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

Annotations cover the safety profile (readOnly=false, idempotent=true, destructive=false), and the description goes well beyond them: per-key outcomes (added/exists/rejected), rejection reasons (plan, active-monitor limit, missing WordPress plugin), and the scheduling consequences before and after ownership proof. The 'exists' outcome is consistent with idempotentHint=true.

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?

Four sentences, front-loaded with the action and key source, with no filler. The final sentence is long and packs several clauses (trigger_scan, scheduleStarts, per-plan cadence, re-run triggers), which would read better split, keeping it from a 5.

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 two-parameter mutation with no output schema, the description covers inputs, per-key result semantics, failure reasons, and post-ownership scheduling behavior. An agent has everything needed to call it and interpret the outcome without opening the schema.

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 100%, so the baseline is 3; the description adds that keys are catalog check keys sourced from list_checks and that each key produces an individual result, which meaningfully informs how the array is consumed. It does not add format or ordering detail beyond that.

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 first sentence states a specific verb+resource+mechanism: 'Add monitors to a site by key (from list_checks).' It implicitly separates itself from add_custom_check and add_heartbeat by scoping to catalog keys sourced from list_checks, so an agent can route correctly.

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?

It tells the agent where keys come from (list_checks) and ties behavior to two prerequisites, site ownership proof and trigger_scan, which is clear usage context. It stops short of an explicit when-not or a direct 'use add_custom_check instead' exclusion, so it is a 4 rather than a 5.

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