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

Beyond the readOnlyHint, openWorldHint, and idempotentHint annotations, the description adds valuable behavioral context: 'The server holds NO long-lived keys — every payment is signed per-request.' It also discloses the header-based auto-resolution behavior. This enriches the agent's understanding of the tool's security model and invocation behavior.

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 description is three sentences, each earning its place: the first states the core purpose, the second explains address resolution via header or parameters, and the third provides a security assurance. No fluff or redundancy, and the main action is front-loaded in the first sentence.

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 simple read-only balance-checking tool with optional parameters, the description is complete. It covers authentication methods, the scope of addresses that can be checked, and a security note. There is no output schema, but the return value is self-evident for a balance check. The annotations already declare safety properties, and the description fills in the remaining context.

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 schema already documents both parameters. The description adds context by connecting them to the header logic ('Otherwise pass `address` or `agentPrivateKey`'), but it doesn't provide new format or syntax details beyond the schema. This matches the baseline of 3 for high schema coverage.

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 specific verb+resource: 'Check the USDC balance for your agent wallet (or any address).' This clearly distinguishes the tool from sibling tools like check_paywall, get_checkout_status, and verify_receipt, which focus on payments, checkouts, and receipts rather than wallet balances.

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 description provides explicit usage guidance on how to specify the address: if `X-Coal-Agent-Key` is set in the header, it auto-resolves; otherwise pass `address` or `agentPrivateKey`. While it does not explicitly name alternatives or exclusions, it clearly explains prerequisites and the two valid invocation patterns, which is more than most tool descriptions offer.

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