session_fill_selector

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

With no annotations, the description carries the full burden and does so well: it discloses the React/Vue-compatible Playwright behavior, the session prerequisite, the replacement semantics of the text field, and the failure codes 409/404/502 with causes. It also states the successful return shape {ok: true}.

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?

The description is front-loaded with the core action, followed by important behavioral context, an Args section, and failure behavior. Despite its length, every sentence adds information not present in the bare schema or annotations.

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 three-required-parameter mutation tool with no annotations, no output schema, and no schema descriptions, the description is complete: it covers prerequisites, parameter meaning, behavioral traits, success return, and error mapping. An agent has enough information to invoke it correctly and interpret common failures.

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%, so the description must compensate, and it does for every parameter. It explains name as an identity exactly matching profiles_list, selector as a CSS selector (not XPath), and text as replacing existing content.

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 verb and resource: fill a text field on the session's current page, located by a CSS selector. It distinguishes this from direct DOM manipulation by noting Playwright's fill() handles React/Vue-controlled inputs correctly, and it explicitly rules out XPath selectors.

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?

It clearly says a session must first be opened with profile_start and gives a 409 fallback instruction. It also notes the selector must be CSS, not XPath. It does not explicitly route the agent to sibling tools like session_click_selector, but the fill-specific context is clear.

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