agent-harness: Use Coding Agents from Rust Applications

Overview

agent-harness is a Rust library for running coding agents from Rust programs through a common Harness interface. It normalizes agent output into events for text, reasoning, tool calls, plans, usage, and lifecycle, so application code can handle different backends with a consistent run loop.

It supports existing agent CLIs and ACP agents, as well as a built-in OpenAI-compatible implementation for chat APIs and local model servers. This suits developers building agent-enabled applications who want to switch providers or integrate tools without coupling their event handling to one agent's output format.

Key Features

• Provides a shared Harness interface and RunEvent stream across supported agents.
• Supports Claude Code, OpenAI Codex, ACP agents, and OpenAI-compatible chat APIs.
• Includes a Rust tool loop for OpenAI-compatible models, including local model endpoints.
• Exposes host functions through ToolServer and HostTool integrations.
• Offers both receiver-based run and callback-based start execution patterns.
• Reports agent readiness and installation hints without installing agent software.
• Includes cli-stream, a standalone subprocess engine for streaming output and cancellation.

Use Cases

• Rust application developers can add coding-agent runs while keeping response handling independent of a specific backend.
• Teams comparing or changing agent providers can reuse one event-processing loop and change the harness implementation.
• Developers using local or hosted OpenAI-compatible models can run tool-enabled agent workflows from Rust.
• Applications with in-process functions can expose those tools to supported agents without deploying a separate tool server.

Project Facts

• Language: Rust
• License: Apache-2.0
• Stars: 1
• Forks: 0
• Topics: none listed
• Archived: No

Getting Started

Add the crate with:

cargo add agent-harness

See the repository README and the crate documentation for provider setup, feature flags, and examples.

Alternatives

• harnessrouter: HarnessRouter unifies multiple agent harnesses behind a hosted-service API, while agent-harness is a Rust library with a typed event stream.

Considerations

• The project describes itself as being in active development, and its API may change.
• Agent CLIs and model providers must be available and configured separately; the crate reports readiness but does not install them.
• Provider support and examples may depend on enabling crate features such as acp or openai-compatible.
• The repository description and README differ in the agents they mention, so check the current crate documentation for the supported integrations.