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

With no annotations, the description carries the burden and does well: it enumerates what is returned (times, levels, minutes-to-event, current level, rising/falling, tide cycle, speech_ja) and states an important caveat — it is JMA astronomical tide calculation, 'not a forecast'. That caveat is genuinely valuable context a schema cannot convey.

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?

One dense but front-loaded paragraph with no filler; the core scope leads and the caveat and speech_ja note trail as supporting detail. Nothing is redundant.

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 appropriately spells out the returned fields and the not-a-forecast caveat, and the params are fully covered by the schema. Minor gaps (units or precision of tide level, error behavior for unknown station) keep it from a 5.

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 both parameters are already documented. The description's 'at で別の時刻も' restates the schema's at parameter rather than adding new syntax or format detail, so the 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?

The description names a specific verb (返す) and precise resource: next high tide, next low tide, current tide level, rising/falling state, and tide cycle, all from Japan time now. It is clearly distinguishable by its 'next + current' scope, though it never explicitly separates itself from the sibling get_tide, so it falls short of full sibling differentiation.

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?

'いま（日本の時刻）から見た' and 'at で別の時刻も' imply when to use it (point-in-time snapshot) and how to shift the reference time. But there is no explicit when-not guidance and no routing to get_tide or list_tide_stations, leaving usage to inference.

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