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Spawn

spawn

Opens an isolated stealth browser tab with its own cookies and identity, plus optional proxy or proxy-pool routing for scraping. Use it to launch separate sessions per site or task.

Instructions

Open stealth tab. browser_id='alice'=isolated Chrome (own cookies/identity, ~1.5s boot). For same-identity new pages prefer navigate (cheaper). stealth_mode='minimal' (default)=mimics vanilla Chrome, 'full'=adds anti-tracking noise.

Proxy: pass proxy='http://user:pass@host:port' for one-off, OR set use_proxy_pool=True (after proxy_pool_load) to pick from the pool — optionally filter by proxy_country='US' / proxy_tag='residential'. Pool wins over proxy if both set. Auth wired via CDP (Chrome flag strips inline auth).

Ex: spawn('https://news.ycombinator.com') → {"tab_id":"t0","browser_id":"default","url":"..."} Ex: spawn('about:blank', browser_id='alice', proxy='http://1.2.3.4:8080') Ex: spawn(use_proxy_pool=True, proxy_country='US', browser_id='scraper-1')

chromium: 'cloak' (default) auto-downloads CloakBrowser's patched chromium (C++ fingerprint patches — beats JS shims). 'stock'=system chromium. Pass an absolute path to use a custom binary. Env: UMBRA_NO_CLOAK=1 forces stock; UMBRA_CLOAK_BINARY= uses that as cloak. See cloak_status.

Tool call template

This tool declares no required parameters. Add optional parameters from the input schema as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {},
    "name": "spawn"
  }
}

Input Schema

Name	Required	Default
`url`	No	about:blank
`proxy`	No
`chromium`	No	cloak
`headless`	No
`timezone`	No
`proxy_tag`	No
`browser_id`	No
`low_memory`	No
`user_agent`	No
`stealth_mode`	No	minimal
`proxy_country`	No
`use_proxy_pool`	No

Output Schema

Name	Required	Description	Default
No arguments

Schema Changelog

Changes observed during successful MCP inspections.

First observedv0.1.0Oct 28, 2026

TDQS

A4.6/5.0

Behavior4/5

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

With no annotations, the description carries the burden and does well: it discloses boot cost (~1.5s), what stealth_mode levels actually do, that proxy auth is wired via CDP because the Chrome flag strips inline auth, and that 'cloak' auto-downloads a patched binary. It stops short of covering resource footprint (memory, low_memory) or when to close the spawned browser, which matters for a process-spawning tool.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loaded with the core action and the sibling alternative, then grouped by concern (proxy, examples, chromium). The three call examples earn their place by showing output shape and proxy/pool combinations, though the chromium/env-var paragraph adds bulk that could be tightened.

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?

An output schema exists and the examples show the {tab_id, browser_id, url} return shape, so return values needn't be re-explained. Given a 12-param process-spawning tool with no annotations, the definition covers selection, isolation, proxy routing, and binary choice well enough to call correctly.

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

Parameters4/5

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

Schema coverage is 0%, so the description must compensate, and it explains 8 of 12 params: url (via examples), browser_id, stealth_mode, proxy, use_proxy_pool, proxy_country, proxy_tag, chromium (including absolute-path and env-var behavior). headless, timezone, low_memory, and user_agent get no mention in either the schema or the description.

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?

"Open stealth tab" is a specific verb+resource, and the definition immediately scopes it with browser_id isolation semantics (~1.5s Chrome boot, own cookies/identity). It explicitly names the sibling it is not for "same-identity new pages" and points to `navigate`, so an agent can route without opening either schema.

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?

Explicit when-to-use/when-not: use spawn for a new isolated identity, prefer `navigate` for cheap same-identity pages. Proxy selection is equally explicit — one-off `proxy` vs `use_proxy_pool=True` (after `proxy_pool_load`), with the precedence rule "Pool wins over `proxy` if both set" spelled out.

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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