AI SDK - Harness Specification and Agent

This package is experimental.

HarnessAgent implementation plus the underlying harness specification, including an expanded network session sandbox interface to support harness sandbox needs.

Setup

npm i ai zod @ai-sdk/harness @ai-sdk/harness-claude-code @ai-sdk/sandbox-vercel

Usage

import { HarnessAgent } from '@ai-sdk/harness/agent';
import { claudeCode } from '@ai-sdk/harness-claude-code';
import { createVercelSandbox } from '@ai-sdk/sandbox-vercel';
import { tool } from 'ai';
import { z } from 'zod/v4';

const agent = new HarnessAgent({
  harness: claudeCode,
  id: 'auth-agent',
  model: 'claude-sonnet-4-5',
  instructions:
    'You are a careful refactoring assistant. Prefer minimal diffs.',
  sandbox: createVercelSandbox({
    runtime: 'node24',
    ports: [4000],
  }),
  sandboxConfig: {
    bootstrapHash: 'ripgrep-v1',
    onBootstrap: async ({ session, abortSignal }) => {
      const streamResult = await session.run({
        command:
          'command -v rg >/dev/null || (apt-get update && apt-get install -y ripgrep)',
        abortSignal,
      });
      if (result.exitCode !== 0) {
        throw new Error(`Failed to install ripgrep: ${result.stderr}`);
      }
    },
    onSession: async ({ session, sessionWorkDir, abortSignal }) => {
      await session.writeTextFile({
        path: `${sessionWorkDir}/README.md`,
        content: 'Workspace notes for this session.',
        abortSignal,
      });
    },
  },
  tools: {
    deploy: tool({
      description: 'Deploy to a target environment',
      inputSchema: z.object({ env: z.enum(['staging', 'production']) }),
      execute: async ({ env }) => ({ url: `https://${env}.example.com` }),
    }),
  },
});

const session = await agent.createSession();

try {
  const generateResult = await agent.generate({
    session,
    prompt: 'Fix the failing test in src/auth.ts',
  });
  console.log(generateResult.text);

  // Streaming
  const streamResult = await agent.stream({
    session,
    prompt: 'Now write a regression test',
  });
  for await (const part of streamResult.stream) {
    if (part.type === 'text-delta') {
      process.stdout.write(part.text);
    }
  }
} finally {
  await session.destroy();
}

Set output on HarnessAgent to require the same typed, schema-backed output on every turn. generate() exposes the validated value as result.output, and stream() additionally exposes partialOutputStream; the JSON also remains on the normal text and stream surfaces.

import { Output } from 'ai';

const agent = new HarnessAgent({
  harness: claudeCode,
  sandbox,
  output: Output.object({
    schema: z.object({ answer: z.string() }),
  }),
});

Use session.detach() to park a bridge-backed session for later attach, session.stop() to save state and stop the sandbox, or session.destroy() to clean up without keeping resume state. Bridge-backed adapters such as Claude Code, Codex, OpenCode, and DeepAgents require a network sandbox session that exposes ports — @ai-sdk/sandbox-vercel is the supported choice today. @ai-sdk/sandbox-just-bash is suitable only for host-runtime or otherwise non-bridge flows, such as Pi.

Set model on HarnessAgent to select the model used when the harness session starts. Model identifiers are harness-specific, so model accepts any string.

sandbox is an optional HarnessV1SandboxProvider. When omitted, pass a HarnessV1NetworkSandboxSession to every agent.createSession({ sandboxSession }) call. Use sandboxConfig for agent specific sandbox configuration that works independently from the sandbox provider that is used:

• Use sandboxConfig.onSession to prepare the acquired sandbox before the harness adapter starts. The hook runs for fresh and resumed sessions, so keep it idempotent.
• Use sandboxConfig.onBootstrap for expensive sandbox setup that should be baked into a reusable snapshot, such as installing tools or cloning a large repository. Provide sandboxConfig.bootstrapHash with it and change that value whenever the bootstrap output should invalidate the cached snapshot.
• Use sandboxConfig.workDir to set a stable working directory for the agent, relative to the sandbox's default working directory; otherwise regular sessions use the existing <harnessId>-<sessionId> directory. In that case, the onBootstrap callback receives the sandbox's default working directory.

