region_changed

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

The description adds meaningful behavioral context beyond the readOnlyHint annotation: it explicitly says the tool 'Polls pixels', 'returns as soon as it changes', and waits. This tells the agent the tool is a polling/observation primitive, not an immediate action. No contradiction with annotations and no misleading claims.

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 sentences, front-loaded with the core behavior, then enriched with relevant use cases and mechanism. Every sentence adds value: one states purpose, one gives when-to-use, one explains the polling approach. No fluff or 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?

The description covers overall purpose, polling behavior, return timing, and main inputs, but no output schema leaves 'show it' somewhat ambiguous. It also does not address what happens when neither window nor region is provided, or how the tool signals a timeout. Given 5 params and low schema coverage, the description is solid but not fully complete.

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 only 40%, with window, timeout, and poll_seconds lacking schema descriptions. The description partially compensates by mentioning 'window or rectangle' and 'Polls pixels', which maps to window vs region and poll_seconds. However, it does not explain timeout semantics, defaults, units, or how window vs region interact, leaving an agent with meaningful gaps.

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 action ('Wait until a window or rectangle CHANGES, then show it') and clearly identifies the resource. It also differentiates itself from the sibling wait_for by saying 'For anything wait_for cannot express', so an agent can tell this tool apart from the most similar alternative.

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 gives concrete use cases ('a reply arriving, a spinner finishing, a download completing') and contrasts itself with taking blind screenshots, implicitly directing the agent away from screenshot-based polling. It does not explicitly name exclusions, but the context is clear enough for an agent to select this tool over wait_for or screenshot.

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