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

Annotations already declare readOnly=false and destructive=false, so the description's real contribution is behavioral: partial-update semantics ('only the fields given change') and that null clears a field. These are non-obvious traits an agent would otherwise get wrong, though write-permission needs and assignee-replacement behavior are unstated.

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?

A single tight sentence with the toggle behavior front-loaded and the null-clearing caveat trailing. Zero wasted words.

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 needn't be described. For a mutation of this size, the description covers the key surprises (partial update, null clears), leaving only assignee-list semantics as a notable gap.

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 coverage is very low (13%), so the description must compensate, and it does partially: it explains `done` toggling and null-clearing. But it leaves `assignees` (whether it replaces or merges), `dueDate`, `title`, and `notes` field semantics unaddressed beyond the schema's format hints.

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: toggling `done` on a list item or changing its details. It's clearly a mutation of an existing item, distinguishing it from `add_list_item`/`remove_list_item`, though it doesn't name those siblings explicitly.

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?

Usage is implied by the examples (tick off/on, edit details) but there is no explicit guidance on when to use this versus `add_list_item` or `remove_list_item`, nor any prerequisites or exclusions.

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