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

Annotations already indicate readOnlyHint=false, destructiveHint=true, idempotentHint=true, and openWorldHint=true, so the safety profile is covered. The description adds 'within its required mailbox scope', which is a constraint but not a behavioral trait beyond what annotations imply. No contradiction, but minimal added value.

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 exceptionally concise: two short sentences that front-load the purpose and provide a usage guideline. No filler or redundancy.

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?

Despite having an output schema, the description lacks parameter details for a 9-parameter tool with low schema coverage. It mentions only one alternative among many siblings. The description does not cover return values, side effects, or prerequisites beyond user approval.

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?

With 9 parameters and only 33% schema description coverage, the description should add meaning to undocumented parameters but does not. It merely restates the purpose. For example, it doesn't explain 'sandbox_outcome' or 'scheduled_at' beyond the schema's type/format.

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 states 'Reply to a stored ShipMail thread within its required mailbox scope' and distinguishes from the sibling 'shipmail_reply_to_inbox_thread' for JMAP inbox threads. This provides a specific verb-resource pair and differentiates from an alternative tool.

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 includes a clear usage condition: 'Use only after the user approves the exact recipients and content.' It also mentions an alternative tool for JMAP inbox threads, offering some guidance on when not to use this tool. However, it does not explicitly list when not to use or other exclusions.

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