Capabilities

A capability is a self-contained unit that extends an agent. Each capability can contribute three kinds of thing:

Tools — functions the agent can invoke during a turn.
System prompt additions — text prepended to the agent’s prompt that teaches the model when and how to use those tools.
Mount points — files, directories, or session state the tools need to operate.

Agents compose capabilities. Enable as many as you need; leave the rest disabled. The runtime resolves capabilities in topological order (dependencies first) and concatenates their system prompt fragments in the order you configured them on the agent.

Why capabilities, not just tools

A bare tool registration is a function with a JSON schema. Capabilities exist because real tools need more than that. To use virtual_bash effectively, the agent needs both the bash tool and the prompt fragment explaining the sandbox model and the session filesystem (a dependency). Capabilities bundle those concerns into one enable/disable unit.

See Why capabilities are first-class for the full rationale.

Where capabilities come from

Source	ID format	Example
Built-in	`snake_case`	`web_fetch`, `session_file_system`
MCP server	`mcp:{uuid}`	`mcp:550e8400-...`
Registry skill	`skill:{uuid}`	`skill:550e8400-...`
Declarative	`declarative:{name}`	`declarative:research_pack`

All four kinds participate in the same merge, dependency resolution, and tool-name prefixing. The agent doesn’t care where a tool came from.

Where to attach them

Capabilities can be attached at three layers, and the layers stack additively:

Harness — defaults for every session that uses this harness.
Agent — capabilities for this specific role.
Session — extras for this one conversation.

See Why three configuration layers for how the merge works.

Browse the catalog

The full list of built-in capabilities, organised by category, lives in the reference:

Capabilities reference — every capability with ID, tools, parameters, and dependencies.

Do something

Equip an agent with tools — the practical recipe.
Give an agent web access — narrower task with network policies.
Customize a harness — bundle capabilities at the harness layer.