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

Annotations already declare readOnlyHint=true, destructiveHint=false, openWorldHint=true, idempotentHint=false, so the safety profile is covered. The description adds genuinely new context: a non-existent block returns 404 uncharged, failed calls are free, and repeat calls are billed — matching idempotentHint=false. It slightly re-states 'reads or computes only and changes nothing', which annotations already convey.

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 return fields, then routing guidance, then error/billing notes. It is dense but slightly overloaded by the pricing sentence ('$0.01/call via x402 USDC or a card key (/credits), first 25 free'), which is useful but verbose for a 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?

For a one-parameter, read-only block lookup with no output schema, the description covers purpose, return fields, error behavior, cost model, and sibling routing — everything an agent needs to call it 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%, so the schema documents the single param, but the description still adds value by specifying 'decimal number' format and an example ('number=latest') plus the implicit default behavior.

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+resource ('Latest block number and block details on Arbitrum One, or any block by decimal number') and enumerates the returned fields. It is clearly distinguishable from the chain-specific siblings (eth_block, base_block, op_block, polygon_block) and from fee tools.

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 explicit when-to-use ('timestamps and confirmation counts') and an explicit alternative for the adjacent need ('for current fees use arb_gas'). It also supplies a usage example ('number=latest'), leaving little inference.

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