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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), and the description layers on real behavioral context: the return shape (JPEG plus a text block with pixel and point size and a conversion formula), the auto-wake-on-black-frame behavior, and the session dependency that explains the speed. With no output schema, this disclosure is essential and it is delivered.

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?

Front-loaded with the 'PREFERRED fast screenshot' signal, and the scale-conversion warning, speed comparison, and session requirement each carry weight. It is dense and slightly long, but no sentence is filler.

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 read-only screenshot tool with no output schema, the description covers return format, the pixel-to-point conversion caveat, precondition (active session), and fallback tool. Nothing an agent needs to call it correctly is missing.

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% for the single udid parameter, so the schema already documents it fully. The description adds nothing about the parameter itself, 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 and resource ('fast screenshot of an iOS device via the MJPEG stream') and explicitly distinguishes itself from siblings ios_screenshot ('much faster than') and ios_fast_screenshot. An agent can pick it out without opening any 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 explicit when-to-use ('use it for routine per-action checks'), the alternative when the precondition is unmet ('with no session, use ios_fast_screenshot instead'), and the precondition itself ('requires an active iOS automation session'). Nothing 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.