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

Adds substantial context beyond annotations: it discloses the `draft_pending_approval` and `blocked` governed outcomes, explains these are expected rather than errors, references the matched rule, and states the required `send_internal` capability. This is exactly the kind of auth/governance detail annotations alone cannot convey.

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 waste, front-loading the core purpose before the governance behavior and capability requirement. 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?

With no output schema, the description carries the return-value burden and does so, explaining the two governed outcome states. Combined with the capability requirement and sandbox rules, an agent has enough to call and interpret results 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 100%, so all 7 parameters are already documented, including the sandbox/recipient semantics and the mode enum. The description's mention of 'reply' loosely maps to thread_id but adds no syntax or format detail beyond the schema, so the baseline 3 applies.

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 specific verbs (send or reply) plus the resource and scope ('from a scoped inbox'), and immediately qualifies it with governance context. It is clearly distinguishable from siblings like mailbuttons_get_message or mailbuttons_list_drafts, which read rather than deliver.

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 clear operating context — sandbox tokens cannot deliver externally, disallowed recipients are blocked, and the `send_internal` capability is required. It does not explicitly name sibling alternatives (e.g., when to use get_draft versus send), but the conditions that govern a send are well specified.

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