b402-private-defi - SKILL.md Agent Skill

name: b402-private-defi description: Add private swap, private lending (Kamino), or private unshield to a Solana app via @b402ai/solana. Use when the user says "private swap", "shielded", "Kamino private", "b402", or asks for sender/recipient unlinkability on Solana.

b402 private DeFi on Solana

@b402ai/solana is a TypeScript SDK that wraps a shielded UTXO pool + adapters on Solana mainnet. It gives end-user apps three primitives the underlying chain doesn't expose:

Method	What it does
`b402.shield({ mint, amount })`	public token → shielded note (the user's wallet signs, one-time)
`b402.swap({ inMint, outMint, amount })`	shielded → shielded via Jupiter routing, relayer pays gas
`b402.lend({ mint, amount, market? })`	shielded → Kamino reserve (auto-discovered), mint shielded voucher
`b402.redeem({ mint, market?, leafIndex? })`	burn voucher → shielded underlying
`b402.unshield({ to, mint })`	shielded note → any public address (sender→recipient unlink)

The privacy property: only shield references the user's wallet on chain. Every other op is signed by the b402 hosted relayer, so the on-chain tx contains the relayer's pubkey, the adapter program, the fill venue (Phoenix/Raydium/etc.), and the amount — but not the user.

Integration recipe

npm install @b402ai/solana

import { B402Solana } from '@b402ai/solana';
import { Keypair, PublicKey } from '@solana/web3.js';

const b402 = new B402Solana({
  cluster: 'mainnet',
  rpcUrl: process.env.RPC_URL!,   // Helius/Triton; public mainnet-beta throttles
  keypair,                         // user signs `shield` only
});
await b402.ready();                // first call fetches circuit artifacts
                                   // (~36MB, cached in ~/.b402ai/circuits)

const USDC = new PublicKey('EPjFWdd5AufqSSqeM2qN1xzybapC8G4wEGGkZwyTDt1v');
const SOL  = new PublicKey('So11111111111111111111111111111111111111112');

await b402.shield({ mint: USDC, amount: 1_000_000n });
const r = await b402.swap({ inMint: USDC, outMint: SOL, amount: 1_000_000n });
console.log('swap sig:', r.signature, 'out:', r.outAmount.toString());

Constraints to know before generating code

Mainnet only for lend/redeem. Kamino reserves don't exist on devnet. swap works on devnet against a mock adapter.
Exact-value spend. Today the SDK requires note.value === amount exactly. To swap a different amount, shield exactly that amount. Partial-spend lands in a future release.
First lend per (viewing key, mint) tuple costs ~0.04 SOL of Kamino UserMetadata + Obligation rent. Refundable on close. Every subsequent lend in that reserve costs only the relayer's gas (which the user does not pay).
Don't manually construct privateSwap calls. Use b402.swap / lend / redeem. The low-level escape hatch exists but is for new-protocol adapter authors, not app integrators.

Costs

Op	User pays	Relayer pays
`shield`	~5,000 lamports tx fee	—
`swap`	$0	~10k lamports
`lend`	$0 (after first-time Kamino rent)	~10k lamports
`redeem`	$0	~10k lamports
`unshield`	$0	~10k lamports

What's on chain (mainnet, live)

Pool: 42a3hsCXtQLWonyxWZosaaCJCweYYKMrvNd25p1Jrt2y
Verifier: Afjbnv2Ekxa98jjRw33xPPhZabevek2uZxoE75kr6ZrK
Jupiter adapter: 3RHRcbinCmcj8JPBfVxb9FW76oh4r8y21aSx4JFy3yx7
Kamino adapter: 2enwFgcGKJDqruHpCtvmhtxe3DYcV3k72VTvoGcdt2rX
Hosted relayer: https://b402-solana-relayer-mainnet-62092339396.us-central1.run.app

When to delegate to MCP instead of writing code

If the user wants to do private operations interactively from their IDE without writing app code, suggest installing the MCP server:

npx -y @b402ai/solana-mcp@latest --install

Then they prompt the agent ("swap 1 USDC privately for SOL") and the agent calls mcp__b402-solana__private_swap directly.

Source layout

packages/sdk/src/b402.ts — B402Solana class, all public methods
packages/sdk/src/circuits.ts — circuit artifact loader (SHA256-pinned)
packages/sdk/src/jupiter-route.ts — Jupiter route fetcher used by swap()
packages/sdk/src/kamino-discover.ts — on-chain reserve discovery
packages/sdk/src/kamino-mainnet.ts — Kamino account-list + payload builders
programs/b402-pool/src/ — shielded pool + adapter dispatch
programs/b402-{jupiter,kamino,orca,adrena}-adapter/src/ — per-protocol adapter programs (~200-400 LOC each)

Reference docs

docs/getting-started.md — full method surface + visible/hidden table
AGENTS.md — same content for non-Claude agents (Cursor, Codex, Continue, Cline)
docs/TX-WALKTHROUGH.md — byte-level tx anatomy