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

Annotations already cover readOnly/idempotent/openWorld, and the description adds substantial beyond-annotation behavior: pre-genesis requests return 422 before_genesis unchained, malformed node answers return an uncharged 5xx, latency is typically 1-3 s, and the `assumed_utc` flag is disclosed. This is exactly the extra context the structured fields cannot carry.

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?

Dense but front-loaded: inputs first, then return shape, then error semantics, then cost/limits. Every sentence carries operational weight with no filler or repetition of the schema.

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 present, the description compensates fully by naming the return fields (`block`, `next_block` {number, timestamp}, `is_latest`) and the edge cases. For a 2-parameter read tool, nothing an agent needs to invoke and interpret it is missing.

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%, so the baseline is 3, but the description adds meaning the schema does not: that a no-offset ISO timestamp is treated as UTC and surfaced via `assumed_utc`, and that timestamps accept integer or digit-string Unix seconds. This is genuine value over the two documented parameters.

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 ('find the block at a point in time') with an explicit chain scope (Ethereum or Base), which inherently separates it from the sibling get_block (block-by-number) without naming it. An agent can tell exactly what this tool resolves.

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 clear operational context: cost (USD 0.003), the shared free pool of 10 chain reads per IP per UTC day, and the conditions that produce errors. It does not explicitly route the agent to an alternative tool when a block number is already known, so it falls short of a full when/when-not statement.

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