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

Annotations already declare it is a non-readonly, non-destructive, non-idempotent mutation, so the description's added value is the side effect: any running timer is stopped at the new start time. That is exactly the kind of context annotations cannot express. It does not say what happens to the stopped timer's entry or whether the stop is recorded automatically.

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 core action first and the important side effect second. No filler, nothing that could be trimmed.

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?

An output schema exists and annotations carry the safety profile, so the description only needed to cover the exclusive-timer behavior, which it does. Minor gap: with zero required parameters it is unclear whether calls without activityId/activityName are valid or what activity they attach to.

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 description coverage is 100%, so all three parameters (startTime formats, activityId precedence, activityName prefix matching) are already fully documented. The description adds nothing about parameters, 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?

States a specific verb and resource ('Start tracking time on an activity'), which is immediately distinguishable from stop_timer and log_time_entry at a glance. It does not explicitly name the sibling it is not, so it stops short of the top mark.

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 second sentence gives a real usage constraint (only one timer at a time), but it never says when to prefer this over log_time_entry for retroactive time or stop_timer for ending a session. Usage is implied rather than guided.

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