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

Annotations already carry readOnlyHint, idempotentHint, and destructiveHint, so the safety profile is covered. The description adds genuinely new behavioral context beyond annotations: the $0.001 USDC per-call cost, the binary-search algorithm, the at-or-after boundary semantics, and the byte-identical REST endpoint equivalence.

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?

Three sentences, zero filler: algorithm/purpose, cost, and chains + endpoint identity. The core semantics are front-loaded and every sentence carries operational value for the agent.

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?

For a 2-param read-only tool with a full output schema and safety annotations, everything an agent needs is present: purpose, semantics, cost, supported chains, and endpoint reference. Return-value explanation is unnecessary given the output schema, and openWorldHint covers auth expectations.

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% — both chain (with the full chain list) and timestamp (unix seconds) are already documented in the schema. The description's example URL (chain=base&timestamp=1758000000) adds a concrete worked instance, but it mostly confirms rather than extends the schema, so the baseline 3 is appropriate.

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 ('Binary-searches the header timestamps for the first block at or after a unix time') with precise matching semantics. This clearly differentiates it from siblings like chain_block_by_hash (lookup by hash), chain_block_number (latest block), and chain_block (by number).

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 a concrete when-to-use: "every 'what was the supply / who held what on that date' question starts here," plus a differentiator ('no public node offers the lookup'). However, it never names sibling alternatives or states when not to use it, so the exclusion guidance is implied rather than explicit.

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