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

Adds substantial behavior beyond annotations: the x402 proof must arrive only in the outer X-PAYMENT HTTP header (never as an argument), a missing proof returns the current 402 challenge, PayPal routes and the human window are web-only, and idempotency semantics are spelled out via request_id reuse. This is exactly the context annotations cannot express.

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?

Front-loaded with purpose, then parameter rules, then retry/payment mechanics, so the most important information lands first. It is dense and slightly repetitive with the schema (e.g. 'never a number or an amount'), which keeps it just short of a 5.

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 two-parameter mutation with no output schema, it covers everything an agent needs: payment header placement, the 402 challenge when proof is absent, retry/idempotency rules, the preflight dependency, and the web-only escape hatches.

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 coverage is 100%, so the baseline is 3, but the description adds real meaning: one dollar buys one credit with no rounding, and request_id must come from credit_preflight rather than be invented. The only minor gap is that it largely restates the schema's 1–10000 string range.

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?

States a specific verb and resource ('Purchase prepaid city fee credit') plus the exclusive channel ('through x402 only'), which separates it from siblings like credit_preflight, credit_gift, withdraw and payment_attempt. An agent knows immediately this is the paid credit-buying action.

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?

Gives explicit routing: obtain suggested_request_id from credit_preflight, retry with the identical request_id/amount after a timeout, never re-pay when a durable response or payment attempt exists, and call front_door when lost. Names the prerequisite tool, the alternative suspects, and the conditions that select them.

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