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

Annotations only supply destructiveHint=true, a low bar. The description adds real behavioral context beyond that: the entry it creates (start_time, no end_time), the single-running-timer constraint, and the 409 conflict response. It doesn't cover auth requirements or what the call returns.

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 tight sentences, front-loaded with the action and definition before the constraint and endpoint. The trailing 'Paymo: POST /api/entries' is marginally useful API trivia rather than load-bearing information.

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?

No output schema exists, so the description could ideally say what the created entry looks like or what is returned. Still, for a well-specified 4-param start-timer tool, the combination of definition, constraint, and error behavior is close to sufficient.

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 task_id, user_id, start_time, and description are already documented with defaults and formats. The description adds nothing per-parameter, 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+resource ('Start a running timer on a task') and defines the resulting artifact as a time entry with start_time and no end_time. This distinguishes it in spirit from paymo_create_time_entry, though that sibling is never named, so the differentiation is inferred rather than explicit.

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 usage context: one running timer per user, and the 409 response if one is already active. It stops short of naming alternatives (e.g. use paymo_stop_timer first, or paymo_create_time_entry for a completed entry), so the agent must infer the routing.

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