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

Annotations already declare readOnlyHint=true and destructiveHint=false, and the description reinforces that with 'reads or computes only and changes nothing.' What it adds beyond annotations is cost behavior: $0.01/call, first 25 free, failed calls free, and the explicit note that repeats are not free — which explains the otherwise puzzling idempotentHint=false. It stops short of describing pagination or response shape, but for a single-block lookup that is minor.

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?

The first sentence front-loads purpose and return fields, following sentences handle alternatives, edge cases, and pricing. It is dense but nearly every clause earns its place; the only slight redundancy is restating the per-call charge twice ('$0.01/call' and 'each call is charged, so repeats are not free'), though the second instance does clarify non-idempotency.

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 by listing the block fields returned. Combined with the 404 behavior, example call, and cost model, an agent has everything needed to invoke this correctly and interpret the result.

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 is already documented, so the baseline is 3. The description still adds value by specifying 'decimal number' format and giving a concrete example (number=latest), which removes ambiguity about hex vs decimal input and confirms 'latest' as the default.

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 states a specific verb+resource (fetch block number or block details) scoped to Avalanche, and enumerates exactly what comes back (hash, timestamp, tx count, gas used/limit, base fee, producer). It is clearly separable from siblings like avax_gas and avax_tx 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?

It names the use cases (timestamps and confirmation counts) and explicitly routes the agent to avax_gas for current fees, which is the main adjacent-sibling ambiguity. It also states the edge case that a future block returns 404 and is uncharged, so the agent knows the failure mode in advance.

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