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

Annotations already declare readOnlyHint=false, destructiveHint=false, and openWorldHint=false, so the mutation/safety profile is covered structurally. The description usefully adds the delivery mechanism (an outbound POST on each event) and enumerates the event triggers, but says nothing about webhook limits, duplicate-URL behavior, delivery retries, or the fact that this creates persistent server-side state.

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 sentences: the first front-loads the core behavior and its event triggers, the second points to the external API reference for deeper detail. The parenthetical event list slightly duplicates the enum in the schema, but it earns its place by translating enum tokens into readable event meanings.

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 three-parameter creation tool with no output schema and annotations already covering mutability, the description plus the linked API reference give an agent enough to call it correctly. What is missing is post-creation context (whether a webhook ID is returned, permission requirements, and any limits), which prevents a full 5.

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 description coverage is 100%, so all three parameters are already documented in the schema, which sets the baseline at 3. The description restates the event set in prose as human-readable labels ("email sent, reply received, bounce..."), which slightly aids mapping to the enum values but adds no new constraints beyond the 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?

The description pairs a specific verb ("Registers") with a specific resource ("a webhook endpoint") and adds the observable effect ("receives a POST request whenever the selected event happens"), plus an enumeration of the subscribable events. It does not explicitly differentiate itself from siblings like update_webhook or delete_webhook, but the create-vs-modify distinction is unambiguous from the verb.

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 (e.g., required permission level, whether a URL must first be verified), and no mention of how this relates to update_webhook/delete_webhook or what to do if a webhook for the same URL already exists. The only usable signal is the implied "call this to start receiving event notifications."

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