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

Adds real behavior beyond the annotations: cost of $0.01/call via x402 or card key, 25 free calls, failed calls uncharged, 404 for future blocks, and that repeat calls are charged (explaining the non-idempotent hint). It does not cover rate limits or response shape details, but the annotation set already carries the safety profile.

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 it returns, then routing guidance, then operational caveats; the example is one token and useful. The pricing clause is dense (x402 USDC, card key, /credits, free tier, failed-free) but each piece affects whether an agent should call it, so little is wasted.

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 tool with no output schema, the description supplies the return-field list, default behavior, error semantics, cost model, and sibling routing. Nothing an agent needs in order to call it 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 schema already documents 'number' plus the 'latest' default, so the baseline is 3. The description adds a real constraint the schema omits: the value must be a decimal block number, and it supplies a concrete example ('number=latest').

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 (fetch) and resource (Ethereum block, latest or by number) and enumerates the returned fields: hash, timestamp, tx count, gas used/limit, base fee, producer. The word 'Ethereum' plus the eth_ naming separates it from the base_block/op_block/polygon_block/arb_block siblings, and eth_gas is explicitly carved out.

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 positive triggers ('timestamps and confirmation counts') and an explicit alternative with its condition ('for current fees use eth_gas'), which is exactly the when-to-use/when-not guidance needed among 30+ eth_* siblings. The 404 behavior for a not-yet-existing block further defines valid usage.

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