opencode

OpenCode — Agent Framework Reference

OpenCode is the AI agent framework running this environment. It manages agents, skills, tools, commands, sessions, providers, and plugins. Everything is configured via files in the config directory (/opt/opencode/ in the Kortix sandbox, or .opencode/ in a project).

Architecture Overview

┌─────────────────────────────────────────────────────┐
│  OpenCode                                            │
│                                                      │
│  ┌──────────┐  ┌──────────┐  ┌──────────┐          │
│  │  Agents   │  │  Skills   │  │  Tools    │          │
│  │ (.md)     │  │ (SKILL.md)│  │ (.ts)     │          │
│  └────┬─────┘  └────┬─────┘  └────┬─────┘          │
│       │              │              │                 │
│  ┌────▼──────────────▼──────────────▼─────┐          │
│  │              Session Engine              │          │
│  │  (prompt → model → tool calls → text)   │          │
│  └────────────────┬───────────────────────┘          │
│                   │                                   │
│  ┌────────────────▼───────────────────────┐          │
│  │            Provider Layer               │          │
│  │  (Anthropic, OpenAI, Kortix router)     │          │
│  └────────────────────────────────────────┘          │
│                                                      │
│  Config: opencode.jsonc  │  Plugins  │  MCP Servers  │
└─────────────────────────────────────────────────────┘

Agents

What is an Agent?

An agent is a persona — a system prompt combined with permission rules, model preferences, and behavioral constraints. Defined as .md files with YAML frontmatter.

Agent Definition File

Location: agents/*.md or agents/**/*.md in the config directory.

---
description: "Short description shown in UI and Task tool"
model: provider/model-id
mode: primary
permission:
  bash: allow
  edit: allow
  read: allow
  task: allow
  skill: allow
  # ... tool permissions
---

# Agent Name

System prompt content goes here. This entire markdown body
becomes the agent's system prompt.

Frontmatter Fields

Field	Type	Required	Description
`description`	string	Yes	Shown in UI and Task tool agent list
`mode`	`primary` / `subagent` / `all`	Yes	Controls where the agent appears
`model`	string	No	Default model as `provider/model-id` (e.g., `anthropic/claude-opus-4-6`)
`permission`	object	No	Tool permission rules (`allow` / `deny` per tool name)
`temperature`	number	No	Model temperature
`topP`	number	No	Top-P sampling
`steps`	number	No	Max tool-use steps per turn
`hidden`	boolean	No	Hide from UI

Agent Modes

Mode	User-selectable	Task tool	Use case
`primary`	Yes	Hidden from list, but can be spawned by name	Main agent the user interacts with
`subagent`	No	Listed and spawnable	Specialist agents for Task tool delegation
`all`	Yes	Yes	Available everywhere

Key insight: The Task tool's description filters out primary mode agents (a.mode !== "primary"), but Agent.get() (the execution path) has no mode guard. A primary agent CAN self-spawn by name via subagent_type: "agent-name".

Agent Name Derivation

The agent name is the filename without .md. For nested paths, only the filename matters:

agents/kortix.md → name: kortix
agents/specialised/my-agent.md → name: my-agent

Agent Loading Order

Built-in agents (compaction, title, summary — hidden infrastructure)
Config directory agents (agents/*.md)
Config overrides from opencode.jsonc "agent" section (can disable built-in agents)

Disabling Built-in Agents

In opencode.jsonc:

{
  "agent": {
    "build": { "disable": true },
    "plan": { "disable": true },
    "explore": { "disable": true },
    "general": { "disable": true }
  }
}

Agent Permission System

Permissions control which tools an agent can use. Rules are evaluated per-tool-call:

permission:
  bash: allow       # Allow all bash commands
  edit: allow       # Allow file editing
  task: allow       # Allow spawning subagents
  skill: allow      # Allow loading skills
  web-search: allow # Allow web search tool

Subagent sessions inherit parent permissions but can have additional restrictions. The Task tool hardcodes todowrite: deny and todoread: deny for all subagent sessions (lines 77-82 of task.ts).

Skills

What is a Skill?

A skill is a knowledge package — domain-specific instructions, workflows, and resources that inject into the agent's context when loaded via the skill() tool. Skills transform a general-purpose agent into a specialist on demand.

Skill Structure

skill-name/
├── SKILL.md          # Required: frontmatter + instructions
├── scripts/          # Optional: executable code
├── references/       # Optional: supplementary docs
└── assets/           # Optional: templates, images

SKILL.md Format

---
name: my-skill
description: "Detailed description of when to load this skill. Include trigger phrases."
---

# Skill Title

Instructions loaded into context when the skill is triggered.
These become part of the agent's working knowledge.

