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

Goes well past the annotations: it discloses that the entry lands on the day the timer started, that time is snapped to the workspace rounding grid, that rating/billable behave as if typed in the app, that the optional description replaces the timer's text, and that refusal leaves the timer running with the reason returned. It even states the result carries the logged entry and the call that removes it.

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?

Front-loaded purpose followed by three information-dense sentences covering parameter effect, failure behavior, and result contents, with no filler or repetition of the schema. Every sentence earns its place.

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 non-idempotent mutation on the signed-in user's timer, the description covers the write effect, the billing/rounding consequences, the failure mode, and a pointer to the undo call; the output schema is present so return shape needn't be spelled out further.

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 the baseline is 3, but the description adds real meaning: 'An optional description replaces the timer's' tells the agent that omitting the parameter preserves the existing description, which the schema does not convey. The userId parameter is only explained in the schema, not the description.

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?

States a specific verb and resource ('Stops the signed-in person's running timer and logs the time as one entry') and pins the scope to the signed-in user's own timer, which separates it from start_timer, cancel_timer and list_running_timer. The logging/billing behavior is described concretely rather than left to inference.

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?

Gives clear operating context: what happens on success, and that when the entry is refused the timer keeps running and the reason is returned, so the agent knows the failure path is non-lossy. It stops short of explicit routing to alternatives (e.g. cancel_timer for discarding a timer without logging), so there is no when-not-to-use statement.

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