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

Adds substantial context beyond annotations: size limits (50 MB video / 20 MB audio), supported formats, a safety check on video, and no credit charge. Annotations only cover non-readonly, non-destructive, non-idempotent, open-world — the description fills in the operational constraints.

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?

Front-loaded with the core action and return value, then constraints. Dense but each sentence carries information. The sibling-tool enumeration is long but necessary because the rejection rule is a hard prerequisite.

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 3-parameter upload tool with no output schema, the description covers what it returns, formats, limits, side effects (none — no credits), and the dependency contract for downstream tools. Nothing an agent needs 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 coverage is 100%, so baseline is 3, but the description adds real value: it clarifies that `url` should be preferred for larger files and that base64 is an alternative for smaller payloads, plus `kind` semantics via the format lists. Slight redundancy with the schema's own descriptions.

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+resource (uploads video/audio file to FATTLY) and its output (returns a fal URL). Clearly distinguished from the sibling fattly_upload_image and from the generate_* tools.

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?

Explicitly says every videoUrl/audioUrl/sampleUrl passed to fattly_edit_video, fattly_actor_swap, fattly_transcribe, fattly_clone_voice must come from this upload, and that other links (even other fal.media links) are rejected. Also advises preferring `url` for larger files.

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