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

Beyond the annotations (which only say openWorld=true, destructive=false), the description discloses cost behavior ('Uses one reel credit per variant'), a content-filtering rule ('Text inside a script or style tag is not spoken'), and the return handoff (job_id → get_job). That is real operational context an agent would otherwise have to discover by trial.

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?

Six short sentences, each carrying a distinct fact: purpose, input mapping, sibling relation, exclusion rule, cost, and next step. The purpose is front-loaded and nothing is padding.

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 no output schema present, the description compensates by stating the return value (job_id) and the polling tool. Cost, filtering rules, and sibling relationship are all covered, and the 100%-covered schema carries the parameter detail, so an agent has everything needed to call and follow up 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 description coverage is 100%, so the baseline is 3, but the description adds meaning on top: it clarifies that html must be structured as a heading plus paragraphs and that variants each consume a credit, which the schema's 'each uses one credit' reinforces but the description ties to billing behavior. The remaining parameters (avatar_id, voice_name, caption_style, aspect, language) are fully documented in the schema.

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 gives a specific verb and resource ('Turn HTML ... into a vertical reel') and explains the domain mapping of the input ('The heading is the subject and the paragraphs are the narration'). It also positions itself relative to the sibling create_reel ('Starts the same render as create_reel'), so an agent can distinguish the HTML-entry variant from the other entry point without opening a 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 names the sibling it duplicates (create_reel) and the mandatory follow-up ('poll it with get_job'), which gives the agent a clear before/after picture. However, it never states explicitly when to pick render_html over create_reel — the choice is left implicit in 'HTML the agent wrote', so it stops short of a full when/when-not statement.

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