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

Annotations already declare the write/non-destructive/non-idempotent/open-world profile, so the bar is lower. The description usefully adds the permission constraint ('a project the signed-in user can add to') and clarifies that only the title is mandatory while all other fields are free-form brief content. It does not say what happens on a duplicate or failed project match, nor what a repeated call produces despite idempotentHint=false.

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 short sentences, front-loaded with the action and scope, then the usage rationale, then the one structural fact about required fields. No filler, though the middle sentence is the weakest and could be merged.

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 12-parameter creation tool with no output schema, the description leaves the return value unstated (does it return the created brief or just confirmation?) and says nothing about project resolution failure or duplicate briefs. The permission note and the required-field note carry most of the load; annotations handle safety.

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 12 parameters are already documented in the schema, including the project-vs-project_id fallback and the name-matching disambiguation behavior. The description only adds that brief_title is the sole required field and the rest constitutes the brief body — helpful framing but no new per-parameter meaning, so 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 ('Write a thumbnail brief') plus a scope condition ('inside a project the signed-in user can add to'). Creation versus the get/list/update-thumbnail-brief siblings is inferable from the name and verb, but the description never names an alternative, so it stops short of a 5.

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?

'Use this to specify what a thumbnail has to communicate before anyone designs it' gives an implied workflow phase (pre-design), which is real but thin guidance. There is no when-not guidance, no mention of update-thumbnail-brief for revising an existing brief, and no prerequisites such as needing a project first.

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