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

Annotations already declare readOnlyHint, idempotentHint, and openWorldHint=false, so safety and repeatability are covered. The description adds behavior the annotations do not: this is a paid, per-call metered endpoint with a concrete price in both sats and USDC, which materially affects how an agent should invoke it. It omits what happens on an unknown block_id, so it is not a 5.

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?

Two sentences with the returned-field inventory front-loaded and the commercial terms second. The long comma-separated field list is dense but each item is informative; nothing is redundant filler.

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 carries the burden of telling the agent what comes back, and it does so by enumerating the header fields. The payment flow is partly deferred to schema descriptions for l402 and pay_with, which is reasonable, though error/not-found behavior is never addressed.

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%, so the baseline is 3, but the description earns a bump by explaining the payment model (2 sats via Lightning or US$0.002 USDC via x402), which gives meaning to the pay_with enum and the l402 token parameter beyond their schema text. It adds nothing new about block_id itself, which the schema already fully documents.

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 names the resource (Bitcoin block header), the lookup key (by height or hash), and enumerates exactly which fields are returned, so an agent knows precisely what it gets. It does not explicitly distinguish itself from close siblings like bitcoin_block_stats or bitcoin_latest_block, which keeps it out of 5 territory.

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?

"Any Bitcoin block by height or hash" implicitly positions this as the arbitrary-block lookup versus bitcoin_latest_block, but no sibling is named and there is no explicit when-to-use/when-not-to-use guidance. The pricing note tells the agent about cost, which is useful selection context, but routing between block-related siblings is left to inference.

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