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

Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered. The description adds genuinely useful non-annotation context — the $0.05 cost and API key requirement — which an agent needs for budgeting and auth decisions, though it says nothing about behavior when a day has no 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 sentences: the payload is front-loaded, then the usage cue and cost/auth notes. No sentence is wasted and there is no redundancy.

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, return values need no prose, and the description covers content, entry-point role, cost, and auth. The only gap is edge-case behavior (e.g., empty days), which is minor for this aggregator.

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 optional parameters (at, currency, timezone) are already fully documented, including the profile-zone-wins precedence rule. The description's mention of "the user's timezone" echoes the schema without adding syntax or format detail, 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 resource and enumerates exactly what is returned: local date/time, today's events, Moments, Food Diary entries, and money totals by category. An agent can distinguish this aggregate 'today' snapshot from narrower siblings like get_events or get_moments without opening the schema.

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?

"Use first when an agent needs the user's current day" gives a clear triggering condition and positions the tool as a preferred entry point. It does not name alternatives or state when NOT to use it, so it falls short of the explicit routing seen in top-tier definitions.

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