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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds valuable behavioral context: it returns decoded/readable numbers rather than raw hex, includes a specific cost per call ($0.001 USDC), and states it is byte-identical to a REST endpoint. It does not mention pagination or error behavior, but for a single-block read tool this is sufficient.

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?

The description is compact and front-loaded: it opens with the output fields, then cost, then chains, then the REST equivalence. Every sentence adds information. No filler or repetition of the schema.

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?

Given the tool's simplicity (2 params, 1 required, no nested objects) and the presence of an output schema, the description covers the essential context: what the output looks like, cost, supported chains, and the block parameter semantics. It does not explicitly state that the output schema describes the return shape, but the output schema exists and the description's field list aligns with it. A minor gap is not naming sibling tools for alternative lookup methods, but the 'by number or tag' phrasing is enough.

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%, so the schema already documents both parameters (chain and block) with their allowed values. The description adds the default for block ('latest') and the list of supported chains, but these are also present in the schema. The description does not add significant new meaning beyond the schema, so baseline 3 is appropriate.

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?

The description states a specific verb ('decoded'), a resource ('block header'), and the exact output fields (height, time, author, gas used vs limit, base fee in gwei, transaction count). It also distinguishes itself from sibling tools like chain_block_by_hash and chain_block_by_timestamp by clarifying it fetches by number or tag. The title 'Block by number or tag' reinforces the scope.

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?

The description clearly states the supported chains and the block parameter options (number or latest|safe|finalized|earliest|pending), which tells an agent when to use this tool. It does not explicitly name alternatives like chain_block_by_hash or chain_block_by_timestamp, but the 'by number or tag' phrasing implies the distinction. It also mentions the cost and the byte-identical REST endpoint, which helps an agent decide if this is the right call.

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