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

描述披露了返回内容为 AX tree + refs，这对快照类工具是核心行为；但没有任何 annotations，且描述未说明该操作是否为只读、是否需要现有会话、失败时的行为等。信息足够基本使用，但不够透明。

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?

描述非常简短，主要信息前置，没有多余罗列。但“阶段 2：转发 BG 引擎”句式含糊，对实际调用帮助有限，略微削弱了表达效果。

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?

工具复杂度低，只有一个可选参数且无输出 schema；描述已覆盖返回值、调用场景和与相关工具的配合方式，足以让 Agent 判断何时调用。主要不足是“阶段 2”语义不明确，以及缺少会话前置条件说明。

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?

input schema 对唯一的 selector 参数有 100% 描述覆盖，描述本身没有对参数增加新的含义。在 schema 已充分说明参数的情况下，基线为 3 是合适的。

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?

描述以“获取页面可访问性快照（AX tree + refs）”明确了动词和资源，并说明其在感知→交互闭环中的角色。不过它没有直接与 bf_extract 或 bf_screenshot 区分，尽管“AX tree”本身暗含了区别。

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?

“Agent 用它感知页面结构，配合 bf_click/bf_fill 形成感知→交互闭环”给出了明确的使用时机和协同工具。但未说明何时不应使用或是否存在替代工具，因此不到满分。

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