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

Annotations already declare readOnlyHint=true and destructiveHint=false, but the description adds real context the annotations don't carry: pricing per call ($0.01 x402 USDC or card key, first 25 free), that failed calls are free, that malformed addresses return 400 uncharged, and that repeats are billed. The 'nothing is changed' statement is redundant with readOnlyHint but harmless. It stops short of describing response shape or rate limits beyond the per-call charge.

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-loads what the tool returns and where, then usage routing, then operational facts. Dense but each sentence earns its place; the pricing/payment clause is the one segment that could be trimmed for a reader who already knows the platform's billing model.

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?

Single-parameter read tool with no output schema: the description covers what is returned, in what units, on which chain, at which block, how to choose it over siblings, and how errors and billing behave. There is nothing an agent needs in order to invoke it correctly that 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% and the single parameter is already documented, so the baseline is 3. The description adds an inline example address and motivates the format ('0x wallet address', malformed rejected with 400), which gives the agent slightly more than 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?

States the specific resource (native ETH balance of a 0x address on Optimism at the latest block), the underlying method (eth_getBalance), and the returned units (wei and ETH). It explicitly distinguishes itself from op_token_balance and wallet_portfolio, so an agent can route correctly without opening the sibling schemas.

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 when-to-use condition (checking whether a wallet can pay gas or a transfer) and names two alternatives with the conditions that select them: op_token_balance for ERC-20s and wallet_portfolio for all holdings. Nothing about tool selection is left to inference.

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