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Browser Session MCP

by xav-ie
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Rust
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scroll

Scroll a browser session's active tab with a real mouse-wheel event at a viewport position, revealing lazy-loaded content or adding human-like behavioral signals. Point inside a scrollable panel to scroll that element instead of the page.

Instructions

Scroll the session's active tab with a real mouse-wheel event at a viewport position; whatever scrollable element is under (x,y) scrolls, so point inside a scrollable panel to scroll it rather than the page. deltaY and deltaX are in pixels; negative values scroll up or left. Useful to reveal lazy-loaded content and to generate human-like behavioral signals. Don't use it just to reach an element before clicking: click scrolls its target into view itself. Returns immediately without waiting for content to load; follow with wait_for if needed.

Input Schema

NameRequiredDescriptionDefault
xNoWheel position from the viewport's left edge in CSS pixels. Default 50.
yNoWheel position from the viewport's top edge in CSS pixels. Default 50.
deltaXNoHorizontal pixels to scroll; negative scrolls left. Default 0.
deltaYNoVertical pixels to scroll; negative scrolls up. Default 600.
sessionIdYesThe id returned by open_browser_session. Errors if no open session has it.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed5 schema fields changed
    • addedInput schema / properties / deltaX / description
      Added value: +"Horizontal pixels to scroll; negative scrolls left. Default 0."
    • addedInput schema / properties / deltaY / description
      Added value: +"Vertical pixels to scroll; negative scrolls up. Default 600."
    • addedInput schema / properties / sessionId / description
      Added value: +"The id returned by open_browser_session. Errors if no open session has it."
    • addedInput schema / properties / x / description
      Added value: +"Wheel position from the viewport's left edge in CSS pixels. Default 50."
    • addedInput schema / properties / y / description
      Added value: +"Wheel position from the viewport's top edge in CSS pixels. Default 50."
  2. First observedv0.1.2

TDQS

A4.7/5.0
Behavior4/5

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

Annotations cover the safety profile (readOnlyHint=false, openWorldHint=true, non-idempotent, non-destructive), and the description adds the timing behavior that annotations cannot express: it returns immediately without waiting for content to load. It does not discuss rate limits or failure modes, but the key operational caveat is present.

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?

Front-loaded with the core mechanism, then the targeting rule, then units, then use cases, then the exclusion. Every sentence carries distinct information and none repeats the schema verbatim.

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 five-parameter interaction tool with no output schema, the description covers the mechanism, targeting, units, timing behavior, follow-up requirement, and anti-pattern. Nothing an agent needs in order to call 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 schema documents defaults and units. The description still adds real semantics beyond the schema: the delta units and sign direction, and crucially that x/y select which scrollable panel receives the wheel event rather than being arbitrary coordinates.

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 (scroll) plus the mechanism (real mouse-wheel event at a viewport position), and explains the targeting rule that the element under (x,y) scrolls rather than the page. This is clearly distinguishable from the sibling click and move_mouse 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?

Explicitly names both when to use it (revealing lazy-loaded content, generating human-like behavioral signals) and when not to ('Don't use it just to reach an element before clicking'), and names the alternative behavior that replaces it (click scrolls its target into view). It also prescribes the follow-up pattern with wait_for.

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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