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

Annotations already declare readOnlyHint=true, destructiveHint=false, openWorldHint=true and idempotentHint=false, so the safety profile is covered. The description adds the useful fact that the payload is a real-time snapshot ('now'), but the claim that it also covers 'a past moment' conflicts with an empty parameter schema and muddies the freshness guarantee.

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?

A single short sentence is appropriately sized, and the geographic/data scope is front-loaded. However, the trailing 'or at a past moment' clause earns no place: with no time parameter, it cannot be acted on and only invites an incorrect invocation.

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, the description is the only source of information about what comes back, and it says nothing about the return shape (vehicle positions vs. departure times vs. route lists) or freshness semantics. For a zero-parameter, open-world sensor tool the description should at least state what the caller receives and how the sibling _at variant differs.

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?

The tool takes zero parameters, so there is nothing for the description to disambiguate and the baseline of 4 applies. The only parameter-adjacent statement ('at a past moment') is unactionable given the schema, but no parameter-level gap is created.

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 the resource (Helsinki public transport: buses, trams, metro, ferries) but supplies no verb, leaving the operation (fetch/stream) implicit. Critically, it does not distinguish itself from the sibling cell_sensor_hsl_at, and its 'or at a past moment' clause blurs the boundary rather than sharpening it.

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?

There is no explicit when-to-use guidance and no named alternative, despite a directly competing sibling (cell_sensor_hsl_at) that appears purpose-built for the historical case. The agent must infer from the naming convention alone which of the two to call.

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