explain-eutxo

name	explain-eutxo
description	Explain Cardano's extended UTxO model to developers. Trigger phrases: "explain datum", "what is a redeemer", "eUTxO vs account model", "how do validators work", "what is a script context", "UTxO model", "how does Cardano differ from Ethereum".
allowed-tools	Read Grep Glob

Explain eUTxO

Help developers understand Cardano's extended UTxO (eUTxO) model, especially those transitioning from account-based chains like Ethereum or from web2 backgrounds.

When to use

• A developer asks what datums, redeemers, or script contexts are
• Someone coming from Ethereum or Solidity wants to understand the Cardano model
• A web2 developer is encountering UTxO concepts for the first time
• Questions about how validators differ from smart contracts on other chains
• Conceptual questions about reference inputs, reference scripts, collateral, or minting policies
• The developer is confused about concurrency, state, or how to "store data on-chain"

When NOT to use

• The developer needs to write a validator or build a transaction — use write-validator or build-transaction instead
• They are asking about a specific CIP standard — use explain-cip
• They need to deploy or test a contract — use setup-devnet or build-transaction
• They need wallet integration — use connect-wallet

Key Principles

1. Think in UTxOs, not accounts

There is no global mutable state. The blockchain is a set of unspent transaction outputs (UTxOs). Each UTxO is an independent, immutable unit containing:

• An address (who can spend it)
• A value (ADA and/or native tokens)
• An optional datum (application data attached to the UTxO)

A transaction consumes UTxOs as inputs and produces new UTxOs as outputs. State changes happen by consuming old UTxOs and creating new ones with updated datums.

2. Validators verify, they do not compute

Cardano validators are predicates: they return true or false. They do not modify state or call other contracts. A validator receives three arguments and decides whether the transaction is allowed:

• Datum — data sitting on the UTxO being spent (the current state)
• Redeemer — data provided by the transaction builder (the action/intent)
• Script Context — the full transaction being validated (inputs, outputs, signatories, mint, validity range, etc.)

The off-chain code (transaction builder) does all the computation: selecting UTxOs, constructing outputs, calculating values. The validator merely checks that the result is valid.

3. The datum/redeemer/script-context triad

Every validator invocation receives exactly these three pieces of data:

Component	Provided by	Purpose
Datum	Attached to the UTxO at the script address	Represents current state
Redeemer	Supplied by the transaction spending the UTxO	Represents the action or intent
Script Context	Constructed by the ledger from the transaction	Full transaction view for validation

4. Deterministic evaluation

Transactions are validated locally before submission. If validation passes locally, it will pass on-chain (assuming the UTxOs have not been spent by another transaction). This means:

• No failed transactions consuming fees on-chain
• Exact fee calculation before submission
• Predictable execution costs via ExUnits (CPU and memory budgets)

Ethereum to Cardano Concept Mapping

When a developer comes from Ethereum, use these mappings to bridge understanding:

Ethereum Concept	Cardano Equivalent	Key Difference
Smart contract	Validator (spending, minting, or withdrawing)	Validators are stateless predicates, not persistent programs
Contract storage	Datum on UTxOs at the script address	No global storage; state is per-UTxO
Function call	Redeemer	The redeemer encodes which "action" is being taken
msg.sender	extra_signatories in script context	Must explicitly check that a required key signed the tx
msg.value	Value in the transaction inputs/outputs	Explicitly check value flow in the script context
ERC-20 token	Native token (minting policy)	Tokens are ledger-native, no contract needed to transfer
require()	expect / fail in Aiken	Pattern matching with expect is idiomatic
Contract-to-contract call	Multiple validators in one transaction	No re-entrancy; all validators run independently
block.timestamp	Validity range (valid_after, valid_before)	Tx specifies a time window; validator checks the range
Mapping(address => uint)	Multiple UTxOs with datums, or a datum containing a data structure	No native key-value store; design around UTxOs
Events / logs	Datum content or transaction metadata	No native event system; off-chain indexers watch the chain
Constructor	Parameterized validator	Validators can take compile-time parameters
Proxy / upgradeable contract	Reference scripts (CIP-33) + governance pattern	Validators are immutable; upgradeability needs explicit design

Workflow

Step 1: Identify the concept and the developer's background

Determine:

• What specific concept are they asking about?
• Are they coming from Ethereum/Solidity, another blockchain, or web2?
• Do they need a high-level overview or a detailed technical explanation?

