element_action

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

With zero annotations the description carries the full burden and does disclose meaningful behavior: ambiguous/stale/disabled targets fail explicitly rather than silently picking a match, and the return is a compact ack rather than a UI tree. It stops short of stating prerequisites (whether a prior snapshot_id is required, which actions demand a `value`) or what timeout/duration_ms actually govern.

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 dense sentences with the action vocabulary front-loaded and no filler; the failure-mode and return-shape points follow in priority order. Phrasing like "pay only for the delta" is idiomatic but still clear in context.

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?

An output schema exists, so the ack need not be expanded, and the description covers the core select-only (no annotation) behavioral story. For a mutation tool with seven parameters it still leaves gaps: text-entry prerequisites (value for set_text/append_text), timeout semantics, and what duration_ms affects are unaddressed.

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 0% at the top level, so the description must compensate — and it does add real value by supplying the `action` enum that is entirely absent from the schema, plus the semantics of `return_ui`. However `value`, `timeout`, `duration_ms`, and `press_enter` are never explained, leaving four of seven parameters undocumented anywhere outside their names.

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 opens with a specific verb-plus-scope ("Act on exactly one matching element") and enumerates the exact action vocabulary (tap/long_click/double_click/set_text/.../read), so an agent knows precisely what surface it covers. It does not name the near-siblings (tap_element, input_text, act) to differentiate itself, which keeps it short of a 5.

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?

There is explicit when/when-not guidance for one parameter: set return_ui=true only when the next step needs the whole page, otherwise pick up the delta via ui_snapshot(since_snapshot_id=...). Nothing tells the agent when to reach for element_action instead of tap_element, input_text, or fill_form, so sibling selection 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.