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

Annotations already declare readOnlyHint=true, openWorldHint=false, destructiveHint=false, so the safety profile is covered. The description adds behavior the annotations cannot convey: each entry carries sync status and last error, and the iCal link — a credential — is never returned. That last point is genuinely useful for an agent reasoning about whether it can read or reuse the subscription URL.

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 sentences, front-loaded with the operation and its return payload, then the security disclosure, then the out-of-scope action. Every sentence adds information an agent needs and none restates the title or annotations.

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 no-parameter, read-only list with no output schema, the description covers what is returned (calendars, sync status, last error), what is withheld (the secret link), and what cannot be done here (connecting). There is no meaningful gap an agent would need filled before calling it.

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?

The tool takes no parameters, which is the baseline-4 case per the rubric; there is nothing for the description to disambiguate. It correctly implies the listing is scoped to the caller's own imported calendars with no filtering options, consistent with the empty schema.

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 ('List the external calendars') and scopes it precisely to calendars the caller imported into Planner, naming the concrete providers (Google, Outlook, iCloud via secret iCal link). It also names what is listed alongside them (sync status, last error), so an agent can distinguish this from the scheduling siblings like planner_get_agenda or planner_free_slots without opening the schema.

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?

The description draws an explicit boundary on a closely related operation: 'Connecting a calendar is done in the Planner page, not over MCP,' which tells the agent there is no add/connect counterpart to call. It does not name a sibling alternative for adjacent read needs, but the use condition (I need to know which external calendars exist and whether they are syncing) is clear from the first sentence.

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