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

Annotations already declare readOnlyHint=true, destructiveHint=false, and openWorldHint=false, so the safety profile is covered. The description adds genuine behavioral value beyond that: it enumerates the returned fields (state, schedule, last ping, next due, plan coverage, missed/failed check-ins) and discloses a security-sensitive trait — ping URLs are exposed only to full-access connections and anyone holding the URL can send pings.

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?

It is a single dense sentence, but the key facts are front-loaded (resource, then return contents, then the ping-URL caveat last). Given there is no output schema, the long clause enumerating return fields earns its place rather than being padding.

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?

With no output schema and only one simple parameter, the description carries the return-value burden and does so thoroughly — state machine values, schedule encoding in seconds, account quota, and the ping-URL access condition. An agent has what it needs to call and interpret this tool.

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?

There is a single parameter (siteId) with 100% schema description coverage, so the schema already documents it fully. The description adds no syntax, format, or ID-source detail beyond the schema, which is the baseline expectation at this coverage level.

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?

The description opens with a specific verb+resource ('A site's heartbeats') and immediately defines the resource as cron jobs, backups and scheduled tasks that check in by requesting their own URL. This distinguishes it cleanly from siblings add_heartbeat and update_heartbeat, which mutate rather than list.

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 only implied by the list verb and the required siteId. It never states when to call this versus list_checks, list_runs, or get_site_health, nor any exclusion or prerequisite (e.g., must the site have monitoring enabled). Adequate but with a clear gap in routing guidance.

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