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

Annotations declare readOnlyHint=false and destructiveHint=false, so safety is partly covered. The description adds meaningful context beyond annotations: it creates a session, requires an authenticated Proof account, and implies a follow-up challenge step. It does not disclose what the session returns or how long it remains valid, so it is adequate but not rich.

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?

Front-loads purpose in one tight sentence, with the auth prerequisite clearly separated in its own ACCESS block. Slightly longer than strictly necessary, but 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?

For a session-starting tool with no output schema, the description covers what happens and who can call it, but omits the natural next steps (the resulting DNS/HTTP challenge and checking via check_user_domain_verification). An agent can call it correctly but cannot plan the full flow from this definition alone.

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% with a documented enum for channel, so the schema already carries the parameter burden. The description's mention of DNS or HTTP challenge maps to the channel enum but adds no syntax or format detail beyond it, making the baseline 3 appropriate.

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 and resource ('Start a ... domain verification session') and clarifies the mechanism (DNS or HTTP challenge). However, it does not differentiate from close siblings like start_domain_verification or start_domain_email_verification, which an agent would need help choosing between.

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 ACCESS block gives real prerequisites (needs a Proof account, authenticate this client or sign in via start_login first), which is genuine when-to-use context. But it offers no guidance on when to pick this self-service flow over start_domain_verification or start_domain_email_verification, so tool selection among siblings is left to inference.

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