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

Annotations already cover read-only and destructive behavior, and the description adds genuinely new operational context: $0.01/call via x402 USDC or card key, first 25 free, failed calls free, and that repeats are not free — which explains the non-obvious idempotentHint=false. It also reinforces that the call reads/computes only and changes nothing.

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-loaded with the resource and return shape, then usage, then example, then cost. The pricing/billing sentence is slightly dense but carries real decision value; nothing is gratuitous 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?

There is no output schema, so the description must describe the return shape, and it does so field by field (base fee, fullness, percentiles, aggregated low/avg/high). Combined with the cost model and sibling routing, an agent has everything needed to call it correctly.

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 parameter's range and default (5-100, default 20) are already documented in the schema, so the description's restatement adds no new constraint. The description does clarify what the window is used for and how it maps to returned statistics, but the parameter meaning itself is fully covered by structured data.

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 Arbitrum One via eth_feeHistory — and enumerates the returned fields (base fee, block fullness, 10/50/90th percentile priority fees, low/avg/high over the window). It explicitly contrasts with the sibling arb_gas for the current price alone, so an agent can pick it without opening the 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?

Names concrete decision contexts (judge whether fees are rising, pick a priority fee, time a transaction) and gives the explicit alternative for the other case: 'for the current price alone use arb_gas'. That is an unambiguous when-to-use / when-not-to-use routing rule.

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