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

Annotations already declare read-only, idempotent, open-world, non-destructive behavior, so the bar is lower. The description adds meaningful context beyond that: it is the authoritative source, it returns the realtime endpoint URL and the linked schedule feed, and the realtime trip IDs it exposes are feed-specific. It does not mention pagination or rate limits, but limit is covered in the schema.

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 sentences, both front-loaded and purposeful. The first defines exactly what is listed and what is returned; the second explains why it matters and when to use it. There is no filler.

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 read-only discovery tool with no output schema, the description gives a sufficiently complete picture of what is returned (endpoint URL, linked schedule feed) and why it is authoritative. It does not explain every filter combination, but the schema covers parameters and annotations cover safety, so the remaining gap is minor.

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 all five parameters are already documented in the input schema. The description adds some high-level semantics by mentioning country, city or operator filtering, but it does not elaborate on entity_type or group_by_agency beyond what the schema provides. Baseline 3 is appropriate when the schema carries the parameter documentation.

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 uses a specific verb ('List') and precisely names the resource ('GTFS-Realtime feeds') along with its sub-types (vehicle positions, trip updates, service alerts). It also distinguishes this tool from schedule-feed siblings by emphasizing that it is authoritative for live data and returns the realtime endpoint plus the schedule feed it is keyed against.

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?

It clearly states the usage context: this is what you need before consuming realtime trip IDs, because trip IDs only resolve against the specific schedule feed named here. It does not explicitly name alternative sibling tools or when not to use this one, but the operational context is clear enough to guide selection.

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