browser-firefox-mcp

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set_storage_item

Set a key-value pair in localStorage or sessionStorage to manipulate browser session state and test storage-related security issues.

Instructions

Write a key-value pair to localStorage or sessionStorage.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "key": "<key>",
      "value": "<value>"
    },
    "name": "set_storage_item"
  }
}

Input Schema

Name	Required	Default
`key`	Yes
`store`	No	localStorage
`value`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

First observedv0.1.0Aug 20, 2026

TDQS

C2.9/5.0

Behavior2/5

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

With no annotations, the description bears the burden of disclosing side effects. It implies a mutation (writing) but does not mention overwrite behavior, persistence across sessions, or potential errors. This minimal transparency is insufficient for a state-changing operation.

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 a single, clear sentence with no redundancy or padding, achieving maximum efficiency.

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

Completeness2/5

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

The description does not mention return values, success/failure indications, error conditions, or security implications. Given the tool's simplicity, some basic completion (e.g., what happens on success) is expected, but it is absent.

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

Parameters2/5

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

The schema lists key, value, and store, but the description adds little beyond restating 'key-value pair' and the storage options. It does not clarify acceptable values for 'store' (e.g., 'localStorage' vs 'sessionStorage') or enforce any constraints, leaving ambiguity.

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 clearly states the action (write) and the resource (key-value pair to localStorage or sessionStorage), making the tool's purpose unambiguous and distinguishable from other browser automation tools.

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

Usage Guidelines1/5

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

No guidance is provided on when to use this tool versus alternatives, such as when localStorage is preferred over sessionStorage or when to use other storage-related tools. The description lacks any contextual or conditional instructions.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.