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

Adds substantial behavioral context beyond the annotations: non-existent blocks return 404, that call is uncharged, and each call costs $0.003 with the first 25 free. It even reconciles the readOnlyHint=false annotation by explaining the tool reads but is charged per call. Return shape is described, though pagination/limits are not relevant here.

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-loads purpose and returned fields before the usage routing and pricing details. Information-dense with little waste, though the pricing/example tail reads as a run-on that could be tightened.

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 compensates by listing all returned fields, and it covers error behavior (404), billing, and the read-only rationale. An agent has everything needed to select and invoke 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 description coverage is 100%, so the single parameter is already documented. The description adds the format hint ('decimal number') and an example (number=latest), which is marginally useful but mostly restates the schema's 'Block number, or latest (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?

States a specific verb (Fetch) and resource (block on Arbitrum One) and enumerates the returned fields (hash, timestamp, transaction count, gas used/limit, base fee, producer). It clearly distinguishes itself from same-chain siblings like arb_tx, arb_receipt, and arb_gas.

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?

Explicitly names when to use it (timestamps and confirmation counts) and points to the sibling alternative for a different need (for current fees use arb_gas). Both the condition and the alternative are stated, 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.