors who want to charge per call without building metering.

• Endpoint (MCP): https://gap-functions.bfurrow26.workers.dev/mcp (all URLs in config.json)
• Tool list: GET /mcp/tools · Health: GET /health
• Contract: schema/mcp_usage_metering.v1.0.0.json (strict input, required idempotency_key, versioned 1.0.0)

Pricing and buying (no signup, no key)

• Call the tool directly. Each IP gets 100 free keyless calls per day.
• After that, tools/call returns isError: true with error_code: PAYMENT_REQUIRED. structuredContent.payment holds a one-click approve_and_pay_url for your human and a grant_token. Your agent asks you once, you click buy, then the agent retries the same call with header X-Grant-Token (or uses the /u/<grant_token>/mcp URL form). Same token forever.
• Usage after purchase: $0.15 per 1,000 events ($0.00015/event), 10,000 free events per month per account.
• Replays with the same idempotency_key are never billed twice.

Inputs

Required: tool_name, tool_version (semver), end_customer_ref, idempotency_key (8–128 chars). Optional: units, unit_label, unit_price_cents, metadata.

Legal

Terms of Service and Privacy Policy are DRAFT, pending counsel review, not yet in effect — see legal/.

License

MIT (this repository: docs and schema). Operated by Dirtsmith LLC.

More tools on the same endpoint (added 2026-11-03)

The same MCP URL (https://gap-functions.bfurrow26.workers.dev/mcp) also serves:

• agent_spend_ledger: hosted shared budget ledger. Actions: set_budget, reserve, settle, release, status. Reserve is atomic: concurrent agents on one budget can never exceed the ceiling. Reservations expire after ttl_s (default 900 s). Amounts are integer micro-USD. $9.00 per 1,000 calls after 1,000 free calls a month.
• agent_lease_lock: hosted lease lock. Actions: acquire, renew, release, status, with ttl_s and a monotonic fencing_token. $0.90 per 1,000 calls after 1,000 free calls a month.
• agent_semaphore: hosted counting semaphore. Actions: acquire (with max_slots, holder_id, ttl_s), release (by permit_id), status. At most N permits at once; each permit carries a monotonic fencing_token; full returns NO_SLOT_FREE with retry_after_s. $0.90 per 1,000 calls after 1,000 free calls a month.
• agent_heartbeat_registry: hosted dead-man switch. Actions: beat (with agent_id, interval_s, grace_s, optional note), status, list_stale (agents past interval + grace, up to 100). $0.90 per 1,000 calls after 1,000 free calls a month. State is shared by namespace (16-128 chars). Any agent that knows the namespace can read and change that state, so treat it like a password. We store only its hash. Buying works the same way as metering: no signup or key, one approve-and-pay link when the free calls run out.