H3 Harness SDK for Python

Python SDK for building H3-compliant agent harnesses.

Install

pip install h3-harness-sdk

Install fallback (source / git)

If a release isn't published to PyPI yet (or you want the latest unreleased changes), install directly from the repository:

# From git
pip install git+https://github.com/get-h3/sdk-python.git

# Editable source install (development)
git clone https://github.com/get-h3/sdk-python.git
cd sdk-python
pip install -e .

Quickstart

from datetime import datetime, timezone

from h3_harness import (
    BaseHarness,
    Decision,
    DecisionType,
    End,
    TextResponse,
    create_router,
)
from fastapi import FastAPI


class MyHarness(BaseHarness):
    def __init__(self):
        # Track sessions so cancel/session lookups 404 on unknown ids
        # (battery: test_5_9b cancel_unknown_session, test_5_10 session_not_found).
        self._sessions: dict[str, dict] = {}

    async def on_process(self, req):
        # Echo conversation history from context (battery: history preserved).
        history = list(req.context.history)
        # Streaming: "do not finish" in message -> unfinished text.
        streaming = "do not finish" in req.message.content
        finished = not streaming
        self._sessions[req.session_id] = {
            "started_at": datetime.now(timezone.utc).isoformat(),
            "turn_count": (
                self._sessions.get(req.session_id, {}).get("turn_count", 0) + 1
            ),
        }
        return Decision(
            decision=DecisionType.TEXT,
            text=TextResponse(
                content=f"Echo: {req.message.content}",
                finished=finished,
            ),
            history=history,
        )

    async def on_result(self, req):
        return Decision(decision=DecisionType.END, end=End(reason="task_complete"))

    def get_session_info(self, session_id: str) -> dict | None:
        return self._sessions.get(session_id)


app = FastAPI()
app.include_router(create_router(MyHarness()))
# Run with: uvicorn my_harness:app --port 9191

Testbed

import asyncio

from h3_harness.testbed import MockHermes
from h3_harness.examples.echo import EchoHarness


async def main() -> None:
    mock = MockHermes(EchoHarness())
    decision = await mock.send_message("Hello!")
    assert decision.text.content == "Echo: Hello!"


if __name__ == "__main__":
    asyncio.run(main())

Examples

• echo.py — Echo harness that mirrors user messages
• minimal.py — Minimal harness with health endpoint, uvicorn runner
• langchain_agent.py — LangChain integration: LLM call with text response

Passing the battery (h3-test compliance)

The gate for any H3 harness is the test battery (test_battery.py from get-h3/shim — 44 tests across 6 categories). Run it against any running harness endpoint:

# The shim is not yet published to PyPI — install from source (get-h3/shim)
pip install git+https://github.com/get-h3/shim
h3-test --endpoint http://localhost:9191   # exit 0 = compliant

The Quickstart harness above implements all four conventions and is fully battery-compliant (44/44). If you modify it, keep the conventions intact — a naive harness that drops them scores 41/44. The four conventions the battery checks (beyond "return a Decision") are:

1. Echo context.history in every Decision returned from on_process. The battery sends a session with prior history and asserts it flows back through the response (test_2_8_process_preserves_history). Pass it through explicitly:

   history = list(req.context.history)
   return Decision(..., history=history)
   

   This applies to decisions returned from on_process — the ProcessRequest carries the context field. on_result receives a ResultRequest, which has no context field (only decision_id/result/session_id); a decision returned from on_result simply omits history. Echoing req.context.history there raises AttributeError.
2. Never issue llm_call when context.models is empty. The battery sends context.models: [] and FAILS any harness that returns an llm_call decision (test_5_8_no_models_available — "hallucinated model"). Only return LLM_CALL when the request actually lists models.
3. Return text.finished=false for "do not finish" prompts. The battery sends "Just start a thought, do not finish it yet." and asserts the response has text.finished == False (test_2_4_process_text_finished_false). Detect streaming/unfinished intent and set finished accordingly.
4. 404 unknown sessions. The battery cancels a nonexistent session (test_5_9b cancel_unknown_session) and GETs one (test_5_10 session_not_found) and asserts a 404. Track sessions in the harness (get_session_info returning None for unknown ids) — the router turns that into the 404.

The canonical battery-ready template is echo.py — it implements all four conventions and scores 44/44. Use it as the starting point for your own harness.

Development

make install   # create venv + install deps
make build     # build wheel (and sdist) into dist/
make test      # run tests
make lint      # ruff check
make fmt       # ruff format

Reference

• Spec: get-h3/h3 — specs/04-SDK-Libraries.md
• Protocol: get-h3/protocol
• API reference: docs/api/index.md