feat(sell): add obol sell mcp — paid MCP tool over x402 in-band _meta

[bussyjd]

Summary

This PR does two things, both on the latest x402 SDK:

1. Migrates the x402 Go SDK github.com/coinbase/x402/go → github.com/x402-foundation/x402/go (the maintained module path) — folded in from chore(deps): migrate x402 SDK to x402-foundation/x402/go (unlocks upto + batch-settlement) #613, so this branch is self-contained off main.
2. Adds obol sell mcp — runs a local x402-paid MCP (Model Context Protocol) server. The paid tool forwards the buyer's JSON arguments to a backend HTTP service and returns the response, so any real service can be resold to agents per call. Buyers settle in-band via the MCP request _meta["x402/payment"] field (per specs/transports-v2/mcp.md).

Why it matters: the MCP server is the application-layer counterpart to the HTTP-402 ForwardAuth gate used by obol sell inference / obol sell http. It mirrors the canonical x402 paid-MCP example — the x402 repo's own examples/go/servers/mcp is a paid get_weather tool built with the same NewPaymentWrapper — except the upstream is generalised from a stub to a real backend. The point is not "pay for inference over MCP" (an agent would just call an inference endpoint directly); it's metered, pay-per-call access to a service an agent doesn't have its own account for. verify → execute → settle runs inside the tool call, so a caller is never charged for a failed tool.

Risk level: low — the migration is a pure module-path swap (types API unchanged); the MCP server is a new, opt-in subcommand with no change to existing sell paths.

Commits

• chore(deps): migrate x402 SDK coinbase/x402/go → x402-foundation/x402/go — folded in from chore(deps): migrate x402 SDK to x402-foundation/x402/go (unlocks upto + batch-settlement) #613 (chore(deps): migrate x402 SDK to x402-foundation/x402/go (unlocks upto + batch-settlement) #613 is superseded by this PR).
• feat(sell): add obol sell mcp — paid MCP tool over x402 in-band _meta.

Scope

• Code
• Charts / manifests
• Flows / QA scripts
• Docs / skills
• Images / dependencies — migrates x402 SDK to x402-foundation/x402/go; adds modelcontextprotocol/go-sdk v1.3.0
• Other

Base branch: main

Implementation notes

• internal/x402mcp/server.go — wraps a tool with x402-foundation/x402/go/mcp's PaymentWrapper over the official modelcontextprotocol/go-sdk. The paid tool (proxyTool) POSTs the buyer's tool arguments verbatim to --upstream and returns the service response; the backend's own auth header (if any) is passed via --upstream-header, set server-side and never sent to buyers. Serves streamable HTTP at /mcp, plus a free ping tool and /health.
• cmd/obol/sell.go — sell mcp subcommand: --pay-to / --price / --chain / --tool-name / --port / --upstream / --upstream-header (repeatable "Key: Value") / --description / --facilitator. Headline example fronts a weather API.

# Front a weather API as a paid MCP tool (the canonical x402 paid-MCP shape):
obol sell mcp weather --pay-to 0x... --price 0.001 --chain base-sepolia \
    --tool-name get_weather \
    --description 'Current weather for a city. Args: {city}' \
    --upstream https://your-weather-service/current

Validation

go build ./... clean; go test ./internal/x402/... ./internal/x402mcp/... ./internal/inference/... ./cmd/obol/... pass; go.sum clean of coinbase/x402. MCP tests cover the upstream proxy (arg forwarding + backend-auth-header injection + service-response passthrough, modeled on a weather backend; upstream error; missing upstream), Serve validation, CAIP-2 mapping, --upstream-header parsing, and result helpers.

[OisinKyne]

Walk me through this. Someone else is going to find the mcp, add it, and their agent is going to decide when to pay for it? what paid mcp tools exist in the wild? what examples of MCP x x402 are out there at the moment? i'm not convinced theres moat in paid tools, but if its just a different type of discovery that's not a massive transition.

What tools are you paying for/is anyone paying for?

[bussyjd]

Reworked the example off the back of this. You're right that paying for inference over MCP is pointless and a bad example.

The one I should've pointed at is in the x402 repo itself: examples/go/servers/mcp is a paid get_weather tool ($0.001, plus get_forecast at $0.005) next to a free ping, built with the same NewPaymentWrapper we use. That's the canonical paid MCP tool — pay per call, settle in-band, server returns the data. obol sell mcp is exactly that, with the upstream generalised: instead of a stub weather function it fronts a real backend service, so an agent gets metered, pay-per-call access to something it doesn't have its own account for.

On the moat: agreed, there's none at the protocol layer — it's standardised discovery + auth. The durable bit is the wrapped capability and aggregating demand — fronting one backend for many agents — not the transport.

Paying today? Basically no — ~$28k/day on Base, ~$0.20 average, half of it tests. Real endpoints exist (Firecrawl/Exa, Cloudflare's paidTool, Coinbase's Bazaar auto-listing anything that settles a payment) and Stripe's MPP is the same bet with Anthropic/OpenAI on board — but early. A cheap bet on agent-native distribution, not a revenue line.

[bussyjd]

Superseded by the v0.10.0-rc14 release train: #616 — this PR's branch is merged there verbatim (merge commit preserved, full union build + test green). Closing in favor of the consolidated release PR.