Set the cover image

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

Annotations provide readOnlyHint=false, destructiveHint=false, and idempotentHint=false, which already indicate this is a mutating, non-idempotent operation. The description adds the mutual-exclusivity constraint and the clear option, but does not disclose whether clearing the cover is reversible, what happens to the existing cover, or any authentication requirements. While not contradictory, it leaves behavioral nuances unaddressed, so a mid-range score is appropriate.

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?

Two sentences with no filler. The first sentence establishes importance, the second delivers all operational detail. The exclusive-options list is compact and front-loaded, making it easy for an agent to parse and act on quickly.

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 tool with five parameters but one required, the description covers the functional behavior well: the mutual-exclusivity rule and the clear action are explained. The game parameter is documented fully in the schema, and there is no output schema to explain. The only minor gap is not mentioning validation errors when multiple options are supplied, but this is not critical for correct invocation.

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 each parameter already has descriptions. The description adds value beyond the schema by explicitly framing screenshotId as promoting an existing screenshot and by mandating mutual exclusivity ('Send exactly one of'), which is not encoded in the schema. This extra semantic clarity justifies a score above the baseline of 3.

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 states a specific verb (set), resource (cover image), and context (grid tile display). It clearly distinguishes from siblings like add_screenshot by explaining that screenshotId promotes an existing screenshot, and it introduces the unique clear option. This is far above a tautology and gives an agent immediate understanding of the tool's role.

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?

The description provides strong guidance on when to use each parameter: 'screenshotId to promote a screenshot you already added, url for a public https image, base64 for raw bytes, or clear: true to remove the cover.' This implicitly tells the agent that for adding a new screenshot, it should use add_screenshot instead. The 'exactly one of' rule is explicit, but it does not explicitly name alternatives like add_screenshot, so it falls short of a 5.

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