Use prepareHarnessSandboxTemplate() to create or refresh the sandbox provider's own reusable template for one harness before serving traffic. This is the replacement for prewarmHarness(), which remains as a deprecated alias.

Use prepareSandboxForHarness() when you own an existing sandbox and want to prepare it before creating your own snapshot. It applies the selected harness bootstrap recipes and sandboxConfig.onBootstrap, returns the computed preparation identity and per-harness recipe identities, and leaves snapshotting or stopping the sandbox to your code. Later, create a sandbox from that snapshot and pass the native sandbox object to createVercelSandbox({ sandbox }) for the HarnessAgent. When several bridge-backed harnesses share a caller-provided sandbox, create that sandbox with one exposed port for each harness. Then pass each harness's assigned port to that harness's create* function.

Available harnesses

See the harness adapters documentation.

Implementing a harness

Implement the HarnessV1 factory and a HarnessV1Session whose doPromptTurn emits events; the agent surface, streaming, tool execution, and multi-turn state are handled for you. Read startOpts.model for the consumer-selected model and startOpts.sandboxSession for the selected network sandbox session. The harness layer stops or destroys sessions it acquires from the provider, while a session passed to agent.createSession({ sandboxSession }) remains caller-owned. Call sandboxSession.restricted() for the tool-safe file-IO/exec/spawn surface.

Each prompt and continuation receives an optional responseFormat. JSON formats carry a caller-provided JSON Schema plus optional name and description; the adapter must enforce the schema and emit the resulting JSON through normal text parts. If the runtime cannot honor the format, throw HarnessCapabilityUnsupportedError before starting the turn.

Bootstrap recipe paths may be absolute or relative. Relative bootstrapDir and file paths are resolved against sandboxSession.defaultWorkingDirectory. The framework creates bootstrapDir before writing files, and bootstrap commands always run from that directory. Prefer a relative directory such as .harness-bootstrap/my-harness so bootstrap assets are kept with the sandbox's snapshot-persistent working tree.

import type { HarnessV1, HarnessV1Session } from '@ai-sdk/harness';

export function myHarness(): HarnessV1 {
  return {
    specificationVersion: 'harness-v1',
    harnessId: 'my-harness',
    builtinTools: {},
    doStart: async startOpts => {
      const usage = {
        inputTokens: { total: 0, noCache: 0 },
        outputTokens: { total: 0, text: 0 },
      };
      const resumeState = {
        type: 'resume-session' as const,
        harnessId: 'my-harness',
        specificationVersion: 'harness-v1' as const,
        data: {},
      };
      const continueState = {
        type: 'continue-turn' as const,
        harnessId: 'my-harness',
        specificationVersion: 'harness-v1' as const,
        data: {},
      };
      const session: HarnessV1Session = {
        sessionId: startOpts.sessionId,
        isResume:
          startOpts.resumeFrom != null || startOpts.continueFrom != null,
        doPromptTurn: async promptOpts => {
          const done = Promise.resolve().then(() => {
            promptOpts.emit({ type: 'text-start', id: 't' });
            promptOpts.emit({ type: 'text-delta', id: 't', delta: 'Hello.' });
            promptOpts.emit({ type: 'text-end', id: 't' });
            promptOpts.emit({
              type: 'finish',
              finishReason: { unified: 'stop', raw: 'stop' },
              totalUsage: usage,
            });
          });
          return { submitToolResult: async () => {}, done };
        },
        doContinueTurn: async continueOpts => {
          const done = Promise.resolve().then(() => {
            continueOpts.emit({
              type: 'finish',
              finishReason: { unified: 'stop', raw: 'stop' },
              totalUsage: usage,
            });
          });
          return { submitToolResult: async () => {}, done };
        },
        doCompact: async () => {},
        doDetach: async () => resumeState,
        doStop: async () => resumeState,
        doDestroy: async () => {},
        doSuspendTurn: async () => continueState,
      };
      return session;
    },
  };
}