Frontmatter fields:

Field	Required	Description
`name`	Yes	Skill identifier (used in `skill({ name: "..." })`)
`description`	Yes	Trigger description — the agent reads this to decide when to load the skill

Skill Discovery

Skills are discovered from multiple locations (loaded in order, later overwrites earlier):

Global external dirs: ~/.claude/skills/**/SKILL.md, ~/.agents/skills/**/SKILL.md
Project external dirs: Walk up from project dir looking for .claude/skills/, .agents/skills/
OpenCode config dirs: {config}/skills/**/SKILL.md or {config}/skill/**/SKILL.md
Additional paths: From opencode.jsonc skills.paths array
URL downloads: From opencode.jsonc skills.urls array

Skill Loading Flow

On startup, all SKILL.md files are scanned and their name + description are extracted (~100 tokens each)
These descriptions are included in the skill tool's description (available_skills list)
The agent reads descriptions and decides when to load a skill
When loaded via skill({ name: "..." }), the full SKILL.md body is injected into context
Bundled files (scripts/, references/) are listed but not loaded — the agent reads them as needed

How the skill tool works

The skill tool (defined in src/tool/skill.ts):

Lists all accessible skills in its description (filtered by agent permissions)
When called with a skill name, reads the full SKILL.md content
Returns <skill_content name="..."> block with the content + skill base directory
Also lists up to 10 files in the skill directory for the agent to reference

Tools

What is a Tool?

A tool is a capability the agent can invoke — bash commands, file operations, web search, etc. Tools are defined in TypeScript and registered with the framework.

Built-in Tools

These are part of OpenCode core:

Tool	Description
`bash`	Execute shell commands
`read`	Read files
`edit`	Edit files (string replacement)
`write`	Write/create files
`glob`	File pattern matching
`grep`	Content search
`task`	Spawn subagent sessions
`skill`	Load skills
`todowrite`	Manage task list
`todoread`	Read task list
`question`	Ask user questions

Custom Tools

Custom tools are TypeScript files in tools/*.ts in the config directory. They use the Tool.define() API:

import { Tool } from "opencode/tool"
import z from "zod"

export default Tool.define("my-tool", async (ctx) => {
  return {
    description: "What this tool does",
    parameters: z.object({
      input: z.string().describe("Input parameter"),
    }),
    async execute(params, ctx) {
      // Do work
      return {
        title: "Short result title",
        output: "Full output text",
        metadata: { key: "value" },
      }
    },
  }
})

Tool Permissions

Tools respect the agent's permission rules. Each tool call checks:

Agent-level permissions (from agent frontmatter)
Session-level permissions (from parent session overrides)
Global permission setting (from opencode.jsonc "permission")

With "permission": "allow" in config, all tools are auto-approved.

MCP Tools

OpenCode supports Model Context Protocol (MCP) servers that provide additional tools:

{
  "mcp": {
    "context7": {
      "type": "remote",
      "url": "https://mcp.context7.com/mcp",
      "headers": { "CONTEXT7_API_KEY": "{env:CONTEXT7_API_KEY}" },
      "enabled": true
    }
  }
}

MCP tools appear alongside built-in and custom tools. The agent uses them the same way.

Commands

What is a Command?

A command is a slash-triggered workflow — a prompt template that routes to a specific agent with predefined instructions. Defined as .md files with YAML frontmatter.

Command Definition

Location: commands/*.md in the config directory.

---
description: "What this command does"
agent: kortix
---

# Command Prompt Template

This content becomes the prompt sent to the specified agent.
The user's input after the slash command is appended.

$ARGUMENTS - placeholder for user's input after the command name

Frontmatter Fields

Field	Type	Description
`description`	string	Shown in UI command palette
`agent`	string	Which agent handles this command
`model`	string	Override model for this command

Command Routing

When user types /research quantum computing:

OpenCode looks up commands/research.md
Reads the frontmatter to get the target agent
Sends the command's prompt template (with $ARGUMENTS replaced) to that agent
The agent executes with its full tool/skill access

Sessions

Session Lifecycle

CREATE → PROMPT → BUSY (tool calls, text generation) → IDLE → PROMPT again...
                                                      → ABORT
                                                      → DELETE

Session Data Model

{
  "id": "ses_...",
  "title": "Session Title",
  "projectID": "global",
  "directory": "/workspace",
  "parentID": "ses_...",           // Only for subagent sessions
  "permission": [...],             // Permission overrides
  "time": { "created": 123, "updated": 456 }
}

Subagent Sessions

When the Task tool spawns a subagent:

Creates a child session with parentID pointing to parent
Applies permission restrictions (todowrite/todoread denied, optionally task denied)
Sends the prompt to the child session
Waits for completion, returns the last text part

Storage Layout

All session data stored as JSON files:

.local/share/opencode/storage/
├── session/global/ses_*.json     # Session metadata
├── message/ses_*/msg_*.json      # Messages per session  
├── part/msg_*/prt_*.json         # Content parts per message
├── todo/ses_*.json               # Todo lists per session
└── tool-output/tool_*            # Large tool outputs

