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

Annotations include openWorldHint and idempotentHint, which already signal side effects and repeatability. The description adds useful behavioral detail: it returns the HMAC secret once, events are signed POSTs, and it covers specific lifecycle events. This goes beyond the annotations without contradicting them.

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?

The description is two sentences but the first is long and information-dense, listing all event types and the polling alternative. It is front-loaded with the core purpose and avoids redundancy, though it could be broken into shorter sentences for readability. Still, every sentence earns its place.

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 an output schema present and annotations covering side effects, the description adequately explains what the tool does, when to use it, and a key return behavior (HMAC secret once). It does not cover error scenarios or security details beyond 'signed POSTs', but these are not critical for invoking the tool correctly given the rich 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 both parameters (id and url) already have meaningful descriptions. The tool description does not add further parameter-level detail, but it does clarify the url's role (receives signed POSTs) and that private/loopback hosts are refused (though this is in the schema). Baseline of 3 is appropriate since the schema does the heavy lifting.

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 states a specific action ('Register a URL') with a precise resource (webhook for instance events), enumerates the exact events covered, and explicitly contrasts itself with the sibling tool get_instance_health. This makes the tool's purpose unmistakable and differentiates it from the alternative without needing to inspect 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 explicitly names get_instance_health as an alternative and indicates this tool should be used 'instead of polling', giving clear guidance on when to prefer it. It does not, however, state conditions when webhooks are inappropriate (e.g., for short-lived instances), but the core usage context is present.

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