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

Annotations already declare readOnlyHint=true, but the description adds substantive context beyond that: nothing is sent and no send-log row is written, and it discloses that unsubstituted placeholders are surfaced in the result. It could still note idempotency or variable fallback behavior, but it meaningfully exceeds the annotation.

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?

Two sentences, front-loaded with the core action and side-effect disclaimer, then the routing guidance. Slightly dense but every clause earns its place; no filler.

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?

An output schema exists, so return values needn't be fully explained, yet the description still names the key outputs (subject, HTML, unsubstituted placeholders). The no-side-effect and routing details cover the main agent ambiguities; minor gaps like error behavior on bad templateId remain.

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%, so both templateId and variables are already documented with format hints ('tmpl-...') and semantics ('Sample values for {{variable}} placeholders'). The description adds the placeholder-substitution concept but no syntax or format detail beyond the schema, so baseline 3 applies.

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 (render) on a specific resource (template) and specifies the return payload (subject, HTML, unsubstituted placeholders). Explicitly distinguishes the side-effect-free nature from send tools, making it clearly separable from volanea_send_test_email.

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?

Gives explicit when-to-use ('while iterating on copy') and names the alternative (volanea_send_test_email) with its selecting condition ('only when you want a real logged message'). This is textbook when/when-not/alternatives guidance.

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