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

Annotations already cover read-only/idempotent/open-world safety, and the description layers on genuinely additive behavior: $0.001 per-call pricing, validation refusing bad input before any charge, the exact endpoint (GET /evm/block), and the auth header format with a key-purchase URL. This is well beyond what structured fields disclose.

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 the use case in the opening clause, then moves to pricing, failure behavior, and auth. The final sentences are dense with nested parentheticals (URL, price, header, purchase link), which slightly hurts scannability but every element is load-bearing.

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 compensates by enumerating the returned header fields and noting the response schema plus example are included. Pricing, validation-before-charge, and both authentication paths are all disclosed, so an agent can call this correctly on the first try.

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 both params are self-documented (chain enum with default base, number accepting decimal/0x hex or latest). The description restates chain names and 'latest or a given number' but adds no new syntax or edge-case meaning, 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 with scope: a block header across five named chains, latest or a given number, plus the fields returned (hash, parent, timestamp, miner, gas used/limit). This cleanly separates it from siblings like evm_transaction, evm_receipt, and evm_code without opening their schemas.

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?

"Use when you need a block header" gives a clear selection trigger, and the description explains the two invocation modes (raw x402 request vs. P0 access key making the paid call itself). It never names a sibling alternative or states an exclusion, so it stops short of a 5.

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