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agent-in-browser

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fill_form

Sets multiple form field values in one browser call—text inputs, textareas, selects, checkboxes, and radios—using refs, selectors, or visible labels; it does not submit the form.

Instructions

Set the values of several form fields in one call: text inputs and textareas (string), selects (option value or visible label), checkboxes and radios (true/false). Each field is located by ref, selector or visible label and scrolled into view; input and change events are fired so React, Vue and Angular see the update. Returns one line per field with the value set, and stops with an error naming the failing fields[i]. It does not submit the form: click the submit button afterwards. For typing key by key use type.

Input Schema

NameRequiredDescriptionDefault
tabNoTab id from tabs_list. Defaults to the current tab.
fieldsYesFields to set, in order. Each needs a target (ref, selector or match) and a value.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed5 schema fields changedv0.1.2
    • changedInput schema / properties / fields / description
      Previous value: -"Fields to set."New value: +"Fields to set, in order. Each needs a target (ref, selector or match) and a value."
    • changedInput schema / properties / fields / items / properties / match / description
      Previous value: -"Visible text / accessible name to match (case-insensitive substring)."New value: +"Visible text or accessible name to match (case-insensitive substring), e.g. \"Sign up\"; use when you have no ref."
    • changedInput schema / properties / fields / items / properties / ref / description
      Previous value: -"Element ref from snapshot/find, e.g. \"e12\". Preferred."New value: +"Element ref from the latest snapshot or find, e.g. \"e12\". Preferred: unambiguous and fastest; stale after navigation."
    • changedInput schema / properties / fields / items / properties / selector / description
      Previous value: -"CSS selector (first match)."New value: +"CSS selector; the first matching element is used."
    • changedInput schema / properties / fields / items / properties / value / description
      Previous value: -"Value: string, number or boolean."New value: +"Value to set: a string or number for inputs and selects, true/false for checkboxes and radios."
  2. First observedv0.1.1

TDQS

A4.9/5.0
Behavior5/5

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

Beyond the annotations, the description discloses scrolling into view, firing input/change events for React/Vue/Angular, returning one line per field, stopping with an error that names `fields[i]`, and not submitting the form. These are meaningful behavioral details for a browser form mutation tool.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is dense but front-loaded, starting with what the tool does and immediately covering value types, targeting, side effects, return behavior, and the non-submission caveat without filler.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a multi-field form mutation tool with no output schema, the description covers the important missing pieces: target resolution, supported value types, event firing, return format, error behavior, and the required follow-up submit action.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so the baseline is 3. The description adds useful semantic detail: selects accept an option value or visible label, checkboxes/radios take true/false, and fields are set in order using ref, selector, or visible label.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific verb and resource—setting values of several form fields in one call—and distinguishes the tool from siblings by explaining that it does not submit the form and that key-by-key typing is handled by `type`.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It explicitly says when to use this tool versus `type`, and it tells the agent to use `click` afterwards to submit the form. It also explains supported field types and how targets are located.

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

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