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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=true, and the description reinforces 'reads or computes only and changes nothing' rather than adding to it. However, it does add genuinely new behavioral facts the annotations cannot carry: a nonexistent block returns 404 uncharged, and failed calls are free, which matters for retry strategy. The pricing detail ('$0.002/call', 'first 25 free') explains why idempotentHint=false ('repeats are not free'), making the non-idempotent hint concrete. With annotations covering the safety profile, this lands at a solid mid-range: useful additions but no return-shape or pagination detail.

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 and returned fields, then usage routing, then error/pricing edge cases in decreasing priority – a good ordering. It is slightly long: 'each call is charged, so repeats are not free' restates 'Price $0.002/call' and the pricing block sits inside the same sentence cluster as the functional description.

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?

There is no output schema, so the description compensates by listing the returned fields (hash, timestamp, tx count, gas used/limit, base fee, producer) and covers the error path (404, uncharged) and cost model. What is missing is minor: no detail on units beyond gwei, and no note on how base fee behaves on a nonexistent future block beyond the 404.

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 as 'Block number, or latest (default)'. The description does add one thing the schema omits: the number must be decimal, and 'number=latest' confirms the accepted sentinel. That is a real but small increment over a fully documented schema.

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 and resource ('Latest block number and block details on Arbitrum One, or any block by decimal number') and enumerates the returned fields, so the agent knows exactly what it gets. It also names the chain and the sibling it is not (arb_gas), distinguishing it from both same-chain and cross-chain (eth_block, base_block) lookalikes.

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?

Explicitly routes the agent: 'Use for timestamps and confirmation counts; for current fees use arb_gas.' That is a named alternative plus the condition that selects it, so no inference is required.

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