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

The annotations are counterintuitive (readOnlyHint=false for what is otherwise a pure read), and the description proactively explains the reason: each call is charged. It additionally discloses billing tiers (first 25 free, $0.003/call via x402 USDC or card key, failed calls free), which annotations cannot express. No contradiction with annotations.

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 usage, error handling, example, and pricing—each sentence carries selection-relevant information. The pricing clause is dense boilerplate but is decision-useful since cost affects tool choice.

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?

With no output schema, the description still covers the return form (wei and ETH), input validation behavior, cost, and sibling alternatives. Nothing an agent needs to invoke this correctly is missing.

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. The description adds value beyond the schema by giving a concrete example address, stating 'any 0x address', and specifying that malformed addresses are rejected with 400 uncharged—relevant semantics not in the schema description.

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 precise verb+resource+scope: read the native ETH balance of a 0x address at the latest block, returned in wei and ETH. It explicitly differentiates from siblings (eth_token_balance for ERC-20, wallet_portfolio for all holdings), so an agent can select it without opening a schema.

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?

Names the concrete use case (checking whether a wallet can pay gas or a transfer) and routes to explicit alternatives for adjacent cases. It also states failure behavior (malformed address rejected with 400, uncharged), removing ambiguity about the invalid-input path.

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