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open_tab

Open a new browser tab with a given URL and get its tab ID, so you can work on another site without losing the current page.

Instructions

Open a new browser tab with the given URL. Returns the new tab ID. When to use: when you need to work on a different site without losing the current page. Related: switch_tab (switch between open tabs), list_tabs (see all open tabs), navigate (change URL in current tab instead). Multi-agent: agent-scoped tabs; cross-agent reject with TAB_NOT_OWNED; cap configurable (default 8, 1-64). Active behavior: defaults to background (active=false); pass active:true to steal focus. RETURNS change_report: when this tool runs, the response includes a change_report field with a compact diff of what the action mutated (URL, dialogs_opened, nodes_added, nodes_removed, attrs_changed, inputs_changed, focus_shift). Use this to learn the consequence without calling read_page next. If the report exceeds the size cap, change_report.truncated:true and change_report_hint are set; call read_page for the full state.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
urlYesURL to open in new tab
activeNoWhen true, the new tab is foregrounded (steals focus). Multi-agent default is FALSE (background) so this agent does not steal focus from the user or another agent. Set true ONLY when the new tab needs immediate user visibility.
clientYesAllowlisted client label. Validated against the shared v0.9.36 server/extension allowlist (see mcp/src/tools/visual-session.ts -- MCP_VISUAL_CLIENT_LABELS). Required.
is_finalNoWhen true, the visual session clears immediately after this tool resolves. Optional (default false).
visual_reasonYesShort human-readable reason shown in the overlay (for example, "Logging in to GitHub"). Required.

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.1.0

TDQS

A4.6/5.0
Behavior5/5

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

No annotations are provided, so the description carries the full burden and does so thoroughly: agent-scoped tabs with TAB_NOT_OWNED rejection, configurable cap (default 8, 1-64), default background vs active:true focus-stealing, and the change_report return semantics including truncation handling. This is exactly the operational context an agent needs for a state-mutating UI action.

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-loaded with the core purpose and return, then organized into labeled subsections (When to use, Related, Multi-agent, Active behavior, RETURNS). Dense but every section is functional; the change_report paragraph is slightly verbose but earns its place given no output schema.

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?

No output schema exists, so the description properly documents the return (change_report fields, truncation, read_page fallback), plus multi-agent constraints and focus semantics. Nothing an agent needs to invoke this correctly is missing.

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

Parameters3/5

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

Schema description coverage is 100%, so the schema already documents url, active, client, is_final, and visual_reason. The description reiterates active's default and adds the change_report return behavior, but adds little parameter-level meaning beyond what the schema provides. Baseline 3 is appropriate.

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?

States a specific verb+resource ('Open a new browser tab with the given URL') and immediately differentiates from siblings by name (switch_tab, list_tabs, navigate). An agent can tell exactly what this does and how it differs from adjacent tab/navigation tools.

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 to use' condition ('work on a different site without losing the current page') plus a Related list that names the alternatives and their distinguishing behavior (navigate changes URL in current tab instead). No inference required to choose between them.

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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