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

Beyond the annotations, which already declare this is mutating, non-idempotent, open-world, and non-destructive, the description discloses the merge-tag replacement rules and the exact validation conditions that cause refusal. That is rich behavioral context an agent would not get from the annotations or schema alone.

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 front-loaded with purpose and alternative, then adds the two pieces of behavioral information that materially affect calling the tool: merge-tag format and validation refusal rules. Every sentence earns its place, and there is no repetition or 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?

For a mutating create tool with no output schema, the description is nearly complete: it covers the source HTML use case, the sibling alternative, merge tags, and validation failures. The remaining gap is that it does not describe success/return behavior or mention the dry_run use case, though dry_run is documented in the schema.

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?

With 100% schema description coverage, the baseline is 3. The description adds meaningful semantics for the html parameter by explaining [[key]] merge tags, the [[name]] example, keys from list_custom_fields, and that {{…}} is not replaced in campaigns. It does not add comparable guidance for dry_run, plain_text, or category_id, so it is not a 5.

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 and resource: 'Create a new email template from HTML the user pasted or supplied.' It explicitly distinguishes this tool from create_dnd_email_template, making the intended use case unambiguous without opening either schema.

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 gives an explicit alternative and the condition that selects it: use this tool for user-supplied HTML, and use create_dnd_email_template for an email you design so it opens in the drag-and-drop editor. It also provides concrete guidance on merge-tag syntax and which tag source to use via list_custom_fields.

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