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

Annotations already declare readOnly=false, openWorld=true and idempotent=false, so the agent knows this is a mutating, non-idempotent call. The description adds substantive context beyond that: the created domain is pending, DNS records are returned, verification must occur before activation, and there is a critical constraint against using a Shopify primary domain. This is genuinely helpful behavioral disclosure.

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?

Three sentences, zero padding, with the core action and its outcome front-loaded and the safety warning placed last for emphasis. 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 no output schema, the description usefully explains the return value (DNS records) and the pending/verification lifecycle, which is what an agent needs to proceed. It omits behavior on duplicate/repeat calls (notably with idempotentHint=false) and any permission prerequisites, leaving minor gaps.

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 0% for both parameters, so the description carries the burden. It implies siteId identifies 'a Replo site' and that domain is a custom domain requiring DNS records, but it gives no field-level meaning, format expectations, or the Shopify-domain constraint tied specifically to the domain argument. It only partially compensates for the coverage gap.

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?

States a specific verb+resource ('Begin connecting a custom domain to a Replo site') and clarifies scope ('This creates a pending domain'), which distinguishes it from the sibling verify_custom_domain and get_custom_domain_status. An agent can identify the tool and its role in the domain lifecycle without opening 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 places the tool as the first step ('Begin...') in a sequence leading to verification, and gives an explicit when-not: 'Never connect a Shopify store's primary domain.' It does not name the follow-up tools (verify_custom_domain, get_custom_domain_status) that would complete the routing guidance, so it falls short of a full 5.

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