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

Annotations only declare the generic profile (readOnlyHint=false, openWorldHint=true, idempotentHint=false, destructiveHint=false); the description adds the material operational fact that each successful call costs $0.001 in USDC on Base via x402, plus the default-to-latest behavior. It does not explain what happens on failure or whether the paid call is charged only on success (it does say 'per successful call'), leaving a small gap.

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?

Three tight sentences in a deliberate order: what it returns, when to use it, what it costs. No filler, no repetition of the name, and the highest-value details (chain, returned fields, price) are front-loaded.

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 correctly compensates by listing the returned fields (hash, timestamp, tx count, gas used/limit, base fee, producer address), and it discloses the x402 payment requirement. Minor omissions remain (error/payment-failure behavior, whether results are cached), but nothing needed to invoke the tool 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 description coverage is 100% for the single 'number' parameter, and the schema already documents the 'latest' default. The description's 'by number, or the latest one' merely restates that, adding no new format or edge-case semantics, so the baseline 3 applies.

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 ('A block on Arbitrum One by number, or the latest one') and enumerates the returned fields, which distinguishes it from the many sibling *_block tools (base_block, eth_block, op_block) by chain. An agent can identify the tool and its output shape without opening any 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 clear context for use ('Use for timestamps, confirmations and chain-activity checks'), which is more than most siblings provide. It stops short of naming an alternative or an exclusion (e.g., when to prefer arb_tx or arb_receipt), so it is clear context without routing.

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