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

Annotations already declare readOnlyHint=true, destructiveHint=false and idempotentHint=false, and the description reinforces that it changes nothing. It adds genuinely new context beyond annotations: the $0.01/call x402 pricing, 25 free calls, failed calls free, and that paid repeats are not free — which explains the non-idempotent hint. Minor repetition of the read-only claim keeps it from 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?

Information is front-loaded: what it returns, then when to use it, then the alternative, then pricing. The pricing sentence is long and partially restates the 'changes nothing' claim already covered by annotations, so it is efficient but not maximally tight.

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 return-value burden and does so: per-block base fee, block fullness, percentile priority fees, plus window-level low/average/high base fee. Cost and failure behavior are also disclosed, so an agent has everything needed to call it correctly and interpret results.

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 the single parameter already documents 'How many recent blocks, 5-100 (default 20)'; the description restates that range and default. It does clarify what the window length governs (which block range the fee percentile summary covers), but adds little syntax or semantics beyond the schema. 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?

States a specific verb and resource (gas price history on Polygon PoS via eth_feeHistory) and enumerates the returned measures: base fee in gwei, block fullness, 10th/50th/90th percentile priority fees. It explicitly names the sibling it is not (polygon_gas) for current-price-only queries, so an agent can route without opening either 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 concrete use cases (judge whether fees are rising, pick a priority fee, time a transaction) and an explicit exclusion routed to polygon_gas. A usage example (blocks=20) is included, leaving nothing to inference.

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