openrein 0.6.2

Minimal harness engine for building LLM agent apps (C++ + pybind11)

openrein

A minimal harness engine for building LLM agent apps in Python, powered by a C++ core via pybind11.

openrein handles the repetitive parts — message state, tool dispatch, sub-agent coordination, context compaction — while leaving model calls entirely to the developer.

Features

• Engine — message state + tool_use execution loop
• SubAgent — nested agent with its own tool registry and system prompt
• Compact — token estimation and context compaction utilities
• 8 built-in tools — Read, Write, Edit, Bash, Grep, Glob, WebFetch, WebSearch
• MCP support — connect stdio or HTTP MCP servers via add_mcp_server()
• ToolBase — pure Python base class for writing custom tools without a C++ build
• Skills — plug in reusable prompt recipes from Markdown files at runtime

Requirements

• Python >= 3.9
• CMake >= 3.18
• A C++17 compiler (MSVC, GCC, Clang)
• pybind11 >= 2.13

Installation

From PyPI

pip install openrein

Build from source

git clone https://github.com/kurt01124/openrein.git
cd openrein
pip install -e ".[dev]"

Build a wheel

Use the provided build scripts to produce a .whl file in dist/:

Windows:

scripts\build.bat

Linux / macOS:

chmod +x scripts/build.sh
./scripts/build.sh

Both scripts install the required build tools (scikit-build-core, pybind11, build) automatically and output the wheel to dist/.

Quick Start

import anthropic
import openrein

client = anthropic.Anthropic()
engine = openrein.Engine(system_prompt="You are a helpful assistant.")

@engine.tool("get_time", "Return the current ISO timestamp")
def get_time() -> str:
    from datetime import datetime
    return datetime.now().isoformat()

engine.add("user", "What time is it?")

while True:
    resp = client.messages.create(
        model="claude-opus-4-5",
        max_tokens=1024,
        system=engine.system_prompt,
        tools=engine.tool_schemas(),
        messages=engine.messages,
    )
    if engine.step(resp.model_dump()):
        break

print(engine.last_answer)

Custom Tools (ToolBase)

Extend ToolBase to add tools in pure Python — no C++ required:

import openrein

class WordCount(openrein.ToolBase):
    def name(self) -> str:
        return "word_count"

    def description(self) -> str:
        return "Count the number of words in a string"

    def input_schema(self) -> dict:
        return {
            "type": "object",
            "properties": {"text": {"type": "string"}},
            "required": ["text"],
        }

    def call(self, input: dict) -> str:
        return str(len(input["text"].split()))

engine = openrein.Engine()
engine.register_tool(WordCount())

SubAgent

import openrein

def my_model(messages, tools):
    # your model call here
    ...

sub = openrein.SubAgent(description="Summarize text", model=my_model)

@sub.tool("summarize", "Summarize the given text")
def summarize(text: str) -> str:
    return text[:100] + "..."

engine = openrein.Engine()
engine.add_subagent("summarizer", sub)

MCP Server

engine = openrein.Engine()

# stdio transport (spawns a process)
engine.add_mcp_server("filesystem", ["npx", "-y", "@modelcontextprotocol/server-filesystem", "/tmp"])

# HTTP transport
engine.add_mcp_server("my-server", "http://localhost:8080/mcp")

Built-in Tools

Tool	Description
Read	Read a file from the filesystem
Write	Write content to a file
Edit	Make targeted edits to a file
Bash	Execute a shell command
Grep	Search file contents with regex
Glob	Find files by pattern
WebFetch	Fetch content from a URL
WebSearch	Search the web

To use built-in tools:

engine = openrein.Engine(tools=True)          # enable all built-ins (default)
schemas = openrein.default_tools()            # get schemas only
engine = openrein.Engine(tools=False)         # disable built-ins

Context Compaction

compact = openrein.Compact()

if compact.should_compact(engine.messages, threshold=80000):
    prompt = compact.make_prompt(engine.messages)
    summary_text = my_model_call(prompt)
    engine.messages = compact.apply(engine.messages, summary_text)

Or use the convenience method on Engine:

engine.maybe_compact(my_model_fn, threshold=80000)

Skills

Skills are reusable prompt recipes stored as Markdown files. Load a file or an entire directory at runtime — no rebuild required.

