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

Annotations only carry destructiveHint, so the description usefully discloses a behavioral trait the agent cannot infer: the single-running-timer invariant and the exact failure mode ('Paymo returns 409'). It stops short of covering auth requirements or what the response contains, but the conflict semantics are the key operational fact.

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 tightly written sentences, front-loaded with the core action before the constraint and error. The trailing 'Paymo: POST /api/entries' is an implementation detail of marginal value to an agent, a minor bit of filler.

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 small 4-parameter creation tool with no output schema, the description covers purpose, the running-timer constraint, and the error case. It omits what the call returns (e.g. the created entry id), which an agent might want, but otherwise nothing essential is missing.

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 four parameters (task_id, user_id, start_time, description) are already documented in the schema. The description reinforces the start_time/no-end_time concept but adds no syntax or default details beyond it — baseline 3.

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+resource ('Start a running timer on a task') and then defines the resource precisely as 'a time entry with a start_time and no end_time', which differentiates it from the sibling paymo_create_time_entry and pairs conceptually with paymo_stop_timer.

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 one-running-timer-per-user constraint and the resulting 409 give real usage context, but it never explicitly says when to prefer this over paymo_create_time_entry or paymo_stop_timer, leaving the sibling routing implied.

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