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

The description adds value beyond the annotations by clarifying that 'Today is the local calendar day' and that a timer started before midnight contributes only post-midnight minutes. This edge-case behavioral detail is not present in the annotations and helps the agent interpret results correctly.

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?

Three sentences, each with a distinct purpose: state the output, explain the calendar-day nuance, and confirm no side effects. It is front-loaded with the main function and contains no filler or redundant content.

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 zero-parameter read-only tool, the description fully covers what is reported, how the day boundary is handled, and that no state is changed. Annotations already cover safety, and the description supplies the operational detail needed for correct invocation and interpretation.

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 input schema has zero parameters, so there is nothing for the description to explain. The description does explicitly state 'No arguments,' which reinforces the schema and removes any doubt for the agent.

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 begins with a specific verb and resource: 'Report the running timer and today's total hours.' It clearly identifies the tool's output and distinguishes it from sibling tools like timer_start and timer_stop by emphasizing it is a status read, not a mutation.

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?

The description clearly implies when to use this tool: to check current timer state and accumulated hours. It does not explicitly name alternatives or exclusions, but the context of sibling tools and the explicit 'No arguments' and 'writes nothing' phrases effectively signal read-only usage.

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