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

With no annotations provided, the description carries the full transparency burden. It discloses key behaviors: actions performed on match, forward/label argument requirements, the webhook_only special case, and priority ordering with first-match-wins. It omits permission requirements and return values but still offers substantial behavioral insight.

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?

The description is three sentences long, front-loaded with the tool's purpose, and each sentence adds critical information: action types, match criteria, priority behavior, and API endpoint. There is zero filler and every word 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 9-parameter creation tool with no annotations and no output schema, the description covers match fields, all actions, priority ordering, and the API endpoint. It does not mention the enabled flag's default behavior or error/duplicate-rule handling, but the provided context is sufficient for an agent to construct a correct request. A 5 would require explicit coverage of these 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 coverage is 100%, so the baseline is 3. The description adds value beyond the schema by explicitly linking action_arg to forward and label actions, clarifying that webhook_only skips forward-to, and explaining the priority field's lower-first ordering. This enriches the parameter understanding without duplicating schema text.

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?

The description clearly specifies the verb 'Add' and the resource 'inbound filtering rule to a mailbox.' It distinguishes itself from siblings like list_mail_rules and remove_mail_rule by describing the exact creation action and enumerating the actions and match fields supported.

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 provides strong usage context by explaining rule execution semantics (priority order, first match wins) and the action_arg dependency for forward and label actions. However, it does not explicitly state when not to use the tool or mention alternative tools like send_email or set_email_forward, so exclusions are only implied.

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