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

No annotations, so the description carries the burden. It does disclose the return ('Returns the new screen'), which is valuable, but says nothing about how far a scroll moves, what happens at page boundaries, or whether the session must already be open (only the schema covers the session param).

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 short sentences, front-loaded with the verb and its two modes, with the return value tacked on last. Nothing 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?

The absence of an output schema is partly compensated by 'Returns the new screen', and the rail/page distinction is covered. However, for a UI-driving tool with no annotations and a required session, missing details on scroll granularity and edge behavior leave a 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 67% and the description adds real meaning the enum alone lacks: down/up are for the page, left/right for a rail, and ref must point at a rail. That maps direction semantics to the ref parameter in a way the schema does not.

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 (scroll) plus the two resource scopes: the page vertically and a horizontal rail. It's clearly distinguishable from siblings like tap, screen, or screenshot, though it doesn't explicitly name those alternatives.

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 parenthetical '(down/up)' vs '(right/left, give the rail's ref)' implicitly tells the agent which direction set applies where, which is useful conditional guidance. There's no explicit when-not-to-use or comparison to alternatives like screenshot/screen for re-checking state.

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