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firefox-devtools-mcp

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screenshot_page

Read-only

Capture a page viewport as a base64 PNG, or save it to a file. Set fullPage to include the entire scrollable document.

Instructions

Capture viewport screenshot as base64 PNG. Set fullPage for the whole scrollable document.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
saveToNoSave the screenshot to a file instead of returning it as image data in the response. Pass a file path, an existing directory (generated file inside), or true (generated file under ~/.firefox-devtools-mcp/output/). Relative paths resolve against the current working directory.
fullPageNoCapture the whole scrollable document (default: false)

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changedv0.10.3
    • addedInput schema / properties / fullPage
      Added value: +{
+  "description": "Capture the whole scrollable document (default: false)",
+  "type": "boolean"
+}
  2. Changed2 schema fields changedv0.9.15
    • changedInput schema / properties / saveTo / description
      Previous value: -"Optional file path to save the screenshot to instead of returning it as image data in the response."New value: +"Save the screenshot to a file instead of returning it as image data in the response. Pass a file path, an existing directory (generated file inside), or true (generated file under ~/.firefox-devtools-mcp/output/). Relative paths resolve against the current working directory."
    • changedInput schema / properties / saveTo / type
      Previous value: -"string"New value: +[
+  "boolean",
+  "string"
+]
  3. Changed1 schema field changedv0.9.3
    • addedInput schema / properties / saveTo
      Added value: +{
+  "description": "Optional file path to save the screenshot to instead of returning it as image data in the response.",
+  "type": "string"
+}
  4. First observed

TDQS

B3.3/5.0
Behavior3/5

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

Annotations already provide readOnlyHint=true, so the read-only nature is covered. The description adds the base64 PNG output format and fullPage behavior, which are useful. It does not mention potential side effects like forced layout or timing, but these are minor for a read-only capture.

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?

Two short sentences front-load the core action and output format, then explain the optional fullPage behavior. There is no filler, redundancy, or unnecessary detail.

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

Completeness3/5

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

For a simple two-optional-parameter tool, the description is adequate and the schema fills parameter details. However, it does not state explicitly that the screenshot targets the active page, and it does not point to screenshot_by_uid for element-level captures, leaving a notable gap in multi-page and sibling-tool contexts.

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%, and the schema already documents saveTo and fullPage well. The description merely restates fullPage's meaning without adding semantics for saveTo or any parameter edge cases, so it meets the baseline but adds no new value.

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

Purpose4/5

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

The description clearly states a specific verb and resource: 'Capture viewport screenshot as base64 PNG'. It also distinguishes full-page mode from the default viewport capture. However, it does not explicitly call out how this differs from the sibling screenshot_by_uid or that it targets the active page.

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

Usage Guidelines2/5

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

There is no guidance on when to use this tool versus screenshot_by_uid or other page-related tools. The only usage instruction is 'Set fullPage for the whole scrollable document,' which is a parameter tip rather than a tool-selection guideline.

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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