Step 2: Search Bundled Documentation

Search the bundled documentation for relevant content:

• ${CLAUDE_SKILL_DIR}/../../docs/sources/plutus/ - Plutus docs
• ${CLAUDE_SKILL_DIR}/../../docs/sources/aiken/ - Aiken language docs
• ${CLAUDE_SKILL_DIR}/../../docs/sources/developer-portal/ - Cardano Developer Portal

Step 3: Explain using analogies from their background

For Ethereum developers, use the mapping table above. For web2 developers, use analogies:

• UTxO = a sealed envelope containing money and a note (datum). To use the money, you must open (consume) the envelope, take the contents, and create new envelopes.
• Validator = a lock on the envelope. It checks conditions before allowing you to open it.
• Redeemer = the key you present to the lock.
• Script Context = security camera footage of the entire operation, so the lock can verify what you are doing with the contents.

Step 4: Provide a practical scenario

Show when a developer encounters this concept. Examples:

• "You want to lock ADA so only a specific person can claim it" -> spending validator with datum containing the beneficiary's key hash
• "You want to create a token" -> minting policy that validates the conditions under which tokens can be minted/burned
• "You want to read on-chain data without spending it" -> reference inputs (CIP-31)

Step 5: Show a code example

Provide a minimal Aiken validator plus the off-chain interaction pattern:

// A simple vesting validator
validator vesting {
  spend(
    datum: Option<VestingDatum>,
    _redeemer: Data,
    _self: OutputReference,
    tx: Transaction,
  ) {
    expect Some(VestingDatum { beneficiary, deadline }) = datum

    // Check that the beneficiary signed the transaction
    let signed = list.has(tx.extra_signatories, beneficiary)

    // Check that the deadline has passed
    let deadline_passed = when tx.validity_range.lower_bound.bound_type is {
      Finite(lower) -> lower >= deadline
      _ -> False
    }

    signed && deadline_passed
  }
}

Off-chain pattern (conceptual):

1. Query UTxOs at the script address
2. Find the UTxO with the matching datum
3. Build a transaction that consumes the UTxO, provides a redeemer, includes the beneficiary's signature, and sets valid_after past the deadline
4. Sign and submit

Step 6: Highlight common mistakes

• Forgetting the datum on outputs: When sending to a script address, always attach a datum. UTxOs without datums at script addresses are unspendable.
• Not checking all conditions: Validators must check every invariant. If you check the signer but not the value, someone can drain the funds to a different address.
• Double satisfaction: When multiple script inputs exist in one transaction, each validator only sees that "its" UTxO is being spent. A malicious user can satisfy one validator's checks using another validator's UTxO. Always verify outputs specific to your script.
• Ignoring the change output: The eUTxO model requires explicit handling of change. The off-chain code must account for leftover ADA.
• Using datum as global state: Datum is per-UTxO. If you need shared state across multiple interactions, you need a design pattern (see reference docs).

Concepts Covered by This Skill

Concept	Quick Definition
Datum	Application data attached to a UTxO. Represents the "state" at a script address.
Redeemer	Data provided by the spender to indicate intent/action. The "key" to unlock.
Script Context	Full transaction data available to the validator during execution.
Validator	A predicate script that authorizes spending, minting, or withdrawing.
Reference Input	A UTxO included in the transaction for reading only (not consumed). CIP-31.
Reference Script	A script attached to a UTxO that other transactions can reference instead of including. CIP-33.
Collateral	A pure-ADA UTxO pledged to cover fees if script execution fails during phase-2 validation.
UTxO Selection	The off-chain process of choosing which UTxOs to use as transaction inputs.
Script Address	An address derived from a validator hash. UTxOs here are governed by the validator.
Stake Credential	A credential for staking/delegation, which can also be a script (enabling withdraw-zero).
Withdraw-Zero Pattern	A technique using a staking validator with a zero-ADA withdrawal to run validation logic once per transaction instead of once per input. Useful for batching.
Minting Policy	A validator that controls creation and destruction of native tokens.

References

• references/eutxo-vs-account.md — detailed comparison of eUTxO and account models
• Shared principles: ../shared/PRINCIPLES.md