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

Annotations already cover read-only, non-destructive, open-world, and non-idempotent. The description goes beyond them by explaining error handling (malformed address → 400, uncharged), the billing model ($0.01/call, first 25 free, failed calls free), and why repeats cost money despite identical results — context that explains the idempotentHint=false flag. Return shape is stated in units rather than format, which is the only thing left thin.

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?

Purpose, semantics, alternatives, error behavior, and pricing are front-loaded in three dense sentences with no filler. The pricing/payment clause is the longest single stretch and could be trimmed, but every element is operationally useful to a caller.

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 one parameter, full schema coverage, and no output schema, the description must supply the return semantics and call cost — it does both (wei and ETH, $0.01/call, first 25 free). Nothing an agent needs in order to call 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?

There is one parameter at 100% schema coverage, so the schema already documents it as a '0x wallet or contract address'. The description adds an explicit accepted example and the failure mode for malformed input (400), which is real added value, but it also narrows the wording to 'wallet address' while the schema permits contracts — a small mismatch rather than added clarity.

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 names a specific verb+resource (native ETH balance via eth_getBalance), the exact chain (Unichain), the block scope (latest), the address format (0x), and the units returned (wei and ETH). It explicitly distinguishes itself from unichain_token_balance and wallet_portfolio, so an agent can separate it from the large sibling set 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?

It states the use case directly (check whether a wallet can pay gas or a transfer) and names the two alternatives with the conditions that select them: unichain_token_balance for ERC-20 tokens, wallet_portfolio for all holdings at once. Both when-to-use and when-to-use-something-else are explicit.

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