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

Annotations already declare readOnly/idempotent/openWorld/non-destructive, yet the description adds substantial behavior the annotations cannot convey: cost ($0.001 per call), the x402/MPP payment flow, the guarantee that bad input is refused before any charge, and the P0 bearer-key path that executes the call server-side. It omits rate limits and any error/payment-failure semantics, which keeps it at 4 rather than 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?

Front-loads purpose, then the return shape, then the payment/auth mechanics in a tight sequence with no filler. It is dense and long for a two-parameter tool – the buy-keys URL plus the P0 explanation could be compressed – but every sentence carries operational information.

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 paid, read-only data endpoint with no output schema, the definition supplies what an agent needs to call it correctly: chains, defaults, returned fields, price, auth path, and pre-charge validation. Remaining gaps are minor (payment failure/rejection behavior, latency or rate limits), so it is nearly but not fully complete.

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 enum, default, and number-format details are already fully documented in the schema. The description mirrors the chain list and the 'latest or a given number' choice but adds no syntax or edge-case meaning (e.g., hex formatting, reorg/orphan handling) beyond that. 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 and resource ('need a block header'), enumerates the five supported chains, and lists the returned header fields (hash, parent, timestamp, miner, gas used/limit). That makes it trivially separable from siblings like evm_transaction, evm_receipt, and evm_code without opening a 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?

Gives clear when-to-use context ('Use when you need a block header on Base, Ethereum, Arbitrum, Polygon or Optimism') and explains the latest-vs-number selector, plus the two invocation paths (paid URL vs P0 key). It never explicitly names a sibling alternative or an exclusion, so it stops short of the top band.

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