harness

A Go library for calling different AI coding agent CLIs (Docker Agent, Claude Code, Pi, Codex, opencode) through a unified interface. Switch between agents by changing a single line of code.

Install

go get github.com/rumpl/harness

Quick start

package main

import (
	"context"
	"fmt"

	"github.com/rumpl/harness"
	"github.com/rumpl/harness/dockeragent"
)

func main() {
	// Create a provider — swap this line to switch agents.
	p := dockeragent.New("coder")

	// Run the agent and handle streaming events.
	harness.Run(context.Background(), p, "Explain goroutines", func(ev harness.Event) {
		switch ev.Type {
		case harness.EventText:
			fmt.Print(ev.Text)
		case harness.EventToolCallStart:
			fmt.Printf("[tool: %s]\n", ev.ToolName)
		case harness.EventToolCallDelta:
			fmt.Print(ev.ToolArgs)
		case harness.EventToolCall:
			fmt.Printf("[tool: %s] %s\n", ev.ToolName, ev.ToolArgs)
		case harness.EventToolResult:
			fmt.Printf("[tool result: %s] %s\n", ev.ToolName, ev.ToolOutput)
		case harness.EventResult:
			fmt.Printf("\nResult: %s\n", ev.Result)
		}
	})
}

Switching providers

// Docker Agent
p := dockeragent.New("coder")

// Claude Code
p := claudecode.New("claude-sonnet-4-6", claudecode.WithEffort(claudecode.EffortHigh))

// Pi
p := pi.New("claude-sonnet-4-6")

// Codex
p := codex.New("gpt-5.4-mini")

// opencode
p := opencode.New("anthropic/claude-sonnet-4-6")

The rest of your code stays exactly the same — all providers implement harness.Provider.

For providers whose CLIs have their own default model, pass an empty model string to omit the model flag entirely (for example, codex.New("") emits codex exec ... without -m).

The Provider interface

type Provider interface {
	Name() string
	PrintCommand(prompt string) string
	InteractiveArgs(prompt string) []string
	ParseStreamLine(line string) []Event
}

Method	Purpose
Name()	Human-readable identifier ("docker-agent", "claude-code", etc.)
PrintCommand()	Shell command for non-interactive mode (sh -c safe)
InteractiveArgs()	Arg list for interactive mode (first element = binary)
ParseStreamLine()	Parse one NDJSON line into []Event

Event types

Type	Fields set
EventText	Text
EventResult	Result, Usage (opt)
EventToolCallStart	ToolID (opt), ToolName
EventToolCallDelta	ToolID (opt), ToolName (opt), ToolArgs raw delta
EventToolCall	ToolID (opt), ToolName, ToolArgs
EventToolResult	ToolID (opt), ToolName (opt), ToolOutput, ToolError

Example CLI

go run ./cmd/harness-example --provider docker-agent --model coder "Hello world"
go run ./cmd/harness-example --provider claude-code --model claude-sonnet-4-6 "Hello world"
go run ./cmd/harness-example --provider pi --model claude-sonnet-4-6 "Hello world"
go run ./cmd/harness-example --provider codex --model gpt-5.4-mini "Hello world"
go run ./cmd/harness-example --provider opencode --model anthropic/claude-sonnet-4-6 "Hello world"

# Just print the command without executing:
go run ./cmd/harness-example --print-cmd --provider docker-agent --model coder "test"

License

MIT