PSBT requirements

A PSBT is the exchange format between a service that builds a transaction and a wallet that signs it. For Atomicals, a PSBT that omits context is a PSBT that cannot be signed safely.

What every input must carry

Field	Why
The previous output script and value	So the wallet can verify what it is spending
Whether the input is coloured, and for which Atomical	So the wallet can warn before spending an asset
The signature hash flag to use	So the signer commits to the intended scope
Derivation information for the signing key	So the wallet can find the key
Tap internal key and merkle root for Taproot inputs	So key-path signing works

What every output must carry

Field	Why
Exact script	So the recipient cannot be redirected
Exact value in satoshis	So allocation is deterministic
The expected coloured assignment	So the signer can compare against their intent

Signature hash flags

A flag that does not commit to all outputs lets someone else change the outputs after you sign. For a swap, the seller’s signature must commit to the exact output that pays them.

A builder must state, per input, which flag it expects. A wallet must show which flag it is about to use. A signer must reject a flag they did not expect.

Ordering

Input order and output order are part of the meaning. A wallet or library that reorders inputs or outputs for canonicalisation changes the allocation result. Do not reorder an Atomicals PSBT.

Protected outputs

A wallet must refuse to select a coloured output for fees unless the user explicitly approved spending that asset. This is the single most valuable wallet feature for Atomicals. See wallet safety.

Validation before signing

Before presenting a PSBT for signature, a builder should:

1. Resolve every input’s coloured state at a stable chain position.
2. Compute the expected allocation for the exact output set.
3. Assert the burn total is zero, or make the intended burn explicit.
4. Assert no unexpected Atomicals operation is present.
5. Assert output totals do not exceed input totals per token.
6. Record the chain position, and re-validate if it moved.

The transaction inspector performs a read-only version of steps one to five locally, without uploading anything.