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mozilla

firefox-devtools-mcp

by mozilla
OverviewSchemaRelated ServersScoreDiscussions
TypeScript
Local

screenshot_page

Read-only

Capture the current page as a base64-encoded PNG screenshot. Optionally save it to a file to avoid returning image data in the response.

Instructions

Capture page screenshot as base64 PNG.

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.

Schema Changelog

Changes observed during successful MCP inspections. Dates show when Glama detected each change.

  1. 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"
+]
  2. 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"
+}
  3. First observed

TDQS

B3.3/5.0
Behavior3/5

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

Annotations already declare readOnlyHint=true, so the description does not need to state that it's a read operation. The description adds output format (base64 PNG), which is useful. However, it does not describe what the return value looks like or any side effects (e.g., does it capture full page or viewport?). With annotations providing some transparency, a score of 3 is appropriate.

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?

The description is extremely concise and front-loaded: one sentence that captures the essence. No superfluous information. Score 5 for efficient structure.

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?

Given the simplicity of the tool (one optional parameter, no output schema), the description provides minimal but adequate context: purpose and output format. However, it lacks details like whether it captures the full page or viewport, and does not mention related tools. For a tool with annotations, it is moderately complete but leaves some gaps.

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?

The input schema has 100% description coverage for the only parameter 'saveTo', with a detailed explanation. The tool description does not add any extra meaning beyond the schema. Thus, baseline 3 is correct.

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 the action ('Capture page screenshot') and the output format ('base64 PNG'), which is specific and informative. However, it does not distinguish from the sibling tool 'screenshot_by_uid', which likely captures a specific element. Score 4 because it is clear but lacks sibling differentiation.

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?

The description provides no guidance on when to use this tool vs. alternatives like 'screenshot_by_uid' or other screenshot-related tools. There is no mention of context, when-not-to-use, or prerequisites. This is a significant gap for an agent to decide between similar tools.

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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