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

Annotations already declare readOnly/idempotent/non-destructive, yet the description still adds real value: it explains that fare_page_text is the agency's authoritative source for prices, discounts and transfer rules, and that the shorter hand-maintained fields are overridden by the fare page. That source-precedence detail materially affects how an agent should interpret and trust the result, though the overall return shape is only hinted at.

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?

The purpose and the source-precedence rule are front-loaded in the first two sentences, the parameter block is compact, and the grounding/unaffiliated disclaimer is a single closing line. Nothing is padded.

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 one-parameter, read-only, open-world lookup with no output schema, the description supplies purpose, parameter format, and result-trust guidance. It stops short of describing what the response actually contains beyond naming fare_page_text and mentioning 'other fields', which would have fully closed the gap.

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 0% for the single agency parameter, so the description has to carry the load, and it does: it specifies slug-or-name and gives five concrete example values (vre, dash, omniride, wmata, metrobus). This meaningfully reduces call errors versus a bare string type.

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 guide-style resource (payment instructions for riding one named agency), which is more precise than the title 'How to pay'. It does not, however, distinguish itself from the sibling get_fare or estimate_trip_cost, so the agent must infer the boundary between payment mechanics and fare amounts.

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?

Usage is implied by the wording 'how to pay to ride one agency', which suggests payment methods/discounts rather than price lookup, but no when-to-use or when-not-to-use rule is stated. The near-twin sibling get_fare is never mentioned, leaving the agent to guess which to call, and 'Answer only from this tool's results' is a grounding rule rather than a selection guideline.

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