Tutorials, comparisons and design patterns for building autonomous agents that self-fund, call 345+ models and orchestrate MCP Tools.
We cloned a2aproject/A2A on 2026-08-15 and read the specification, the proto and the SDK compatibility layers. A2A 1.0 unified Part types, renamed every enum, moved errors to google.rpc.Status and made version negotiation a header. It also added signed Agent Cards and a two-tier extension governance framework that, as of today, holds zero official extensions. The x402 payment extension still speaks the 0.3 wire format.
Between 6 and 13 August the supply side of the agent stack advanced on four fronts at once: a vendor-neutral packaging format for skills and MCP servers, EC2-backed agent sessions that live for two weeks, an open source model router with published cost curves, and a browser engine that reads the web for a fraction of Chromium's memory. In the same week, the only public demand-side number moved the other way: x402 daily settlement volume is down 93% year to date.
We cloned open-telemetry/semantic-conventions-genai on 2026-08-14 and read every model file. Sixty-three gen_ai attributes, five agent span types, MCP context propagation through params._meta — and zero attributes for money. PR #443, opened five days ago, would add gen_ai.usage.cost.* as a float. That is the exact place where telemetry stops and settlement begins.
Three purpose-built payment chains went live or were dated in 2026: Tempo (Stripe, mainnet March 18), Kite (April 30) and Arc (Circle, September 16). We cloned the x402 reference implementation and read each chain's docs. The binding constraint is not the chain — it is the token interface underneath it.
x402's fourth scheme adds the one thing exact never had: a way to get money back. We read both spec documents, the shipped TypeScript, and the escrow contract itself — then swept 24 hours of Base mainnet to see who is actually using it. The scheme is client-only, its fee semantics have already drifted from upstream, and the refund machinery it exists for fires on 0.28% of payments.
The x402 spec everyone reads is 4,252 words. The extension specs nobody reads are 14,820. We audited all nine — sign-in-with-x, offer-receipt, auth-hints, payment-identifier, builder-code, gas sponsoring, HTTP message signatures, Bazaar — mapped which SDKs actually implement them (TypeScript 7 of 9, Python and Go 4 of 9), traced a month of security patches, and found three verifiable inconsistencies between the specs and the shipped code.
We cloned tempoxyz/mpp-specs and read all 23 specification documents of the Machine Payments Protocol, the Stripe + Tempo rival to x402. MPP registers a real HTTP authentication scheme, ships payment channels as a first-class primitive, carries fiat rails through Shared Payment Tokens, and defines an MCP transport. We also found wire examples that contradict their own prose, a credential example missing a required field, and an MCP binding pinned to a superseded spec revision.
Our facilitator API deep dive ended with a one-line escape hatch: self-host with x402-rs. This is the follow-up. We cloned the repo at HEAD, read the config parser, probed the maintainer's live testnet instance, and found both the strongest open-source settlement plane in the ecosystem and three documentation bugs that will bite anyone deploying it.