render_template

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

The description explicitly says 'No side effects,' reinforcing the readOnlyHint/idempotentHint annotations. It also discloses detailed behavior: message_id auto-populates recipient_name, recipient_email, original_subject, and today, while user-supplied vars override auto-fills. This goes beyond the annotations and gives the agent a clear picture of invocation effects.

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 three dense paragraphs with front-loaded purpose, followed by side-effect posture and auto-fill details. Every sentence carries operational weight, with 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?

With annotations already covering safety and an output schema present, the description supplies the remaining decision-relevant context: how output should be used downstream, when message_id affects placeholders, and the precedence of vars. An agent has everything needed to call this tool 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 coverage is 100%, so the baseline is met. The description adds substantial meaning: message_id is not just 'reply context' but drives specific variable auto-fills, and vars override those auto-fills. This is critical semantics that the schema alone does not convey.

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 opens with a specific verb-resource pair: 'Render a template into ready-to-send subject and body text.' It also distinguishes the tool from sending siblings by explicitly framing the output as text for create_draft/update_draft and noting there are no side effects.

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?

It explicitly warns 'No side effects' and instructs the caller to pass rendered text to create_draft or update_draft, including send_now=True for sending. This tells the agent when to use the tool (prepare content) and what it is not responsible for (sending/mutation), distinguishing it from send/reply/forward siblings.

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