Skill file format

---
description: stage and commit changes
when_to_use: when the user asks to commit
allowed_tools: [Read, Bash]
---

Stage all modified files and create a commit with a concise message.

Frontmatter key	Required	Description
description	yes	One-line summary shown in the skill listing
when_to_use	no	Hint for the LLM on when to trigger this skill
allowed_tools	no	Comma-separated or [bracket] list of tool names

The skill name is the filename without the .md extension.

Usage

import openrein

engine = openrein.Engine(system_prompt="You are a helpful assistant.")

# Load a directory of .md skill files
engine.skill_add("skills/")

# Or load a single file
engine.skill_add("skills/commit.md")

# Append the skill listing to your system prompt
system = engine.system_prompt + "\n\n" + engine.skill_prompt_listing()

# Inspect registered skills
for skill in engine.skill_list():
    print(skill.name, "—", skill.description)

# Inject a skill's prompt as a user message before the model call
engine.skill_inject("commit")

# Pass extra context along with the skill prompt
engine.skill_inject("commit", "Push to the main branch afterwards.")

skill_prompt_listing() produces text like:

Available skills:
- commit: stage and commit changes (use when: when the user asks to commit)
- review: review the current diff for issues (use when: when the user asks for a code review)

Removing skills

# Remove a previously added path (file or directory).
# If another loaded path contains a skill of the same name, that earlier
# definition is automatically restored.
engine.skill_remove("skills/commit.md")

Example: rein-code

example/ contains rein-code, a Claude Code-style AI coding agent CLI built on openrein.
It uses GLM (ZhipuAI) as the model backend via an OpenAI-compatible API.

Setup

rein-code depends on openrein, which must be built and installed first.

Step 1 — Build and install openrein:

# Windows
scripts\build.bat
pip install dist/openrein-*.whl

# Linux / macOS
./scripts/build.sh
pip install dist/openrein-*.whl

Step 2 — Install rein-code:

cd example
pip install -e .

Step 3 — Set your API key:

export GLM_API_KEY=<your-key>        # Linux / macOS
set GLM_API_KEY=<your-key>           # Windows CMD
$env:GLM_API_KEY = "<your-key>"      # Windows PowerShell

Run

Interactive mode (REPL):

rein-code
# or if rein-code is not on PATH:
python -m rein_code

Single task:

rein-code "Refactor the auth module to use JWT"
rein-code "Write tests for utils.py"
# or if rein-code is not on PATH:
python -m rein_code "Refactor the auth module to use JWT"

Options:

Flag	Description
--cwd PATH	Set working directory (default: current)
--new	Force a new session
--session ID	Resume a specific session

Model configuration

Environment variable	Default	Description
GLM_API_KEY	—	ZhipuAI API key (required)
REIN_MAIN_MODEL	glm-5-turbo	Main agent model
REIN_SUB_MODEL	glm-4.5-air	Sub-agent model

---

Legal notice — rein-code example

The system prompt used in example/ is derived from prompts that have been publicly disclosed and widely circulated. It is included here solely for educational and illustrative purposes to demonstrate how openrein can be used to build a coding agent.

This example is provided as-is, without warranty of any kind. The author makes no representations regarding the intellectual property status of the prompt text, and accepts no liability for any commercial use, derivative works, or legal claims arising from use of the example. Anyone who uses this example in a commercial product or service does so entirely at their own risk and responsibility.

Ecosystem

openrein-contrib — community Skills and Tools for openrein.

Install it alongside openrein to get a collection of ready-made prompt recipes and ToolBase implementations:

pip install openrein openrein-contrib

import openrein
from openrein.contrib.tools import GitHubTool, SlackTool
from openrein.contrib import SKILLS_DIR

engine = openrein.Engine(system_prompt="You are a helpful assistant.")

# Drop in all contrib skills
engine.skill_add(SKILLS_DIR)

# Register a contrib tool
engine.register_tool(GitHubTool(token="..."))

contrib is a separate package that plugs into the openrein.* namespace — no rebuild of openrein required.
See the openrein-contrib repository for the full list of available Skills and Tools.

License

MIT