From APIs to Agent Markets: Governed Capability Acquisition and Qualified Demand in a Machine-Payable Agent Utility Grid

Autonomous agents can invoke tools, but invocation alone does not define a market. Before software can economically acquire an external capability, it must discover a relevant operation, observe current price and requirements, determine whether payment is authorized, cross a payment boundary, receive an outcome, and preserve enough evidence to distinguish discovery from realized commercial activity. This preprint presents a capability-acquisition layer added to the SALT19 Agent Utility Grid after the earlier Governed Decision-Commerce Control Plane record (DOI 10.5281/zenodo.22102248). The new architecture introduces two free front-door capabilities (find-salt19-tool and salt19-pricing), twelve one-cent Fresh-State acquisition products, explicit qualified-demand telemetry, and a zero-spend public-market observer while preserving x402 v2 exact payments on Base mainnet with USDC and validated_external_revenue as the sole revenue authority. At the pinned evidence cutoff, source state was commit 39c5e0e2be09e57a59c6343a45468e3584e636a9 (2026-08-28T22:38:18Z). That exact revision completed the production deployment workflow successfully, including exact-revision validation, D1 migration checks, production Worker deployment, external-payability certification, live surface verification, comprehensive advertised production contracts, and zero-spend transaction readiness. Same-revision MCP Registry publication, Phase-1 live smoke, and registry public-identity synchronization also completed successfully. The paper explicitly distinguishes free selection, qualified discovery, payment capability, paid retry, settlement, and validated revenue. A crawler, registry probe, HTTP 402 challenge, free selection, or seller-run test is not treated as commercial success. Fresh-State source/fetch provenance is not represented as a signed Proof402 receipt unless the receipt finalizer actually creates one. The evidence is first-party production and source validation. This record does not claim product-market fit, generalized willingness-to-pay, repeat external buyers, optimal economic routing, external scientific replication, or a stable authoritative external tool count.