render_template

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

Annotations already declare readOnlyHint=true, so the no-side-effect behavior is partially covered. The description adds valuable specifics: it returns rendered subject, html_body, and text_body, and reinforces the non-sending guarantee. This goes beyond what the annotations alone provide and helps the agent understand observable results.

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?

A single sentence conveys the action, target, output fields, and side-effect guarantee with no filler. The key distinction from sending tools is placed at the end for emphasis but remains immediately clear.

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 simple preview tool with only two parameters, full schema coverage, and readOnly annotations, the description covers inputs, outputs, and non-side effects completely. No critical information is missing, and the lack of an output schema is compensated by explicitly naming the rendered fields.

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%, with both template and template_model already described in the input schema. The description's phrase 'variable model' loosely maps to template_model but adds no new semantic detail beyond the schema. This meets the baseline for schema-covered parameters but does not elevate it.

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 states a specific verb ('Preview') and resource ('stored template rendered against a variable model'), and clearly lists the return values: rendered subject, html_body, and text_body. It explicitly differentiates itself from sending tools with the phrase 'without sending anything,' making its purpose distinct from siblings like send_email and send_email_with_template.

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 clearly implies this tool is for testing/previewing templates before sending, and the 'without sending anything' clause rules out the sending alternatives. It does not explicitly name sibling tools or provide a when-not-to-use list, but the context is unambiguous enough for an agent to select it correctly.

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