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

Goes far beyond the annotations, disclosing that signingSecret is only returned on this call and cannot be retrieved later, the exact HMAC-SHA256 verification scheme, and the 5-minute timestamp rejection window. It also warns that enabledEvents are workspace-global with a validation constraint. No contradiction with annotations — readOnlyHint=false, idempotentHint=false, and openWorldHint=true all align with a stateful create operation that has external side effects.

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?

Long but earned — every sentence carries either a critical caveat (one-time secret), a security contract (HMAC verification), a routing rule (subscription vs endpoint), or a documentation pointer. The CRITICAL warning is front-loaded right after the purpose sentence. Minor deductions for the somewhat run-on enabledEvents sentence and the trailing llmContext advice, which could be tightened.

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 an 8-parameter creation tool with nested objects and a close sibling, this is remarkably complete: the one-time secret, the verification contract, the event-scoping trap, and an escape hatch to documentation are all present. The output schema covers the response structure, so the description doesn't need to explain return values. Nothing an agent needs to call this correctly is missing.

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?

The schema already covers all 8 parameters at 100%, so the baseline is 3. The description adds genuine meaning for the trickiest parameter, enabledEvents, by explaining the workspace-global vs resource-scoped distinction that the schema only gestures at, and by naming the subscription tool that handles the rejected event types. Other parameters need nothing more, but the description adds per-parameter value only for this one.

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?

Opens with a specific verb and resource — 'Register an HTTPS URL to receive Extend events' — and immediately names the webhooks group. It also distinguishes itself from its closest sibling, create_webhook_subscription, by explaining which event types each handles, so an agent can tell them apart without opening either 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?

Explicitly states when NOT to use this tool: resource-run events such as workflow_run.completed are rejected here and must go to create_webhook_subscription, including the exact two-step workflow (create endpoint with [], then subscribe). It also routes manual setup to get_documentation with a concrete URL. This is textbook when-to-use guidance with a named alternative.

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