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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds substantial context beyond them: credit cost, the weekday 07:00-19:00 coverage window, that counts are raw survey-day data rather than averages, and that many count points are only surveyed every few years (a real data-vintage caveat). The response shape (per-direction hours, busiest hour, 12-hour total) is also disclosed.

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?

Four tight sentences, front-loaded with what is returned, followed by use case, coverage caveats, and cost. No filler; each sentence contributes either scope, routing, or a limitation the agent would otherwise miss.

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 and only three simple parameters, the description carries its weight by explaining the return structure, temporal coverage, survey-frequency limitation, and cost. An agent has everything needed to decide to call it and to interpret the result.

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 all three parameters (year, direction, count_point_id) are already documented with defaults and enum-like values. The description reinforces that data is tied to a single survey year and one count point, but adds no syntax or format detail beyond the schema — baseline 3 applies.

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 (returns hour-by-hour manual traffic counts from DfT) at one count point, broken down by direction and including busiest hour and 12-hour totals. The scope language ('manual counts', 'one count point', 'survey day') distinguishes it from the aggregate siblings like region_traffic_by_vehicle_type and local_authority_traffic_totals without needing extra lookups.

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 use case ('peak-hour questions such as how busy is this road at 8am') and target audience, plus clear exclusions: not averages, not night-time, single weekday 07:00-19:00 only. It does not explicitly name which sibling to use for averages or region-wide totals, so routing to alternatives is left partly to inference.

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