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

readOnlyHint=true is consistent with a rendering operation, and the description adds context the annotations don't carry: the custom-vs-default template fallback, the fact that output is fully rendered message content, and a non-obvious auth prerequisite. It stops short of noting side effects or rate limits, but the key behavioral traits are disclosed.

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?

Front-loaded with the core purpose in the first sentence, then the access caveat in a short second paragraph. Both earn their place, though the parenthetical UI instruction is slightly more detail than strictly needed.

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 4-parameter tool with a nested object and no output schema, the description covers what is rendered, where the template comes from, and the auth gate. Parameter-level detail is delegated to a fully covered schema, leaving little an agent would still need.

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 channel, message_type, profile_id, and the nested variables object are already documented in the schema. The description only gestures at 'provided variables' and adds no syntax, defaults, or format meaning beyond the schema — 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) and resource (the template a sub-account sends), plus the resolution rule — custom template if present, otherwise the default — and the return value. It clearly separates rendering from adjacent operations like preview/get, though it never names those siblings explicitly.

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 ACCESS paragraph gives a real precondition (a Proof account must exist and this client must be authenticated; start_login does not unlock this tool). However, there is no guidance on when to use render_template versus preview_template or get_template, which are the obvious alternatives in the sibling list.

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