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

Annotations already cover readOnlyHint=true and destructiveHint=false, and the description reinforces this with 'reads or computes only and changes nothing.' It adds non-obvious commercial context (per-call charge, 25 free calls, failed calls free, repeats billed again), which is genuinely useful and not contradicted by idempotentHint=false. However it does not describe return shape, error behavior, or pagination, so it is solid but not rich beyond the annotations.

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 core purpose and return contents, then alternatives, then example and pricing. It is a single dense paragraph that mixes tool semantics with payment marketing, but every sentence carries information an agent could act on.

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 one-parameter read tool with full annotation coverage, the description explains what the call returns in enough detail that no output schema is needed. Range, default, example, cost model, and the sibling alternative are all present.

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 description's 'last N blocks (5-100, default 20)' plus 'blocks=20' merely restate what the schema already documents. Baseline 3 applies since the schema carries parameter meaning and the description adds no format or edge-case detail beyond it.

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 zkSync Era via eth_feeHistory) and enumerates the exact returned quantities: base fee in gwei, block fullness, 10th/50th/90th percentile priority fees, and low/average/high base fee. It explicitly distinguishes itself from the sibling zksync_gas (current price only), so the agent can route 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 concrete decision contexts (judge whether fees are rising, pick a priority fee, time a transaction) and names the alternative tool plus the condition that selects it: for the current price alone use zksync_gas. This is explicit when-to-use and when-to-use-something-else guidance.

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