A real-time debugger for Solana payment flows. Groups HTTP 402 challenge-response cycles into single payment flows and visualizes them with a sequence diagram, so developers can see exactly what happened (and what went wrong) at each step.

Supports two payment protocols:

• MPP (Monetized Payment Protocol) — www-authenticate / payment-receipt headers
• x402 — X-PAYMENT header, with facilitator verify/settle

Runs against the Solana Payment Sandbox — no real funds needed.

• Backend — Express API (api/index.ts) with payment-gated demo endpoints, an embedded x402 facilitator, and a correlation engine that groups raw HTTP requests into payment flows streamed over SSE.
• Frontend — React SPA (src/) with Vite. Compact flow list with protocol badge, resource path, status, and latency. Expanded view shows a step-by-step sequence diagram alongside a timestamped event log.
• Embedded mode — The frontend and backend are also compiled into the pay Rust binary (crates/pdb). Run pay --sandbox server start --debugger spec.yml to get the debugger alongside any gateway proxy.

For source packaging, build or unpack the frontend before compiling Rust:

cd web-ui && pnpm install --frozen-lockfile && pnpm build
cd ../rust && cargo build --release

Release builds also publish pay-pdb-dist-<version>.tar.gz. Packagers such as Homebrew can unpack that artifact and set PAY_PDB_DIST=/path/to/dist before running Cargo. build.rs intentionally does not fetch the latest GitHub release so builds stay pinned, reproducible, and compatible with offline build systems.

pnpm install
pnpm dev        # starts Express (port 3000) + Vite (port 5173)

Open http://localhost:5173, then in another terminal:

# Trigger a 402 challenge
curl -i http://localhost:3000/mpp/quote/GOOG

# Or let the CLI handle the full payment flow
pay --sandbox curl http://localhost:3000/mpp/quote/GOOG

vercel          # preview
vercel --prod   # production

On cold start the serverless function bootstraps the fee payer with 100 SOL + 1000 USDC via Surfnet cheatcodes.

Set these in your Vercel project settings (all optional):

The embedded x402 facilitator is mounted at /facilitator/*.