fill

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

With no annotations, the description carries the transparency burden and does well by disclosing the replace-always behavior and the Firefox-specific select-all workaround. It is a non-obvious behavioral detail that agents need to avoid surprising append behavior.

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 tight sentences front-load the main purpose and then add a valuable edge-case detail. Every clause earns its place; there is no filler 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?

For a simple two-parameter mutation with no annotations or output schema, the description covers the critical behavior (replacement and the Firefox nuance). It leaves minor unknowns such as return value or event dispatch, but these are not essential for a straightforward fill operation.

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 coverage is 100%, so a baseline of 3 applies. The description adds useful semantic context: ref is a ref ID targeting input/textarea elements, and value is used as replacement text, which goes beyond the schema's bare 'Element ref' and 'Text to fill'.

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 ('Fill') with a resource ('input/textarea') and a targeting method ('by ref ID'), making the core action clear. It does not explicitly differentiate from related siblings like type_text or smart_fill, so it falls 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?

It conveys an important usage trait: the tool always replaces existing content rather than appending. However, it provides no explicit guidance on when to choose this tool over siblings such as type_text, smart_fill, or fill_form.

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