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

Annotations already declare readOnly/idempotent/non-destructive, but the description adds substantial extra context: latency (0.1-2 s), price (USD 0.05), free quota (5 lookups per IP per UTC day), 404 billing behavior, and an explicit prompt-injection warning that endpoint strings are untrusted data. These are traits annotations cannot express.

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 purpose, then routing, output shape, cost/quota, and a safety warning; each sentence carries distinct information. It is dense and the safety boilerplate is somewhat long, but nothing is redundant.

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?

An output schema exists so return values need not be spelled out, yet the description still sketches them (record plus dated presence rows and price-change points). Combined with annotations and explicit cost/quota/safety context, an agent has everything needed to 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 coverage is 100%, so the baseline is 3 and the schema already documents id, url, and source. The description adds meaning by stressing that the lookup targets ONE record, which tells the agent this is not a bulk operation and that url/id identify a single indexed entry.

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 (presence and price history of ONE indexed record) and immediately distinguishes itself from the sibling it is not by naming query_agent_index as the way to find records. An agent can tell this apart from the index-query and scan tools 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?

Explicitly routes the agent: 'To find records use query_agent_index', which implies this tool is used only after an id/url is known. It also covers the failure/edge case (404 for unindexed records, not charged) and the free-tier limit. No explicit when-not-to-use or prerequisite statement beyond the implicit routing.

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