Tools · L3 — ecash & scaling layers

Cashu

Chaumian bearer ecash on Bitcoin — instant, private, lightweight tokens an agent can hold and spend without channels or accounts.

What it is

Cashu is a free, open protocol for Chaumian ecash backed by Bitcoin and Lightning. A mint issues blind-signed bearer tokens against a Lightning deposit; because the signatures are blinded, the mint cannot link issuance to redemption, so payments are private by design. Tokens are bearer instruments — possession is title, and value transfers simply by handing over the token, with no on-chain or routing-layer footprint per transfer.

For agents, Cashu is the lightest-weight layer in the stack: no channel management, instant transfer, and a token small enough to pass around like a string. The reference implementation is Nutshell (mint + wallet); the protocol itself is defined by the NUTs specifications.

When to use it

• Agent payments that need privacy or must be offline-capable.
• Lightweight wallets where running or managing Lightning channels at the agent layer is overkill.
• Bearer-token API access patterns (see Routstr, where the token is the API key).
• Paying for a priced MCP tool call — Cashu is a first-class rail in ContextVM’s CEP-8 (the bitcoin-cashu method), and its bearer model lets an agent attach the token directly to the request, with change for any overpayment returned in the response.

Dependencies

A Cashu wallet, a mint to issue and redeem tokens (whose solvency you trust), and a Lightning connection to fund the mint and cash out. Diversify across independent mints to spread the custodial trust.

Quick start

Run or connect to a mint with the reference implementation, Nutshell (github.com/cashubtc/nutshell), or build on a client SDK such as CDK (Rust, github.com/cashubtc/cdk). The NUTs spec at cashubtc.github.io/nuts defines token format and mint operations. Check the repo’s releases page for the current version before deploying.

Gotchas

• Mints are custodial trust points — a malicious or insolvent mint can lose the ecash it issued. Diversify across independent mints rather than concentrating balance in one. For how to judge a specific mint (solvency proofs, ratings/vouches, uptime boards), see Evaluating an ecash mint.
• Bearer model: losing the token means losing the funds, with no chargeback or recovery without the mint’s cooperation.
• Mint upgrades can involve database migrations — back up before upgrading, per release notes.