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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds meaningful non-schema behavior: the observation yields ref numbers that are the addressing scheme for the act tool, which is exactly the workflow an agent needs. It does not mention snapshot freshness or timing limits.

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, front-loaded with what the tool returns and followed by the routing rule. Every phrase earns its place with no boilerplate or redundancy.

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?

With no output schema, the description must describe the return shape, and it does so concretely (elements, ref, text, center, screenshot). It is nearly complete for a two-parameter read tool; only snapshot/timing behavior and the phone-state prerequisite are 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%: phone_id and screenshot are both fully documented in the schema, including the default value and the 720px width. The description's "plus a screenshot when asked" merely echoes the schema, adding no new syntax or semantics.

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 ("Read the screen") and then defines exactly what that returns: visible elements with a ref number, text, and center, plus an optional screenshot. It also distinguishes itself from the primary confusable sibling by explicitly routing tapping to the act tool.

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 second sentence gives a clear workflow rule: observe to read the screen, then use act to tap elements by ref. It does not state any exclusions or prerequisites (e.g., needing an already-started phone), so it stops short of full when/when-not guidance.

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