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

Annotations already state readOnlyHint=false, destructiveHint=false, and openWorldHint=false, so the safety profile is known. The description adds that the effect is a user-visible project thumbnail on the projects screen, which is useful context. It does not say whether an existing thumbnail is replaced, whether the change is reversible, or what is returned, leaving gaps beyond the annotations.

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?

A single front-loaded sentence with no filler; the target (frame at `time`) and the outcome (project thumbnail on the projects screen) both land immediately. It is arguably too terse for a mutation tool, but nothing in it is redundant.

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 two-parameter mutation with no output schema and half of its parameters undocumented, the description covers purpose but not the details needed for reliable invocation: the unit convention for `time`, replacement behavior for any existing thumbnail, and confirmation of the default project. Adequate but with 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 coverage is 50%: projectId is documented in-schema as defaulting to the open project, but `time` has no schema description. The description clarifies that `time` identifies a frame, yet omits its units/format (seconds, frames, ticks), which is exactly the detail an agent needs. This matches the baseline for partial coverage.

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 gives a concrete verb and resource: set the project thumbnail from the frame at `time`, visible on the projects screen. That disambiguates the terse name `set_cover`, which alone could read as audio-cover or cover-art. It does not explicitly contrast with siblings like set_project_settings or set_project_brand, which keeps it short of a 5.

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?

There is no guidance on when to use this versus other project-level tools, nor any prerequisites (e.g., the frame must exist, a project must be open). The only implied usage cue is the 'projects screen' mention. An agent must infer the selection context entirely.

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