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

Goes far beyond the readOnly/idempotent/openWorld annotations: discloses the search strategy and cost ('batched k-ary search ... about 7 round trips'), a real limitation ('very old history may be unavailable on public nodes'), and the full x402 payment flow including the PaymentRequired retry via params._meta['x402/payment']. This is exactly the operational context an agent needs before invoking.

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 semantics and purpose before pricing/transport detail. It is dense but nearly every clause carries needed information; the chain enumeration duplicates the enum and is the one mildly wasteful element.

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 still describes the return ('with its timestamp and the next block's'), plus coverage limits and payment mechanics. Nothing an agent needs to call and interpret this correctly 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 50% with only two params; the description restates the time formats and chain list (overlapping the schema/enum) but also surfaces the implicit _meta['x402/payment'] parameter that is not in the schema at all, which is genuinely additive. The chain enum being self-documenting keeps the remaining gap minor.

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 precise verb+resource: 'the last block at or before a given time' across eight named chains. No sibling tool does this, and the description makes the semantics (last block AT OR BEFORE) unambiguous, which is the key distinction an agent needs.

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 the intended use cases explicitly: 'pin eth_call reads, audits and snapshots to a moment.' It does not name alternative tools or state when NOT to use it, but no sibling overlaps this capability, so the practical routing gap is small.

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