kestrion 0.2.0

A durable-execution-first framework for building production AI agents

Kestrion

A durable-execution-first framework for building production AI agents.

Status: pre-alpha (0.1.0). Core engine, decorator API, and three LLM providers are built and tested (35 passing tests). MCP integration, scheduler, CLI, and Postgres support are designed but not yet implemented — see Roadmap below.

Why Kestrion

Most agent frameworks are strong at authoring an agent loop. Kestrion is built around a narrower, specific bet: state is never mutated directly — it's derived by folding an immutable log of events. That single decision is what makes the following true by construction, not by careful discipline on the part of whoever writes a given agent:

• Crash recovery is the default. Any process — the original one or a brand new one — can reconstruct a run's exact state from the store and continue it.
• Human-approval gates pause the run itself, not just a function call. A tool marked as requiring approval can't be invoked without it, enforced centrally by the engine.
• Observability comes from the same log everything else does — token counts, cost, and full trace history, not a separate system bolted on after.

Install

pip install kestrion[anthropic]   # or [openai], [ollama], or [all]

Each LLM provider is an optional extra. If you only use Ollama, you never need the anthropic or openai packages installed.

Quickstart

import asyncio
from kestrion.agent.agent import Agent
from kestrion.agent.decorators import tool
from kestrion.llm.anthropic_provider import AnthropicProvider

@tool
def get_cluster_state() -> dict:
    """Read current deployment replica counts."""
    return {"deployment": "checkout-api", "replicas": 2}

@tool(requires_approval=True)
def apply_manifest(yaml: str) -> dict:
    """kubectl apply a manifest against the cluster."""
    # real kubectl call would go here
    return {"applied": True}

async def main():
    agent = Agent(
        provider=AnthropicProvider(model="claude-sonnet-4-6"),
        tools=[get_cluster_state, apply_manifest],
        store="sqlite:///agent_runs.db",
    )
    result = await agent.run("Check checkout-api and scale it up by one if it's under 3 replicas")
    print(result.status)   # "waiting_on_human" — paused before the mutating call
    print(result.output)

asyncio.run(main())

The run pauses with status=waiting_on_human the moment the model decides to call apply_manifest, since that tool is marked requires_approval=True. Nothing executes against the real cluster until that's explicitly approved.

Resuming a paused run

Resuming works from a completely independent process — this is the actual crash-recovery guarantee, not just a convenience method:

# Anywhere else, any time later, sharing only the same store file:
state.scratch["_approved_tools"] = {"apply_manifest": True}
# (persist that as a checkpoint — see examples/kubectl_agent for the full pattern;
#  Agent.approve() is not yet a polished one-liner, see Known Gaps below)

result = await agent.resume(run_id)
print(result.status)  # "completed"

Calling REST or SOAP APIs from a tool

@tool wraps any Python function, so calling an external API is no different from any other tool — there's no special Kestrion API for this:

import httpx
from kestrion.agent.decorators import tool

@tool
def get_order_status(order_id: str) -> dict:
    """Fetch order status from the orders API."""
    response = httpx.get(f"https://api.example.com/orders/{order_id}", timeout=10.0)
    response.raise_for_status()
    return response.json()

Any exception the function raises — a timeout, a 4xx/5xx, a connection error — is automatically caught and turned into a clean ToolResult.error rather than crashing the run, exactly like every other tool. What's not automatic: timeouts, retries, and secrets handling are on you to write explicitly. See examples/rest_api_tool for the patterns that matter in practice (explicit timeouts, gating a mutating call, reading secrets from the environment instead of hardcoding them, writing your own retry loop). SOAP follows the identical shape with zeep instead of httpx.

What you can build with this today

• Tool-calling agents where some actions are safe to auto-run and others need a human in the loop first — infrastructure agents, ops bots, anything touching a database or cluster.
• Agents that need to survive a crash or restart mid-task. agent.resume(run_id) works from a totally different process than the one that started the run.
• Long-running approval workflows — start a run, let it sit paused for hours, approve it from a Slack bot or web UI later, resume it from anywhere with access to the same store.
• Multi-turn tool use — the agent loop keeps calling tools and feeding results back to the model until it produces a final answer with no more tool calls.

Known gaps (honest, not aspirational)

• MCP client is live-verified. kestrion.mcp.client.MCPClient connects to real MCP servers (stdio or streamable-HTTP) and exposes their tools as ordinary Kestrion Tool objects. tests/unit/test_mcp_client.py runs against a real test-fixture MCP server over a real stdio subprocess connection — including the full approval-gating flow, proving an MCP-sourced tool pauses an Agent run exactly like a @tool(requires_approval=True) function does. Not yet tested against a third-party MCP server (e.g. a real kubectl-over-SSH server) — only against the purpose-built fixture.
• No MCP server side yet. Exposing a Kestrion Agent itself as an MCP server (so it's callable from Claude Code or Codex CLI) is designed but not implemented.
• Anthropic and OpenAI providers are implemented against documented API shapes but not yet smoke-tested against a live API call — no API key has been used to verify them in practice. Ollama is verified live — tests/unit/test_smoke_ollama.py runs a real agent against a real local Ollama server and passes, confirming the request-building and response-parsing logic actually works against a live model, not just against fakes.
• No real concurrency control. Running many agents at once against a shared rate limit isn't implemented.
• No CLI or deploy story. kestrion deploy --target k8s doesn't exist yet — you'd containerize and deploy this yourself today.
• Agent.approve() is a stub. Approving a paused run currently means manually setting state.scratch["_approved_tools"] and saving a checkpoint by hand (see examples/kubectl_agent), not a polished one-line call.
• SQLite only. A CheckpointStore Protocol exists so Postgres can be added without touching the engine, but that implementation doesn't exist yet.

Examples

• examples/kubectl_agent — the original worked example, demonstrating pause-on-approval and resume-after-restart using the raw Engine/Node primitives directly (useful for understanding what Agent builds on top of).
• examples/rest_api_tool — patterns for calling REST/SOAP APIs from a tool: explicit timeouts, gating a mutating call, reading secrets from the environment, and writing your own retry loop.
• tests/unit/test_smoke_ollama.py — a live, real smoke test against a local Ollama server. Useful as a template if you want to verify your own Ollama setup works with Kestrion. Skips automatically if Ollama isn't running.

Development

git clone https://github.com/VinayakDubey07/kestrion.git
cd kestrion
python3 -m venv .venv
source .venv/bin/activate
pip install -e ".[dev]"
pytest tests/ -v

Roadmap

See the full phased build plan in the repo for the path to 1.0.0. Short version: MCP client/server integration, a scheduler for safe concurrent execution, a CLI with Kubernetes deploy support, Postgres-backed storage, and a docs site are next.

License

Apache 2.0