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

Annotations already declare destructiveHint=true, idempotentHint=false, and readOnlyHint=false, so the safety profile is covered. The description adds genuinely new behavior beyond that: completing a task also mutates recurrence, which is a side effect an agent would not predict from the annotations or schema alone.

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 short sentences, front-loaded with the core action and zero padding. The second sentence is a little vague ('supports confirm and idempotency validation') but costs almost nothing.

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, and the annotations carry the destructive safety profile, so the description need not explain return values. It still leaves the key mutation semantics unexplained: what 'update recurrence' actually does (spawn the next occurrence? alter the schedule?) and what completing does to the task record.

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 six parameters including confirm, idempotencyKey, and hoursLogged are documented in the schema. The description adds no syntax or semantic detail beyond restating that confirm and idempotency are supported, 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 ('Mark a task as Completed') and discloses a side effect ('update recurrence'), so the agent knows this is a state transition rather than a creation or deletion. However it never names or distinguishes itself from the sibling task tools (create_task, delete_task, get_tasks), leaving that routing 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?

There is no statement of when to use this versus delete_task or get_tasks, nor any prerequisites beyond the schema's confirm flag. The clause about confirm/idempotency is a capability note, not usage guidance.

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