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DB-25

arc-control-mcp

by DB-25
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JavaScript
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Fill Field

fill

Set the value of an input, textarea, or contenteditable element and fire input/change events so React and other frameworks register the change.

Instructions

Set the value of an input, textarea or contenteditable. Uses the native setter and fires input and change, so React and similar frameworks register it. Fails with an error naming the tag when the target cannot be filled: a heading or other non-input, a disabled or readonly field, a checkbox, radio or button (use click), a (use select_option), or a field that rejects the value (a number input given text). "text=Label" matches on visible text as a substring, with exact matches ranked first and visible elements ahead of hidden ones, so a short label also matches longer ones. Check the returned "matches" count, and pass exact when it is above 1. Also accepts "ref=e12" from snapshot (re-resolved and reported as reResolved when the page re-rendered, a clear stale-ref failure when the element is truly gone), "role=button[name="Save"]", "label=Email" and "placeholder=Search". On success it waits for the page to go quiet and reports settledMs (and settled false if the DOM was still changing at the cap), so the next read sees the result of the action.

Input Schema

NameRequiredDescriptionDefault
nthNoWhich match to fill, 0-based
exactNoFor "text=", "label=" and "placeholder=" selectors, require the whole trimmed text to equal the label instead of containing it. No effect on CSS, ref= or role= selectors.
valueYesValue to set
submitNoPress Enter, then submit the form if the page did not. Fails with ok false when nothing was submitted, or when the form fails its own validation (the invalid fields are listed). The tab usually navigates, so follow with wait_for_load: the returned tab snapshot may predate the navigation, and urlBefore reports the url from just before the key press.
tab_idNoArc tab id from list_tabs. Omit to use this agent's current tab. A tool that changes a tab will not fall back to the tab the user is looking at, so pass this or call open_url first.
verboseNoInclude the bulky element rect and longer attribute values
selectorYesCSS selector, or one of: "text=Some label" (visible text, case-insensitive substring, exact matches ranked first); "ref=e12" (a ref from snapshot, re-resolved if the page re-rendered); "role=button[name="Save"]" (ARIA or implicit role, name an exact match, or name~="sav" for a substring); "label=Email" (the control a label names); "placeholder=Search"
settle_msNoAfter acting, wait until the page's DOM has been quiet for this many ms, then report settledMs. 0 skips the wait. The whole wait is capped at 1500ms; a page that never goes quiet reports settled false.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed5 schema fields changedv0.4.0
    • changedInput schema / properties / exact / description
      Previous value: -"For \"text=\" selectors, require the whole trimmed text to equal the label instead of containing it. No effect on CSS selectors."New value: +"For \"text=\", \"label=\" and \"placeholder=\" selectors, require the whole trimmed text to equal the label instead of containing it. No effect on CSS, ref= or role= selectors."
    • changedInput schema / properties / selector / description
      Previous value: -"CSS selector, or \"text=Some label\" to match on visible text (substring, exact matches ranked first)"New value: +"CSS selector, or one of: \"text=Some label\" (visible text, case-insensitive substring, exact matches ranked first); \"ref=e12\" (a ref from snapshot, re-resolved if the page re-rendered); \"role=button[name=\"Save\"]\" (ARIA or implicit role, name an exact match, or name~=\"sav\" for a substring); \"label=Email\" (the control a label names); \"placeholder=Search\""
    • addedInput schema / properties / settle_ms
      Added value: +{
+  "default": 300,
+  "description": "After acting, wait until the page's DOM has been quiet for this many ms, then report settledMs. 0 skips the wait. The whole wait is capped at 1500ms; a page that never goes quiet reports settled false.",
+  "maximum": 1500,
+  "minimum": 0,
+  "type": "number"
+}
    • changedInput schema / properties / submit / description
      Previous value: -"Press Enter and request form submit afterwards. The tab usually navigates, so follow with wait_for_load: the returned tab snapshot may predate the navigation, and urlBefore reports the url from just before the key press."New value: +"Press Enter, then submit the form if the page did not. Fails with ok false when nothing was submitted, or when the form fails its own validation (the invalid fields are listed). The tab usually navigates, so follow with wait_for_load: the returned tab snapshot may predate the navigation, and urlBefore reports the url from just before the key press."
    • changedInput schema / properties / tab_id / description
      Previous value: -"Arc tab id from list_tabs. Omit to use this agent's current tab, falling back to whatever tab is active in Arc."New value: +"Arc tab id from list_tabs. Omit to use this agent's current tab. A tool that changes a tab will not fall back to the tab the user is looking at, so pass this or call open_url first."
  2. First observedv0.3.0

TDQS

A4.6/5.0
Behavior4/5

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

Annotations cover the safety profile (non-destructive, not read-only, open-world), and the description adds substantial behavior beyond them: native setter, input+change events for React, specific failure modes, ref re-resolution with reResolved/stale reporting, and settledMs/settled=false on the post-action quiet wait. It stops short of describing auth or rate-limit concerns, but for a DOM action tool that is not a meaningful gap.

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

Conciseness4/5

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

Front-loads the action and the routing to alternatives, and nearly every sentence carries actionable detail. However it is a single very dense paragraph with long compound sentences, and a few clauses (settle cap details, verbose) restate schema content, so it is slightly overpacked rather than tight.

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 mutation tool with no output schema, the description covers the action, framework compatibility, all named failure modes, ref staleness handling, and the post-action settle contract including settledMs and settled=false. Nothing an agent needs to invoke and interpret it correctly is missing.

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 already 100%, so the baseline is 3, but the description adds real meaning: ranking rules for text= matches (exact first, visible before hidden), what exact affects and does not affect, and reResolved behavior for ref=. Some of this duplicates the schema descriptions, so it is a modest lift over baseline rather than a full one.

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?

Opens with a precise verb+resource ("Set the value of an input, textarea or contenteditable") and immediately distinguishes itself from siblings by naming click for checkboxes/radios/buttons and select_option for <select>. An agent can route correctly without opening another schema.

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?

Explicit when-not-to-use guidance (checkbox, radio, button → click; <select> → select_option) plus a workflow rule: check the returned matches count and pass exact when it exceeds 1. Alternatives and the condition that selects them are stated outright.

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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