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

Annotations already declare readOnly/destructive/openWorld, and the description goes well beyond them: it explains the idempotentHint=false rationale ('each call is charged, so repeats are not free'), states pricing ($0.003/call via x402 USDC or card key, first 25 free, failed calls free) and confirms 'reads or computes only and changes nothing.' This is exactly the added context annotations cannot carry.

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 the core action and scope, then alternatives, then validation, then pricing. Every sentence is load-bearing, though the pricing/free-tier clause is dense enough that the description runs longer than strictly needed for tool selection.

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 compensates by naming the return units (wei and ETH) and the block height semantics (latest). Combined with error handling and cost, an agent has everything needed to call and interpret this 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?

Schema coverage is 100% and the single parameter is documented there, so the baseline is 3. The description adds real value on top: it specifies the accepted format ('0x address', contract addresses included), supplies a valid example address, and defines the invalid-input contract (400, uncharged).

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 (read), resource (native ETH balance), scope (any 0x address on Optimism at the latest block) and return units (wei and ETH). It explicitly names what it is not (op_token_balance for ERC-20, wallet_portfolio for all holdings), so an agent can distinguish it from the ~50 sibling chain/token tools 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?

Gives a concrete use case ('check whether a wallet can pay gas or a transfer') and routes to named alternatives for the two adjacent needs (ERC-20 balances, all holdings at once). Failure behavior is also scoped: malformed addresses are rejected with 400 and uncharged.

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