Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With readOnlyHint=false and openWorldHint=true already declared, the description adds genuinely new behavioral context: pricing (0.10 USDC/call on Base, 10 free signed-wallet calls per rolling 30 days), the instruction to read the backend 402 body for x402 terms, and required preconditions (product credentials, signed mandates, single-use action grants). It also explicitly disclaims authority and server credential supply, which is important for a POST-based tool that is non-idempotent. It does not describe the response payload or failure behavior in any detail.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The pricing/x402 sentence is front-loaded ahead of the core statement of what the tool does, delaying purpose behind billing metadata. The no-authority, bounded-computation point is stated twice ('grants no authority and never supplies server credentials' and 'a bounded computation on caller-supplied data'), which is redundant padding in an otherwise compact block.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a six-required-parameter, paid, non-idempotent POST with no output schema and zero schema coverage, the description covers cost and credential preconditions well but leaves the return shape ('itemized total') and the meaning of half the parameters unexplained. An agent knows the commercial terms and the operating boundary but still has to guess at several field values.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 0% across six required parameters, so the description must carry the burden and largely does not. It gestures at 'items' and 'USD estimates' and vaguely at references via 'signed mandates, and single-use action grants,' but never explains agent_id, mode='prepare_checkout', or the distinction between payment_reference and allowance_reference, all of which are opaque identifiers an agent must populate correctly.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a concrete action: 'Itemize caller-supplied USD estimates and calculate a total for supported computations' against POST /api/unified-agent-checkout. It also pre-empts the misleading name by clarifying this 'grants no authority' and is 'a bounded computation on caller-supplied data' rather than an actual checkout, which distinguishes it from purchase-execute/payout-execute siblings. It stops short of naming a specific alternative tool.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The 'Operating boundary' line tells the agent exactly what to supply (computation items and USD estimates) and what to read (the itemized total), and implies sequencing by noting a human compares items 'before requesting a provider checkout.' However, no sibling tool is named as the alternative for the actual checkout step, so routing among the 50+ siblings is only implied.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.