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

Annotations already declare the mutation/safety profile (readOnlyHint=false, destructiveHint=false, idempotentHint=false), so the bar is lower, and the description adds real context on top: the PROJECT-SCOPED constraint (acts only on explicit project_id, never the active-project pointer), a hard 'Up to 64' tile cap, and the fact that the project identity is returned with the result. It does not say what happens to existing elements or the required permissions, but multiple concrete behaviors beyond the annotations are disclosed.

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?

The PROJECT-SCOPED constraint is front-loaded, which is good, but the body is dense marketing prose ('A living wall of REAL footage', 'tiles pop on ... breathe, collapse tile by tile') and ends with the cryptic fragment 'video plays'. Some of the descriptive language does carry information, but the structure buries concrete parameter guidance inside it.

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 13-parameter mutation tool with 8% schema coverage and no output schema, the description is not complete enough: most parameters are undocumented, return behavior is only hinted at ('returns the project identity'), and effects on existing timeline content are unstated. It covers the highest-value fields (kind, tiles) but leaves clear gaps.

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 only 8% (just project_id), so the description must carry the load and only partially does. It documents the critical 'tiles' syntax ('asset:<key> | source:<seconds> | <storage key>'), the 64-tile limit, and the three 'kind' enum meanings, and hints at 'at' (beats) and 'hero' (hero tile) — but 8 of 13 parameters (id, start, end, title, kicker, layout, backdrop, collapse_at) go unexplained in both schema and description.

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?

The description conveys the resource (a collage of 'REAL footage') and enumerates the three modes (mosaic, scroll_wall, flatlay), so an agent knows roughly what gets created. However, it never states the core act plainly — that a collage element is added to the timeline between 'start' and 'end' — instead leading with the impressionistic 'A living wall of REAL footage'. There is no differentiation from the many sibling add_* 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?

No when-to-use or when-not-to-use guidance is given, and no alternatives are named despite dozens of sibling add_* tools (add_overlay, add_motion_graphic, compose_panels, etc.) that a user might confuse this with. Usage is only implied by the description of the visual effect, leaving the agent to infer when a collage is the right choice.

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