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

Annotations only declare readOnlyHint=false, destructiveHint=false, so the description must add the behavioral context — and it does: this 'Spends credits', it returns a task id, and progress is checked via get_task. It doesn't explain failure/refund behavior or what wait_for_completion's 'short while' means. Note the idempotentHint=false annotation is not contradicted: the description's idempotency_key is an opt-in per-request safety mechanism, not a claim of tool-level idempotency.

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 sentences, each doing distinct work: what it makes, its cost and async return path, and the alternative tool. The cost and task-id facts are front-loaded where they matter.

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 9-parameter, credit-spending, async generation tool with no output schema, the description covers the essentials an agent needs: cost, task id return, and the polling tool. It omits error/partial-failure behavior and the interaction of wait_for_completion with polling, which keeps it short of complete.

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 schema already documents all 9 parameters, including character_ids limits and the atmosphere_id/opening_image_upload exclusivity. The description adds only light reinforcement ('up to 4 of your saved characters', 'saved or uploaded images'), which is baseline 3 territory rather than new meaning.

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 ('Generates one short video clip from a written brief') and immediately bounds the optional inputs (up to 4 saved characters, one image as opening frame). It explicitly distinguishes itself from the sibling generate_video, so an agent can route 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?

Names two concrete alternates and their conditions: use generate_video for 'full control over a single shot', and use get_task to check the returned task id. The guidance is clear, though 'full control over a single shot' is a somewhat fuzzy criterion and no other siblings (frame_chain, generate_image) are addressed.

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