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

Annotations already declare readOnly/openWorld/non-idempotent/non-destructive, and the description adds genuinely new behavior: 404 on future blocks with no charge, per-call pricing ($0.002 via x402 USDC or card key), 25 free calls, failed calls free, and the warning that repeats are not free (reinforcing idempotentHint=false). The only redundancy is restating that it 'changes nothing', which mildly duplicates readOnlyHint but does not contradict it.

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 resource and return fields, then usage routing, then edge cases and pricing — a logical order with no filler sentences. The pricing sentence is long and could be trimmed, but it carries decision-relevant cost information rather than padding.

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 carries the burden of describing returns and does so field-by-field (hash, timestamp, tx count, gas, base fee, producer). Combined with error behavior, cost model, and default handling for a single optional parameter, nothing an agent needs to call this correctly 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 sole parameter is already documented as 'Block number, or latest (default)', so the baseline is 3. The description adds real meaning beyond that by specifying decimal (not hex) numbering and giving the example number=latest, which prevents the common hex-block-tag mistake.

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 Ethereum') and enumerates exactly what comes back: hash, timestamp, transaction count, gas used/limit, base fee in gwei, producer. It also distinguishes itself from the sibling eth_gas by naming the fee use case, so an agent can separate eth_block from eth_gas and from the many other eth_* 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?

Explicitly says what it is for ('timestamps and confirmation counts') and routes the alternative ('for current fees use eth_gas'), which is the most likely confusion in this large sibling set. It adds an edge-case rule ('A block that does not exist yet returns 404, uncharged') and a concrete invocation example, leaving little to inference.

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