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

Annotations declare readOnlyHint and idempotentHint and destructiveHint=false, but the description adds significant behavioral details: no message is sent, queued, or stored; recipient receives nothing; validates against the exact sendEmail schema; returns an envelope with demo:true; and enforces a 16KB total body cap. All this context is beyond what annotations provide and is consistent with them.

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 moderately lengthy but every sentence adds value: simulation warning, when-not-to-use, alternative tool path, validation coverage, size limit. It is front-loaded with the critical simulation warning and well-structured. Could trim slightly but is effectively composed.

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?

Given the tool's complexity (nested object parameter, many fields, output schema exists), the description covers purpose, usage constraints, behavioral details, and size limits. It refers to the output schema implicitly by describing the synthetic envelope. The presence of an output schema reduces the need to describe return values in detail, so the description is complete for an agent to correctly select and invoke this tool.

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%, but the description provides meaningful context about the requestBody parameter: it validates against the sendEmail schema (including cc/bcc, reply_to, tags, attachments, scheduled_at) and has a 16KB total body limit that can reject large attachments. This adds practical constraints not captured in the schema alone, earning a score above the baseline of 3.

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 simulation that previews a send by validating the request body against the same schema as sendEmail and returning a synthetic envelope with demo:true. It explicitly distinguishes from the actual sendEmail tool and provides the correct alternative using createEmailAddress and sendEmail. Scope and resource are unambiguous.

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 explicitly warns when NOT to use this tool (when the user wants an email delivered) and provides the correct alternative path (createEmailAddress then sendEmail). It also clarifies that the tool exists only to preview the response shape, giving clear context for appropriate invocation.

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