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

Annotations only give the generic safety profile (non-read-only, non-idempotent, non-destructive, closed-world); the description adds the operationally critical details: self-contained HTML is mandatory, remote assets are refused with the 'external_url' error, animation is sampled on a virtual clock 0..duration_ms, and the response is a queued job_id rather than files.

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?

Dense but front-loaded: purpose first, then the two required inputs, then constraints, then return shape. A couple of the trailing 'It does not / It cannot' sentences are near-duplicates of earlier statements, costing a little tightness.

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 non-idempotent job-submitting tool with an output schema, the description covers the full contract: inputs and their self-containment rule, timing semantics, hard limits, the async job_id return, and the required follow-up call. Nothing an agent needs to invoke it correctly is missing.

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 schema already documents every parameter including defaults, ranges and the even-pixel rule. The description restates those same bounds and only marginally adds meaning by tying cover_ms to 'the video time the still is paired to' and explaining the virtual clock, so baseline 3 is appropriate.

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 (Apple Live Photo / animated card / MP4 or GIF) plus the concrete input model (cover_html still + motion_html animation). It implicitly separates itself from live_card_status by noting it does not return the files and that the caller must poll status instead.

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?

Opens with an explicit trigger ('Use this when the user wants an Apple Live Photo, a short animated card, or an MP4 or GIF'), and closes with explicit exclusions (cannot fetch a URL, cannot import into Photos, cannot exceed 3 seconds). It also routes the agent to the sibling live_card_status for retrieving results.

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