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Capabilities

The registry of tools and behaviors an agent can use — execution, sandboxes, storage, data, safety, and optimization — as composable units.

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What a capability is

A capability is a self-contained unit that extends an agent. A single capability can contribute tools the model can call, additions to the system prompt, and mount points the agent operates against. Instead of hard-coding what an agent can do, you attach capabilities and the agent gains exactly those behaviors.

This keeps the agent definition small. An agent is configuration — a prompt, an optional model override, and the set of capabilities it is allowed to use.

Where capabilities come from

Capabilities are resolved from four origins, so the same attachment mechanism covers built-in behavior and your own extensions:

  • Built-in — first-party capabilities shipped with the platform, named in snake_case (for example web_fetch, bashkit_shell, session_file_system).
  • MCP servers — tools and context exposed over the Model Context Protocol, surfaced as capabilities.
  • Registry skills — reusable instruction-and-tool packages discovered from the workspace.
  • Declarative — capabilities defined as configuration rather than code.

Categories

Every built-in capability declares a category. The main groups are:

  • Core — session basics, current time, and message metadata.
  • Execution and Sandboxes — bashkit_shell for in-process shell, plus isolated sandboxes via Daytona, E2B, Deno, and Docker. See Sandboxes.
  • File Operations and Storage — session_file_system for a per-session filesystem and session_storage for key/value and encrypted secrets.
  • Data and Knowledge — session-scoped SQL databases and knowledge bases.
  • Orchestration — sub-agents, schedules, and agent handoff.
  • Optimization — tool search, budgeting, parallel tool calls, and extended context.
  • Safety — guardrails, prompt-injection canaries, and tool-call repair.
  • MCP Servers — external tools and context attached over the Model Context Protocol.

Attachment layers

Capabilities are attached at three levels, each narrowing the one above it:

  • Harness — defaults that every agent on the harness inherits.
  • Agent — capabilities for a specific role, on top of the harness defaults.
  • Session — capabilities scoped to a single conversation.
  • Sandboxes — the isolated execution environments behind the sandbox capabilities
  • Integrations — external services Everruns connects to
  • Capability reference — every built-in capability with its tools, parameters, and dependencies