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

The annotations already declare destructiveHint=true, openWorldHint=true and idempotentHint=false, and the description genuinely adds beyond them: new-mail-only effect, first-enabled-match-wins precedence, pausing via enabled=false, pausing while billing is past due or the mailbox is offline, and the fresh boundary on re-enable (which explains the non-idempotency). It does not spell out that unmentioned fields are cleared by a replace, though 'Replace' hints at it.

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?

Four sentences, front-loaded with the purpose, then behavior, then the exclusions/contrast note. Every sentence carries information and none is padding. Slightly denser than needed for an update tool but well structured.

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 6-required-parameter mutation with nested objects and no output schema, the behavioral side is well covered, but the parameter side is thin: the description never explains the action union or conditions fields. Annotations carry the safety profile, but an agent still needs the schema alone to construct a valid call.

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 only 33% across 6 required parameters plus nested action/conditions objects. The description gives real meaning for 'enabled' (pause, billing, offline) but says nothing about id, name, accountId, conditions, or the move/label/trash/spam action union, so it fails to compensate 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 and resource ('Replace a SuperMailbox rule'), which an agent can distinguish from create_mail_rule/delete_mail_rule in the sibling list. It does not explicitly name those siblings, but the replace semantics make the update role clear. The product-specific term 'SuperMailbox' adds identity but no further disambiguation.

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?

Usage is only implied: the description explains runtime behavior (new incoming mail, first matching rule wins) and how to pause via enabled=false, plus a separation note ('not a provider-native filter'). There is no explicit guidance on when to choose update over create_mail_rule, delete_mail_rule, or list_mail_rules, nor prerequisites such as needing an id from list_mail_rules.

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