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

The annotations already declare readOnly, openWorld, and idempotent hints, but the description goes further by exposing the paid x402 flow: no payment returns the 402, invalid addresses return 400 without charge, and RPC errors return 5xx without charge. This is exactly the kind of behavioral detail that annotations cannot replace.

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 compact and information-dense: it covers payment, resource, network, units, challenge handling, explicit failure modes, and charge guarantees in under three sentences. No sentence is wasted.

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?

Given that an output schema exists and the input schema is highly descriptive, the tool description covers all essential non-schema context: payment requirements, the exact two-step x402 flow, error behavior, and what is and is not charged. An agent has enough information to call the tool correctly.

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?

The input schema already covers both parameters thoroughly, including the Base64 payload format and the two-call protocol. The description adds useful semantic context by linking payment to the x402 challenge and by noting the npm package's automatic-pay behavior, which helps an agent understand the optional payment parameter beyond the schema alone.

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 clearly states the operation: it reads the USDC balance of any Base address. It adds specific scoping details such as 'on Base', 'settlement asset for x402', and 'atomic units and USDC', which makes the tool's purpose precise and easily distinguishable from siblings like eth_balance.

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 gives clear invocation context: omit payment to get the 402 challenge, send payment on the second call, and note that the fiatdock-mcp npm package handles payment automatically. It does not explicitly contrast this tool with sibling balance/token tools, so a small exclusionary step is missing.

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