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

Goes well beyond the annotations: discloses plan gating and the failure mode (upgrade offer), discloses the side effect (routing/TLS provisioning), discloses the return value (the DNS record to set), and discloses the activation condition (domain only serves once the record is live). This is exactly the behavioral context annotations cannot express.

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 dense sentences, zero filler, with the core action front-loaded and the gating/return/activation details following in priority order. Every sentence carries information the agent needs.

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 two-parameter mutation with no output schema, the description covers action, prerequisites, error behavior, return payload, and when the result takes effect. Nothing required to call it correctly or interpret the result 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?

Schema coverage is 100%, so both parameters are already documented, and the baseline is 3. The description adds one genuine constraint not in the schema — the domain must be one the user already owns — but gives no format guidance beyond the schema's 'app.example.com' example.

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?

Specific verb+resource: 'Attach a domain the user owns to a published project and provision routing/TLS' — an agent immediately knows this writes a custom-domain binding plus TLS routing. It implicitly separates itself from get_custom_domain, remove_custom_domain and update_project_subdomain, though it never names those alternatives.

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?

Preconditions are stated up front: paid plan required ('the API answers with an upgrade offer otherwise') and the project must be published. It does not explicitly route the agent to remove_custom_domain or update_project_subdomain, but the read/write/subdomain split is inferable from the wording.

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