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

Annotations already declare the write/idempotency profile (readOnlyHint=false, idempotentHint=false, destructiveHint=false, openWorldHint=true). The description adds two genuinely useful behavioral facts not in the annotations: the event scope (all planets Mercury through Pluto) and the cost (10 credits, Tier 1). It says nothing about delivery semantics, persistence, retry behavior, or how to remove the webhook.

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?

Two short, front-loaded lines with no filler; the trigger condition comes first and the metadata brackets are compact. The cost/group metadata is arguably structured data rather than description prose, but it costs little space.

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?

An output schema exists so return values need not be explained, but for a stateful webhook registration the description omits the essentials: whether the endpoint must be publicly reachable, what HTTP payload is delivered, whether multiple registrations are allowed, and how the subscription is cancelled. Given the dense webhook/stream sibling set, this is under-specified.

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 67%: fields and precision are well documented in the schema, while the required url parameter carries only format/maxLength constraints and no description. The description adds no parameter meaning at all, so it neither compensates for the url gap nor improves on the documented params. Baseline 3 is appropriate 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?

Combined with the title, the description identifies a specific event source ('Mercury–Pluto stations direct'), which distinguishes it from the sibling astroway_webhooks_retrograde_start. However, the sentence describes the event rather than the tool action ('registers a webhook'), so the agent must rely on the name to know what the tool actually does.

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?

There is no when-to-use guidance, no prerequisites (auth, endpoint requirements), and no routing to alternatives such as astroway_webhooks_retrograde_start or astroway_stream_retrograde_alerts, even though the sibling list shows several near-overlapping event subscription tools.

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