harness

harness is a Go library for driving AI coding CLIs in headless mode. It supports Claude Code, Codex, GitHub Copilot CLI, and OpenCode through one interface while leaving process placement to the caller. A command can run on the host, inside a container, or through a remote runner with the same arguments and event parser.

Supported backends

Name	Binary	Credential environment	Staged skill path	Project instructions	Model API hosts
claude	claude	ANTHROPIC_API_KEY, CLAUDE_CODE_OAUTH_TOKEN	.claude/skills/<name>	CLAUDE.md	*.anthropic.com
codex	codex	CODEX_API_KEY	skills/<name>	AGENTS.md	api.openai.com, auth0.openai.com, chatgpt.com
copilot	copilot	COPILOT_GITHUB_TOKEN, GH_TOKEN, GITHUB_TOKEN	.github/skills/<name>	.github/copilot-instructions.md	github.com, api.github.com, api.mcp.github.com, *.githubcopilot.com
opencode	opencode	OPENAI_API_KEY, ANTHROPIC_API_KEY, OPENCODE_CONFIG_CONTENT, OPENCODE_AUTH_CONTENT	.opencode/skill/<name>	AGENTS.md	models.dev, api.openai.com, *.anthropic.com

The Copilot parser targets CLI 1.0.75 or later, where --output-format json emits JSONL in prompt mode.

The library owns the details that differ between CLIs: binary names, arguments, credential and state environment variables, project instruction files, skill directories, model API hosts, JSONL parsing, account-limit errors, default models, and token prices.

Install

go get github.com/alpha-omega-security/harness

Go 1.26 or later is required.

Core API

A Job contains resolved values for one invocation. Callers apply their own configuration defaults before constructing it.

type Job struct {
    Workspace string
    SrcDir    string
    SkillName string

    Prompt       string
    SystemPrompt string

    Model    string
    Effort   string
    MaxTurns int

    OutputFile  string
    AllowedTools string
    BaseURL      string

    ResumeSessionID string
    ResumePrompt    string
}

Workspace is the command's working directory. SrcDir is the repository directory relative to it and defaults to src; set it to . when Workspace is already the repository root. SkillName selects a staged SKILL.md; when Prompt is empty, the backend builds a short activation prompt. SystemPrompt uses --system-prompt with Claude and the backend's project instruction file for the other CLIs.

MaxTurns uses the backend default when set to zero. Effort and AllowedTools currently apply only to Claude. ResumeSessionID and ResumePrompt continue an existing conversation.

The Harness interface exposes the parts needed by local, container, and remote runners:

type Harness interface {
    Binary() string
    Args(Job) []string
    Prompt(Job) string
    ParseStream(io.Reader, func(Event))
    SkillDir(workspace, name string) string
    GuideFilename() string
    SystemPromptViaArgs() bool
    EgressHosts() []string
    Env(baseURL string) []string
    StateEnv(dir string) []string
    AccountErrorText(string) string
    DefaultModels() []ModelDefault
}

Use ByName to select a backend. An empty name selects Claude.

h, err := harness.ByName("codex")
name := harness.Name(h)
available := harness.Names() // "claude, codex, copilot, opencode"

Each parser produces the same event type:

type Event struct {
    Kind      string
    Tool      string
    Text      string
    CostUSD   float64
    Turns     int
    Usage     Usage
    SessionID string
    RateLimit *RateLimitInfo
}

Kinds are thinking, text, tool, result, error, session, and rate_limit. FormatEvent renders an event for a plain-text log. CostFromUsage calculates a list-price estimate when the CLI reports tokens without a dollar amount.

Run a local subprocess

Run starts the selected binary in the workspace, applies its environment, writes a project instruction file when needed, and parses combined output as it arrives.

package main

import (
    "context"
    "fmt"
    "log"

    "github.com/alpha-omega-security/harness"
    "github.com/alpha-omega-security/harness/egress"
    "github.com/alpha-omega-security/harness/skills"
)

func main() {
    ctx := context.Background()
    workspace := "/work/project"

    h, err := harness.ByName("claude")
    if err != nil {
        log.Fatal(err)
    }
    instructions, err := skills.Parse("/work/instructions/review.md")
    if err != nil {
        log.Fatal(err)
    }

    job := harness.Job{
        Workspace:    workspace,
        SrcDir:       ".",
        Prompt:       "Review this project for security defects.",
        SystemPrompt: skills.Concat(instructions),
        Model:        "claude-sonnet-4-6",
        MaxTurns:     20,
    }

    if err := egress.WriteSandboxSettings(workspace, h.EgressHosts()); err != nil {
        log.Fatal(err)
    }
    err = harness.Run(ctx, h, job, func(event harness.Event) {
        fmt.Println(harness.FormatEvent(event))
    })
    if err != nil {
        log.Fatal(err)
    }
}

When a non-zero exit contains a provider account-limit message, Run returns an *harness.AccountError. Its optional reset time can be used to schedule a later retry.

Use another process runner

Callers that own process creation can use the same API without Run. This is useful for containers, job queues, and remote execution.

h, err := harness.ByName("codex")
if err != nil {
    return err
}
skill, err := skills.Parse("/skills/security-review/SKILL.md")
if err != nil {
    return err
}
job := harness.Job{
    Workspace:  "/work",
    SrcDir:     ".",
    SkillName:  skill.Name,
    Model:      "gpt-5.3-codex",
    OutputFile: "report.json",
}
if err := skills.Stage(h, job, skill); err != nil {
    return err
}
if err := harness.WriteSystemPrompt(h, job); err != nil {
    return err
}
argv := append([]string{h.Binary()}, h.Args(job)...)
env := append(h.Env(job.BaseURL), h.StateEnv("/state")...)

// Pass argv and env to the process runner. Entries such as "CODEX_API_KEY"
// use the `docker run -e KEY` passthrough form.
stdout, err := startContainer(ctx, argv, env)
if err != nil {
    return err
}
h.ParseStream(stdout, func(event harness.Event) {
    fmt.Println(harness.FormatEvent(event))
})

WriteSystemPrompt is needed only when SystemPrompt is non-empty. Backends whose SystemPromptViaArgs method returns true receive that value in their arguments, so the helper does not write a guide file.

Process isolation

The generated arguments allow unattended tool use. Claude uses bypassPermissions unless AllowedTools is set. Codex uses danger-full-access, OpenCode uses --auto, and Copilot uses --allow-all. Run these commands only in a workspace and execution environment where those permissions are acceptable. Container and remote callers should apply their own filesystem, process, secret, and network limits.

The egress package can restrict outbound HTTP and HTTPS by hostname. Its proxy checks the resolved destination immediately before connecting and rejects loopback, private, link-local, carrier-grade NAT, unspecified, and multicast addresses. This closes the usual DNS rebinding path after a hostname has passed the allowlist.

Packages

skills parses SKILL.md files following the Agent Skills specification. YAML frontmatter is optional, so plain markdown instruction files parse too. Parse returns a Skill with the spec fields (name, description, license, compatibility, allowed-tools, metadata), the body, a sibling schema.json when present, and a content hash covering both. Walk finds skills under a directory; Stage writes one into the selected backend's discovery directory; Concat joins bodies for a system prompt; Render produces SKILL.md bytes from a Skill built in memory. ValidateNamespace and the Match/PathIncluded glob helpers support callers that add their own metadata keys and path filters.

egress contains the authenticated allowlist proxy and WriteSandboxSettings, which writes Claude's .claude/settings.json domain allowlist.

llm sends a single schema-constrained request to the Anthropic Messages API. It accepts a caller-owned HTTP client and permits plain HTTP only for local development endpoints.

License

MIT