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httptoolkit-mcp

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Intercept Firefox Browser

intercept_firefox

Launch an isolated Firefox window to intercept and inspect HTTP(S) traffic for debugging purposes.

Instructions

Launch a fresh independent Firefox window with all HTTP(S) traffic intercepted. Uses an isolated profile.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
proxyPortYesProxy port to route traffic through

Implementation Reference

  • src/index.ts:148-158 (registration)
    Registration of the 'intercept_firefox' tool, which uses 'client.activateInterceptor' to perform the action.
    server.registerTool(
  'intercept_firefox',
  {
    title: 'Intercept Firefox Browser',
    description: 'Launch a fresh independent Firefox window with all HTTP(S) traffic intercepted. Uses an isolated profile.',
    inputSchema: z.object({
      proxyPort: z.number().describe('Proxy port to route traffic through'),
    }),
  },
  async ({ proxyPort }) => jsonResult(await client.activateInterceptor('fresh-firefox', proxyPort))
);

Tool Definition Quality

A3.5/5.0
Behavior3/5

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

With no annotations provided, the description carries the full burden. It discloses key behavioral traits: launching a fresh window, intercepting HTTP(S) traffic, and using an isolated profile. However, it lacks details on permissions needed, whether it modifies system settings, error handling, or what happens after launch (e.g., persistence, cleanup).

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, consisting of two sentences that directly convey the tool's purpose and key features without any wasted words. Every sentence earns its place by adding value.

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 tool's complexity (launching a browser with interception) and lack of annotations and output schema, the description is incomplete. It covers the core action but omits critical details like what 'intercepted' entails, how to access captured traffic, error conditions, or dependencies. More context is needed for safe and effective use.

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 the 'proxyPort' parameter. The description does not add any meaning beyond what the schema provides, such as explaining why this port is needed or how it integrates with interception. Baseline 3 is appropriate given high schema coverage.

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 specific action ('Launch a fresh independent Firefox window') and resource ('Firefox') with additional scope details ('with all HTTP(S) traffic intercepted', 'Uses an isolated profile'). It distinguishes from siblings like 'intercept_chrome' by specifying Firefox and from 'intercept_existing_terminal' by emphasizing a fresh window.

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?

No explicit guidance on when to use this tool versus alternatives is provided. While the description implies it's for intercepting Firefox traffic, it doesn't mention when to choose it over 'intercept_chrome' or other browser tools, nor does it specify prerequisites or exclusions.

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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