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

Goes well beyond the annotations by disclosing a substantial refusal policy (credential/shell-history/mail/browser-profile files, case-insensitive), the path validity requirement, and the side effect that the page receives normal input and change events. The readOnlyHint=false / destructiveHint=false annotations are consistent with this being a page-mutating but non-destructive action.

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?

Front-loads the purpose in the first sentence, then layers constraints. The security-refusal list is long but each entry is functionally load-bearing for an agent that will otherwise retry with a rejected path.

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 mutation tool with annotations and a fully documented schema, the description covers prerequisites (DevTools engine), failure modes (refused paths), and side effects (input/change events). No output schema exists, but the return value is not needed to invoke it correctly.

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 already 100%, so baseline is 3, but the description adds genuine meaning: paths must be absolute and exist, and the selector is advised to target the hidden input element on real-world sites. tab_id/nth/exact semantics are left to the schema.

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?

States a precise verb+resource: 'Set the files of an <input type=file>, as if chosen in the file picker.' This distinguishes it clearly from siblings like fill, type, and fill_form, which set text values rather than file inputs.

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?

Gives real operational guidance: paths must be absolute and point at existing regular files, it needs the DevTools engine, and for sites that hide the input behind a button you must select the hidden input element itself. It stops short of explicitly naming alternative tools for non-file form filling.

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