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

Annotations already declare readOnlyHint=true, so the safety profile is covered. Beyond that, the description discloses a non-obvious behavior the schema and annotations do not: the two-step x402 payment handshake (unpaid call returns terms, signed retry executes) and that the block is returned 'exactly as the node gives it'. That is meaningful operational context.

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?

Purpose is front-loaded and the payment steps are compact, but the trailing 'Measured output' block dumps a full raw block object with dozens of hash fields. While it illustrates the return shape in the absence of an output schema, it is far larger than needed and hurts overall conciseness.

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 with no output schema, the description covers purpose, chain scope, the x402 payment protocol, and an example return payload, which is close to complete. It only lacks explicit sibling-selection guidance, which is minor given the clearly namespaced method name.

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% and the single `block` parameter is already documented as 'a block number, or latest'. The description merely restates that same semantics, adding no format detail (e.g., hex vs decimal) beyond the schema, 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, and chain scope: 'Ethereum mainnet on-chain read: a block by number or latest', reinforced by the underlying method name eth_getBlockByNumber. Naming the chain (Ethereum mainnet) plus the method lets an agent separate it from the Base/Gnosis and eth_reads_v1 sibling variants 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?

It gives clear invocation guidance for the x402 flow: 'Call once with no payment. The result is the payment terms. Retry the same call with the signed payment at _meta["x402/payment"].' That is useful procedural context, but there is no guidance on when to choose this tool over sibling reads (e.g., Base getBlockByNumber, or eth_blockNumber), so selection guidance is only implied.

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