OpenCode

The OpenCode harness adapter connects HarnessAgent to OpenCode through @opencode-ai/sdk. The adapter runs a bridge inside the sandbox, starts an OpenCode server in that sandbox, and streams OpenCode session events back to the host over a sandbox-exposed WebSocket.

Setup

pnpm add @ai-sdk/harness @ai-sdk/harness-opencode @ai-sdk/sandbox-vercel

npm install @ai-sdk/harness @ai-sdk/harness-opencode @ai-sdk/sandbox-vercel

yarn add @ai-sdk/harness @ai-sdk/harness-opencode @ai-sdk/sandbox-vercel

bun add @ai-sdk/harness @ai-sdk/harness-opencode @ai-sdk/sandbox-vercel

The adapter bootstraps the OpenCode bridge dependencies inside the sandbox when the first session starts. The bridge package depends on @opencode-ai/sdk and opencode-ai.

Import

import { openCode, createOpenCode } from '@ai-sdk/harness-opencode';

openCode is equivalent to createOpenCode() with its default configuration.

Basic Usage

import { HarnessAgent } from '@ai-sdk/harness/agent';
import { openCode } from '@ai-sdk/harness-opencode';
import { createVercelNetworkSandboxSession } from '@ai-sdk/sandbox-vercel';

const agent = new HarnessAgent({
  harness: openCode,
  model: 'anthropic/claude-sonnet-4-6',
});

const sandboxSession = await createVercelNetworkSandboxSession({
  runtime: 'node24',
  ports: [4000],
  template: await agent.getSandboxTemplate(),
});
const session = await agent.createSession({ sandboxSession });

let exitCode = 0;
try {
  const result = await agent.stream({
    session,
    prompt: 'Check the test failures and fix the production code.',
  });

  for await (const part of result.stream) {
    if (part.type === 'text-delta') {
      process.stdout.write(part.text);
    }
  }
} catch (err) {
  exitCode = 1;
  console.error(err);
} finally {
  await session.destroy();
  await sandboxSession.destroy();
  process.exit(exitCode);
}

To use this agent, ensure environment variables include VERCEL_OIDC_TOKEN for Vercel Sandbox, and one of the variables listed under authentication for OpenCode.

Adapter Settings

Use createOpenCode() to configure the runtime:

const harness = createOpenCode({
  reasoningVariant: 'high',
  openCodeConfig: {
    agent: {
      general: {
        model: 'openai/gpt-5.4-mini',
      },
    },
  },
});

Settings:

• auth: authentication mode (auto, anthropic, openai, or ai-gateway) or an isolated authentication environment.
• credentialForwarding: optional synchronous or asynchronous callback that customizes each credential immediately before the harness adapter forwards it into a sandbox process. It receives the credential value that would otherwise be forwarded (either the real credential or a masked value) and the environment variable name used to expose it. This callback only controls the value forwarded into the sandbox process. It does not restrict which credentials the harness adapter can discover, read, or otherwise access in the host process.
• mcpServers: MCP server definitions keyed by server name.
• openCodeConfig: additional native OpenCode configuration. Adapter-managed settings take precedence. Agent-local permission and deprecated tools settings are ignored so they cannot bypass harness permissions or built-in tool filtering.
• provider: provider id to use when model on HarnessAgent is unprefixed.
• reasoningVariant: OpenCode reasoning/thinking variant for supported models, such as low, medium, or high.
• port: bridge port override.
• startupTimeoutMs: maximum time to wait for the bridge to start.
• reconnect: reconnect timing after an established bridge WebSocket connection drops. maxElapsedMs controls the total retry window, including connection establishment and backoff delays, and defaults to 30 seconds. initialDelayMs defaults to 50 milliseconds, and maxDelayMs defaults to 2 seconds. These retries use exponential backoff and are separate from startupTimeoutMs. They cannot recover when the sandbox, bridge process, or bridge endpoint is permanently unavailable.
• mintBridgeToken: synchronous function that receives the sandbox id and returns the bridge authentication token. By default, the adapter generates a random 32-byte token. Custom implementations must return a suitably secret token.

Structured Output

OpenCode supports schema-backed HarnessAgent structured output. The adapter uses OpenCode's json_schema prompt format and returns the validated structured result as JSON text.

Authentication

The auth setting selects how credentials are resolved from the host environment:

• auto (default): use AI Gateway credentials when available, then use the credentials for the selected model provider.
• anthropic: use Anthropic credentials.
• openai: use OpenAI credentials.
• ai-gateway: use AI Gateway credentials.

When the sandbox supports additive request transformations, the bridge receives placeholders and the adapter injects credentials into matching outbound requests. Sandboxes without that capability retain direct credential forwarding.

Supported environment variables:

• AI_GATEWAY_API_KEY
• AI_GATEWAY_BASE_URL
• VERCEL_OIDC_TOKEN
• ANTHROPIC_API_KEY
• ANTHROPIC_AUTH_TOKEN
• ANTHROPIC_BASE_URL
• OPENAI_API_KEY
• OPENAI_BASE_URL
• OPENAI_ORGANIZATION
• OPENAI_PROJECT

If no applicable credential environment variable is set, the adapter attempts to resolve a native subscription from the host system unless AI Gateway authentication is selected.

Select a specific authentication mode when you do not want automatic detection:

const anthropicHarness = createOpenCode({ auth: 'anthropic' });
const openAIHarness = createOpenCode({ auth: 'openai' });
const gatewayHarness = createOpenCode({ auth: 'ai-gateway' });

Pass an authentication environment to use programmatically resolved credentials without reading process.env:

const harness = createOpenCode({
  auth: { OPENAI_API_KEY: await resolveOpenAIToken() },
  provider: 'openai',
});

The supplied record replaces the host environment for authentication discovery. Only recognized authentication variables are forwarded.

For OpenAI-compatible endpoints, select openai and set OPENAI_BASE_URL.

Sandbox

OpenCode requires a network sandbox with at least one exposed port, e.g. @ai-sdk/sandbox-vercel:

const sandboxSession = await createVercelNetworkSandboxSession({
  runtime: 'node24',
  ports: [4000],
  template: await agent.getSandboxTemplate(),
});

Built-in Tools

The adapter exposes these common OpenCode built-ins through agent.tools:

• read
• write
• edit
• bash
• glob
• grep
• ls
• webfetch
• skill
• todowrite
• agent

Additional OpenCode built-ins may also appear in agent.tools when they do not fit a common tool shape.

OpenCode supports built-in tool approval requests when permissionMode is allow-reads or allow-edits. Host-executed AI SDK tool approvals also work.

Built-in Tool Filtering

The adapter supports activeTools and inactiveTools for OpenCode built-in tools. It uses OpenCode's native permission: 'deny' rules, which keep inactive built-ins out of the model-facing tool list.

OpenCode groups its native write and edit tools under the same edit permission. Keep the Harness write and edit tools in the same active or inactive state. Filtering only one of them causes session startup to throw a HarnessCapabilityUnsupportedError.

Related

• HarnessAgent
• Harness tools
• Harness adapters