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

Annotations only declare readOnly/idempotent/openWorld/non-destructive; the description goes well beyond that, disclosing provenance and methodology limits (DefiLlama TVL/stablecoin supply vs growthepie transactions/active addresses, 'never added together'), staleness fields (stale_after, changed_since), null-with-reason behavior, the reading rules of the mcp-facts/1 shape, the untrusted_text safety contract, and 'Free, no key.' This is exactly the extra behavioral context annotations 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?

Front-loaded with USE WHEN, then NOT FOR, then RETURNS and READING RULES — every block is load-bearing and skimmable. The READING RULES paragraph is somewhat long for a definition body, but it supplies interpretive context (fact shape, untrusted_text, changed_since) that is not available elsewhere, so the length is largely earned.

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?

No output schema exists, yet the description specifies the full return envelope (chain, as_of, stale_after, changed_since, seven named sections, not_covered, next, meta) and what null means. For a 'everything about a chain in one call' tool with one required param, an agent has everything needed to call and interpret 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 coverage is 100%, so both parameters are already documented, including the chain id/CAIP-2/name forms and the concise/detailed enum semantics. The description's 'Example: { chain: "base" }' and the section list add marginal confirmation but no syntax or format meaning beyond the schema. Baseline 3 applies when the schema does the heavy lifting.

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 ('get its dated facts in one call') and enumerates the exact fact sections returned (registry, USDC, CCTP domain, ERC-8004, RPC state-override support, x402 facilitators, activity). It explicitly separates itself from the nearest siblings by name, so an agent can distinguish it from get_contract_facts, get_x402_facilitators and list_chains without opening any schema.

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?

The 'USE WHEN you are about to build for or deploy to a chain' trigger is concrete, and the 'NOT FOR ...' clause names four alternatives (get_contract_facts, get_x402_facilitators, get_agent_economy, list_chains) with the condition that selects each. This is the explicit when/when-not/alternatives pattern.

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