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T3rnel Browser — Session Bridge

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by t3ratech
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JavaScript
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session_screenshot

Read-onlyIdempotent

Capture the current visible viewport of a browser tab and return it as a base64 PNG or JPEG data URL. Use it to see exactly what is on screen in the signed-in session.

Instructions

Capture what is currently on screen in a tab. Returns a data: URL holding a base64 PNG or JPEG. This is the visible viewport only — it does not scroll or stitch, so a long page needs the extension's full-page capture instead.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
tabIdNoTab to capture; uses the active tab when omitted
formatNopng (default, lossless) or jpeg (smaller, and the only format quality applies to)png
qualityNoJPEG quality from 0 to 100; ignored for png
windowIdNoWindow to capture from; uses the current window when omitted

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed7 schema fields changedv1.2.0
    • addedInput schema / properties / format / default
      Added value: +"png"
    • addedInput schema / properties / format / description
      Added value: +"png (default, lossless) or jpeg (smaller, and the only format quality applies to)"
    • changedInput schema / properties / quality / description
      Previous value: -"JPEG quality 0-100"New value: +"JPEG quality from 0 to 100; ignored for png"
    • addedInput schema / properties / quality / maximum
      Added value: +100
    • addedInput schema / properties / quality / minimum
      Added value: +0
    • changedInput schema / properties / tabId / description
      Previous value: -"Tab to capture (defaults to the active tab)"New value: +"Tab to capture; uses the active tab when omitted"
    • addedInput schema / properties / windowId / description
      Added value: +"Window to capture from; uses the current window when omitted"
  2. First observedv1.1.1

TDQS

A4.7/5.0
Behavior5/5

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

Beyond the annotations, the description adds valuable behavioral context: the output is a data: URL containing base64 PNG or JPEG, and the capture is strictly the visible viewport with no scrolling or stitching. This meaningfully enriches the tool's behavior beyond what readOnlyHint/idempotentHint convey.

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?

Three compact sentences with no filler: the action comes first, then the return format, then the critical limitation and alternative. Every sentence earns its place.

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?

With no output schema, the description appropriately explains the return value. Combined with complete parameter schemas and supportive annotations, nothing needed for correct invocation 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 all four parameters including defaults and enum behavior. The description only restates the PNG/JPEG output, so it adds little beyond the structured parameter documentation.

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: 'Capture what is currently on screen in a tab.' It also clarifies the exact scope ('visible viewport only') and distinguishes itself from full-page capture, so an agent can understand what the tool does at a glance.

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?

The description explicitly says when not to use the tool ('does not scroll or stitch') and points to the alternative ('a long page needs the extension's full-page capture instead'). This gives clear usage guidance without relying on inference.

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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