Start with approved packets
The API should not ask the agent to invent legal language. It should ask for a template id, a recipient, known variables, optional metadata, and a webhook destination. The packet is already approved; the agent only supplies the operational context.
That single choice removes a large class of failures. If the agent cannot rewrite terms, the API does not need to guess whether a generated clause is acceptable.
Make the request inspectable
Every send command should have a dry-run equivalent. In an agent workflow, dry runs are not a developer convenience; they are the approval surface. The human operator needs to inspect exactly what will be sent before the agent takes the irreversible step.
The response should include the chosen template, resolved recipient, field list, metadata, and the endpoint that would receive the real request. If the dry run is hard to read, the human will skip it.
Return records, not vibes
Once the agreement exists, the API should return stable identifiers and status. Once it is signed, it should return the signed PDF, a SHA-256 hash, structured field values, and audit events.
Agents and backend jobs are good at continuing from structured state. They are bad at interpreting ambiguous UI states like, "it probably sent." Good contract APIs should never require that kind of guess.
Build in cancellation and key revocation
Agent workflows need an obvious stop button. If a key starts sending the wrong packet, revoking that key should stop future sends and make it easy to cancel in-flight agreements associated with that key.
Rate limits, ownership metadata, and audit events are not polish. They are the controls that make it reasonable to let an agent operate in a workflow that touches legal documents.