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

Annotations declare readOnlyHint=true, and the description confirms read-only behavior. It adds behavioral details like cursor-based pagination, the exact response fields, and default values for filters (includeNonreply, archived). No contradictions.

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 a single dense paragraph with no fluff. Every sentence adds value: pagination format, response fields, filters, and sibling tool recommendation. Well-structured and front-loaded.

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 no output schema, the description specifies the response structure. It covers all 8 parameters adequately, notes pagination behavior, and provides sibling differentiation. For a read-only list tool, it is complete.

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 0%, so the description compensates by explaining all filter parameters (mailbox, campaign, label, read, includeNonreply, archived) and their meanings (e.g., includeNonreply includes bounces/OOO, archived enum values). It adds value beyond the bare 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?

The description clearly states the tool is a 'Unified reply inbox across mailboxes' that lists threads, and distinguishes it from the sibling tool 'thread' by saying 'Use thread for one thread's history.' It provides a specific verb and resource.

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?

Explicit guidance on when to use this tool vs. the 'thread' tool is provided: 'Use thread for one thread's history.' The description also lists all available filters, helping the agent choose appropriate usage.

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