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

Adds substantial behavior beyond the annotations: media replacement, cover picture retention with keep_cover: false clearing it, that the old clip's cover moment is cleared, the pending outcome for in-progress films, the 80 MB / MP4-WebM-MOV constraint, and that url videos are stored in the Library first. It matches destructiveHint=true rather than contradicting it. Missing only what the response actually returns beyond the pending case.

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-loads the action and the exactly-one-of rule, then walks through the three sources in order. Dense but well organized; the bare 'Free.' at the end is terse and the parenthetical cross-references make it a bit heavy, yet nothing is filler.

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 7-parameter destructive mutation with no output schema and open-world behavior, the description covers inputs, side effects, cover handling and one failure mode. It never describes the success response shape, so an agent still doesn't know what a successful attach returns — a minor but real gap.

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 the baseline is 3, but the description contributes real value the schema lacks: the mutual-exclusivity constraint across the three source parameters, the pending/retry semantics of project_id, and that asset_id refers to the owner's own Library video attached as-is. It doesn't explain the user_goal parameter or brand_id/post_id sourcing beyond what the schema already says.

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?

Opens with a specific verb+resource+effect: 'Attach a video to a planned post, replacing its media.' It clearly contrasts itself with create_motion_project for unchanged clips. However, it never names the near-identical sibling attach_library_media_to_post, so an agent comparing the two must still open both schemas to tell them apart.

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 routing rule — 'Send exactly one of' — followed by the condition for each of project_id, asset_id and video_url, including when to prepare a film first and what to expect while it is pending. It also states a negative rule: publish unchanged clips via asset_id/video_url, 'not create_motion_project'.

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