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

Annotations already declare this is a non-readonly, idempotent, open-world, potentially destructive mutation. The description adds substance beyond that: it confirms PATCH-style partial semantics (supporting the idempotency hint) and warns that credential fields are rejected server-side, which the annotations cannot convey. It says nothing about what 'destructive' means here or about reversibility, so it is helpful but not complete.

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 short sentences: the change scope is front-loaded, then the partial-update rule, then the exclusion. Every clause earns its place and nothing is padding.

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 13-parameter mutation with nested warmup/tracking objects, no output schema, and annotations covering the safety profile, the description supplies the key missing behavior: partial updates and the credential rejection. It could still say more about reset/clear semantics and error behavior, but it is sufficient to call the tool correctly.

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 77%, so the schema already documents most parameters including constraints and the empty-string clears replyToAddress. The description names parameter groups in prose but adds no format, unit, or default detail beyond the schema's own text. Baseline 3 for a high-coverage 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?

States a specific verb ('Change') on a specific resource ('one mailbox') and enumerates the editable attribute families (sender name, campaign cap, sending gap, signature, reply-to, BCC, tracking domain, warmup, language). The phrase 'one mailbox' also separates it from the sibling bulk_update_mailboxes, so an agent can route 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?

Gives real usage context: partial-update semantics ('Only the fields you pass change') and an explicit when-not ('SMTP/IMAP credentials are not accepted here ... the user changes them in the MailBeast dashboard'). It does not state when to choose this over bulk_update_mailboxes or note prerequisites/permissions, so it stops short of full routing guidance.

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