gmail_render_template

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

Annotations already declare readOnly, idempotent, non-destructive, closed-world, so the safety profile is covered. The description adds genuinely new context beyond that: 'returns subject/body' discloses the shape of the result despite no output schema, and 'no I/O' signals it performs no Gmail API call or side effect. It does not mention behavior for unknown template names or missing variables, which keeps it from a 5.

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?

Three short sentences, all front-loaded with the core mechanic first and the return value and safety note after. No filler; every clause carries information.

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 two-parameter transform tool with a nested object, the description is close to sufficient: it names the input (template + placeholders) and the output (subject/body) even without an output schema. Minor gaps remain around error handling for missing templates or unused variables, but nothing blocks correct invocation.

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% and both parameters are documented in-schema (name sourced from gmail_list_templates, variables with an example). The description's mention of {{variable}} placeholders lightly reinforces how the variables object maps to the template but adds little beyond the schema's own wording.

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?

States a specific verb and resource ('renders a built-in template substituting {{variable}} placeholders') and adds the outcome ('returns subject/body ready for draft/send tools'), which separates it from gmail_create_draft, gmail_send_message_strict, and gmail_list_templates. An agent can tell this is a pure transform tool, not a mailbox writer.

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?

'Returns subject/body ready for draft/send tools' gives clear workflow context: render here, then hand the result to a draft or send tool. It stops short of naming the exact sibling or stating when-not to use it (e.g., listing templates first via gmail_list_templates), so it is clear but not exhaustive.

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