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

Annotations already declare readOnlyHint=false, idempotentHint=false, and destructiveHint=false, so the write/safe profile is covered. The description adds the non-obvious consequence — "it never merges with an existing one" — which tells the agent that duplicate days produce duplicate entries, something idempotentHint alone does not convey.

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?

Four short sentences, front-loaded with the action, then the input choice, then the key non-merge behavior, then the pre-check. The opening field enumeration slightly restates the schema but is compact and orienting rather than wasteful.

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, so return values need not be explained, and annotations cover the safety profile. Combined with the no-merge warning and the list_entries pre-check, the definition is sufficient for correct invocation; only the differing behavior for already-logged days is left unstated.

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 the schema already documents project, date, minutes, duration, and note. "Give minutes or a duration text" restates the mutual exclusivity already stated in the schema ("use this or minutes"), adding no format or precedence guidance, 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?

Specific verb + resource ("Add one time entry") with the fields it carries, which implicitly separates it from bulk/day-level siblings like log_time_on_days and set_day_totals. It does not explicitly name those siblings, so differentiation is 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 a concrete when-to-use condition and a named alternative: "Check list_entries first if the day may already be logged." It stops short of stating what to do instead when a day is already logged (update_entry vs set_day_totals), so the routing is clear but not exhaustive.

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