REST API

Base URL: http://localhost:3111

Method	Endpoint	Description
`GET`	`/session`	List all sessions
`POST`	`/session`	Create session
`GET`	`/session/{id}`	Get session
`DELETE`	`/session/{id}`	Delete session
`GET`	`/session/{id}/message`	All messages with parts
`POST`	`/session/{id}/message`	Send message (synchronous)
`POST`	`/session/{id}/prompt_async`	Send prompt (fire-and-forget)
`GET`	`/session/{id}/children`	List subagent sessions
`POST`	`/session/{id}/abort`	Abort running session
`GET`	`/session/status`	Map of busy sessions
`GET`	`/config`	Full config
`GET`	`/agent`	All agents
`GET`	`/provider`	Providers and models
`GET`	`/skill`	All skills with content
`GET`	`/event`	SSE event stream

SSE Events

GET /event returns a Server-Sent Events stream:

data: {"type":"session.status","properties":{"sessionID":"ses_...","status":{"type":"busy"}}}
data: {"type":"message.part.updated","properties":{"part":{"type":"text",...},"delta":"word "}}
data: {"type":"session.idle","properties":{"sessionID":"ses_..."}}

Event types: server.connected, session.status, session.idle, session.updated, message.updated, message.part.updated, session.diff, file.edited, command.executed

Configuration

opencode.jsonc

Main config file. In the Kortix sandbox: /opt/opencode/opencode.jsonc

{
  "$schema": "https://opencode.ai/config.json",
  "permission": "allow",                    // Auto-approve all tool calls
  "default_agent": "kortix",               // Default agent for new sessions
  "autoupdate": true,                       // Auto-update OpenCode
  "plugin": [                               // Plugins to load
    "opencode-pty",                         // PTY terminal support
    "./plugin/worktree.ts",                 // Git worktree management
    "./plugin/memory.ts"                    // Memory observation system
  ],
  "agent": {                                // Agent overrides
    "build": { "disable": true },           // Disable built-in agents
    "plan": { "disable": true },
    "explore": { "disable": true },
    "general": { "disable": true }
  },
  "provider": {                             // LLM providers
    "kortix": {
      "npm": "@ai-sdk/openai-compatible",
      "options": { "baseURL": "{env:KORTIX_API_URL}/v1/router", "apiKey": "{env:KORTIX_TOKEN}" },
      "models": {
        "kortix/basic": { "name": "Kortix Basic", "cost": { "input": 3, "output": 15 } },
        "kortix/power": { "name": "Kortix Power", "cost": { "input": 5, "output": 25 } }
      }
    }
  },
  "mcp": {                                  // MCP server connections
    "context7": { "type": "remote", "url": "https://mcp.context7.com/mcp" }
  }
}

Config Discovery

OpenCode searches for config in these directories (merged in order):

.opencode/ in the current directory and parent directories
$OPENCODE_CONFIG_DIR (e.g., /opt/opencode/)
Global: ~/.config/opencode/

Environment Variable Interpolation

Config values support {env:VAR_NAME} syntax for environment variable interpolation:

{ "baseURL": "{env:KORTIX_API_URL}/v1/router" }

Plugins

Plugins extend OpenCode with custom tools, hooks, and functionality:

npm plugins: Referenced by package name (e.g., "opencode-pty")
local plugins: Referenced by path (e.g., "./plugin/memory.ts")

Plugins can register tools, listen to events, modify behavior.

Provider System

How Providers Work

Providers connect OpenCode to LLM APIs. They use the Vercel AI SDK provider pattern:

{
  "provider": {
    "provider-name": {
      "npm": "@ai-sdk/openai-compatible",
      "name": "Display Name",
      "options": { "baseURL": "...", "apiKey": "..." },
      "models": {
        "provider/model-id": {
          "name": "Model Name",
          "cost": { "input": 3, "output": 15 },
          "limit": { "context": 200000, "output": 8192 }
        }
      }
    }
  }
}

Model References

Models are referenced as provider/model-id throughout the system:

In agent frontmatter: model: anthropic/claude-opus-4-6
In API calls: { "providerID": "kortix", "modelID": "kortix/basic" }
In config: "model": "anthropic/claude-sonnet-4-20250514"

Key Files

Path	Description
`opencode.jsonc`	Main configuration
`agents/*.md`	Agent definitions
`skills/*/SKILL.md`	Skill definitions
`tools/*.ts`	Custom tool implementations
`commands/*.md`	Slash command definitions
`plugin/*.ts`	Local plugins

About this skill

Architecture Overview

Agents

What is an Agent?

Agent Definition File

Frontmatter Fields

Agent Modes

Agent Name Derivation

Agent Loading Order

Disabling Built-in Agents

Agent Permission System

Skills

What is a Skill?

Skill Structure

SKILL.md Format

Skill Discovery

Skill Loading Flow

How the skill tool works

Tools

What is a Tool?

Built-in Tools

Custom Tools

Tool Permissions

MCP Tools

Commands

What is a Command?

Command Definition

Frontmatter Fields

Command Routing

Sessions

Session Lifecycle

Session Data Model

Subagent Sessions

Storage Layout

REST API

SSE Events

Configuration

opencode.jsonc

Config Discovery

Environment Variable Interpolation

Plugins

Provider System

How Providers Work

Model References

Key Files