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

Annotations already cover the safety profile (read-only, idempotent, open-world, non-destructive), so the bar is lower. The description still adds real value: it discloses that results are limited to the current week and that times of 24:00 or later belong to the next calendar date, which prevents a common misreading of returned data.

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?

Two tightly written sentences with the core behavior front-loaded and the 24:00 edge case appended as a clarifying note. Nothing is redundant and nothing needs trimming.

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 an output schema present, annotations covering safety, and full schema description coverage, the description only needs to carry behavior and scoping, which it largely does. The one remaining gap is routing relative to transit_list_arrivals (real-time) and transit_get_route (direction lookup), which an agent would have to work out from sibling names alone.

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%, so the schema already documents all six parameters including date range, kind, language, and direction. The description repeats the current-week constraint and the single-direction scope but contributes no syntax or format detail beyond the schema, making the baseline 3 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?

The description names a specific verb and resource — planned departure times for one direction of a route at one stop — plus a scoping constraint (current week). The word 'planned' implicitly contrasts with transit_list_arrivals, but the sibling is never named, so the distinction relies on inference.

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 rather than stated: the current-week restriction and the fact that a single direction is returned suggest when this tool fits, but there is no explicit when-to-use or when-not guidance relative to transit_list_arrivals or transit_plan_trip. The 'direction required when the route has two directions' rule lives in the schema rather than the description's guidance.

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