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

Annotations already declare destructiveHint=true, idempotentHint=true and openWorldHint=true, so the safety profile is covered. The description adds real value beyond them: the change is a full-document replacement (not a merge), it becomes immediately visible for human review, and it requires a specific plan and auth scope.

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?

Three tight sentences, front-loaded with the operation, then the workflow, then the constraints. Every sentence carries distinct information with no repetition.

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 2-required-param nested-object mutation with no output schema, the description covers operation semantics, workflow, side effects (immediate visibility for review), and auth/plan requirements. Only failure/error behavior and the relationship to update_screenshot_set are left unstated.

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 100%, and the schema's 'layout' description already spells out full-replacement semantics and the inline-image-placeholder caveat. The description reinforces the whole-document replace but adds no new syntax or format detail beyond the schema, so the baseline 3 applies.

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 ('Replace') and resource ('one screen's layout in a screenshot set'), and the scope ('one screen') implicitly distinguishes it from the sibling sonar_update_screenshot_set. An agent can pick between them without opening either schema.

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 clear context: whole-document replace, the fetch-modify-send workflow, and prerequisites (Indie plan, authorized account or write-scope key). It does not explicitly name update_screenshot_set as the alternative for set-level edits, so routing between the two is left to inference.

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