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

Beyond the readOnly/idempotent/openWorld annotations, it discloses the pending-block contract ('answers found:false with the reason rather than an error'), that output is raw and uninterpreted, the pricing model ($0.002 per call over x402 on Base), the prepaid x-credit-token auth header, the 1 free call/day allowance, and that no API key is needed. That is a dense, non-obvious behavioral payload the annotations cannot convey.

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?

Purpose is front-loaded in the first clause, and the pricing/auth tail is genuinely useful rather than filler. It is slightly redundant at the head ('Any Base block, by number, hash or tag — One block on Base') and the em-dash-chained sentences make it denser than it needs to be, but nothing is wasted.

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 an output schema present, the description is not obligated to enumerate return fields, yet it does so anyway along with the pending-block edge case, the unit details (age in seconds, base fee in gwei), and the full access/payment model. An agent has everything needed to call it correctly and to understand 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 schema already says 'Block number, hash, or tag (default latest)', so the baseline is 3. The description adds real value on top: it enumerates the exact accepted tags (latest / finalized / safe / earliest) and pins the hash format to 0x…, which the schema does not spell out.

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?

It names a concrete resource (one Base block) and enumerates exactly what the result contains — height, hash, parent, timestamp/age, tx count, gas used vs limit, base fee — which is far more specific than the bare name. The phrase 'Raw chain data, no interpretation' cleanly separates it from the interpretive ai_* and explain-style siblings, and the single-block scope separates it from base_tx/base_receipt.

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 states the accepted inputs and the latest/finalized/safe/earliest tags and the quirky 'not mined yet' case, which is useful operating context. But it never states when to reach for this tool versus base_tx, base_receipt, or base_nonce, so the choice between siblings is left to inference.

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