> ## Documentation Index
> Fetch the complete documentation index at: https://docs.51ultron.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Execution Engine

> Ultron's sandboxed runtime: 50-minute sessions, 50+ tools, 50+ MCP servers, full browser access.

Ultron's execution engine is a custom-built sandboxed runtime that runs in isolated cloud environments. It's what makes long, complex, multi-step tasks possible — not a 3-minute timeout, but a 50-minute window with a full toolchain available from the start.

## What the sandbox provides

```text theme={null}
Sandbox session starts
├── 50+ built-in tools loaded
├── 70+ slash commands available
├── 50+ MCP servers pre-configured
│   ├── Fixed: Brave Search, Puppeteer, GitHub, Apify, Tavily, Fetch, Filesystem, Memory
│   └── User-connected: Notion, Slack, Stripe, Linear, HubSpot, Figma, and more
├── Full filesystem access (read, write, execute)
├── Full browser automation (Playwright via Puppeteer MCP)
└── 50-minute runtime window
```

## Tool execution pipeline

Every tool call follows the same pipeline:

```text theme={null}
Tool call received from Kimi K2
    ↓
validateInput()      — Zod schema validation against tool definition
    ↓
checkPermissions()   — Rule matching against allowed tool set
    ↓
call()               — Execute the tool (API call, shell command, browser action, etc.)
    ↓
onProgress()         — Stream live update to chat UI via SSE
    ↓
result()             — Formatted output returned for next loop iteration
```

## Parallel tool execution

When Kimi K2 returns multiple tool calls in a single turn, they execute concurrently. Independent operations don't wait for each other:

```typescript theme={null}
// Three searches fire simultaneously
const [companyData, ceoBackground, recentNews] = await Promise.all([
  web_search("Acme Corp overview"),
  web_search("Acme Corp CEO background"),
  web_search("Acme Corp news 2026")
]);
```

This is what makes research tasks run in minutes instead of sequentially chaining one search after another.

## Budget control

Large tool outputs are handled gracefully:

* Results exceeding **30,000 characters** are persisted to disk; the file path is returned instead
* The engine tracks token accumulation across all turns
* Compression triggers automatically before context limits are hit
* Sandbox sessions with heavy output (browser screenshots, large scraped pages) manage their own storage

## Sandbox vs API mode

|                | API Mode            | Sandbox Mode    |
| -------------- | ------------------- | --------------- |
| Runtime limit  | 3 minutes           | 50 minutes      |
| MCP servers    | Selected subset     | 50+ pre-loaded  |
| Browser access | Via Browserbase API | Full Playwright |
| Filesystem     | None                | Full read/write |
| Plans          | All plans           | Growth, Scale   |

<Tip>
  Complex multi-step tasks — building a full competitive report, running a lead enrichment batch, generating a content calendar with research — run in sandbox mode to take advantage of the extended runtime.
</Tip>