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

Annotations carry only a title, so the description bears the full burden and does so well: it discloses billing ('Paid per extracted frame through the normal approval/credit flow'), a determinism guarantee ('never estimates the endpoint from FPS'), and retry safety ('stable idempotency_key is required outside the assistant'). These are exactly the traits an agent needs and none are inferable from the schema.

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 dense sentences with zero filler, and the most important fact — that it extracts the real endpoint frames rather than an FPS estimate — is front-loaded. Every clause adds actionable 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 paid, four-parameter tool with no output schema, the description covers inputs, billing, retry semantics, and the nature of the returned assets. The one notable omission is whether the operation is synchronous or job-based (siblings like var2_get_video_result and var2_check_join_status suggest async patterns), which an agent would want to know before calling.

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 coverage is only 25% — only idempotency_key is described in the schema, and name/projectId are undocumented everywhere. The description compensates for video_url by enumerating accepted forms (Share link, ID, storage URL) and for idempotency_key with the deployment caveat, but leaves two of four parameters entirely unexplained, so it only partially closes the gap.

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 ('Extract the ACTUAL first and last image frames') with a clear qualifier ('completed owned video') and states the output artifact ('reusable image assets for shared-frame shot chaining'). An agent can distinguish this from create_image, modify_image, get_image_result, and trim_audio 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?

Gives an explicit input condition ('Pass an owned video Share link, ID, or storage URL') and a prerequisite for the failure case ('import external videos first'), plus a deployment condition for idempotency_key ('required outside the assistant'). It does not name a sibling alternative or state when NOT to use it, so it stops short of a 5.

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