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P21 is designed for agents that need an inspectable answer to a precise question, not a blanket assurance that another agent is safe.

A complete interaction

A requester supplies the instruction, policy version, operation identifier, and evidence requirements. A producer collects the permitted evidence and performs supported checks. A consumer verifies the resulting artifact, reproduces the relevant checks, and decides whether to accept the result.

Example: a service claims an instructed payout succeeded. A P21 payment profile would connect the authorized instruction to the observed transaction, then check the chain, token, recipient, amount, execution status, and required finality. Paying the service's x402 fee is not proof that the requested payout succeeded.

Second example: a marketplace commits a job set before requesting a future selection source. A P21 sampling profile would make selection reproducible. It would not, by itself, establish that the marketplace included every eligible job.

Reading order

Read Quickstart to understand what exists today. Read Vision for the broader product thesis. Read Status and roadmap before treating any interface as available. The Protocol section defines the trust boundaries that every capability must preserve.

A useful first integration has one operator, one supported operation, a small set of fixtures, and a counterparty that can independently consume the result. Start in shadow mode: findings inform people and existing systems without moving customer funds.